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Oil palm plantations are rapidly expanding in the Brazilian eastern Amazon. Further expansion of irrigated oil palm plantations may occur because they may improve productivity during drought events compared to rainfed-only plantations. To investigate the importance of irrigation to alleviate drought-induced stress on palm trees, and its potential as a management alternative, we monitored transpiration (T, mm day–1) in a five-year-old oil palm plantation during an extreme drought caused by the El-Nino Southern Oscillation (ENSO) event in the Amazon basin. Our study spanned over the dry season (DS) and the wet season (WS), using an ongoing irrigation experiment (control with no irrigation, drip and sprinkler irrigation), to identify how differences in water supply affect the oil palm T during a period of reduced precipitation induced by ENSO. Our results indicate that independently of the irrigation system, T was higher in the DS, compared to the WS. Higher soil moisture in both the irrigation treatments, relative to the control plot, allowed for 23% higher average T relative to the control plot. The vapor pressure deficit (VPD) variation presented a higher effect over oil palm transpiration than photosynthetic active radiation. In both irrigation systems, a higher rate of transpiration with increases in VPD was observed than in the control plot during ENSO drought. Additionally, within individual palm fronds, we also observed differences in VPD tolerance (higher for lower and older leaves) and avoidance (higher for upper and newer leaves). Results indicate that irrigation increased fruit production by 35% and 26% in the sprinkler and drip irrigation systems, respectively. Overall, these results demonstrate the potential role of irrigation to alleviate water stress and maintain productivity of palm oil plantations during periods of moisture stress, while highlighting the possible feedback of increasing transpiration on water balance and hydrologic regulation.

Understanding relations among forest carbon (C) uptake and water use is critical for predicting forest-climate interactions. Although the basic properties of tree-water relations have long been known, our understanding of broader-scale patterns is limited by several factors including: 1) incomplete understanding of drivers of change in coupled C and water fluxes, and water use efficiency (WUE); 2) difficulty in reconciling WUE estimates obtained at different scales; and 3) uncertainty in how evapotranspiration (ET) and WUE vary with other important resources such as nitrogen (N). To address these issues, we examined ET, gross primary production (GPP) and WUE at eleven AmeriFlux sites across North America. Our analysis spanned leaf and ecosystem scales, and included foliar δ13C, δ18O and %N measurements, eddy covariance estimates of GPP and ET, and remotely sensed estimates of canopy %N. We used flux data to derive ecosystem WUE (WUEe) and foliar δ13C to infer intrinsic WUE (iWUE). We found that GPP, ET and WUEe scaled with canopy %N, even when environmental variables were considered, and discuss the implications of these relationships for forest-atmosphere-climate interactions. We observed opposing patterns of WUE at leaf and ecosystem scales, and examined uncertainties to help explain these opposing patterns. Nevertheless, significant relationship between C isotope-derived ci/ca and GPP indicates that δ13C can be an effective predictor of forest GPP. Finally, we show that incorporating species functional traits - wood anatomy, hydraulic strategy and foliar %N -into a conceptual model improved the interpretation of Δ13C and δ18O vis-a-vis leaf to canopy water-carbon fluxes.

Tropical montane cloud forests (TMCFs) are dynamic ecosystems defined by frequent, but intermittent, contact with fog. The resultant microclimate can vary considerably over short spatial and temporal scales, affecting the ecophysiology of TMCF plants. We synthesized research to date on TMCF carbon and water fluxes at the scale of the leaf, plant and ecosystem and then contextualized this synthesis with tropical lowland forest ecosystems. Mean light-saturated photosynthesis was lower than that of lowland forests, probably due to the effects of persistent reduced radiation leading to shade acclimation. Scaled to the ecosystem, measures of annual net primary productivity were also lower. Mean rates of transpiration, from the scale of the leaf to the ecosystem, were also lower than in lowland sites, likely due to lower atmospheric water demand, although there was considerable overlap in range. Lastly, although carbon use efficiency appears relatively invariant, limited evidence indicates that water use efficiency generally increases with altitude, perhaps due to increased cloudiness exerting a stronger effect on vapour pressure deficit than photosynthesis. The results reveal clear differences in carbon and water balance between TMCFs and their lowland counterparts and suggest many outstanding questions for understanding TMCF ecophysiology now and in the future.

Throughout a single day, the microclimatic conditions in tropical montane cloud forests vary from strong solar radiation with simultaneous strong atmospheric water demand (high vapor pressure deficit, VPD, values), followed quickly by clouds and fog events drastically reducing both of these environmental variables. Due to the complex topography and weather patterns in these regions, microclimate, including fog events, can vary strongly across very small spatial scales as well, leading to a highly dynamic and compelling environment to examine how environmental variables influence tree water use across space and time. Due to this variation, the present study examines how environmental drivers of tree water use varies across three slope positions (upslope, midslope, low-slope) in a tropical montane cloud forest in Veracruz, Mexico. Measurements of sap flow using the heat ratio method were conducted on three dominant canopy species along with simultaneous measurements of microclimate within each site. To assess the relative importance of microclimatic variables in explaining tree water use across diurnal periods, data were separated into day and night periods and fog and clear events. Multiple regression models were conducted for each tree with input variables of VPD, solar radiation, air temperature, shallow soil moisture, deep soil moisture, and leaf wetness. We found that VPD explained a large majority of the variation in tree water use during daytime fog periods, particularly at the upslope and midslope sites. During nighttime periods, VPD was the dominant driver of water use variation during clear periods while a combination of VPD and leaf wetness explained variation during night, fog periods. Additionally, tree water use was more decoupled from environmental variables at the low-slope site. Finally, a separation of model components into fog and clear periods improved model outputs particularly at low flow conditions, highlighting the differential interactions between tree water use and environment during night and fog periods. Results from this study provide new insight into the importance of fog events, low VPD, and leaf surface wetting at controlling tree water use in cloud forests. The variation in drivers of water use across short spatial scales demonstrate the importance of considering individual and species level variation across fog and clear periods in predicting physiological responses of species to climate in cloud forests.

It is generally assumed that forests improve ecosystem service (ES) provisioning within landscapes. These assumptions drive policies (e.g. Payment for Ecosystem Services) that affect land-use without knowing if the desired services are achieved. Here we use a data-intensive approach to explore the synergies and tradeoffs between three regulating (hydrologic regulation, water quality, carbon storage) and one supporting ES (biodiversity). Using field-based measurements for ten ES indicators collected within eight land use/land cover (LULC) types we assess: (1) the relationship between ES indicators and LULC type and (2) the synergies and tradeoffs across ES indicators. For objective one, we found that primary forests promote more favorable hydrological services, including having greater base flow, flow regulation, and soil conductivity. For objective two, we observed synergies across many ES where management of one would improve provisioning for several other ES, specifically between low flow, carbon storage, and biodiversity. However, many ES parameters (e.g. water quality) had no relationship with other ES parameters. Our results underscore the value of site-specific research in addressing assumptions about the relationship between LULC and ES provisioning. More site-specific data is needed for more informed design of management strategies that can maximize ES benefits.

A number of studies show that payments for ecosystem services (PES) programs contribute to reducing deforestation, but few have measured the impact on ecosystem service provision or compared economic benefits and costs of these programs. We integrate impact evaluation and ecosystem service measurement to examine the effects of PES programs in Veracruz, Mexico. We use quasi-experimental methods to measure the effect of enrolling in PES on forest cover. We link these changes in forest cover to field-calibrated measures of water regulation and carbon storage. After converting these changes in ecosystem services to monetary values we calculate the net benefits of the programs. We find that PES reduces losses of forest cover but that the additional gains in forest cover are small. However, these gains in forest lead to positive water regulation and carbon storage services. The net benefits of the PES programs over a 10-year period are as large as USD 3.2 million for watershed services and USD 4.9 million for water plus carbon storage. However, when there is no additionality in forest cover due to PES, the programs result in economic losses. Adopting this type of integrated modeling of PES impacts sheds light on the economic efficiency of these approaches.

The northeastern U.S. is projected to experience more frequent short-term (1−2 month) droughts interspersed among larger precipitation events. Agroforestry practices such as silvopasture may mitigate these impacts of climate change while maintaining economic benefits of both agricultural and forestry practices. This study evaluated the effects of forest-to-silvopasture (i.e., 50 % thinning) conversion on the components of evapotranspiration (transpiration, rainfall interception, and soil evaporation) during the growing season of 2016. The study coincided with a late-summer drought throughout the northeastern U.S., which allowed us to also evaluate the effects of forest-to-silvopasture conversion on drought responses of multiple tree species, including Pinus strobus, Tsuga canadensis, and Quercus rubra. In the reference forest and silvopasture, we observed declining soil moisture and tree water use during the drought for all three tree species. However, the decline in P. strobus water use in response to declining soil moisture in the silvopasture was not as steep as compared with the reference forest, resulting in greater water use in the silvopasture for this species. In contrast, we did not detect different water-use responses between forest and silvopasture in T. canadensis or Q. rubra. This suggests that forest-to-silvopasture conversion via thinning can alleviate drought stress for P. strobus and that this species may be more sensitive to moisture stress when competition for water is high in denser stands. Evapotranspiration was 35 % lower in the silvopasture compared with the reference forest, primarily a result of lower transpiration and rainfall interception. While soil evaporation was greater in the silvopasture, this was not enough to offset the considerably lower transpiration and interception. We observed greater radial tree growth 1–3 years following conversion in the silvopasture as compared with the reference forest for T. canadensis and Q. rubra, but not for P. strobus. Overall, our results suggest that forest conversion to silvopasture (in lieu of clearcutting for new pasture) may mitigate the impacts of agricultural land use intensification and climate change on ecosystem services, especially in terms of sustaining hydrologic regulation functions. Further study is required to determine the generality of these results and whether these benefits extend beyond the first few years post-conversion.

Tree transpiration is important in the recycling of precipitation in the Amazon and might be negatively affected by El Niño–Southern Oscillation (ENSO)–induced droughts. To investigate the relative importance of soil moisture deficits versus increasing atmospheric demand (VPD) and determine if these drivers exert different controls over tree transpiration during the wet season versus the dry season (DS), we conducted sap flow measurements in a primary lowland tropical forest in eastern Amazon during the most extreme ENSO-induced drought (2015/2016) recorded in the Amazon. We also assessed whether trees occupying different canopy strata contribute equally to the overall stand transpiration ( T stand ). Canopy trees were the primary source of T stand . However, subcanopy trees are still important as they transpired an amount similar to other biomes around the globe. Tree water use was higher during the DS, indicating that during extreme drought trees did not reduce transpiration in response to low soil moisture. Photosynthetically active radiation and VPD exerted an overriding effect on water use patterns relative to soil moisture during extreme drought, indicating that light and atmospheric constraints play a critical role in controlling ecosystem fluxes of water. Our study highlights the importance of canopy and subcanopy trees to the regional water balance and highlights the resilience to droughts that these trees show during an extreme ENSO event. This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.

1. Context. Precipitation regimes are changing in response to climate change, yet understanding of how forest ecosystems respond to extreme droughts and pluvials remains incomplete. As future precipitation extremes will likely fall outside the range of historical variability, precipitation manipulation experiments (PMEs) are critical to advancing knowledge about potential ecosystem responses. However, few PMEs have been conducted in forests compared to short‐statured ecosystems, and forest PMEs have unique design requirements and constraints. Moreover, past forest PMEs have lacked coordination, limiting cross‐site comparisons. Here, we review and synthesize approaches, challenges, and opportunities for conducting PMEs in forests, with the goal of guiding design decisions, while maximizing the potential for coordination.2. Approach. We reviewed 63 forest PMEs at 70 sites worldwide. Workshops, meetings, and communications with experimentalists were used to generate and build consensus around approaches for addressing the key challenges and enhancing coordination.3. Results. Past forest PMEs employed a variety of study designs related to treatment level, replication, plot and infrastructure characteristics, and measurement approaches. Important considerations for establishing new forest PMEs include: selecting appropriate treatment levels to reach ecological thresholds; balancing cost, logistical complexity, and effectiveness in infrastructure design; and preventing unintended water subsidies. Response variables in forest PMEs were organized into three broad tiers reflecting increasing complexity and resource intensiveness, with the first tier representing a recommended core set of common measurements.4. Conclusions. Differences in site conditions combined with unique research questions of experimentalists necessitate careful adaptation of guidelines for forest PMEs to balance local objectives with coordination among experiments. We advocate adoption of a common framework for coordinating forest PME design to enhance cross‐site comparability and advance fundamental knowledge about the response and sensitivity of diverse forest ecosystems to precipitation extremes.

We assessed the effects of heater wattage on sap flux estimates from heat dissipation sensors and generated calibrated equations for 1-year-old Eucalyptus grandis Hill ex Maiden trees. We used a total of eight trees ranging from 3 to 6 cm in diameter. Our calibration experiment was performed with a modified tree-cut approach, which allowed us to estimate gravimetric water use manually weighing 20 l buckets every 15 min while sap flux was monitored on each tree. Our results indicate that changes the current supplied to the heaters from 0.15 to 0.25 W does not significantly influence sap flux estimates, as long as the maximum temperature (Tmax) is properly determined for each period when wattage is different, and natural temperature gradients are corrected. Using the original parameters developed for this method, sap flux density and sap flow had an average underestimation of 53%, which according to our analysis had a reduced but relevant correlation with tree diameter (R2 = 0.3, linear regression). These results may allow researchers to supply different currents to heat dissipation sensors to increase sensitivity or to reduce power consumption. They also provide evidence in favor of the correction and use of raw data collected when unwanted changes in wattage occur. The relationship observed between estimation error and tree diameter, while not strongly significant, suggests that diameter plays an important role in the estimation errors that has not been previously considered, and requires further research.

Payments for watershed services (PWS) as a policy tool for enhancing water quality and supply have gained momentum in recent years, but their ability to lead to sustainable watershed outcomes is uncertain. Consequently, the demand for effective monitoring and evaluation (M&E) of PWS impacts on coupled human and natural systems (CHANS) and their implications for watershed sustainability (WS) is increasing. The theoretical foundations and practical applications of WS frameworks, which integrate biophysical and socioeconomic indicators to assess progress toward WS goals, have been extensively explored but rarely applied to PWS. We develop the PWS–WS framework as an approach for guiding indicator selection to improve knowledge about the complex drivers, interactions, and feedback between PWS and CHANS. A review of the PWS and WS literatures provides a basis for comparing and contrasting indicators. Using two case studies, we illustrate how applying the PWS-WS framework using a place-based, contextualized approach enhances potential for sustainable watershed outcomes.

Large-scale bioenergy production will affect the hydrologic cycle in multiple ways, including changes in canopy interception, evapotranspiration, infiltration, and the quantity and quality of surface runoff and groundwater recharge. As such, the water footprints of bioenergy sources vary significantly by type of feedstock, soil characteristics, cultivation practices, and hydro-climatic regime. Furthermore, water management implications of bioenergy production depend on existing land use, relative water availability, and competing water uses at a watershed scale. This paper reviews previous research on the water resource impacts of bioenergy production-from plot-scale hydrologic and nutrient cycling impacts to watershed and regional scale hydro-economic systems relationships. Primary gaps in knowledge that hinder policy development for integrated management of water-bioenergy systems are highlighted. Four case studies in the Americas are analyzed to illustrate relevant spatial and temporal scales for impact assessment, along with unique aspects of biofuel production compared to other agroforestry systems, such as energy-related conflicts and tradeoffs. Based on the case studies, the potential benefits of integrated resource management are assessed, as is the need for further case-specific research.

La provision y regulacion de flujos de agua en cuencas es probablemente el servicio ecosistemico mas importante de los bosques de niebla, sin embargo, su funcionamiento hidrologico y como este es alterado por el cambio en el uso de suelo es aun muy poco entendido. Este estudio evaluo los efectos hidrologicos causados por la conversion del bosque mesofilo de montana sobre suelo volcanico a otros tipos de vegetacion en Veracruz, Mexico. Para ello, se realizaron mediciones micrometeorologicas, ecofisiologicas e hidrologicas combinadas con informacion isotopica. Los resultados mostraron mayores rendimientos hidricos anuales en el pastizal y plantaciones de Pinus patula joven y maduro debido a menores tasas de evapotranspiracion comparados con el bosque maduro y secundario. El caudal total anual y estacional fue similar en los bosques, sugiriendo que con 20 anos de regeneracion natural es posible restaurar la funcionalidad hidrologica de microcuencas. En contraste, la microcuenca de pastizal reporto un mayor caudal anual (10 %), pero flujos 50 % en promedio mas bajos al final de la epoca de estiaje, asociado probablemente a una topografia mas suave y una menor capacidad de infiltracion. Aun en sustratos volcanicos altamente permeables, se observo que la conversion de bosque a pastizal puede conducir a incrementos importantes en los flujos superficiales en respuesta a eventos maximos de precipitacion. El efecto de la reforestacion con P. patula a escala de cuenca se desconoce, pero tasas de infiltracion mas altas que el pastizal sugieren una probable recuperacion hidrica del suelo en el corto a mediano plazo.

Key message Highly variable nighttime transpiration, with higher rates generally observed after preceding fog, is prevalent in dominant tree species of the nutri- ent-poor tropical montane cloud forest environment of central Veracruz, Mexico. Abstract Although nighttime transpiration (En) is pre- valent in a wide range of species from cloud-affected for- ests, its magnitude relative to total daily transpiration (Ed) as reported in the literature is generally small (En/Ed is 0.12 on average). In the present study, we observed high dry- season En/Ed ratios with large night-to-night variation in dominant species from the tropical montane cloud forest (TMCF) zone of central Veracruz, Mexico: 0.22 ± 0.18 for Quercus lancifolia (old-growth TMCF); 0.26 ± 0.14 and 0.16 ± 0.16 for Alchornea latifolia and Alnus jorull- ensis, respectively (regenerating post-fire TMCF); and 0.30 ± 0.20 to 0.12 ± 0.21 for Pinus patula (young and mature pine plantations). En was determined as the dif- ference between observed nocturnal sap flow and estimated refilling of stem water storage, the latter of which was on average: 21-25 % of nocturnal sap flow for Q. lancifolia ;6 and 5 % for A. latifolia and A. jorullensis, respectively; and 21-23 % for P. patula. Night-to-night variation in En was mostly due to large variation in vapor pressure deficit (VPD) related in turn to the alternation of cold fronts (producing fog events) and high pressure weather (pro- ducing nights with VPD up to 2 kPa). Moreover, in the hours following fog events without concurring rainfall, En was often higher as compared to fog-free nights with similar VPD across all species examined. Low-nutrient availability and high water content of the soils in the study area suggest a nutrient uptake benefit associated with the relatively high En rates observed.

The disease complex white pine needle damage (WPND), first reported in 2006, has now escalated to an epidemic state across the northeastern United States. Although this complex is composed of several fungal species, Lecanosticta acicola is considered to be the primary causal agent. Knowledge regarding the epidemiology, specific climatic factors that affect the spread of L. acicola on eastern white pine (Pinus strobus) in natural forest settings, and potential risks repeated defoliation may have on tree health is limited. Therefore, this study examined how climatic variables affect the abundance and distance of spore dispersal of L. acicola and compared litterfall caused by defoliation versus natural needle abscission. Conidia were observed on spore traps from May through August, with a peak in abundance occurring in June, corresponding to the defoliation of second- and third-year foliage measured in litter traps. During peak spore production, relative humidity and the occurrence of rainfall was found to have the greatest influence on spore abundance. Our results will aid managers in determining how far from infected trees natural regeneration will likely be affected and predicting future disease severity based on climatic conditions.

In the Northeastern U.S., drought is expected to increase in frequency over the next century, and therefore, the responses of trees to drought are important to understand. There is recent debate about whether land-use change or moisture availability is the primary driver of changes in forest species composition in this region. Some argue that fire suppression from the early twentieth century to present has resulted in an increase in shade-tolerant and pyrophobic tree species that are drought intolerant, while others suggest precipitation variability as a major driver of species composition. From this debate, an emerging hypothesis is that mesophication and increases in the abundance of mesophytic genera (e.g., Acer, Betula, and Fagus) resulted in forests that are more vulnerable to drought. This review examines the published literature and factors that contribute to drought vulnerability of Northeastern U.S. forests. We assessed two key concepts related to drought vulnerability, including drought tolerance (ability to survive drought) and sensitivity (short-term responses to drought), with a focus on Northeastern U.S. species. We assessed drought-tolerance classifications for species, which revealed both consistencies and inconsistencies, as well as contradictions when compared to actual observations, such as higher mortality for drought-tolerant species. Related to drought sensitivity, recent work has focused on isohydric/anisohydric regulation of leaf water potential. However, based on the review of the literature, we conclude that drought sensitivity should be viewed in terms of multiple variables, including leaf abscission, stomatal sensitivity, turgor pressure, and dynamics of non-structural carbohydrates. Genera considered drought sensitive (e.g., Acer, Betula, and Liriodendron) may actually be less prone to drought-induced mortality and dieback than previously considered because stomatal regulation and leaf abscission in these species are effective at preventing water potential from reaching critical thresholds during extreme drought. Independent of drought-tolerance classification, trees are prone to dieback and mortality when additional stressors are involved such as insect defoliation, calcium and magnesium deficiency, nitrogen saturation, and freeze-thaw events. Overall, our literature review shows that multiple traits associated with drought sensitivity and tolerance are important as species may rely on different mechanisms to prevent hydraulic failure and depleted carbon reserves that may lead to mortality.

We evaluated the applicability of tree-ring δWood cores (n = 4 per tree) were sampled from ten trees per species. Cores were cross-dated within and across trees at each site, and for the last 30 years. Seven years, including the driest on record, were selected for this study. The δWe found offsets of 1.1‰ and 5.6‰ between bulk and α-cellulose for δExtraction of α-cellulose was not necessary to detect climate signals in tree rings across the four investigated sites. Furthermore, the physiological information inferred from the dual isotope approach was similar for most of the species regardless of the material considered.

In stands with a broad range of diameters, a small number of very large trees can disproportionately influence stand basal area and transpiration (Et). Sap flow-based Et estimates may be particularly sensitive to large trees due to nonlinear relationships between tree-level water use (Q) and tree diameter at breast height (DBH). Because Q is typically predicted on the basis of DBH and sap flow rates measured in a subset of trees and then summed to obtain Et, we assessed the relative importance of DBH and sap flow variables (sap velocity, Vs, and sapwood depth, Rs) in determining the magnitude of Et and its dependence on large trees in a tropical montane forest ecosystem. Specifically, we developed a data-driven simulation framework to vary the relationship between DBH and Vs and stand DBH distribution and then calculate Q, Et and the proportion of Et contributed by the largest tree in each stand. Our results demonstrate that variation in how Rs is determined in the largest trees can alter estimates up to 26% of Et while variation in how Vs is determined can vary results by up to 132%. Taken together, these results highlight a great need to expand our understanding of water transport in large trees as this hinders our ability to predict water fluxes accurately from stand to catchment scales.

RationaleWe evaluated the applicability of tree-ring C-13 and O-18 values in bulk wood - instead of the more time and lab-consuming -cellulose C-13 and O-18 values, to assess climate and physiological signals across multiple sites and for six tree species along a latitudinal gradient (35 degrees 97'N to 45 degrees 20'N) of the northeastern United States.MethodsWood cores (n=4 per tree) were sampled from ten trees per species. Cores were cross-dated within and across trees at each site, and for the last 30years. Seven years, including the driest on record, were selected for this study. The C-13 and O-18 values were measured on two of the ten trees from the bulk wood and the -cellulose. The offsets between materials in C-13 and O-18 values were assessed. Correlation and multiple regression analyses were used to evaluate the strength of the climate signal across sites. Finally the relationship between C-13 and O-18 values in bulk wood vs -cellulose was analyzed to assess the consistency of the interpretation, in terms of CO2 assimilation and stomatal conductance, from both materials.ResultsWe found offsets of 1.1 and 5.6 parts per thousand between bulk and -cellulose for C-13 and O-18 values, respectively, consistent with offset values reported in the literature. Bulk wood showed similar or stronger correlations to climate parameters than -cellulose for the investigated sites. In particular, temperature and vapor pressure deficit and standard precipitation-evaporation index (SPEI) were the most visible climate signals recorded in C-13 and O-18 values, respectively. For most of the species, there was no relationship between C-13 and O-18 values, regardless of the wood material considered.ConclusionsExtraction of -cellulose was not necessary to detect climate signals in tree rings across the four investigated sites. Furthermore, the physiological information inferred from the dual isotope approach was similar for most of the species regardless of the material considered.

There is a growing interest in the use of native prairie vegetation to restore the hydrologic balance in agricultural watersheds. The objectives of this study were to investigate the effects of establishing strips of native prairie vegetation (SNPV) in agricultural fields (CROP) on soil water dynamics and assess the potential value of SNPV as an agricultural conservation practice to regulate soil water dynamics. Soil water storage (SWS) was monitored biweekly at four soil depth intervals (0–30, 30–60, 0–60, 0–100) during the growing seasons of 2007, 2008, 2009 and 2010 at the Neal Smith National Wildlife Refuge, Prairie City, Iowa. The influence of land cover (SNPV, CROP), topographic position (summit, sideslope footslope), season, and precipitation on SWS was analyzed. Results indicated that SWS was higher under prairie vegetation than CROP the year the SNPV was established (2007), while a reverse pattern was observed for the following three post-treatment years. More pronounced differences among land covers were observed in the first 30 cm of soil. Overall, June–July had the highest SWS and August–September the lowest, but the trend varied within years. In 2009 and 2010, years of low and high precipitation, respectively, the differences in SWS among land covers were minimal and no statistical differences were observed. Overall, results indicate that the introduction of SNPV can contribute to the regulation of the water balance in agricultural watersheds by increasing water holding capacity, and hence, reducing runoff during high rainfall events.

The goal of this study was to determine the effects of atmospheric demand on both plant water relations and daily wholetree water balance across the upper limit of a cloud forest at the mean base height of the trade wind inversion in the tropical trade wind belt. We measured the microclimate and water relations (sap flow, water potential, stomatal conductance, pres sure–volume relations) of Metrosideros polymorpha Gaudich. var. polymorpha in three habitats bracketing the cloud forest’s upper limit in Hawai‘i to understand the role of water relations in determining ecotone position. The subalpine shrubland site, located 100 m a bove the cloud forest boundary, had the highest vapor pressure deficit, the least amount of rainfall and the highest levels of nighttime transpiration (EN) of all three sites. In the shrubland site, on average, 29% of daily whole-tree transpiration occurred at night, while on the driest day of the study 50% of total daily transpiration occurred at night. While EN occurred in the cloud forest habitat, the proportion of total daily transpiration that occurred at night was much lower (4%). The average leaf water potential (Ψleaf) was above the water potential at the turgor loss point (ΨTLP) on both sides of the ecotone due to strong stomatal regulation. While stomatal closure maintained a high Ψleaf, the minimum leaf water potential (Ψleafmin) was close to ΨTLP, indicating that drier conditions may cause drought stress in these habitats and may be an important driver of current landscape patterns in stand density.

a b s t r a c t Reductions of nitrogen (N) export from agricultural lands because of changes in specific N stocks and fluxes by incorporation of small amounts of prairie vegetation strips (PVS) are poorly understood. The primary objective of this study was to evaluate the effect of the presence and topographical position of PVS on soil and plant carbon (C) and N stocks relative to annual crop and native prairie vegetation. The study was implemented within three small adjacent watersheds, treated with one of the following cover types: (1) 100% row-crop agriculture (CROP); (2) 20% prairie vegetation (PVS) distributed along the contour across three topographical positions: upslope, sideslope and footslope position; and (3) 100% 17-year old reconstructed native prairie (RNP) as the control condition. Total soil organic C (SOC), total soil N (TN), inorganic N availability as indexed by ion exchange resins, N stocks in plant biomass and litter, and the ratio of C3:C4 plant species were measured during the 2010 growing season. Results showed that over five years of treatment, PVS footslope improved soil quality by increasing TN by almost 100% and SOC by 37%; while CROP footslope TN decreased by 31% and SOC decreased by 28%. Overall, N stocks in plant biomass and litter were higher in PVS compared with RNP, except in the footslope where the lower N plant stocks was associated with higher C3 abundance in RNP. Nitrogen availability was higher in CROP (25.4 ± 1.4), followed by PVS (10.2 ± 1.3), and RNP (2.2 ± 1.4); with the highest values recorded in the upslope position for PVS and RNP, and the footslope for CROP. These findings are important for designing watersheds with PVS to reduce N accumulation in the footslope position and promote additional N retention in soil organic

Global climate models suggest enhanced warming of the tropical mid and upper troposphere, with larger temperature rise rates at higher elevations. Changes in fire activity are amongst the most significant ecological consequences of rising temperatures and changing hydrological properties in mountainous ecosystems, and there is a global evidence of increased fire activity with elevation. Whilst fire research has become popular in the tropical lowlands, much less is known of the tropical high Andean region (2000 masl, from Colombia to Bolivia). This study examines fire trends in the high Andes for three ecosystems, the Puna, the Paramo and the Yungas, for the period 1982-2006. We pose three questions: (i) is there an increased fire response with elevation? (ii) does the El Niño- Southern Oscillation control fire activity in this region? (iii) are the observed fire trends human driven (e.g., human practices and their effects on fuel build-up) or climate driven? We did not find evidence of increased fire activity with elevation but, instead, a quasicyclic and synchronous fire response in Ecuador, Peru and Bolivia, suggesting the influence of high-frequency climate forcing on fire responses on a subcontinental scale, in the high Andes. ENSO variability did not show a significant relation to fire activity for these three countries, partly because ENSO variability did not significantly relate to precipitation extremes, although it strongly did to temperature extremes. Whilst ENSO did not individually lead the observed regional fire trends, our results suggest a climate influence on fire activity, mainly through a sawtooth pattern of precipitation (increased rainfall before fire-peak seasons (t-1) followed by drought spells and unusual low temperatures (t0), which is particularly common where fire is carried by low fuel loads (e.g., grasslands and fine fuel). This climatic sawtooth appeared as the main driver of fire trends, above local human influences and fuel build-up cyclicity.

Demand for locally-produced food is growing in areas outside traditionally dominant agricultural regions due to concerns over food safety, quality, and sovereignty, rural livelihoods, and environmental integrity. Strategies for meeting this demand rely upon agricultural land use change, in various forms of either intensification or extensification (converting non-agricultural land, including native landforms, to agricultural use). The nature and extent of the impacts of these changes on non-food-provisioning ecosystem services are determined by a complex suite of scale-dependent interactions among farming practices, site-specific characteristics, and the ecosystem services under consideration. Ecosystem modeling strategies which honor such complexity are often impenetrable by non-experts, resulting in a prevalent conceptual gap between ecosystem sciences and the field of sustainable agriculture. Referencing heavily forested New England as an example, we present a conceptual framework designed to synthesize and convey understanding of the scale- and landscape-dependent nature of the relationship between agriculture and various ecosystem services. By accounting for the total impact of multiple disturbances across a landscape while considering the effects of scale, the framework is intended to stimulate and support the collaborative efforts of land managers, scientists, citizen stakeholders, and policy makers as they address the challenges of expanding local agriculture.

Nitrogen (N) and phosphorus (P) from agricultural landscapes have been identified as primary sources of excess nutrients in aquatic systems. The main objective of this study was to evaluate the effectiveness of prairie filter strips (PFS) in removing nutrients from cropland runoff in 12 small watersheds in central Iowa. Four treatments with PFS of different spatial coverage and distribution (No-PFS, 10% PFS, 10% PFS with strips, and 20% PFS with strips) were arranged in a balanced incomplete block design across four blocks in 2007. A no-tillage two-year corn (Zea mays L.) –soybean (Glycine max [L.] Merr.) rotation was grown in row-cropped areas beginning in 2007. Runoff was monitored by H flumes, and runoff water samples were collected during the growing seasons to determine concentrations of nitrate-nitrogen (NO3-N), total nitrogen (TN) and total phosphorus (TP) through 2011. Overall, the presence of PFS reduced mean annual NO3-N, TN, and TP concentrations by 35%, 73%, and 82%, respectively, and reduced annual NO3-N, TN, and TP losses by 67%, 84%, and 90%, respectively. However, the amount and distribution of PFS had no significant impact on runoff and nutrient yields. The findings suggest that utilization of PFS at the footslope position of annual row crop systems provides an effective approach to reducing nutrient loss in runoff from small agricultural watersheds.

Tree transpiration is important in the recycling of precipitation in the Amazon and might be negatively affected by El Niño-Southern Oscillation (ENSO)-induced droughts. To investigate the relative importance of soil moisture deficits versus increasing atmospheric demand (VPD) and determine if these drivers exert different controls over tree transpiration during the wet season versus the dry season (DS), we conducted sap flow measurements in a primary lowland tropical forest in eastern Amazon during the most extreme ENSO-induced drought (2015/2016) recorded in the Amazon. We also assessed whether trees occupying different canopy strata contribute equally to the overall stand transpiration (

Rapid expansion in biomass production for biofuels and bioenergy in the Americas is increasing demand on the ecosystem resources required to sustain soil and site productivity. We review the current state of knowledge and highlight gaps in research on biogeochemical processes and ecosystem sustainability related to biomass production. Biomass production systems incrementally remove greater quantities of organic matter, which in turn affects soil organic matter and associated carbon and nutrient storage (and hence long-term soil productivity) and off-site impacts. While these consequences have been extensively studied for some crops and sites, the ongoing and impending impacts of biomass removal require management strategies for ensuring that soil properties and functions are sustained for all combinations of crops, soils, sites, climates, and management systems, and that impacts of biomass management (including off-site impacts) are environmentally acceptable. In a changing global environment, knowledge of cumulative impacts will also become increasingly important. Long-term experiments are essential for key crops, soils, and management systems because short-term results do not necessarily reflect long-term impacts, although improved modeling capability may help to predict these impacts. Identification and validation of soil sustainability indicators for both site prescriptions and spatial applications would better inform commercial and policy decisions. In an increasingly inter-related but constrained global context, researchers should engage across inter-disciplinary, inter-agency, and international lines to better ensure the long-term soil productivity across a range of scales, from site to landscape.

The ecophysiology of tropical montane cloud forest (TMCF) trees is influenced by crown-level microclimate factors including regular mist/fog water inputs, and large variations in evaporative demand, which in turn can significantly impact water balance. We investigated the effect of such microclimatic factors on canopy ecophysiology and branch-level water balance in the dry season of a seasonal TMCF in Veracruz, Mexico, by quantifying both water inputs (via foliar uptake, FU) and outputs (day- and night-time transpiration, NT). Measurements of sap flow, stomatal conductance, leaf water potential and pressure-volume relations were obtained in Quercus lanceifolia, a canopy-dominant tree species. Our results indicate that FU occurred 34% of the time and led to the recovery of 9% (24 ± 9.1 L) of all the dry-season water transpired from individual branches. Capacity for FU was independently verified for seven additional common tree species. NT accounted for approximately 17% (46 L) of dry-season water loss. There was a strong correlation between FU and the duration of leaf wetness events (fog and/or rain), as well as between NT and the night-time vapour pressure deficit. Our results show the clear importance of fog and NT for the canopy water relations of Q. lanceifolia.

Intensively managed annual cropping systems have produced high crop yields but have often produced significant ecosystem services alteration, in particular hydrologic regulation loss. Reconversion of annual agricultural systems to perennial vegetation can lead to hydrologic function restoration, but its effect is still not well understood. Therefore, our objective was to assess the effects of strategic introduction of different amounts and location of native prairie vegetation (NPV) within agricultural landscapes on hydrological regulation. The study was conducted in Iowa (USA), and consisted of a fully balanced, replicated, incomplete block design whereby 12 zero-order ephemeral flow watersheds received four treatments consisting of varying proportions (0%, 10%, and 20%) of prairie vegetation located in different watershed positions (footslope vs. contour strips). Runoff volume and rate were measured from 2008 to 2010 (April–October) with an H-Flume installed in each catchment, and automated ISCO samplers. Over the entire study period, we observed a total of 129 runoff events with an average runoff volume reduction of 37% based on the three treatments with NPV compared to watersheds with row crops. We observed a progressively greater reduction across the 3 years of the study as the perennial strips became established with the greatest differences among treatments occurring in 2010. The differences among the watersheds were attributed mainly to NPV amount and position, with the 10% NPV at footslope treatment having the greatest runoff reduction probably because the portion of NPV filter strip that actually contacted watershed runoff was greater with the 10% NPV at footslope. We observed greater reductions in runoff in spring and fall likely because perennial prairie plants were active and crops were absent or not fully established. High antecedent soil moisture sometimes led to little benefit of the NPV treatments but in general the NPV treatments were effective during both small and large events. We conclude that, small amounts of NPV strategically incorporated into corn-soybean watersheds in the Midwest US can be used to effectively reduce runoff., Funding for this project was provided by the Leopold Center for Sustainable Agriculture, Iowa State University College of Agriculture and Life Sciences, USDA Forest Service Northern Research Station, Iowa Department of Agriculture and Land Stewardship, USDA North Central Region SARE program, and USDA-AFRI Managed Ecosystems program

Characterizing the variability of the radial profiles of sap velocity (v s) is a critical step to improve upscaling point measurements of v s to whole-tree sap flow. One promising approach is the use of a probability distribution function (pdf) to model radial profiles of v s, because shape parameters could potentially be generalized to trees of the same species based on the premise that the shape remain consistent regardless of the tree size and age, and over time, even though the magnitude of v s may vary with environmental conditions. The objective of this study was to characterize and assess the variability of the radial profile and to examine the validity of the premises underlying this approach by applying it to Pinus patula, one of the most widely planted tree species in the uplands of central-eastern Mexico. We measured v s with the Heat Ratio method at various sapwood depths in 18 P. patula trees with a dbh between 7.3 and 59.7cm and age between of 10 and 34 years, over a period of 1.5 years. Trees were growing in two stands: a mature forest stand and a young plantation. By fitting the Beta-pdf to hourly radial profiles of v s, we derived a lumped shape parameter (Ï?) to denote the radial position relative to sapwood depth with average v s and a scaling parameter (c s). The typical radial profile was unimodal, asymmetrical and with peak v s generally within the outermost 20-33% of the sapwood depth. However, tree-to-tree variability in Ï? was considerable among trees within the same stand and also across stands. Long-term and day-to-day variation of Ï? was marginal. The hourly dynamics of the radial profile, characterized by c s, can be explained by a linear combination of incoming shortwave radiation, vapor pressure deficit, the hour of day and their interaction (r 2=0.74). An independent field evaluation confirmed that a radial profile of fixed shape can be effectively used to estimate whole-tree sap flow with relatively low bias (4-26% underestimation) relative to cut-tree water uptake, particularly for trees for which v s observations covered at least 60% of the sapwood depth. Our findings emphasize the importance of conducting multiple v s point measurements covering most of the sapwood depth for accurate characterization of the radial profile, and demonstrate the utility of fitting a pdf to point v s measurements in order to assess the variability of v s radial profiles as well as to compute sap flow at the whole-tree level.

Twelve small watersheds in central Iowa were used to evaluate the eff ectiveness of prairie filter strips (PFS) in trapping sediment from agricultural runoff. Four treatments with PFS of different size and location (100% rowcrop, 10% PFS of total watershed area at footslope, 10% PFS at footslope and in contour strips, 20% PFS at footslope and in contour strips) arranged in a balanced incomplete block design were seeded in July 2007. All watersheds were in bromegrass (Bromus L.) for at least 10 yr before treatment establishment. Cropped areas were managed under a no-till, 2-yr corn (Zea mays L.)-soybean [Glycine max. (L.) Merr.] rotation beginning in 2007. About 38 to 85% of the total sediment export from cropland occurred during the early growth stage of rowcrop due to wet field conditions and poor ground cover. The greatest sediment load was observed in 2008 due to the initial soil disturbance and gradually decreased thereafter. The mean annual sediment yield through 2010 was 0.36 and 8.30 Mg ha-1 for the watersheds with and without PFS, respectively, a 96% sediment trapping efficiency for the 4-yr study period. The amount and distribution of PFS had no significant impact on runoff and sediment yield, probably due to the relatively large width (37-78m) of footslope PFS. The findings suggest that incorporation of PFS at the footslope position of annual rowcrop systems provides an effective approach to reducing sediment loss in runoff from agricultural watersheds under a no-till system.

Upwind advective energy occurring in edge environments may increase tree water use. In humid agricultural landscapes, advection-enhanced transpiration in riparian buffers may provide hydrologic regulation. The objectives of this study were to determine how water use by trees growing in a riparian buffer in central Iowa is influenced by 1) advective energy, 2) tree position, and 3) thinning (40% LAI reduction). We measured meteorological variables and sap flux density from July to September in 13 trees (2009) and 12 trees (2010) in one thinned plot (TP, treated in Aug 2010) and one untreated (control) plot (UP). The difference in Qs between edge and interior trees (39% higher in 2009) was attributed to the advective energy at the buffer edge. After thinning, maximum Qs increased was greater in TP compared to UP, explained primarily by solar radiation (R=0.7, p

Summary This paper compares the water budgets of two adjacent micro-catchments covered by mature (MAT) and 20-year-old secondary (SEC) lower montane cloud forests, respectively, in central Veracruz, Mexico over a 2-year period. Rainfall ( P ) and streamflow ( Q ) were measured continuously, whereas dry canopy evaporation (transpiration E t ), wet canopy evaporation (rainfall interception I ), and cloud water interception ( CWI ) were quantified using a combination of field measurements and modeling. Mean annual P was 3467 mm, of which typically 80% fell during the wet season (May–October). Fog interception occurred exclusively during the dry season (November–April), and was ⩽2% of annual P for both forests. Rainfall interception loss was dominated by post-event evaporation of intercepted water rather than by within-event evaporation. Therefore, the higher overall I (i.e. including CWI ) by the MAT (16% of P vs. 8% for the SEC) reflects a higher canopy storage capacity, related in turn to higher leaf area index and greater epiphyte biomass. Annual E t totals derived from sapflow measurements were nearly equal for the MAT and SEC (∼790 mm each). Total annual water yield calculated as P minus ( E t + I ) was somewhat higher for the SEC (2441 mm) than for the MAT (2077 mm), and mainly reflects the difference in I . Mean annual Q was also higher for the SEC (1527 mm) than for the MAT (1338 mm), and consisted mostly of baseflow (∼90%). Baseflow recession rates were nearly equal between the two forests, as were stormflow coefficients (4% and 5% for MAT and SEC, respectively). The very low runoff response to rainfall is attributed to the high infiltration and water retention capacities of the volcanic soils throughout the ∼2 m deep profile. The water budget results indicate that ∼875 and 700 mm year −1 leave the SEC and MAT as deep groundwater leakage, which is considered plausible given the fractured geology in the study area. It is concluded that 20 years of natural regeneration following cloud forest disturbance in central-eastern Mexico is capable of producing near-original hydrological behavior.

Despite their critical role as freshwater resources and their vulnerability to anthropogenic pressures, our knowledge of the ecohydrology of tropical montane cloud forests remains limited. Here, we use a dual stable isotope approach (δ2H and δ18O) to trace water inputs, fluxes, and pools through a seasonally dry tropical montane cloud forest in central Veracruz, Mexico. We found strong seasonal variation in the isotope composition of precipitation inputs (δ2H ~130‰ and δ18O ~18‰), with significantly more depleted wet-season values as compared with dry-season values. These seasonal differences are subject to inter-annual variation, as reflected by shifts in the local meteoric water line. Even at the peak of the dry season, stable isotope values of plant xylem water from six different deciduous and evergreen species occurring in the understory and canopy of mature and secondary forests were all consistent with the use of shallow soil water (20–60 cm). Alternative limiting factors, such as nutrient distribution along the soil profile, may be strongly contributing to plant rooting patterns. Stable isotope values of soil water were evaporatively enriched compared with precipitation inputs, whereas stream values plotted with precipitation on the local meteoric water line. In contrast to current hydrologic models, this indicates the presence of two distinct soil water pools, one highly mobile pool of precipitation that contributes to streams and a second less mobile pool of soil water that contributes to plant fluxes. This comprehensive perspective can provide the basis for generating process-based models of ecohydrology in the future. Copyright © 2011 John Wiley & Sons, Ltd.

Midwestern savannas historically covered >10 M ha in central North America, but are now rare due to agricultural conversion and anthropogenic modifications to disturbance regimes – particularly fire suppression. Throughout this range, Midwestern savannas are characterized by scattered overstory trees; however, with fire suppression, these systems are invaded by non-savanna trees. Restoration of encroached savannas involves removal of invading trees, yet little is known about the impacts of encroachment or encroachment removal on the relict savanna overstory trees, which define these systems. Here, we use tree ring analysis to investigate savanna tree growth rates in encroached, non-encroached, and experimentally restored Midwestern oak savannas in central Iowa. We found that woody encroachment led to pronounced declines in growth rate (ring width) of relict overstory white oak ( Quercus alba ), relative to Q. alba trees in competition-free, open-grown conditions, or in an encroachment-free remnant woodland. To further understand effects of encroachment removal on relict Q. alba savanna trees, we conducted a large-scale restoration experiment, where encroaching trees were mechanically removed from four encroached savannas, with an additional four savannas retained as encroached controls. Restoration led to elevated tree growth rates, with these changes generally persistent through 7 years post-restoration (2003–2009). Over the course of this post-restoration study period, ring width, basal area increment, and relative basal area increased by 49%, 59%, and 55%, respectively, in trees from restored sites, relative to trees from encroached, control sites. These results suggest that woody encroachment has strong influence on overstory savanna trees, through increased competitive dynamics; however, woody encroachment removal may help to restore relict savanna tree growth rates, even after prolonged periods of encroachment (>40 years). To restore the oak savannas at our sites, and perhaps elsewhere, we advocate a three step process: (1) mechanical woody encroachment removal, (2) maintenance of the encroachment-free state through prescribed fire, and (3) promotion of a diverse understory layer, characteristic of oak savanna in our region. While promoting oak regeneration will be important for the long-term maintenance of these sites as oak savanna, relict savanna trees appear responsive to restoration and should maintain overstory conditions through the near-term.

Fire in tropical montane cloud forests (TMCFs) is not as rare as once believed. Andean TMCFs sit immediately below highly flammable, high-altitude grasslands (Puna/Paramo) that suffer from recurrent anthropogenic fire. This treeline is a zone of climatic tension where substantial future warming is likely to force upward tree migrations, while increased fire presence and fire impacts are likely to force it downwards. TMCFs contain large carbon stocks in their peat soils and their loss through fire is a currently unaccounted for regional source of CO2. This study, conducted in the southern Peruvian Andes (>2800 m), documents differences in live tree biomass, fine root biomass, fallen and standing dead wood, and soil organic carbon in 4 paired-sample plots (burned versus control) following the severe ground fires that occurred during the 2005 Andean drought. Peat soils contributed the most to biomass burning emissions, with lower values corresponding to an 89% mean stock difference compared to the controls (mean ± SE) (54.1 ± 22.3 vs. 5.8 ± 5.3 MgC ha−1). Contrastingly, carbon stocks from live standing trees differed by a non-significant 37% lower value in the burned plots compared to the controls, largely compensated by vigorous resprouting (45.5 ± 17.4 vs. 69.2 ± 13.4 MgC ha−1). Both standing dead trees and fallen dead wood were significantly higher in the burned plots with a three-fold difference from the controls: dead Trees 45.2 ± 9.4 vs. 16.4 ± 4.4 MgC ha−1, and ca. a 2 fold difference for the fallen dead wood: 11.2 ± 5 vs. 6.7 ± 3.2 MgC ha−1 for the burned plots versus their controls. A preliminary estimate of the regional contribution of biomass burning emissions from Andean TMCFs for the period 2000–2008, resulted in mean carbon emission rates of 1.3 TgC yr−1 (max-min: 1.8–0.8 TgC yr−1). This value is in the same order of magnitude than South American annual fire emissions (300 TgC yr−1) suggesting the need for further research on Andean forest fires. On-going projects on the region are working on the promotion of landowner participation in TMCFs conservation through REDD+ mechanism. The heart of the proposed initiative is reforestation of degraded lands with green fire breaks enriched with economically valuable Andean plant species. The cultivation of these species may contribute to reduce deforestation pressure on the Amazonian cloud forest by providing an alternative income to local communities, at the same time that they prevent the spread of fire into Manu National Park and adjacent community-held forests, protecting forest and reducing CO2 emissions.

Riparian buffers are designed as management practices to increase infiltration and reduce surface runoff and transport of sediment and nonpoint source pollutants from crop fields to adjacent streams. Achieving these ecosystem service goals depends, in part, on their ability to remove water from the soil via transpiration. In these systems, edges between crop fields and trees of the buffer systems can create advection processes, which could influence water use by trees. We conducted a field study in a riparian buffer system established in 1994 under a humid temperate climate, located in the Corn Belt region of the Midwestern U.S. (Iowa). The goals were to estimate stand level transpiration by the riparian buffer, quantify the controls on water use by the buffer system, and determine to what extent advective energy and tree position within the buffer system influence individual tree transpiration rates. We primarily focused on the water use response (determined with the Heat Ratio Method) of one of the dominant species (Acer saccharinum) and a subdominant (Juglans nigra). A few individuals of three additional species (Quercus bicolor, Betula nigra, Platanus occidentalis) were monitored over a shorter time period to assess the generality of responses. Meteorological stations were installed along a transect across the riparian buffer to determine the microclimate conditions. The differences found among individuals were attributed to differences in species sap velocities and sapwood depths, location relative to the forest edge and prevailing winds and canopy exposure and dominance. Sapflow rates for A. saccharinum trees growing at the SE edge (prevailing winds) were 39% greater than SE interior trees and 30% and 69% greater than NW interior and edge trees, respectively. No transpiration enhancement due to edge effect was detected in the subdominant J. nigra. The results were interpreted as indicative of advection effects from the surrounding crops. Further, significant differences were document in sapflow rates between the five study species, suggesting that selection of species is important for enhancing specific riparian buffer functions. However, more information is needed on water use patterns among diverse species growing under different climatic and biophysical conditions to assist policy and management decisions regarding effective buffer design.

William J. McConnell, James D. A. Millington, Nicholas J. Reo, Marina Alberti, Heidi Asbjornsen, Lawrence A. Baker, Nicholas Brozov, Laurie E. Drinkwater, Scott A. Drzyzga, Jose, Fragoso, Daniel S. Holland, Claire A. Jantz, Timothy Kohler, Herbert D. G. Maschner, Michael Monticino, Guillermo Podesta, Robert Gilmore Pontius, Jr., Charles L. Redman, David Sailor, Gerald Urquhart, and Jianguo Liu. (2011). Research on Coupled Human and Natural Systems (CHANS): Approach, Challenges, and Strategies. Bulletin of the Ecological Society of America April: 218-228.

Close relationships among climatic factors and soil respiration (Rs) are commonly reported. However, variation in Rs across the landscape is compounded by site-specific differences that impede the development of spatially explicit models. Among factors that influence Rs, the effect of ecosystem age is poorly documented. We hypothesized that Rs increases with grassland age and tested this hypothesis in a chronosequence of tallgrass prairie reconstructions in central Iowa, U.S.A. We also assessed changes in root biomass, root ingrowth, aboveground net primary productivity (ANPP), and the strength of soil temperature and moisture in predicting Rs. We found a significant increase in total growing season Rs with prairie age (R2 = 0.79), ranging from 714 g C m−2 in the youngest reconstruction (age 4) to 939 g C m−2 in the oldest prairie (age 12). Soil temperature was a strong predictor of intra-seasonal Rs among prairies (R2 = 0.78–0.87) but mean growing season soil temperature and moisture did not relate to total Rs. The increase in Rs with age was positively correlated with root biomass (r = 0.80) and ANPP (r = 0.87) but not with root ingrowth. Our findings suggest that growing season Rs increases with tallgrass prairie age, root biomass, and ANPP during young grassland development.

Montane ecosystems are known for their high numbers of endemic species, unique climate conditions, and wide variety of ecosystem services such as water supply and carbon storage. Although many ecohydrological and climatic studies of montane environments have been carried out in temperate and boreal regions, few have been done in Neotropical regions. Hence, the objective of this review is to synthesize the existing literature on the main factors (biotic and abiotic) that influence vegetation distribution, functional traits, and ecohydrological processes and feedbacks in tropical montane ecosystems and to identify key knowledge gaps. Most of the literature used includes work conducted in Neotropical montane rainforests, cloud forests, and grass/scrublands (e.g., páramos, punas, and campos de altitude/rupestres). Fog is a major climatic attribute in tropical montane habitats. We found that fog regimes (frequency and intensity of fog events) influence both water inputs (i.e., canopy interception and foliar water uptake) and outputs (evapotranspiration) and represent an important driver of local species composition, dominance of plant functional types, and ecological functioning. The stability and conservation of tropical montane ecosystems depends on such ecohydrological fluxes, which are sensitive to increases in air temperature and changing precipitation and fog regimes. Furthermore, to better inform effective conservation and restoration strategies, more work is needed to elucidate how key ecohydrological processes are affected by land use conversion to agriculture and pasture lands, as human activities influence the water budgets in Neotropical montane watersheds not only at regional‐scales but also globally.

Rainfall and cloud water interception (CWI) were determined for a mature and a 19-year old secondary lower montane cloud forest in central Veracruz, Mexico. Cloud water was measured using a passive fog gauge, and consisted most likely of a mixture of fog and wind-driven drizzle. CWI by the canopy was derived from the wet canopy water budget as throughfall + stemflow + calculated interception loss minus rainfall. Rainfall interception loss was calculated using the Liu model, parameterized for events with rain-only. Precipitation events with cloud water input occurred exclusively during the dry season (November-April), and were primarily associated with cold fronts. CWI was estimated at 6% of dry season rainfall (640 mm on average) for the secondary forest vs. 8% for the mature forest, whereas annual values were ≤2% of total rainfall (3180 mm). Infrequent fog occurrence and low wind speeds were the most important reasons for the observed low values of CWI. Total apparent interception loss (i.e. including CWI) was 17% of annual rainfall for the mature forest and 8% for the secondary forest. Post-event evaporation of intercepted water stored in the canopy rather than within-event evaporation dominated interception loss at both forests. Hence, the higher loss observed for the mature forest is considered to reflect a higher canopy storage capacity, related in turn to a higher Leaf Area Index and larger epiphyte biomass. © 2010 Elsevier B.V. All rights reserved.

Summary 1. Savannas throughout the world are characterized by spatial gradients of resources created by scattered overstorey trees. These gradients are important for maintenance of understorey biodiversity; however, they may be disrupted by woody encroachment, resulting in alterations to understorey vegetation. Little is known about the status of understorey gradients in encroached savannas, or whether they redevelop during restoration and if so, by what mechanism. 2. We used a large-scale restoration experiment with Midwestern oak savannas (USA) to address these issues. We established understorey transects radiating from overstorey Quercus alba L. trees to inter-canopy gaps in four control and four restoration treatment sites. Along each transect, we sampled understorey vegetation over three field seasons and we recorded physical factors in the final year of study. 3. Restoration produced a strong relationship between visible sky (e.g. light penetrating to the understorey) and distance from overstorey trees, while none existed in control sites. Restored sites had greater variability in soil moisture due to higher levels immediately after rain at all distances from trees, coupled with greater drying rates, particularly at farther distances from overstorey trees. With restoration, a positive relationship between cover by understorey vegetation and distance from overstorey trees developed and strengthened over time, whereas no relationship existed in control sites. Cover by each of the major functional groups, forbs, graminoids, and woody species, contributed to this pattern. Furthermore, after restoration, species richness increased with distance from overstorey trees in the final year of study. 4. Nonmetric multidimensional scaling (NMS) showed that common understorey species were correlated with gradients of canopy cover and soil moisture, which were associated with restoration plots, and gradients of soil texture and N, which were associated with both restoration and control plots. Furthermore, restoration strengthened correlations between NMS plot scores and distance from overstorey trees. 5. Synthesis and applications. Re-establishing overstorey structure was an important step during restoration of the oak savannas in this study. While encroached savannas contained a homogenized understorey, restored sites contained understorey patterning typical of intact savannas. The timeframe for re-establishment ‐ within years of restoration, after decades of degradation ‐ demonstrates high resiliency and suggests restorability of other highly degraded sites; however, we recognize the importance of prescribed fire for maintaining open savanna structure and, probably, promoting further understorey pattern development.

The scattered tree layer that defines savannas is important for structuring the understory community and determining patterns of overstory recruitment. However, encroachment by woody plants has altered overstory tree densities and regeneration dynamics. We characterized seedling success of the savanna-forming species Quercus alba within Midwestern (USA) oak savannas that had been degraded by encroachment (control; n = 4) or experimentally restored by removal of encroaching woody vegetation (treatment; n = 4). In early 2004, 981 seedlings were transplanted along transects radiating from tree boles of overstory Q. alba trees to inter-canopy gaps and monitored for three growing seasons. Seedlings in restored sites had greater survival (>2×), height growth (by >50%), and basal diameter growth (by >20%). In general, seedling survival and growth parameters increased with distance from overstory trees and were greatest in inter-canopy gaps of restored sites. By the final growing season (2006), the seedling survival-by-distance from tree correlation was stronger in control (r2 = 0.25) than treatment sites (r2 = 0.18), due to relatively uniform (and greater) survival at all distances from trees in treatment sites. In 2006, growth parameters (seedling height, diameter, Δ height, Δ diameter, and # leaves) were significantly (and more strongly) positively correlated with distance from trees in treatment sites. However, seedling herbivory was also greater after treatment and increased with distance from overstory trees. To understand seedling/microenvironment relationships, we created logistic (survival) and linear regression models (Δ height, Δ basal diameter, # leaves in 2006). Control seedling models had consistently greater predictive power and included more variables, suggesting that savanna restoration may decouple seedlings from their microenvironments, potentially by decreasing competition for limiting resources. Encroachment of the savannas in this study is limiting regeneration of Q. alba, suggesting substantially altered regeneration dynamics from those under which these savannas originally formed. Initial responses from our test of restoration, however, were promising and mechanical encroachment removal may be a means to promote overstory regeneration of this species. Finally, the savannas in this study appear inherently unstable and a scattered canopy tree configuration is unlikely to persist without regular disturbance, even in the restoration sites. Repeated mechanical thinning treatments with selected retention of recruiting Q. alba individuals or reintroduction of understory fire or grazing animals may be potential mechanisms for promoting long-term persistence of savannas at these sites.

In agricultural landscapes, variation and ecological plasticity in depth of water uptake by annual and perennial plants is an important means by which vegetation controls hydrological balance. However, little is known about how annual and perennial plants growing in agriculturally dominated landscapes in temperate humid regions vary in their water uptake dynamics. The primary objective of this study was to quantify the depth of water uptake by dominant plant species and functional groups growing in contrasting annual and perennial systems in an agricultural landscape in Central Iowa. We used stable oxygen isotope techniques to determine isotopic signatures of soil water and plant tissue to infer depth of water uptake at five sampling times over the course of an entire growing season. Our results suggest that herbaceous species (Zea mays L., Glycine max L. Merr., Carex sp., Andropogon gerardii Vitman.) utilized water predominantly from the upper 20 cm of the soil profile and exhibited a relatively low range of ecological plasticity for depth of water uptake. In contrast, the woody shrub (Symphoricarpos orbiculatus Moench.) and tree (Quercus alba L.) progressively increased their depth of water uptake during the growing season as water became less available, and showed a high degree of responsiveness of water uptake depth to changes in precipitation patterns. Co-existing shrubs and trees in the woodland and savanna sites extracted water from different depths in the soil profile, indicating complementarity in water uptake patterns. We suggest that deep water uptake by perennial plants growing in landscapes dominated by rowcrop agriculture can enhance hydrologic functioning. However, because the high degree of ecological plasticity allows some deep-rooted species to extract water from surface horizons when it is available, positive effects of deep water uptake may vary depending on species’ growth patterns and water uptake dynamics. Knowledge about individual species’ and plant communities’ depth of water uptake patterns in relation to local climate conditions and landscape positions can provide valuable information for strategically incorporating perennial plants into agricultural landscapes to enhance hydrologic regulation.

Savannas’ defining feature is a scattered tree layer. However, woody plants have encroached savannas throughout the world, altering tree densities and potentially modifying regeneration dynamics. We used a replicated large-scale restoration experiment with Midwestern oak savannas (USA) to understand spatial patterns of regeneration by the dominant overstory species, Quercus alba . Q. alba is not only of interest as a savanna-forming species, but also a species of concern due to widespread low rates of regeneration. In this experiment, four sites received a restoration treatment, whereby all encroaching mesophytic trees and large shrubs were mechanically removed, and four sites operated as encroached controls. Within each site, we monitored naturally occurring Q. alba seedlings in plots under overstory trees and in plots in inter-canopy gaps for 4 years, including 1 year before and 3 years after treatment. Seedlings were ∼5× more abundant below overstory Q. alba trees, which we attribute to seed-rain. There was no effect of restoration on seedling densities. Year-to-year survival and survival of seedlings tracked over the course of the study were greater in sites undergoing restoration. Four-year survival rates were 31% in treatment sites and 10% in control sites. Across all sites, seedlings survived equally as well under canopy trees as in inter-canopy gaps. Mean seedling height, basal diameter, and number of leaves were greater in sites undergoing restoration, relative to control sites, and in inter-canopy gaps, relative to under canopy trees. Furthermore, there was an interaction between treatment and plot type, whereby mean seedling attributes were greatest in inter-canopy gaps of treatment sites. Seedlings tracked over the course of this study displayed greater survival, Δ (change in) height, Δ basal diameter, and Δ number of leaves in treatment sites, relative to control sites, and greater survival, Δ basal diameter, and Δ number of leaves in inter-canopy gaps, relative to under canopy trees. Individual seedling survival, Δ basal diameter, and Δ number of leaves were greatest in inter-canopy gaps of treatment sites. Our study suggests that woody encroachment is limiting regeneration of Q. alba in savannas; however, this may be ameliorated by removal of encroaching woody vegetation. Thus, the act of restoring these savannas may hold promise for promoting regeneration of Q. alba . Finally, the restored savannas in this study appear inherently unstable, as seedlings performed best in inter-canopy gaps. Further work with prescribed fire and/or grazing may elucidate stable tree–herbaceous understory coexistence in Midwestern oak savannas.

Landscape-scale conversion of deeply rooted perennial native vegetation to annual crops has altered the ecohydrological balance in the Midwestern U.S. One of the major ways this may occur is through plant species’ abilities to access and take up water from different depths in the soil profile. In this study, we use stable isotope techniques to assess plant water uptake patterns for plants growing in four contrasting annual and perennial ecosystems in central Iowa: row-cropped cornfield, prairie, oak savanna, and woodland. To address limitations of conventional approaches to analyzing isotopic data, especially when isotopic profiles in the soil are complex, we employ an alternative ‘multiple source mass-balance isotopic’ approach suitable for determining probable contributions of multiple potential sources to total plant water uptake. Observed differences in isotopic profiles among the plant communities were attributed to differences in infiltration rates, plant water uptake rates, the degree of evaporative isotopic fractionation under the different vegetative covers, and processes of hydraulic redistribution. Savanna ( Quercus macrocarpa Mixch) and woodland ( Q. macrocarpa , Umus americana L.) trees had the lowest stem water δ 18 O values (−6.12‰, −7.77‰, and −7.61‰, respectively), whereas corn ( Zea mays L.) and prairie ( Andropogon gerardii ) exhibited higher stem water δ 18 O values (−5.49‰ and −5.85‰, respectively), suggesting more shallow sources of water uptake for the latter. Frequency histographs produced from the multiple source mass-balance analysis confirmed that the corn and prairie species obtained up to 45% and 36% of their water from the upper 0–20 cm soil horizon, respectively. In contrast, oak trees growing in the savanna and woodland obtained up to 40% and 20% of their water from the upper 20 cm, and up to 60% and 80% of their water from depths >60 cm, respectively. Lack of strong isotopic gradients in the soil profiles combined with irregularities in the slope of the isotopic gradient by depth complicated interpretation of the data. However, an important advantage of the alternative multiple source mass-balance isotopic approach over conventional approaches was its capability of producing more quantitative and objective estimates for the probable contributions of multiple sources to total plant water uptake. This study highlights how changes in land use and vegetative cover in the Midwest affects plant water uptake patterns, and suggests how these changes may influence larger landscape scale alterations in the hydrologic balance.

Oak savannas, once common in the Midwest, are now isolated remnants within agricultural landscapes. Savanna remnants are frequently encroached by invasive trees to becomewoodlands. Thinning and prescribed burning can restore savanna structure, but the ecohydrological effects of managing these remnants are poorly understood. In this study, we measured sap flow (Js) to quantify transpiration in an Iowa bur oak (Quercus macrocarpa) savanna woodland encroached by elms (Ulmus americana), and in an adjacent restored savanna after thinning to remove elms, during summer 2004. Savanna oaks had greater mean daily Js (35.9 L dm � 2 day � 1 ) than woodland oaks (20.7 L dm � 2 day � 1 ) and elms (12.4 L dm � 2 day � 1 ). The response of Js to vapor pressure deficit (D) was unexpectedly weak, although oaks in both stands showed negative correlation between daily Js and D for D > 0.4 kPa. An earlier daily peak in Js in the elm trees showed a possible advantage for water uptake. As anticipated, thewoodland’s stand transpiration was greater (1.23 mm day � 1 ) than the savanna’s (0.35 mm day � 1 ), yet the savanna achieved 30% of the woodland’s transpiration with only 11% of its sapwood area. The difference in transpiration influenced water table depths, which were 2 m in the savanna and 6.5 m in the woodland. Regionally, row-crop agriculture has increased groundwater recharge and raised water tables, providing surplus water that perhaps facilitated elm encroachment. This has implications for restoration of savanna remnants. If achieving a savanna ecohydrology is an aim of restoration, then restoration strategies may require buffers, or targeting of large or hydrologically isolated remnants. # 2007 Published by Elsevier B.V.

Woody encroachment has altered oak savannas throughout much of the Midwestern United States. To help understand restoration options, we assessed the impacts of mechanically removing encroaching mesophytic trees and shrubs on structure, composition, and regeneration dynamics in Quercus alba dominated oak savannas in Iowa (n = 4), relative to control sites (n = 4). We monitored stand structure and species composition for the seedling, shrub, sapling, and overstory tree strata for 1 year prior to and 3 years following mechanical removal of encroaching woody vegetation. There was no evidence for differences between treatment and control sites for any study variable prior to treatment implementation. The removal treatment resulted in increased cover by understory vegetation and concomitant reductions in cover by leaf litter and bare ground. Treatment altered overstory tree species composition (assessed by multi-response permutation procedure [MRPP]) in favor of oak species in all 3 years following removal, while shrub and sapling compositions were not statistically different from control sites until the third year following the removal treatment. Seedling composition was unaffected by treatments. We observed a recruitment pulse, with treated sites displaying increased density of shrubs 2 years after and saplings 3 years after removal of encroaching vegetation. Advanced regeneration (saplings size class) was dominated by two species: Ostrya virginiana and Cornus racemosa, which we attribute to vigorous vegetative reestablishment by stump resprouting. Seedlings of Q. alba increased in density throughout the course of this study; however, Q. alba shrub and sapling size classes were unaffected by treatment. We suggest that the encroached savannas in this study represent an alternative stable state, whereby regeneration is dominated by encroaching species even shortly after removal treatments. Continued research during future stages of restoration, which will include prescribed fire, may help to identify effective management options for controlling encroaching woody vegetation and promoting oak regeneration.