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During daylight hours, the isotope composition of leaf water generally approximates steady-state leaf water isotope enrichment model predictions. However, until very recently there was little direct confirmation that isotopic steady-state (ISS) transpiration in fact exists. Using isotope ratio infrared spectroscopy (IRIS) and leaf gas exchange systems we evaluated the isotope composition of transpiration and the rate of change in leaf water isotopologue storage (isostorage) when leaves were exposed to variable environments. In doing so, we developed a method for controlling the absolute humidity entering the gas exchange cuvette for a wide range of concentrations without changing the isotope composition of water vapour. The measurement system allowed estimation of 18O enrichment both at the evaporation site and for bulk leaf water, in the steady state and the non-steady state. We show that non–steady-state effects dominate the transpiration isoflux even when leaves are at physiological steady state. Our results suggest that a variable environment likely prevents ISS transpiration from being achieved and that this effect may be exacerbated by lengthy leaf water turnover times due to high leaf water contents.

Predicting how individual taxa will respond to climatic change is challenging, in part because the impacts of environmental conditions can vary markedly, even among closely related species. Studies of chipmunks (Tamias spp.) in Yosemite National Park provide an important opportunity to explore the reasons for this variation in response. While the alpine chipmunk (T.alpinus) has undergone a significant elevational range contraction over the past century, the congeneric and partially sympatric lodgepole chipmunk (T.speciosus) has not experienced an elevational range shift during this period. As a first step toward identifying the factors underlying this difference in response, we examined evidence for dietary changes and changes in cranial morphology in these species over the past century. Stable isotope analyses of fur samples from modern and historical museum specimens of these species collected at the same localities indicated that signatures of dietary change were more pronounced in T.alpinus, although diet breadth did not differ consistently between the study species. Morphometric analyses of crania from these specimens revealed significant changes in cranial shape for T.alpinus, with less pronounced changes in shape for T.speciosus; evidence of selection on skull morphology was detected for T.alpinus, but not for T.speciosus. These results are consistent with growing evidence that T.alpinus is generally more responsive to environmental change than T.speciosus, but emphasize the complex and often geographically variable nature of such responses. Accordingly, future studies that make use of the taxonomically and spatially integrative approach employed here may prove particularly informative regarding relationships between environmental conditions, range changes, and patterns of phenotypic variation.

It is commonly assumed that transpiration does not occur at night because leaf stomata are closed in the dark. We tested this assumption across a diversity of ecosystems and woody plant species by various methods to explore the circumstances when this assumption is false. Our primary goals were: (1) to evaluate the nature and magnitude of nighttime transpiration, E(n), or stomatal conductance, g(n); and (2) to seek potential generalizations about where and when it occurs. Sap-flow, porometry and stable isotope tracer measurements were made on 18 tree and eight shrub species from seven ecosystem types. Coupled with environmental data, our findings revealed that most of these species transpired at night. For some species and circumstances, nighttime leaf water loss constituted a significant fraction of total daily water use. Our evidence shows that E(n) or g(n) can occur in all but one shrub species across the systems we investigated. However, under conditions of high nighttime evaporative demand or low soil water availability, stomata were closed and E(n) or g(n) approached zero in eleven tree and seven shrub species. When soil water was available, E(n) or g(n) was measurable in these same species demonstrating plasticity for E(n) or g(n). We detected E(n) or g(n) in both trees and shrubs, and values were highest in plants from sites with higher soil water contents and in plants from ecosystems that were less prone to atmospheric or soil water deficits. Irrespective of plant or ecosystem type, many species showed E(n) or g(n) when soil water deficits were slight or non-existent, or immediately after rainfall events that followed a period of soil water deficit. The strongest relationship was between E(n) or g(n) and warm, low humidity and (or) windy (> 0.8 m s(-1)) nights when the vapor pressure deficit remained high (> 0.2 kPa in wet sites, > 0.7 kPa in dry sites). Why E(n) or g(n) occurs likely varies with species and ecosystem type; however, our data support four plausible explanations: (1) it may facilitate carbon fixation earlier in the day because stomata are already open; (2) it may enhance nutrient supply to distal parts of the crown when these nutrients are most available (in wet soils) and transport is rapid; (3) it may allow for the delivery of dissolved O(2) via the parenchyma to woody tissue sinks; or (4) it may occur simply because of leaky cuticles in older leaves or when stomata cannot close fully because of obstructions from stomatal (waxy) plugs, leaf endophytes or asymmetrical guard cells (all non-adaptive reasons). We discuss the methodological, ecophysiological, and theoretical implications of the occurrence of E(n) or g(n) for investigations at a variety of scales.

© 2015. American Geophysical Union. All Rights Reserved. Use of the stable isotopes of water (δD, δ18O) to determine vegetative water sources, runoff paths, and residence times generally assumes that, other than shallow evaporative enrichment, the isotopic composition of precipitation is conserved as it travels through the subsurface to the stream channel. Here we follow rainfall through a thick (up to 25m) vadose zone of soil, saprolite, and weathered bedrock mostly composed of argillite, and underlying a steep (32°) forested hillslope. We discover a persistent structured heterogeneity of water isotopes inside the hillslope. Summer dry season causes evaporative enrichment of the soil, but not in the saprolite and weathered bedrock. In winter, the mobile water, generated by successive rainstorms with widely varying isotopic composition, mixes in the vadose zone, elevating soil and rock moisture content, and eventually recharging the groundwater with isotopically invariant water similar to the seasonally averaged rainfall. Yet throughout the winter the less mobile winter soil and rock moisture remains relatively light, and water extracted from the interior of argillite lies well to the left of the local meteoric water line. This persistently light composition of soil and rock moisture and the deviation from average meteoric values suggest that subsurface fractionation, or the inheritance of paleo-meteoric rock moisture associated with rock uplift may lead to large enduring isotopic differences between high and low mobility water. These differences suggest that the use of water isotopes as tracers must consider the possibility of subsurface isotopic evolution and the influence of exchange with more tightly held water.

Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca -induced changes in ci /ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca - ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization towards any single strategy, particularly maintaining a constant ci . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca , when photosystems are saturated and water loss is large for each unit C gain. This article is protected by copyright. All rights reserved. Rising atmospheric [CO2], c(a), is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2], c(i), a constant drawdown in CO2 (c(a)-c(i)), and a constant c(i)/c(a). These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying c(a). The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to c(a). To assess leaf gas-exchange regulation strategies, we analyzed patterns in c(i) inferred from studies reporting C stable isotope ratios (C-13) or photosynthetic discrimination () in woody angiosperms and gymnosperms that grew across a range of c(a) spanning at least 100ppm. Our results suggest that much of the c(a)-induced changes in c(i)/c(a) occurred across c(a) spanning 200 to 400ppm. These patterns imply that c(a)-c(i) will eventually approach a constant level at high c(a) because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant c(i). Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low c(a), when additional water loss is small for each unit of C gain, and increasingly water-conservative at high c(a), when photosystems are saturated and water loss is large for each unit C gain.

Across land plants there is a general pattern of xylem conduit diameters widening towards the stem base thus reducing the accumulation of hydraulic resistance as plants grow taller. In conifers, xylem conduits consist of cells with closed end-walls and water must flow through bordered pits imbedded in the side walls. As a consequence both cell size, which determines the numbers of walls that the conductive stream of water must cross, as well as the characteristics of the pits themselves, crucially affect total hydraulic resistance. Because both conduit size and pit features influence hydraulic resistance in tandem, we hypothesized that features of both should vary predictably with one another. To test this prediction we sampled a single tall (94.8 m) Sequoiadendron giganteum tree (giant sequoia), collecting wood samples from the most recent annual ring progressively downwards from the tree top to the base. We measured tracheid diameter and length, number of pits per tracheid, and the areas of pit apertures, tori, and margos. Tracheid diameter widened from treetop to base following a power law with an exponent (tracheid diameterstem length slope) of approximately 0.20. A similar scaling exponent was found between tracheid length and distance from tree top. Additionally, pit aperture, torus, and margo areas all increased (again with a power of ~0.20) with distance from tree top, paralleling the observed variation in tracheid diameter and length. Pit density scaled isometrically with tracheid length. Within individual tracheids, total permeable area of pits, measured as the sum of the margo areas, scaled isometrically with lumen area. Given that pores of the margo membrane are believed to increase in parallel with membrane area, from a strictly anatomical perspective, our results support the interpretation that pit resistance remains a relatively constant fraction of total resistance along the hydraulic pathway.

Angiosperms dominate almost every ecosystem globally, and for the vast majority, flowering is vital to successful reproduction, yet little work has characterized the dynamics of water use during flowering and its impacts on the reproductive biology of wild plants. Here we demonstrate a new implementation of the heat ratio method to measure sap flow dynamics of leaves and flowers. We employed this new method to estimate the water requirements of flowering in four tropical species: the understory tree, Hybanthus prunifolius (Violaceae), the liana Clitoria javitensis (Fabaceae), the canopy tree Cordia alliodora (Boraginaceae), the shrub Annona acuminate (Annonaceae). Our results suggest that there is substantial diversity in the timing and magnitude of water transport to flowers. Some species show almost no water flow to flowers, while flowers of other species show sap velocities approximately 30-50% that of nearby leaves. This variability is likely related to the pathway and mechanism of water transport to flowers, to floral morphology, and to evaporative demand in the floral microenvironment.

Hydrogen-isotopic abundances of lipid biomarkers are emerging as important proxies in the study of ancient environments and ecosystems. A decade ago, pioneering studies made use of new analytical methods and demonstrated that the hydrogen-isotopic composition of individual lipids from aquatic and terrestrial organisms can be related to the composition of their growth (i.e., environmental) water. Subsequently, compound-specific deuterium/hydrogen (D/H) ratios of sedimentary biomarkers have been increasingly used as paleohydrological proxies over a range of geological timescales. Isotopic fractionation observed between hydrogen in environmental water and hydrogen in lipids, however, is sensitive to biochemical, physiological, and environmental influences on the composition of hydrogen available for biosynthesis in cells. Here we review the factors and processes that are known to influence the hydrogen-isotopic compositions of lipids—especially n-alkanes—from photosynthesizing organisms, and we provide a framework for interpreting their D/H ratios from ancient sediments and identify future research opportunities.

Previous research suggests that the lifetime carbon gain of a leaf is constrained by a tradeoff between metabolism and longevity. The biophysical reasons underlying this tradeoff are not fully understood. We used a photosynthesis-leaf water balance model to evaluate biophysical constraints on carbon gain. Leaf hydraulic conductance (K(Leaf)), carbon isotope discrimination (Δ(13)C), leaf mass per unit area (LMA) and the driving force for water transport from stem to leaf (ΔΨ(Stem-Leaf)) were characterized for leaves spanning three orders of magnitude in surface area and two orders of magnitude in lifespan. We observed positive isometric scaling between K(Leaf) and leaf area but no relationship between Δ(13)C and leaf area. Leaf lifespan and LMA had minimal effect on K(Leaf) per unit leaf area, but a negative correlation exists among LMA, lifespan, and K(Leaf) per unit dry mass. During periods of leaf water loss, ΔΨ(Stem-Leaf) was relatively constant. We show for the first time that K(Leaf, mass), an index of the carbon cost associated with water use, is negatively correlated with lifespan. This highlights the importance of characterizing K(Leaf, mass) and suggests a tradeoff between resource investment in liquid phase processes and structural rigidity.

Information about the sulfur stable isotope composition (δ(34) S value) of organic materials and sediments, in addition to their nitrogen (δ(15) N value) and carbon (δ(13) C value) stable isotope compositions, can provide insights into mechanisms and processes in different areas of biological and geological research. The quantification of δ(34) S values has traditionally required an additional and often more difficult analytical procedure than NC dual analysis. Here, we report on the development of a high-throughput method that simultaneously measures the elemental and isotopic compositions of N, C and S in a single sample, and over a wide range of sample sizes and C/N and C/S ratios.We tested a commercially available CHNOS elemental analyzer in line with an isotope ratio mass spectrometer for the simultaneous quantification of N, C and S stable isotope ratios and contents, and modified the elemental analyzer in order to overcome the interference of (18) O in δ(34) S values, to minimize any water condensation that could also influence S memory, and to achieve the complete reduction of nitrogen oxides to N2 gas for accurate measurement of δ(15) N values. A selection of organic materials and soils was analyzed with a ratio of 1:1.4 standards to unknowns per run.The modifications allowed high quality measurements for N, C and S isotope ratios simultaneously (1 SD of ±0.13‰ for δ(15) N value, ±0.12‰ for δ(13) C value, and ±0.4‰ for δ(34) S value), with high throughput (75 unknowns per run) and over a wide range of element amount per capsule (25 to 500 μg N, 200-4000 μg C, and 8-120 μg S).This method is suitable for widespread use and can significantly enhance the application of δ(34) S measurements in a broad range of soils and organic samples in ecological and biogeochemical research. Copyright © 2016 John WileySons, Ltd.

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.

Previous studies have demonstrated the potential for large errors to occur when analyzing waters containing organic contaminants using isotope ratio infrared spectroscopy (IRIS). In an attempt to address this problem, IRIS manufacturers now provide post-processing spectral analysis software capable of identifying samples with the types of spectral interference that compromises their stable isotope analysis. Here we report two independent tests of this post-processing spectral analysis software on two IRIS systems, OA-ICOS (Los Gatos Research Inc.) and WS-CRDS (Picarro Inc.). Following a similar methodology to a previous study, we cryogenically extracted plant leaf water and soil water and measured the δ(2)H and δ(18)O values of identical samples by isotope ratio mass spectrometry (IRMS) and IRIS. As an additional test, we analyzed plant stem waters and tap waters by IRMS and IRIS in an independent laboratory. For all tests we assumed that the IRMS value represented the "true" value against which we could compare the stable isotope results from the IRIS methods. Samples showing significant deviations from the IRMS value (>2σ) were considered to be contaminated and representative of spectral interference in the IRIS measurement. Over the two studies, 83% of plant species were considered contaminated on OA-ICOS and 58% on WS-CRDS. Post-analysis, spectra were analyzed using the manufacturer's spectral analysis software, in order to see if the software correctly identified contaminated samples. In our tests the software performed well, identifying all the samples with major errors. However, some false negatives indicate that user evaluation and testing of the software are necessary. Repeat sampling of plants showed considerable variation in the discrepancies between IRIS and IRMS. As such, we recommend that spectral analysis of IRIS data must be incorporated into standard post-processing routines. Furthermore, we suggest that the results from spectral analysis be included when reporting stable isotope data from IRIS.

Background and Aims Cerradao (Brazilian woodland savannas) and seasonally dry forests (SDF) from southeastern Brazil occur under the same climate but are remarkably distinct in species composition. The objective of this study was to evaluate the role of soil origin in the initial growth and distribution of SDF and Cerradao species. Methods We conducted a greenhouse experiment growing Cerradao and SDF tree seedlings over their soil and the soil of the contrasting vegetation type. We evaluated soil nutrient availability and seedling survivorship, growth and leaf functional traits. Results Despite the higher nutrient availability in SDF soils,soilorigindidnotaffectseedlingsurvivorship.The three SDF species demonstrated home-soil advantage, enhanced growth with increasing soil nutrient availability and had higher growth rates than Cerradao species, even on Cerradao soils. Growth of Cerradao seedlings was not higher on Cerradao soil and, overall, was not positively correlated with soil nutrient availability. Conclusions SDF species are fast-growing species while Cerradao trees tend to be slow-growing species. Although savanna soil reduces growth of forest species, our findings suggest that soil chemical attributes, alone, does not exclude the occurrence of SDF seedlings in Cerradao and vice-versa.

Coast redwood (Sequoia sempervirens) ecosystems are strongly influenced by the presence of summer marine fog, and variation in fog frequency is closely linked to climate variation in the NE Pacific region. Because oxygen isotope composition (δ18O) of organic matter records distinct water sources (e.g. summertime fog vs. winter precipitation) and carbon isotopes (δ13C) are typically sensitive to humidity and water status, it then follows that inter-annual variation in tree-ring isotope ratios, which are coherent across multiple sites, should preserve a potentially powerful proxy for climate reconstruction. Here we present an analysis of a 50-year time series for both δ18O and δ13C values from subdivided tree rings obtained from multiple redwood trees at multiple sites. Within-site and between-site correlations were highly significant (p < 0.01) for the δ18O time series indicating a regionally coherent common forcing of δ18O fractionation. Within-site and between-site correlation coefficients were ...

The stable hydrogen isotope ratios (δD) of leaf wax n-alkanes record valuable information on plant and ecosystem water relations. It remains, however, unknown if leaf wax n-alkane δD values record only environmental variation during the brief period of time of leaf growth or if leaf wax n-alkane δD values are affected by environmental variability throughout the entire lifespan of a leaf. To resolve these uncertainties, we irrigated Populus trichocarpa trees with a pulse of deuterium-enriched water and used compound-specific stable hydrogen isotope analyses to test if the applied tracer could be recovered from leaf wax n-alkanes of leaves that were at different stages of their development during the tracer application. Our experiment revealed that only leaf wax n-alkanes from leaves that had developed during the time of the tracer application were affected, while leaves that were already fully matured at the time of the tracer application were not. We conclude from our study that under controlled environmental conditions, leaf wax n-alkanes are synthesized only early in the ontogeny of a leaf. Our experiment has implications for the interpretation of leaf wax n-alkane δD values in an environmental context, as it suggests that these compounds record only a brief period of the environmental variability that a leaf experiences throughout its life.

Nitrogen availability and N-cycling dynamics across ecosystems play a critical role in plant functioning and species distribution. Measurements of 15N natural abundance provides a way to assess ecosystem N dynamics, and the range of nitrogen stable isotope values (δ15N) for plants in an ecosystem can indicate divergent strategies for N uptake. We tested the hypotheses that the N-rich seasonally dry forest would have higher soil and leaf δ15N and a smaller range of leaf δ15N values compared to the N-poor cerradao (savanna woodland). We measured N concentration and δ15N in two soil depths and leaves of 27 woody species in cerradao and 26 in seasonally dry forest. As expected, total soil N concentration decreased while soil δ15N value increased with soil depth. Regardless of soil depth, seasonally dry forest soils had higher δ15N and total N concentration compared to cerradao soils. Foliar δ15N values varied from −6.4‰ to 5.9‰ in cerradao and from −2.3‰ to 8.4‰ in seasonally dry forest plants. Phylogenetically independent contrasts analysis and comparisons of δ15N mean values of the most abundant species and species co-occurring in both sites confirmed the hypothesis of higher δ15N for seasonally dry forest in comparison to cerradao. These results corroborate the expectation of higher soil and leaf δ15N values in sites with higher soil N availability. However, except for the most abundant species, no across-site leaf–soil (δ15N leaf –δ15N soil) differences (Δδ15N) were found suggesting that differences in leaf δ15N between cerradao and seasonally dry forest are driven by differences in soil δ15N. Variation of leaf δ15N was large in both sites and only slightly higher in cerradao, suggesting high diversity of N use strategies for both cerradao and seasonally dry forest communities.

Cellulose in plants contains oxygen that derives in most cases from precipitation. Because the stable oxygen isotope composition, δ 18 O, of precipitation is associated with environmental conditions, cellulose δ 18 O should be as well. However, plant physiological models using δ 18 O suggest that cellulose δ 18 O is influenced by a complex mix of both climatic and physiological drivers. This influence complicates the interpretation of cellulose δ 18 O values in a paleo-context. Here, we combined empirical data analyses with mechanistic model simulations to i ) quantify the impacts that the primary climatic drivers humidity ( e a ) and air temperature ( T air ) have on cellulose δ 18 O values in different tropical ecosystems and ii ) determine which environmental signal is dominating cellulose δ 18 O values. Our results revealed that e a and T air equally influence cellulose δ 18 O values and that distinguishing which of these factors dominates the δ 18 O values of cellulose cannot be accomplished in the absence of additional environmental information. However, the individual impacts of e a and T air on the δ 18 O values of cellulose can be integrated into a single index of plant-experienced atmospheric vapor demand: the leaf-to-air vapor pressure difference (VPD). We found a robust relationship between VPD and cellulose δ 18 O values in both empirical and modeled data in all ecosystems that we investigated. Our analysis revealed therefore that δ 18 O values in plant cellulose can be used as a proxy for VPD in tropical ecosystems. As VPD is an essential variable that determines the biogeochemical dynamics of ecosystems, our study has applications in ecological-, climate-, or forensic-sciences.

Summary 1. Seasonal and inter-annual variability in ecosystem carbon dioxide (CO2) exchange is attributed to numerous climate drivers. However, climate effects on metabolism often override ecological functions. This study seeks insight into which biological and ecological processes influence temporal patterns of ecosystem productivity in natural ecosystems. 2. The specific objectives of this study are to (i) identify seasonal and inter-annual patterns of ecosystem-level photosynthesis in relation to climatic conditions, (ii) examine and compare seasonal and inter-annual variations in leaf traits for annual grasses and oak trees across multiple years, and (iii) explore interactions among leaf traits and ecosystem-level photosynthesis across multiple seasons and years. 3. We conducted this study in a woody savanna and open grassland in California, USA. Ecosystem-level photosynthetic rates of annual grasses (Agrass) and oak tree canopy (Acanopy) were deduced from eddy covariance measurements over a 7-year period (2001 and 2007). In conjunction, we sampled grass and oak leaves at weekly to monthly intervals and constructed a multiyear time series of leaf nitrogen concentration (N), leaf mass per unit area (LMA), leaf carbon concentration (C), and leaf carbon stable isotope discrimination (D). 4. Given the same grass community age or tree canopy age, inter-annual variations of the photosynthetic rates were up to 1–2 gC m )2 day )1 for annual grasses and oak trees while the two types of vegetation were exposed to different, wide ranges of inter-annual climate fluctuations: up to 5 � C in daily mean soil temperature, 15% in soil moisture, and 10 mol m )2 day )1 in photosynthetically active radiation. 5. While both grass and oak leaf traits varied seasonally and inter-annually, they experienced temporal patterns and seasonal peaks that were distinct from one another. Multi-year means of grass leaf N, C, D, and LMA were 2AE3%, 40AE8%, 22AE6& and 71AE 3gm )2 , respectively; multi-year means of oak leaf N, C, D, and LMA were 1AE9%, 45AE1%, 20AE5& and 132 g m )2 , respectively. 6. Based on the analysis of variance, seasonal and inter-annual terms were associated with Agrass or Acanopy up to 90% or 81%. On the other hand, variations in leaf N, LMA, C, D, and their interactions could statistically explain about 53% and 26% of variations in Agrass and Acanopy, respectively. 7. We discussed possible biological and ecological processes involved in regulating seasonal and inter-annual variability in ecosystem-level photosynthesis. Clearly, seasonal and inter-annual variation in ecosystem photosynthesis was strongly associated with the dynamics of leaf traits.

Stem photosynthesis can contribute significantly to woody plant carbon balance, particularly in times when leaves are absent or in ‘open’ crowns with sufficient light penetration. We explored the significance of woody tissue (stem) photosynthesis for the carbon income in three California native plant species via measurements of chlorophyll concentrations, radial stem growth, bud biomass and stable carbon isotope composition of sugars in different plant organs. Young plants of Prunus ilicifolia, Umbellularia californica and Arctostaphylos manzanita were measured and subjected to manipulations at two levels: trunk light exclusion (100 and 50%) and complete defoliation. We found that long-term light exclusion resulted in a reduction in chlorophyll concentration and radial growth, demonstrating that trunk assimilates contributed to trunk carbon income. In addition, bud biomass was lower in covered plants compared to uncovered plants. Excluding 100% of the ambient light from trunks on defoliated plants led to an enrichment in 13C of trunk phloem sugars. We attributed this effect to a reduction in photosynthetic carbon isotope discrimination against 13C that in turn resulted in an enrichment in 13C of bud sugars. Taken together our results reveal that stem photosynthesis contributes to the total carbon income of all species including the buds in defoliated plants.

Water transport in conifers occurs through single-celled tracheids that are connected to one another via intertracheid pit membranes. These membranes have two components: the porous margo, which allows water to pass through the membrane, and the impermeable torus, which functions to isolate gas-filled tracheids. During drought, tracheids can become air filled and thus hydraulically dysfunctional, a result of air entering through the pit membrane and nucleating cavitation in the water column. What are the hydraulic tradeoffs associated with cavitation resistance at the pit level, and how do they vary within the structural components of the intertracheid pit? To address these questions, we examined pit structure in 15 species of Cupressaceae exhibiting a broad range of cavitation resistances. Across species, cavitation resistance was most closely correlated to the ratio of the torus to pit aperture diameter but did not vary systematically with margo porosity. Furthermore, our data indicate that constraints on pit hydraulic efficiency are shared: the pit aperture limits pit conductivity in more drought-resistant taxa, while increased margo resistance is more likely to control pit conductivity in species that are more vulnerable to cavitation. These results are coupled with additional data concerning pit membrane structure and function and are discussed in the context of the evolutionary biogeography of the Cupressaceae.

The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotope analysis of water is increasing. While IRIS has many advantages over traditional isotope ratio mass spectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Of particular concern is the potential for contaminants in the water sample to interfere with the spectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soil samples may often contain organic contaminants. The extent to which contaminants may interfere with IRIS and thus impact upon data quality is presently unknown. We tested the performance of IRIS relative to IRMS for water extracted from 11 plant species and one organic soil horizon. IRIS deviated considerably from IRMS for over half of the samples tested, with deviations as large as 46 per thousand (delta(2)H) and 15.4 per thousand (delta(18)O) being measured. This effect was reduced somewhat by using activated charcoal to remove organics from the water; however, deviations as large as 35 per thousand (delta(2)H) and 11.8 per thousand (delta(18)O) were still measured for these cleaned samples. Interestingly, the use of activated charcoal to clean water samples had less effect than previously thought for IRMS analyses. Our data show that extreme caution is required when using IRIS to analyse water samples that may contain organic contaminants. We suggest that the development of new cleaning techniques for removing organic contaminants together with instrument-based software to flag potentially problematic samples are necessary to ensure accurate plant and soil water analyses using IRIS.

Biogeographical, physiological, and paleoecological evidence suggests that the coast redwood [ Sequoia sempervirens (D. Don) Endl.] is closely associated with the presence of summer marine fog along the Pacific coast of California. Here we present a novel record of summer fog frequency in the coast redwood region upon the basis of direct hourly measurements of cloud ceiling heights from 1951 to 2008. Our analysis shows that coastal summer fog frequency is a remarkably integrative measure of United States Pacific coastal climate, with strong statistical connections to the wind-driven upwelling system of the California Current and the broad ocean temperature pattern known as the Pacific Decadal Oscillation. By using a long-term index of daily maximum land temperatures, we infer a 33% reduction in fog frequency since the early 20th century. We present tree physiological data suggesting that coast redwood and other ecosystems along the United States west coast may be increasingly drought stressed under a summer climate of reduced fog frequency and greater evaporative demand.

Over the past few decades, wireless technologies have revolutionized personal communications by improving flexibility and solving myriad logistical constraints. Recently, wireless sensor technologies have begun offering similar enhancements to communications between the scientist and the environment. This includes monitoring of civil structures and natural ecosystems. Although forest biology has benefited from novel technologies such as remote sensing and GIS, many ‘on the ground’ sensing technologies have not changed in many decades. What can wireless sensor networks offer the forest and agricultural scientist? We suggest at least three benefits: improved experimental design via flexible equipment deployment, improved monitoring access in logistically challenging environments (i.e. tall tree canopies) and increased density of observations for better validation of models and hypotheses. Here we briefly describe the current state of wireless environmental monitoring from an end-user point of view and report our initial attempts to adapt this cutting-edge technology to the realm of forest research.

© 2017 The Author(s). Published by IOP Publishing Ltd. Tropical dry forests (TDFs) undergo a substantial dry season in which plant species must endure several months of drought. Although TDFs support a diverse array of plant growth forms, it is not clear how they vary in mechanisms for coping with seasonal drought. We measured organic tissue stable isotopic composition of carbon (δ 13 C) and nitrogen (δ 15 N) across six plant growth forms including epiphytes, terrestrial succulents, trees, shrubs, herbs, and vines, and oxygen (δ 18 O) of four growth forms, to distinguish among patterns of resource acquisition and evaluate mechanisms for surviving annual drought in a lowland tropical dry forest in Yucatan, Mexico. Terrestrial succulent and epiphyte δ 13 C was around -14‰, indicating photosynthesis through the Crassulacean acid metabolism pathway, and along with one C 4 herb were distinct from mean values of all other growth forms, which were between -26 and -29‰ indicating C 3 photosynthesis. Mean tissue δ 15 N across epiphytes was -4.95‰ and was significantly lower than all other growth forms, which had values around +3‰. Tissue N concentration varied significantly among growth forms with epiphytes and terrestrial succulents having significantly lower values of about 1% compared to trees, shrubs, herbs and vines, which were around 3%. Tissue C concentration was highest in trees, shrubs and vines, intermediate in herbs and epiphytes and lowest in terrestrial succulents. δ 18 O did not vary among growth forms. Overall, our results suggest several water-saving aspects of resource acquisition, including the absolute occurrence of CAM photosynthesis in terrestrial succulents and epiphytes, high concentrations of leaf N in some species, which may facilitate CO 2 drawdown by photosynthetic enzymes for a given stomatal conductance, and potentially diverse N sources ranging from atmospheric N in epiphytes with extremely depleted δ 15 N values, and a large range of δ 15 N values among trees, many of which are legumes and dry season deciduous.

The stable oxygen isotope ratio (delta(18)O) of plant material has been shown to contain essential information on water and carbon fluxes at the plant and ecosystem scales. However, the effective path length (L(m)), a parameter introduced to leaf-water models still requires a comprehensive biological characterization to allow interpretation of delta(18)O values in plant material with confidence. Here, we tested the variability of L(m) across and within three species that developed leaves in environments with different relative humidity. We also tested whether the L(m) of fully developed leaves is affected by short-term fluctuations in relative humidity. We determined that significant differences in L(m) exist among Phaseolus vulgaris, Rizinus communis and Helianthus annuus. Within a given species, however, L(m) values did not differ significantly among individuals. These findings indicate that L(m) is species specific and a relatively constant parameter and that L(m) will not obscure the interpretation of delta(18)O values in plant material of a given species. We urge caution, however, because values for L(m) are derived from fitting leaf-water models to measured values of delta(18)O, so care must be taken in assigning a 'cause' to values of L(m) as they likely capture a combination of different biological leaf properties.

Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.

We photo-verified the presence of a wolverine (Gulo gulo) in California for the first time in 86 years during February 2008. Herein we document the process of determining the origin of this wolverine using genetic, stable carbon (δ13C) and stable nitrogen (δ15N) isotope information. The wolverine's origin was significant because it is a state-threatened species and California represents a historically unique genotype of wolverines in North America. We obtained both photographs and noninvasively-collected genetic evidence (scat and hair). DNA analysis revealed the animal was a male and not a remnant of a historical California population. Comparison with available data revealed the individual was most closely related to populations from the western edge of the Rocky Mountains. This represents the first evidence of connectivity between wolverine populations of the Rocky and Sierra Nevada Mountain Ranges.

Characterizing the use of carbon (C) reserves in trees is important for understanding regional and global C cycles, stress responses, asynchrony between photosynthetic activity and growth demand, and isotopic exchanges in studies of tree physiology and ecosystem C cycling. Using an inadvertent, whole-ecosystem radiocarbon (14C) release in a temperate deciduous oak forest and numerical modeling, we estimated that the mean age of stored C used to grow both leaf buds and new roots is 0.7 years and about 55% of new-root growth annually comes from stored C. Therefore, the calculated mean age of C used to grow new-root tissue is ∼0.4 years. In short, new roots contain a lot of stored C but it is young in age. Additionally, the type of structure used to model stored C input is important. Model structures that did not include storage, or that assumed stored and new C mixed well (within root or shoot tissues) before being used for root growth, did not fit the data nearly as well as when a distinct storage pool was used. Consistent with these whole-ecosystem labeling results, the mean age of C in new-root tissues determined using ‘bomb-14C’ in three additional forest sites in North America and Europe (one deciduous, two coniferous) was less than 1–2 years. The effect of stored reserves on estimated ages of fine roots is unlikely to be large in most natural abundance isotope studies. However, models of root C dynamics should take stored reserves into account, particularly for pulse-labeling studies and fast-cycling roots (

Summary • Access to deeper soil water and water-conserving traits should reduce water stress for ectomycorrhizal fungi, permitting function during drought. Here, we explored whether epigeous fruiting of ectomycorrhizal fungi during drought was facilitated by access to deep soil water, how much water was lost from sporocarps, and how sporocarp surface to volume ratios affected water-loss rates.  We used oxygen stable isotope analysis of water combined with modeling of water sources used by ectomycorrhizal fungi; measured sporocarp water loss using a transient porometer, and related water loss to vapor pressure deficit (VPD) and sporocarp morphology.  In drier soils sporocarps likely derived a significant portion (25–80%) of their water from deep (> 30 cm) or hydraulically lifted water. Amanita muscaria had water-loss rates over twice those of Suillus sp., Boletus edulis, Tricholoma spp. and Russula albonigra. Vapor pressure deficit was an excellent predictor of water-loss rates for individual mushrooms. Sporocarp surface to volume ratios explained much of the variation among mushrooms in the slope of VPD–water loss relationships.  Access to deeper soil water might be a significant driver of ectomycorrhizal symbiotic function, sporocarp distribution, fruiting habit and morphology. Sporocarp morphology can affect water-loss rates and hence influences fungal ability to fruit during summer drought.

Fog is a seasonally variable hydrologic input to coast redwood ( Sequoia sempervirens, D. Don) ecosystems with a stable isotopic composition (δ 18O, δ2H) that is distinct from rainfall. The intra-annual variation in tree ring cellulose δ18O of coast redwood was measured to determine what portion of the tree ring should be sampled to best target fog signals. Ten years of rings were subdivided into ten equal units for three trees at two sites in northern California. Intra-annual variation in cellulose δ18O was as high as 3‰ with a consistent pattern of enriched (in 18O) values from the annual ring boundary and depleted values from the central portion of the ring. These patterns show a strong coherence between replicate trees at the same site as well as significant correlations between individuals at sites over 250 km apart. Intra-annual variation in stable carbon isotope ratios (δ13C) was as high as 2‰ and produced a pattern with depleted (in 13C) values from the ring boundary and enriched values from the central portion of the ring. Although these δ13C patterns were not as strongly correlated between replicate trees as the patterns in δ18O variation, they did produce significant correlations with the variation in fog frequencies recorded at local airports. This study highlights the importance of quantifying intra-annual variation in tree ring stable isotope signals as a guide to further investigations on historic variability from long chronologies especially if the signal of interest is seasonally variable (eg, fog).

The diet of Acorn Woodpeckers (Melanerpes formicivorus) in central coastal California consists of a mixture of high-quality (insects) and low-quality (stored acorns) food during the spring breeding season. We used stable isotope ratios obtained from blood samples to estimate possible differences in the proportion of these items being fed to nestlings and being consumed by breeding adults, as well as the extent to which the diet of nestlings shifts between insects and acorns during the nestling period. Based on both feeding observations and items recovered from nestlings, older nestlings were fed 28% acorns. Using this value as a baseline for the isotope analyses, the diet of breeding adults was estimated to consist of 90% acorns. Based on repeated samples from the same nestlings over time, the estimated proportion of acorns in their diet increased from 19% in 9- to 12-d-old nestlings to 42% in 23- to 26-d-old nestlings. There was a significant correlation between the isotope values of adults and the nestlings they fed, indicating that different groups of Acorn Woodpeckers have significantly different diets. Although important for successful reproduction in this species, stored acorns are not the primary food resource used to feed young. Instead, the availability of stored acorns allows adult Acorn Woodpeckers to provide nestlings with more protein-rich insects while maintaining themselves on relatively protein-poor, low-quality acorns. Differences in the diets of adults and their nestlings are likely widespread among species that feed, at least in part, on low-quality foods, such as fruit and seeds, during the breeding season. SINOPSIS La dieta de Melanerpes formicivorus en la parte central costera de California, consiste de una mezcla de insectos y de bellotas que las aves almacenan durante la epoca reproductiva primaveral. Utilizamos tasas de isotopos estables obtenidos de muestras de sangre para estimar la posible diferencia en la proporcion de los articulos mencionados en la dieta de los adultos y con los cuales se alimentaron a pichones, ademas del cambio entre insectos y bellotas durante el periodo de cria. Basados en observaciones de los articulos con los cuales se alimentaron a pichones y en articulos recuperados de pichones, encontramos que la dieta de los pichones mas viejos era en un 28% bellotas. Usando dicho valor como base para el analisis de isotopos, encontramos que la dieta de los adultos consistia en un 90% de bellotas. Basado en muestras repetidas del mismo pichon, estimamos que la proporcion de bellotas en la dieta de los pichones jovenes incremento de 19% (en pichones de 9–12 dias) a 42% en pichones de edad mas avanzada (23–26 dias). Hubo una correlacion significativa entre los valores de los isotopos en adultos y los pichones que estos alimentaban, lo que indico que diferentes grupos de estas aves tenian significativamente dietas diferentes. Aunque las bellotas son importantes para la reproduccion de los adultos, estas no son la fuente principal de alimento para los pichones. La disponibilidad de bellotas permite que los adultos le puedan proveer a los pichones con una dieta insectivora mas rica en proteinas, mientras que ellos se mantienen en una dieta relativamente pobre en proteinas, al ingerir bellotas de baja calidad. Diferencias en la dieta de adultos y pichones es amplia, particularmente entre especies que se alimentan (al menos en parte), durante la epoca de reproduccion, de alimentos de baja calidad, tales como frutas y semillas.

After a century of fire suppression, prescribed fire and mechanical thinning are widely used to restore mixed-conifer forests in California's Sierra Nevada, yet after these treatments, trees sometimes fail to regenerate on many sites, for several possible reasons. Notably, competition between shrubs and tree seedlings for scarce water during prolonged summer dry seasons is suspected to influence seedling survival, yet this hypothesis has been tested in few manipulated field experiments. We investigated the effects of vegetation patch types, root competition, putative mycorrhizal connections, soil moisture, and microclimate on the establishment of sugar pine (Pinus lambertiana) and white fir (Abies concolor) seedlings in an old-growth, mixed-conifer forest in the southern Sierra Nevada. Seedling survival rates were significantly higher under closed tree canopies than in patches dominated by manzanita (Arctostaphylos patula) shrubs. Treatments that allowed seedlings to connect with existing ectomyc...

Simple but realistic modeling of radiation transfer within heterogeneous canopy has been a challenging research question for decades and is critical for predicting ecological processes such as photosynthesis. The Markov model proposed by [Nilson, T., 1971. A theoretical analysis of the frequency of gaps in plant stands. Agric. Meteorol. 8, 25–38] is theoretically sound to meet this challenge. However, it has not been widely used because of the difficulty of determining the clumping factor. We propose an analytical approach to calculate clumping factors based on the average characteristics of vegetation distributed across a landscape. In a savanna woodland in California, we simulate the photosynthesis of the landscape in three different ways: (1) the crown envelope and location of each tree is spatially explicitly specified, (2) the canopy is assumed to be horizontally homogeneous within which leaves are randomly dispersed as a Poisson process, and (3) the canopy is horizontally homogeneous but leaves are clumped and distributed with a Markov process. We find that the Markov model can achieve much better performance than the Poisson model by incorporating the crown-level clumping. The results indicate that our approach of calculating clumping factors has applications in terrestrial ecosystem modeling, particularly where accurate representation of “system heterogeneity” (e.g., savannas and woodlands) is required.

Stable oxygen isotope ratios (delta18O) have become a valuable tool in the plant and ecosystem sciences. The interpretation of delta18O values in plant material is, however, still complicated owing to the complex interactions among factors that influence leaf water enrichment. This study investigated the interplay among environmental parameters, leaf physiological properties and leaf water relations as drivers of the isotopic enrichment of leaf water across 17 Eucalyptus species growing in a common garden. We observed large differences in maximum daily leaf water delta18O across the 17 species. By fitting different leaf water models to these empirical data, we determined that differences in leaf water delta18O across species are largely explained by variation in the Péclet effect across species. Our analyses also revealed that species-specific differences in transpiration do not explain the observed differences in delta18O while the unconstrained fitting parameter 'effective path length' (L) was highly correlated with delta18O. None of the leaf morphological or leaf water related parameters we quantified in this study correlated with the L values we determined even though L was typically interpreted as a leaf morphological/anatomical property. A sensitivity analysis supported the importance of L for explaining the variability in leaf water delta18O across different species. Our investigation highlighted the importance of future studies to quantify the leaf properties that influence L. Obtaining such information will significantly improve our understanding of what ultimately determines the delta18O values of leaf water across different plant species.

In this study we use stable isotope analysis to investigate the climate in southern coastal California during the last 10,000 years of the Pleistocene by comparing the carbon and oxygen stable isotope ratios of preserved Douglas-fir ( Pseudotsuga menziesii ) wood from Santa Cruz Island to modern Douglas-fir wood from northern California. AMS age determination indicates that the preserved trees grew at about 16,600 ± 270 cal. yr. B.P. After isolating cellulose from both the modern and Pleistocene wood to remove any potential bias due to differential decomposition rates of wood components [Loader, N.J., Robertson, I., McCarroll, D., 2003. Comparison of stable carbon isotope ratios in whole wood, cellulose, and lignin of oak tree-rings. Palaeogeography, Palaeoclimatology, Palaeoecology 196, 395–407.], we find that the Pleistocene Douglas-fir wood has significantly greater carbon isotope discrimination and significantly more enriched δ 18 O than the modern Douglas-fir wood, which suggests that the Douglas-fir trees growing on Santa Cruz Island at ~ 16,600 cal. yr. B.P. experienced less moisture stress and used more enriched water from fog than Douglas-fir trees at Point Reyes today.

Thermometric sap flow sensors are widely used to measure water flow in roots, stems and branches of plants. Comparison of the timing of flow in branches and stems has been used to estimate water capacitance of large trees. We review studies of sap flow in branches and present our own data to show that there is wide variation in the patterns and timing of sap flow of branches in different parts of the crown, owing to the course of daily solar illuminance. In contiguous forest, east-facing and upper branches are illuminated earlier than west-facing and lower branches and most capacitance studies do not include adequate information about branch sampling regimes relative to the overall pattern of crown illuminance, raising questions about the accuracy of capacitance estimates. Measuring only upper branches and normalising these results to represent the entire crown is dangerous because flows at the stem base likely peak in response to maximum crown illuminance (and transpiration) and this will differ compared to the timing of peak flows in upper branches. We suggest that the magnitude of flow lags between branches and stems needs further study, with careful attention to branch position and method application before a robust understanding of capacitance, particularly in woody tissues of large trees, can be formed. We did not detect flow lags in the world’s tallest and largest tree species Sequoia sempervirens and Sequoiadendron giganteum, despite measurement along large pathlengths (∼57 and 85 m), which raises questions as to why large flow lags are often recorded for much smaller species. One conspicuous possibility is the different methods used among studies. Constant-heating methods such as the thermal dissipation probe (and also heat balance methods) include heat capacitance behaviour due to warming of wood tissues, which delays the response of the sensors to changing sap flow conditions. We argue that methods with intrinsic heat-capacitance present dangers when trying to measure water-capacitance in trees. In this respect heat pulse methods hold an advantage.

Summary • Leaf morphology and physiological functioning demonstrate considerable plasticity within tree crowns, with various leaf traits often exhibiting pronounced vertical gradients in very tall trees. It has been proposed that the trajectory of these gradients, as determined by regression methods, could be used in conjunction with theoretical biophysical limits to estimate the maximum height to which trees can grow. • Here, we examined this approach using published and new experimental data from tall conifer and angiosperm species. • We showed that height predictions were sensitive to tree-to-tree variation in the shape of the regression and to the biophysical endpoints selected. We examined the suitability of proposed end-points and their theoretical validity. We also noted that site and environment influenced height predictions considerably. • Use of leaf mass per unit area or leaf water potential coupled with vulnerability of twigs to cavitation poses a number of difficulties for predicting tree height. Photosynthetic rate and carbon isotope discrimination show more promise, but in the second case, the complex relationship between light, water availability, photosynthetic capacity and internal conductance to CO2 must first be characterized.

Species distributions may be limited by substrate boundaries due to constraints at either the germination or seedling establishment life history stages. In a community of alpine tundra plants from the Sierra Nevada, some species are widespread, occurring across all four substrates studied: diorite, granite, hornfels, and marble. Other species are restricted to fewer soil types. We collected seeds from 12 species and performed a reciprocal transplant experiment in growth chambers using native soils. Eleven species germinated and established equally well across all soil types, indicating substrate chemistry did not alone determine species field distributions. However, two patterns emerged that may contribute to our understanding of species composition and dominance across substrates. First, species more dominant on the drier substrates reached at least half their total germination within two weeks of planting. Species more dominant on the wetter substrates required a longer period in wet soil to ge...

Flowers face desiccating conditions, yet little is known about their ability to transport water. We quantified variability in floral hydraulic conductance (Kflower) for 20 species from 10 families and related it to traits hypothesized to be associated with liquid and vapor phase water transport. Basal angiosperm flowers had trait values associated with higher water and carbon costs than monocot and eudicot flowers. Kflower was coordinated with water supply (vein length per area, VLA) and loss (minimum epidermal conductance, gmin) traits among the magnoliids, but was insensitive to variation in these traits among the monocots and eudicots. Phylogenetic independent contrast (PIC) correlations revealed that few traits had undergone coordinated evolution. However, VLA and the desiccation time (Tdes), the quotient of water content and gmin, had significant trait and PIC correlations. The near absence of stomata from monocot and eudicot flowers may have been critical in minimizing water loss rates among these clades. Early-divergent, basal angiosperm flowers maintain higher Kflower due to traits associated with high rates water loss and water supply, while monocot and eudicot flowers employ a more conservative strategy of limiting water loss and may rely on stored water to maintain turgor and delay desiccation.

Flowers face desiccating conditions, yet little is known about their ability to transport water. We quantified variability in floral hydraulic conductance (Kflower ) for 20 species from 10 families and related it to traits hypothesized to be associated with liquid and vapour phase water transport. Basal angiosperm flowers had trait values associated with higher water and carbon costs than monocot and eudicot flowers. Kflower was coordinated with water supply (vein length per area, VLA) and loss (minimum epidermal conductance, gmin ) traits among the magnoliids, but was insensitive to variation in these traits among the monocots and eudicots. Phylogenetic independent contrast (PIC) correlations revealed that few traits had undergone coordinated evolution. However, VLA and the desiccation time (Tdes ), the quotient of water content and gmin , had significant trait and PIC correlations. The near absence of stomata from monocot and eudicot flowers may have been critical in minimizing water loss rates among these clades. Early divergent, basal angiosperm flowers maintain higher Kflower because of traits associated with high rates water loss and water supply, while monocot and eudicot flowers employ a more conservative strategy of limiting water loss and may rely on stored water to maintain turgor and delay desiccation.

In his 7 August In Depth News story “The dimming of NEON” (p. [574][1]), J. Mervis reported cuts to the National Science Foundation's (NSF's) National Ecological Observatories Network (NEON) program. NEON is a highly ambitious research program that attempts to observe biological and ecological phenomena at unprecedented scales ([ 1 ][2]). We served as science advisers, evaluating a series of budget and technical problems that compromised the envisioned capabilities of the Observatory. We recommended changes that will leave an Observatory of extraordinary value, provide continental coverage, and deliver transformational science ([ 2 ][3]). Mervis focused on STREON, one of the most important conceptual components of NEON despite accounting for only a few percent of the budget. The STREON design adds an otherwise missing element to NEON by altering stream ecosystems, and NSF should identify a funding path to restore it. Deferment of STREON places crucial science at risk. Despite his generally upbeat appraisal of the Observatory, Mervis raised serious concerns of a broader nature, including “high staff turnover and conflicts created by giving scientists what they felt was insufficient authority.” NSF's model for project management emphasizes early science involvement in the design, but execution by project management professionals ([ 3 ][4]). This approach limited NEON's ability to adapt scientifically as unforeseen technical and scientific challenges arose. Many of the challenges that derailed NEON were evident long ago, and some could have been addressed by peer-to-peer engagement between science and project management. Re-engaging the scientific community at this time will provide benefits for NSF, NEON, and the broader ecological community. A renewal of science leadership, and scientific peer review, are required to lead to final project success, allowing NEON to realize its benefits to science and society. 1. [↵][5]1. M. Keller, 2. D. S. Schimel, 3. W. W. Hargrove, 4. F. M. Hoffman , Front. Ecol. Environ. 6, 282 (2008). [OpenUrl][6][CrossRef][7] 2. [↵][8]1. K. Sand , Special report: Recommendations related to NSF Scope Management Directives (2015); [www.neoninc.org/updates-events/update/special-report-recommendations-response-nsf-scope-management-directives][9]. 3. [↵][10]1. D. Schimel, 2. M. Keller , Oecologia 177, 925 (2015). [OpenUrl][11][CrossRef][12][PubMed][13] [1]: /lookup/doi/10.1126/science.349.6248.574 [2]: #ref-1 [3]: #ref-2 [4]: #ref-3 [5]: #xref-ref-1-1 "View reference 1 in text" [6]: {openurl}?query=rft.jtitle%253DFront.%2BEcol.%2BEnviron.%26rft.volume%253D6%26rft.spage%253D282%26rft.atitle%253DFRONT%2BECOL%2BENVIRON%26rft_id%253Dinfo%253Adoi%252F10.1890%252F1540-9295%25282008%25296%255B282%253AACSFTN%255D2.0.CO%253B2%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [7]: /lookup/external-ref?access_num=10.1890/1540-9295(2008)6[282:ACSFTN]2.0.CO;2&link_type=DOI [8]: #xref-ref-2-1 "View reference 2 in text" [9]: http://www.neoninc.org/updates-events/update/special-report-recommendations-response-nsf-scope-management-directives [10]: #xref-ref-3-1 "View reference 3 in text" [11]: {openurl}?query=rft.jtitle%253DOecologia%26rft.volume%253D177%26rft.spage%253D925%26rft_id%253Dinfo%253Adoi%252F10.1007%252Fs00442-015-3236-3%26rft_id%253Dinfo%253Apmid%252F25680334%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [12]: /lookup/external-ref?access_num=10.1007/s00442-015-3236-3&link_type=DOI [13]: /lookup/external-ref?access_num=25680334&link_type=MED&atom=%2Fsci%2F349%2F6253%2F1176.3.atom

Continuous measurements of sap flow have been widely used to estimate water flux through tree stems and branches. However, stem-level measurements lack the resolution necessary for accurately determining fine-scale, leaf-level responses to environmental variables. We used the heat ratio method to measure sap flow rates through leaf petioles and leaflet petiolules of saplings of the tropical tree Tabebuia rosea (Bignoniaceae) to determine how leaf and leaflet sap flow responds to variation in photosynthetically active radiation (PAR) and vapor pressure deficit (VPD). In the morning sap flow rates to east-facing leaves increased 26 minutes before adjacent west-facing leaves. Integrated daily sap flow was negatively correlated with daily mean VPD, and sap flow declined when VPD exceeded 2.2 kPa whether this occurred in the morning or afternoon, consistent with a feedforward response to humidity. Indeed, changes in VPD lagged behind changes in sap flow. In contrast, changes in PAR often preceded changes in sap flow. The sap flow-VPD relationship was characterized by two distinct patterns of hysteresis, while the sap flow-PAR relationship displayed three types of hysteresis, and the type of VPD hysteresis that occurred on a given day was correlated with the type of PAR hysteresis occurring on that day. These patterns highlight how multiple environmental drivers interact to control leaf sap flux and that the development of sap flow sensors capable of measuring individual leaves could drastically influence the amount of data recordable for these structures.

Stable isotope analysis has provided insights into the trophic ecology of a wide diversity of animals. Knowledge about isotopic incorporation rates and isotopic discrimination between the consumer and its diet for different tissue types is essential for interpreting stable isotope data, but these parameters remain understudied in many animal taxa and particularly in aquatic invertebrates. We performed a 292-day diet shift experiment on 92 individuals of the predatory mantis shrimp, Neogonodactylus bredini, to quantify carbon and nitrogen incorporation rates and isotope discrimination factors in muscle and hemolymph tissues. Average isotopic discrimination factors between mantis shrimp muscle and the new diet were 3.0 ± 0.6 ‰ and 0.9 ± 0.3 ‰ for carbon and nitrogen, respectively, which is contrary to what is seen in many other animals (e.g. C and N discrimination is generally 0–1 ‰ and 3–4 ‰, respectively). Surprisingly, the average residence time of nitrogen in hemolymph (28.9 ± 8.3 days) was over 8 times longer than that of carbon (3.4 ± 1.4 days). In muscle, the average residence times of carbon and nitrogen were of the same magnitude (89.3 ± 44.4 and 72.8 ± 18.8 days, respectively). We compared the mantis shrimps’ incorporation rates, along with rates from four other invertebrate taxa from the literature, to those predicted by an allometric equation relating carbon incorporation rate to body mass that was developed for teleost fishes and sharks. The rate of carbon incorporation into muscle was consistent with rates predicted by this equation. Our findings provide new insight into isotopic discrimination factors and incorporation rates in invertebrates with the former showing a different trend than what is commonly observed in other animals.

© 2015 McCauley et al. The common hippopotamus, Hippopotamus amphibius, transports millions of tons of organic matter annually from its terrestrial feeding grounds into aquatic habitats. We evaluated whether carbon stable isotopes (δ13C) can be used as tracers for determining whether H. amphibius-vectored allochthonous material is utilized by aquatic consumers. Two approaches were employed to make this determination: (1) lab-based feeding trials where omnivorous river fish were fed a H. amphibius dung diet and (2) field sampling of fish and aquatic insects in pools with and without H. amphibius. Lab trials revealed that fish fed exclusively H. amphibius dung exhibited significantly more positive δ13C values than fish not fed dung. Fish and aquatic insects sampled in a river pool used for decades by H. amphibius also exhibited more positive δ13C values at the end of the dry season than fish and insects sampled from an upstream H. amphibius-free reference pool. Fish sampled in these same pools at the end of the wet season (high flow) showed no significant differences in δ13C values, suggesting that higher flows reduced retention and use of H. amphibius subsidies. These data provide preliminary evidence that δ13C values may be useful, in certain contexts, for quantifying the importance H. amphibius organic matter.

In order to understand the timing of leaf wax synthesis in higher plants, we analysed the variability in leaf wax n-alkane concentration, composition (expressed as average chain length (ACL)), and delta H-2(wax) values as well as plant source water delta H-2 values (xylem and leaf water) in the evergreen tree Quercus agrifolia over a period of 9 months, beginning with leaf flush. We identified three distinct periods of leaf development with the first month following leaf flush being characterized by de novo synthesis and possibly removal of n-alkanes. During the following 3 months, n-alkane concentrations increased sevenfold and delta H-2(wax) and ACL values increased, suggesting this period was the major leaf wax n-alkane formation period. During the remaining 4 months of the experiment, stable values suggest cessation of leaf wax n-alkane formation. We find that n-alkane synthesis in Q. agrifolia takes place over 4 months, substantially longer than that observed for deciduous trees.

Woody plant encroachment into semiarid ecosystems has become a global trend in recent decades. Due to stream channel incision, the semiarid riparian montane meadows of the southern Sierra Nevada Mountains, USA are experiencing long-term declines in soil moisture. A woody shrub, Artemisia rothrockii A. Gray (Rothrock sagebrush, Asteraceae) is invading these herbaceous meadows. We used an analysis of the stable oxygen isotope ratios of plant and soil water to measure the depth of plant water acquisition during the early stages of this woody plant encroachment. Sagebrush used deeper water on average than most herbs, but it also acquired 10–30% of its water from shallow (

Summary Under well-watered conditions leaf hydraulic conductance increases with transpiration rate. This reduces the water potential gradient in leaves and potentially improves productivity under daily variation in evaporative demand., Leaf hydraulic conductance (k leaf) is a central element in the regulation of leaf water balance but the properties of k leaf remain uncertain. Here, the evidence for the following two models for k leaf in well-hydrated plants is evaluated: (i) k leaf is constant or (ii) k leaf increases as transpiration rate (E) increases. The difference between stem and leaf water potential (ΔΨstem–leaf), stomatal conductance (g s), k leaf, and E over a diurnal cycle for three angiosperm and gymnosperm tree species growing in a common garden, and for Helianthus annuus plants grown under sub-ambient, ambient, and elevated atmospheric CO2 concentration were evaluated. Results show that for well-watered plants k leaf is positively dependent on E. Here, this property is termed the dynamic conductance, k leaf(E), which incorporates the inherent k leaf at zero E, which is distinguished as the static conductance, k leaf(0). Growth under different CO2 concentrations maintained the same relationship between k leaf and E, resulting in similar k leaf(0), while operating along different regions of the curve owing to the influence of CO2 on g s. The positive relationship between k leaf and E minimized variation in ΔΨstem–leaf. This enables leaves to minimize variation in Ψleaf and maximize g s and CO2 assimilation rate over the diurnal course of evaporative demand.

About half of the Amazon rainforest is subject to seasonal droughts of 3 months or more. Despite this drought, several studies have shown that these forests, under a strongly seasonal climate, do not exhibit significant water stress during the dry season. In addition to deep soil water uptake, another contributing explanation for the absence of plant water stress during drought is the process of hydraulic redistribution; the nocturnal transfer of water by roots from moist to dry regions of the soil profile. Here, we present data on patterns of soil moisture and sap flow in roots of three dimorphic-rooted species in the Tapajós Forest, Amazônia, which demonstrate both upward (hydraulic lift) and downward hydraulic redistribution. We measured sap flow in lateral and tap roots of our three study species over a 2-year period using the heat ratio method, a sap-flow technique that allows bi-directional measurement of water flow. On certain nights during the dry season, reverse or acropetal flow (i.e.,in the direction of the soil) in the lateral roots and positive or basipetal sap flow (toward the plant) in the tap roots of Coussarea racemosa (caferana), Manilkara huberi (maçaranduba) and Protium robustum (breu) were observed, a pattern consistent with upward hydraulic redistribution (hydraulic lift). With the onset of heavy rains, this pattern reversed, with continuous night-time acropetal sap flow in the tap root and basipetal sap flow in lateral roots, indicating water movement from wet top soil to dry deeper soils (downward hydraulic redistribution). Both patterns were present in trees within a rainfall exclusion plot (Seca Floresta) and to a more limited extent in the control plot. Although hydraulic redistribution has traditionally been associated with arid or strongly seasonal environments, our findings now suggest that it is important in ameliorating water stress and improving rain infiltration in Amazonian rainforests. This has broad implications for understanding and modeling ecosystem process and forest function in this important biome.

Our common view on water uptake by terrestrial plants is that it occurs via absorption by roots from the soil substrate. However, it has long been known that plants exhibit alternative water-absorption strategies, particularly in drought-prone environments. Examples include many tropical epiphytic orchids which use a specialized structure called velamen radicum around their aerial roots for moisture absorption directly from the air (Capesius & Barthlott 1975), specialized trichomes in bromeliads (Andrade 2003, Benzing 1990), uptake by hydathodes into leaves of species inhabiting dry desert ecosystems of Namibia (Martin & von Willert 2000) and foliar absorption by coastal California redwoods during the summer fog season (Burgess & Dawson 2004). One of the most intriguing and yet, least-studied examples of adaptations to severe water limitation is found with desiccation-tolerant plants (also called resurrection plants). During drought periods, the water content of these plants can equilibrate with the low relative humidity of the air to the point that the plants appear dead. However, when water is supplied, these plants fully rehydrate (Alpert 2000, Bewley & Krochko 1982) and become physiologically active. Desiccation-tolerant vascular plants are rare in most ecosystems but diverse in tropical inselbergs (granitic outcrops; Porembski & Barthlott 2000). Relatively little is known about inselberg species particularly from an ecophysiological perspective (see Lüttge 1997 and Klüge & Brulfert 2000 for reviews).