Search

© 2018 The US and Mexico share a common history in many areas, including language and culture. They face ecological changes due to the increased frequency and severity of droughts and rising energy demands; trends that entail economic costs for both nations and major implications for human wellbeing. We describe an ongoing effort by the Environment Working Group (EWG), created by The University of California's UC-Mexico initiative in 2015, to promote binational research, teaching, and outreach collaborations on the implications of climate change for Mexico and California. We synthesize current knowledge about the most pressing issues related to climate change in the US-Mexico border region and provide examples of cross-border discoveries and research initiatives, highlighting the need to move forward in six broad rubrics. This and similar binational cooperation efforts can lead to improved living standards, generate a collaborative mindset among participating universities, and create an international network to address urgent sustainability challenges affecting both countries.

California experienced severe drought from 2012 to 2016, and there were visible changes in the forest canopy throughout the State. In 2014, unprecedented foliage dieback was recorded in giant sequoia (Sequoiadendron giganteum) trees in Sequoia National Park, in the southern California Sierra Nevada mountains. Although visible changes in sequoia canopies can be recorded, biochemical and physiological responses to drought stress in giant sequoia canopies are not well understood. Ground-based measurements provide insight into the mechanisms of drought responses in trees, but are often limited to few individuals, especially in trees of tall stature such as giant sequoia. Recent studies demonstrate that remotely measured forest canopy water content (CWC) is a general indicator of canopy response to drought, but the underpinning leaf- to canopy-level causes of observed variation in CWC remain poorly understood. We combined field and airborne remote sensing measurements taken in 2015 and 2016 to assess the biophysical responses of giant sequoias to drought. In 49 study trees, CWC was related to leaf water potential, but not to the other foliar traits, suggesting that changes in CWC were made at whole-canopy rather than leaf scales. We found a non-random, spatially varying pattern in mapped CWC, with lower CWC values at lower elevation and along the outer edges of the groves. This pattern was also observed in empirical measurements of foliage dieback from the ground, and in mapped CWC across multiple sequoia groves in this region, supporting the hypothesis that drought stress is expressed in canopy-level changes in giant sequoias. The fact that we can clearly detect a relationship between CWC and foliage dieback, even without taking into account prior variability or new leaf growth, strongly suggests that remotely sensed CWC, and changes in CWC, are a useful measure of water stress in giant sequoia, and valuable for assessing and managing these iconic forests in drought.

Hotter droughts are becoming more common as climate change progresses, and they may already have caused instances of forest dieback on all forested continents. Learning from hotter droughts, including where on the landscape forests are more or less vulnerable to these events, is critical to help resource managers proactively prepare for the future. As part of our Leaf to Landscape Project, we measured the response of giant sequoia, the world’s largest tree species, to the extreme 2012–2016 hotter drought in California. The project integrated leaf-level physiology measurements, crown-level foliage dieback surveys, and remotely sensed canopy water content (CWC) to shed light on mechanisms and spatial patterns in drought response. Here we summarize initial findings, present a conceptual model of drought response, and discuss management implications; details are presented in the other four articles of the special section on Giant Sequoias and Drought. Giant sequoias exhibited both leaf- and canopy-level responses that were effective in protecting whole-tree hydraulic integrity for the vast majority of individual sequoias. Very few giant sequoias died during the drought compared to other mixed conifer tree species; however, the magnitude of sequoia drought response varied across the landscape. This variability was partially explained by local site characteristics, including variables related to site water balance. We found that low CWC is an indicator of recent foliage dieback, which occurs when stress levels are high enough that leaf-level adjustments alone are insufficient for giant sequoias to maintain hydraulic integrity. CWC or change in CWC may be useful indicators of drought stress that reveal patterns of vulnerability to future hotter droughts. Future work will measure recovery from the drought and strengthen our ability to interpret CWC maps. Our ultimate goal is to produce giant sequoia vulnerability maps to help target management actions, such as reducing other stressors, increasing resistance to hotter drought through prescribed fire or mechanical thinning, and planting sequoias in projected future suitable habitat, which may occur outside current grove distributions. We suggest that managers compare different types of vulnerability assessments and combine vulnerability maps with other sources of information to inform decisions.

Drought is expected to become an increasingly important stressor on forests globally, and understanding the physiological mechanisms driving tree drought response is essential for developing effective mitigation and conservation measures for these ecosystems. In 2014, during California’s 2012–2016 “hotter” drought in which higher temperatures exacerbated the effects of low water availability, many giant sequoia trees in the Sierra Nevada mountains exhibited foliage dieback at levels previously unreported. We hypothesized that this apparent drought-induced foliage dieback was associated with spatial patterns of site water balance and consequently tree water status and physiology. As part of an ongoing collaborative project aimed at understanding and mapping giant sequoia drought response and vulnerability at leaf to landscape scales, in 2015, 2016 and 2017 we climbed and measured leaf-level water status and physiology of mature and seedling giant sequoia trees at sites exhibiting low and high average foliage dieback in Giant Forest, Sequoia National Park. We also compared our 2015–2017 measurements with similar measurements made in giant sequoias prior to the commencement of the 2012–2016 drought. We found that during the drought, leaf water potentials of both mature and seedling sequoias were as low or lower than any previous measurements, and leaf water potentials of some seedlings were as low as the tops of mature trees. In contrast to our expectations, we found similar water potentials in both foliage dieback classes, although there was a high degree of within-site variability and in some measurement periods trees and seedlings growing in flat or meadow-side topographic positions had higher water potentials than those growing on mid- or upslope positions with presumably less favorable water supply. Leaf-level adjustments included multiple mechanisms to reduce water loss and resist the effects of desiccation at both seasonal and annual time scales, including stomatal closure, redistribution of leaf water to less mobile storage, and a shift in leaf carbon fractions to build tougher, more drought-resistant foliage. Our results suggest that giant sequoia employs a drought avoidance water regulation strategy and leaf-level adjustments are sufficient to maintain relatively isohydric water potentials above critical thresholds under most conditions. Because of the high severity of the 2012–2016 drought, however, the capacity for leaf-level compensation in some giant sequoias was overwhelmed and both leaf- and crown-level adjustments were necessary to maintain favorable water status to protect whole-tree hydraulic functioning. Additional research aimed at understanding and monitoring the consequences of future hotter droughts is necessary in order to determine the long-term persistence of giant sequoias within their current geographic distribution.

Desiccation tolerance (DT) is the ability of plants to tolerate low leaf water content from which they can recover when water becomes available. DT species mostly occur in seasonally-dry environments, tropical rocky outcrops such as the campos rupestres. Velloziaceae, a prominent family in the campos rupestres, comprises both DT and non-DT species. We tested if DT Vellozia nivea was associated with low productivity under mild drought conditions and survival under extreme drought. We measured traits related to drought resistance (turgor loss point – ΨTLP), plant functioning and productivity (e.g., gas exchange, biomass), non-structural carbohydrate dynamics and mortality in two species, one DT and one non-DT Vellozia species in two water-stress experiments simulating mild and extreme drought. Both species had similar survival rates under mild drought conditions, but the DT species had a lower productivity. Under extreme drought, the dynamics of non-structural carbohydrates was similar in both species but only the non-DT species died, and the cause of mortality was likely hydraulic dysfunction, as they died after reaching ΨTLP. All individuals of the DT species desiccated when they reached ΨTLP but rehydrated after re-watering. We show that the DT strategy in V. nivea implies a costs in terms of productivity, specially under mild drought conditions, but it allows survival under extreme drought, indicating a trade-off between productivity and survival in the DT species. Climate change predictions with more frequent and severe droughts will likely affect the abundance and distribution of DT and non-DT taxa.

Quantitative, well-dated, local paleoenvironmental records are necessary to 1) evaluate responses to regional to global-scale climate change at the scale of human habitats, and 2) test hypotheses regarding the effects of environmental change and human biological and cultural evolution. Ostrich eggshell (OES) fragments are common in African archaeological sequences, are amenable to 14C and 230Th/U dating, and their stable carbon (δ13C values) and nitrogen (δ15N values) isotopic compositions track local vegetation and mean annual precipitation (MAP), respectively. We review previous interpretations of the stable isotopic composition of OES, apply a novel calibration to estimate paleo-MAP (PMAP) from δ15N values, and show that oxygen isotopes (δ18O values) record evapotranspiration, which is controlled by temperature, relative humidity, and/or photosynthetic performance, if other components of the water cycle are constrained. The stable isotopic compositions of the organic fraction of OES remain unaltered to at least ∼50 ka, indicating potential to examine even older OES. We present a ∼50–4 ka record of OES δ13C, δ15N, and δ18O values from archaeological sites recording the Middle to Later Stone Age (MSA/LSA) transition at Lukenya Hill (Kenya) and Kisese II (Tanzania). Stable isotope proxies indicate contrasting but subtle changes in local paleoenvironment throughout the records at both sites, likely explained by local ecological and climatological effects that are not resolved by regional-scale paleoclimate records. These records highlight the need for additional local studies to assess the covariance of paleoenvironments and material culture. Furthermore, they indicate that the MSA/LSA transition at the two sites did not result from paleoenvironmental change.

Summary In isotope-enabled hydrology, soil and vadose zone sediments have been generally considered to be isotopically inert with respect to the water they host. This is inconsistent with knowledge that clay particles possessing an electronegative surface charge and resulting cation exchange capacity (CEC) interact with a wide range of solutes which, in the absence of clays, have been shown to exhibit δ 18 O isotope effects that vary in relation to the ionic strength of the solutions. To investigate the isotope effects caused by high CEC clays in mineral–water systems, we created a series of monominerallic-water mixtures at gravimetric water contents ranging from 5% to 32%, consisting of pure deionized water of known isotopic composition with homoionic (Mg, Ca, Na, K) montmorillonite. Similar mixtures were also created with quartz to determine the isotope effect of non-, or very minimally-, charged mineral surfaces. The δ 18 O value of the water in these monominerallic soil analogs was then measured by isotope ratio mass spectrometry (IRMS) after direct headspace CO 2 equilibration. Mg- and Ca-exchanged homoionic montmorillonite depleted measured δ 18 O values up to 1.55‰ relative to pure water at 5% water content, declining to 0.49‰ depletion at 30% water content. K-montmorillonite enriched measured δ 18 O values up to 0.86‰ at 5% water content, declining to 0.11‰ enrichment at 30% water. Na-montmorillonite produces no measureable isotope effect. The isotope effects observed in these experiments may be present in natural, high-clay soils and sediments. These findings have relevance to the interpretation of results of direct CO 2 -water equilibration approaches to the measurement of the δ 18 O value of soil water. The adsorbed cation isotope effect may bear consideration in studies of pedogenic carbonate, plant-soil water use and soil-atmosphere interaction. Finally, the observed isotope effects may prove useful as molecular scale probes of the nature of mineral–water interactions.

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.

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.

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.

The influence of litter quality on soil organic matter (SOM) stabilization rate and pathways remains unclear. We used 13C/15N labeled litter addition and Curie-point pyrolysis gas chromatography–mass spectrometry combustion-isotope ratio mass spectrometry (Py–GC–MS–C–IRMS) to explore the transformation of litter with different composition and decay rate (ponderosa pine needle vs. fine root) to SOM during 18 months in a temperate conifer forest mineral (A horizon) soil. Based on 13C Py–GC–MS–C–IRMS the initial litter and bulk soil had ∼1/3 of the total pyrolysis products identified in common. The majority was related either to carbohydrates or was non-specific in origin. In bulk soil, carbohydrates had similar levels of enrichment after needle input and fine root input, while the non-specific products were more enriched after needle input. In the humin SOM fraction (260 yr C turnover time) we found only carbohydrate and alkyl C-derived compounds and greater 13C enrichment in the “carbohydrate” pool after fine root decomposition. 15N Py–GC–MS–C–IRMS of humic substances showed that root litter contributed more than needle litter to the enrichment of specific protein markers during initial decomposition. We found little evidence for the selective preservation of plant compounds considered to be recalcitrant. Our findings suggest an indirect role for decomposing plant material composition, where microbial alteration of fine root litter seems to favor greater initial stabilization of microbially derived C and N in SOM fractions with long mean turnover times, such as humin, compared to needles with a faster decay rate.

The stable hydrogen isotopic ratio (δD) of stem water is shown to deviate from source water values in young, nonsuberized woody stems, reflecting an isotopic enrichment associated with cuticular water loss and/or transpiration. The extent of this enrichment was greater in trees which produce a new set of leaves annually (e.g., deciduous-leaved species) than in trees which retain their leaves for two or more years (e.g., coniferous or evergreen-leaved species). Stem-water enrichment was also greater in rapidly growing stem tissues than in mature, fully suberized or lignified stems which had ceased growing and showed little or no isotopic enrichment. These results are discussed in terms of their implications for studies which use xylem-sap δD values to evaluate plant water sources and uptake. We also discuss our findings in the context of long-term investigations that use cellulose or cellulose nitrate δD values for climatic reconstruction or to infer past water-use or distributional patterns in woody vegetation.