Site Water Balance Research Articles

Interspecific, conspecific, and ontogenetic responses of tree rings to climate: A case study utilizing Carya glabra, Carya ovata, Carya tomentosa, and Quercus montana from an Oak-Hickory Forest in Southeastern Ohio

Climate change in eastern North America is likely to impact the abundance and distribution of the region’s tree species. However, determining the degree to which species will be impacted by altered climates is challenging. Dendrochronology research aimed at understanding relationships between climate and annual ring-width is one way of understanding how climate change may impact forest communities. Oak (Quercus spp.) and hickory (Carya spp.) are two foundational groups of trees likely to undergo changes in abundance and distribution due to climate change. The goal of this study was to compare the radial growth climate sensitivity of three common and co-occurring hickory species (Carya glabra, Carya ovata, and Carya tomentosa) to ecologically similar Quercus montana in southeast Ohio. Also, this study compared conspecific radial-growth climate responses between canopy and subcanopy trees to assess the impacts of climate and drought on subcanopy forest layers. All four species in the forest canopy demonstrated significant positive relationships to growing season precipitation, significant negative relationships with growing season temperature, and significant positive relationships with growing season site water balance. Subcanopy chronologies for all four species demonstrated weaker growth responses to climate, with only Carya glabra demonstrating significant growth relationships with May precipitation and site water balance. Additionally, the increased drought resistance of subcanopy trees provided some evidence of the forest overstory buffering the impacts of climate variability on understory trees. Overall, ontogenetic differences in tree sensitivity to climate variability and drought show that climate change likely has the potential to influence the forest understory, but the degree to which systems are impacted may be highly species-specific.

Phytodiversity is associated with habitat heterogeneity from Eurasia to the Hengduan Mountains.

The geographic distribution of plant diversity matches the gradient of habitat heterogeneity from lowlands to mountain regions. However, little is known about how much this relationship is conserved across scales. Using the World Checklist of Vascular Plants and high-resolution biodiversity maps developed by species distribution models, we investigated the associations between species richness and habitat heterogeneity at the scales of Eurasia and the Hengduan Mountains (HDM) in China. Habitat heterogeneity explains seed plant species richness across Eurasia, but the plant species richness of 41/97 HDM families is even higher than expected from fitted statistical relationships. A habitat heterogeneity index combining growing degree days, site water balance, and bedrock type performs better than heterogeneity based on single variables in explaining species richness. In the HDM, the association between heterogeneity and species richness is stronger at larger scales. Our findings suggest that high environmental heterogeneity provides suitable conditions for the diversification of lineages in the HDM. Nevertheless, habitat heterogeneity alone cannot fully explain the distribution of species richness in the HDM, especially in the western HDM, and complementary mechanisms, such as the complex geological history of the region, may have contributed to shaping this exceptional biodiversity hotspot.

Global climate-related predictors at kilometer resolution for the past and future

Abstract. A multitude of physical and biological processes on which ecosystems and human societies depend are governed by the climate, and understanding how these processes are altered by climate change is central to mitigation efforts. We developed a set of climate-related variables at as yet unprecedented spatiotemporal detail as a basis for environmental and ecological analyses. We downscaled time series of near-surface relative humidity (hurs) and cloud area fraction (clt) under the consideration of orography and wind as well as near-surface wind speed (sfcWind) using the delta-change method. Combining these grids with mechanistically downscaled information on temperature, precipitation, and solar radiation, we then calculated vapor pressure deficit (vpd), surface downwelling shortwave radiation (rsds), potential evapotranspiration (pet), the climate moisture index (cmi), and site water balance (swb) at a monthly temporal and 30 arcsec spatial resolution globally from 1980 until 2018 (time-series variables). At the same spatial resolution, we further estimated climatological normals of frost change frequency (fcf), snow cover days (scd), potential net primary productivity (npp), growing degree days (gdd), and growing season characteristics for the periods 1981–2010, 2011–2040, 2041–2070, and 2071–2100, considering three shared socioeconomic pathways (SSP126, SSP370, SSP585) and five Earth system models (projected variables). Time-series variables showed high accuracy when validated against observations from meteorological stations and when compared to alternative products. Projected variables were also highly correlated with observations, although some variables showed notable biases, e.g., snow cover days. Together, the CHELSA-BIOCLIM+ dataset presented here (https://doi.org/10.16904/envidat.332, Brun et al., 2022) allows improvement to our understanding of patterns and processes that are governed by climate, including the impact of recent and future climate changes on the world's ecosystems and the associated services on societies.

Forest treatment effects on wood production in ponderosa pine

Forest treatments reduce wildfire risk and can promote the vigor and production of remaining trees, but they are also a disturbance. Understanding the type, timing, and longevity of tree response to treatment, as well as the potential for interactive effects of treatments and drought, could help managers plan and evaluate forest management practices. Environmental drivers, biological modifiers, and tree capacity to respond to prior disturbances were concurrently tested to predict ponderosa pine basal area increment (BAI) in a lowland and upland dry pine forest in south central Oregon, USA. Environmental drivers included current year and lags or running averages of a drought index, SPEI, and the sum or count of growing degree days >0°C or 10°C. Biological modifiers of environmental drivers considered pre-treatment response to disturbance, tree vigor, and tree-to-tree competition. A model was developed to predict BAI in both topographic positions for applicability to the landscape level, and then was used to test for specific differences in BAI between paired forest treatments differing by one treatment. Forest treatments tested included no management (NM), undercut and even spacing harvest (HE), prescribed fire (Rx), and their combinations. HE significantly increased BAI shortly after treatment. Post-harvest, one or two Rx did not provide additional BAI benefits, nor in the absence of HE, did 2Rx vs. 1Rx treatment. The 1Rx treatment was imposed between multi-year droughts; BAI significantly increased after the treatment and was resistant to droughts. Upland trees were affected by a single year of drought; lowland trees responded only after sequential drought years. A single treatment, HE or 1Rx appeared to be as effective as multiple or mixed treatments in improving BAI in dry pine forest stands. HE appeared to generate the largest effect. Timing of forest treatments relative to site water balance may affect short term (decadal) wood production.

A Review on Bryophytes as Key Bio-indicators to Monitor Heavy Metals in the Atmosphere

Bryophytes are one of the simplest autotrophic cryptogams invading land and characterized either by simple thalloid or erect habit lacking true leaves, root and stem within the plant body. Although they are ubiquitous in distribution, yet becomes sensitive to certain environmental conditions that can be natural or induced due to anthropogenic activity. Due to their versatile tolerance and resistance capability they can be categorically used as potential bioindicators for monitoring pollution. Bryophytes can be utilized as ‘environmental specimen bank’ due to their unique capacity of indicating the presence of metal and their concentration gradient in the substratum. Apart from their utility in pharmaceutical products, horticulture, household purposes they are also ecologically important. As multidimensional applications of the flora are being increasingly standardized universally, their potential in the biomapping of atmospheric pollution as well as ecological biodegradation is also enormous. Currently, a change in global climate is intensified that affected Earth’s biomes and vegetation zones redistribution. At higher altitudes, this alteration is more promising with rapid consequences. Elevated temperatures are expected to produce a drier environment that affect site water balance and cause shifts in the distribution of ecosystems on a universal scale. These effects are rather evident in such ecosystems as peatlands which are sensitive to both climate and water level fluctuations. Decrease in epiphytic bryophytes because of gaseous and particulate pollutants as well as the greenhouse gases is also a serious problem. Besides this, very specific and unique responses are generated by the bryophytes; studies have proven that they act as potential monitoring bio-agents for heavy metal pollution. In the present paper, an attempt is done to do a comprehensive study on bryophytes that reflects their role as promising indicators in monitoring pollution.

Land use and water availability drive community-level plant functional diversity of grasslands along a temperature gradient in the Swiss Alps

Functional traits of mountain grassland communities strongly depend upon temperature variation along elevational gradients. However, little is known to what degree the direction of such trait-temperature relationships is shaped by other environmental factors or land-use types. Here, we investigated context-dependent patterns of plant functional trait variation in alpine grassland communities. Specifically, we tested whether temperature (degree-days) variation along an elevational gradient, interacts with water availability, soil properties and land-use type to moderate such patterns. We used cover-abundance and plant-trait data from 236 grassland relevés of the Swiss Alps along an elevational range of 500–2400 m a.s.l. with plant traits being specific leaf area (L), seed releasing height (H) and seed mass (S). We used indices capturing different dimensions of plant functional diversity as response variables, i.e. community weighted mean (CWM), trait range (TR) and functional dispersion (FDis). Land-use type and water availability interacted significantly with degree-days determining the responses of multiple plant traits community attributes. Specific leaf area (CWML) and seed releasing height (CWMH) increased with temperature in meadows and pastures, while no significant trend was detected in fallows. In meadows, seed mass (CWMS) increased and was at the same time less constrained (higher TRS) with increasing temperature. In pastures and fallows, by contrast, no seed trait-temperature trends were detected. In addition, water availability interacted with increasing temperature affecting functional dispersion: FDisL decreased only in sites with higher site water balance and TRS and FDisS increased in sites with low mean summer precipitation. Our findings suggest that functional diversity of grasslands might respond to climate warming with strong ecological differences depending on land-use types and water availability. Based on our results, managed meadows and pastures most likely change in direction to species with more acquisitive strategies, whereas in fallows, no specific trajectory of change is expected.

Use of Sentinel-2 MSI data to monitor crop irrigation in Mediterranean areas

The availability of accurate information on the water consumed for crop irrigation is of vital importance to support compatible and sustainable environmental policies in arid and semi-arid regions. This has promoted several studies about the use of remote sensing data to monitor irrigated croplands, which are mostly based on statistical classification and/or regression techniques. The current paper proposes a new semi-empirical approach that relies on a water balance logic and does not require local tuning. The method stems from recent investigations which demonstrated the possibility of combining standard meteorological data and Sentinel-2 (S-2) Multi Spectral Instrument (MSI) NDVI images to estimate the actual evapotranspiration (ETa) of irrigated Mediterranean croplands. This ETa estimation method is adapted to drive a simplified site water balance which, for each 10-m S-2 MSI pixel, predicts the irrigation water (IW), i.e. the water which is consumed in addition to that naturally supplied by rainfall. The new method, fed with ground and satellite data from two years (2018–2019), is tested in a Mediterranean area around the town of Grosseto (Central Italy), that is covered by a particularly complex mosaic of rainfed and irrigated crops. The results obtained are first assessed qualitatively for some fields grown with known winter, spring and summer crops. Next, the IW estimates are evaluated quantitatively versus ground measurements taken over two irrigated fields, the first grown with processing tomato in 2018 and the second with early corn in 2019. Finally, the IW estimates are statistically analyzed against various datasets informative on local agricultural practices in the two years. All these analyses indicate that the proposed method is capable of predicting both the intensity and timing of the IW supply in the study area. The method, in fact, correctly identifies rainfed and irrigated crops and, in the latter case, accurately predicts the IW actually supplied. The results of the quantitative tests performed on tomato and corn show that over 50 % and 70 % of the measured IW variance is explained on daily and weekly bases, respectively, with corresponding mean bias errors below 0.3 mm/day and 2.0 mm/week. Similar indications are produced by the qualitative tests; reasonable IW estimates are obtained for all winter, springs and summer crops grown in the study area during 2018 and 2019.

Burn weather and three-dimensional fuel structure determine post-fire tree mortality

Post-fire tree mortality is a spatially structured process driven by interacting factors across multiple scales. However, empirical models of fire-caused tree mortality are generally not spatially explicit, do not differentiate among scales, and do not differentiate immediate from delayed mortality. We aimed to quantify cross-scale linkages between forest structure—including spatial patterns of trees—and the progression of mortality 1–4 years post-fire in terms of rates, causes, and underlying demography. We used data from a long-term study site in the Sierra Nevada, California to build a post-fire tree mortality model predicted by lidar-measured estimates of structure. We calculated structural metrics at scales from individual trees to 90 × 90 m neighborhoods and combined them with metrics for topography, site water balance, and burn weather to predict immediate and delayed post-fire tree mortality. Mortality rates decreased while average diameter of newly killed trees increased each year post-fire. Burn weather predictors as well as interactive terms across scales improved model fit and parsimony. Including landscape-scale information improved finer-scale predictions but not vice versa. The amount of fuel, fuel configuration, and burning conditions predicted total mortality at broader scales while tree group-scale fuel connectivity, tree species fire tolerance, and local stresses predicted the fine-scale distribution, timing, and agents of mortality. Landscape-scale conditions provide the template upon which finer-scale variation in post-fire tree mortality is arranged. Post-fire forest structure is associated with the etiologies of different mortality agents, and so landscape-level heterogeneity is a key part of ecosystem stability and resilience.

Spatial variation in oak (Quercus spp.) radial growth responses to drought stress in eastern North America

Dendroecology provides a means to evaluate how mature trees have responded to climate stresses in the recent past and provides one approach for projecting how existing forests will respond to future climate change. This study documented spatial variation in the strength of growth–climate associations for six oak (Quercus) species at 284 sites in eastern North America that span substantial gradients of temperature and site water balance. Radial growth of oaks was more strongly related to growing-season precipitation and the ratio of precipitation to potential evapotranspiration at sites in the western part of the study region where drought conditions occur more frequently. Growth was more strongly related to growing-season mean maximum temperature in the warmer, southern part of the study region. Growth of oaks was not strongly related to site water balance or temperature in the northeastern part of the study region. These results indicate that if climate change results in increased growing-season drought stress, this will adversely affect mature oak trees growing in the southern and western parts of eastern North America, but oaks growing in northeastern North America have more safe space for change before they will suffer reduced growth and vigor.

A Semiempirical Method to Estimate Actual Evapotranspiration in Mediterranean Environments

Actual evapotranspiration (ETA) is a major term of site water balance whose knowledge is essential for numerous purposes. The classical ETA estimation approach based on the use of multitemporal crop coefficients (Kc) cannot be applied in water-limited environments without proper correction. Such correction can be theoretically obtained by means of soil water content (SWC) measurements, which, however, are affected by several drawbacks, due to both their technical and operational characteristics. The current paper proposes a method to normalize annual SWC datasets and integrate them in an ETA estimation procedure suitable for monitoring both agricultural and natural Mediterranean ecosystems. Differently from previous approaches, the SWC normalization is obtained using data-specific information, which renders the new method mostly insensitive to the mentioned problems. The method is first described and then applied in three case studies representative of different Mediterranean ecosystems (i.e., grassland, coniferous, and deciduous forests). The results are evaluated versus latent heat measurements taken by eddy covariance flux towers. Satisfactory accuracy is obtained in all three case studies, with advantages and limitations which are discussed in the final concluding sections.

Hydrology of Drained Peatland Forest: Numerical Experiment on the Role of Tree Stand Heterogeneity and Management

A prerequisite for sustainable peatland forestry is sufficiently low water table (WT) level for profitable tree production. This requires better understanding on controls and feedbacks between tree stand and its evapotranspiration, drainage network condition, climate, and WT levels. This study explores the role of spatial tree stand distribution in the spatiotemporal distribution of WT levels and site water balance. A numerical experiment was conducted by a three-dimensional (3-D) hydrological model (FLUSH) applied to a 0.5 ha peatland forest assuming (1) spatially uniform interception and transpiration, (2) interception and transpiration scaled with spatial distributions of tree crown and root biomass, and (3) the combination of spatially scaled interception and uniform transpiration. Site water balance and WT levels were simulated for two meteorologically contrasting years. Spatial variations in transpiration were found to control WT levels even in a forest with relatively low stand stem volume (<100 m3/ha). Forest management scenarios demonstrated how stand thinning and reduced drainage efficiency raised WT levels and increased the area and duration of excessively wet conditions having potentially negative economic (reduced tree growth) and environmental (e.g., methane emissions, phosphorus mobilization) consequences. In practice, silvicultural treatment manipulating spatial stand structure should be optimized to avoid emergence of wet spots.

Cave drip water solutes in south-eastern Australia: Constraining sources, sinks and processes

Constraining sources and site-specific processes of trace elements in speleothem geochemical records is key to an informed interpretation. This paper examines a 10-year data set of drip water solutes from Harrie Wood Cave, south-eastern Australia, and identifies the processes that control their response to El Niño-Southern Oscillation events which varies the site water balance. The contributions of aerosol and bedrock end-members are quantified via hydrochemical mass balance modelling. The parent bedrock is the main source for the drip water solutes: Mg, Sr, K and trace elements (Ba, Al, V, Cr, Mn, Ni, Co, Cu, Pb and U), while atmospheric aerosol inputs also contribute significantly to drip water trace elements and Na, K and Zn. A laboratory investigation evaluating water-soluble fractions of metals in soil samples and soil enrichment factors provided a basis for understanding metal retainment and release to solution and transport from the soil zone. These results identified the role of the soil as a sink for: trace metals, Na and K, and a secondary source for Zn. Further, soil processes including: cation exchange, K-fixation, metal adsorption to colloids and the release of Zn associated with organic matter degradation further modify the chemical composition of the resultant drip waters. This research is significant for the south-eastern Australian region, as well as other sites in a karst setting with clay-rich soil. In particular these results reveal that the response of drip water chemistry to hydroclimatic forcing is non-linear, with the greatest response observed when the long-term gradient in the cumulative water balance reverses. This longer-term drip water monitoring dataset is significant because it provides the pivotal framework required to reliably identify suitable trace element proxies for interpretation in geochemical speleothem records on multi-decadal timescales.

Leaf to landscape responses of giant sequoia to hotter drought: An introduction and synthesis for the special section

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.

Mine Water Management – A study at Edikan Gold Mine (EGM)

Water is an essential part of all mining activities and the availability of quality water year round is critical to mining operations.Mine water is largely employed in drilling, dust suppression, mineral processing and other in-pit operations. Shortage of water ina mine results in poor drilling control, dust pollution, and mill shutdowns, hence a critical need to optimize the use of water invarious mining processes. This paper seeks to establish an effective mine water management system at Edikan Gold Mine (EGM)to have enough water in store for drought conditions. A site water balance was established primarily taking into accounts theinflows and outflows of water at the mine. Results show that rainfall and the Asuafa River are the principal sources of water forEGM mining operations, while the processing plant was the main water user on the mine site. Annual net water balance is alsoestimated to help determine the required capacities of future water storage facilities.

Leaf- and crown-level adjustments help giant sequoias maintain favorable water status during severe drought

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.

Upscaling transpiration in diverse forests: Insights from a tropical premontane site

AbstractUpscaling water use of individual trees to stands using sap flux techniques is a common method for partitioning site water balance, but few such studies have occurred in the tropics. Increasing interests in the role of tropical forests in global cycles have spurred upscaling studies in natural tropical forests, which present challenges from greater tree species and functional diversity, and potential factors that would reduce transpiration, such as frequent cloud cover and wet canopy conditions. In a premontane wet tropical forest in central Costa Rica, sap flow was measured in 15 trees stratified into 5 size classes based on tree diameters. None of the trees belonged to the same species, genus, or even family. We also accounted for potential radial variation in sap flux density. Data were scaled to estimate transpiration within a small 2.2‐ha watershed using stand surveys of sapwood area. Stand transpiration averaged only 1.4 ± 0.7 mm day−1 within this forested watershed due to persistent low radiation, evaporative demand, and frequent wet canopy conditions. Our systematic approach used tree size attributes to scale water use to the stand, given difficulties to quantify species differences in such a diverse ecosystem. Contrary to previous evidence on temperate trees, the large trees sampled did not exhibit flow reductions in deeper sapwood, which warrants further study. These results highlight some unique aspects of measuring transpiration in wet tropical forests that are important to consider for future studies in diverse stands.

What mediates tree mortality during drought in the southern Sierra Nevada?

Severe drought has the potential to cause selective mortality within a forest, thereby inducing shifts in forest species composition. The southern Sierra Nevada foothills and mountains of California have experienced extensive forest dieback due to drought stress and insect outbreak. We used high-fidelity imaging spectroscopy (HiFIS) and light detection and ranging (LiDAR) from the Carnegie Airborne Observatory (CAO) to estimate the effect of forest dieback on species composition in response to drought stress in Sequoia National Park. Our aims were (1) to quantify site-specific conditions that mediate tree mortality along an elevation gradient in the southern Sierra Nevada Mountains, (2) to assess where mortality events have a greater probability of occurring, and (3) to estimate which tree species have a greater likelihood of mortality along the elevation gradient. A series of statistical models were generated to classify species composition and identify tree mortality, and the influences of different environmental factors were spatially quantified and analyzed to assess where mortality events have a greater likelihood of occurring. A higher probability of mortality was observed in the lower portion of the elevation gradient, on southwest- and west-facing slopes, in areas with shallow soils, on shallower slopes, and at greater distances from water. All of these factors are related to site water balance throughout the landscape. Our results also suggest that mortality is species-specific along the elevation gradient, mainly affecting Pinus ponderosa and Pinus lambertiana at lower elevations. Selective mortality within the forest may drive long-term shifts in community composition along the elevation gradient.

Patterns and correlates of giant sequoia foliage dieback during California’s 2012–2016 hotter drought

Hotter droughts – droughts in which unusually high temperatures exacerbate the effects of low precipitation – are expected to increase in frequency and severity in coming decades, challenging scientists and managers to identify which parts of forested landscapes may be most vulnerable. In 2014, in the middle of California’s historically unprecedented 2012–2016 hotter drought, we noticed apparently drought-induced foliage dieback in giant sequoias (Sequoiadendron giganteum Lindl. [Buchholz]) in Sequoia and Kings Canyon national parks, California. Characteristics of the dieback were consistent with a controlled process of drought-induced senescence: younger (distal) shoots remained green while older (proximal) shoots were preferentially shed. As part of an ongoing interdisciplinary effort to understand and map sequoia vulnerability to hotter droughts, we reviewed historical records for evidence of previous foliage dieback events, surveyed dieback along trail corridors in eight sequoia groves, and analyzed tree-ring data from a high- and a low-foliage-dieback area. In sharp contrast to the greatly elevated mortality of other coniferous species found at low and middle elevations, we estimate that <1% of sequoias died during the drought. Foliage dieback was notably elevated in 2014 – the most severe single drought year in our 122-year record – but much lower in subsequent years. We found no historical records of similar foliage dieback during previous droughts. Dieback in 2014 was highly variable both within and among groves, ranging from virtually no dieback in some areas to nearly 50% in others. Dieback was highest (1) at low elevations, probably due to higher temperatures, reduced snowpack, and earlier snowmelt; (2) in areas of low adult sequoia densities, which likely reflect intrinsically more stressful sites; and (3) on steep slopes, probably reflecting reduced water availability. Average sequoia ring widths were narrower at the high-dieback than the low-dieback tree-ring site, but for reasons that remain unclear the sites did not differ in their proportional ring-width responses to past droughts. Collectively, our results suggest that giant sequoia vulnerability to hotter droughts may be spatially quite variable, and that at least some of that variability can be explained by metrics related to site water balance. Future research will focus on integrating our results with physiological and remote-sensing data, including tracking sequoias as they recover from the drought.

Distinct genecological patterns in seedlings ofNorway spruce and silver fir from a mountainous landscape

Understanding the genecology of forest trees is critical for gene conservation, for predicting the effects of climate change and climate change adaptation, and for successful reforestation. Although common genecological patterns have emerged, species-specific details are also important. Which species are most vulnerable to climate change? Which are the most important adaptive traits and environmental drivers of natural selection? Even though species have been classified as adaptive specialists vs. adaptive generalists, large-scale studies comparing different species in the same experiment are rare. We studied the genecology of Norway spruce (Picea abies) and silver fir (Abies alba), two co-occurring but ecologically distinct European conifers in Central Europe. For each species, we collected seed from more than 90 populations across Switzerland, established a seedling common-garden test, and developed genecological models that associate population variation in seedling growth and phenology to climate, soil properties, and site water balance. Population differentiation and associations between seedling traits and environmental variables were much stronger for Norway spruce than for silver fir, and stronger for seedling height growth than for bud phenology. In Norway spruce, height growth and second flushing were strongly associated with temperature and elevation, with seedlings from the lowlands being taller and more prone to second flush than seedlings from the Alps. In silver fir, height growth was more weakly associated with temperature and elevation, but also associated with water availability. Soil characteristics explained little population variation in both species. We conclude that Norway spruce has become an adaptive specialist because trade-offs between rapid juvenile growth and frost avoidance have subjected it to strong diversifying natural selection based on temperature. In contrast, because silver fir has a more conservative growth habit, it has evolved to become an adaptive generalist. This study demonstrates that co-occurring tree species can develop very different adaptive strategies under identical environmental conditions, and suggests that Norway spruce might be more vulnerable to future maladaptation due to rapid climate change than silver fir.

Germination sensitivity to water stress in four shrubby species across the Mediterranean Basin.

Mediterranean shrublands are generally water-limited and fire-driven ecosystems. Seed-based post-fire regeneration may be affected by varying rainfall patterns, depending on species sensitivity to germinate under water stress. In our study, we considered the germination response to water stress in four species from several sites across the Mediterranean Basin. Seeds of species with a hard coat (Cistus monspeliensis, C.salviifolius, Cistaceae, Calicotome villosa, Fabaceae) or soft coat (Erica arborea, Ericaceae), which were exposed or not to a heat shock and smoke (fire cues), were made to germinate under water stress. Final germination percentage, germination speed and viability of seeds were recorded. Germination was modelled using hydrotime analysis and correlated to the water balance characteristics of seed provenance. Water stress was found to decrease final germination in the three hard-seeded species, as well as reduce germination speed. Moreover, an interaction between fire cues and water stress was found, whereby fire cues increased sensitivity to water stress. Seed viability after germination under water stress also declined in two hard-seeded species. Conversely, E.arborea showed little sensitivity to water stress, independent of fire cues. Germination responses varied among populations of all species, and hydrotime parameters were not correlated to site water balance, except in E.arborea when not exposed to fire cues. In conclusion, the species studied differed in germination sensitivity to water stress; furthermore, fire cues increased this sensitivity in the three hard-seeded species, but not in E.arborea. Moreover, populations within species consistently differed among themselves, but these differences could only be related to the provenance locality in E.arborea in seeds not exposed to fire cues.