Soil Water Availability Research Articles

Early Growth Response of Khaya senegalensis Seedlings to Water Requirement

Water availability in quantity and quality is an important factor required during growth, development and for nursery seedling productivity. The study determined the response of Khaya senegalensis seedlings to water requirement and it effect on the growth. Levels of water required was determined by subjecting seedlings to four watering frequencies vis-à-vis Daily watering, watering every two days, watering every four days and once a week. The experiment was laid out in a complete randomized design (CRD) and parameters such as stem height, collar diameter, leaf area, leaf number were collected fortnightly while, dry weight, fresh weight, root to shoot ratio, root length, root weight, shoot length, turgidity weight and relative water content were evaluated after Twelve (12) weeks of growth. Data collected was subjected to One Way Analysis of Variance on SAS software and significant means was separated using Duncan Multiple Range Test (DMRT). From the result, morphological variables such as collar diameter (10.80mm), number of leaf (32.77), leaf area (403.65 cm2) were significantly (p<0.05) increased in seedlings with highest requirement for water (daily watering). Also, physiological variables such as dry weight (17.35 g), fresh weight (36.28 g), turgid weight (47.93 g), root weight (14.02 g) and shoot weight (24.38 g) increased significantly (p<0.05) in seedling water daily. The early growth of Khaya senegalensisseedlings was enhanced due to availability of soil water in response to its requirement.

Thermography captures the differential sensitivity of dryland functional types to changes in rainfall event timing and magnitude.

Drylands of the southwestern United States are rapidly warming, and rainfall is becoming less frequent and more intense, with major yet poorly understood implications for ecosystem structure and function. Thermography-based estimates of plant temperature can be integrated with air temperature to infer changes in plant physiology and response to climate change. However, very few studies have evaluated plant temperature dynamics at high spatiotemporal resolution in rainfall pulse-driven dryland ecosystems. We address this gap by incorporating high-frequency thermal imaging into a field-based precipitation manipulation experiment in a semi-arid grassland to investigate the impacts of rainfall temporal repackaging. All other factors held constant, we found that fewer/larger precipitation events led to cooler plant temperatures (1.4°C) compared to that of many/smaller precipitation events. Perennials, in particular, were 2.5°C cooler than annuals under the fewest/largest treatment. We show these patterns were driven by: increased and consistent soil moisture availability in the deeper soil layers in the fewest/largest treatment; and deeper roots of perennials providing access to deeper plant available water. Our findings highlight the potential for high spatiotemporal resolution thermography to quantify the differential sensitivity of plant functional groups to soil water availability. Detecting these sensitivities is vital to understanding the ecohydrological implications of hydroclimate change.

Afforestation reduced the deep profile soil water sustainability on the semiarid Loess Plateau

Insight into the response of deep soil moisture (SM) to land cover change is crucial for the sustainable management of regional water resources and ecosystems. However, the deep soil water budget and resilience after revegetation are not clear. In this study, seven typical land-cover patterns (artificial arbor, shrub, economic tree, pasture, abandon grassland, and natural grassland) were selected in four different study sites on the Loess Plateau of China. From May 2019 to December 2020, SM was monitored daily at ten depths along a 10 m profile in situ to assess the effect of revegetation on the vertical distribution, availability, and sustainability of soil water. The results indicated that the minimum SM moved to the deeper depth in the planted forest, while the maximum SM gradually moved to a shallower and deeper depth after the conversion of cropland to forestland and grassland, respectively. The difference in soil water use strategies under different revegetation types generated the opposite effect on deep SM availability, resilience, and sustainability. Conversion of cropland to grassland realized the deep soil available water (SAW) increment by 53.3% over the entire measurement period. Besides, the grassland promoting the deep soil water resilience (SWR) and soil water storage variation (ΔSWS) enhanced by 41.6% and 708.7% below 2 m depth, respectively. Whereas, compared to cropland, afforestation decreased the SAW, SWR, and ΔSWS by 17.2%, 10.5%, and 570.7% below 2 m depth, respectively. Furthermore, the deep SWR and ΔSWS were basically negative in the forestland, resulting in an unbalanced deep soil water budget within the year. Despite the significant disadvantages of sustaining the availability and water budget of deep soil water in artificial vegetation, constructing a reasonable land cover and vegetation structure could achieve the soil–water environment balance and the sustainable utilization of ecosystem water resources in the semiarid Chinese Loess Plateau (CLP). The findings of this study provide valuable references and guidance for sustainable vegetation conservation and water resource management and provide design and optimization solutions for vegetation restoration in similar areas of arid and semiarid regions around the world.

Thinning effectively mitigates the decline of aging Mongolian pine plantations by alleviating drought stress and enhancing plant carbon balance

Intensifying drought around the globe poses strong challenges to the growth and survival of trees, emphasizing the urgency of understanding the physiological mechanisms of drought-induced tree decline and mortality and the necessity of using silvicultural approaches to mitigate drought effects on tree growth. We experimentally explored whether thinning enhances tree growth vigor and drought resilience in early aging Mongolian pine (Pinus sylvestris var. mongolica Litv.) plantations in a water-limited area of Northeast China using dendrochronological methods. Meanwhile, we compared plant hydraulic traits, water relations, and non-structural carbohydrate concentrations between trees growing in the thinned and un-thinned stands with the aim of unraveling the physiological mechanisms underlying the density effects on tree growth performance. The results showed that basal area increments of Mongolian pine trees in the high-density plantations started to decline continuously at tree cambium age of around 15 years over about a decade, while thinning effectively reversed their growth decline. Trees in thinned stands exhibited significantly higher predawn leaf water potentials, higher stem hydraulic conductivities, and larger carbon reserves than trees in un-thinned stands. Although trees growing in high-density stands maintained as low risks of xylem hydraulic dysfunction as trees growing in low-density stands, that was at the cost of substantially reducing tree crown size through branch mortality that unavoidably undermined their carbon balance at the whole-plant level. Consistently, trees growing in the thinned low-density stands showed significantly higher levels of non-structural carbohydrate reserve in stems and enhanced growth resilience to extreme drought than trees growing in the high-density reference stands. Our results suggest that thinning can effectively enhance xylem hydraulic transport efficiency and reduce the risk of carbon imbalance by increasing soil water availability and hence mitigates the early aging of Mongolian pine plantations in water-limited environments.

Effects of soil moisture and vapor pressure deficit on canopy transpiration for two coniferous forests in the Loess Plateau of China

Soil moisture (VWC) and atmospheric dryness (vapor pressure deficit, VPD) have an increasing impact on tree transpiration and productivity with climate change. However, the interaction of VWC and VPD on canopy transpiration (Ec) of plantations is still unclear, especially in water-scarce areas. Here, we selected Pinus tabulaeformis Carr. and Platycladus orientalis (L.) Franco plantations in the semiarid loess hills of China to examine the impacts of soil and atmospheric water conditions on Ec during the growing seasons (May–September) of 2015–2020. The results showed that the magnitudes of Ec were similar in the two plantations throughout the study period. The lowest Ec occurred in September because of little precipitation and low atmospheric evaporative demand, while Ec was highest in July due to the increase in soil water availability and atmospheric evaporative demand. There were obvious inflection points in the response relationship between VWC and VPD with Ec for both plantations. Under non-drought conditions, Ec reached a maximum in both plantations and was mainly impacted by solar radiation (Rs) and air temperature. As VPD increased, Rs still was the dominant factor, but canopy conductance decreased significantly, resulting in negative correlations between Ec and VPD. Under soil drought and compound drought conditions, Ec was mainly controlled by Rs and VWC, and the inhibitory effect of VPD on Ec was enhanced. In addition, the ratio of Ec to precipitation was close to 50% for both plantations at the interannual scale. Considering that soil water depletion in the Loess Plateau will accelerate with climate change and increasing drought events, the increasing water use stress will further hinder tree gas exchange, inhibiting tree growth. It is thus necessary to pay special attention to the impact of soil and atmospheric drought on water use for forest management in the Loess Plateau and similar regions of the world.

Soil texture mediates the surface cooling effect of urban and peri-urban green spaces during a drought period in the city area of Hamburg (Germany)

Urban green spaces (UGS) and peri-urban green spaces (P-UGS) play a crucial role in reducing the land surface temperature within the urban environment, especially during heat waves. Although their cooling effect generally is due to shading and evaporation, the role of soil texture and soil water availability on surface cooling remains largely unexplored. This study investigated the impact of soil texture on the spatio-temporal patterns of LST in different UGSs and P-UGSs in Hamburg (Germany) during a hot summer drought period. The LST and the Normalized Differentiated Moisture and Vegetation Indices (NDMI, NDVI) were calculated based on two Landsat 8 OLI/TIRS images from July 2013. Non-spatial and spatial statistical approaches such as stepwise backward regression or Hotspot (Getis-Ord Gi*) analyses were applied explaining LST distributions in relation to soil texture within each UGS and P-UGS. All GSs were clearly characterized as surface cooling islands whereas, for each GS, an individual thermal footprint was observed. Within all GSs, the LST patterns showed a significant negative relationship to NDMI values, whereas the NDVI values and the elevation were of minor importance. Soil texture was found to influence the LST distribution significantly in most UGSs and P-UGSs, where sites on clay-rich soils showed the highest LST values compared to sites on sand- or silt-rich soils. For example, in parks, clayey soils showed a mean LST of 25.3 °C whereas sand-dominated sites had a mean LST of only 23.1 °C. This effect was consistent throughout all statistical approaches, for both dates and across most GSs. This unexpected result was explained by the very low unsaturated hydraulic conductivity in clayey soils which limits plant water uptake and transpiration rates responsible for the evaporative cooling effect. We concluded that soil texture has to be considered for understanding and managing the surface cooling capacity of UGSs and P-UGSs.

Estimating Future Surface Water Availability Through an Integrated Climate‐Hydrology‐Management Modeling Framework at a Basin Scale Under CMIP6 Scenarios

AbstractClimate change and increasing water demand due to population growth pose serious threats to surface water availability. The biggest challenge in addressing these threats is the gap between climate science and water management practices. Local water planning often lacks the integration of climate change information, especially with regard to its impacts on surface water storage and evaporation as well as the associated uncertainties. Using Texas as an example, state and regional water planning relies on the use of reservoir “Firm Yield” (FY)—an important metric that quantifies surface water availability. However, this existing planning methodology does not account for the impacts of climate change on future inflows and on reservoir evaporation. To bridge this knowledge gap, an integrated climate‐hydrology‐management (CHM) modeling framework was developed, which is generally applicable to river basins with geographical, hydrological, and water right settings similar to those in Texas. The framework leverages the advantages of two modeling approaches—the Distributed Hydrology Soil Vegetation Model (DHSVM) and Water Availability Modeling (WAM). Additionally, the Double Bias Correction Constructed Analogues method is utilized to downscale and incorporate Coupled Model Intercomparison Project Phase 6 GCMs. Finally, the DHSVM simulated naturalized streamflow and reservoir evaporation rate are input to WAM to simulate reservoir FY. A new term—“Ratio of Firm Yield” (RFY)—is created to compare how much FY changes under different climate scenarios. The results indicate that climate change has a significant impact on surface water availability by increasing reservoir evaporation, altering the seasonal pattern of naturalized streamflow, and reducing FY.

Spatiotemporal Variability of Alpine Meadow Aboveground Biomass and Sustainable Grazing in Light of Climate Warming

Understanding spatiotemporal variability of aboveground biomass (AGB) in alpine grassland ecosystems is required to better formulate sustainable grazing strategies, especially in climate-sensitive areas such as the Qinghai-Tibet Plateau. Drawing from recent modeling results, we analyzed spatial variability in grassland AGB over seasonal and annual timescales. Theoretical livestock carrying capacity (LCC) was predicted in relation to projected warming and simulated pika disturbance. We found that elevation and slope were key determinants of the distribution of AGB and its change. Although grasses and shrubs occupied an equivalent area, the latter generated approximately double the amount of total biomass, but the former generated slightly more available biomass. Although pika habitat extended over 45.3% of the study area, the impacts of pika only reduced the total available AGB (AGBV) by < 0.07%. The actual LCC (2.16 SU · ha−1 [sheep unit per hectare]) of the study area was lower than the theoretical LCC (2.77−2.96 SU · ha−1). Under climate warming scenarios of 1.0°C, 1.5°C, and 2.0°C, the actual LCC can be increased by 0.82-1.25, 0.99-1.53, and 0.77−1.23 SU · ha−1, respectively. Although systematic climate warming increased AGB, impacts were especially marked at lower elevations and on north-facing slopes because of higher soil water availability. Results highlight that rather than implementing LCC in a static manner in light of climate warming, deeper understanding of biomass availability and the combined effects of climate change and topographic heterogeneity is required to support better management of grasslands.

Analysis of Principal Components for the Assessment of Silage Corn Hybrid Performance under Water Deficit

Corn is an important crop for animal production systems. Owing to climate change, it is important to understand the behavior of commercial corn hybrids under low soil water availability. The objective of this study was to evaluate the morphogenic characteristics and yields of silage corn hybrids subjected to different water conditions using univariate and multivariate analyses. The experimental design was randomized blocks in a 5 × 2 factorial scheme with four replications. Corn hybrids (AGRI104, AGRI320, AGRI330, AGRI340, and AGRI360) and water replacement levels (50% and 100% replacement of crop evapotranspiration—ETc) were considered as factors. A water replacement level of 50% ETc was defined as a severe water deficit that persisted for 46 days from V7 until harvesting (R3). Plant height, stem diameter, leaf area, chlorophyll index, water productivity, fresh and dry mass, and dry matter of the shoot and root systems were evaluated. An analysis of variance and a mean test of isolated variables were carried out, and a multivariate perspective, through principal components was performed. In general, the replacement of 50% ETc changed the morphophysiological characteristics of the plants; however, it was only possible to classify the hybrids through a principal components analysis. The best performing hybrids, regardless of the water replacement levels, were AGRI340 and AGRI360.

Impact of Nanoparticles from Ball-Milled Date Palm Biochar on the Hydro-Physical Characteristics of Sandy Soils

Water management in sandy soils (Typic Torripsamments) is crucial in sustaining agricultural production. The main goal of this research was to assess the impact of date palm biochar on the physical properties of sandy soil with different particle sizes of biochar (macro and nano). For nano-biochar preparation, stick chips were established into a tubular furnace with nitrogen air and heated to 400–450 °C, which was accompanied by a holding period of 4 h. The ball-milled biochar was inclined via ball grinding in a model number PQN2.110 planetary mill and within jars (500 mL), and the biochar-to-sphere mass ratio was 1:100. The sphere-milling apparatus was processed at a speed of 300 rpm for 13 h. Laboratory experiments were carried out at one rate—biochar 5%—and three depths (0.0–5, 5–10, and 10–15 cm). Applying macro-biochar reduced cumulative evaporation compared to the control by 4%, 24%, and 14% for the macro-biochar particles at soil depths. In contrast, biochar reduced cumulative evaporation compared to the control by 8%, 12%, and 4% for the nano-biochar particles at the soil depths tested. Adding biochar significantly raised the amount of retained water, with the highest level recorded at the 5–10 cm depth, while the variations were significantly lower between the macro and nano-biochar when in the direction of the soil surface (0–5 cm), indicating the significance of mixing biochar with the top 10 cm of the soil to increase its ability to reduce evaporation and increase the amount of water retained in the soils. It could be concluded that applying at the top of the coarse soil can positively impact soil hydro-physical properties and increase soil water availability to plants.

Comparing actual transpiration fluxes as measured at leaf-scale and calculated by a physically based agro-hydrological model

The main purpose of this paper is to compare the actual transpiration rates from tomato crop, as measured at leaf scale and estimated by a macroscopic approach in an agro-hydrological model, named FLOWS-HAGES, under variable soil properties and water availability. To this aim, sixteen plots were cultivated with tomatoes in Metaponto, Southern Italy. Soil hydraulic properties (SHP) were obtained using a fast in-situ characterization method. Leaf-area index (LAI) was measured using a leaf-area meter. SHP and LAI were then used in the physically-based FLOWS-HAGES which allowed calculating the macroscopic transpiration rates, Ta,m. Single-leaf transpiration rates, Ta,l, and stomatal conductance, gs,l, were measured in situ. For comparing with Ta,m, gs,l was upscaled by Big-Leaf approach to canopy scale stomatal conductance, gs,c, which was applied to Penman-Monteith model to obtain the canopy-scale transpiration, Ta,c. Finally, multiple linear regression (MLR) was used to find the statistical relationship between Ta,m and Ta,c, and the SHP and gs,c. Results showed that the macroscopic approach smooths the spatial variability of transpiration rates. Ta,c increased with the saturated water content, θs, and the slope of the water retention curve, n, while Ta,m decreased with increasing θs and n. MLR improved significantly by introducing gs,c to predict Ta,m.

Long-term under-vine coverage by spontaneous vegetation changed plant community and soil dynamics without impacting yield at two South Australian vineyards

Under-vine management practices have traditionally consisted of herbicide applications or cultivation to manage weeds, thus limiting competition for resources with grapevines. However, increasing awareness of detrimental impacts of herbicides and cultivation on agricultural landscapes are shifting practices towards alternative options, including an unmanaged under-vine area where spontaneous vegetation proliferates. How this practice may affect the suite of soil, water, and nutrient relations and corresponding grapevine performance in a Mediterranean climate in the long-term is relatively unknown. Thus, this study utilised a unique opportunity to explore how undisturbed under-vine spontaneous vegetation compared to traditional herbicide (Eden Valley, loamy sand soil) or cultivation (McLaren Vale, silty loam soil) practices at two commercial vineyards. After five years of treatments, assessments were made on under-vine plant community and soil dynamics, grapevine responses, and juice composition. At Eden Valley, the spontaneous vegetation changed the plant community by increasing total plant coverage and biomass, while at McLaren Vale there was a lesser effect; however, the plant communities at both sites were similarly dominated by perennial grasses compared to higher proportions of broad-leaf, fast-growing species in the traditional herbicide and cultivation treatments. Soil water and nutrient availability were decreased by spontaneous vegetation in Eden Valley, while in McLaren Vale, the spontaneous vegetation enhanced soil phosphorus, water infiltration, and increased organic carbon by 40%. In Eden Valley, spontaneous vegetation decreased juice yeast available nitrogen; yet phenolics, anthocyanins, and total soluble solids were higher. At both sites, there was no significant impact of maintaining under-vine spontaneous vegetation on yields, however it decreased pruning weights at Eden Valley, while no effects were quantified on pruning weights or juice composition at McLaren Vale. We demonstrate that in the long-term, maintaining undisturbed spontaneous vegetation as an alternative to using herbicides or cultivation in the under-vine row can benefit the vineyard agroecosystem, while not adversely impacting grapevine yields depending on vineyard soil type and target winemaking outcomes.

Nitrogen uptake preference and allocation in Populus cathayana in response to drought stress

The preference for different nitrogen (N) forms may enhance plant survival and fitness. However, little information is known about how soil water availability, N addition, and their interaction dynamically impact plant N uptake preference and allocation. Here, we conducted a pot experiment using Populus cathayana cuttings, consisting of soil water regime (well watering control/mild drought/intensive drought: 80/60/40 % of field capacity) combined with N addition (N0 as control/N4/N8: 0/4/8 g N m−2 yr−1) treatments based on 15N isotope tracer technology. Two tracer solution (15NH4NO3/NH415NO3, both at 10.2 atom%) were used in N4/N8 treatments. Root, stem and leaf were collected at 6 h, 1, 3, 7, 14, and 31 days after labeling. Results showed that N addition significantly increased plant biomass and N accumulation, and decreased N allocation to leaf. Drought significantly decreased plant biomass, N accumulation, and N uptake (indicated by δ15N), and intensive drought reduced more than mild drought did. Nitrogen allocation to leaf was increased by drought. NO3- uptake by P. cathayana (indicated by δ15N in NH415NO3 treatment) was correlated significantly positively with total root length (TRL), total root surface area (TRA), specific root length (SRL), specific root area (SRA), root vitality, and root nitrate reductase (NR) activity. Intensive drought significantly reduced TRL, TRA, SRL, SRA, root vitality, and root NR activity under N4 and N8 treatments, which resulted δ15N in NH415NO3 treatment reduction and lower than in 15NH4NO3 treatment, so P. cathayana exhibited ammonium preference under intensive drought treatment. Our study provides an essential insight into how plants adapt and prefer different N forms under drought stress in nature.

Genetic control underlying the flowering-drought tolerance trade-off in the Antarctic plant Colobanthus quitensis.

Plants inhabiting environments with stressful conditions often exhibit a low number of flowers, which can be attributed to the energetic cost associated with reproduction. One of the most stressful environments for plants is the Antarctic continent, characterized by limited soil water availability and low temperatures. Induction of dehydrins like those from the COR gene family and auxin transcriptional response repressor genes (IAAs), which are involved in floral repression, has been described in response to water stress. Here, we investigated the relationship between the water deficit-induced stress response and the number of flowers in Colobanthus quitensis plants collected from populations along a latitudinal gradient. The expression levels of COR47 and IAA12 genes in response to water deficit were found to be associated with the number of flowers. The relationship was observed both in the field and growth chambers. Watering the plants in the growth chambers alleviated the stress and stimualted flowering, thereby eliminating the trade-off observed in the field. Our study provides a mechanistic understanding of the ecological constraints on plant reproduction along a water availability gradient. However, further experiments are needed to elucidate the primary role of water availability in regulating resource allocation to reproduction in plants inhibiting extreme environments.

Deep roots mitigate drought impacts on tropical trees despite limited quantitative contribution to transpiration

Deep rooting is considered a central drought-mitigation trait with vast impact on ecosystem water cycling. Despite its importance, little is known about the overall quantitative water use via deep roots and dynamic shifts of water uptake depths with changing ambient conditions. Knowledge is especially sparse for tropical trees. Therefore, we conducted a drought, deep soil water labeling and re-wetting experiment at Biosphere 2 Tropical Rainforest. We used in situ methods to determine water stable isotope values in soil and tree water in high temporal resolution. Complemented by soil and stem water content and sap flow measurements we determined percentages and quantities of deep-water in total root water uptake dynamics of different tree species. All canopy trees had access to deep-water (max. uptake depth 3.3 m), with contributions to transpiration ranging between 21 % and 90 % during drought, when surface soil water availability was limited. Our results suggest that deep soil is an essential water source for tropical trees that delays potentially detrimental drops in plant water potentials and stem water content when surface soil water is limited and could hence mitigate the impacts of increasing drought occurrence and intensity as a consequence of climate change. Quantitatively, however, the amount of deep-water uptake was low due to the trees' reduction of sap flow during drought. Total water uptake largely followed surface soil water availability and trees switched back their uptake depth dynamically, from deep to shallow soils, following rainfall. Total transpiration fluxes were hence largely driven by precipitation input.

Divergent effects of intensified precipitation on primary production in global drylands

Amplification of hydrological cycle under warming climate is anticipated to result in intensified precipitation characterized by fewer, more intense events and correspondingly longer dry intervals between events, even without major changes in annual total precipitation. Vegetation gross primary production (GPP) in drylands is highly responsive to intensified precipitation, however, how intensified precipitation influences GPP in global drylands is not well understood. Based on multiple satellite datasets from 2001 to 2020 and in-situ measurements, we investigated the effects of intensified precipitation on global drylands GPP under diverse annual total precipitation along the bioclimate gradient. Dry, normal, and wet years were identified as years with annual precipitation anomalies below, within, and above the range of one standard deviation. Intensified precipitation led to increases or decreases of GPP during dry or normal years, respectively. However, such effects were largely weakened during wet years. The responses of GPP to intensified precipitation were mirrored by soil water availability, as intensified precipitation enhanced root zone soil moisture, and thus vegetation transpiration and precipitation use efficiency during dry years. During wet years, root zone soil moisture showed less response to changed precipitation intensity. Land cover types and soil texture regulated the magnitude of the effects along the bioclimate gradient. Under intensified precipitation, shrubland and grassland distributed in drier region with coarse soil texture showed greater increases of GPP during dry years. As global precipitation will likely further intensify, the impacts of intensified precipitation on dryland carbon uptake capacity will be highly diverse along the bioclimate gradients.

Experimental drier climates affect hydraulics and induce high mortality of seedlings of three northern conifer species

The future climate of northern temperate forests is projected to be drier and warmer by the end of this century. As a result, more drought-induced forest dieback events are anticipated in northeastern North America, and assessing the vulnerability of dominant tree species to drought is critical for understanding the future composition of these forests. In a greenhouse experiment, we exposed two-year-old seedlings of Picea glauca (Moench) Voss, Picea mariana (Mill.) B.S.P. and Pinus strobus L. to three future climate treatments for southern Quebec, Canada, and evaluated their mortality, growth, and foliage water status responses to soil water availability and atmospheric drought. Using a unique approach, climate treatments emulated droughts of different frequencies, durations, and intensities. Treatments closely simulated one growing season, with changes in air temperature and relative humidity every six hours and daily adjustment in the amount of water delivered to the seedlings. The three species experienced high mortality (75%) in all water-limited treatments compared to a control treatment that provided non-limiting soil moisture (0% mortality). The biomass of the seedlings that survived was 40% lower than that of control seedlings. Our results confirmed that the hydraulic safety margins, defined as the difference between seasonal minimum water potential and xylem water potential leading to 12, 50 and 88% of hydraulic conductivity loss, were good predictors of probability of tree mortality. Therefore, hydraulic safety margins are useful functional traits that can be used to compare the vulnerability of various species to drought and then provide crucial information to practitioners and policymakers to adjust forest management to climate change. We showed that three dominant conifer species of northern temperate forests were highly vulnerable to drought in future climates. Because drought is projected to be a significant threat to forests, understanding potentially adaptive physiological responses to drought, such as hydraulic safety margins of tree seedlings, is important for predicting the response of forest regeneration and composition in warmer and drier climates.

Responses of two Acacia species to drought suggest different water-use strategies, reflecting their topographic distribution.

Soil water availability is a key factor in the growth of trees. In arid deserts, tree growth is limited by very dry soil and atmosphere conditions. Acacia tree species are distributed in the most arid deserts of the globe, therefore they are well adapted to heat and long droughts. Understanding why some plants do better than others in some environments is a key question in plant science. Here we conducted a greenhouse experiment to continuously and simultaneously track the whole-plant water-balance of two desert Acacia species, in order to unravel their physiological responses to low water availability. We found that even under volumetric water content (VWC) of 5-9% in the soil, both species maintained 25% of the control plants, with a peak of canopy activity at noon. Moreover, plants exposed to the low water availability treatment continued growing in this period. A. tortilis applied a more opportunistic strategy than A. raddiana, and showed stomatal responses at a lower VWC (9.8% vs. 13.1%, t4= -4.23, p = 0.006), 2.2-fold higher growth, and faster recovery from drought stress. Although the experiment was done in milder VPD (~3 kPa) compared to the natural conditions in the field (~5 kPa), the different physiological responses to drought between the two species might explain their different topographic distributions. A. tortilis is more abundant in elevated locations with larger fluctuations in water availability while A. raddiana is more abundant in the main channels with higher and less fluctuating water availability. This work shows a unique and non-trivial water-spending strategy in two Acacia species adapted to hyper-arid conditions.

Vanilla insignisAmes. (Orchidaceae): morphological variation of the labellum in Quintana Roo, Mexico

AbstractThe shape of the flower can vary based on the type of pollinator or the environment in which the plant develops. InVanilla insignis, there are no studies that analyse the shape of the labellum of the flower as has been done inV. planifolia. Therefore, the study aimed to determine the variation in the shape of the labellum ofV. insignisthrough a morphometric analysis in different environmental conditions in the state of Quintana Roo, Mexico. The results showed that there were significant differences in the variables analysed. Principal component analysis and dendrogram analysis reveal that fourV. insignismorphotypes were possibly associated with soil water availability conditions because there were significant differences between the variables that define the apical region. In addition, the distribution of the morphotypes corresponded with the presence of humidity regardless of geographical distances such as in the populations of Tenampulco, Puebla and Caobas, Quintana Roo. The presence of these morphotypes allows the development of conservation programmes and genetic improvement of the species ofV. insignisand related commercial species.

Disentangling the Role of Forest Structure and Functional Traits in the Thermal Balance of the Mediterranean–Temperate Ecotone

AbstractThe thermal balance of forests is the result of complex land–atmosphere interactions. Different climate regimes and plant functional types can have contrasting energy budgets, but little is known about the influence of forest structure and functional traits. Here, we combined spaceborne measurements of surface temperature from ECOSTRESS with ground‐based meteorological data to estimate the thermal balance at the surface (∆Tcan−air) during four summers (2018–2021), at the Mediterranean–temperate ecotone in the NE Iberian Peninsula. We analyzed the spatiotemporal drivers of ∆Tcan−air by quantifying the effects of meteorology, forest structure (stand density, tree height) and ecophysiology (hydraulic traits), during normal days and hot spells. Canopy temperatures (Tcan) fluctuated according to changes in air temperature (Tair) but were on average 4.2 K warmer. During hot spells, ∆Tcan−air was smaller than during normal periods. We attribute this decrease to the advection of hot and dry air masses from the Saharan region resulting in a sudden increase in Tair relative to Tcan. Vapor pressure deficit (VPD) was negatively correlated with ∆Tcan−air, since the highest VPD values coincided with peaks in heat advection. Nonetheless, Tcan increased with VPD due to decreased transpiration (following stomatal closure), even though sufficient soil water availability enabled some degree of evaporative cooling. Our findings demonstrate that plot‐scale forest structural and hydraulic traits are key determinants for the forest thermal balance. The integration of functional traits and forest structure over relevant spatial scales would improve our ability to understand and model land–atmosphere feedbacks in forested regions.