Water Availability Gradient Research Articles

Helechos y licofitas del sudeste del Sistema de Tandilia (Buenos Aires, Argentina): composición, afinidad biogeográfica y patrones de diversidad ecológica

Background and aims: The Tandilia system is a fundamental refuge for a highly diverse biota throughout the wide climatic variability of the mid-late Cenozoic that affected the Pampean steppe, especially for fern and lycophyte flora. The geology and proximity to the sea in the southeast of the Tandilia system raise questions about the composition and biogeographical affinities of the fern and lycophyte flora. Furthermore, given the anthropogenic landscape transformation, it is necessary to inventory the status of the frequency and microenvironments in which these species develop. Thus, we present an inventory and frequency of ferns and lycophytes of the southeast of Tandilia associated with different microenvironments and biogeographical affinity and develop a dichotomous key for the area. M&M: We collected fern and lycophyte between 2006 and 2024 in Balcarce and General Pueyrredon hill ranges and they were compared with those fern and lycophyte flora from Ventania and Tandilia. Results: This study shows that 51% of the ferns and licophytes reported at Tandilia rangeland System develope in the southeastern of Tandilia. An increase of AustroBrazilian elements and decrease of Austral-Antarctic elements were observed, linked to latitudinal and biogeographical aspects. Adaptive traits related to a water availability gradient and different microenvironments were established. Eight species were cataloged as rare due to their low frequency. Conclusions: The results highlight the need to plan conservation areas for the fern and lycophyte flora of the Tandilia mountain system.

Medium-term biotic interactions in Olea europaea seedlings along an abiotic stress gradient

Plant-plant interactions are important drivers for the functioning and structuring of dryland plant communities. Stressful conditions, mainly related to water availability, determine whether plant interactions result in net facilitation or competition. Many studies have been conducted on the impacts of mature individuals, trees, shrubs or tussock grasses, on woody seedlings but little is known about the biotic interactions when they all are at the same developmental stage under field conditions. Here we present a spatially replicated study with Olea europaea as the target species and Pistacia lentiscus and Macrochloa tenacissima as neigbors. We also implemented field techniques to increase water availability and, hence, reproduce an abiotic gradient of stress. Wild olive performance was sensitive to neighbors, water availability and site. Improved microcatchments, which in addition to site recreated a gradient of water availability, improved wild olive performance as well as planted neighbors negatively impacted olive seedlings. We observed increasing negative interacting effects as conditions become less stressful. This is in line with the stress-gradient-hypothesis but also provides important knowledge regarding plant interactions in drylands at the same development stage.

Pure and mixed Scots pine forests showed divergent responses to climate variation and increased intrinsic water use efficiency across a European-wide climate gradient

AbstractThe present study examined Pinus sylvestris L. growth responses to climatic variations and its relationship with intrinsic water-use efficiency (iWUE) across a water availability gradient and also in pure P. sylvestris and P. sylvestris-Quercus species mixed forests. Study sites were selected in the Mediterranean, temperate, and temperate continental climates in Spain, Italy, and Poland, respectively. A combined tree-ring dendrochronological and stable carbon isotope analysis was used to assess the relationship between tree growth and climate variation. Results showed that P. sylvestris growth is critically affected by summer water availability, regardless of study site and species mixing. Warming temperatures during the early growing season benefit tree growth in Mediterranean and temperate continental climates, while no significant effect was observed in the temperate climatic conditions. At the Mediterranean site, trees in mixed stands showed enhanced growth during wet years when moisture is not limiting. At the temperate continental site, trees in the mixed stand grew at a lower rate than those in pure stands, which suggests that intense interspecific competition for water could overwhelm the benefits of species mixing. Also, we found a divergent growth-iWUE relationship of non-significant and significantly positive and significantly negative correlations at the Polish, Italian, and Spanish sites, respectively. Overall, the negative growth-iWUE relationship at the drier Mediterranean site signifies the risk of tree growth decline, particularly in drier climate conditions. Despite that, elevated iWUE levels would benefit tree radial growth when water is not limited and the admixing tree species have compatible light and water use strategies.

Sex-specific phosphorus acquisition strategies and cycling in dioecious Populus euphratica forests along a natural water availability gradient.

Soil phosphorus (P) availability affects plant growth and distribution. However, it is still unknown how sex-specific variation in functional traits affects plants' P acquisition and soil P transformation. On wet sites, female poplars had a greater specific root length (SRL), and a higher diversity of arbuscular mycorrhizal fungi (AMF) and phosphate-solubilizing bacteria (PSB). Male poplars living on wet sites increased the abundance of AMF and PSB communities and enhanced moderately labile and highly resistant organic P mineralisation via increased phosphatase activity. In contrast, on the dry site, the abundance and diversity of AMF and PSB communities increased in females, enhancing moderately labile and highly resistant organic P mineralisation via elevating phosphatase activities. Males maintained greater SRL and promoted Ca-P mobilisation via the release of root carboxylic acids and rhizosphere acidification on the dry site. The AMF community diversity followed a similar pattern as that of the PSB community when altering the P availability of different-sex plants. Our results indicated that organic P and Ca-P are the major sources of plant-available P in natural P. euphratica forests. Seasonal shifts and geographic locations affected the share of organic and inorganic P pools, and AMF and PSB diversities, ultimately altering sex-specific P acquisition strategies of plants.

Interspecies diversity in morphological responses to water stress: Study on a panel of weed and crop species

Climate change modifies the dynamics and the quantity of plant water supply, and plant morphological response to environmental factors plays a key role in crop-weed interactions. This study investigated the interspecies diversity related to the morphological responses of annual herbaceous species to water stress. Key morphological traits were measured at two growth stages on five weed and two crop (soft wheat, rapeseed) species grown on a gradient of water availability in a greenhouse experiment. For each trait, response curves to water stress were defined, and their parameters were used to quantify interspecies diversity. Generic morphological response patterns were identified across all species and plant stages. Water stress reduced leaf area per unit leaf biomass (SLA, lowering water demand) and increased the ratio of plant height to aboveground biomass for all species (HBR, keeping access to light). In most situations, the ratio of root biomass over total biomass (RBR) increased (improving water uptake). Variability in parameter values of morphological traits was primarily explained by the species, followed by growth stage. Geranium dissectum L. and Abutilon theophrasti Medik. were the most responsive species to water stress, especially at the flowering stage, with strong RBR and HBR increase and slight SLA decrease. Species differences were not related to clade (monocotyledonous/dicotyledonous) nor status (weed/crop), despite a near-significant clade effect on allocation of aboveground biomass, with grass species allocating more biomass to stems (vs leaves), while no general tendency was observed in broadleaved species. These findings provide new insights on comparative ecology of weed and crop species response to water limitation, and more research is expected to cover a wider range of weed and crop species.

Extremophile hypolithic communities in the Vestfold Hills, East Antarctica

Abstract The Vestfold Hills are a 400 km2, isolated ice-free oasis in eastern Antarctica featuring large areas with translucent quartz rocks that provide habitat for hypolithic microbial communities underneath. We used high-throughput DNA sequencing of 16S and 18S ribosomal RNA amplicons to characterize bacterial and eukaryotic hypolithic communities across the Vestfold Hills. We found high-level, local heterogeneity in community structure consistent with limited dispersal between hypoliths. Hypolithic communities were dominated by heterotrophic Bacteroidetes (mean bacterial relative read abundance: 56%) as well as Cyanobacteria (35%), with the eukaryote component often dominated by Chlorophyta (43%). Small but significant proportions of the variation in microbial community composition and function were explained by soil salinity (5–7%) and water availability (8–11%), with distinct taxa associated with different salinities and water availabilities. Furthermore, many inferred bacterial metabolic pathways were enriched in hypolithic communities from either dry or high-salinity sites. Vestfold Hills hypolithic habitats are likely to be local refuges for bacterial and eukaryotic diversity. Gradients in soil salinity and water availability across the Vestfold Hills, in addition to the number and diversity of lake types and fjords as potential source populations, may contribute to the observed variation in the extremophile, hypolithic microbial community composition.

Inoculated biocrust cover and functions diverged over a gradient of soil textures and water availability

Restoring biological crust (biocrust) in disturbed drylands is challenging due to the difficult environmental conditions, such as limited soil moisture, low soil nutrients, and extreme temperatures, that impede growth. Understanding how the key components of biocrust—mosses, lichens, and cyanobacteria—react to different environmental factors informs the optimal timing, locations, and species composition for biocrust reintroduction, thereby increasing the likelihood of establishment. Here, we inoculated soils with a diverse range of biocrust organisms, analogous to seeding an area with diverse vascular plant seeds, and varied environmental conditions to observe how these changes influenced the development and functions of reintroduced biocrust. We found that by manipulating soil texture and time spent wet, we can change the proportional cover of biocrust within a restoration‐like setting. Specifically, we found that 4 months after inoculation, finer textured soils that received more water become dominated by moss cover, while coarser textured soils with less water remained dominated by cyanobacteria cover, and the interactions between texture and time spent wet strongly influenced cover. We found biocrust morphological group cover had a small, but detectable, effect on ecosystem functions (soil stability and nitrogenase activity, a proxy for nitrogen fixation), but that environmental conditions had a stronger impact on the functions we measured. Manipulative experiments in controlled environments, like this one, can help elucidate the mechanisms underlying the establishment rate and patterns of biocrusts post‐inoculation, and inform implementation of inoculations in the field.

Microbe‐induced plant resistance against insect pests depends on timing of inoculation but is consistent across climatic conditions

Abstract To cope with abiotic and biotic stressors, plants have developed mutualistic associations with beneficial soil microbes, but little is known about how extreme abiotic conditions impact on microbe‐induce resistance to insect herbivores. Extreme temperatures are often accompanied by extremes in plant water availability, which together reduce plant growth and change plant physiology. There are potential consequences for increasing plant susceptibility to biotic stresses, and this poses a real challenge for plant productivity. We evaluated how the effects of beneficial soil bacteria (Acidovorax radicis N35e) on barley plant growth and resultant resistance against aphid infestation (Sitobion avenae) were impacted by a single heatwave event across a plant water availability gradient. We also tested if timing of bacterial inoculation (before or after the temperature treatment) affected bacteria‐plant interactions on aphids. We found that heatwaves affected plant biomass allocation from above‐ground to below‐ground tissues. Inoculation with A. radicis led to reduction of aphid numbers, but depended on timing of inoculation, and led to stronger resistance when inoculations occurred closer to aphid infestation. Remarkably, microbe‐induced resistance against aphids was consistent across heatwave and water availability treatments. This study provides evidence that beneficial plant‐bacteria interactions may represent a potential solution for sustainable agricultural practices to enhance plant growth and response to insect pests under climate change. Future field trials should investigate the consistency of beneficial effects reported here for a better understanding of multispecies interactions in the context of global change. Read the free Plain Language Summary for this article on the Journal blog.

Genomic prediction for root and yield traits of barley under a water availability gradient: a case study comparing different spatial adjustments

BackgroundIn drought periods, water use efficiency depends on the capacity of roots to extract water from deep soil. A semi-field phenotyping facility (RadiMax) was used to investigate above-ground and root traits in spring barley when grown under a water availability gradient. Above-ground traits included grain yield, grain protein concentration, grain nitrogen removal, and thousand kernel weight. Root traits were obtained through digital images measuring the root length at different depths. Two nearest-neighbor adjustments (M1 and M2) to model spatial variation were used for genetic parameter estimation and genomic prediction (GP). M1 and M2 used (co)variance structures and differed in the distance function to calculate between-neighbor correlations. M2 was the most developed adjustment, as accounted by the Euclidean distance between neighbors.ResultsThe estimated heritabilities (h^2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\widehat{h}}^{2}$$\\end{document}) ranged from low to medium for root and above-ground traits. The genetic coefficient of variation (GCV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$GCV$$\\end{document}) ranged from 3.2 to 7.0% for above-ground and 4.7 to 10.4% for root traits, indicating good breeding potential for the measured traits. The highest GCV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$GCV$$\\end{document} observed for root traits revealed that significant genetic change in root development can be achieved through selection. We studied the genotype-by-water availability interaction, but no relevant interaction effects were detected. GP was assessed using leave-one-line-out (LOO) cross-validation. The predictive ability (PA) estimated as the correlation between phenotypes corrected by fixed effects and genomic estimated breeding values ranged from 0.33 to 0.49 for above-ground and 0.15 to 0.27 for root traits, and no substantial variance inflation in predicted genetic effects was observed. Significant differences in PA were observed in favor of M2.ConclusionsThe significant GCV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$GCV$$\\end{document} and the accurate prediction of breeding values for above-ground and root traits revealed that developing genetically superior barley lines with improved root systems is possible. In addition, we found significant spatial variation in the experiment, highlighting the relevance of correctly accounting for spatial effects in statistical models. In this sense, the proposed nearest-neighbor adjustments are flexible approaches in terms of assumptions that can be useful for semi-field or field experiments.

Constrained trait variation by water availability modulates radial growth in evergreen and deciduous Mediterranean oaks

Spatial and temporal variation in functional traits allows trees to adjust to shifting environmental conditions such as water stress. However, the change of traits, both mean and variances, along water availability gradients and across growing seasons, as well as their covariation with tree performance, have been rarely assessed. We examined intraspecific trait variation in coexisting evergreen (Quercus ilex ssp. ilex and Q. ilex ssp. ballota) and deciduous (Quercus faginea and Quercus humilis) Mediterranean oaks along a wide water availability gradient in northeastern Spain during six years. We measured leaf area (LA), shoot twig mass (Sm), leaf mass per area (LMA) and the ratio of shoot twig to leaf biomass (Sm:Lm). We characterized tree performance through basal area increment (BAI) and drought resilience indices. Higher variation was found within individuals than between individuals across populations and years. Within species, we found trait adjustments toward more conservative water-use (low LA and Sm and high LMA) with increasing drier conditions. Intraspecific trait variation was constrained by water availability, particularly on the deciduous species. In Q. ilex, trait variance of LMA positively covaried with annual BAI, whereas variance of LA, Sm and Sm:Lm was positively related to resistance and resilience against the severe 2012 drought in deciduous oaks. Our results support a tradeoff between the ability to tolerate drought and the capacity to cope with unpredictable changes in the environment through increased intraspecific trait variation, which may have implications on tree performance in the face of increased extreme events.

Macroinvertebrate communities reveal regional asymmetries in riverine ecosystems along a gradient of water availability in hydrologically distinct years

Climate change has been increasing the frequency and severity of drought periods. There is a need to understand how this water scarcity affects aquatic communities. The main aim of this work was to compare the macroinvertebrate communities and their bioindicator value (ecological status sensu the Water Framework Directive) in three Regions [northeast (N), central (C) and south (S)] of Portugal along a water availability and climate gradient, in two consecutive years that reflect hydrologically distinct scenarios (2018 versus 2019). The period prior to sampling (summer and autumn 2017) was extremely dry and hot, whereas climatic fluctuations in 2018–2019 (when the sampling occurred) were closer to the norm. A total of 28 sampling sites were surveyed in streams of Regions where water scarcity is differentially constitutive. The results showed a consistent environmental and ecological gradient in both years, despite the background differences. A coinciding mineralization, temperature, riparian cover and water availability gradient clearly separated Regions C (wettest) and S (driest), with Region N occupying an intermediate position. Region C had overall higher ecological quality (prevalence of sensitive organisms) in both years. Despite our a priori expectations that 2018 would represent a much more stressful condition, with larger differences across Regions (because of their constitutive background), the data suggest that macroinvertebrate communities may be resilient or adapted, to some degree, to hydrological fluctuations. Signs of more favourable conditions in 2019 were also observed (general improvement in ecological status). Studies across hydrological and climatic gradients (in time and space) are particularly important where water availability is becoming more challenging.

Drought‐tolerant grassland species are generally more resistant to competition

Abstract Plant populations are limited by resource availability and exhibit physiological trade‐offs in resource acquisition strategies. These trade‐offs may constrain the ability of populations to exhibit fast growth rates under water limitation and high cover of neighbours. However, traits that confer drought tolerance may also confer resistance to competition. It remains unclear how fitness responses to these abiotic conditions and biotic interactions combine to structure grassland communities and how this relationship may change along a gradient of water availability. To address these knowledge gaps, we estimated the low‐density growth rates of populations in drought conditions with low neighbour cover and in ambient conditions with average neighbour cover for 82 species in six grassland communities across the Central Plains and Southwestern United States. We assessed the relationship between population tolerance to drought and resistance to competition and determined if this relationship was consistent across a precipitation gradient. We also tested whether population growth rates could be predicted using plant functional traits. Across six sites, we observed a positive correlation between low‐density population growth rates in drought and in the presence of interspecific neighbours. This positive relationship was particularly strong in the grasslands of the northern Great Plains but weak in the most xeric grasslands. High leaf dry matter content and a low (more negative) leaf turgor loss point were associated with high population growth rates in drought and with neighbours in most grassland communities. Synthesis: A better understanding of how both biotic and abiotic factors impact population fitness provides valuable insights into how grasslands will respond to extreme drought. Our results advance plant strategy theory by suggesting that drought tolerance increases population resistance to interspecific competition in grassland communities. However, this relationship is not evident in the driest grasslands, where above‐ground competition is likely less important. Leaf dry matter content and turgor loss point may help predict which populations will establish and persist based on local water availability and neighbour cover, and these predictions can be used to guide the conservation and restoration of biodiversity in grasslands.

Drought-adapted leaves are produced even when more water is available in dry tropical forest.

Species in dry environments may adjust their anatomical and physiological behaviors by adopting safer or more efficient strategies. Thus, species distributed across a water availability gradient may possess different phenotypes depending on the specific environmental conditions to which they are subjected. Leaf and vascular tissues are plastic and may vary strongly in response to environmental changes affecting an individual's survival and species distribution. To identify whether and how legumes leaves vary across a water availability gradient in a seasonally dry tropical forest, we quantified leaf construction costs and performed an anatomical study on the leaves of seven legume species. We evaluated seven species, which were divided into three categories of rainfall preference: wet species, which are more abundant in wetter areas; indifferent species, which are more abundant and occur indistinctly under both rainfall conditions; and dry species, which are more abundant in dryer areas. We observed two different patterns based on rainfall preference categories. Contrary to our expectations, wet and indifferent species changed traits in the sense of security when occupying lower rainfall areas, whereas dry species changed some traits when more water was available, such as increasing cuticle and spongy parenchyma thickness, or producing smaller and more numerous stomata. Trischidium molle, the most plastic and wet species, exhibited a similar strategy to the dry species. Our results corroborate the risks to vegetation under future climate change scenarios as stressed species and populations may not endure even more severe conditions.

Leaf trait coordination and variation of blue oak across topo-environmental scales.

Trees are arguably the most diverse and complex macro-organisms on Earth. The equally diverse functions of trees directly impact fluxes of carbon, water and energy from the land surface. A number of recent studies have shed light on the substantial within-species variability across plant traits, including aspects of leaf morphology and plant allocation of photosynthates to leaf biomass. Yet, within-tree variability in leaf traits due to microclimatic variations, leaf hydraulic coordination across traits at different physiological scales and variations in leaf traits over a growing season remain poorly studied. This knowledge gap is stymieing the fundamental understanding of what drives trait variation and covariation from tissues to trees to landscapes. Here, we present an extensive dataset measuring within-tree heterogeneity in leaf traits in California's blue oak (Quercus douglasii) across an edaphic gradient and over the course of a growing season at an oak-grass savanna in Southern CA, USA. We found a high level of within-tree crown leaf area:sapwood area variation that was not attributable to sample height or aspect. We also found a higher level of trait integration at the tree level, rather than branch level, suggesting that trees optimize water use at the organismal level. Despite the large variance in traits within a tree crown and across trees, we did not find strong evidence for adaptive plasticity or acclimation in leaf morphological traits (e.g., changes to phenotype which increased fitness) across temporal and spatial water availability gradients. Collectively, our results highlight strong variation in drought-related physiology, but limited evidence for adaptive trait plasticity over shorter time scales.

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.

Through eagle eyes—the potential of satellite-derived LAI time series to estimate masting events and tree-ring width of European beech

Masting is a well-known phenomenon in forest science in which trees recurrently produce exceptional high seed amounts throughout their lifespan. Occurring simultaneously across wide regions, masting may impact carbon sequestration and carbon cycling of forest ecosystems. To identify masting-induced changes in tree canopies, remote sensing methods may offer a suitable tool, though so far only few studies have used RS methods to investigate the relationship between masting and tree-ring width (TRW). Therefore, we employed a regional network of 19 European beech sites along a precipitation gradient in north-eastern Germany, providing time series of TRW (1961–2017) and categorical masting data (1992–2020). Combined with leaf area index (LAI) from the medium-resolution satellite sensors SPOT-VGT/PROBA-V and MODIS, we assessed the relationship between all three parameters by correlation analysis. Furthermore, we identified common climatic drivers and spatial masting patterns along gradients of water availability. We found negative relationships for both TRW and LAI with masting and a positive correlation for TRW with LAI for SPOT-VGT/PROBA-V data. Contrary, MODIS LAI provided no significant relationships. Water availability did not impact masting occurrence or intensity. The climate correlations showed that masting and TRW are commonly driven by summer climate in June and July, while we found no correlations between climate and LAI. We conclude that medium-resolved SPOT-VGT/PROBA-V LAI is suitable to observe masting but not to estimate TRW. Nonetheless, the results should be thoroughly interpreted in the context of uncertainties of RS measurements and complex interactions of different environmental drivers.

Root trait variation along water gradients in the Cape Floristic Region

AbstractQuestionsRoots are responsible for essential plant functions including water uptake. However, the extent to which root traits (size and structure) determine plant presence in water‐limited environments is still poorly understood. Here we investigated how root traits vary across water availability gradients within a dry South African biome.LocationSouth Africa.MethodsWe measured six below‐ground (root) and eight above‐ground (leaf + stem) traits of 124 individuals of nine dominant woody shrub species from wetter and drier sites (600–700 vs 250–300 mm annual precipitation) in the Fynbos biome of the Cape Floristic Region. Within sites, we sampled from recently burnt and unburnt/more mature vegetation and at three locations along topographical gradients.ResultsDrier regions showed greater maximum rooting depth, length, root dry matter content and root to shoot ratio. These trait patterns were consistent at an intraspecific level, along locally drier topographical locations and in post‐fire environments. Roots accounted for significant whole‐plant trait variation. Additionally, in drier conditions, we found increased root allocation deviating from expected global allometric relationships. Our study also demonstrates that the combination of fire and drought in the driest locations results in poor above‐ground vegetation recovery in terms of plant size, cover and individual counts with only resprouters persisting.ConclusionsOur research suggests that root investment in Fynbos shrubs will likely be key for coping with a drier and warmer future and should be a focus of more research for dryland biomes.

The role of chronic anthropogenic disturbances in plant community assembly along a water availability gradient in Brazil’s semiarid Caatinga region

The effects of anthropogenic disturbances on the structure and function of seasonally dry tropical forests (SDTFs) are mediated both by disturbance type and intensity, and the environmental conditions, including climate. In a climate change scenario, the interaction between low water availability and chronic anthropogenic disturbances may potentially have drastic consequences on SDTF dynamics and the ecosystem services it provides. We hypothesized that in these naturally stressed systems, the effects of human disturbance are strongest in the most water-stressed sites, implying a worrisome synergism between harsh abiotic conditions (lower precipitation, higher temperature) and human effects, ultimately resulting in a less developed community structure and reduced diversity. By integrating information for the adult and regenerating vegetation strata in 21 sites distributed across climatic, plant cover and disturbance gradients in the Caatinga (seasonally dry tropical) forest of Brazil, we analyzed the individual and combined effects of chronic anthropogenic disturbances, as measured through the Cattle Pressure Index and the Human Activity Index, along climate gradients, on community structure, diversity, and composition. As expected, the Cattle Pressure Index was negatively related to diameter at ground level, basal area, and diversity in the adult stratum, whereas for the regenerating stratum, litter stock had the strongest effect on vegetation structure. Climatic factors were poor predictors of the variation in the three true diversity measures (0D, 1D, and 2D) of the regenerating community. Our results revealed important cross-site differences in both vegetation strata over a small range of climatic variation. We conclude that the most stressful conditions associated with a very low precipitation regime enhance the effects on chronic disturbances, gradually driving SDTF communities into new alternative states and rendering these environments more prone to desertification.

Clinal variations in seedling traits and responses to water availability correspond to seed-source environmental gradients in a foundational dryland tree species.

In dryland ecosystems, conifer species are threatened by more frequent and severe droughts, which can push species beyond their physiological limits. Adequate seedling establishment will be critical for future resilience to global change. We used a common garden glasshouse experiment to determine how seedling functional trait expression and plasticity varied among seed sources in response to a gradient of water availability, focusing on a foundational dryland tree species of the western USA, Pinus monophylla. We hypothesized that the expression of growth-related seedling traits would show patterns consistent with local adaptation, given clinal variation among seed source environments. We collected P. monophylla seeds from 23 sites distributed across rangewide gradients of aridity and seasonal moisture availability. A total of 3320 seedlings were propagated with four watering treatments representing progressively decreasing water availability. Above- and below-ground growth-related traits of first-year seedlings were measured. Trait values and trait plasticity, here representing the degree of variation among watering treatments, were modelled as a function of watering treatment and environmental conditions at the seed source locations (i.e. water availability, precipitation seasonality). We found that, under all treatments, seedlings from more arid climates had larger above- and below-ground biomass compared to seedlings from sites experiencing lower growing-season water limitation, even after accounting for differences in seed size. Additionally, trait plasticity in response to watering treatments was greatest for seedlings from summer-wet sites that experience periodic monsoonal rain events. Our results show that P. monophylla seedlings respond to drought through plasticity in multiple traits, but variation in trait responses suggests that different populations are likely to respond uniquely to changes in local climate. Such trait diversity will probably influence the potential for future seedling recruitment in woodlands that are projected to experience extensive drought-related tree mortality.

Precipitation is the main axis of tropical plant phylogenetic turnover across space and time.

Early natural historians-Comte de Buffon, von Humboldt, and De Candolle-established environment and geography as two principal axes determining the distribution of groups of organisms, laying the foundations for biogeography over the subsequent 200 years, yet the relative importance of these two axes remains unresolved. Leveraging phylogenomic and global species distribution data for Mimosoid legumes, a pantropical plant clade of c.3500 species, we show that the water availability gradient from deserts to rain forests dictates turnover of lineages within continents across the tropics. We demonstrate that 95% of speciation occurs within a precipitation niche, showing profound phylogenetic niche conservatism, and that lineage turnover boundaries coincide with isohyets of precipitation. We reveal similar patterns on different continents, implying that evolution and dispersal follow universal processes.