Germination strategies under climate change scenarios along an aridity gradient

由于年降雨量减少和变异性的增加，地中海东部地区的气候变化将对生态系统功能和植物群落动态产生重大影响。我们旨在了解种子库作为应对气候变化所导致的气候不确定性的潜在缓冲作用。我们研究了沿干旱梯度出现的18种常见物种的萌发策略。数据由干旱、 半干旱、地中海和中等地中海生态系统连续九年内萌发的土壤种子库获得。在半干旱和地中海地区，采用了模拟30%干旱和30%降雨增加的降雨处理方法。在连续三个萌发季的最佳灌溉条件下检测了萌发策略，以确定每种土壤样品的总体种子萌发能力。使用一种新颖的统计方法研究了萌发策略的变化，该方法考虑了可能影响种子发芽性的气候和生物因素。研究结果表明，优势种通过产生具有不同年度发芽率概率的种子来控制其发芽率。可产生种子的降雨量导致了关于可萌发性的两种主要种子类型：高萌发性(可形成短暂种子库的种子)和低萌发性(可形成持久种子库的种子)。我们得出的结论是，两种类型的干湿年之间种子产生的差异沿干旱梯度建立了一个稳定的平衡，使土壤种子库可以充当稳定机制，以防止降雨的不可预测性。此外，我们提出了地中海和干旱生态系统中占主导地位的一年生物种萌发策略的一般模型，该模型加强了土壤种子库可以作为应对该地区气候变化引起的气候不确定性的缓冲剂的概念。     

The metabolism of social subterranean rodents: adaptation to aridity

Summary The social Damara mole-rat Cryptomys damarensis (124 g), has a mean (±SD) resting metabolic rate of 0.57±0.09 cm³ O₂ g^-1 h^-1, within a thermoneutral zone of 27–31° C. This rate of metabolism is 43% lower than that predicted by the curve for rodents, and 29% lower than that predicted by the subterranean rodent curve. These data support the hypothesis that the resting metabolic rates of social and solitary subterranean rodents are lower than those of solitary species inhabiting mesic habitats. These low resting metabolic rates may represent an energy-saving adaptation to aridity. The energetic cost of burrowing, in relation to the dispersion patterns of food in arid habitats, may explain these low metabolic rates.     

Annual plant–shrub interactions along an aridity gradient

Annual plants in semi-arid and arid areas are often closely associated with shrubs. The degree of association largely depends on the balance of negative and positive effects between these contrasting plant life-forms, ranging from interference to facilitation. Since positive interactions are predicted to become less important with increasing rainfall, the interaction balance is expected to shift along aridity gradients. However, this prediction has not been tested on a community level and for different life-history stages across large geographical gradients. Here, we show such changes for annual plant populations and communities in four contrasting sites along a steep climatic gradient, ranging from the arid desert to mesic Mediterranean regions in Israel. Above-ground productivity, richness, seedling density, and seed bank density of the annual plant community, as well as fecundity of annual plant populations, were generally higher under shrubs than in areas between shrubs at the arid end of the gradient, but significantly lower at the humid end. Net effects of shrubs on annuals expressed as relative interaction intensity indicated a steady and consistent shift from net positive or neutral effects in the desert to net negative effects in the mesic part of the gradient. These findings emphasize the usefulness of studies along large-scale gradients encompassing a wide range of environmental conditions for understanding community level interactions among coexisting species.     

Height differences in two eucalypt provenances with contrasting levels of aridity

Huge investments are fed into repairing the world's degraded land, placing unparalleled pressure on delivering large quantities of quality seed. One of the most pressing issues is to identify which region to collect seed from and specifically whether local seed has a home‐site advantage, particularly given the pressures of climate change. Recent theoretical recommendations have supported supplementing local seed with seed transferred in an arid‐to‐mesic direction to improve climate resilience of plantings. We tested this recommendation by establishing a reciprocal transplant trial in June 2010 of two seed provenances with contrasting aridity of Eucalyptus socialis, a tree widely used for restoration in Southern Australia. We recorded survival and height over 5 years. The years 2010 and 2011 were particularly wet years at both sites (>1.8 times historical rainfall), but the years 2012–2015 were consistent with long‐term rainfall trends, with the arid site receiving 12–48% less annual rainfall than the mesic site. Only the arid provenance showed a home‐site advantage, and only for height after the two wet years followed by the three drier years. Provenances had similar levels of survival at both sites and did equally well at the mesic site. These results only provide initial evidence to support the recommendation that restoration plantings aiming to incorporate climate resilience should include arid‐to‐mesic transferred seed. Further work is needed to fully explore potential confounding site‐specific effects. Supplementing locally collected seeds with arid‐to‐mesic transferred seed could be important to increase climate resilience of plantings and demands further studies to explore its costs versus benefits.     

Increasing aridity causes larger and more severe forest fires across Europe

Area burned has decreased across Europe in recent decades. This trend may, however, reverse under ongoing climate change, particularly in areas not limited by fuel availability (i.e. temperate and boreal forests). Investigating a novel remote sensing dataset of 64,448 fire events that occurred across Europe between 1986 and 2020, we find a power-law relationship between maximum fire size and area burned, indicating that large fires contribute disproportionally to fire activity in Europe. We further show a robust positive correlation between summer vapor pressure deficit and both maximum fire size (R² = .19) and maximum burn severity (R² = .12). Europe's fire regimes are thus highly sensitive to changes in future climate, with the probability for extreme fires more than doubling by the end of the century. Our results suggest that climate change will challenge current fire management approaches and could undermine the ability of Europe's forests to provide ecosystem services to society.     

Reproductive patterns in annual legume species on an aridity gradient

Reproductive patterns are analysed in annual legumes of west Asia, and their relationships to increasing aridity determined by multivariate analysis. Dormancy, seed size, dispersal and fecundity are shown to be partially substitutable in terms of their effect on survival and population growth.The range of patterns show greatest diversity under mesic conditions in coastal, mediterranean areas with high winter rainfall, low incidence of frost and long growing season. Increasing aridity leads to increasing reproductive homogeneity, in which a subset of patterns-those composed of high levels of seed dormancy, high seed to pod ratios, restricted dispersal capabilities and early flowering-become predominant. These findings corroborate earlier theoretical and empirical evidence concerning desert annuals.The majority of widespread species are shown to possess arid-type patterns. This exerts considerable influence on both the type and degree of ecotypic differentiation within species. The only feature that responds consistently to climatic change is flowering time, particularly among species characterized by high seed dormancy. In the few widespread species that do not display arid-type characteristics, ecotypic differentiation appears more frequently in a larger suite of traits.The unequal environmental demands made on species showing different levels of dormancy (e.g. variation in length of growing season from year to year and place to place) leads to strong asymmetries governing the relationships between reproduction and ecological amplitude. In this respect arid-type strategists with high seed dormancy appear to have greater chances of expansion than others. This has implications when choosing pasture legumes to improve mediterranean grasslands.     

Biomass-density relationship varies with water use efficiency across an aridity gradient

Accumulating evidence has shown that the scaling exponent (α) of the aboveground biomass-density relationship is not a constant value. Debate continues over what determines the variation in α. By measuring foliar stable carbon isotope, plant morphological traits and α along an aridity gradient from eastern to western China, we confirmed that the variation in α was accounted for by changes in plant morphological traits which are adaptive strategies for enhancing water use efficiency during drought stress. This information can be crucial for understanding and predicting community and ecosystem processes.     

Threshold responses of soil gross nitrogen transformation rates to aridity gradient

The responses of soil nitrogen (N) transformations to climate change are crucial for biome productivity prediction under global change. However, little is known about the responses of soil gross N transformation rates to drought gradient. Along an aridity gradient across the 2700 km transect of drylands on the Qinghai-Tibetan Plateau, this study measured three main soil gross N transformation rates in both topsoil (0–10 cm) and subsoil (20–30 cm) using the laboratorial ¹⁵N labeling. The relevant soil abiotic and biotic variables were also determined. The results showed that gross N mineralization and nitrification rates steeply decreased with increasing aridity when aridity was less than 0.5 but just slightly decreased with increasing aridity when aridity was larger than 0.5 at both soil layers. In topsoil, the decreases of the two gross rates were accompanied by the similar decreased patterns of soil total N content and microbial biomass carbon with increasing aridity (p < .05). In subsoil, although the decreased pattern of soil total N with increasing aridity was still similar to the decreases of the two gross rates (p < .05), microbial biomass carbon did not change (p > .05). Instead, bacteria and ammonia oxidizing archaea abundances decreased with increasing aridity when aridity was larger than 0.5 (p < .05). With an aridity threshold of 0.6, gross N immobilization rate increased with increasing aridity in wetter region (aridity < 0.6) accompanied with an increased bacteria/fungi ratio, but decreased with increasing aridity in drier region (aridity > 0.6) where mineral N and microbial biomass N also decreased at both soil layers (p < .05). This study provided new insight to understand the differential responses of soil N transformation to drought gradient. The threshold responses of the gross N transformation rates to aridity gradient should be noted in biogeochemical models to better predict N cycling and manage land in the context of global change.     

Habitat aridity and urine concentrating ability of nearctic, insectivorous bats

Urine concentrating ability of nearctic, insectivorous bats determined from renal anatomy was significantly correlated to total annual precipitation and potential evapotranspiration to precipitation ratio of the habitat where the animal was collected. Habitat aridity measured by water input accounted for as much variation in urine concentrating ability as habitat aridity measured by both water and energy input. Habitat aridity, regardless of how it was measured, explained only 25% of the among-species variation in urine concentrating ability. Factors other than habitat aridity significantly affect the urine concentrating ability of insectivorous bats.     

Physiological adaptation to aridity in the bushveld gerbil, Tatera leucogaster

Oxygen consumption, rectal temperature, thermal conductance, and evaporative water loss (EWL) were determined in resting captive Tatera leucogaster at ambient temperatures of between 14 and 38 °C. Basal metabolic rate (BMR) was 0.86 ml O₂. min⁻¹ (S.D.=0.15, n = 6), 45% of that expected for a rodent of the same body mass (106.2 g). Minimum wet thermal conductance was 0.21 ml O₂. min⁻¹, °C⁻¹ (S.D. = 0.01, n = 6), 113% of that expected for a mammal of the same body mass. Wet thermal conductance increased exponentially at temperatures greater than 32 °C. Mean rectal temperature was 35.3 °C below 35 °C (S.D. = 0.5, n = 6) and 39.3 (S.D. = 0.6, n = 5) at 38 °C. Mean resting EWL was 1.43 mg. min⁻¹ (S.D. = 0.14, n = 6) between 15 and 32 °C and increased dramatically at temperatures above 32 °C. Combining our data with data from the literature suggests that gerbils (Family Muridae; subfamily Gerbillinae) have, on average, low BMR and average minimum wet thermal conductance when compared to other rodents and other mammals, respectively, of the same body mass. Similarly, rodents (including gerbils) from arid habitats have, on average, lower rates of EWL when at rest below thermoneutrality than do other rodents of the same body mass from mesic habitats.     

Adaptation of metabolism and evaporative water loss along an aridity gradient

Broad-scale comparisons of birds indicate the possibility of adaptive modification of basal metabolic rate (BMR) and total evaporative water loss (TEWL) in species from desert environments, but these might be confounded by phylogeny or phenotypic plasticity. This study relates variation in avian BMR and TEWL to a continuously varying measure of environment, aridity. We test the hypotheses that BMR and TEWL are reduced along an aridity gradient within the lark family (Alaudidae), and investigate the role of phylogenetic inertia. For 12 species of lark, BMR and TEWL decreased along a gradient of increasing aridity, a finding consistent with our proposals. We constructed a phylogeny for 22 species of lark based on sequences of two mitochondrial genes, and investigated whether phylogenetic affinity played a part in the correlation of phenotype and environment. A test for serial independence of the data for mass-corrected TEWL and aridity showed no influence of phylogeny on our findings. However, we did discover a significant phylogenetic effect in mass-corrected data for BMR, a result attributable to common phylogenetic history or to common ecological factors. A test of the relationship between BMR and aridity using phylogenetic independent constrasts was consistent with our previous analysis: BMR decreased with increasing aridity.     

Allelic diversity associated with aridity gradient in wild emmer wheat populations

The association between allelic diversity and ecogeographical variables was studied in natural populations of wild emmer wheat [ Triticum turgidum ssp. dicoccoides (Körn.) Thell.], the tetraploid progenitor of cultivated wheat. Patterns of allelic diversity in 54 microsatellite loci were analyzed in a collection of 145 wild emmer wheat accessions representing 25 populations that were sampled across naturally occurring aridity gradient in Israel and surrounding regions. The obtained results revealed that 56% of the genetic variation resided among accessions within populations, while only 44% of the variation resided between populations. An unweighted pair-group method analysis (UPGMA) tree constructed based on the microsatellite allelic diversity divided the 25 populations into six major groups. Several groups were comprised of populations that were collected in ecologically similar but geographically remote habitats. Furthermore, genetic differentiation between populations was independent of the geographical distances. An interesting evolutionary phenomenon is highlighted by the unimodal relationship between allelic diversity and annual rainfall ( r  = 0.74, P  < 0.0002), indicating higher allelic diversity in populations originated from habitats with intermediate environmental stress (i.e. rainfall 350–550 mm year⁻¹). These results show for the first time that the 'intermediate-disturbance hypothesis', explaining biological diversity at the ecosystem level, also dominates the genetic diversity within a single species, the lowest hierarchical element of the biological diversity.     

Evolutionary shifts in habitat aridity predict evaporative water loss across squamate reptiles

Aridity is an important determinant of species distributions, shaping both ecological and evolutionary diversity. Lizards and snakes are often abundant in deserts, suggesting a high potential for adaptation or acclimation to arid habitats. However, phylogenetic evidence indicates that squamate diversity in deserts may be more strongly tied to speciation within arid habitats than to convergent evolution following repeated colonization from mesic habitats. To assess the frequency of evolutionary transitions in habitat aridity while simultaneously testing for associated changes in water‐balance physiology, we analyzed estimates of total evaporative water loss (EWL) for 120 squamate species inhabiting arid, semiarid, or mesic habitats. Phylogenetic reconstructions revealed that evolutionary transitions to and from semiarid habitats were much more common than those between arid and mesic extremes. Species from mesic habitats exhibited significantly higher EWL than those from arid habitats, while species from semiarid habitats had intermediate EWL. Phylogenetic comparative methods confirmed this association between habitat aridity and EWL despite phylogenetic signal in each. Thus, the historical colonization of arid habitats by squamates is repeatedly associated with adaptive changes in EWL. This physiological convergence, which may reflect both phenotypic plasticity and genetic adaptation, has likely contributed to the success of squamates in arid environments.     

Trends in savanna structure and composition along an aridity gradient in the Kalahari

Abstract. The Kalahari sand sheet occupies 2.5 million ha in southern Africa. It is an area with relatively similar deep aeolian soils, and a strong south to north gradient in rainfall, from 200 to 1000 mm mean annual precipitation (MAP) in the region studied. This provides an excellent basis for gradient studies at the subcontinental scale. This paper briefly reviews the literature on the vegetation of the Kalahari and describes the vegetation structure and composition at 11 new sites. There is a clear gradient in woody plant biomass (as indexed by basal area) from south to north. Above the minimum level of 200 mm MAP, the woody basal area increases at a rate of ca. 2.5 m².ha^‐1 per 100 mm MAP. Mean maximum tree height also increases along the gradient, reaching 20 m at ca. 800 mm MAP. The number of species to contribute > 95% of the woody basal area increases from one at 200 mm to 16 at 1000 mm MAP. Members of the Mimosaceae (mainly Acacia) dominate the tree layer up to 400 mm MAP. They are replaced by either the Combretaceae (Combretum or Terminalia) or Colophospermum mopane of the Caesalpinaceae between 400 and 600 mm MAP, and by other representatives of the Caesalpinaceae above 600 mm MAP. The vegetation is largely deciduous up to 1000 mm MAP, except for species that apparently have access to groundwater, which may be locally dominant above about 600 mm MAP.     

Plant neighbor effects mediated by rhizosphere factors along a simulated aridity gradient

Aims

We investigated how rhizosphere factors (total rhizosphere, roots, arbuscular mycorrhizal fungal hyphae [AMF], and soil solution) and water availability affect interactions between neighboring Medicago sativa plants.

Methods

A three-compartment mesocosm was used to test the effects of rhizosphere factors on plant–plant interactions. A relative interaction index (RII) was calculated to indicate whether effects of neighbor plant on target plant were positive or negative (facilitative or competitive). Isotope tracers were used to test whether AMF hyphae mediated competition for nitrogen (N) between target and neighbor plants.

Results

The effects of rhizosphere factors on the interactions between neighboring M. sativa plants depended on water availability. The effects of total rhizosphere shifted RII from negative to positive as water availability increased. Interaction with the roots and rhizosphere soil solution of neighbor plants shifted RII from negative to positive as water availability increased but the opposite was true for AMF hyphae. AMF hyphae helped neighbor plants compete for ¹⁵N when water was available but not when water was limiting.

Conclusions

The effect of total rhizosphere on plant–plant interaction of M. sativa shifted from competitive to facilitative as water availability increased. Competition was reduced by neighboring soil solution and roots but was increased by AMF hyphae.     

Estimates of nitrogen fixation by trees on an aridity gradient in Namibia

Summary Nitrogen (N₂) fixation was estimated along an aridity gradient in Namibia from the natural abundance of ¹⁵N (¹⁵N value) in 11 woody species of the Mimosacease which were compared with the ¹⁵N values in 11 woody non-Mimosaceae. Averaging all species and habitats the calculated contribution of N₂ fixation (N _f ) to leaf nitrogen (N) concentration of Mimosaceae averaged about 30%, with large variation between and within species. While in Acacia albida N _f was only 2%, it was 49% in Acacia hereroensis and Dichrostachys cinerea, and reached 71% in Acacia melifera. In the majority of species N _f was 10–30%. There was a marked variation in background ¹⁵N values along the aridity gradient, with the highest ¹⁵N values in the lowland savanna. The difference between ¹⁵N values of Mimosaceae and non-Mimosaceae, which is assumed to result mainly from N₂ fixation, was also largest in the lowland savanna. Variations in ¹⁵N of Mimosaceae did not affect N concentrations, but higher ¹⁵N-values of Mimosaeae are associated with lower carbon isotope ratios (¹³C value). N₂ fixation was associated with reduced intrinsic water use efficiency. The opposite trends were found in non-Mimosaceae, in which N-concentration increased with ¹⁵N, but ¹³C was unaffected. The large variation among species and sites is discussed.This paper is prepared in memory of J. Visser, who took part in the collection of species, but died in 1990     

中值温度和干燥度指数计算及其应用的简便方法

利用合肥2003年逐日温度资料进行相关分析和回归分析,发现中值温度与最低、最高及平均温度之间存在极显著的相关性,回归模型的拟合优度均在96．8％以上。依据合肥多年逐日中值温度和平均温度的回归分析,建立了一元回归方程Y=1．0026x-0．4683,并通过了α=0.001水平检验;对其他地区资料进行分析也发现有同样规律,这充分表明了日中值温度与日平均温度之间存在极显著回归关系。另外,根据合肥多年气象资料,对“蒸降比”与积温干燥度指数进行相关分析和回归分析,建立了干燥度指数计算的新公式E=0．282054＋0．552118E／r。方法可直接利用蒸发量和降水量常规气象数据,简便地计算出各年份干燥度指数;也可用在低于10℃期间,以便计算年内各季节或月份干燥度指数;经分析验证,效果良好。