For evaluating climate change impacts on biodiversity, extensive experiments are urgently needed to complement popular non-mechanistic models which map future ecosystem properties onto their current climatic niche. Here, we experimentally test the main prediction of these models by means of a novel multi-site approach. We implement rainfall manipulations—irrigation and drought—to dryland plant communities situated along a steep climatic gradient in a global biodiversity hotspot containing many wild progenitors of crops. Despite the large extent of our study, spanning nine plant generations and many species, very few differences between treatments were observed in the vegetation response variables: biomass, species composition, species richness and density. The lack of a clear drought effect challenges studies classifying dryland ecosystems as most vulnerable to global change. We attribute this resistance to the tremendous temporal and spatial heterogeneity under which the plants have evolved, concluding that this should be accounted for when predicting future biodiversity change.

There is growing evidence for rapid adaptive evolution in response to climate change, including phenological transitions such as earlier flowering with climate warming. The consequences of these evolutionary changes for population dynamics and shifts in species ranges remain, however, quite unexplored. Here, we propose that inter-population differences in patterns of flowering across geographic precipitation gradients can be considered a proxy for changes in flowering time due to variation in rainfall resulting from climate change. To this end, we analyze trends of variation in flowering time across rainfall gradients in the eastern Mediterranean region in three main plant life-forms present in the local vegetation: winter annuals, geophytes, and perennial grasses. These life-forms cope with the hot and dry summer via a drought escape strategy. The analysis is based on published and unpublished data from common-garden experiments in which plants from populations sampled along rainfall gradients were grown under similar conditions, thus allowing detection of genetic differences in flowering time along the gradient. The data clearly indicate that decreasing rainfall across a Mediterranean-desert transect is associated with earlier flowering in winter annual species. In contrast, the limited available data shows no consistent trend of change in flowering time with decreasing rainfall in geophytes and perennial grasses. The phenological shift to early flowering in winter annuals coping with terminal drought appears to be a widespread method for adaptation to arid environments by stress avoidance, diminishing the risk of early death before seed production. However, changes in flowering time associated with the reduction in precipitation predicted by climate change models are relatively small, suggesting that additional traits are involved in the adaptation to increasing aridity. The hypothesis that low water availability is an environmental signal inducing earlier flowering of annual plants under drought conditions is not supported by experimental data.

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Aims: Primary biomass production is a fundamental process for ecosystem functioning. Yet, little is known on the mechanisms driving temporal stability of biomass production in annual plant communities, particularly when subjected to highly variable environments and undergoing temporal changes in species composition. We aimed to disentangle the relative importance of biomass production, species diversity, dominance and asynchrony of species fluctuations as drivers of biomass stability in mediterranean and semi-arid annual plant communities.Location: Mediterranean (31°42′ N; 35°03′ E) and semi-arid (31°23′ N; 34°54′ E) sites, Israel.Methods: Above-ground biomass and species abundance were monitored in 15 plots of 250 m 2 per site during eight consecutive years. Relationships between stability drivers and community stability were studied at the regional (between sites) and local (within sites) spatial scales.Results: Community biomass stability (mean biomass/SD) increased from the semiarid to the mediterranean site concomitantly with higher biomass production, richness, and evenness. Differences in stability between sites were due to opposite effects of site conditions on the mean and SD of community biomass, leading to higher stability in the mediterranean site. Within sites, species asynchrony was the key driver of stability at the local spatial scale. Richness and biomass production affected stability indirectly through asynchrony, but in different ways at each site. At the mediterranean site, these factors had indirect negative effects on stability by reducing asynchrony, but did not rescind a positive effect of asynchrony on community stability. At the semi-arid site, biomass production had indirect positive effects on stability through asynchrony, while richness had no effect on asynchrony and stability. Stability was not driven by species evenness in either site. Conclusions:Our study provides new insights into the complex control of biomass stability in the dynamics of mediterranean and semi-arid annual plant communities, with different mechanisms driving stability across the regional vs local spatial scales.

Kigel,J.,Konsens, I.andOfir,M. l99l.Branching,floweringandpod-setpatternsinsnap-bean (Phaseolus vulgaris L.) as affected by temperature. Can. J. Plant Sci. 7l: [1233][1234][1235][1236][1237][1238][1239][1240][1241][1242]. The effect of temperature on branching, flowering and pod-set patterns of determinate snap-bean (Phaseolus vulgaris L.) cultivars has not been well documented. In this work, well-defined rigid gradients of branching and flowering were found in detern-rinate snap-bean. However, in spite of the great developmental stability of these gradients, pod production and distribution along the main axis of the bean plant was highly plastic in its response to environmental conditions. Under controlled conditions, high temperature (32127' C) drastically reduced pod production even though branching and flowering were greatly increased. Under lower temperatures most (>50%) mature pods were produced by branches of the first and second trilbliolate leaves. Low night temperature (12'C) increased branching and relative pod production by more basal nodes, mainly by primary leaves branches. Large shifts in the relative contribution to mature pod production by main inflorescence and branches at different nodal positions along the main shoot were observed also in the field. lt appears that in determinate cultivars of snapbean the rigid gradients in branching and flowering do not limit pod production because of compensatory rapid developmental adaptation to transient environmental stresses. (32127'C) ont durement r6duit la production de gousses, mOme si la ramifrcation et la floraison se trouvaient grandement stimul6es. Par temp6ratures plus fiaiches, la plupart (> 50%) des gousses mtres provenaient des ramifications des premidres et deuxibmes feuilles trifoliol6es. Des temp6ratures noctumes fraiches (12"C) ont accru la ramification et la production relative de sousses ir partir des noeuds inf6rieurs. surtout par Ies ramifications des leuilles primaires. Des tldpla"cements importants de la contribution relative 2r la production des gousses m0res par les inflorescences et les ramifications principales aux diff6rentes positions nodales le long de la tige principale, ont 6t6 observ6s 6galement en culture au champ. Tout indique que chez les vari6t6s d croissance d6terminde, les gradients rigides gouvernant les m6canismes de ramification et de floraison n'entravent pas la production des gousses! apparemment grAce ir une adaptation compensatrice rapide de la plante aux stress ambiantaux passagers.