Multiple strategies for drought survival among woody plant species

Drought-induced mortality and regional dieback of woody vegetation are reported from numerous locations around the world. Yet within any one site, predicting which species are most likely to survive global change-type drought is a challenge. We studied the diversity of drought survival traits of a community of 15 woody plant species in a desert-chaparral ecotone. The vegetation was a mix of chaparral and desert shrubs, as well as endemic species that only occur along this margin. This vegetation boundary has large potential for drought-induced mortality because nearly all species are at the edge of their range. Drought survival traits studied were vulnerability to drought-induced xylem cavitation, sapwood capacitance, deciduousness, photosynthetic stems, deep roots, photosynthetic responses to leaf water potential and hydraulic architecture. Drought survival strategies were evaluated as combinations of traits that could be effective in dealing with drought. The large variation in seasonal predawn water potential of leaves and stem xylem ranged from -6·82 to -0·29 MPa and -6·92 to -0·27 MPa, respectively. The water potential at which photosynthesis ceases ranged from -9·42 to -3·44 MPa. Architecture was a determinant of hydraulic traits, with species supporting large leaf area per sapwood area exhibiting high rates of water transport, but also xylem that is vulnerable to drought-induced cavitation. Species with more negative midday leaf water potential during the growing season also showed access to deeper water sources based on hydrogen isotope analysis. Drought survival mechanisms comprised of drought deciduousness, photosynthetic stems, tolerance of low minimum seasonal tissue water potential and vulnerability to drought-induced xylem cavitation thus varied orthogonally among species, and promote a diverse array of drought survival strategies in an arid ecosystem of considerable floristic complexity

Multiple strategies for drought survival among woody plant species

Drought-induced mortality and regional dieback of woody vegetation are reported from numerous locations around the world. Yet within any one site, predicting which species are most likely to survive global change-type drought is a challenge. We studied the diversity of drought survival traits of a community of 15 woody plant species in a desert-chaparral ecotone. The vegetation was a mix of chaparral and desert shrubs, as well as endemic species that only occur along this margin. This vegetation boundary has large potential for drought-induced mortality because nearly all species are at the edge of their range. Drought survival traits studied were vulnerability to drought-induced xylem cavitation, sapwood capacitance, deciduousness, photosynthetic stems, deep roots, photosynthetic responses to leaf water potential and hydraulic architecture. Drought survival strategies were evaluated as combinations of traits that could be effective in dealing with drought. The large variation in seasonal predawn water potential of leaves and stem xylem ranged from -6·82 to -0·29 MPa and -6·92 to -0·27 MPa, respectively. The water potential at which photosynthesis ceases ranged from -9·42 to -3·44 MPa. Architecture was a determinant of hydraulic traits, with species supporting large leaf area per sapwood area exhibiting high rates of water transport, but also xylem that is vulnerable to drought-induced cavitation. Species with more negative midday leaf water potential during the growing season also showed access to deeper water sources based on hydrogen isotope analysis. Drought survival mechanisms comprised of drought deciduousness, photosynthetic stems, tolerance of low minimum seasonal tissue water potential and vulnerability to drought-induced xylem cavitation thus varied orthogonally among species, and promote a diverse array of drought survival strategies in an arid ecosystem of considerable floristic complexity

Carbon isotopes in terrestrial ecosystem pools and CO 2

Stable carbon isotopes are used extensively to examine physiological, ecological, and biogeochemical processes related to ecosystem, regional, and global carbon cycles and provide information at a variety of temporal and spatial scales. Much is known about the processes that regulate the carbon isotopic composition (delta(13)C) of leaf, plant, and ecosystem carbon pools and of photosynthetic and respiratory carbon dioxide (CO(2)) fluxes. In this review, systematic patterns and mechanisms underlying variation in delta(13)C of plant and ecosystem carbon pools and fluxes are described. We examine the hypothesis that the delta(13)C of leaf biomass can be used as a reference point for other carbon pools and fluxes, which differ from the leaf in delta(13)C in a systematic fashion. Plant organs are typically enriched in (13)C relative to leaves, and most ecosystem pools and respiratory fluxes are enriched relative to sun leaves of dominant plants, with the notable exception of root respiration. Analysis of the chemical and isotopic composition of leaves and leaf respiration suggests that growth respiration has the potential to contribute substantially to the observed offset between the delta(13)C values of ecosystem respiration and the bulk leaf. We discuss the implications of systematic variations in delta(13)C of ecosystem pools and CO(2) fluxes for studies of carbon cycling within ecosystems, as well as for studies that use the delta(13)C of atmospheric CO(2) to diagnose changes in the terrestrial biosphere over annual to millennial time scales

Outbreeding depression in the common frog, Rana temporaria

Theory suggests that parental relatedness is a continuous variable with a fitness optimum that we heretoforth will refer to as 'optimal outbreeding'. In the present paper, we test this proposition from a conservation (translocation) perspective. Amphibians are facing a global decline and many amphibian populations are today small and threatened by extinction. Because genetic differentiation is often high between amphibian populations, they could be particularly sensitive to outbreeding depression, e.g. due to breakdown of locally adapted gene complexes. We tested if outbreeding would reduce fitness in common frogs, Rana temporaria, crossed from a large and an isolated, small population, separated by 130 km, using artificial fertilization. For females from the large population, tadpoles were significantly smaller and more malformed in crosses with males from the small population, than with males from the large population. For the small population, however, no significant paternal genetic effects could be found. The difference in response to outbreeding between populations was accompanied with significant differences in the importance of maternal effects. We conclude that care should be taken when translocating frogs between distantly related populations to avoid outbreeding depression. © Springer 2005

Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes

Unger S, Maguas C, Pereira JS, Aires LM, David TS, Werner C. Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes. Oecologia. 2010;163(4):1043-1057.Combining C flux measurements with information on their isotopic composition can yield a process-based understanding of ecosystem C dynamics. We studied the variations in both respiratory fluxes and their stable C isotopic compositions (delta C-13) for all major components (trees, understory, roots and soil microorganisms) in a Mediterranean oak savannah during a period with increasing drought. We found large drought-induced and diurnal dynamics in isotopic compositions of soil, root and foliage respiration (delta C-13(res)). Soil respiration was the largest contributor to ecosystem respiration (R (eco)), exhibiting a depleted isotopic signature and no marked variations with increasing drought, similar to ecosystem respired delta(CO2)-C-13, providing evidence for a stable C-source and minor influence of recent photosynthate from plants. Short-term and diurnal variations in delta C-13(res) of foliage and roots (up to 8 and 4aEuro degrees, respectively) were in agreement with: (1) recent hypotheses on post-photosynthetic fractionation processes, (2) substrate changes with decreasing assimilation rates in combination with increased respiratory demand, and (3) decreased phosphoenolpyruvate carboxylase activity in drying roots, while altered photosynthetic discrimination was not responsible for the observed changes in delta C-13(res). We applied a flux-based and an isotopic flux-based mass balance, yielding good agreement at the soil scale, while the isotopic mass balance at the ecosystem scale was not conserved. This was mainly caused by uncertainties in Keeling plot intercepts at the ecosystem scale due to small CO2 gradients and large differences in delta C-13(res) of the different component fluxes. Overall, stable isotopes provided valuable new insights into the drought-related variations of ecosystem C dynamics, encouraging future studies but also highlighting the need of improved methodology to disentangle short-term dynamics of isotopic composition of R (eco)