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1. Climate warming alters photosynthetic responses to elevated CO 2 in prairie plants

   https://doi.org/10.1002/ajb2.1532  

   Sage, Emma ; Heisler‐White, Jana ; Morgan, Jack ; Pendall, Elise ; Williams, David G. ( September 2020 , American Journal of Botany)

   The impact of elevated CO₂concentration ([CO₂]) and climate warming on plant productivity in dryland ecosystems is influenced strongly by soil moisture availability. We predicted that the influence of warming on the stimulation of photosynthesis by elevated [CO₂] in prairie plants would operate primarily through direct and indirect effects on soil water.

   We measured light‐saturated photosynthesis (A_net), stomatal conductance (g_s), maximum Rubisco carboxylation rate (V_cmax), maximum electron transport capacity (J_max) and related variables in four C₃plant species in the Prairie Heating and CO₂Enrichment (PHACE) experiment in southeastern Wyoming. Measurements were conducted over two growing seasons that differed in the amount of precipitation and soil moisture content.

   A_netin the C₃subshrubArtemisia frigidaand the C₃forbSphaeralcea coccineawas stimulated by elevated [CO₂] under ambient and warmed temperature treatments. Warming by itself reducedA_netin all species during the dry year, but stimulated photosynthesis inS. coccineain the wet year. In contrast,A_netin the C₃grassPascopyrum smithiiwas not stimulated by elevated [CO₂] or warming under wet or dry conditions. Photosynthetic downregulation under elevated [CO₂] in this species countered the potential stimulatory effect under improved water relations. Warming also reduced the magnitude of CO₂‐induced down‐regulation in this grass, possibly by sustaining high levels of carbon utilization.

   Direct and indirect effects of elevated [CO₂] and warming on soil water was an overriding factor influencing patterns ofA_netin this semi‐arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change.

    

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2. Long‐term elevated precipitation induces grassland soil carbon loss via microbe‐plant–soil interplay

   https://doi.org/10.1111/gcb.16811  

   Wang, Mengmeng ; Sun, Xin ; Cao, Baichuan ; Chiariello, Nona R. ; Docherty, Kathryn M. ; Field, Christopher B. ; Gao, Qun ; Gutknecht, Jessica L. M. ; Guo, Xue ; He, Genhe ; et al ( June 2023 , Global Change Biology)

   Global climate models predict that the frequency and intensity of precipitation events will increase in many regions across the world. However, the biosphere‐climate feedback to elevated precipitation (eP) remains elusive. Here, we report a study on one of the longest field experiments assessing the effects of eP, alone or in combination with other climate change drivers such as elevated CO₂(eCO₂), warming and nitrogen deposition. Soil total carbon (C) decreased after a decade of eP treatment, while plant root production decreased after 2 years. To explain this asynchrony, we found that the relative abundances of fungal genes associated with chitin and protein degradation increased and were positively correlated with bacteriophage genes, suggesting a potential viral shunt in C degradation. In addition, eP increased the relative abundances of microbial stress tolerance genes, which are essential for coping with environmental stressors. Microbial responses to eP were phylogenetically conserved. The effects of eP on soil total C, root production, and microbes were interactively affected by eCO₂. Collectively, we demonstrate that long‐term eP induces soil C loss, owing to changes in microbial community composition, functional traits, root production, and soil moisture. Our study unveils an important, previously unknown biosphere‐climate feedback in Mediterranean‐type water‐limited ecosystems, namely how eP induces soil C loss via microbe‐plant–soil interplay.

    

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3. Compensatory dynamics drive grassland recovery from drought

   https://doi.org/10.1111/1365-2745.14096  

   Luo, Wentao ; Ma, Wang ; Song, Lin ; Te, Niwu ; Chen, Jiaqi ; Muraina, Taofeek O. ; Wilkins, Kate ; Griffin‐Nolan, Robert J. ; Ma, Tianxiao ; Qian, Jianqiang ; et al ( March 2023 , Journal of Ecology)

   Grasslands are expected to experience droughts of unprecedented frequency and magnitude in the future. Characterizing grassland responses and recovery from drought is therefore critical to predict the vulnerability of grassland ecosystems to climate change. Most previous studies have focused on ecosystem responses during drought while investigations of post‐drought recovery are rare. Few studies have used functional traits, and in particular bud or clonal traits, to explore the mechanisms underlying grassland responses to and recovery from drought.

   To address this issue, we experimentally imposed a four‐year drought in a C₃‐dominated grassland in northeastern China and monitored recovery for 3 years post‐drought. We investigated the immediate and legacy effects of drought on total above‐ground net primary productivity (ANPP), ANPP of functional groups (rhizomatous grasses, bunch grasses and forbs), and how the legacy effects were driven by plant species diversity, clonal traits and vegetative traits.

   We found that drought progressively reduced total ANPP over the 4‐year period. The reductions in total ANPP in the first and third drought years were caused by the decrease in ANPP of bunch grasses only, while that of the second year was caused by declines in ANPP of bunch grasses and forbs, and the fourth year decline was linked to all three functional groups. The post‐drought recovery of ANPP, which occurred despite the continued loss of plant species diversity, was mainly driven by rapid recovery of rhizomatous and bunch grasses, which compensated for the slow response by forbs. The rapid post‐drought recovery of these grasses can be attributed to their relatively large, intact bud and shoot densities post‐drought, as well as the recovery of plant height and specific leaf area. The rapid recovery of grasses possibly restricted the growth and distribution of forbs, resulting in reduced forb ANPP and, consequently, lower species diversity during the recovery period.

   Synthesis. These results highlight the potential for positive legacy effects of drought on ANPP as well as the important and complementary roles of plant reproductive and vegetative traits in mediating ecosystem recovery from drought in a C₃‐dominated grassland.

    

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4. The ecosystem wilting point defines drought response and recovery of a Quercus‐Carya forest

   https://doi.org/10.1111/gcb.16582  

   Wood, Jeffrey D. ; Gu, Lianhong ; Hanson, Paul J. ; Frankenberg, Christian ; Sack, Lawren ( April 2023 , Global Change Biology)

   Abstract Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem‐scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (Ψ EWP ), a property that integrates the drought response of an ecosystem's plant community across the soil–plant–atmosphere continuum. Specifically, Ψ EWP defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the Ψ EWP of a Quercus‐Carya forest from an “ecosystem pressure–volume (PV) curve,” which is analogous to the tissue‐level technique. When community predawn leaf water potential (Ψ pd ) was above Ψ EWP (=−2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψ pd fell below Ψ EWP , the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, Ψ EWP is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall‐induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψ pd observations fell below Ψ EWP , the forest is commonly only 2–4 weeks of intense drought away from reaching Ψ EWP , and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom‐up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of Ψ EWP , and species in an ecosystem require compatible leaf‐level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil. 

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5. Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO 2 enrichment, independent of nitrogen supply

   https://doi.org/10.1111/gcb.14714  

   Pastore, Melissa A. ; Lee, Tali D. ; Hobbie, Sarah E. ; Reich, Peter B. ( June 2019 , Global Change Biology)

   Uncertainty about long‐term leaf‐level responses to atmospheric CO₂rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO₂(eCO₂) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long‐lived perennials. Here, we report the effects of eCO₂on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free‐Air CO₂Enrichment experiment, BioCON. Monocultures of species belonging to C₃grasses, C₄grasses, forbs, and legumes were exposed to two levels of CO₂and nitrogen supply in factorial combinations over 21 years. eCO₂increased photosynthesis by 12.9% on average in C₃species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO₂was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO₂was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C₄grasses the least as expected, but did not further diverge over time. Leaf‐level water‐use efficiency increased by 50% under eCO₂primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO₂in nitrogen‐limited systems, and that significant and consistent declines in stomatal conductance and increases in water‐use efficiency under eCO₂may allow plants to better withstand drought.

    

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