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1. Rising CO ₂ and warming reduce global canopy demand for nitrogen

   https://doi.org/10.1111/nph.18076  

   Dong, Ning; Wright, Ian J.; Chen, Jing M.; Luo, Xiangzhong; Wang, Han; Keenan, Trevor F.; Smith, Nicholas G.; Prentice, Iain Colin (April 2022, New Phytologist)

   Summary Nitrogen (N) limitation has been considered as a constraint on terrestrial carbon uptake in response to rising CO₂and climate change. By extension, it has been suggested that declining carboxylation capacity (V_cmax) and leaf N content in enhanced‐CO₂experiments and satellite records signify increasing N limitation of primary production. We predictedV_cmaxusing the coordination hypothesis and estimated changes in leaf‐level photosynthetic N for 1982–2016 assuming proportionality with leaf‐levelV_cmaxat 25°C. The whole‐canopy photosynthetic N was derived using satellite‐based leaf area index (LAI) data and an empirical extinction coefficient forV_cmax, and converted to annual N demand using estimated leaf turnover times. The predicted spatial pattern ofV_cmaxshares key features with an independent reconstruction from remotely sensed leaf chlorophyll content. Predicted leaf photosynthetic N declined by 0.27% yr⁻¹, while observed leaf (total) N declined by 0.2–0.25% yr⁻¹. Predicted global canopy N (and N demand) declined from 1996 onwards, despite increasing LAI. Leaf‐level responses to rising CO₂, and to a lesser extent temperature, may have reduced the canopy requirement for N by more than rising LAI has increased it. This finding provides an alternative explanation for declining leaf N that does not depend on increasing N limitation. 

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2. Empirical evidence and theoretical understanding of ecosystem carbon and nitrogen cycle interactions

   https://doi.org/10.1111/nph.20178  

   Stocker, Benjamin_D; Dong, Ning; Perkowski, Evan_A; Schneider, Pascal_D; Xu, Huiying; de_Boer, Hugo_J; Rebel, Karin_T; Smith, Nicholas_G; Van_Sundert, Kevin; Wang, Han; et al (October 2024, New Phytologist)

   Summary Interactions between carbon (C) and nitrogen (N) cycles in terrestrial ecosystems are simulated in advanced vegetation models, yet methodologies vary widely, leading to divergent simulations of past land C balance trends. This underscores the need to reassess our understanding of ecosystem processes, given recent theoretical advancements and empirical data. We review current knowledge, emphasising evidence from experiments and trait data compilations for vegetation responses to CO₂and N input, alongside theoretical and ecological principles for modelling. N fertilisation increases leaf N content but inconsistently enhances leaf‐level photosynthetic capacity. Whole‐plant responses include increased leaf area and biomass, with reduced root allocation and increased aboveground biomass. Elevated atmospheric CO₂also boosts leaf area and biomass but intensifies belowground allocation, depleting soil N and likely reducing N losses. Global leaf traits data confirm these findings, indicating that soil N availability influences leaf N content more than photosynthetic capacity. A demonstration model based on the functional balance hypothesis accurately predicts responses to N and CO₂fertilisation on tissue allocation, growth and biomass, offering a path to reduce uncertainty in global C cycle projections. 

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3. Intra-canopy leaf trait variation facilitates high leaf area index and compensatory growth in a clonal woody encroaching shrub

   https://doi.org/10.1093/treephys/tpac078  

   Tooley, E Greg; Nippert, Jesse B; Bachle, Seton; Keen, Rachel M (July 2022, Tree Physiology)

   Cavaleri, Molly (Ed.)

   Abstract Leaf trait variation enables plants to utilize large gradients of light availability that exist across canopies of high leaf area index (LAI), allowing for greater net carbon gain while reducing light availability for understory competitors. While these canopy dynamics are well understood in forest ecosystems, studies of canopy structure of woody shrubs in grasslands are lacking. To evaluate the investment strategy used by these shrubs, we investigated the vertical distribution of leaf traits and physiology across canopies of Cornus drummondii, the predominant woody encroaching shrub in the Kansas tallgrass prairie. We also examined the impact of disturbance by browsing and grazing on these factors. Our results reveal that leaf mass per area (LMA) and leaf nitrogen per area (Na) varied approximately threefold across canopies of C. drummondii, resulting in major differences in the physiological functioning of leaves. High LMA leaves had high photosynthetic capacity, while low LMA leaves had a novel strategy for maintaining light compensation points below ambient light levels. The vertical allocation of leaf traits in C. drummondii canopies was also modified in response to browsing, which increased light availability at deeper canopy depths. As a result, LMA and Na increased at lower canopy depths, leading to a greater photosynthetic capacity deeper in browsed canopies compared to control canopies. This response, along with increased light availability, facilitated greater photosynthesis and resource-use efficiency deeper in browsed canopies compared to control canopies. Our results illustrate how C. drummondii facilitates high LAI canopies and a compensatory growth response to browsing—both of which are key factors contributing to the success of C. drummondii and other species responsible for grassland woody encroachment. 

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4. Save or spend? Diverging water‐use strategies of grasses and encroaching clonal shrubs

   https://doi.org/10.1111/1365-2745.14276  

   Keen, R_M; Helliker, B_R; McCulloh, K_A; Nippert, J_B (February 2024, Journal of Ecology)

   Abstract Shrub encroachment is one of the primary threats to mesic grasslands around the world. This dramatic shift in plant cover has the potential to alter ecosystem‐scale water budgets and responses to novel rainfall regimes. Understanding divergent water‐use strategies among encroaching shrubs and the grasses they replace is critical for predicting shifts in ecosystem‐scale water dynamics as a result of shrub encroachment, particularly if drought events become more frequent and/or severe in the future.In this study, we assessed how water‐use traits of a rapidly encroaching clonal shrub (Cornus drummondii) and a dominant C₄grass (Andropogon gerardii) impact responses to changes in water availability in tallgrass prairie. We assessed intra‐annual change in depth of water uptake, turgor loss point and stomatal regulation in each species. Sampling took place at Konza Prairie Biological Station (northeastern KS, USA) during the 2021 and 2022 growing seasons.Cornus drummondiishifted from shallow to deep soil water sources across the 2021 and 2022 growing seasons. This plasticity in depth of water uptake facilitated a ‘wasteful’ water‐use strategy inC. drummondii, where stomatal conductance and transpiration rates continued to increase even when no further gain in photosynthetic rate occurred.A. gerardiiphotosynthetic rates and stomatal conductance were more variable through time and were more responsive to changes in leaf water potential thanC. drummondii. However, intra‐annual adjustment of turgor loss point was more pronounced inC. drummondii(Δπ_TLP = −0.48 MPa ± 0.15 SD) than inA. gerardii(Δπ_TLP = −0.29 MPa ± 0.19 SD).Synthesis. These results suggest thatC. drummondiiis highly resilient to changes in water availability in surface soils and will likely remain unaffected by future droughts unless they are severe enough to reduce the availability of deep soil water. Given that clonal shrubs are key invaders of grasslands world‐wide, increased leaf‐level water loss is expected to accelerate ecosystem‐level drying as clonal shrub encroachment proceeds in mesic grasslands. 

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5. A moving target: trade‐offs between maximizing carbon and minimizing hydraulic stress for plants in a changing climate

   https://doi.org/10.1111/nph.20127  

   Quetin, Gregory_R; Anderegg, Leander_D_L; Boving, Indra; Trugman, Anna_T (September 2024, New Phytologist)

   Summary Observational evidence indicates that tree leaf area may acclimate in response to changes in water availability to alleviate hydraulic stress. However, the underlying mechanisms driving leaf area changes and consequences of different leaf area allocation strategies remain unknown.Here, we use a trait‐based hydraulically enabled tree model with two endmember leaf area allocation strategies, aimed at either maximizing carbon gain or moderating hydraulic stress. We examined the impacts of these strategies on future plant stress and productivity.Allocating leaf area to maximize carbon gain increased productivity with high CO₂, but systematically increased hydraulic stress. Following an allocation strategy to avoid increased future hydraulic stress missed out on 26% of the potential future net primary productivity in some geographies. Both endmember leaf area allocation strategies resulted in leaf area decreases under future climate scenarios, contrary to Earth system model (ESM) predictions.Leaf area acclimation to avoid increased hydraulic stress (and potentially the risk of accelerated mortality) was possible, but led to reduced carbon gain. Accounting for plant hydraulic effects on canopy acclimation in ESMs could limit or reverse current projections of future increases in leaf area, with consequences for the carbon and water cycles, and surface energy budgets. 

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