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Abstract Photosynthetic acclimation to both warming and elevated CO₂of boreal trees remains a key uncertainty in modelling the response of photosynthesis to future climates. We investigated the impact of increased growth temperature and elevated CO₂on photosynthetic capacity (V_cmaxandJ_max) in mature trees of two North American boreal conifers, tamarack and black spruce. We show thatV_cmaxandJ_maxat a standard temperature of 25°C did not change with warming, whileV_cmaxandJ_maxat their thermal optima (T_opt) and growth temperature (T_g) increased. Moreover,V_cmaxandJ_maxat either 25°C,T_optorT_gdecreased with elevated CO₂. TheJ_max/V_cmaxratio decreased with warming when assessed at bothT_optandT_gbut did not significantly vary at 25°C. TheJ_max/V_cmaxincreased with elevated CO₂at either reference temperature. We found no significant interaction between warming and elevated CO₂on all traits. If this lack of interaction between warming and elevated CO₂on theV_cmax,J_maxandJ_max/V_cmaxratio is a general trend, it would have significant implications for improving photosynthesis representation in vegetation models. However, future research is required to investigate the widespread nature of this response in a larger number of species and biomes. 

Abstract Warming shifts the thermal optimum of net photosynthesis (T_optA) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO₂) conditions. It is unclear whether shifts inT_optAof field-grown trees will keep pace with the temperatures predicted for the 21^stcentury under elevated atmospheric CO₂concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO₂levels, we show thatT_optAof mature boreal conifers increased with warming. However, shifts inT_optAdid not keep pace with warming asT_optAonly increased by 0.26–0.35 °C per 1 °C of warming. Net photosynthetic rates estimated at the mean growth temperature increased with warming in elevated CO₂spruce, while remaining constant in ambient CO₂spruce and in both ambient CO₂and elevated CO₂tamarack with warming. Although shifts inT_optAof these two species are insufficient to keep pace with warming, these boreal conifers can thermally acclimate photosynthesis to maintain carbon uptake in future air temperatures. 

Summary Deep‐water access is arguably the most effective, but under‐studied, mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep‐water access may delay plant dehydration. Here, we tested the role of deep‐water access in enabling survival within a diverse tropical forest community in Panama using a novel data‐model approach.We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990–2015) to vapor pressure deficit, water potentials in the whole‐soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water‐access depths.Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981–2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress‐related mortality risk through deep‐water access.The role of deep‐water access in mitigating mortality of hydraulically‐vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.