Stomatal behaviour moderates the water cost of CO2 acquisition for 21 boreal and temperate species under experimental climate change.

The linkage of stomatal behaviour with photosynthesis is critical to understanding water and carbon cycles under global change. The relationship of stomatal conductance (gs ) and CO2 assimilation (Anet ) across a range of environmental contexts, as represented in the model parameter (g1 ), has served as a proxy of the marginal water cost of carbon acquisition. We use g1 to assess species differences in stomatal behaviour to a decade of open-air experimental climate change manipulations, asking whether generalisable patterns exist across species and climate contexts. Anet -gs measurements (17 727) for 21 boreal and temperate tree species under ambient and +3.3°C warming, and ambient and ~40% summer rainfall reduction, provided >2700 estimates of g1 . Warming and/or reduced rainfall treatments both lowered g1 because those treatments resulted in lower soil moisture and because stomatal behaviour changed more in warming when soil moisture was low. Species tended to respond similarly, although, in species from warmer and drier habitats, g1 tended to be slightly higher and to be the least sensitive to the decrease in soil water. Overall, both warming and rainfall reduction consistently made stomatal behaviour more conservative in terms of water loss per unit carbon gain across 21 species and a decade of experimental observation.

Assessing the relevant time frame for temperature acclimation of leaf dark respiration: A test with 10 boreal and temperate species.

Plants often adjust their leaf mitochondrial ("dark") respiration (Rd ) measured at a standardized temperature such as 20°C (R20 ) downward after experiencing warmer temperatures and upward after experiencing cooler temperatures. These responses may help leaves maintain advantageous photosynthetic capacity and/or be a response to recent photosynthate accumulation, and can occur within days after a change in thermal regime. It is not clear, however, how the sensitivity and magnitude of this response change over time, or which time period prior to a given measurement best predicts R20 . Nor is it known whether nighttime, daytime, or 24-hour temperatures should be most influential. To address these issues, we used data from 1620 Rd temperature response curves of 10 temperate and boreal tree species in a long-term field experiment in Minnesota, USA to assess how the observed nearly complete acclimation of R20 was related to past temperatures during periods of differing lengths. We hypothesized that R20 would be best related to prior midday temperatures associated with both photosynthetic biochemistry and peak carbon uptake rates that drive carbohydrate accumulation. Inconsistent with this hypothesis, prior night temperatures were the best predictors of R20 for all species. We had also hypothesized that recent (prior 3-10 days) temperatures should best predict R20 because they likely have stronger residual impacts on leaf-level physiology than periods extending further back in time, whereas a prior 1- to 2-day period might be a span shorter than one to which photosynthetic capacity and Rd adjust. There was little to no support for this idea, as for angiosperms, long time windows (prior 30-60 nights) were the best predictors, while for gymnosperms both near-term (prior 3-8 nights for pines, prior 10-14 nights for spruce/fir) and longer-term periods (prior 45 nights) were the best predictors. The importance of nighttime temperatures, the relatively long "time-averaging" that best explained acclimation, and dual peaks of temporal acclimation responsiveness in some species were all results that were unanticipated.

Short- and long-term responses of photosynthetic capacity to temperature in four boreal tree species in a free-air warming and rainfall manipulation experiment.

High latitude forests cope with considerable variation in moisture and temperature at multiple temporal scales. To assess how their photosynthetic physiology responds to short- and long-term temperature variation, we measured photosynthetic capacity for four tree species growing in an open-air experiment in the boreal-temperate ecotone `Boreal Forest Warming at an Ecotone in Danger' (B4WarmED). The experiment factorially manipulated temperature above- and below-ground (ambient, +3.2 °C) and summer rainfall (ambient, 40% removal). We measured A/Ci curves at 18, 25 and 32 °C for individuals of two boreal (Pinus banksiana Lamb., Betula papyrifera Marsh.) and two temperate species (Pinus strobus L., Acer rubrum L.) experiencing the long-term warming and/or reduced-rainfall conditions induced by our experimental treatments. We calculated the apparent photosynthetic capacity descriptors VCmax,Ci and Jmax,Ci and their ratio for each measurement temperate. We hypothesized that (i) VCmax,Ci and Jmax,Ci would be down-regulated in plants experiencing longer term (e.g., weeks to months) warming and reduced rainfall (i.e., have lower values at a given measurement temperature), as is sometimes found in the literature, and that (ii) plants growing at warmer temperatures or from warmer ranges would show greater sensitivity (steeper slope) to short-term (minutes to hours) temperature variation. Neither hypothesis was supported as a general trend across the four species, as there was not a significant main effect (across species) of either warming or rainfall reduction on VCmax,Ci and Jmax,Ci. All species markedly increased VCmax,Ci and Jmax,Ci (and decreased their ratio) with short-term increases in temperature (i.e., contrasting values at 18, 25 and 32 °C), and those responses were independent of long-term treatments and did not differ among species. The Jmax,Ci:VCmax,Ci ratio was, however, significantly lower across species in warmed and reduced rainfall treatments. Collectively, these results suggest that boreal trees possess considerable short-term plasticity that may allow homeostasis of VCmax,Ci and Jmax,Ci to a longer term temperature treatment. Our results also caution against extrapolating results obtained under controlled and markedly contrasting temperature treatments to responses of photosynthetic parameters to more modest temperature changes expected in the near-term with climate warming in field conditions.

Surprising lack of sensitivity of biochemical limitation of photosynthesis of nine tree species to open-air experimental warming and reduced rainfall in a southern boreal forest.

Photosynthetic biochemical limitation parameters (i.e., Vcmax , Jmax and Jmax :Vcmax ratio) are sensitive to temperature and water availability, but whether these parameters in cold climate species at biome ecotones are positively or negatively influenced by projected changes in global temperature and water availability remains uncertain. Prior exploration of this question has largely involved greenhouse based short-term manipulative studies with mixed results in terms of direction and magnitude of responses. To address this question in a more realistic context, we examined the effects of increased temperature and rainfall reduction on the biochemical limitations of photosynthesis using a long-term chamber-less manipulative experiment located in northern Minnesota, USA. Nine tree species from the boreal-temperate ecotone were grown in natural neighborhoods under ambient and elevated (+3.4°C) growing season temperatures and ambient or reduced (≈40% of rainfall removed) summer rainfall. Apparent rubisco carboxylation and RuBP regeneration standardized to 25°C (Vcmax25°C and Jmax25°C , respectively) were estimated based on ACi curves measured in situ over three growing seasons. Our primary objective was to test whether species would downregulate Vcmax25°C and Jmax25°C in response to warming and reduced rainfall, with such responses expected to be greatest in species with the coldest and most humid native ranges, respectively. These hypotheses were not supported, as there were no overall main treatment effects on Vcmax25°C or Jmax25°C (p > .14). However, Jmax :Vcmax ratio decreased significantly with warming (p = .0178), whereas interactions between warming and rainfall reduction on the Jmax25°C to Vcmax25°C ratio were not significant. The insensitivity of photosynthetic parameters to warming contrasts with many prior studies done under larger temperature differentials and often fixed daytime temperatures. In sum, plants growing in relatively realistic conditions under naturally varying temperatures and soil moisture levels were remarkably insensitive in terms of their Jmax25°C and Vcmax25°C when grown at elevated temperatures, reduced rainfall, or both combined.