LETTER https://doi.org/10.1038/s41586-018-0582-4 Effects of climate warming on photosynthesis in boreal tree species depend on soil moisture Peter B. Reich 1,2 *, Kerrie M. Sendall 1,3 , Artur Stefanski 1 , Roy L. Rich 1,4 , Sarah E. Hobbie 5 & Rebecca A. Montgomery 1 Climate warming will influence photosynthesis via thermal effects and by altering soil moisture 1–11 . Both effects may be important for the vast areas of global forests that fluctuate between periods when cool temperatures limit photosynthesis and periods when soil moisture may be limiting to carbon gain 4–6,9–11 . Here we show that the effects of climate warming flip from positive to negative as southern boreal forests transition from rainy to modestly dry periods during the growing season. In a three-year open-air warming experiment with juveniles of 11 temperate and boreal tree species, an increase of 3.4 °C in temperature increased light- saturated net photosynthesis and leaf diffusive conductance on average on the one-third of days with the wettest soils. In all 11 species, leaf diffusive conductance and, as a result, light-saturated net photosynthesis decreased during dry spells, and did so more sharply in warmed plants than in plants at ambient temperatures. Consequently, across the 11 species, warming reduced light- saturated net photosynthesis on the two-thirds of days with driest soils. Thus, low soil moisture may reduce, or even reverse, the potential benefits of climate warming on photosynthesis in mesic, seasonally cold environments, both during drought and in regularly occurring, modestly dry periods during the growing season. A changing climate will influence plants by altering temperature, precipitation and soil moisture, as well as their variability and sea- sonality 1–11 . In temperate and boreal climates, temperatures switch seasonally from cold (and limiting to biological processes) to warm and periodically dry, during which time moisture can be limiting 2–6,9–11 . Both the ‘law of the minimum’ and multiple limitation theory 12–14 pro- vide a conceptual basis for predicting climate warming interactions with soil moisture. Although higher temperatures may alleviate enzy- matic limits on the biochemistry of photosynthesis, realized rates of CO 2 assimilation may decrease if and when low soil water causes sto- matal closure and limitation of the CO 2 substrate for photosynthesis. As growing season conditions in temperate and boreal forests are likely to become effectively drier than in the past 3,8,9 , because climate warming will increase evapotranspiration more than precipitation 3,9 and increase variability in the amount of precipitation per event 1,9 , the importance of water availability to  forest responses to rising temperature may increase in the future 3–6,9–11,15–18 . Mid- and high-latitude plants will therefore probably experience both positive and negative effects of climate warming on photosynthe- sis within and across years—we propose that these will be positive when soil moisture is ample but negative when soils are drier 4–6,9–11,15–17 . Whether such effects are in aggregate positive or negative is likely to depend on the balance of time that warming alleviates low temperature limitations to plant function as opposed to causing limitations to func- tion through decreased soil moisture. However, direct tests of the effects of climate warming across a range of soil moisture conditions, caused by seasonal or interannual variation or by manipulations of tempera- ture or moisture, are rare, and it remains unclear how plant responses to climate warming will be influenced by these indirect effects of soil moisture 4–6,9–11,16–18 . Here we provide evidence from 11 co-occurring boreal and temperate tree species (Fig. 1) in support of the overarching hypothesis that low soil moisture status has a dampening effect on the photosynthetic enhancement that results from experimental warming. This moisture regulation of the response to climate warming was consistent for all 11 species and occurred in response to reductions in soil moisture due to typical seasonal variation and in response to further reductions in soil moisture due to experimental warming. Results are from the free-air B4WarmED experiment 19–22 , in which juveniles (3–5 years old at the time of measurements) of local ecotypes of the 11 tree species were grown under ambient and seasonally elevated (+3.4 °C, April– November) temperatures from 2009 to 2011 at two southern boreal sites in Minnesota, USA (Extended Data Table 1 and Methods). The 11 species co-occur in forests in northern Minnesota; however, five are boreal with southern range limits in or near Minnesota and six are temperate with northern range limits not far north of the Minnesota– Canada border 19 . Fluctuations in soil moisture levels (volumetric water content (VWC), m 3 H 2 O per m 3 soil) occurred at both sites and across all years (Extended Data Fig. 1 and Extended Data Table 2), and spanned from 0.27 to 0.05 VWC, representing a range from slightly wetter than field capacity to slightly drier than the permanent wilting point (of approximately -1.5 MPa) for these sandy loam soils 23,24 . Leaf temperature (T leaf ) and vapour pressure gradient (VPG) also varied considerably across all photosynthetic measurements (Extended Data Fig. 2). All species responses were consistent with the hypothesis that effects of experimental warming on carbon gain would be less positive or more negative during periods of low soil moisture (Fig. 1, Table 1 and Extended Data Table 3). In moist soils, all angiosperm species (and no gymnosperms) showed higher maximum carboxylation capacity at 25 °C (V cmax-25 ) when grown at increased temperature compared to ambient temperatures (Extended Data Fig. 3), helping to explain the higher light-saturated net photosynthesis (A net ) in warmed plants when soil water limitations were modest (Fig. 1). This higher maximum carboxylation capacity in well-watered, warmed angiosperms assessed at a standardized temperature is indicative of an acclimation response (upregulation of V cmax-25 ) to growth in elevated temperatures.  However, every species showed marked sensitivity of A net to drying soil moisture (Fig. 1). More relevant to our overarching hypothesis, A net in all species declined more steeply with decreasing soil moisture in warmed than in ambient conditions (Fig. 1); therefore, when compared at a common soil moisture, plants showed the most positive (or least negative) effects of experimental warming on A net when soil moisture availability was high, whereas positive effects decreased (or negative effects increased) as soil moisture availability declined (Fig. 1). In other words, we found a significant interaction between the increased temperature treatment and VWC for A net (Table 1; F 1,553 = 40.9, P < 0.0001) in a model that included treatment (increased or ambient temperature), species, VWC and two other environmental drivers (T leaf and VPG). Moreover, although species differed from each other in A net , they did not differ in how VWC influenced their response 1 Department of Forest Resources, University of Minnesota, St. Paul, MN, USA. 2 Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales, Australia. 3 Department of Biology, Georgia Southern University, Statesboro, GA, USA. 4 Smithsonian Environmental Research Center, Edgewater, MD, USA. 5 Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN, USA. *e-mail: preich@umn.edu N AT U R E | www.nature.com/nature © 2018 Springer Nature Limited. All rights reserved.