Department of Forest Resources, University of Minnesota, St. Paul, MN, USA.
Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales, Australia.
Nature. 2018 Oct;562(7726):263-267. doi: 10.1038/s41586-018-0582-4. Epub 2018 Oct 3.
Climate warming will influence photosynthesis via thermal effects and by altering soil moisture. 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. 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.
气候变暖将通过热效应和改变土壤湿度来影响光合作用。这两种效应对于全球森林的广阔地区可能都很重要，这些地区在凉爽温度限制光合作用的时期和土壤湿度可能限制碳吸收的时期之间波动。在这里，我们表明，随着南方北方森林在生长季节从多雨期过渡到适度干燥期，气候变暖的影响从正面变为负面。在一项为期三年的开放空气增温实验中，对 11 种温带和北方树种的幼树进行了实验，温度升高 3.4°C 平均增加了三分之一最湿润土壤日的光饱和净光合速率和叶片扩散导度。在所有 11 种物种中，叶片扩散导度（因此，光饱和净光合速率）在干旱期下降，而在增温植物中比在环境温度下的植物下降得更剧烈。因此，在 11 个物种中，变暖减少了最干燥土壤日的三分之二的光饱和净光合速率。因此，在季节性寒冷的湿润环境中，无论是在干旱期间还是在生长季节经常出现的适度干燥期，低土壤湿度可能会降低甚至逆转气候变暖对光合作用的潜在益处。
