Abstract
Terrestrial plant production varies across climate gradients, but the mechanisms driving this variation are a subject of debate. Specifically, it is unclear whether production is primarily influenced by “direct” climate effects on the kinetics of plant metabolism, or “indirect” climate effects on plant size, stand biomass, stand age structure, and growing season length. I address this debate by deriving metabolic theory that links hypothesized climate influences with production, and assessing these hypothesized relationships using a global compilation of ecosystem woody plant biomass and production data. Notably, age and biomass explained most of the variation in production whereas temperature and precipitation explained almost none, suggesting that climate indirectly (not directly) influences production. This lack of a direct kinetic effect may arise from covariation of leaf traits and climate, which decouples plant and air temperatures. This yields a general thermoregulation of leaves, which I suggest originates from selection on leaf traits to maximize carbon gain across and within variable environments. Using global data for leaf temperatures, traits and photosynthesis, I evaluate predictions from a theory of thermoregulation that synthesizes energy budget and carbon economics theories. These results provide a novel mechanistic understanding of constraints on evolution of plant form and function, and have critical implications for earth system models that almost always assume plant temperatures equal ambient air temperature.

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