Excision increases branch water potential but generally reduces leaf gas exchange

Measurement of leaf gas exchange is often complicated by the difficulty of sampling leaves from tall or otherwise inaccessible plants. Branch excision is widely used to enable such measurements, but the magnitude, variability and mechanisms of its effects on water potential and gas exchange are not well characterized across species and environments. We quantified the effects of excision on branch water potential, stomatal conductance, photosynthesis and stomatal behavior (parameter g1) in four angiosperm and one gymnosperm tree species differing in hydraulic traits and water-use strategies. To assess generality, we also conducted a meta-analysis of 26 species spanning a broad range of hydraulic traits, water-use strategies and biomes. Excision consistently increased branch water potential in all species. In angiosperms, excision reduced stomatal conductance and photosynthesis, with decreases in g1 for some species, whereas in the gymnosperm, excision increased stomatal conductance, photosynthesis and g1. The meta-analysis showed that excision generally decreased stomatal conductance and photosynthesis, with effect sizes varying by species, water-use strategy and hydraulic traits. These results demonstrate that the effects of excision are strongly species- and trait-dependent. Careful consideration and study-specific corrections are needed when interpreting gas exchange data from excised branches.