Foliage development and resource allocation determine the growth responses of silver birch (Betula pendula) to elevated environmental humidity.

This article examines the effects of artificially elevated environmental humidity—specifically increased air relative humidity and soil moisture—on the physiology, growth, and phenology of silver birch (Betula pendula) trees in a hemiboreal forest zone in eastern Estonia. Using a free air humidity manipulation (FAHM) experimental site, the study found that elevated air humidity did not significantly alter leaf water relations or photosynthetic capacity but increased stomatal sensitivity to vapor pressure deficit, reduced height growth and foliage biomass, shifted biomass allocation toward stem radial growth, and prolonged leaf retention in autumn. Increased soil moisture alone did not produce these growth effects. The findings suggest that rising atmospheric humidity at high latitudes may slow silver birch growth and forest productivity by delaying foliage development and altering resource allocation, potentially counteracting expected growth benefits from climate warming.