Forest microclimate during the winter-spring transition in montane conifer forests

Seasonality in climate is a major driver of forest carbon cycling. Besides the direct influence on phenology, the length of the active season impacts annual productivity. With changing climate, the timing of the winter-spring transition is advancing, changing the active season. Earlier and longer active seasons are generally associated with increased ecosystem carbon gain, but not always. A longer growing season can instead mean summer drought: forests that rely on winter precipitation via a slowly-melting snowpack to supply summer transpiration can have lower annual productivity. Moreover, conifers do photosynthesize in winter, and in milder locations can perform as much as 50-70% of total annual carbon uptake outside the traditional "growing season".

Here we describe a study focused on the biophysical factors of the winter-spring transition in conifer forests of Colorado and Utah. Observations included forest microclimate, canopy visible, near- and thermal-infrared imagery, sapflux, and tower-based eddy covariance. We show that Abies concolor at 2050 m elevation was active during favorable periods in winter, but Abies lasiocarpa, Pinus contorta, and Picea engelmanii at 2630 m and 3050 m were dormant. We describe the forest microclimate conditions associated with initiation of early- and late-winter transpiration, including within the soil, stem, and vegetation canopy.