Canopy structure modulates the sensitivity of subalpine forest stands to interannual snowpack and precipitation variability

Abstract. Declining spring snowpack is expected to have widespread effects on montane and subalpine forests in western North America and across the globe. However, the effect of this forcing at the species and hillslope scale are difficult to predict from remote sensing or eddy covariance. Here, we present data from a network of sap velocity sensors and xylem water isotope measurements from three common subalpine tree species (Picea engelmannii, Abies lasiocarpa, Populus tremuloides) across a hillslope transect in a subalpine watershed in the Upper Colorado River Basin. We use these data to compare tree- and stand-level responses to the historically high spring snowpack but low summer rainfall of 2019 against the low spring snowpacks but high summer rains of 2021 and 2022. From the sap velocity data, we found that only 40 % of the trees showed an increase in cumulative transpiration in response to the large snowpack year (2019), illustrating the absence of a common response to a major decline in snowpack. The trees that benefited from the large snow year were all found in dense canopy stands – irrespective of species – while trees in open canopy stands were more active during the years with modest snow and higher summer rains. This pattern reflects how persistent access to soil moisture recharged by snowmelt in topographically-mediated convergence zones shapes stand density. These locally dense canopies also experience high levels of summer rainfall interception that reduce summer precipitation inputs to the soil perpetuating their greater sensitivity to snowmelt inputs. The results illustrate that the progression towards a low snowpack future will manifest at the sub-hillslope scale in dense stands with significant rainfall interception and high water demands reflecting their historical reliance on snowmelt water.