The influence of environmental variables on intra-seasonal radial stem growth dynamics at the Arctic forest-tundra ecotone using point dendrometers

Abstract:

The Arctic forest-tundra ecotone (FTE) is vulnerable to climate change and increases in Arctic temperatures are expected to promote tree growth across this extensive transition zone. However, throughout the boreal forest, the response of tree growth to climate change has been varied, often linked to differences in other environmental variables such as vapor pressure deficit (VPD) and soil water availability. A more detailed understanding of how these drivers influence growth will allow us to more accurately predict how the FTE will respond to continued climate change in the 21st century. Therefore, this study aims to identify the extent to which environmental variables control dynamics in intra-seasonal radial stem growth that ultimately determine the magnitude of annual stem growth in trees at the FTE. We recorded microclimate (i.e. air temperature, VPD, solar irradiation, soil moisture, and soil temperature) and radial growth using point dendrometers at 60 Picea glauca and P. mariana trees at the FTE in Alaska, USA and Northwest Territories, CA from 2016 - 2019. By using a linear mixed-effects model and applying a backward, step-wise removal of covariates, we ranked the effect of the environmental variables on radial growth. Preliminary results show that low VPD and high air temperature were associated with high radial growth at both daily- and seasonal-scales. Soil temperature did not improve predictions of radial growth at either scale, however, may be biologically significant over longer timescales and may trigger the onset and cessation of radial growth. Our full model accounted for approximately 70% of both daily and seasonal variations in the dynamics of stem growth. These findings suggest that, in addition to air temperature, VPD is important to consider when predicting the stem radial growth at the FTE in a changing climate.