Causes of plant mortality from extreme winter events: model insights into desiccation processes during frost droughts

Abstract:

Arctic and boreal vegetation has experienced significant change during the last decades. Shrubs have grown larger, and trees expand northwards and move upwards along mountain slopes. Although the overall living biomass has gone up, there are significant areas across the Arctic that experienced a decrease in vegetation productivity in recent years – a phenomenon, called "Arctic browning". Part of the reason for arctic browning is a vulnerability to a growing number of extreme weather events associated with climate change. Extreme winter events can initiate icing, loss of frost tolerance, and frost droughts that lead to vegetation damage and death of tissues. While frost droughts are not as well-documented as summer droughts, they are the cause of a considerable proportion of observed damage. They have been suggested to occur when sudden warm events trigger leaf transpiration combined with deeply frozen soils due to the lack of snow that prevent plants from replenishing leaf water loss from soil.

Most terrestrial biosphere models represent plant water transport as one single resistance term, or ignore plant hydraulics completely. As a result, leaf transpiration is too strongly regulated by soil water stress, and the disparity between leaf transpiration and soil water stress as expected in frost droughts can hardly be depicted by the models. Recent incorporation of much more detailed plant hydraulic modules, based on tissue (root,stem,leaf) level traits, however, opens up the possibility to properly represent frost droughts experienced by plants, and to investigate how frost droughts impact land-atmosphere interactions.

In this study, we used the FATESHydro, a cohort model of vegetation coexistence and competition, driven by high resolution atmospheric forcing derived from COSMO-REA6, to evaluate how frost droughts impact vegetation mortality in northern Norway over the period 2012-2020. We established a clear link between snow depth and drought intensity. We show that root water exudation at low soil water potentials, rather than leaf transpiration loss at high vapor pressure deficit, explained tissue desiccation during shallow snow covered winters. We describe which areas of our domain (northern Norway) are most vulnerable and have been strongly hit by frost droughts over the period 2012-2020.