Quantifying the Hydraulic Health of Fennoscandian Boreal Forests in a Changing Climate

Drought-induced tree mortality in the Anthropocene has increased at an unprecedented rate and is expected to increase even more sharply in the next few decades due to anthropogenic climate change. Fennoscandian boreal forests are particularly at risk of drought-related mortality because they are less adapted to drought conditions compared with lower latitude forests. Currently, tree mortality research is inhibited by a lack of data on tree deaths, but we can still anticipate mortality events by monitoring forest health. In this study we determine indicators of forest ‘hydraulic health’ which we define as the ability of a forest to withstand and recover from drought events. Ultimately, this may help to predict and prevent drought-induced tree deaths under future climatic shifts.  

 To describe the hydraulic health of Fennoscandian forests we examine vegetation water content at both short (sub-daily) and intermediate (weekly) timescales using remotely sensed vegetation optical depth (VOD) measurements. Whilst short-timescale VOD is more tightly linked to water stress, intermediate-timescale VOD can demonstrate forest resilience and recovery to drought events. Here, we use a multivariate time series model to integrate VOD measurements at different timescales, presenting a novel opportunity to disentangle the drivers and responses of forests to drought. 

 The method introduced here is a first step towards quantifying abiotic hazards for forests. By focusing on smaller time steps of VOD data we can minimise the noise attributed to biomass changes over time and can better detect changes in vegetation water content. We hope that by quantifying hydraulic health indicators, they can serve as an early warning system for drought-induced mortality or help in predicting other hazards such as wildfire. Although we focus on Fennoscandian forests, in principle, these methods could be applied to other biomes.