Introduction

Background

Trees are critical for maintaining healthy ecosystems, sequestering carbon, and supporting economies (Gamfeldt, et al., 2013). However, their roles are at risk since forests are vulnerable under a changing global climate (Lindner, et al., 2010). Amongst other threats, drought is expected to become one of the most critical factors in limiting tree growth and survival (Babst, et al., 2019). Droughts are expected to increase both in severity and frequency as average temperatures increase (Allen, et al., 2010). Thus, it is especially important to understand the relationship between drought and tree growth to more accurately predict how growth will change so better management decisions can be made to preserve trees and the services they provide.

Figure 1. Predicted changes in the ratio of evapotranspiration to precipitation under Shared Socioeconomic Pathway 3. Represented using the Climate Moisture Index (CMI) which is equal to precipitation minus evapotranspiration (Wang, Hamann, Spittlehouse, & Carroll, 2016).

Boreal forest in Canada is expected to be particularly vulnerable to drought under climate change (Peng, et al., 2011). Specifically, the moisture limitations of Picea glauca have been demonstrated in the southwest and west central Canadian boreal forest (Hynes & Hamann, 2020). However, determining tree drought response can be challenging, especially when identifying drought over large areas without fine scale and site specific data. Ultimately the attribute of interest is soil moisture at the root level (Liu, et al., 2020), but this is time consuming and expensive to measure directly; generally proxies like climatic moisture must be used instead. Additionally, mortality is equally important as reductions in growth, but historical growth can be measured by extracting cores and measuring the widths of tree rings, while any evidence of mortality will decompose over time. However, it has been shown that reduced growth in trees is directly linked to risk of future mortality (DeSoto, et al., 2020). Drought responses can also vary depending on a variety of factors, such as species, climate, and other stressors (Hacke, Sperry, Wheeler, & Castro, 2006), so predicting the effect of drought requires understanding the influence of these as well.

Therefore, this study aims to quantify the differences in growth response of 8 tree species in western Canada to drought from 1980 to 2010, stratifying responses by climate and species. By analyzing historical data, the study aims to better understand how these tree species have responded to drought in the past and how different climates and species may influence their growth response. 

Research Objectives