Sequential droughts: A silent trigger of boreal forest mortality

Abstract

Despite great concern for drought‐driven forest mortality, the effects of frequent low‐intensity droughts have been largely overlooked in the boreal forest because of their negligible impacts over the short term. In this study, we used data from 6876 permanent plots distributed across most of the Canadian boreal zone to assess the effects of repeated low‐intensity droughts on forest mortality. Specifically, we compared the relative impact of sequential years under low‐intensity dry conditions with the effects of variables related to the intensity of dry conditions, stand characteristics, and local climate. Then, we searched for thresholds in forest mortality as a function of the number of years between two forest surveys affected by dry conditions of any intensity. Our results showed that, in general, frequent low‐intensity dry conditions had stronger effects on forest mortality than the intensity of the driest conditions in the plot. Frequent low‐intensity dry conditions acted as an inciting factor of forest mortality exacerbated by stand characteristics and environmental conditions. Overall, the mortality of forests dominated by shade‐tolerant conifers was significantly and positively related to frequent low‐intensity dry conditions, supporting, in some cases, the existence of thresholds delimiting contrasting responses to drought. In mixtures with broadleaf species, however, sequential dry conditions had a negligible impact. The effects of frequent dry conditions on shade‐intolerant forests mainly depended on local climate, inciting or mitigating the mortality of forests located in wet places and dominated by broadleaf species or jack pine, respectively. Our results highlight the importance of assessing not only climate‐driven extreme events but also repeated disturbances of low intensity. In the long term, the smooth response of forests to dry conditions might abruptly change leading to disproportional mortality triggered by accumulated stress conditions. Forest and wildlife managers should consider the cumulative effects of climate change on mortality to avoid shortfalls in timber and habitat.

Climate change may increase Quebec boreal forest productivity in high latitudes by shifting its current composition

“…However, in our simulations, this co-occurrence allowed white birch and trembling aspen species to partially compensate for the decline in coniferous species biomass carbon stock. Overall, the mortality of forests dominated by shade-tolerant conifers is positively related to dry conditions (warm temperature) (Sánchez-Pinillos et al, 2022). Our model overestimated the dispersal of white birch and trembling aspen since there are several edaphic limitations for their regeneration and establishment.…”

Soil Respiration Response to Simulated Precipitation Change Depends on Ecosystem Type and Study Duration

“…Additionally, our results are sensitive to the limited amount of data from long‐term experiments, especially in desert ecosystems where only one study (Li et al., 2020) contributed data for more than 1 year. This limitation highlights a well‐known need within ecological literature for long‐term manipulative studies (Beier et al., 2012; Knapp et al., 2018; Parts et al., 2019; Sánchez‐Pinillos et al., 2022; Walker et al., 2020). However, our results indicate the clear potential for differential long‐term responses to precipitation of different ecosystem types, which we hope will be further investigated by future studies.…”

Floods and droughts: a multivariate perspective on hazard estimation

“…several extreme events occurring in close temporal succession (Figure 2b). An example for such a consecutive extreme event is the 'multi-year' drought 2018-2020 characterized by multiple dry summers over Central Europe (Rakovec et al, 2022), which severely impacted water supply and agriculture (Stephan et al, 2021) and had severe ecological consequences such as forest die backs (Sánchez-Pinillos et al, 2022). A third type of multivariate extreme is hydrological extremes described by multiple characteristics such as flood peak and volume as in the case of the 2021 flood event in Germany (Kreienkamp et al, 2021) (Figure 2c).…”