Sequential droughts: A silent trigger of boreal forest mortality.

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.

Temporal dimension of forest vulnerability to fire along successional trajectories.

Understanding ecosystem vulnerability is essential in risk management to anticipate disasters. While valuable efforts have been made to characterize vulnerability components (exposure, sensitivity, and response capacity) at particular ecosystem stages, there is still a lack of context-specific studies accounting for the temporal dimension of vulnerability. In this study, we developed a procedure to identify the main natural dynamics of monospecific and mixed forests and to assess the variations of sensitivity and response capacity to fire along successional dynamics. In the procedure, we generated forest chronosequences by summarizing the dynamics between consecutive surveys of permanent plots into a set of longer successional trajectories represented in a multidimensional space. Then, we calculated several variables of sensitivity and response capacity to fire of forest stages associated with each trajectory and we assessed their variation along succession. The procedure was applied to Mediterranean forests in Spain dominated by a pine species poorly adapted to severe crown fires. We found that forest vulnerability components varied differently among successional trajectories, which depended on the composition and structure of their initial stages and the environmental context in which they occurred. Autosuccessional dynamics of pine forests showed relatively low sensitivity to fire along trajectories. However, their response capacity was related to the changes in shrub cover. In contrast, diversifying dynamics showed an increasing sensitivity to fire, but also a higher response capacity the greater the functional diversity along succession. These results highlight the need for considering the temporal dimension of vulnerability in risk management and the importance of assessing sensitivity and response capacity as independent components of vulnerability that can be modified through management at critical forest stages.