Sapling recruitment does not compensate for increased tree mortality under climate change

Abstract

Sapling recruitment within established stands plays an important role in the long-term functionality of forest ecosystems. However, the specific impacts of climate change on sapling recruitment and its underlying mechanisms remain unclear. We hypothesized that sapling recruitment is directly affected by climate change and indirectly through the responses of tree mortality and growth to climate change as well as their induced modification of neighbourhood conditions. We employed a dataset of 173 permanent sample plots with individual stems that were spatially mapped and monitored at 5-year intervals from 1986 to 2010 in the central boreal forests of North America. By constructing structural equation models (SEMs), we examined how sapling recruitment probability may, directly and indirectly, respond to climate change through the mortality and growth of established trees, neighbourhood crowding, and functional and phylogenetic dissimilarities. We used the middle calendar year of a 5-year census period to represent recent systematic temporal trends in climate change. Our SEMs revealed that sapling recruitment probability did not respond significantly directly to temporal climate change in the studied forests, where atmospheric CO2 concentration, air temperature, and climatic water availability simultaneously increased temporally. However, the temporally increased mortality and growth of neighbourhood established trees (all stems except for new recruits) exerted indirect negative effects on sapling recruitment probability. Furthermore, temporally increased neighbourhood crowding had an indirect adverse effect, while temporally increased neighbourhood shade tolerance and phylogenetic dissimilarities had weak indirect positive effects on sapling recruitment probability. Collectively, temporal climate change had a significant total negative effect on sapling recruitment probability. Our analysis demonstrated multiple mechanisms that simultaneously regulated the effect of temporal climate change on sapling recruitment. Our results emphasized that sapling recruitment does not compensate for increased tree mortality. We suggest that even so-called favourable climate change could threaten the long-term functionality of boreal forest ecosystems.