Retention forestry enhances the resilience of the moss Hylocomium splendens to extreme drought in boreal forests

Understorey biodiversity is increasingly impacted by extreme climate events. Retention forestry, which involves preserving small patches of live and dead trees from preharvest forests within clearcuts, can help mitigate these extremes by creating more favourable microclimates than traditional clearcutting practices. Despite their importance in buffering climate extremes, it remains unclear whether, and to what extent, the microclimates in retention patches enhance the growth response and recovery of the understorey after extreme droughts in boreal managed forests. We retrospectively investigated the annual growth response from 2016 to 2022 of the mat-forming understorey moss Hylocomium splendens, in relation to micro- and macroclimate, including an extreme drought in 2018, in retention patches relative to clearcuts and mature forests, across 130 plots distributed across 30 forest sites in a boreal landscape in Sweden. The 2018 summer drought reduced the annual growth rates of H. splendens. Clearcuts experienced the greatest climatic impact from the 2018 drought and exhibited the lowest growth rates, followed by retention patches, while mature forests maintained the highest growth rates. This pattern persisted subsequent two post-drought years. Closer alignment of below-canopy temperature and vapour pressure deficits (VPDs) with those of mature forests enhanced moss growth in retention patches, bringing it closer to the levels observed in mature forests. In clearcuts and mature forests, where variation in forest canopy and microclimate was minimal, biological legacies did not influence annual moss growth. In retention patches, however, a greater basal area of large living trees and the presence of standing deadwood contributed to higher canopy closure, which reduced microclimate VPDs and increased H. splendens growth. Increasing volumes of lying deadwood positively contributed to H. splendens growth, likely by creating favourable microhabitats and microclimates near the logs. Synthesis and applications. This study demonstrates that drought reduced the growth of mat-forming understorey H. splendens in boreal forest ecosystems, but drought effects in clearcuts are mitigated in retention patches. By preserving large living trees, standing and lying deadwood, retention patches can be further optimized. Foresters and policymakers can use these findings to minimize the impact of drought after clearcutting on understorey biodiversity and functionality.