Extreme climatic events, such as droughts, can have large effects on biodiversity. Drought effects in forest understories are variable over small spatial scales and can be exacerbated, or buffered, by the local vegetation structure, distance to forest edges, hydrology, and soil characteristics. Boreal forest landscapes are intensively managed, which affects several of these factors and many boreal forest species are confined to small forest fragments in an otherwise managed landscape. 

In this thesis, I investigated how summer drought affects different taxa in spruce-dominated forest understories, including vascular plants, bryophytes, lichens and fungi. I assessed if drought sensitivity can be linked to taxonomic group and species traits, and how drought effects vary over space and time. I conducted both observational studies, after the drought in 2018, and experimental studies in which I used rainout shelters to exclude all precipitation for 45 days. First, I examined if spatial variation in the 2018 drought was correlated with old-growth forest affiliated species richness and community composition, and tested if drought effects on understory species were stronger in edge exposed forest patches (chapter I). I also investigated how the 2018 drought affected the depth and magnitude of microclimatic edge effects, using the annual growth of an understory moss (chapter II). Second, I assessed how the experimental drought affected the performance of transplanted understory plants (chapter III) and soil fungal communities in terms of biomass, community composition and sporocarp production (chapter IV), and investigated how spatial variation in canopy cover, soil moisture and soil nutrients modified responses to drought (chapter III and IV). Finally, I suggest tools to optimize forest management and biodiversity conservation in a changing climate with a higher drought prevalence (chapter I – IV).

My results indicate that summer droughts can have significant impacts on forest understory species, both above and belowground, and that these impacts vary across landscapes. The groups that correlated most strongly with spatial variation in the 2018 drought were cyanolichens, epiphytes on high-pH bark, and species on logs and boulders (chapter I). After the experimental drought, particularly bryophytes, and the orchid Goodyera repens, experienced long-lasting negative effects on growth and reproduction (chapter III). Belowground, the experimental drought negatively affected species richness of saprotrophic fungi in the phylum Basidiomycota and ectomycorrhizal fungi with extensive and hydrophobic extramatrical mycelia (chapter IV). My results suggest that understory species are more vulnerable to extreme drought in edge exposed forest patches (chapter I), and edge effects were twice as strong during the 2018 drought compared to regular years (chapter II). Within forest patches, higher canopy cover and soil moisture levels reduced negative drought responses to some extent (chapter II, III, IV).  

In order to maintain the conservational value of small forest remnants in future climates with a higher frequency of droughts, the amount of edge habitat surrounding these forest patches needs to be reduced. This can be achieved by adding buffer zones with high shade levels or by moving away from clearcutting as the dominant harvesting practice. Furthermore, minimizing canopy opening and restoring hydrological networks can buffer drought impacts on understory species.