Forest microclimates and climate change: importance, drivers and future research agenda

Forest microclimates contrast strongly with the climate outside forests. To fully understand and better predict how forests' biodiversity and functions relate to climate and climate change, microclimates need to be integrated into ecological research. Despite the potentially broad impact of microclimates on the response of forest ecosystems to global change, our understanding of how microclimates within and below tree canopies modulate biotic responses to global change at the species, community and ecosystem level is still limited. Here, we review how spatial and temporal variation in forest microclimates result from an interplay of forest features, local water balance, topography and landscape composition. We first stress and exemplify the importance of considering forest microclimates to understand variation in biodiversity and ecosystem functions across forest landscapes. Next, we explain how macroclimate warming (of the free atmosphere) can affect microclimates, and vice versa, via interactions with land-use changes across different biomes. Finally, we perform a priority ranking of future research avenues at the interface of microclimate ecology and global change biology, with a specific focus on three key themes: (1) disentangling the abiotic and biotic drivers and feedbacks of forest microclimates; (2) global and regional mapping and predictions of forest microclimates; and (3) the impacts of microclimate on forest biodiversity and ecosystem functioning in the face of climate change. The availability of microclimatic data will significantly increase in the coming decades, characterizing climate variability at unprecedented spatial and temporal scales relevant to biological processes in forests. This will revolutionize our understanding of the dynamics, drivers and implications of forest microclimates on biodiversity and ecological functions, and the impacts of global changes. In order to support the sustainable use of forests and to secure their biodiversity and ecosystem services for future generations, microclimates cannot be ignored.Peer reviewe

Effects of drought on boreal forest understory species

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

Effects of drought on boreal forest understory species

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

Interactive effects of drought and edge exposure on old-growth forest understory species

Context Both climatic extremes and land-use change constitute severe threats to biodiversity, but their interactive effects remain poorly understood. In forest ecosystems, the effects of climatic extremes can be exacerbated at forest edges. Objectives We explored the hypothesis that an extreme summer drought reduced the richness and coverage of old-growth forest species, particularly in forest patches with high edge exposure. Methods Using a high-resolution spatially explicit precipitation dataset, we could detect variability in drought intensity during the summer drought of 2018. We selected 60 old-growth boreal forest patches in central Sweden that differed in their level of drought intensity and amount of edge exposure. The year after the drought, we surveyed red-listed and old-growth forest indicator species of vascular plants, lichens and bryophytes. We assessed if species richness, composition, and coverage were related to drought intensity, edge exposure, and their interaction. Results Species richness was negatively related to drought intensity in forest patches with a high edge exposure, but not in patches with less edge exposure. Patterns differed among organism groups and were strongest for cyanolichens, epiphytes associated with high-pH bark, and species occurring on convex substrates such as trees and logs. Conclusions Our results show that the effects of an extreme climatic event on forest species can vary strongly across a landscape. Edge exposed old-growth forest patches are more at risk under extreme climatic events than those in continuous forests. This suggest that maintaining buffer zones around forest patches with high conservation values should be an important conservation measure