Due to climate change, droughts are increasingly frequent and intense. Yet, their impact on boreal forest fungal communities remains unclear, especially across different fungal functional and taxonomic groups. We induced an experimental rainfall exclusion for 45 summer days, using a paired design of 1 × 1 m treatment and control plots replicated in 25 sites in a boreal forest landscape in Sweden. Immediately after the experiment, we assessed the effects on soil fungal biomass, community composition and, after 2 months, sporocarp production. We did not detect significant effects of the rainfall exclusion on soil fungal biomass, but the fungal community composition was affected. In the rainfall exclusion plots, richness of ectomycorrhizal species with extensive extramatrical mycelia and saprotrophic basidiomycetes was reduced, while richness of ascomycetes was not affected. Sporocarp production of both saprotrophic and ectomycorrhizal fungi was reduced. The clear effects of a small-scale rainfall exclusion demonstrated in our study suggest that belowground fungal communities in boreal forests may be vulnerable to drought.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

Extreme droughts are globally increasing in frequency and severity. Most research on drought in forests focuses on the response of trees, while less is known about the impacts of drought on forest understory species and how these effects are moderated by the local environment. We assessed the impacts of a 45-day experimental summer drought on the performance of six boreal forest understory plants, using a transplant experiment with rainout shelters replicated across 25 sites. We recorded growth, vitality and reproduction immediately, 2 months, and 1 year after the simulated drought, and examined how differences in ambient soil moisture and canopy cover among sites influenced the effects of drought on the performance of each species. Drought negatively affected the growth and/or vitality of all species, but the effects were stronger and more persistent in the bryophytes than in the vascular plants. The two species associated with older forests, the moss Hylocomiastrum umbratum and the orchid Goodyera repens, suffered larger effects than the more generalist species included in the experiment. The drought reduced reproductive output in the moss Hylocomium splendens in the next growing season, but increased reproduction in the graminoid Luzula pilosa. Higher ambient soil moisture reduced some negative effects of drought on vascular plants. Both denser canopy cover and higher soil moisture alleviated drought effects on bryophytes, likely through alleviating cellular damage. Our experiment shows that boreal understory species can be adversely affected by drought and that effects might be stronger for bryophytes and species associated with older forests. Our results indicate that the effects of drought can vary over small spatial scales and that forest landscapes can be actively managed to alleviate drought effects on boreal forest biodiversity. For example, by managing the tree canopy and protecting hydrological networks.

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.

Forest fragmentation increases the amount of edges in the landscape. Differences in wind, radiation, and vegetation structure create edge-to-interior gradients in forest microclimate, and these gradients are likely to be more pronounced during droughts and heatwaves. Although the effects of climate extremes on edge influences have potentially strong and long-lasting impacts on forest understory biodiversity, they are not well understood and are not often considered in management and landscape planning. Here we used a novel method of retrospectively quantifying growth to assess biologically relevant edge influences likely caused by microclimate using Hylocomium splendens, a moss with annual segments. We examined how spatio-temporal variation in drought across 3 years and 46 sites in central Sweden, affected the depth and magnitude of edge influences. We also investigated whether edge effects during drought were influenced by differences in forest structure. Edge effects were almost twice as strong in the drought year compared to the non-drought years, but we did not find clear evidence that they penetrated deeper into the forest in the drought year. Edge influences were also greater in areas that had fewer days with rain during the drought year. Higher levels of forest canopy cover and tree height buffered the magnitude of edge influence in times of drought. Our results demonstrate that edge effects are amplified by drought, suggesting that fragmentation effects are aggravated when droughts become more frequent and severe. Our results suggest that dense edges and buffer zones with high canopy cover can be important ways to mitigate negative drought impacts in forest edges.

Conservation of biodiversity in managed forest landscapes needs to be complemented with new approaches given the threat from rapid climate change. Most frameworks for adaptation of biodiversity conservation to climate change include two major strategies. The first is the resistance strategy, which focuses on actions to increase the capacity of species and communities to resist change. The second is the transformation strategy and includes actions that ease the transformation of communities to a set of species that are well adapted to the novel environmental conditions. We suggest a number of concrete actions policy makers and managers can take. Under the resistance strategy, five tools are introduced, including: identifying and protecting forest climate refugia with cold-favored species; reducing the effects of drought by protecting the hydrological network; and actively removing competitors when they threaten cold-favored species. Under the transformation strategy, we suggest three tools, including: enhancing conditions for forest species favored by the new climate, but currently disfavored by forest management, by planting them at suitable sites outside their main range; and increasing connectivity across the landscape to enhance the expansion of warm-favored species to sites that have become suitable. Finally, we suggest applying a landscape perspective and simultaneously managing for both retreating and expanding species. The two different strategies (resistance and transformation) should be seen as complementary ways to maintain a rich biodiversity in future forest ecosystems. 

La conservación de la biodiversidad en los paisajes forestales gestionados necesita complementarse con estrategias nuevas debido a la amenaza del cambio climático acelerado. La mayoría de los marcos de trabajo para la adaptación de la conservación de la biodiversidad ante el cambio climático incluye dos estrategias principales. La primera es la estrategia de resistencia, la cual se enfoca en acciones para incrementar la capacidad de las especies y comunidades para resistir el cambio. La segunda es la estrategia de transformación e incluye acciones que facilitan la transformación de las comunidades a un conjunto de especies que están bien adaptadas a las nuevas condiciones ambientales. Sugerimos un número de acciones concretas que los gestores y los formuladores de políticas pueden tomar. Bajo la estrategia de resistencia, introducimos cinco herramientas, incluyendo: identificación y protección de los refugios climáticos forestales con especies favorecidas por el frío, reducción de los efectos de la sequía mediante la protección de la red hidrológica y extirpación activa de los competidores cuando amenacen a las especies favorecidas por el frío. Bajo la estrategia de transformación, sugerimos tres herramientas, incluyendo: mejorar las condiciones para las especies forestales favorecidas por el nuevo clima pero actualmente desfavorecidas por la gestión forestal, mediante su siembra en sitios adecuados fuera de su distribución principal e incrementando la conectividad en el paisaje para incrementar la expansión de las especies favorecidas por el calor hacia sitios que se han vuelto más adecuados. Finalmente, sugerimos aplicar una perspectiva de paisaje y gestionar simultáneamente tanto para las especies en retirada y en expansión. Las dos estrategias diferentes (resistencia y transformación) deberían considerarse como maneras complementarias para mantener una biodiversidad rica en los ecosistemas forestales del futuro.

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.

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.

With the current deforestation rates in tropical ecosystems, optimizing biodiversity in managed systems has become fundamental for conservation. Agroforestry has been suggested to conserve biodiversity and buffer deforestation rates, while also sustaining local livelihoods. While many studies have focused on the relation between local management intensity and biodiversity, processes at the landscape scale are often overlooked and remain a knowledge gap. In this study we identified drivers behind woody plant regeneration in coffee agroforestry on both local and landscape scale. We used univariate-, multivariate- and structural equation models to relate seedling species richness, diversity, density, community composition and height to local management intensity and location in the landscape of 60 coffee agroforestry sites in southwestern Ethiopia. Local management intensity, which simplifies and reduces canopy cover, negatively impacted species richness, diversity and density, presumably due to altered microclimatic conditions and a reduction in local seed sources. Seedling height was also reduced by management intensity, including slashing frequency and canopy cover. On the landscape scale, species richness and diversity of seedlings was higher at sites adjacent to continuous forests where seed sources are abundant, and declined with distance to the forest. In particular, late successional species were negatively affected, whereas common shade tree species and pioneers occurred as seedlings throughout the landscape and in more managed systems. This suggests that dispersal limitation is detrimental for the regeneration of late successional species, especially in agroforestry systems where the standing woody plant diversity is largely reduced. Our results indicate that natural regeneration of woody plants still occurs in coffee agroforestry systems, primarily when the canopy structure is dense and diverse and/or when sites are located nearby continuous forests. Management intensification and deforestation will limit the potential for many woody plant species to regenerate in coffee agroforestry sites, by altering the local microclimate, reducing local seed sources and disrupting seed dispersal from the surrounding landscape. This will likely result in a positive feedback loop, as a reduction in woody plant regeneration reduces future seed sources. We therefore stress that both a local and a landscape perspective should be incorporated in conservation and restoration approaches.