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- Christiansen, Ditte Marie, 1990-
(författare)
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Responses of boreal forest understory plant communities to climate and forestry
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A warming climate is altering species distributions and community compositions. To understand and predict changes in species distributions to climate change, we often use species occurrences together with large-scale regional climate data. This can be problematic for several reasons. Species living near the ground experience small-scale spatial variation in temperatures, i.e., microclimate, that are influenced by topography and vegetation and can therefore deviate a lot from regional temperatures. Further, climate often affects species indirectly via species interactions, and such interactions can also change with climate. And last, species may respond slower than climate changes. Ignoring these aspects can complicate our understanding of species-climate relationships.In this thesis, I examined how microclimate and changes in microclimate due to forest management impact performances, interactions, and distributions of plant species in boreal forest understory communities. First, I quantified the importance of microclimate for species performances and distributions. Specifically, I compared the effects of spring temperatures measured on local and regional scales on the population dynamics of a southern forest herb (I). I also tested how small-scale spatial microclimate variation contributed to the regional co-existence of northern and southern understory plant species (II). Second, I examined the role of species interactions in driving abundance patterns of two moss species with different temperature niches across their Swedish ranges by transplanting them separately and together across a climate gradient (III). Lastly, I investigated how understory plant communities respond to changes in microclimate caused by forest management (IV), and how past microclimates influence current patterns of species occurrence, abundance, and reproduction (II).I found that local spring temperatures had a significant effect on the population dynamics of the southern forest herb that could not be detected using regional spring temperatures (I). Spatial variation in microclimate explained the regional co-existence of two northern and two southern species, where the northern species were favoured by cold microclimates and the southern species by warm microclimates (II). In the transplant experiment (III), I found that climate-mediated competition can override the direct effects of climate and limit abundances across ranges. Lastly, I found that microclimate changes caused by forest management activities had a large effect on understory communities (IV), and that current abundances of northern and southern species were partly explained by past microclimate (II).Overall, I demonstrated that, to understand how species (particularly understory plants) respond to climate, we need to replace the standard use of regional climate data with locally measured climate data or down-scaled gridded climate data that account for variation in topography as well as vegetation. To predict how species will respond to climate change, we also need to include species interactions and how these interactions change with a changing climate. Finally, changes in microclimate following changes in forest structure have large effects on understory species. The last finding is important to consider when studying changes in understory communities in a climate context and could be used to mitigate climate effects on forest biodiversity.
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| 2. |
- Greiser, Caroline, 1987-
(författare)
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Microclimate at range margins : Consequences for boreal forest understory species
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A warmer climate will shift species distributional range margins poleward, but near-ground microclimates may modify these shifts. Cold-adapted northern species at their rear edge may survive locally in microrefugia with a colder microclimate, and warm-adapted southern species at their leading edge may colonize stepping stone habitats with a warmer microclimate. However, we do not always know if species ranges are limited by climate and which role microclimate variation plays in modifying range margins. This is especially true for lowland forests, where forest structure and composition have relatively large influences on near-ground microclimates.In this thesis, I explored patterns and drivers of forest microclimate at the southern margin of the boreal zone in central Sweden, where many northern and southern species meet. First, I measured, modelled and mapped near-ground temperatures across ca. 20 000 km2 of forested land (Paper I). Second, I tested if cold and warm microclimates favour northern and southern understory species, respectively. To answer this, I investigated the occurrence and performance patterns of understory vascular plants, bryophytes and lichens across microclimate gradients at the species’ northern or southern range margins (Paper II-IV). I performed both correlational analyses on natural populations and experimental testing with transplanted populations. Third, I derived recommendations and tools for biodiversity conservation and forest management (Paper I-IV).I found high spatial and temporal variation of forest microclimate, which was in the summer mainly linked to differences in forest density and in the cold season to terrain effects (Paper I). Cold and warm microclimates were occupied by natural edge populations of northern and southern species, respectively (Paper II and IV). However, in the transplant experiments with removed competition other factors were more important for the species performance. The southern herb appeared to cope well with the range of microclimate at its current northern range margin and instead seems to be limited by soil and light in northern conifer-dominated forests (Paper IV). The northern transplanted bryophytes and lichens showed no or a positive response to warmer temperature, but also to higher moisture, to more conifers in the overstory and to less gastropod grazing (Paper III). The results indicate that competition with southern species, herbivory, leaf litter and water scarcity might be more important than temperature as direct limiting factors at the species’ current southern range margin. To conclude, microclimate influences the occurrence and performance of range edge populations, but it likely does so indirectly via effects on water availability and biotic interactions.Forest management heavily modifies near-ground temperature and humidity and hence likely impacts the climate-driven range shifts of understory species. I call for considering these effects in conservation and management actions, e.g. by protecting valuable microclimates, moving from clear-cutting to selective logging, reducing forest fragmentation and drainage and favouring either broad-leaved or coniferous trees in the overstory - depending on the local conservation target (Paper I-IV). Climate-change induced biodiversity loss may thus be slowed down by responsible forest management that provides stepping stone habitats for advancing southern species as well as microrefugia for retreating northern species.
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