Effects of Terrain on Climate and Consequences for Boreal Songbird Distribution and Refugia
Creators: Estevo, Cesar A
Publisher: name: University of Alberta Library
Year: 2024

<p>Local climates have been increasingly recognized in ecological and climatological studies, particularly because anthropogenic climate change poses threats to biodiversity and ecosystems. Local climates exist at scales of meters to up to a few kilometers and are defined by the set of properties that influence atmospheric conditions at a small scale. These may include biotic properties and topography. Topography and vegetation cover can create thermally heterogeneous landscapes that affect ecological processes and can be strong enough that local climatic trends deviate from conditions at larger (meso or synoptic) spatial scales; this has been proposed as one of the key features of climate-change refugia. Climate change refugia are areas relatively buffered from anthropogenic climate change and have played a role in the historic persistence of species. Identification of these refugia is therefore of considerable importance in light of contemporary climate change. Areas with high refugia potential may warrant greater protection from other sources of human-induced change for conservation purposes (e.g., land-use change, increased wildfire activity). In the western boreal forest, the boreal plains to the east of the Rocky Mountains are marked by limited relief, suggesting limited thermal heterogeneity and refugia potential on the landscape. Despite that, local climate conditions in hills and valley systems favored the persistence of boreal forest in otherwise unfavourable climate for millennia, indicating the potential for refugia at small scales. For forest-associated wildlife, such as boreal songbirds, climate-change refugia is related to the composition and structure of the forests the species inhabit, but also the role that vegetation cover plays in regulating climate at the local scale. Broad-scale bioclimatic models have been used to identify changes in species distribution due to climate change and refugia, but they use coarse resolution or “macroclimate” data that ignore the effects of topoedaphic and vegetation on local climate. Downscaling of macroclimate, which consists of increasing the resolution of climate grids, may be essential in overcoming shortfalls that arise from that and help quantify refugia. Statistical downscaling correlates in-situ climate measurements (e.g., subcanopy temperature) with remotely sensed information and topoclimatic metrics and could be a way forward to identify refugia potential driven by local climate across the limited relief of the boreal plains. I sought to determine the degree to which climate varies at small scales (meters to up to a few kilometers) in the western boreal plains and the impacts that local climate has on species distribution and refugia. First, I examined the impact that topography and vegetation cover have on the local climate by sampling climate conditions with temperature dataloggers across a series of hill and river valley systems for several ecoregions in Alberta. I found that summer maximum temperatures (Tmax) varied nearly 6 °C across the elevation gradient sampled. Local summer mean (Tmean) and maximum (Tmax) temperatures in slopes with low levels of potential solar radiation were up to 0.70 °C and 2.90 °C cooler than highly exposed areas, respectively. Tmax in incised valleys was between 0.26 and 0.28 °C cooler than other landforms, while areas with greater terrain roughness experienced maximum temperatures that were up to 1.62 °C cooler. I also found that forest cover buffered temperatures locally, with coniferous and mixedwood forests decreasing summer Tmean from 0.23 to 0.72°C, and increasing winter temperature (Tmin) by up to 2 °C. I then applied these results to develop climate offsets to correct and adjust long-term climate predictions for Alberta for local terrain effects (i.e., adjusted climate). I then built four models for the occurrence of 48 songbird species, grouped by their nesting habitat preference, representing all combinations of adjusted vs. unadjusted climate (broad-scale) and climate-only vs. climate + vegetation variables (full model). I applied these models to predict the future distribution of songbirds under a moderate climate scenario to quantify refugia. We observed that the explanatory power of models improved using adjusted vs unadjusted climate for 5°°C) with occurrence of spruce (Picea spp.) trees, 3-4 times larger than the effect of summer precipitation, while Tmin had consistently negative effects across tree species. I also found that bird occurrence was primarily driven by vegetation characteristics than climate. Coniferous- and wetland-associated species were particularly influenced by spruce biomass. The influence of climate was mostly direct, rather than indirectly mediated through vegetation. For deciduous-associated bird species, vegetation characteristics were more limiting than climate. Relative biomass of spruce trees were the strongest mediators of the indirect effects of climate. Our results suggest that climate change impacts on boreal birds may manifest via direct effects on their resources rather than physiology as well as via indirect effects on vegetation. I conclude that coniferous- and wetland-associated species may be particularly at risk to habitat loss because they may lack the ability to adapt to climate change and be more impacted by climate directly. Coniferous- and wetland-associated species may also be especially at risk because key tree species, such as spruce, may be replaced by fast-growing deciduous trees in the future. I conclude that identification of refugia potential at smaller scales may be crucial for coniferous- and wetland-associated species and may indicate that areas with highest refugia potential could be prioritized for conservation, such as the creation of protected areas. </p>