Mapping Lichen in Caribou Ranges

Reindeer lichens are an important part of caribou diets.

Lichens, a fascinating group of species in which fungi and algae grow together, have been declining in arctic tundra and northern alpine areas due to effects related to climate warming, including shrub expansion and increased fire frequency. Lichens are important forage for caribou (Rangifer tarandus), whose populations are declining throughout most of North America. To better understand how caribou use and select for lichens, biologists need better maps that show the distribution and abundance of lichens. A large collaborative research group, including botanists and biologists from the NPS, used a combination of field data and satellite imagery to model lichen cover over a huge area (583,000 km²) of interior Alaska and the Yukon, including the ranges of 9 caribou herds. The best model was used to create a lichen cover map of the area and was tested against existing data. The researchers then used the map to test the importance of lichen cover on Fortymile Herd caribou distribution from 2012 to 2018, during summer and winter. In both seasons, caribou avoided areas with little (0-5%) lichen cover and showed stronger selection as lichen cover increased to ∼30%. The results suggest that ground lichen cover is an important factor influencing caribou selection in northern boreal forests across seasons. This new lichen cover map goes beyond existing maps because it was based upon extensive field data and covers a much larger area. Large-scale maps, such as this one, should be useful for understanding trends in lichen abundance and distribution, as well as for caribou research, management, and conservation.

Lichen cover mapping for caribou ranges in interior Alaska and Yukon

Abstract

Previous research indicates that the effects of climate warming, including shrub expansion and increased fire frequency may lead to declining lichen abundance in arctic tundra and northern alpine areas. Lichens are important forage for caribou (Rangifer tarandus), whose populations are declining throughout most of North America. To clarify how lichen cover might affect caribou resource selection, ecologists require better data on the spatial distribution and abundance of lichen. Here, we use a combination of field data and satellite imagery to model lichen cover for a 583 200 km² area that fully encompasses nine caribou ranges in interior Alaska and Yukon. We aggregated data from in situ vegetation plots, aerial survey polygons and unmanned aerial vehicle (UAV) imagery to align with 30 m resolution Landsat pixels. We used these data to train a random forest model with a suite of environmental and spectral predictors to estimate lichen cover. We validated our lichen cover model using reserved training data and existing external datasets, and found that reserved data from aerial survey polygons (R ² = 0.77) and UAV imagery (R ² = 0.71) provided the best fit. We used our lichen cover map to evaluate the influence of estimated lichen cover on caribou resource selection in the Fortymile Herd from 2012 to 2018 during summer and winter. In both seasons, caribou avoided lichen-poor areas (0%–5% lichen cover) and showed stronger selection as lichen cover increased to ~30%, above which selection leveled off. Our results suggest that terrestrial lichen cover is an important factor influencing caribou resource selection in northern boreal forests across seasons. Our lichen cover map goes beyond existing maps of lichen abundance and distribution because it incorporates extensive field data for model training and validation and estimates lichen cover over a much larger spatial extent. We expect our landscape-scale map will be useful for understanding trends in lichen abundance and distribution, as well as for caribou research, management, and conservation.

Macander, M. J., E. C. Palm, G. V. Frost, J. D. Herriges, P. R. Nelson, C. Roland, K. L. M. Russell, M. J. Suitor, T. W. Bentzen, K. Joly, S. J. Goetz, and M. Hebblewhite. 2020. Lichen cover mapping for caribou ranges in interior Alaska and Yukon. Environmental Research Letters 15: 055001