Past Forest Response to Climate Change Driven by Soils and Local Climate

pastforest-map
Figure 1. The study area – Wisconsin’s Northwest Sands Ecological Landscape (purple) – in relation to nearby US national parks.
A new paper (Tweiten et al. 2015 [PDF]) shows that soils and local climate can strongly influence how forests respond to climate change. This page briefly summarizes the findings, which are part of ongoing work of the National Park Service Climate Change Response Program and collaborators to support park adaptation to changing conditions. Consult the full project brief [PDF] for more in-depth information.

Background
Climate change impacts to North American forests are already evident, and act alongside – and sometimes amplify – impacts from nonnative pests and disease, pollution, and habitat fragmentation. Stewarding natural resources in an era of continuous change will require a long-term, strategic view that adapts management goals and approaches to the effects of both shifting climate baselines and increased variability. Better understanding of how local factors at the management-unit level influence climate change impacts can inform stewardship decisions.

Paleoecological studies based on pollen and charcoal records from lake-sediment cores can be used to reconstruct past responses to climate variability, and suggest where and how to focus monitoring, management, and research. This study characterized soils, modern climate, and differences in past fire regime around 12 lakes in northwestern Wisconsin (see Figure 1) to determine whether observed landscape patterns in geophysical factors or local climate correspond to differences in past variability in forest composition.

Results

Local factors – specifically soils and local climate – strongly influenced forest response to past climate change, and these influences were clearly evident despite the fact that study sites all occur within a small area on a relatively homogeneous glacial outwash landscape. Sites with finer-textured soils and higher water-holding capacity experienced greater long-term (century-to-century) forest community change, whereas forests on sites with poorer soils (coarser texture and lower water- and nutrient-holding capacity) changed much less overall and were more consistently dominated by a single forest type (see Figures 2 & 3).

pastforest-fig2
Figure 2: Vegetation dynamics over the past two millennia differed consistently between sites with different soil types. Click the image for a full-size graphic and caption.
Soils and local climate also strongly influenced forest response to the warm/dry-to-cool/moist climatic transition 750 years ago from the Medieval Climate Anomaly to the Little Ice Age. A landscape-wide increase in white pine during the Little Ice Age was most pronounced on sites with soils of greater water-holding capacity and on sites further from Lake Superior with a warmer, drier local climate, suggesting that a moisture-related establishment threshold for white pine may have been crossed at these sites during the Little Ice Age.

Similarly, fire frequency and intensity were influenced by local geophysical factors. Fires became generally more frequent and less intense during the Little Ice Age. Fire frequency increased most on sites with lower water-holding capacity, where they were associated with rapid pollen assemblage shifts over a few decades and high rates of short-term (decadal to century) forest composition change within the same overall jack pine-dominated forest type.

Implications - Adapting to Change

This study of historical change shows that soil attributes and local climate shape forest response to climate changes and disturbance at very local scales, even within a sandy glacial outwash plain with limited soil variation. The prominence of these relationships on a relatively geologically homogenous landscape suggests that this understanding may be relevant to resource management on a broad range of landscapes.

Past Forest Response Figure 3
Figure 3: Forest compositional variability at two contrasting sites over the past two millennia; samples plotted closer together are more similar in pollen assemble, and samples from a site that are adjacent in time are linked with a solid line. Click the image for a full-size graphic and caption.
Results also show that system responses can be counterintuitive. In the case of the sand plain, for example, the most xeric, high-disturbance sites might be expected to change in response to a changing climate, but the paleo-record reveals much greater overall change on less xeric sites during the Little Ice Age.

Observed relationships between specific soil attributes and past changes in forest composition may not hold under the unique conditions of future climate, but these long-term observations show that soil differences are important influences on vegetation response to climate change. These findings support the call to consider the full range of biophysical features when planning for climate adaptation across regions, and suggest that managers tailor monitoring and management to the full range of soil and geophysical features at the landscape or management-unit level. A manager or management partnership, for example, could encourage stratification of forest research and monitoring across soil types to develop consistent baseline and trend data. This understanding is important because the effect of alternative management regimes and climate adaptation efforts on different soil types may be the most significant predictor of future forest composition changes.

More Information

This project is part of ongoing work of the National Park Service Climate Change Response Program and collaborators to support park adaptation to changing conditions. You can download the full publication: Tweiten, M.A., Calcote, R.R., Lynch, E.A., Hotchkiss, S.C., and Schuurman, G.W. 2015. Geophysical features influence the climate change sensitivity of northern Wisconsin pine and oak forests. Ecological Applications 25:1984-1996 [PDF].

Last updated: October 16, 2015

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