Current and future distributions of four tree species in the Northern Rocky Mountains (USA): Geographic shifts with respect to geology, soils, and land management status

Background/Question/Methods

Climate-based predictions of species distributions provide evidence to understand impacts of climate change. However, such models lack inclusion of other ecological (disturbance) or geo-edaphic factors that govern plant species distributions. Additionally, predicted geographic shifts fail to incorporate considerations of management zones where certain activities could facilitate or impede responses to climate change. For instance, assisted migration to overcome dispersal barriers could be challenged in areas designated for “untrammeled” character (e.g., Wilderness). Alternatively, large tracts of land with uninterrupted disturbance regimes may offer opportunities for disturbance-facilitated establishment of species into new areas. We focused on current and future distributions of four trees across Montana: whitebark pine (Pinus albicaulis), a keystone species of timberline; western larch (Larix occidentalis), a fire-dependent, deciduous conifer; quaking aspen (Populus tremuloides), a disturbance-dependent, broadleaved tree; and ponderosa pine (Pinus ponderosa), a seral species sustained by low intensity fires. Current and future “climate space” of species distributions were overlaid onto geology and soils data to investigate relationships between trees and geo-edaphic conditions. Similarly, we mapped current and predicted extent to determine possible changes in composition of land management zone. We also used different thresholds of predicted presence to assess the sensitivity of patterns to varying levels of confidence.

Results/Conclusions

Geographic distribution of viable “climate space” tended to contract for whitebark pine (88% reduction), western larch (-37%), and ponderosa pine (-40%) by 2060 based on an inclusive estimate of species presence. The most inclusive estimate of range size for aspen predicted an expansion of area by 19% in Montana, but a more conservative estimate predicted a 14% decline. Composition of soils from different parent materials varied little for western larch, and more so for whitebark pine, aspen, and ponderosa pine with dominant underlying parent materials changing in rank abundance from current to predicted distributions. The distribution of species with respect to management zones may also shift with climate change. The percentage of ranges for whitebark pine, western larch, aspen, and ponderosa pine occurring within Wilderness boundaries increased from 28 to 61%, 5 to 15%, 6 to 13%, and 1 to 6%, respectively. Predicted geographic shifts of trees due to climate change may result in new soil conditions with subsequent novel plant-soil feedbacks. Additionally, the percent of a species’ range occurring in Wilderness is expected to more than double. Climate adaptation strategies need to consider the influence of geo-edaphic conditions and management zones as species shift in response to climate change.