Climatic and hydrologic influences on tree regeneration and distribution in the western United States.
Though rapid climate change is widely recognized as a major threat to ecosystem stability, there is little agreement about how resilient species and communities will be to future conditions. The consequences of climate changes will be particularly pronounced for dry-region ecosystems such as forests of the western US due to strong elevation and climatic gradients. Predicting patterns of tree species regeneration will be central to understanding the impact of climate change on western forests. This research will examine the climatic and hydrologic limitations on tree reproduction, and ultimately the limitations on tree distributions, in the western US by developing a statistical framework for assimilating diverse, publicly-available datasets across space and time.
The Fellow will be mentored by Dr. William Lauenroth, an expert in semiarid plant community dynamics, and Dr. Andrew Finely, an ecologist and statistical modeler. This project stresses the interdisciplinary nature needed to address pressing issues such as the influence of global change on species distribution and the development of methods for utilizing extensive, publicly-available datasets. The research benefits society by identifying tree species with the greatest risk of local and regional population decline as well as highlighting geographic areas where tree regeneration is likely to fail under future predicted climate regimes. This information will help guide land managers and policymakers in managing forests on public and private lands across the western US.
Predicting potential changes in forest ecosystems is a major challenge for ecological research, requiring both extensive data and sophisticated statistical techniques. In particular, demographic processes, such as tree regeneration or death, contribute to shifting patterns of forest type (e.g., replacement of drought intolerant trees by drought-tolerant trees as climates become drier) or loss of forest cover altogether. These changes then impact our ability to manage for a variety of ecosystem services, from timber and carbon sequestration to recreation. This research used large, publicly available data sets from the US Forest Service Forest Inventory and Analysis (FIA) program and advanced statistical modeling techniques to assess tree demographic responses to recent and predicted climate change in the western United States. Here, some of the outcomes of this work are briefly summarized as they relate to ecological change and statistical modeling. The results of this research highlight important ecological advances associated with regional forest change in the western United States. Examination of the climatic controls of adult and juvenile trees indicated that high-elevation, compared to low-elevation, tree species are more at risk under future climate change. Firstly, seedlings of high elevation tree species were not consistently found at higher elevations compared to adult trees as might be expected if they were migrating upslope in response to recent climate change. Secondly, the results indicated that predicted climate change during the 21st century would likely reduce the climatic suitability for high-elevation tree species both where species occur currently and in the landscapes to which they might be able to migrate in the future. The lack of a contemporary response of tree recruitment to recent climate change and the predicted need to migrate great distances under future climate change may hint at future regional recruitment failures in lodgepole pine and spruce-fir forests of the Southern Rocky Mountains. In addition to recruitment failures, large mortality events are becoming increasingly common and are often related to drought. In the case of quaking aspen, our research highlighted the potential for forests to vary in their resilience to climate change. Specifically, forest stand structure (e.g., number of trees per acre, tree sizes, etc.), productivity (i.e., growth), and age affected mortality responses to climate. Because forest structure seemed to impact how aspen mortality responded to climate, forest management strategies might be developed to maximize aspen resilience in the face of change. This research also examined some of the methodological limitations of existing tools for assessing forest change. The development of new models and the assessment of existing tools improves our understanding of the strengths and weaknesses of predictions of forest change. For example, species distribution models (i.e., a statistical/correlative framework for relating species presence and absence to some set of environmental conditions) are commonly used to understand current geography of forests as well as predict forest geographies under some set of future conditions, but are limited in a variety of ways. This research (1) developed new species distribution models that incorporated regional differences in responses to different environmental variables and (2) assessed the ability of different models to predict current and future species occurrences. By improving and assessing methodologies, our research helped us to interpret ecological results of similar studies and improve predictions communicated to the scientific community, forest managers, and the public. This research provides useful ecological insights for forest managers, helped in the training of ecologists and computer scientists, and contributed to the development of both data products and analytical tools. To aid forest managers in the western United States, it is our hope that specific results from our research can be incorporated into forest management planning by helping to identify species and/or regions at the greatest risk of experiencing changes in tree recruitment and mortality patterns during the 21st century. We have worked to communicate our results at scientific venues, public forestry seminar series, and in personal communications with members of the public. Furthermore, the fellow mentored several graduate students at the University of Wyoming. As a result of this research, maps of climatic suitability for four major tree species in the western United States were developed and have been made available to other scientists. Finally, this research also helped to fund the development of new versions of an existing hydrologic model.
