Since the late 1990s severe mountain pine beetle outbreak (MPB; Dendroctonus ponderosae) and wildland fires have disturbed large expanses of northern Colorado's subalpine forests. The primary MPB host in the region, lodgepole pine (Pinus contorta var. latifolia), is considered a classic fire-adapted species due to its serotinous cone habit (cones which open in response to heat) and ability to establish rapidly following stand replacing fires. However, there is a substantial knowledge gap in how serotinous cones and lodgepole seedling recruitment respond to non-fire related disturbances (MPB epidemics), and how this response may further influence tree regeneration following future fires.
Doctoral student Teresa Chapman, under the supervision of Professor Thomas Veblen in the Department of Geography at the University of Colorado Boulder examines how conditioning factors (e.g. site conditions, stand structure, seed availability and climate variability) determine the response of lodgepole pine regeneration to recent disturbance by MPB and fire, and how the current patterns of lodgepole pine regeneration following MPB outbreak may affect forest ecosystem responses to future fire. These questions are answered through a series of four objectives at multiple scales: 1) to compare lodgepole pine regeneration at sites either affected by severe fire in 2002 or by the MPB epidemic since 1996, 2) to investigate through field experimentation the reaction of serotinous cones and lodgepole pine germination to variable litter depth and canopy conditions, 3) to conduct broad scale sampling of post-MPB sites across northern Colorado to generate predictive maps of both regeneration patterns and lodgepole serotiny, and 4) to synthesize the empirically generated results through both the implementation of a stand scale predictive model of tree regeneration and a spatial overlay analysis of current MPB effects and their predicted consequences following future fires. The underlying hypothesis of the research is that current patterns of regeneration following the extensive MPB outbreak could lead to less fire resilient subalpine ecosystems.
Understanding the ecological consequences of lodgepole pine tree mortality due to the current MPB outbreak affecting over one million hectares is a major concern among the general public and land managers in the Rocky Mountain region. Basic knowledge of the natural regeneration dynamics of lodgepole pine following severe, infrequent disturbance is essential for both short- and long-term predictions of subalpine ecosystem dynamics. The empirical data generated by this research will provide land managers with an initial basis for evaluating the potential for shifts in forest composition due to the interactions of the MPB outbreak with future fires. The immense extent and severity of both the MPB epidemic and 2002 fires provide a rare, natural experiment to study the regeneration response to such large, infrequent mortality events which may have important impacts on tree species distributions and regional-scale carbon budgets. This research will provide numerous undergraduates at the University of Colorado with field and laboratory research experience and with opportunities to develop senior honors theses.
Both fire and mountain pine beetle (MPB) outbreaks are important disturbances, capable of causing wide spread tree mortality in Rocky Mountain subalpine forests. Basic knowledge of the regeneration dynamics of lodgepole pine following severe, infrequent disturbance is essential for both short- and long-term predictions of subalpine ecosystem patterns. The focus of this dissertation research was to generate a greater understanding of the natural regeneration dynamics of lodgepole pine (Pinus contorta var. latifolia), the primary MPB host species, following recent MPB outbreak and wildfires in subalpine forests in northern Colorado. The impetus for the proposed dissertation research was twofold: 1) the substantial knowledge gap in the extent to which fire-adapted lodgepole pine regenerates following a non-fire related disturbance (i.e. MPB outbreak), and 2) the potentially dominant influence of recent warm and dry weather conditions (drought) on successful conifer seedling establishment following beetle outbreak and, especially, following fire. This dissertation research quantified and compared lodgepole pine regeneration following current (i.e.post-1996) MPB outbreak and recent (2002-2005) wildfires across a broad range of lodgepole pine dominant forests in northern Colorado. Regeneration response of lodgepole pine was quantified in a paired plot sampling design at six sites with recent fires or recent MPB outbreak, resulting in five to seven paired sampling stands per site for 2002 fires and two paired sampling stands for 2005 fires. Data were collected on forest structure, tree seedling attributes (densities, sizes, ages), forest floor attributes (litter, ground cover), and tree serotiny (i.e. trees with closed cones). Initial results show that post-fire lodgepole pine regeneration is significantly positively influenced by amount of fine litter on the forest floor eight years after the 2002 fires and negatively influenced by amount of bare mineral soil eight years after the 2002 fires. Density of pre-fire serotinous trees was not a significant predictor of lodgepole pine post-fire establishment. Following MPB attack, subalpine fir was the most abundant regenerating tree species. Lodgepole pine regeneration following MPB outbreak was negatively influenced by litter depth and positively influenced by canopy openness. Following MPB outbreak, in mixed stands of lodgepole pine and quaking aspen (Populus tremuloides), regeneration of the latter species was abundant. Post-fire lodgepole pine regeneration patterns were compared for fires that occurred during the relatively cool-moist period of 1952-1994 with fires that occurred during the warm-dry period of 2002-2005. Preliminary results suggest that recent (2002-2005) post-fire regeneration of lodgepole pine is less abundant than post-fire regeneration during 1952-1994. In addition to the comparisons of lodgepole pine regeneration following fire and MPB outbreak, a field experiment was conducted to determine which factors control successful germination and establishment of lodgepole pine. The capacity of serotinous cones to open on the forest floor and the seeds to subsequently germinate were determined under various seedbed and canopy conditions. The findings indicate that serotinous cones from beetle attacked trees and live trees open and release seeds on the forest floor under open canopy conditions (> 65% open) and under all litter depths (bare ground, 2cm litter, 4cm litter). The observation that serotinous cones open and release seeds even in the absence of fire (the typical mechanism by which closed cones are opened) indicates some potential of lodgepole pine to regenerate following MPB outbreak even when most trees are serotinous. However, unprotected cones were heavily predated by squirrels (> 80%) and few seeds or seedlings survived. The broader-scale implications of the field-based findings of the research will be assessed through the use of remote sensing and GIS analyses to both estimate MPB mortality and tree regeneration patterns across the subalpine zone of Colorado in relation to biophysical predictors. Patterns of MPB mortality were related to biophysical predictors (interannual climate variability, forest stand attributes, topographic variables) across this extensive region in the Chapman et al. 2012 publication. Thirty additional post-disturbance sites (in addition to those sampled for the objectives described above) have been sampled for tree regeneration to fully represent the range of variability in the lodgepole pine cover type to support a spatial analysis of the biophysical predictors of post-MPB tree regeneration. Data analyses and writing of manuscripts are ongoing. The broader impacts of the research have included contributions to undergraduate education through the involvement of 15 undergraduate research assistants in the project and the support of four honors theses. Broader impacts in the form of public outreach included video interviews for the University of Colorado’s LearnMoreAboutClimate outreach website and numerous public lectures to land managers and environmental organizations.
