Periodic eruptions of bark beetle populations cause widespread tree mortality, which affects stand structure and can create a lasting ecological legacy. The current (post-1995) mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak in ponderosa pine (Pinus ponderosa) forests of northern Colorado is spatially heterogeneous and appears to be influenced by numerous factors operating across a range of spatial and temporal scales. Researchers do not fully understand biotic and abiotic constraints of MPB-caused mortality in ponderosa pine as well as how those constraints change as an outbreak advances over time and space. This doctoral dissertation research project will address how biotic (tree and vegetation attributes) and abiotic (terrain) factors have affected the spread of recent MPB activity in ponderosa pine in the montane zone of the eastern slope of the Colorado Front Range at the tree, stand, and landscape scales. It will examine how previous fires affect the probability of occurrence and severity of MPB activity in ponderosa pine stands in a heterogeneous landscape of different community types and vegetation structures affected by a complex disturbance history. The doctoral student will collect field data from 20 sites nested within areas previously sampled for fire history (tree-ring and fire-scar data), and he will analyze these and other data to elucidate spatio-temporal influences of biotic and abiotic factors affecting ponderosa pine susceptibility to MPB at the tree scale (e.g. tree diameter) and stand scale (e.g. adjacent thinning, fire history). High-resolution satellite imagery will be used to analyze landscape-scale spatio-temporal patterns of MPB-caused tree mortality. The student will use logistic regression or weights of evidence to produce predictive models of MPB infestation at an early (2002) and a more advanced (2008) stage of the current outbreak for the central and northern Colorado Front Range. He hypothesizes that as an outbreak advances, the tree and stand-level constraints on susceptibility of ponderosa pine to MPB will become less important. Another hypothetical relationship to be explored is that that increased time since the last severe fire will elevate the susceptibility of ponderosa pine to MPB attack, because previous studies have identified tree density and size as well as the time since the last severe fire as determinants of ponderosa pine susceptibility to MPB infestation.
This project's multi-scale research approach will analyze the complex temporal and spatial variation of biotic and abiotic drivers of MPB infestation, and it will provide new information and insights regarding the effects of previous fires on MPB risk enhancing our knowledge of disturbance interactions in Colorado's forests. Understanding such non-linear, cross-scale interactions is essential to predict the spatio-temporal dynamics of forests under changing climate conditions. In Colorado's Front Range, increased exurban development into the wildland urban interface has highlighted the importance of understanding forest dynamics in areas surrounding rural communities. There currently is great public concern over MPB-caused tree mortality and its potential effects on fire hazard. This project will inform land managers and owners about areas of high risk to MPB infestation and will aid in maximizing the effectiveness of fire and MPB management policies. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.
The current mountain pine beetle (MPB) outbreak in ponderosa pine forests of northern Colorado is spatially heterogeneous and appears to be influenced by numerous factors operating across a range of spatial and temporal scales. For example, previous disturbance by fire creates spatially heterogeneous stand conditions that may affect the distribution and severity of subsequent beetle outbreaks. The research project addressed the susceptibility of ponderosa pine to mountain pine beetle infestation across a range of spatial scales in the Colorado Front Range by integrating spatial analyses of tree mortality patterns, vegetation and terrain variables, field data collection, and high-resolution aerial imagery. Two main research questions were investigated. Preliminary results are presented below, however, processing of field data is ongoing so not all sites were included in the presented analysis and results should be considered preliminary. How have biotic (tree and vegetation attributes) and abiotic (terrain) factors affected the spread of recent MPB activity in ponderosa pine in the montane zone of the eastern slope of the Colorado Front Range at the tree, stand, and landscape scales? To investigate susceptibility of ponderosa pine to MPB at tree and stand scales, we sampled 20 paired plots (one subplot in an area with MPB activity and one without MPB activity). Data on tree characteristics (e.g. species, tree age, growth rates, diameter at breast height, position in canopy, etc.) and stand structure (tree density, species composition, etc.) were collected and compared. Preliminary results indicate MPB preferentially attacked trees with larger diameter (trees in the 30-35 cm size class had greatest proportion of infestation) which may be explained by the benefit of thick bark and abundant cambium for larval survival. Stands with higher tree densities do not show greater susceptibility to MPB infestation. High rates of infestation of MPB in ponderosa pine when coexisting lodgepole pine have been previously attacked by MPB indicate there may be spillover of MPB into ponderosa pine stands from the outbreak in the lodgepole pine. To investigate MPB activity at the landscape scale, we developed a method to identify tree mortality from high resolution (1 m) aerial imagery. Using this technique we created maps of MPB-caused tree mortality in the landscape which are being used to investigate the landscape-sale influences on the mortality (e.g. elevation, slope, aspect, etc.). We will compare the maps of MPB-caused mortality from earlier in the outbreak (2002) and later in the outbreak (2011) to examine how terrain (e.g. aspect, elevation) and vegetation constraints may change as the outbreak amplifies. In a landscape that is a mosaic of vegetation affected by a complex disturbance history, how do previous fires affect the probability of occurrence and severity of MPB activity in ponderosa pine stands? The 20 paired plots were sampled within areas previously sampled for fire history. We are investigating how the time since the last stand-replacing or surface fire will affect the susceptibility of the forests to MPB. We hypothesize that stands that originated after stand-replacing fires over 120 years ago are more susceptible to MPB infestation than younger stands. This analysis is ongoing.
