Doctoral Dissertation Research: Spatio-Temporal Variability of Ponderosa Pine Susceptibility to Mountain Pine Beetle in the Colorado Front Range

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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.