Complex interactions between dwarf mistletoe, fuel loading, and fire in the lodgepole pine dominated forests of central Colorado

Abstract

Lodgepole pine dwarf mistletoe (Arceuthobium americanum Nutt. ex Engelm) is an obligate hemiparsite that infects lodgepole pine (Pinus contorta Dougl. Ex. Loud) throughout the large majority of lodgepole pine’s range. Lodgepole pine dwarf mistletoe increases mortality rates, alters tree biomass distributions, and slows overall tree growth, which results in substantial losses to stand productivity and wood quality. In lodgepole pine dominated forests, dwarf mistletoe and wildfire are fundamental disturbances that may interact with each other in complex ways. This interaction is bidirectional as wildfire can either positively or negatively affect post-fire dwarf mistletoe populations, and pre-fire dwarf mistletoe populations may influence wildfire severity. Though it has long been assumed that dwarf mistletoe increases potential wildfire severity in lodgepole pine forests through modifications to the fuels complex, empirical data to support this conclusion is lacking. The overall goal of this project was to enhance the understanding of both sides of the fire-dwarf mistletoe interaction through a combination of long-term post-fire data, forest measurements, and simulation of dwarf mistletoe impacts and intensification. Chapter one provides background into dwarf mistletoe biology and pathology, and reviews the existing literature on interactions between fire and dwarf mistletoe. The second chapter documents the results of research into the influence of dwarf mistletoe infestation level on stand structure and fuel parameters that influence potential fire behavior. To evaluate the relationship between infestation severity and stand structure and forest fuels plots were randomly located within stands containing a range of dwarf mistletoe infestation severities. Of primary interest were impacts to canopy base height and the loading of fuels both on the forest floor and in the canopy. Chapter three is a case study documenting the impact of three prescribed crown fires on dwarf mistletoe populations thirty years post-fire. These fires burned across a range of mortality levels allowing for a detailed evaluation of the influence of fire severity on dwarf mistletoe populations. This chapter combines field measurements with forest growth and yield simulations from the United States Forest Service’s Forest Vegetation Simulator to understand longer-term impacts to both the dwarf mistletoe population and stand productivity. Field data from randomly located plots indicate that dwarf mistletoe may have conflicting impacts on parameters influencing crown fire potential and wildfire severity. This finding suggests that the impact of dwarf mistletoe infestation of potential wildfire severity may not conform to the positive linear relationship assumed by many forest pathologists. Infestation level was found to have a strong positive relationship with the loading of surface fuels of all sizes, and was negatively related to canopy base height, and calculated canopy fuel load and canopy bulk density. Impacts to stand structure include significant reductions to live basal area and average tree size, and significant increases to the density and basal area of standing dead trees. The results from the long-term post-fire data set provide experimental evidence showing that fire severity negatively influences future dwarf mistletoe populations, and that long-term population reductions are possible without complete stand replacement. Over multiple fire cycles, feedbacks between fire and dwarf mistletoe may enhance heterogeneity in burn patterns, infestation severity, and stand structures across the landscape.