Assessment of the Current Condition and Future Vulnerability of Sugar Pine (Pinus lambertiana) in the Sierra Nevada: Evaluating the Drivers of Change in a Declining Species

Battles, John; Foster, Daniel

doi:10.36967/2317028

Sugar pine (Pinus lambertiana) is the largest Pinus species, an important timber species, and a component of several dry conifer forest types of western North America, in particular the extensive Sierra Nevada mixed conifer forest in California. The species faces several challenges in the Anthropocene, including a disrupted fire regime, an invasive pathogen, forest structure changes, and drought with ensuing bark beetle epidemics. Managers are concerned about the conservation outlook for sugar pine, but it is unclear where and how to best invest conservation resources. In this study, we used data from the USDA Forest Service’s Forest Inventory and Analysis dataset to assess how these various stressors shape the vital rates (survival, growth, and fecundity) of sugar pine across the vast majority of its range. We augmented this regionwide perspective with long-term (30+ years) trends of sugar pine composition, structure, and population dynamics at three research sites in the Sierra Nevada. We synthesized the vital rate functions by constructing an integral projection model that predicts the effects of various stressors on the asymptotic population growth rate. Regionwide, the asymptotic population growth rate is near or slightly below one, even under undisturbed conditions, and the actual abundance (in terms of both stem density and basal area) slightly declined over the duration of the study. A similar analysis for the United States Geological Survey Tree Demography Study located in Sequoia, Kings Canyon, and Yosemite national parks indicates that sugar pine abundance has been decreasing for the last three decades. Projections from the population models confirm this empirical finding: sugar pine populations are declining throughout much of their range. The analysis reveals that wildfire, white pine blister rust, and forest density are key drivers of the demographic rates of sugar pine across its range. Drought and site dryness had lesser, but still meaningful, effects. Fire has strong negative effects on survival, resulting in a strongly negative population trajectory on burned sites. Conversely, lower than average forest density (neighborhood basal area) results in a positive population growth rate via beneficial effects on growth. These results highlight the value of fire hazard mitigation, particularly where it also reduces forest density, in the conservation of this important species.

Assessment of the Current Condition and Future Vulnerability of Sugar Pine (Pinus lambertiana) in the Sierra Nevada: Evaluating the Drivers of Change in a Declining Species

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