Effects of climate change on vegetation: Oregon Caves National Monument and Preserve

Peterson, David; Halofsky, Jessica; Morgan, Harriet; Krosby, Meade

Climate change is likely to have far-reaching impacts on vegetation throughout the Pacific Northwest. Vegetation assemblages are likely to be affected directly through shifts in growth, mortality, and regeneration and indirectly through changes in hydrologic systems, snowpack, spread of nonnative species, and shifts in other ecosystem processes. This assessment of climate change effects on vegetation in Oregon Caves National Monument and Preserve is completed within the context of historical management actions and disturbances. The vegetation assemblages of southwest Oregon and northern California have evolved over time, shaped by past changes in climate and the biophysical environment. While pine, oak, and chaparral species were common during the warm post-glacial period of the early Holocene, Douglas-fir and hemlock dominated during the cooler, wetter conditions of the late Holocene. The mixed-severity fire regime and irregular fire return interval of southwest Oregon have significantly influenced vegetation patterns, diversity, and fuel conditions across the landscape. There is strong observational evidence that historical climate was the main driver of regional fire regimes, with major fire years coinciding with summer drought years. Over the past 100 years, climate and fire observations from the Pacific Northwest indicate that warm and dry conditions typically result in larger fires and greater area burned. There have been a number of mixed-severity fires in southwestern Oregon over the past several decades. For example, the 2002 Biscuit Fire was the largest recorded forest fire in the state, with 500,000 acres (200,000 ha) burned; in 2017, over 100,000 acres (40,000 ha) of this fire area was reburned in the Chetco Bar fire. Increasing air temperatures, decreasing snowpack, and drier conditions will affect fire frequency, extent, and severity in southwest Oregon. Climate-driven shifts in insect outbreaks will also affect future fire regimes. Modeling studies agree that area burned is projected to increase in the western United States as the climate warms. For example, with a mean temperature increase of 3.6 °F (2 °C), the area burned by wildfire in most western states is projected to increase by a factor of 1.4–5. Warming temperatures are expected to affect insect species directly by altering insect species distribution and population dynamics, such as enhanced winter survival and shortened generation times, and indirectly via shifts in host tree distribution and natural defenses of trees, and interactions with competitors. As temperatures warm and forest drought stress occurs more regularly, insect-mediated disturbances are expected to become more frequent and more intense throughout the western United States. Native forest insects that currently are major disturbance agents in southwest Oregon include the bark beetle species, fir engraver, and western pine beetle. Warming temperatures may also facilitate the northward migration of introduced and native insects from regions south of southwest Oregon. Fungal pathogens and other disease outbreaks are projected to change in frequency and location as pathogen suitability increases or as forests become more vulnerable to infection. However, it is difficult to predict how climate change will affect specific pathogens because responses are likely to be species-specific. The growth and spread of Swiss needle cast and white pine blister rust is influenced directly by climate and local site conditions and will likely be affected by climate change. Laminated root rot and dwarf mistletoe appear to be indirectly associated with local climate and are less likely to be affected by climate change. The most significant implication of climate change for forest disease outbreaks will likely be increased host vulnerability driven by......

Effects of climate change on vegetation: Oregon Caves National Monument and Preserve

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