Climate change impacts on Bay Area vegetation

The distribution of major vegetation types in the Bay Area captures the strong climatic gradients from coastal to inland areas, the variable topography of valleys and mountain ranges, and the impact of more than two centuries of logging, grazing, agriculture and urbanization. As the dominant vegetation creates the template for other plant and animal species, it plays a central role in conservation planning and in the ways that we as humans understand and interact with the landscape. In the Conservation Lands Network planning process, dominant vegetation was used as a ‘coarse filter’, with the goal of stratifying conservation priorities across the region in a way that captures a significant portion of existing vegetation types.

One of the most consistent lessons of ecological history is that episodes of significant climate change, such as the end of the last ice age, cause widespread shifts in species distributions, and the plant communities occupying any one location change dramatically. For example, around Clear Lake (Lake Co.), the forests changed from cedar-dominanted about 20,000 years ago, to a period of pine dominance 15-18,000 years ago, and then oaks in the past 10,000 years. The rate of climate change in the next century is expected to be considerably faster than the changes that occurred at the end of the last ice age, but we don’t know how quickly vegetation will be able to respond. Increased fragmentation of the landscape due to agriculture and urbanization will also slow down responses, as it disrupts seed dispersal and provides fewer places for species to establish as conditions change.

In the short term (decades), the impacts of climate change may be felt primarily as episodes of drought, wildfire and disturbance to local ecosystems. In the long-term (100s of years), vegetation is expected to shift in response to climate change, and the landscape as we know it today, including our favorite parks and open space reserves, may look very different. As part of our TBC3 research projects, we have modeled vegetation distributions under a range of future climates. These projections force us to consider the long-term context of our conservation efforts today, and the kinds of vegetation transitions that we may want to accommodate or even actively promote as we re-evaluate conservation and stewardship practices in an era of climate change.

Overall, the model projects an expansion of vegetation types that currently occupy hotter and drier, interior portions of the Bay Area. Under different modeling approaches, this expanding vegetation may be dominated by woody communities including blue oak woodlands and chamise chaparral, or by hot, interior grasslands that currently occupy the fringes of the Central Valley. Many factors will influence the actual changes that transpire over the coming decades, and fire, fire suppression and grazing practices may be especially important influences on the future distribution of woody vs. grassland vegetation. Given the uncertainties inherent in climate change projections and vegetation modeling, we feel that the model results should be considered at regional scales, such as counties and landscape units, rather than drilling down to individual landscape facets or parcels.