Conifers in the Face of Climate Change

Thursday, October 20 at 10:00-11:40 am, Fir Room

Session Description: Coniferous vegetation occurs from the Coastal zone (coast redwood, coastal cypress) to high-elevations (whitebark pine, bristlecone pine) and areas in between in the foothills and mountains (coastal Douglas-fir, Pacific ponderosa pine). The risks they face due to climate change, insects, and pathogens, intensive logging, largescale and severe wildfires, and other stressors are immense. This session focuses on research and management efforts underway today in California to examine the status and trends of coniferous vegetation in response to climate change and other interacting stressors.

Session Chairs: Marc Meyer (USDA Forest Service, Region 5 Ecology Program, Bishop, CA, USA), and Sarah Bisbing (University of Nevada, Reno, NV, USA)

2.1    Biomass stocks in California’s fire-prone coniferous forests: Mismatch in ecology and policy

Alexis Bernal (University of California, Berkeley, Department of Environmental Science, Policy, and Management, Ecosystem Sciences Division, Berkeley, CA, USA), Scott Stephens (University of California, Berkeley, Department of Environmental Science, Policy, and Management, Ecosystem Sciences Division, Berkeley, CA, USA), Brandon Collins (USDA Forest Service, Pacific Southwest Research Station, Davis, CA, USA), John Battles (University of California, Berkeley, Department of Environmental Science, Policy, and Management, Ecosystem Sciences Division, Berkeley, CA, USA)

Restoration of fire-prone coniferous forests can promote resiliency to disturbances, yet such activities may reduce biomass stocks to levels that conflict with climate mitigation goals. Using a set of large-scale historical inventories across the Sierra Nevada/southern Cascade region, we identified underlying climatic and biophysical drivers of historical forest characteristics and projected how restoration of these characteristics manifest under future climate. Historical forest conditions varied with climate and site moisture availability but were generally characterized by low tree density (~53 trees/ha), low live basal area (~22 m²/ha), low biomass (~34 Mg/ha), and high pine dominance. Our predictions reflected broad convergence in forest structure, frequent fire is the most likely explanation for this convergence. Under projected climate (2040-2069), hotter sites become more prevalent, nearly ubiquitously favoring low tree densities, low biomass, and high pine dominance. Based on these projections, this region may be unable to support aboveground biomass >40 Mg/ha by 2069, a value approximately 25% of current average biomass stocks. Ultimately, restoring resilient forests will require adjusting carbon policy to match limited future aboveground carbon stocks in this region.

2.2    Wildfire-mediated upslope advance in conifers may depend on species regeneration niche

Emily Brodie (USDA Forest Service, Pacific Southwest Research Station, University of California, Davis, Davis, CA, USA)

Many montane tree species distributions are likely to move upslope in the coming decades as temperatures increase with climate change. However, long-lived species such as trees might lag in colonizing climatically suitable habitat because adult trees can persist for long periods in stressful environments. By removing adult trees and reducing the “biological inertia” of an intact forest, disturbances such as fire are predicted to facilitate changes in species composition. In this study we asked whether high severity fire facilitates regeneration of non-resident lower elevation tree species in subalpine forest in California. California subalpine forest is of particular interest because, while cool temperatures and high precipitation limited fire in most subalpine ecosystems in the past, modern increases in atmospheric aridity correlate with rapidly increasing area burned in historically infrequent-fire subalpine forests. In contrast to our expectations, our analysis of tree regeneration in 248 plots across 13 fires in subalpine forest suggests that high severity fire does not facilitate regeneration of lower elevation species over higher elevation species. Our combined model showed that regeneration of lower elevation species was four times higher in unburned (rather than burned) plots, while fire severity had little effect on postfire occurrence of subalpine seedlings. However, species specific models revealed that while fire severity decreased encroachment of the more common and shade-tolerant red fir seedlings, it facilitated encroachment of the less common and shade-intolerant Jeffrey pine seedlings. Our results suggest that small patches of high severity fire such as those sampled in this study might help to maintain high elevation white pine stands in the face of upward movement of lower elevation species that are shade tolerant, but not those that are shade-intolerant.

2.3    Altered fire regimes threaten remaining populations of rare Baker cypress (Hesperocyparis bakeri) 

Kyle Merriam (USDA Forest Service, Sierra Cascade Ecology Program, Quincy, CA, USA)

The extent and frequency of high severity fires has increased dramatically over the past four decades. Although some species depend on high severity fire to reproduce, they cannot survive when these fires occur too frequently. Baker cypress (Hesperocyparis bakeri) is a rare conifer known from approximately 11 widely scattered locations in northern California and southern Oregon. Baker cypress possess serotinous cones that open in response to high severity fire, and they regenerate from cones accumulated in the canopy of mature trees. Baker cypress do not begin producing cones until the trees are at least 14 years of age or older, and require between 35 and 50 years to develop sufficient canopy seed storage to re-establish the population after fire. A 2007 status survey of Baker cypress found that fire had been excluded from most populations for many decades, and reintroduction of fire to promote cypress regeneration was recommended. Seven of the eleven Baker cypress populations have since burned in wildfires, including the southernmost population at Mud Lake, which burned in 2007 and again in 2021. After only 14 years, this population had not yet produced cones and was almost completely extirpated by the second fire. Six other recently burned populations are now characterized by seedlings ranging from 1 to 8 years of age, and these populations are highly vulnerable to reburn in the near-term. Given that current trends of increasing fire activity and severity are predicted to continue into the future, we call for immediate action to conserve Baker cypress throughout its range. We recommend cone collection and seed banking from remaining mature individuals and populations, active replanting efforts to restore extirpated sites, fuel treatments around immature populations to protect them from subsequent fires, and coordination with fire suppression agencies and personnel to include Baker cypress as a priority for protection during wildfire incidents.

2.4    Persistence potential of Great Basin high-elevation conifers under novel climates and disturbance regimes

Lacey Hankin (University of Nevada, Reno, NV; Yosemite National Park, Mariposa, CA, USA)

Forest persistence under global change will depend upon the capacity of species to persist in situ by tolerating water and temperature stress, rapidly adjusting to novel conditions or migrating to track ecological niches. Contemporary climate change outpaces the adaptive capacity of long-lived, geographically isolated tree species, therefore differential regeneration success and proactive management will shape the future composition and distribution of forests. Focusing on the high-elevation pines of the Great Basin, we evaluate seedling success across a range of establishment conditions to assess the potential for ongoing natural regeneration and future reforestation interventions. Using observational and experimental approaches, we a) characterized the natural regeneration niche for each species, b) identified advantageous seedling traits and assessed evidence for local adaptation and plasticity while testing the utility of direct seeding, and c) evaluated the ecophysiological limitations to seedling survival. Our research shows species-specific differences in the regeneration niche and in seedling strategies for success that may lead to differential survival under novel conditions. We highlight the critical role that snowpack-driven soil moisture plays in natural regeneration, seedling emergence from direct seeding, and early seedling survival for all species. Climatic and edaphic specialization may limit regeneration success compared to more generalist strategies, however, plasticity may provide one mechanism for persistence in specialists. Drought-adapted traits varying by seed origin may be valuable for climate-informed reforestation, however, extreme water and temperature stress led to rapid mortality for all populations. Our studies highlight the importance of natural regeneration for persistence, however, expected increases in water stress will reduce establishment opportunities and leave regenerating seedlings more vulnerable to drought-driven mortality.

2.5    Managing coast redwood forests for a changing future

Phillip van Mantgem (U.S. Geological Survey, Western Ecological Research Center, Arcata, CA, USA)
Forests dominated by coast redwood (Sequoia sempervirens) are of tremendous biological and social importance, each year drawing hundreds of thousands of visitors to Redwood National and State Parks (RNSP). These forests provide multiple ecosystem services, including the maintenance of biodiversity, watershed hydrologic integrity, and carbon storage. While these forests appear timeless, they face multiple threats from a recent history of intense logging, flooding, and disease. These stressors are likely to be compounded by expected changes in climate, which may increase the potential for drought and fire. We present recent results showing sensitivity to moisture stress in conifer species in northern coastal California and how active management in second-growth stands may ameliorate some of these stressors.