California’s forests are beset by multiple threats from climate change, including increased fire and drought severity. A single tree can be thought of as a victim to the influences of its climate, at risk of mortality from any number of climate-related stresses. However, forests collectively are a key determinant of some of the very same processes that lead to these risks. In order to effectively manage forest ecosystems and support the human communities that live in and near them, we must examine the causes and effects of forest patterns and process, across scales of space and time.The spatial structure of a forest—the size and arrangement of its constituent trees—can be more or less resistant and resilient to wildfire. Fire in turn can, abruptly or gradually, shape a forest structure towards more heterogeneous stands, which are well adapted to frequent, low-intensity fire. The era of fire exclusion (staring around 1850) has prevented this self-regulating feedback from taking place, leading to crowded, homogeneous forest stands, with high fuel loads that make them vulnerable to stand-replacing fires. Studying the forest structure from a time before the end of frequent, low-intensity fire can give managers a reliable target for the spatial and structural characteristics of a fire-adapted forest. We apply dendrochronological reconstruction methods to Jeffrey pine (Pinus jeffreyi) forests in the Eastern Sierra Nevada, in order to estimate the conditions of fire-adapted forests specific to this region. This is the first spatially explicit stand reconstruction study to take place in the Eastern Sierra, which experiences a unique climate and fire regime compared to its more mesic western-slope counterpart. We present findings indicating that Eastern Sierra stands have increased in density, with fewer trees existing as singletons and fewer, smaller gaps between clumps of trees. But our results also indicate that as few as one to two moderate-severity fires (prescribed fire or wildfire) can meaningfully restore this forest type to its fire-adapted state.
California’s forests and woodlands can change gradually, as they did through hundreds of years of fire suppression; or they can change rapidly, as seen in the waves of mortality caused by the state’s 2012-2017 drought. Water stress, whether cumulative or acute, has always been and will always be one of the greatest challenges to terrestrial plant life. Because plants lose water through their leaves in the process of acquiring CO2 for photosynthesis, it is thought that rising atmospheric CO2 levels could ameliorate the effects of water stress in trees, allowing them to reduce their water losses while increasing their net CO2 uptake. We wished to investigate whether, how, or to what extent high CO2 levels help the youngest trees endure or avoid water stress. Using a newly constructed free-air CO2 enrichment (FACE) system at Quail Ridge Reserve, we planted 384 acorns of two native California oak species (Quercus lobata and Q. wislizeni) directly into the soil. We subjected half of the seedlings to continuous treatment with elevated CO2 (ambient + 137 ppm). Half of the seedlings received supplemental water during the summer. These two factors were combined to yield four treatment levels. By comparing the differences between well-watered and under-watered plants at ambient CO2 vs. elevated CO2, we were able to test for interactions between the CO2 treatment and the watering treatment. We found that in the cases where watering’s effect differed by CO2 treatment, it was only the well-watered seedlings that benefited from the extra CO2 (for Q. wislizeni, the benefit came in the form of increased photosynthetic rates; for Q. lobata, the benefit was an increase in post-grazing resprouting rates). This result may indicate that future climates will widen, rather than narrowing, the fitness gap between oak seedlings with access to abundant water, and those without it.
