Effects of climate on competitive dynamics in mixed conifer forests of the Sierra Nevada

Trees in more competitive environments appear to respond to climate differently than trees in less competitive environments. In turn, climate patterns may affect inter- or intra-specific competition, favoring certain individuals over others. Using dendrochronological methods, we sampled cores from dominant pine trees and their nearby competitors in 40 stands in the northern Sierra Nevada. Our dendroclimatological analyses showed complex responses to climate for the dominant pine and their competitors. Pinus lambertiana Dougl. and Pinus ponderosa Dougl. Ex P. & C. Laws benefited from cooler temperatures in the past May; all species benefited from warmer winter temperatures; Calocedrus decurrens (Torr.) Florin benefited from cooler temperatures in the past summer, while Abies concolor (Gord. & Glend.) Lindl.) and the two pine species benefited from cooler temperatures in the present summer. All species showed consistently positive effects from increased precipitation and increased moisture availability. A competition ratio calculated as the ratio between the dominant tree's radial increments to the competitor's indicated that climate could influence competition through one of two mechanisms. One, negative responses to climate conditions that were negative for one species but neutral for another tended to drive the competition ratios. Two, conditions that were negative for both competing trees tended to hurt competitors more than dominant trees, while conditions that were generally positive tended to help competitors more than dominant trees. These results suggest that the dominant pine trees may be more resilient to climate stress than competitors. Reducing competition via management such as thinning may increase climatic resilience for all species and possibly have positive competitive results for competitors. Increasing temperatures will tend to harm Sierra Nevada conifers in the summer and spring, but may show some benefit over the winter as long as precipitation is not reduced.