Stand development and tree response in mixed-species forest ecosystems affected by introduced pathogens

Introduced forest pests have caused widespread mortality and decline of many native North American tree species. Developing management and restoration strategies in these forests requires an understanding of basic forest processes, such as stand development patterns and responses to disturbance. The research objectives were: (1) to describe stand development patterns and changes in two mixed-species forest types of California that have been affected by an introduced pathogen; (2) quantify vigor of trees susceptible to the introduced pathogen and neighboring trees; and (3) develop the role of silviculture in the restoration and management of affected ecosystems. The two ecosystems studied were mixed-evergreen coastal forests (redwood ( Sequoia sempervirens (D. Don) Endl.) and tanoak (Lithocarpus densiflorus (Hook&Arn.) Rehd.)) where high amounts of tanoak mortality are attributed to Phytophthora ramorum S. Werres, A.W.A.M. de Cock, and the mixed-conifer forests of the Sierra Nevada Mountains where sugar pine (Pinus lambertiana Dougl.) succumbs to Cronartium ribicola J.C. Fisch.ex. Rabenh. In the mixed-evergreen forests, a stratified two-layer canopy was found with redwood in the upper canopy layer and tanoak below. Tanoak surviving on infected sites were growing significantly faster than those on control sites; no differences were seen in redwood. Redwood may have been forming a new cohort in the understory of infected sites. The overall trajectory of these stands may be toward one dominated by redwood. However ring anomalies caused by missing and discontinuous rings made aging redwood difficult and indicate tree ring studies in redwood must be done carefully. In the Sierra Nevada mixed-conifer forest, pines tended to be found in the upper canopy strata over Douglas-fir (Pseudotsuga mensiezii (Mirb.) Franco var. mensiezii) and white fir (Abies concolor var. lowiana (Gord.) Lemm.) and incense-cedar (Calocedrus decurrens Torr.). Height stratification patterns did not always follow the same trajectory and appeared to depend on management history, spatial pattern and age. White fir seemed to benefit the most from sugar pine mortality, growing faster with higher growth efficiency on plots with mortality than plots without. Tree vigor (as measured by growth efficiency) was higher for surviving individuals of both susceptible species, indicating that these species may benefit from thinning and/or burning treatments.