Stand demography as an indicator of ecological resilience in an old-growth mixed conifer forest

Background/Question/Methods

Tree species richness in forests is associated with higher levels of ecosystem services and greater resilience in the face of anthropogenic global change. California’s Sierra Nevada mixed conifer forests have historically been a uniquely resilient system due to the evenness of their mixed composition. However, fire exclusion in the region has led to changes in the physical structure of the forest as well as a sharp decrease in the species richness of recruits in the forest understory. This shift from a mixed to homogenous system is likely to increase the susceptibility of the ecosystem to perturbations and to slow recovery after disturbance, resulting in decreased resilience of the forest in the face of natural and anthropogenic disturbances. What are the drivers of this forest reorganization? Answering this question allows us to more accurately model forest trajectories and better manage for a desired resilient state. We measured changes in forest composition and structure over a 20-year period in a 4-ha mapped stand in the northern Sierra Nevada. We used hierarchical models of growth, mortality, and recruitment to estimate changes in stand dynamics of understory and dominant trees to assess which processes were dominating the shift to a more homogenous forest composition.

Results/Conclusions

While the overstory of this forest remains relatively evenly mixed, there is an obvious imbalance in the species composition of understory recruits. White fir and Douglas fir recruited into the understory at a rate of 1.8±0.5 and 0.6±0.2 trees ha^-1 yr^-1, respectively. Ponderosa pine recruited at a rate of 0.02±0.02 trees ha^-1 yr^-1, and sugar pine showed no recruitment. Preliminary analyses show that average diameter growth of white fir and Douglas fir did not differ significantly from ponderosa pine and sugar pine for either dominant or understory trees. Mortality of these four species was also similar. These results suggest that the main driver of forest reorganization in this system is the lack of recruitment of pines to the forest understory, and eventually to the forest canopy. This mixed conifer forest appears to be nearing a regime shift from a resilient, mixed system to a homogenous system less able to resist disturbance; however, accurately quantifying resilience of slowly changing systems composed of long-lived species has proven difficult. By using proxies for resilience such as species richness, we may be able to better manage forest systems with resilience goals in mind.