Stand Dynamics and Its Relationship to Fire and Climate History in the Cottonwood Creek Drainage, Boise Front Range, Idaho

The structure and composition of a forest influences the way in which it responds to disturbance. At the same time disturbances such as fire, human activity, and climate variability affect vegetation structure and composition. (Amo and Hartwell, 1995; Mast et al., 1998; Ehle and Baker, 2003). By understanding how a forest has responded to past disturbance and climate, one can better predict how it might respond to, as well as influence, future disturbances. The objective of this study was to characterize the relationship between stand structure and age classes of ponderosa pine and Douglas-fir, disturbance, and climate along the Boise Front Range, located to the north and east of Boise, Idaho. Specifically, this study intended to answer two research questions. First, what is the state (age classes, species present) of individual and collective mixed conifer stands in the Cottonwood Creek drainage? Second, are tree recruitment periods and growth related to disturbance events, specific periods of certain climatic conditions, or elements of both? Over the summer of 2004, trees in each of twelve 30x30 meter plots within the 31 km2 Cottonwood Creek drainage were sampled. Cores were processed according to standard dendrochronological methods, and tree ages, as well as stand composition, were compared with instrumental climate records and fire history for the area. It does appear that there are several influences on stand and age structure in the Cottonwood Creek drainage. First, the ratio of ponderosa pine to Douglas-fir decreases as elevation increases. Second, both precipitation and Palmer Drought Severity Index (PDSI) influence tree growth in both species. Douglas-fir growth and yearly average precipitation had an r= .82 (p\u3c.01) correlation value and an r= .53 (p\u3c.02) value for PDSI. Ponderosa pine growth was most highly correlated with prior-year spring precipitation (April-June) (r= .41) (p\u3c.05), and correlated with PDSI at almost the same level (r= .43) (p\u3c.05). This relationship indicates that both species respond positively to both high precipitation years and years with high (wet) PDSI values. The influence of these climate variables may also be tied to recruitment pulses for both species, but further work should be done to see if a statistical correlation actually exists. Evidence for tree recruitment pulses being tied to the occurrence of wildfires is inconclusive at this time and should be examined further, as should the potential influence of other climate signals, such as the EI Nino-Southern Oscillation and the Pacific Decadal Oscillation