Proximity to grasslands influences fire frequency and sensitivity to climate variability in ponderosa pine forests of the Colorado Front Range

This study examines the influence of grasslands on fire frequency and occurrence in the ponderosa pine (Pinus ponderosa)-dominated forests of the central and northern Colorado Front Range. Fire frequency based on tree-ring fire-scar data was compared between 34 fire history sites adjacent to grasslands and 34 fire history sites not adjacent to grasslands for the time period 1675-1920. Relationships were examined between fire occurrence and values of the Palmer Drought Severity Index and sea-surface temperatures from the NINO3 region of the tropical Pacific Ocean (positive values indicating El Nino-like conditions and negative values La Nina-like conditions). Ponderosa pine stands adjacent to grasslands experienced more frequent fire than stands not adjacent to grasslands (P < 0.05) owing to proximity to prevalent fine fuels able to support relatively frequent surface fires. Fire activity adjacent to grasslands showed a lagged positive relationship with moist years (positive Palmer Drought Severity Index and positive NINO3) antecedent to fire events whereas fire occurrence at sites not adjacent to grasslands showed no relationship to antecedent moist years. This study illustrates how the presence of grasslands in a ponderosa pine landscape results in increased fire frequency (a bottom-up influence) and also increases the sensitivity of fire activity to interannual climate variability (a top-down influence).