Disturbance and stand dynamics in ponderosa pine forests in Rocky Mountain National Park, USA

Fire is thought to be the dominant disturbance agent in pure ponderosa pine (Pinus ponderosa Laws.) forests, but fire severity and disturbances other, than fire, and the effects of these events over time, have rarely been analyzed. We first created systematic criteria to (1) identify the causes of tree regeneration and mortality events and (2) classify the severity of these events. These criteria were then applied to understand the effect of events on ponderosa pine forests in Rocky Mountain National Park (RMNP), Colorado. For each of nine intensive study plots sampled in the pure ponderosa pine zone of RMNP, we mapped and dated live and dead trees and the spatial extent of fire and non-fire events using dendrochronology. Events were identified using evidence of disturbance agents, such as blue-stain fungus in the case of mountain pine beetles, fire scars, and climatic reconstructions. Disturbance severity was classified based on amounts and spatial distribution of regeneration, mortality, and survivors. We studied the temporal and spatial relationships between events and tree age structures to determine historical and contemporary stand dynamics. We identified 103 events among the nine study plots; 97% of these events were fire. High-severity events were 7% of events, 2% of events were mixed-severity, and the other 70% were low-severity. The severity of 20% of events could not be determined. All but one of the high-severity events were fire. In seven plots, at least one crown fire occurred within the last 300 years. In RMNP, pre-EuroAmerican settlement crown fires led to dense, even-aged stands. Subsequent surface fires killed some trees and facilitated the regeneration of others, leading to less dense, uneven-aged stands after similar to200-300 years. Climate may be the cause or co-cause of one or two regeneration pulses within the last 300 years, but most regeneration and mortality is associated with fire. The historical occurrence of crown fires, as well as surface fires, in pure ponderosa pine forests in RMNP indicates that the fire regime is more variable than previously thought. Suppression of crown fires, though not completely possible, would move the fire regime outside its historical range of natural variability.