DISTURBANCE REGIME AND DISTURBANCE INTERACTIONS IN A ROCKY-MOUNTAIN SUB-ALPINE FOREST

1 The spatial and temporal patterns of fire, snow avalanches and spruc e beetle outbreaks were investigated in Marvine Lakes Valley in the Co lorado Rocky Mountains in forests of Picea engelmannii, Abies lasiocar pa, Pseudotsuga menziesii and Populus tremuloides. Dates and locations of disturbances were determined by dendrochronological techniques. A geographic information system (GIS) was used to calculate areas affect ed by the different disturbance agents and to examine the spatial rela tionships of the different disturbances. 2 In the Marvine Lakes Valley , major disturbance was caused by fire in the 1470s, the 1630s and the 1870s and by spruce beetle outbreak in c.. 1716, 1827 and 1949. 3 Sin ce c. 1633, 9% of the Marvine Lakes Valley has been affected by snow a valanches, 38.6% by spruce beetle outbreak and 59.1% by fire. At sites susceptible to avalanches, avalanches occur at a near-annual frequenc y. The mean return intervals for fire and spruce beetle outbreaks are 202 and 116.5 years. respectively. Turnover times for fire and spruce beetle outbreaks are 521 and 259 years, respectively. 4 Several types of disturbance interaction were identified. For example, large and sev ere snow avalanches influence the spread of fire. Similarly, following a stand-devastating fire or avalanche, Picea populations will not sup port a spruce beetle outbreak until individual trees reach a minimum d iameter which represents at least 70 years' growth. Thus, recent fires and beetle outbreaks have nonoverlapping distributions.