The effects of fire and tephra deposition on forest vegetation in the Central Cascades, Oregon

a r t i c l e i n f o High-resolution charcoal and pollen analyses were used to reconstruct a 12,000-yr-long fire and vegetation history of the Tumalo Lake watershed and to examine the short-term effects that tephra deposition have on forest composition and fire regime. The record suggests that, from 12,000 to 9200 cal yr BP, the watershed was dominated by an open Pinus forest with Artemisia as a common understory species. Fire episodes occurred on average every 115 yr. Beginning around 9200 cal yr BP, and continuing to the present, Abies became more common while Artemisia declined, suggesting the development of a closed forest structure and a decrease in the frequency of fire episodes, occurring on average every 160 yr. High-resolution pollen analyses before and after the emplacement of three distinct tephra deposits in the watershed suggest that nonarboreal species were most affected by tephra events and that recovery of the vegetation community to previous conditions took between 40 and 100 yr. Changes in forest composition were not associated with tephra depositional events or changes in fire-episode frequency, implying that the regional climate is the more important control on long-term forest composition and structure of the vegetation in the Cascade Range. Introduction Natural disturbances are an important component of ecosystem processes within forest environments. The interactive effects of multiple disturbance agents may increase their overall impact on forest composition and structure but may be difficult to interpret or predict (Turner et al., 2003). In the Cascade Range of the Pacific Northwest (PNW), two dominant disturbance agents are fire and volcanic eruptions. Fire regimes range from those with frequent low-severity surface burns in dry Pinus contorta (lodgepole pine) forests of the eastern slopes to those with infrequent high severity crown fires in wet Tusga heterophylla (western hemlock) forests on the western slopes (Franklin and Dyrness, 1988). A large portion, N50%, of Cascade Range forests fall between these two extremes, and can be characterized as having a mixed-severity fire regime such as that in the Abies grandis (grand fir) forests found the eastern Cascade Range and Sierra Nevada crest (Agee, 1993). Volcanic activity is an episodic disturbance agent that may affect vegetation in several ways ranging from high severity plant-mortality impacts in the areas proximal to blast zones to inconsequential impacts of trace amounts of tephra falling on forest communities thousands of kilometers from an eruption source (Dale et …