Fire and the Forest History of the North Cascade Range

Abstract

Postglacial vegetation changes are often ascribed to the direct effects of climate change. I studied pollen, plant macrofossils, and sediment charcoal in order to determine the potential role of changes in the disturbance (fire) regime in the postglacial development of local vegetation at Kirk Lake in the foothills of the North Cascade Range in northwestern Washington. Five pollen assemblage zones are recognized: a Pinus–Populus zone >12 000 BP, a Picea—Alnus sinuata zone from >12 000 to 11 030 BP, an Alnus rubra—Pteridium zone from 11 030 to 6830 BP, a Cupressaceae zone beginning at 6830 BP, and a late Holocene Pinus—Alnus rubra zone from 2400 to 900 BP. The first forests (>12 000 BP) were an open mixture of conifers and deciduous trees, chiefly Tsuga mertensiana, Abies, Pinus contorta, and Populus on a landscape subject to erosion. Just before 12 000 BP, the pioneer species Picea sitchensis, Alnus rubra, and A. sinuata became important constituents of the forest. Although pollen accumulation rates were high, the abundance of Alnus sinuata indicates an open—canopy forest. Beginning °11 200 BP, climatic warming initiated major changes in forest composition and the fire regime. Tsuga heterophylla migrated into the region, rapidly expanded, then declined shortly thereafter, while Pseudotsuga menziesii, Alnus rubra, and Pteridium expanded, and Pinus contorta, Picea sitchensis, Populus, and Alnus sinuata declined. The abundance of Pseudotsuga, Alnus rubra, and pteridium between 11 030 and 6830 BP corresponds with increased influxes of charcoal into the sediment; this zone is interpreted as a closed forest with a relatively high fire frequency and composed of a mosaic of postfire successional communities in which fire—adapted Pseudotsuga and Alnus rubra predominated over fire—sensitive Tsuga heterophylla. Pinus monticola became locally important °800 BP. Between 6800 and 6400 BP Thuja plicata arrived, Tsuga heterophylla expanded, and Alnus rubra, Pseudotsuga and Pteridium declined. These changes are accompanied by a reduced fire frequency, inferred from lower charcoal accumulation rates, and they indicate a shift to wet—temperate climate similar to today's. The late Holocene fossil record shows the development of the adjacent peatland, which Pinus contorta eventually invaded.