Forest colonization of Puget Lowland Grasslands at Fort Lewis, Washington
Foster, Jeffrey R.
Shaff, Scott E.
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As a result of fire exclusion since the mid-19th century, Douglas-fir has been invading the grasslands of the southern Puget Lowland, Washington, converting them to colonization forests. We studied a chronosequence of these forests, including some that received periodic commercial thinnings, at Fort Lewis Military Reservation to elucidate successional patterns and changes in ecosystem properties associated with tree invasion. In unmanaged colonization forests (ages 27-120 yr), overstory cover and tree density remained constant after 27 yr, while basal area increased. Shrub cover increased with age and, after an initial decrease compared to grasslands, ground cover also increased. Shrub species richness increased over time, while ground-layer richness was much less than in grasslands, and there was a rapid shift from grassland to forest floras. Dead wood (snags and logs) accumulated gradually. The only change in soil properties was lower A-horizon nitrogen. In managed colonization forests (ages 55-91 yr), tree density, overstory cover, and total log length decreased over time, but shrub cover and A-horizon cation-exchange capacity, carbon, and nitrogen increased over time. Although structural and floristic conversion of grassland to forest requires <30 yr, soil changes are much more gradual, such that the soils underlying colonization forests up to 400 yr old are still classified as grassland soils. Higher tree densities in colonization forests compared to the pre-settlement forests of Fort Lewis mean an increased risk of crown fires during droughts. Even with active management, colonization forests will require many decades to provide northern spotted owl habitat because they lack shade-tolerant conifers and large live trees, snags, and logs