Stand development, fire and growth of old-growth and young forests in southwestern Oregon, USA

Abstract

We studied stand development in three distinct forest types in southwestern Oregon using six stands each in uncut and clear-felled old-growth stands and nearby young stands (18 total). Old-growth stands showed a wide range of tree ages (>300years) and low tree densities for several centuries; rapid early growth produced trees with large crowns and diameters, as well as low height-to-diameter ratios. In contrast, young stands established much quicker and at higher tree densities; beyond their initial 20years, trees had smaller diameters at equivalent ages, slower growth rates, smaller crowns and higher H:D than trees in old-growth stands. Low-intensity disturbance, likely dominated by fire, was common in old-growth stands during their early development. Fire scars showed these stands burned frequently from 1700 to 1900, and low levels of tree recruitment occurred in a complex relationship with fire during this 200years. There was no evidence of fire, however, in either old-growth or young stands after 1909, and their densities were well above that of 1900; in old-growth stands, 15–25% of the basal growth occurred from 1950 to 1990, and it appears that they are on a development pathway different from what they experienced from 1700 to 1900. Furthermore, tree recruitment has been limited in both old-growth and young stands since 1950 while biomass and fuels continue to accumulate rapidly. Past stand dynamics can be emulated by prescribed fire and light thinning to reduce risk of loss from severe fire or insects, as well as to partially restore stand conditions that existed prior to fire exclusion. Our results suggest that young stands can be grown to produce high levels of biomass/wood, or their development can be altered to more closely follow that of old-growth stands depending on management objectives.