Pinus contorta understory vegetation dynamics following clearcutting in west-central Alberta, Canada

Abstract

A sequence consisting of 143 post-clearcut (<46 years old) and 24 unmanaged (48–107 years old) forest stands from west-central Alberta was analyzed to evaluate understory vegetation change as a function of overstory (>1 m tall) cover and stand age. Species composition and abundance data were obtained from stands by plot sampling. The abundance of understory plants designated as early-seral species was not correlated with stand age and was only weakly correlated with overstory cover. The relative abundance of early-seral plants decreased linearly in response to increasing overstory cover ( r = −0.502, P < 0.001) within clearcuts, whereas plants more commonly associated with mid-seral stands increased linearly in cover ( r ∼ 0.745, P < 0.001) based on absolute values. Twenty-three percent of the plant cover in young clearcuts consisted of mid-seral species, but early-seral species represented <4% of the plant cover in unmanaged stands. Percent overstory cover and stand age consistently explained more variance in mid-seral than early-seral plant abundance. Overstory cover was more frequently a better predictor of understory vegetation cover than stand age. A 100-year chronosequence of Pinus contorta clearcut and unmanaged stands differed from a recognized model of natural forest development, due to the lack of a stem exclusion and a subsequent understory reinitiation stage. These differences in development were probably related to the low density of planted trees and the correspondingly open overstory that occurred during the early stages of stand development. Despite these differences, the composition of the understory vegetation within clearcuts appeared to be changing toward that of the unmanaged stands, although the initially more open overstory may have prolonged the presence of early-seral understory species. An overstory cover-based sequence suggests that growth-form cover and composition vary in response to the degree of understory shading. Understory vegetation that developed beneath a 30–70% overstory cover reflected characteristics of a tolerance interaction model, whereas greater overstory cover inhibited the development of ericaceous shrubs. Models of early forest community development should be stratified according to both the amount of overstory cover and the stand age to appropriately represent understory development trends.