Twenty-five years of vegetation change along a putative successional chronosequence on the Tanana River, Alaska

Along the Tanana River floodplain, several turning points have been suggested to characterize the changes in ecosystem structure and function that accompany plant community changes through primary succession. In the past, much of this research focused on a presumed chronosequence that uses space for time substitutions. Within this chronosequence, permanent vegetation plots repeatedly measured over time provide an excellent test of the turning points model. We analyzed both canopy and understory vegetation data collected since 1987 in the Bonanza Creek Experimental Forest at the Bonanza Creek Long Term Ecological Research site to address the following questions: (i) Do long-term changes in the densities of seedling, sapling, and mature trees and shrubs of the dominant woody taxa at each successional stage support the turning points model? (ii) How does the entire plant community change with time at each hypothesized turning point? (iii) Do we see evidence of directional and synchronous shifts in species composition across successional stages? We conclude that some aspects of vegetation change during the last 25 years were consistent with the turning points model; however, many changes were not consistent, indicating the potential roles of biological, environmental, landscape, and climate controls in vegetation patterns.