Understory turnover and species disappearance in second growth forests - patterns across three spatial scales

Background/Question/Methods Though much has been written about short- and long-term dynamics of forest communities, and upland succession more generally, very little has been reported for the understory layer that is spatailly explicit. This is especially true at the micro- and meso-scales for the eastern forests of North America. We re-measured 210 permanent quadrats (1 m²) in 2008 that had been established by the U.S. Forest Service in 1991.  These quadrats were spread among 21 second growth forests in four counties of north-central Wisconsin. In addition, we completed a careful 'walk-through' survey to document additional species in an approximate 0.5 ha area centered on the quadrats. This scale is what I refer to as the meso-scale.

Results/Conclusions At the meso-scale, richness increased on average by 3.7 species over the 17-yr period. This increase was weakly and positively related (r = 0.41) to 1991 richness. The average belies the magnitude of change for most sites: site-specific values ranged from -22 to +29 species. Species turnover at the micro-scale was almost as large (+3.6 spp.). The influence of original richness was weaker and negative (r = -0.36), indicating that richness at this scale was more likely to decline as 1991 richness increased. Despite the overall net increase, more than half of the quadrats lost > 50% of their species from 1991 to 2008.  The number lost per quadrat was positively and significantly related to site-level (r = 0.64, p < .01) and quad-level (r = 0.55, p < .05) richness. However, among the species that disappeared, 57-65% were present in another quadrat or noted during the walk-through. These forest understories exhibitied a high level of spatial variation in dynamics from a micro-scale to the landscape. This level of fluctuation and magnitude of species loss in mature forests needs to be more fully considered in monitoring and successional studies.