PS 8-79 - Understory turnover and species disappearanceĀ in second growth forests - patterns across three spatial scales

Monday, August 8, 2011
Exhibit Hall 3, Austin Convention Center
James E. Cook, College of Natural Resources, UW-Stevens Point, Stevens Point, WI
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. 

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