Using forest vegetation inventory data to examine shifts in species distribution and composition

Background/Question/Methods

The Forest Inventory and Analysis Program of the U.S. Forest Service has collected a full census of vascular species on a subset of national inventory plots. There are 507 plots that have been sampled and subsequently revisited 5 – 10 years later. The greatest extent of the vegetation indicator inventory is situated in the Northern and Eastern Temperate Forest ecoregions. Even at a coarse scale, these data provide useful insights into regional patterns and trends. With a full census of vascular plants for the forested areas of each subplot, we are able to assess the influence of forest fragmentation, regional climate, and pollutant gradients on the changes observed in species distribution.  For example we expect introduced species to be most common in fragmented forests and regional climate regimes to either limit or promote the spread of certain species.  In addition, we expect modest level nitrogen (N) deposition to increase vascular species richness, but once it exceeds a certain level, decreases species richness is expected. Sites with limited natural N levels are expected to be more strongly influenced by additional N than sites already saturated.

Results/Conclusions

A total of 2204 species were identified, including 292 introduced species and 28 species with uncertain nativity. Of the top 20 most commonly recorded introduced species, eight increased in constancy by at least 3%, with Microstegium vimineum increasing from 2 to 8%, Rosa multiflora increasing from 35 to 42%, and Plantago major increasing from 5 to 11%. Only Polygonum convolvulus decreased in constancy, from 7 to 5%. Of the top 40 most commonly recorded native species, 18 increased in constancy by at least 3%; the species with the greatest increases included Vitis aestivalis, from 9 to 29 %; Taraxacum officinale, from 13 to 30%, and Rubus idaeus, from 13 to 28%. Four of the top 40 native species decreased slightly (1% or less).  N deposition across the remeasured plots ranged from about 3 to over 20 kg N ha^-1 yr^-1, with regional differences.  Forested conditions have not changed on 80% of the plots; of these about half are intact forests.  We examine the relationship of shifting species distribution and composition with forest conditions, pollution gradients, and regional climatic regimes.