Shrub encroachment across North America: A multi-site synthesis of patterns, mechanisms, and consequences

TitleShrub encroachment across North America: A multi-site synthesis of patterns, mechanisms, and consequences
Publication TypeConference Paper
Year of Publication2006
AuthorsBriggs J.M, Knapp A., Archer SR, Bret-Harte M.S., Collins S.L., Ewers B., Peters DC, Young D.
Conference Name91st Ecological Society of America Meetings
Date PublishedAugust 6-11, 200
Conference LocationProCite field[13]: Memphis, TN
ARIS Log Number200293
Keywordsgrasslands, shrub, sythesis
AbstractIn North America, the expansion of shrubs within ecosystems previously dominated by herbaceous species has been documented in barrier islands off the Virginia coast, mesic grasslands of the Great Plains, sub-tropical savannas of Texas, desert grasslands of the Southwest, across the Intermountain West and in the Artic tundra. We synthesized data from four LTER sites and seven sites in total (in Virginia, Kansas, Texas, New Mexico, Wyoming and Alaska) to assess the mechanisms and consequences of this widespread pattern of growth-form substitution. Mean annual temperature at these sites ranged from -12.5C at the tundra site (Arctic LTER) to 22C at the La Copita, TX site and mean annual precipitation (MAP) ranging from 259mm at the WY site to 1065mm at the Virginia Coast Reserve LTER site. Overall, ANPP was increased when shrubs invaded herbaceous-dominated ecosystems, but this pattern was driven by large increases at the mesic sites. At the driest sites (NM and WY), ANPP decreased with shrub expansion. Across all sites, MAP was more strongly correlated with shrub ANPP (r2 = 0.71) than herbaceous ANPP (r2 = 0.55). In contrast to ANPP responses, species richness declined with the expansion of shrubs into these herbaceous-dominated ecosystems. These shifts in ecosystem structure clearly affect function across these biomes, with potentially dramatic and long-term regional implications given the extent of this phenomenon and the low probability of a return to dominance by herbaceous species in many of these ecosystems.