Local increases in diversity accompany community homogenization in floodplain forest understories

Abstract

Questions Changes in land use, altered flow regimes, invasions by pests and pathogens, and climate change are, separately and together, altering ecological dynamics of floodplain forests. The complex changes these forces impose often go unnoticed due to a lack of reliable baseline data. Using historical data from the 1950s and re-survey data on floodplain forest understories, we ask: (1) what is the direction, magnitude and nature of changes in local and regional diversity; (2) have these floodplain communities become more homogenous in composition due to losses of native species and increases in non-natives; (3) has the relationship between the ground layer and overstorey community weakened over time; and (4) are changes in local and regional diversity among these floodplain forests more or less pronounced than in nearby upland forests? Location Floodplain forests throughout southern Wisconsin, USA. Methods We quantified composition of vegetation in the understorey and overstorey of 40 sites and compared these data with data from the 1950s using a combination of uni- and multivariate analyses. We focus on changes in local vs regional diversity. Results Although average site-level diversity has increased by 18.5%, sites are now 30% more similar to each other, reflecting a pronounced decline in among-site (beta) diversity. Although exotic species have increased in site occupancy and abundance, increases in native woody plants (+30%), graminoids (+24%) and forbs (+16%) drive most of these changes in local and regional diversity. Sites have experienced an average species turnover rate of 39% since the 1950s, and understories are now 39% more correlated with canopy composition than in the 1950s. Biotic homogenization among these floodplain forests since the 1950s is 23.3% higher than the increase in similarity observed in nearby upland forests. Conclusions Vegetation changes in floodplain understories differ from changes previously documented in nearby upland forests. Floodplain forests have experienced a faster rate of biotic homogenization. Unlike upland forests the homogenization trend in these floodplain forests is accompanied by increases in site-level diversity. These may reflect widespread colonization by certain native species. Both trends may also reflect relatively high connectivity among floodplain forests and control of hydrologic fluctuations via dams.