Flood Regime Alters The Abiotic Correlates Of Riparian Vegetation

Questions Predicting the influence of climate change on riparian plant communities improves management strategies. The sensitivity of riparian vegetation to climate and other abiotic factors depends on interactions between properties of the ecosystem, like flood regime, and plant characteristics. To explore these interactions, we addressed three questions: (a) does the composition and diversity of riparian vegetation vary with the flood regime; (b) do abiotic correlates of vegetation, including climate and groundwater, differ between sites that flood compared to locations that did not experience floods; and (c) which plant functional groups account for differential plant community sensitivity to abiotic factors between flood regimes? Location Middle Rio Grande Valley, New Mexico. Methods We used long-term observations of plant community composition, groundwater depth, precipitation and interpolated temperature from 24 sites spanning 210 km of the Rio Grande riparian cottonwood-willow forest to explore the relative importance of climate and hydrologic correlates of riparian vegetation diversity and composition. Results Riparian plant diversity was higher at sites flooding compared to non-flooding sites. Plant diversity positively tracked shallower groundwater depth at flooding sites, but was best predicted by intra-annual groundwater variability at non-flooding sites. Plant community composition correlated with groundwater depth and air temperature at all sites, but at non-flooding sites, also with intra-annual groundwater variability and precipitation. Relationships between native plant cover and potential environmental drivers diverged strongly between the two flood regimes; non-native plant cover had only weak relationships with most environmental predictors. Conclusions The current flood regime of a site determined the climate and hydrologic factors that best predicted riparian plant community composition and diversity. Relationships between plant diversity or total cover and groundwater, temperature, precipitation, or groundwater variability can change in strength or direction depending on a site's flood history, highlighting the importance of flood regime to predicting the sensitivity of riparian woodlands to future environmental change.