Vegetation dynamics in a 2-yr old restored floodplain during drought conditions

Background/Question/Methods

Full restoration of a degraded floodplain involves intensive efforts including  severe alteration of the abiotic conditions.  Vegetation response to these activities is important for ecological and practical reasons.  Recent meta-analyses of wetland system recovery indicated considerable variation among sites, a high degree of compositional change early, and full recovery may require 20+ years.   The recovery within 5-yr is critical, but we do not know how the dynamics are affected by stressful  growing season (below-average precipitation).   A heavily degraded stream and associated floodplain (7.4 ha) in central Wisconsin were restored in summer and early fall of 2010 by lowering and re-contouring the floodplain, and relocation of the channel. The final step was seeding a wetland mix of 14 graminoids and 14 forbs.  Late in the 2011 growing season, we established a systematic array of plots consisting of 103 quadrats along 24 transects.  These quadrats were re-surveyed in July 2012, and in October 2012 a subset (N=40) was inventoried.  The cover and identity of all plants were determined.  Dominance at the quadrat level was determined and scored on a 5-3-1 scale for first through third most abundant species.  This was done for each inventory period, and a ‘dominance ranking’  derived by summing the points by species.  Proportional gain, loss and turnover  (Anderson 2004) were calculated from October to October.

Results/Conclusions

During the growing season (Apr.-Sept.) of 2012, the area experienced a rain deficit of 16 cm.  Despite a doubling of richness in the floodplain from Oct., 2011 to July, 2012, the average cover (37%, 30%) and species density (7.3, 5.8) decreased from October to October.   Contrary to other studies, the proportional loss was much greater (2.5X) than the gain from October to October.  The combined effect was a 40%+  turnover rate.  The average turnover was higher than noted in other studies, perhaps due to the small size of the wetland and fate of opportunistic (facultative wetland status) species.  Contrary to expectations, species with a strong wetland affiliation increased in relative importance- wetland obligates increased from 36% in 2011 to 53% in 2012 and the top two dominants in July and Oct., 2012, were wetland obligates.   This suggests a relatively high degree of resistance/tolerance  to moisture stress among some wetland species,  and a surprising level of resiliency in the community as a whole.