Plant communities are altered via simultaneous disturbances (either independently or interactively) and exotic species invasions. Outcomes of disturbances through secondary succession change which species characterize community assemblages. The objective of research was to address how communities shift due to differing combinations of disturbances. A strip-strip block design was established in 1996 in a successional old-field at Touch of Nature Environmental Center (Makanda, IL). Parallel strips were randomly assigned in each block with crossed fertilizer (control, five-year, and annual application) and mowing (control, spring, spring-fall) treatments. Species abundance was recorded for plots within each of 7 blocks during 9 surveys over 22 years. Nonmetric multidimensional scaling (NMDS) displayed species within plots based on Bray-Curtis dissimilarity following Wisconsin double-standardization and vectors displayed the greatest rate of change in species richness, exotic invasive cover, and time (as year). Confidence ellipses were displayed for treatment levels based on site dispersion. Repeated-measures permutational analyses of variance (PERMANOVA) and tests for homogeneity of dispersion (PERMDISP) quantified differences amongst treatments and their interactions. Post-hoc pairwise-Adonis tests characterized pairwise contrasts among treatment levels. Furthermore, indicator species analyses (ISA) identified which species significantly characterized mowing and fertilizer treatments through time.
Results/Conclusions
PERMANOVA and PERMDISP showed a significant interactive effect of fertilizer and mowing on community composition (F4,617=1.637; p=0.01) and dispersion (F8,617=23.07; p=0.001). This interaction was evident in a three-dimensional NMDS ordination in which confidence ellipses for both fertilizer and mowing treatments indicated overlaps in composition for all three fertilizer treatments and both spring and spring-fall mowing treatments separate to un-mowed plots regardless of fertilizer treatment. Pairwise-Adonis indicated significant differences between combinations of fertilizer and mowing treatment levels occurred in which 28 of 36 pairs of treatment combinations were significant. Indicator species analyses identified 50 species as indicators for at least one treatment for one survey. Of these indicators, there were six exotic invasive species, each displaying unique patterns of dominance for different years and combinations of treatments. Over time, exotic invasives became more frequently indicator species (especially in 2013 and 2018 for mowing treatments). The only treatment levels without exotic invasives as indicators for any survey were five-year and annual fertilizer treatments. Both varying levels of mowing and fertilizer over 22 years have led to differences in both composition and dispersion of communities as different native and exotic species (increasingly) can dominate under different regimes of these disturbances.