Temporal change in the herbaceous understory community of an old-growth forest: from seasons to decades

Abstract

Studies focusing on the herbaceous groundlayer of forest communities have become increasingly common in the literature, yet rarely do these studies adequately capture the temporal dynamics that typify this stratum. Using long-term data from an old-growth remnant stand in southeastern Ohio, we compare seasonal, annual, and decadal changes in herb composition and abundance on a north- and south-facing slope. Herbs were sampled within 70, 2-m2 circular plots at three times throughout the growing season in 1997, 1998, 2011, and 2012. Slope, season, and year all had significant effects on species composition, as did their interactions. Species richness and diversity varied considerably throughout seasons, yet remained consistent over the fifteen-year study interval. A dramatic reduction in overall abundance, from an average of 63 % in 1997 to just 12 % in 2012, was observed. This decline was most severe from 1997 to 1998. A nonmetric multidimensional scaling ordination for all 4 years showed a collapse in phenological distinction among seasonal-sampling units. Finally, we looked at year-to-year changes in individual species abundances by comparing occurrence frequencies. The frequency of a species in year i was usually a good indicator of its frequency in year j; however, a few species showed considerable increases or decreases, such as the invasive Alliaria petiolata, which increased from zero plots in 1997 to 15 plots in 2012. These results suggest the importance of adequately and simultaneously documenting vegetation changes at short, intermediate, and long-term timescales—especially for understanding the dynamics of temperate deciduous forest herb communities.