The importance of multiple, interacting disturbances on the diversity and abundance of forest understory plant communities

Multiple disturbances may interact to promote forest herbaceous layer diversity, yet most studies focus on this layer’s response to single disturbance events.  In general, studies report that gaps increase the abundance of pre-existing individuals, understory fires favor recruitment and dominance of seed-banking and fire-tolerant species, and ungulate herbivory diminishes richness and abundance of browse-sensitive species.  In nature these disturbances often co-occur and likely influence the herbaceous community simultaneously.  To assess the interactive effects of disturbance on this community we established a total of sixty-four, 400 m² plots containing a full-factorial manipulation of Fires (burned/unburned), Gaps (present/absent), and Browsing (fenced/unfenced) at four upland-hardwood sites in West Virginia.  Within all plots we sampled herb layer richness and abundance from 2000-2006.  Repeated-measures ANOVA’s and planned comparisons were utilized to detect differences over time and among treatments.  A significant Fire*Gap*Time interaction revealed increases in overall richness (5.55 species/m² vs.1.45 species/m²) and percent cover (66.28% vs. 3.88%) in Fire+Gap plots relative to Controls by 2006.  This interaction was consistent when examining the forb, grass, or fern groups individually.  Shrubs (predominantly Rubus spp.) were absent initially but dominated the Fire+Gap+Fenced plots by 2006.  The synergistic impact of fires and gaps on understory composition is corroborated by a significant decline in community similarity between Control and Fire+Gap plots (Chao-Sorensen Index: 0.56 vs. 0.16; 2000 and 2006, respectively).  Our results suggest that although low-intensity fires stimulate plant recruitment, gap creation is essential for their subsequent expansion.  Controlling herbivory enhanced this dynamic only for the most browse-sensitive species.