Foster, B.L. and K.L. Gross
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
Over a six year period, tree and shrub recruitment dynamics were studied in all six replicates of the KBS LTER native succession treatment (treatment 7). This study was carried out to document the temporal and spatial patterns of woody plant establishment and mortality occurring in early succession in old-fields of the southwest Michigan region.From 1991 (two years following abandonment from agriculture) to 1996 (final year of the study), annual seedling surveys were conducted during the month of May within each of the six replicates of the native succession treatment. In May 1991, a 10 × 20 meter plot was permanently marked near the center of each of the replicates. In the initial survey, all woody plant seedlings within the plots were marked and mapped to the nearest square meter. Each year thereafter, all newly established stems were marked, and stems marked in previous years were accounted for as dead or alive.Woody plant succession in these plots was characterized by increases in both the number of woody species (Fig. 1) and the number of woody stems (Fig. 2) over time. The strong increase in stem numbers between the fifth (1994) and seventh year (1996) following abandonment reflects both an increase in the establishment of new genetic individuals and the proliferation of clonal stems by the bird-dispersed shrub, Rhus typhina.Accompanying changes in species and stem numbers over time in these plots was a shift in dominance from wind to bird dispersed species (Fig. 3). In the initial survey (1991, two years following abandonment), the wind dispersed species, Populus deltoides, represented 66% of all stems marked. From examination of bud scars in 1992, it appears that the majority of the Populus seedlings present in the initial survey (1991) had established a year earlier during the first year of succession (1989-90). The average number of new Populus seedlings establishing in the plots declined from a high of 12 seedlings per plot in the first year, to zero by the fifth year of succession (Fig. 4). The number of stems of wind dispersed species remained relatively constant over the entire study period. In contrast, the number of stems of bird dispersed species increased by 123% from the second to the seventh year following abandonment (Fig. 3).There was considerable variation in woody plant recruitment among the six replicates. In the last year of the study (1996, 7 years following abandonment), stem numbers ranged from 43 in Replicate 3, to 387 in Replicate 4. Stem numbers where lowest in the two most interior replicates (Reps 3 and 5), which suggests that landscape position may have influenced recruitment patterns across the site. Woody plant species richness was negatively correlated with grass cover across replicates, suggesting that variation in competition with herbaceous vegetation may explain some of the variation in recruitment observed among the replicates.Although there was a high degree of spatial variability in woody plant recruitment patterns, these surveys document directional changes over time in the diversity and abundance of woody species, as well as clear temporal shifts in recruitment with respect to mode of dispersal. Future analyses of these data will focus on temporal and spatial patterns in woody plant mortality, and a more thorough assessment of recruitment patterns in relation to landscape factors.
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