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Individual study: Competition effects on wiregrass Aristida beyrichana seedling growth and survival in experimental restoration of a longleaf pine plantation at the J.W. Jones Ecological Research Center, Georgia, southeast USA

Published source details

Mulligan M.K. & Kirkman l.K. (2002) Competition effects on wiregrass (Aristida beyrichana) growth and survival. Plant Ecology, 167, 39-50


Understanding plant competitive interactions during seedling establishment may be useful in developing reintroduction strategies for target species. The restoration of a species-rich ground cover in longleaf pine Pinus palustris-wiregrass Aristida spp. ecosystems of southeastern USA is an example; restoring degraded longleaf and wiregrass habitat is of conservation interest because less than 3% of the original area, post European settlement, remains. This study examined the effects of neighboring plants on wiregrass seedling establishment of two seedling ages (3 weeks and 6 months) in two commonly encountered initial restoration site situations; abandoned fields and longleaf pine plantations. Summarised here are wiregrass restoration experiments undertaken in a longleaf pine plantation.

Study site: The study was undertaken in a 20-year-old, 49 ha longleaf pine plantation within the J.W. Jones Ecological Research Center (31°15’N, 84°30’E) at Ichauway, Georgia. The relatively closed canopy (75% canopy cover; natural pine savannas typically have <50% canopy cover) and sandy soil resulted in little understory or ground vegetation. Wiregrass seedlings were grown in a greenhouse. Seeds were sown to establish two age groups of seedlings (6 month, 3 week) at planting.

Canopy reduction: In 1994, plots were established for canopy reduction and wiregrass density treatments. Each of 10 blocks contained three, 25 × 25 m plots, randomly assigned one of the following treatments:

i) control - no canopy removal (basal area (ba) = 25 m²/ha);
ii) 33% reduction in overstory basal area (remaining ba = 16 m²/ha);
iii) 66% reduction in overstory basal area (ba = 8 m²/ha).

Effect of wiregrass seedling density on establishment: Each plot in five of the blocks was divided into two 10 × 10 m split-plots. In April 1994, 6-month old seedlings were planted at 5 seedlings/m² (based upon natural wiregrass density) or low density (5 seedlings/10m²). The remaining five blocks had the same canopy treatments but no seedlings were planted.

Effect of neighboring plants: In the canopy reduction + wiregrass density split-plots, three replicates of all possible combinations including: i) seedling age at planting (3-week and 6-month); and ii) competitive exclusion (root exclusion, aboveground wiregrass exclusion, and no exclusion) (540 seedlings in total), were applied.

Two types of exclusion device (to eliminate root competition with established wiregrass and pines) were installed in January 1999: i) PVC pipe (30 cm diameter x 30 cm length) driven into the soil to 30 cm; ii) sharpened metal soil corer (30 × 30 cm). To eliminate competition for light by neighboring wiregrass plants, cloth collars were installed to prevent adjacent, established wiregrass foliage from overlapping planted seedlings.

Planting and monitoring: In March 1999, one wiregrass seedling was planted in each plot and watered immediately after planting. Seedlings that died within the first week were replaced. Severe drought occurred in the two months (April and May) following planting. Therefore to prevent seedlings dying, they were watered at a rate equal to about half the average weekly rain amount during this period. Plots were hand-weeded weekly. Tiller number per seedling was recorded at planting; tiller number and survivorship were recorded in October 1999.

Survival of 6-month old seedlings was unaffected by competition treatment, but 3-week old seedling survival was greatest where neighbouring plant roots were excluded. Where wiregrass was present, both 3- week and 6-month old seedlings increased growth with root exclusion; but for 3-week old seedlings in plots lacking neighbouring wiregrass, root exclusion (i.e. of pine roots) did not enhance growth. Where neighboring wiregrass plants were absent, increasing tree canopy density resulted in decreased seedling size, but did not affect seedling survival.

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