Individual study: Competition effects on wiregrass Aristida beyrichana growth and survival in experimental restoration of wiregrass on field margins at the J.W. Jones Ecological Research Center, Ichauway, Georgia, southeast USA
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 particular 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 on old fallow field margins.
Study areas: Six old cultivated fields were selected within the J.W. Jones Ecological Research Center at Ichauway (31º15'N, 84º30'E), Georgia, USA. The outer 15-20 m of each had been cut annually for several decades to reduce shrub encroachment, but left fallow. Experimental plots where established in these fallow margins.
Wiregrass seedling cohorts were grown in a greenhouse at the Center. Seeds were sown to establish two age groups of seedlings (6 month, 3 week) at the time of planting.
Experimental design: In October 1998, the margins were triple-harrowed. Four blocks were established in each field (total 24 blocks). Each block contained 12, 1 mÂ² plots. Three factors were examined: fertilization, weeding and age of seedlings at planting. Fertilizer application consisted of a single application of 5-10-15 NPK fertilizer (March 1999) at a rate of 272g/mÂ². Weeds were removed by hand from half of the treatment plots prior to planting, and were weeded weekly thereafter. Within each block, every 1mÂ² plot was randomly assigned a treatment combination (fertilizer × weeding × seedling age at planting = 12 treatment combinations per block).
Five seedlings per 1mÂ² plot were planted (total of 1,440 seedlings) and watered immediately after planting. Seedlings that died within the first week were replaced. All seedlings were watered five times in April and May. Initial seedling tiller numbers were counted, and in October 1999, tiller number and survivorship were recorded.
Survival of 6-month old (at time of planting) wiregrass seedlings was much higher (typically around 90%) in all cases than the 3-week seedlings (survival >< 40-60%).
Weeding clearly benefited 3-week seedling survival (c.45% in unweeded plots; 60% in weeded plots) but 6-month old seedling survivorship remained high (>90%) regardless of weeding treatment. Seedlings were smaller where neighbouring weedy vegetation was present regardless of wiregrass seedling age and average tiller number was much reduced for both seedling age classes (3-week seedlings: unweeded - 1 tiller weeded - 14 tillers; 6-month seedlings: unweeded - 11 tillers, weeded - 53 tillers).
Fertilizer application appeared not to benefit seedling survival or growth, and in fact reduced survivorship of 3-week old seedlings (3-week seedlings: not fertilized - 58%, fertilized - 44%; 6-month seedlings: not fertilized - 94%, fertilized - 90%). Despite this, the authors consider that it is still unclear whether fertilizer application is beneficial to the successful establishment of wiregrass.
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