Abiotic and biotic factors affecting first-year seedling growth and survival in Acer grandidentatum, bigtooth maple

To address issues surrounding management of native relict populations of Acer grandidentatum on the Edwards Plateau of Central Texas, factors affecting growth and survival of first-year A. grandidentatum seedlings were considered. Woody plant community composition was compared between a thirty year-old deer exclosure at Lost Maples State Natural Area in the Edwards Plateau region of Central Texas and at two adjacent sites, one xeric and one mesic, using the quadrat method. Fifteen first-year seedlings were planted in the exclosure and fifteen in the xeric site. Growth and survival rates were monitored for one year. A full factorial greenhouse experiment evaluated the effects of nutrient availability, competition and simulated herbivory on A. grandidentatum growth. Eighty plants were planted with high or low nutrients, with or without competition from the perennial grass Bouteloua curtipendula, and herbivory simulated by 0 %, 25 %, 50 %, or 75 % leaf tissue removal. Growth and survival rates were recorded for one year. The effects of light intensity on A. grandidentatum survival and growth were evaluated by growing five seedlings at 20 %, 40 %, 60 % and 100 % of ambient light levels for six months. Survival, growth rates and dry mass were compared. Photosynthetic rates of five full sun leaves and five shaded leaves of juvenile maples were measured in the field at varying light levels and photosynthetic parameters were calculated and compared. In terms of density and basal area, the community within the exclosure was dominated by Quercus buckleyi, A. grandidentatum, and Juniperus ashei. The mesic site was dominated by Platanus occidentalis, Diospyros texana, and Prunus serotina, while the xeric site contained primarily Juniperus ashei. The exclosure had the highest density of seedlings, saplings, and mature A. grandidentatum. Monitored seedlings planted inside the exclosure had a lower mortality rate than those outside the exclosure, though relative growth rates did not differ. Greenhouse plants that experienced high nutrients, no competition, and 0 % simulated herbivory had the highest relative growth rates and dry mass. Seedlings exposed to 40 % ambient sunlight had the lowest mortality and attained the highest dry mass, while those exposed to full sun had the highest mortality and the lowest dry mass. Photosynthetic light response curves were generated, but no significant differences were seen between the curves for sun and shade leaves. The only significantly different photosynthetic parameter was maximum photosynthetic rate (A_max). Acer grandidentatum is an extremely shade tolerant, early succession plant with low growth rates. Herbivory seems to have a significant effect on A. grandidentatum seedling growth and survival, and can thus affect community composition both through direct damage and by altering abiotic conditions such as available light and competition. Addressing A. grandidentatum recruitment failure in Central Texas requires protection from large vertebrate herbivory, and in areas undergoing restoration, the plant's need for at least partial shade in its early stages should be considered.