Effects of herbivory on growth and survival of seedlings of a rainforest tree, Alphitonia whitei (Rhamnaceae)

This study examined the effects of insect herbivory on growth and mortality of seedlings of a mid-successional rainforest tree, Alphitonia whitei Brai d. Two experiments were conducted in which seedlings were exposed to 0% and 50% natural defoliation by insect herbivores and placed in light gaps in s imple notophyll vine forest at Paluma, near Townsville, North Queensland. I n the first experiment, insect herbivory significantly increased mortality of 2-month-old seedlings. Smaller seedlings had significantly greater morta lity rates than larger seedlings, irrespective of herbivory. A significantl y greater proportion of smaller seedlings died from being smothered by fall en leaves and soil as a result of digging by vertebrates than for larger se edlings. In the second experiment, the effects of seedling age were examine d by comparing 2-month and 4-month-old seedlings. Mortality rates were sign ificantly influenced by seedling age, with eight rimes greater survival of older seedlings than of younger seedlings. Although insect herbivory was co rrelated with a significant decrease in shoot mass and a significant increa se in root:shoot ratio, there was no effect of insect herbivory on seedling survival in the second experiment. Thus, mortality rates were greater for seedlings if they were young or small (which indirectly results from insect herbivory), because small, young seedlings were more susceptibile to other mortality factors, such as burial by fallen debris and digging by vertebra tes.