LITTERFALL DYNAMICS AND ECOSYSTEM RECOVERY DURING FOREST DEVELOPMENT

A field study was conducted at the Hubbard Brook Experimental Forest (USA) to quantify changes in the mass, chemistry and timing of litterfall during stand development of the northern hardwood forest and to relate changes to other ecologically meaningful forest dynamics. Leaf fall on three contiguous watersheds, harvested 1, 15 and 66 years ago, was measured with litter traps from 1984 to 1988. Woodfall into the 15-year-old stand was monitored on permanent plots. During the first 17 years of forest development, fallen leaves and wood of Prunus pensylvanica accounted for 32-59% and more than 97%, respectively, of total mass litter inputs. In the older forest, Acer saccharum, Fagus grandifolia, and Betula alleghaniensis contributed 53%, 25%, and 21%, respectively, of the total leaf fall mass of the stand. Changes in the leaf mass of each species during forest development were the result of an increased number of leaves rather than increased mass per leaf. Leaves fell first in the oldest stand and last in the youngest stand. The nitrogen (N) content and non-structural carbohydrates of fallen leaves were highest in the youngest stand, but there was no overall trend in later stages of forest development. In 1985, leaves of all species in all stands had significantly reduced N contents (only 23-80% of N in leaves falling in 1984). In the oldest stand, this difference resulted in leaf fall N of 30.3 kg ha-1 (1984) and only 15.6 kg ha-1 (1985). Leaves of different species varied considerably in organic fractions but, with few exceptions, cellulose and lipin were significantly higher in 1985 than in 1984. The large difference between 1984 and 1985 in organic fractions and N suggests that leaf litter quality may vary greatly from year to year and may partially account for unbalanced nitrogen budgets. Large pulses of wood fell each year into the mid-aged stand (1151-1283 kg ha-1 year-1). This woodfall, high in lignin (28-3 1 %) but low in nitrogen (0.1-0.2%), supplemented leaf fall and yielded total litterfall lignin: nitrogen ratios of 18.9 (1984) and 30.8 (1985). The timing of this deposition coincides with a transition to accretion of forest floor biomass from a protracted decline following disturbance. Apparently, the combined effects of high litterfall and low substrate quality lead to a sustained accumulation of the forest floor until a steady state is reached, about 50 years after disturbance.