Community Structure, Dynamics and Nutrient Cycling in the Okefenokee Cypress Swamp‐Forest

Abstract

Aspects of community structure and nutrient circulation are described for the cypress (Taxodium distichum) forest in Okefenokee Swamp, Georgia, USA. This bog environment is characterized by low nutrient availability and large peat accumulations. The tree stratum of the forest is dominated by cypress, which is probably due to recurrent understory fires which eliminate other swamp species. In stands throughout the cypress forest, the density and total basal area of living stems >4 cm diameter vary greatly, but mean values (1,465 stems/ha; 52 m2/ha) are high compared to upland forests. Natural thinning appears to be unimportant. Abundant standing dead trees suggest that differences in density among stands are due to past differences in the frequency and intensity of forest fires during periodic droughts. At an intensive study site, biomass and net primary production were measured using techniques of dimension analysis. Cypress trees compose 98% of the total forest above—water biomass of 307 tonnes/ha. Most of the cypress biomass is in tree boles (96%), little is in foliage (0.8%). Presumably due to the acid, nutrient—poor conditions in the swamp, total above—water net primary productivity is low (692 g°m—2°yr—1); the high biomass is the result of the old age (150 yr) of the stand and the high density of trees. Cypress net production is largely channeled into bole wood (41%) and current twigs with needles (41%). It appears that in recent years, above—water biomass has been maintained at a constant level by fire—induced thinning. The nutrient pools in the above—water community are large (666 kg/ha Ca, 111 kg/ha Mg, 230 kg/ha K, 996 kg/ha N and 46 kg/ha P). These nutrients are largely contained in cypress boles and do not freely circulate. The annual nutrient uptake in the forest is rather small compared to that in upland forests. The nutrient uptake (54 kg°ha—1°yr—1 Ca, 11 each for Mg and K and 2.3 for P) is largely channeled to the small amount (231 g/m2) of very efficient cypress foliage. Foliage abscission and foliar leaching by rainfall return 73 to 91% of the annual nutrient uptakes each year; therefore, there are only small permanent additions of nutrients to the above—water pools each year. Except for K which is apparently reabsorbed from foliage before abscission, the cypress trees do not appear to conserve foliar nutrients. However, nutrient conservation may be effected by minimizing the amount of foliage. Litterfall from the forest has resulted in a large accumulation of peat. Although some nutrient regeneration apparently occurs, these peat deposits contain large, permanent nutrient losses from the community. Nutrients cycling is dissolved form (e.g., foliar leachates) and nutrients received from atmospheric precipitation are likely to be of special importance to the nutrition of this closed—basin swamp forest community.