Coarse woody debris in undisturbed and logged forests in the eastern Brazilian Amazon

摘要

Coarse woody debris (CWD) is an important component of the carbon cycle in tropical forests. We measured the volume and density of fallen CWD at two sites, Cauaxi and Tapajos in the Eastern Amazon. At both sites we studied undisturbed forests (UFs) and logged forests 1 year after harvest. Conventional logging (CL) and reduced impact logging (RIL) were used for management on areas where the geometric volumes of logs harvested was about 25-30 m(3) ha(-1). Density for five classes of fallen CWD for large material (>10 cm diameter) ranged from 0.71 to 0.28 Mg m(-3) depending upon the degree of decomposition. Density of wood within large fallen logs varied with position relative to the ground and with distance from the center of the log. Densities for materials with diameters from 2 to 5 and 5 to 10 cm were 0.36 and 0.45 Mg m(-3), respectively. The average mass (+/-SE) of fallen CWD at Cauaxi was 55.2 (4.7), 74.7 (0.6), and 107.8 (10.5) Mg ha(-1) for duplicate UF, RIL, and CL sites, respectively. At Tapajos, the average mass of fallen CWD was 50.7 (1.1) Mg ha(-1) for UF and 76.2 (10.2) Mg ha(-1) for RIL for duplicate sites compared with 282 Mg ha(-1) for live aboveground biomass. Small- and medium-sized material (<10 cm dia.) accounted for 8-18% of the total fallen CWD mass. The large amount of fallen CWD at these UF sites relative to standing aboveground biomass suggests either that the forests have recently been subjected to a pulse of high mortality or that they normally suffer a high mortality rate in the range of 0.03 per year. Accounting for background CWD in UF, CL management produced 2.7 times as much CWD as RIL management. Excess CWD at logging sites would generate a substantial CO2 emission given the high rates of decay in moist tropical forests.