20 YEARS OF ECOSYSTEM REORGANIZATION FOLLOWING EXPERIMENTAL DEFORESTATION AND REGROWTH SUPPRESSION

Watershed 2 of the Hubbard Brook Experimental Forest was experimentally deforested in the winter of 1965-1966, and subsequent plant regeneration was suppressed by herbicide application until the 1969 growing season. Changes in species structure, plant abundance, aboveground primary productivity and biomass, and aboveground nutrient pools were recorded on 70 permanent plots distributed across the watershed during 1, 2, 3, 5, 11, and 20 yr of subsequent succession. Species richness increased rapidly over time but equitability and the Shannon-Wiener diversity index declined with succession. Plant densities of herb, shrub, sapling, and tree strata increased, then decreased, in successive waves as the vegetation increased in height. Basal area (stems >5.1 cm dbh) initially increased exponentially, then linearly after the 5th yr to attain a total of 18.7 M2 /ha by the 20th yr. Primary productivity also increased exponentially at first, but then increased linearly through the 20th yr at which time annual aboveground primary productivity was 20 Mg/ha. Above ground biomass increased linearly after the 5th yr until by the 20th yr it was 52 Mg/ha, 38% of the reference forest biomass. Above ground nutrient pools of some elements accumulated faster than biomass by the 20th yr when calculated as percentages of the 55-yr-old reference forest pools and biomass. These included: potassium-52%, phosphorus-44%, magnesium-42%. Others accumulated more slowly than biomass: sulfur-32%, and nitrogen and calcium each-29%. While recovery rate on Watershed 2 was delayed in the first decade of regrowth, and composition was somewhat altered because of the 3-yr suppression, this forest nevertheless displayed a vigorous capacity for regeneration after suppression ceased.