DIVERSITY OF THE UNDERSTORY VASCULAR VEGETATION IN 40-YEAR-OLD AND OLD-GROWTH FOREST STANDS ON VANCOUVER-ISLAND, BRITISH-COLUMBIA

We studied plant diversity of the understory vascular vegetation in 40 yr-old plantations (immature stands) and old-growth forest stands on southwestern Vancouver Island, British Columbia, Canada. Site-specific comparisons using several indices of species diversity were made betw een: (1) immature stands segregated according to the canopy cover and dominant canopy tree species; and (2) immature and old-growth stands. There were no significant differences (P < 0.05) among immature stands in species richness (S) and the Shannon-Wiener index (H'), in relatio n to the canopy cover or in S, H' and evenness (E) in relation to the dominant canopy tree species. Using the same indices, the plant divers ity varied with edaphic conditions (represented by five site associati ons) and time (represented by two developmental stages). At both stand -and site levels, plant diversity increased with increasing soil moist ure, from slightly dry to moist sites, and with increasing plant-avail able soil nitrogen in both immature and old-growth stands; and the pla nt diversity of immature stands across the sites studied was considera bly lower than in old-growth stands, regardless of site association. T he indices of plant diversity, floristic similarity indices, and speci es turnover rates indicated that the immature stands had their plant d iversity at a minimum, but a drastic loss of diversity expected in the stem exclusion stage had not materialized. We attributed decline in p lant diversity to the absence of old-growth structural features in imm ature stands. Several measures to foster the stand-level diversity wer e proposed.