Twenty-five years of shifting communities and biogeochemistry in a plantation ecosystem in 300 seconds
Experimental conifer plantations established in 1986 in the sub-boreal region of Ontario, Canada, have evolved to contrasting canopy dominant species under different early silvicultural treatments. In those plantations treated with herbicide to control competing vegetation, the canopy is now dominated by the planted native pine (Pinus strobus) or spruce (Picea glauca), with a dominantly moss understory. Plantations that did not receive herbicide treatment essentially succeeded to native hardwoods (dominantly Populus tremuloides in the canopy) and a diverse shrub and herb understory, since mortality of planted conifers was almost 100%. We hypothesized that at 20 and 25 years, measured indicators of C and N cycles (Corg:NT, Cmic:Corg, Nmic:Norg and Cmic:Nmic and incubation of soil under controlled conditions) during the 25-year succession of these contrasting vegetation types would indicate an increasing feedback of the vegetation community.
Over this period we noted shifting ratios of total Corg:NT, Cmic:Corg, Nmic:Norg and Cmic:Nmic, in both the humus and superficial (0-10 cm) mineral horizons. NT gradually decreased in the humus over 20 years, however was consistently lower under the conifer canopy; at 20 years the Corg:NT was higher under the conifer (32) compared to the deciduous canopy (27). Nmic tended to be constantly lower under the conifer canopy at all sampling years, and at 20 years, Cmic:Nmic was higher under the conifer (6.2) compared to the deciduous canopy (5.0). In the mineral soil, Nmic was also lower and under the conifer canopy (all years). Since measures taken shortly after initial disturbances, the N cycle has shifted from a cycle driven by soil microclimate and specific disturbances (herbicide), to a cycle increasingly controlled by surface soil pH (positive relation to NO3-N, negative relation to NH4-N) under the influence of contrasting vegetation communities (structure and composition). Both soil organic C and total N in the same horizon were also related to the pH, underlining the strong controlling nature of this abiotic factor on ecosystem function in these acid forest soils.