Ecosystem processes in boreal conifer plantations: Effects of tree species, site type, and age

Plantation forests are becoming increasingly important suppliers of fibre worldwide. In order to understand the consequences of converting natural forests to plantations, investigation of their ecological properties is warranted. We studied various ecosystem processes, including net primary productivity (aboveground; ANPP), coarse woody debris (CWD) production, litter decomposition, and net N mineralization in jack pine and black spruce plantations, ranging in age from 10 to 55 years, in the Lake Nipigon region of northern Ontario, Canada. Selected results: ANPP (biomass) ranged from 3.1 to 5.9 t ha^-1 y^-1 in jack pine stands, and from 1.8 to 4.4 t ha^-1 y^-1 in black spruce stands, depending upon age and site type. CWD accumulation followed a U-shaped curve over time, and was greatest in productive jack pine stands on mesic sites that had begun to self-thin (at age 35, 1.7 to 5.4 t ha^-1 CWD was found in jack pine on mesic sites compared to 0.3 to 0.6 t ha^-1 in jack pine on dry sites and 0.1 to 1.6 t ha^-1 in black spruce on mesic sites of the same age). Litter decomposition rates increased from young (10 y) to mid-aged (35 y) stands and were higher in jack pine compared to black spruce stands. In jack pine, net N mineralization rates (in the forest floor and top 15 cm of mineral soil combined) ranged from 16 to 51 kg ha^-1 y^-1 of NH₄⁺-N, and from -1 to 6 kg ha^-1 y^-1 of NO₃^--N. In black spruce, values ranged from 39 to 42 kg ha^-1 y^-1 of NH₄⁺-N, and from 3 to 6 kg ha^-1 y^-1 of NO₃^--N. In general, rates of ecosystem processes were influenced by overstory tree species composition, as imposed by forest management, and also varied by age and site type.