Response of immature aspen ecosystems to cutting and burning in relation to vernal leaf-flush

Abstract

Vegetative reproduction, above-ground biomass and nutrient pools, and litterfall and substrate nutrient conditions were evaluated in eastern Ontario immature (age 20 years) aspen ( Populus tremuloides) Michx. and P. grandidentata (Michx.) ecosystems which had been subjected to the following four treatments in relation to vernal leaf flushing: burning before; burning after; cutting before; and cutting after flushing. An untreated control area was set aside for appropriate comparisons. Three years after treatment, the greatest numbers of stems ha −1 were supported by the pre-flush cutting treatment (11 00 stems ha −1) followed in decreasing order by post-flush cut (9000), post-flush burn (4000), and pre-flush burn (2000). No suckering was observed on control plots. Above-ground aspen biomass and nutrient-pool values reflected stem densities and these results were discussed in light of known physiological responses of the species to disturbance. Litterfall mass and nutrient inputs over the 3-year observation period were also a function of treatment and reflected stand breakup. There was reduced suckering on the two burning treatments compared with more-vigorous suckering on the cuts. Thus, 3-year totals for litterfall mass (kg ha −1) were: 29 470, 21 393, 10 182, 3022, and 1762 on post-flush, pre-flush burn, control, pre-flush cut, and post-flush cut, respectively. High litterfall biomass values on the burning treatments were a result of overstorey mortality, which reached 100% after three years. Nutrient returns through litterfall followed litterfall biomass input trends. Forest-floor and mineral-soil nutrient pools on the burns showed treatment effects one month after burning which were interpreted in terms of removal of part of the forest floor, changed N-mineralization rates, cation leaching from ash, and differences in nutrient-uptake patterns by surviving overstorey. After three years some treatment effects were still noticeable in forest-floor and, to a lesser extent, in mineral-soil nutrient pools.