Interspecific responses in tree foliar nutrient resorption following fire and logging along a chronosequence in a central Canadian boreal forest

Background/Question/Methods

Biomass harvesting for energy production is a recent practice in forest management, but there are concerns for the resiliency of these forests following nutrient removal. Plants resorb approximately 60% of the nutrients in senescing foliage back to living tissues prior to abscission to the forest floor layer. This process carries great significance for its role in the overall nutrient budget of a tree occupying nutrient-poor habitats in a northern climate. Experimental studies quantifying the effects of biomass harvesting on nutrient resorption of different species over time are critical for sustainable forest management practices. In this study we use the chronosequence approach to investigate how disturbance influences nutrient resorption of 5 tree species. By sampling green foliage in the growing season and senesced foliage in the fall season, we determined the nitrogen resorption efficiency (NRE) and phosphorus resorption efficiency (PRE) in plots representing post-disturbance conditions along an age chronosequence. We then sampled N and P in the soil to quantify the relationship between living tissues and soil environment. An analysis of variance (ANOVA) was performed to quantify the effects of stand age, disturbance origin, and overstory species dominance on NRE and PRE of each of the 5 species of trees.

Results/Conclusions

Our results show that NRE decreased with time since disturbance, with young trees (7 and 15 years since disturbance) demonstrating the highest NRE, compared with older (33-209 years since disturbance), regardless of the origin of disturbance. NRE was significantly higher (P<0.01) in the youngest stands (7 and 15 years) following biomass harvest, compared with stands with wildfire origin of the same age. NRE was also relatively higher for deciduous species (Populus tremuloides and Betula papyrifera) than coniferous species (Pinus banksiana, Picea mariana, Abies balsamea) in all treatments. Phosphorus resorption efficiency (PRE) was not influenced by disturbance origin and did not change through time since disturbance across any tree species. NRE and PRE were negatively correlated (R²>0.8) with soil %N and %P. Given that NRE patterns are negatively correlated with soil %N, these results suggest that young deciduous trees in boreal ecosystems require a higher amount of N than older and coniferous trees, and that biomass harvesting enhances NRE due to changes in the soil environment post-disturbance. Our findings indicate that origin of disturbance is an important driver for nutrient resorption processes and should be included in any biogeochemical cycling modelling.