Assessing the contribution of Red Alder (Alnus rubra) to forest stand nitrogen budgets




Nehring, Lise

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Red Alder (Alnus rubra) is a native coastal hardwood in British Columbia and has evolved a symbiotic relationship with the nitrogen-fixing actinomycete, Frankia. This research uses δ15N signatures in soils, wood and litter to assess the contribution of nitrogen-fixing Red Alder to the components of stand nitrogen budgets. The stands used in this study are part of the B.C. Ministry of Forests’ long-term Experimental Project 1121.01 which examines the interactions between conifers and Red Alder. Planted in 1994, the Holt Creek site contains stands of Douglas-fir and Red Alder in five proportions (Red Alder: Douglas-fir proportions: 100/0, 50/50, 25/75, 11/89, 0/100). Increment cores from 5 trees per species per plot were taken along with soil and litter samples and analyzed for essential mineral elements and δ15N. I hypothesized that Red Alder would enhance soil nitrogen stocks and elevate δ15N signatures and that these changes would be observable in the δ15N signature of the tree rings of both species. Forest floor soil under Red Alder in the 100/0 plot was enriched in total nitrogen, and δ15N was elevated. This was due to the addition of nitrogen-rich litter, like followed by nitrogen discrimination in the forest floor during the process of nitrate leaching or denitrification. The litter of the two species did not differ in δ15N. The effect of forest floor nitrogen enrichment was visible in the tree rings of Douglas-fir in the 50/50 stand confirming that the effect of fixed-nitrogen can be observed in non-fixing species. Red Alder tree ring δ15N exhibited an unexpected non-linear relationship with time that could be due to reduced nitrogen fixation associated with declining tree vigour or negative feedback from low soil pH. This research provides insight into nitrogen fixation by Red Alder over time and its influence on pure and mixed stand nitrogen budgets.



Red Alder, Stand nitrogen budget, Nitrogen fixation, Tree rings, Stable isotopes, Douglas fir