CARBON AND NITROGEN DYNAMICS OF BOREAL JACK PINE STANDS WITH AND WITHOUT A GREEN ALDER UNDERSTORY

We compared the species composition, structure and selected components of the carbon (C) and nitrogen (N) budgets of similar-aged, mature bo real jack pine (Pinus banksiana Lamb.) forests with and without green alder [Alnus crispa (Ait.) Pursh.] in two different boreal environment s. The C and N content of the overstory biomass components (for exampl e, stem, branch, and foliage), total vegetation, forest floor, and min eral soil were greater (P = 0.05 to P = 0.10) for jack pine with alder (JPA) stands than for jack pine without alder (JP) stands at both stu dy areas. Jack pine foliage N isotopic discrimination (delta(15)N) and annual litterfall N content were significantly greater (P < 0.05) in the JPA than the JP stands at both study areas, suggesting that alder was fixing N and that N availability was greater in the JPA than the J P stands. The greater leaf area index (LAI) and overstory C accumulati on in the JPA than the JP stands (P < 0.05) is likely because of the g reater N availability in the JPA stands, but the effect of soil textur e discontinuity on water availability in the JPA stands can not be dis missed. Percent ground cover by feathermoss varied among the jack pine communities and was positively correlated with overstory LAI (r(2) = 0.83, P < 0.05). One index of N-use efficiency (NUE), defined as above ground net primary productivity (ANPP) per litterfall N, was significa ntly greater (P < 0.05) for the JP than the JPA stands, but a second i ndex of NUE, ANPP/N uptake, did not differ between the two jack pine c ommunities. Jack pine trees growing without alder produced more organi c matter per unit of N, but percent N retranslocation from senescing f oliage and N mean residence time in the overstory did not differ betwe en the JPA and the JP stands. A conceptual model is presented that ill ustrates the potential influence of alder on the species composition, structure, and function of boreal jack pine forests.