Soil carbon, nitrogen, and phosphorus and their stoichiometric ratios are affected by stand age and overstory type in the fire-driven boreal forest
Carbon, nitrogen, and phosphorus are intrinsically linked, yet these nutrients also cycle through ecosystems in unique ways, and thus are found in the environment in varying abundance. Total concentrations of soil nutrients are important indicators of site fertility, but optimal plant growth and ecosystem functioning are more strongly correlated with nutrient stoichiometry, since evolution has dictated the use of elements by plants in particular proportions. Under rapid climate change, C, N, and P cycles may become decoupled, with unknown consequences for ecosystem functioning. Forest fires, a critical driver of nutrient cycles, may also be affected by global change processes, with increases in frequency predicted for the boreal biome. Despite this, long-term successional trends in soil C, N, and P and their stoichiometric ratios in the fire-driven boreal mixed-wood forest are not yet fully understood. We used a carefully constructed, 209-year chronosequence, with replicate stands representing each of three dominant upland forest types in our study area (n=54), to examine the successional dynamics of C, N, and P in the forest floor, upper (0-15 cm), and lower (15-30 cm) mineral soil, and to determine the extent to which temporal trends were influenced by overstory type.
In the forest floor, C and N required ~2 to 3 decades to recover following fire, while P did not reach peak concentrations until 209-years after fire, when it was far more abundant than at any other successional stage. In the upper mineral soil, P was strongly affected by competition between trees; we found dramatic declines in concentrations at stem exclusion (33-years after fire) in all overstory types. Between 33- and 209-years-old, P trends were influenced by overstory type, with broadleaf stands accumulating ~30% more P than others. All overstory types were characterized by nutrient limitation during early canopy transition phase (98-years since fire), but conifer stands were far more N- and P-deficient than broadleaf stands, demonstrated not only by low concentrations, but also by extremely high C:nutrient ratios in the forest floor, and high C:N in the upper mineral soil. Conifer stands remained nutrient-limited throughout late succession (98-, 146- and 209-years since fire); longer than broadleaf stands. Mixed-wood stands were intermediate. Our results indicate forest floor P-deficiency as an important concern in boreal conifer stands, particularly with reduced fire cycle length. In addition, harvesting of stands during periods of N- and P-scarcity may impact the productivity of subsequent stands.