The Root dynamics of mixed and single species stands in the boreal forest of central and eastern Canada

Abstract

I examined trends of fine root biomass and necromass in different mixed- and single-species stands within the central and eastern regions of the North American boreal forest to determine how annual fine root production, mortality, and decomposition and seasonal patterns of fine root biomass, necromass, and spatial heterogeneity within the soil profile vary with stand species composition. I conducted root excavations in the central region of the North American boreal forest of mature trees of Abies balsamea L., Picea mariana (Mill.) BSP, Pinus banksiana Lamb., and Populus tremuloides Michx. to develop allometric equations relating stem diameter at breast-height and height to coarse root biomass. In the first fine root study, annual fine root production and total fine root biomass in July and October were higher in stands of P. tremuloides, P. mariana, Picea glauca (Moench) Voss, and A. balsamea (mixed-species stands) than relatively pure stands of P. tremuloides (single-species stands). Furthermore, the mixed-species stands had lower horizontal and higher vertical fine root biomass heterogeneity, respectively, compared to the single-species stands. In the second fine root study, annual fine root production and total fine root biomass for most sampling dates (May to October) were higher in both mixed-species stand types (stands of P. banksiana, P. mariana, P. glauca, and A. balsamea (mixed conifer stands) and stands of P. banksiana and P. tremuloides (mixedwood stands)) than the single-species stands (relatively pure stands of P. banksiana (conifer stands)). Furthermore, horizontal fine root biomass heterogeneity was lower in the mixed- than single-species stands in July, August, and September, but similar among the three stand types for the other sampling dates. By contrast, vertical fine root biomass heterogeneity was higher in the mixed conifer than conifer stands from June to September, whereas mixedwood stands differed significantly from conifer stands for only a single sampling date. There were distinct temporal trends of fine root biomass, necromass, and spatial biomass heterogeneity in the second fine root study. Total fine root biomass followed an inverse U-shaped pattern with sampling date (i.e., highest in the summer and lower in spring and fall), while total fine root necromass followed a U-shaped pattern (i.e., lowest in the summer and higher in spring and fall) in all three stand types, respectively. In the two mixed,species stand types, horizontal fine root biomass heterogeneity followed a U-shaped trend with sampling date, while vertical fine root biomass heterogeneity had an inverse U-shaped trend in mixed conifer stands and a U-shaped trend in mixedwood stands, respectively. However, neither horizontal nor vertical fine root biomass heterogeneity differed with sampling date in the conifer stands. The findings of both fine root studies support the theory that the differences in crown structures and rooting traits between component species in the mixed-species and mixed conifer stands in the first and second fine root studies, respectively (niche differentiation), versus increased nutrient availability resulting from the P. tremuloides leaf litter in the mixedwood stands in the second fine root study (facilitation), were promoting greater soil space filling of fine root biomas and fine root productivity in the mixed- than single-species stands for both studies. All regressions for coarse root biomass using diameter at breast-height (DBH) or height alone, or both DBH and height as predictors were significant. The DBH - coarse root biomass models had higher R2 values than the height - coarse root biomass models for all four species, indicating that DBH was a better predictor for coarse root biomass than height. Furthermore, the DBH-height - coarse root biomass models did not have higher R 2 values than the DBH - coarse root biomass models. All but one DBH - coarse root biomass model from the published literature with similar DBH range underestimated or overestimated coarse root biomass using the data from this study. Coarse root biomass allometric equations, therefore, are probably site-specific as above- and below-ground biomass allocation differs with site condition.