Productivity Relationships and Developmental Dynamics Over 26 Years Within Mixed Aspen-White Spruce Stands in Minnesota

Abstract

The productivity and dynamics of mixed aspen (Populus tremuloides)-white spruce (Picea glauca) forests were examined over a 26-year period across a series of replicated long-term silvicultural trials within northern Minnesota. Long-term growth and biomass production rates were compared between pure aspen, pure white spruce, and mixed aspen-white spruce stands established through planting and/or natural regeneration following clear-cut harvesting in 1982. Interestingly, there were no differences in spruce annual mortality rates between pure and mixed stands, with spruce averaging 1.2 ± 0.26 and 1.4 ± 0.20 % mortality in mixed and pure stands, respectively. In contrast, spruce in pure stands had significantly greater height growth rates than those found in mixtures. Aspen height growth and mortality rates were unaffected by spruce, as there were no differences between aspen grown in pure and mixed stands. Spruce height growth rates were much lower than aspen within mixed stands, resulting in rapid stratification into two-storied stands that contained a lower canopy stratum of white spruce at 4.9 ± 0.3 m and an upper canopy stratum of aspen at 13.8 ± 0.5 m after 26 years of development. Comparisons of productivity rates between pure and mixed stands indicated that there were no significant differences in biomass production rates between pure aspen and mixed aspen-white spruce stands (7.84 ± 0.08 and 6.61 ± 0.97 Mg/ha/yr), whereas pure white spruce stands had much lower levels of productivity (0.98 ± 0.05 Mg/ha/yr). Notably, the highest level of productivity observed across the study was within a stratified, mixed aspen white spruce stand (8.71 Mg/ha/yr); suggesting the potential for higher levels of production within these species mixtures relative to pure populations. In addition, productivity levels associated with spruce within mixtures was largely additive to aspen production, highlighting the potential for increasing productivity in pure aspen systems through spruce underplanting