Effects of organic matter removal, soil compaction and vegetation control on 10th year biomass and foliar nutrition: LTSP continent-wide comparisons

We examined 10th year above-ground planted tree and total stand biomass, and planted tree foliar N and P concentrations across gradients in soil disturbance at 45 North American Long-Term Soil Productivity (LTSP) installations. While ranging across several climate regions, these installations all share a common experimental design with similar measurement protocols. Across all installations planted tree biomass with stem-only harvest (OM0), no compaction (C0) and chemical vegetation control (VC), ranged from 2 to 90 Mg ha−1. When compared with the OM0, full-tree harvest (OM1) had little consistent effect on any response variable. Full-tree harvest plus forest floor removal (OM2) also demonstrated few consistent effects on planted tree biomass, although Boreal – Great Lakes conifers showed some positive effects, reflecting high survival, but also negative effects on foliar nutrition. Compaction (C2), regardless of OM treatment, increased planted tree stand biomass consistently in Warm Humid climates, and compaction with intact forest floors (OM0C2) did so across all regions. However, most installations had medium – or coarse-textured soils and compaction did not achieve theoretical growth-limiting bulk densities. Combining OM2 with C2 resulted in lesser gains in planted tree biomass. Planted tree biomass gains with the OM0C2 were attributed largely to changes in physical soil characteristics, not to vegetation control or nutrient availability. Total stand biomass (Mg ha−1) was either unaffected or, with aspen, reduced by compaction. Vegetation control (VC) consistently enhanced planted tree biomass, regardless of climate, and also enhanced foliar nutrient concentrations on Warm Humid and Mediterranean sites. VC also increased total stand biomass on sites without abundant woody competitors, but decreased it on shrub-dominated Mediterranean sites. For many of the site types and species investigated, harvest-related organic matter removal and soil compaction (excepting aspen vegetative reproduction) have not resulted in large losses in stand biomass 10 year after harvest. Most stands, however, have not yet reached canopy closure, and treatment effects may continue to evolve.

► Stem-only and full tree harvesting produced similar effects on biomass and nutrition.
► Effects of forest floor removal on stand biomass and nutrition varied among sites.
► Compaction with intact forest floors increased planted tree but not stand biomass.
► Compaction effects were not strongly related to foliar nutrition or competition.
► Vegetation control increased planted tree biomass and often foliar nutrition.