Long-Term Site Productivity program: 10 year results

The North American Long-Term Site Productivity program (LTSP) addresses short- and long-term effects of harvest-related disturbance on fundamental soil productivity (i.e., the capacity to capture carbon and produce biomass). The program includes replicated experiments at many locations that are measuring effects on soil productivity of treatments such as stem-only harvest, whole-tree harvest, forest floor removal, and soil compaction. Early results indicate that forest soil productivity is more resilient to these treatments than previously believed – i.e., effects of treatments on tree seedling growth have been smaller than expected and more often beneficial than detrimental.

The LTSP program reached an important milestone earlier this year with the publication of “Effects of organic matter removal, soil compaction, and vegetation control on 10th year biomass and foliar nutrition: LTSP continent-wide comparisons” (Forest Ecology and Management 278:35-54). The authors are Felix Ponder (USDA Forest Service, deceased), Robert Fleming (Canadian Forest Service), and several of their LTSP colleagues. The abstract follows.

“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 90Mg 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.”

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