Old-growth redwood forests II: Accurately quantifying the endpoint of above-ground carbon accumulation

Background/Question/Methods

As part of a larger study addressing the response of Sequoia sempervirens to changing environmental conditions throughout its range, three 1-hectare plots (plot dimensions were 10:1, 316.23 m x 31.623 m) were installed in upland portions of old-growth rain forests at the northern end of the species range.  In these rain forest environments, upland forests are not only wetter, but tend to have greater accumulations of downed wood.  The three plots were divided among primary rain forest reserves – Redwood National Park, and Jedediah Smith and Prairie Creek Redwoods State Parks, California.  Whole-tree quantities were predicted from trees within plots, so plots were chosen to include a wide variety of tree sizes and levels of suppression, including individuals that were near endpoints of structural complexity.  All above-ground plant material, live or dead, was quantified by species, although 94–99 percent of the total mass was in three Sequoia categories – living trees, snags, and logs.  At this extreme end of global biomass, few existing allometrics were adequate.  Even some understory species (e.g. Vaccinium with basal diameters up to 22 cm and heights over 9 m, or Polysticum with > 230 cm frond lengths) were well beyond the range of existing equations.  Equations for all major tree and understory species had to be generated for this study.  Fully crown-mapped trees and whole-tree samples were used to develop aboveground predictors for Sequoia, Pseudotsuga, Tsuga, and Notholithocarpus.  The 5 decay-class log system used so effectively throughout the Pacific Northwest had to be modified to better address the often large diameter and extremely persistent heartwood of Sequoialogs.  All downed wood > 30 cm in diameter was measured and mapped within each plot. 

Results/Conclusions

Stand basal area for live Sequoia exceeded 450 m² ha^-1, representing an oven dry mass of nearly 5 Gg ha^-1.  Understories in these stands are extremely wet – even at the height of summer, condensation accumulates at night and the moisture often does not evaporate until mid-afternoon, if at all.  Thus saturated and decay-resistant heartwood accumulates, particularly in the three upland plots, where dead wood dry masses approached or exceeded 1 Gg ha^-1.  Our intensive effort to quantify accurately these large forests makes it clear that insufficient sampling can lead to large errors.  In these and other high-biomass forests, the estimation errors alone can be greater than the total mass of other forested ecosystems.