Global litter production, pools, and turnover times: Estimates from measurement data and regression models

Abstract

Systematic and compatible databases to quantify composition, distribution, and turnover times of carbon in global litter were developed and evaluated. The study employs an integrated approach, estimating related litter pools and fluxes using a variety of data‐based and model‐based techniques. The analysis includes direct estimates and indirect, or proxy, estimates of litter production and pools; steady‐state turnover times are estimated from the two. Proxies for litter production include net primary productivity and root respiration‐soil respiration relationships. In addition to implementing a suite of regression models, >1100 published measurements of litter components, along with site characteristics, were integrated into a baseline data set and used to estimate litter production and pools. Historically, global estimates of litter production have ranged from 75 to 135 Pg dm/yr; several estimates from this study suggest values in the middle of this range, from 90 to 100 Pg dm/yr. The estimate of aboveground litter production from the compiled measurements, 39 Pg dm/yr, includes mainly forest, woodland, and wooded grassland; other grassland, shrubland, and xeromorphic communities that occupy ∼25% of the ice‐free land surface are unrepresented in the present compilation. Aboveground litter production may be 5–10 Pg dm/yr higher with the inclusion of these ecosystems, and the total, including belowground production, may approach 90–110 Pg dm/year. Two novel production estimates derived from soil‐ and root‐respiration relationships are 93 Pg and 100 Pg dm/yr. These estimates have the major advantage of accounting for both aboveground and belowground litter; the latter is rarely included and can account for a substantial fraction of total production. Production of coarse woody detritus may add ∼12 Pg dm/yr to the fine litter total. The global litter pool has previously been estimated at ∼100 to 400 Pg dm. The fine litter pool estimated here from the measurement compilation is 136 Pg dm. Although this partial estimate includes ecosystems covering just under half the ice‐free land surface, it encompasses forests and woodlands which have the largest pools. Inclusion of the remaining ecosystems may add ∼25 Pg, raising the total to ∼160 Pg dm. An additional ∼150 Pg dm is estimated for the coarse woody detrital pool. Global mean steady state turnover times of litter estimated from the pool and production data range from 1.4 to 3.4 years; mean turnover time from the partial forest/woodland measurement compilation is ∼5 years, and turnover time for coarse woody detritus is ∼13 years. By encompassing spatial distribution, composition, and magnitude, along with numerous field measurements, this integrated approach has begun to yield compositional and ecosystem constraints on modeled global and regional litter fields and NPP allocation schemes in ecosystem models.