The limited contribution of large trees to annual biomass production in an old‐growth tropical forest

Abstract

Although the importance of large trees regarding biodiversity and carbon stock in old-growth forests is undeniable, their annual contribution to biomass production and carbon uptake remains poorly studied at the stand level. To clarify the role of large trees in biomass production, we used data of tree growth, mortality, and recruitment monitored during 20 yr in 10 4-ha plots in a species-rich tropical forest (Central African Republic). Using a random block design, three different silvicultural treatments, control, logged, and logged + thinned, were applied in the 10 plots. Annual biomass gains and losses were analyzed in relation to the relative biomass abundance of large trees and by tree size classes using a spatial bootstrap procedure. Although large trees had high individual growth rates and constituted a substantial amount of biomass, stand-level biomass production decreased with the abundance of large trees in all treatments and plots. The contribution of large trees to annual stand-level biomass production appeared limited in comparison to that of small trees. This pattern did not only originate from differences in abundance of small vs. large trees or differences in initial biomass stocks among tree size classes, but also from a reduced relative growth rate of large trees and a relatively constant mortality rate among tree size classes. In a context in which large trees are increasingly gaining attention as being a valuable and a key structural characteristic of natural forests, the present study brought key insights to better gauge the relatively limited role of large trees in annual stand-level biomass production. In terms of carbon uptake, these results suggest, as already demonstrated, a low net carbon uptake of old-growth forests in comparison to that of logged forests. Tropical forests that reach a successional stage with relatively high density of large trees progressively cease to be carbon sinks as large trees contribute sparsely or even negatively to the carbon uptake at the stand level.

Effects of stand age, richness and density on productivity in subtropical forests in China

“…In addition, we found that the increase in tree size with species richness and stand age had a significantly positive effect on forest productivity. This agrees with previous studies where large‐diameter trees could enhance forest productivity in these forests because large‐diameter trees have the ability to capture more light and compete against neighbours (Ligot et al, ; Xu et al, ). Stand age may affect biomass and productivity via changes in stand structure (i.e.…”

Effects of stand age on carbon storage in dragon spruce forest ecosystems in the upper reaches of the Bailongjiang River basin, China

“…33 and Ligot et al . 34 . This was likely due to SD decreasing as DBH increased with stand age (Table 1).…”

The relationship of woody plant size and leaf nutrient content to large‐scale productivity for forests across the Americas

“…In agreement with theory, there is evidence that stand biomass of tropical and temperate forest plots, primarily determined by the size of the largest tree (Stegen et al, ), is one of the major drivers of forest productivity (Michaletz et al, ; Michaletz, Kerkhoff, & Enquist, ). Furthermore, given that tall trees sequester carbon at higher absolute rates compared to smaller trees (Stephenson et al, ), they are often strong contributors to the whole‐stand carbon flux (Fauset et al, ; Gholz, ; but see Ligot et al, ). These theoretical predictions, however, are expected to be modified by human land‐use or natural disturbances (Kerkhoff & Enquist, ; McDowell et al, ; see also Coomes, Holdaway, Kobe, Lines, & Allen, ; Muller‐Landau et al, ).…”