Global importance of large‐diameter trees

Abstract

Aim To examine the contribution of large‐diameter trees to biomass, stand structure, and species richness across forest biomes. Location Global. Time period Early 21st century. Major taxa studied Woody plants. Methods We examined the contribution of large trees to forest density, richness and biomass using a global network of 48 large (from 2 to 60 ha) forest plots representing 5,601,473 stems across 9,298 species and 210 plant families. This contribution was assessed using three metrics: the largest 1% of trees ≥ 1 cm diameter at breast height (DBH), all trees ≥ 60 cm DBH, and those rank‐ordered largest trees that cumulatively comprise 50% of forest biomass. Results Averaged across these 48 forest plots, the largest 1% of trees ≥ 1 cm DBH comprised 50% of aboveground live biomass, with hectare‐scale standard deviation of 26%. Trees ≥ 60 cm DBH comprised 41% of aboveground live tree biomass. The size of the largest trees correlated with total forest biomass (r2 = .62, p < .001). Large‐diameter trees in high biomass forests represented far fewer species relative to overall forest richness (r2 = .45, p < .001). Forests with more diverse large‐diameter tree communities were comprised of smaller trees (r2 = .33, p < .001). Lower large‐diameter richness was associated with large‐diameter trees being individuals of more common species (r2 = .17, p = .002). The concentration of biomass in the largest 1% of trees declined with increasing absolute latitude (r2 = .46, p < .001), as did forest density (r2 = .31, p < .001). Forest structural complexity increased with increasing absolute latitude (r2 = .26, p < .001). Main conclusions Because large‐diameter trees constitute roughly half of the mature forest biomass worldwide, their dynamics and sensitivities to environmental change represent potentially large controls on global forest carbon cycling. We recommend managing forests for conservation of existing large‐diameter trees or those that can soon reach large diameters as a simple way to conserve and potentially enhance ecosystem services.

Big‐sized trees overrule remaining trees' attributes and species richness as determinants of aboveground biomass in tropical forests

“…Although species richness was not strongly related to aboveground biomass, it had a potentially strong indirect positive effect on aboveground biomass via big-sized trees effect. Furthermore, this result suggests that, as natural forests increase in tree diameter, height, and crown area, their vegetation quantity (i.e., initial biomass stocks) also increases, which in turn may promote absolute annual biomass gain (Lohbeck et al, 2015;Lutz et al, 2018;Yuan et al, 2018). Black color arrows represent direct effect, while gray color arrows represent the indirect effects.…”

“…Across the globe, big-sized trees constitute a huge quantity (approximately 50%) of aboveground biomass in natural forests, thereby holding much stronger abiotic and biotic controls on forest carbon sequestration and stock (Bastin et al, 2015;Lutz et al, 2018;Stephenson et al, 2014). The contribution of a few large-diameter trees to forest diversity, structure, and functioning cannot be ignored, because big-sized trees relative to medium-and small-sized trees take many years, decades, or even centuries to occupy the canopy of the forests (Ali, Lohbeck, & Yan, 2018;Bastin et al, 2018;Lindenmayer & Laurance, 2017;Lutz et al, 2018;Slik et al, 2013). Yet, despite advances in our understanding of global consequences of large-diameter trees to aboveground biomass (Lutz et al, 2018), no strong consensus exists about the importance of big-sized trees attributes relative to species diversity and remaining trees attributes on aboveground biomass at the community level along large-scale natural forests.…”

“…In tropical forests, stand-level aboveground biomass can be predicted from a few big-sized trees (Slik et al, 2013), and more specifically from 20 large-diameter trees per hectare (Bastin et al, 2018), and top 5% (Bastin et al, 2015) and top 1% large-diameter trees (Lutz et al, 2018). From an ecological theoretical point of view, vegetation quantity (i.e., initial biomass stocks) compared to vegetation quality (i.e., species diversity, functional trait diversity, and trait composition) has a strong positive effect on productivity in natural forests (i.e., the "vegetation quantity hypothesis") because steep biomass build-up during succession overrides more subtle effects of species diversity on forest functioning.…”

“…Moreover, stand basal area compared to functional trait diversity promotes productivity (Paquette & Messier, 2011), and as such, the response of aboveground biomass productivity to species richness is largely mediated by stand basal area in natural forests (Vilà et al, 2013). In this understanding, the overruling effects of stand basal area, stand density, functional dominance, and initial biomass stock on forest functioning might be attributable to the fact that top 1%-20% largediameter trees largely explain variation in aboveground biomass at local, regional, continental, and global scales (Ali, Lohbeck, et al, 2018;Bastin et al, 2018;Lutz et al, 2018;Malhi et al, 2006;Slik et al, 2013). In this understanding, the overruling effects of stand basal area, stand density, functional dominance, and initial biomass stock on forest functioning might be attributable to the fact that top 1%-20% largediameter trees largely explain variation in aboveground biomass at local, regional, continental, and global scales (Ali, Lohbeck, et al, 2018;Bastin et al, 2018;Lutz et al, 2018;Malhi et al, 2006;Slik et al, 2013).…”

“…Similarly, aboveground biomass is more strongly explained by functional dominance (i.e., community-weighted mean of maximum tree height or diameter) in tropical forests compared to functional trait diversity, probably due to the niche overlap or functional redundancy effect of the canopy trees (Cavanaugh et al, 2014;Prado-Junior et al, 2016). However, the relative contribution of big-sized trees to the stand-level aboveground biomass varies among the regions of tropical forests (Feldpausch et al, 2012), probably due to the fact that all species or individuals are not capable of reaching large-diameter threshold (e.g., diameter ≥ 60 cm) in many tropical forest sites (Lutz et al, 2018). However, the relative contribution of big-sized trees to the stand-level aboveground biomass varies among the regions of tropical forests (Feldpausch et al, 2012), probably due to the fact that all species or individuals are not capable of reaching large-diameter threshold (e.g., diameter ≥ 60 cm) in many tropical forest sites (Lutz et al, 2018).…”