Changes in tree size, not species diversity, underlie the low above‐ground biomass in natural forest edges

Abstract

Questions: Forest's above-ground biomass is an important component of the global carbon cycle and, in forest fragments, it tends to decrease near the edges. This edge effect on biomass can be due to the number and identity of species residing at edges or due to the context of edges constraining plant functioning regardless of the species. Here, we took advantage of species-rich natural forest edges to test the role of species richness, composition, and context dependency in explaining why forest edges stock less above-ground biomass than interiors.Location: Gallery forests in Brazilian savanna (latitude 21° S, longitude 44° W). Methods:We tested the relationship between species richness and above-ground biomass in 49 forest plots (10,142 trees). We used a novel adaptation of the Price equation to calculate the impact of species composition (losses and gains) and context dependency on the differences of above-ground biomass between edges and interiors.Results: Differences in species richness and composition did not explain the lower above-ground biomass of forest edges when compared to interiors. Biomass responses were context-dependent, in which the same species achieved smaller sizes when it grew in the edge than in the interior. Conclusions:Our study shows that edge effects on biomass may operate independently of biodiversity as edges and interiors had a similar number of species but distinct above-ground biomass. Indeed, in the studied natural forest patches, the edge context itself limits plant size and, thus, the above-ground biomass stock. Beyond unveiling the underlying causes of edge effects on forest biomass, our results call for distinct policies to conserve both the carbon stocks in forest interiors but also the unique diversity of natural edges.

Weak edge effects on trees in Bornean rainforest remnants bordering oil palm

“…Given the absence of edge effects on stand‐level structure and composition, but declines in maximum tree dbh and height, we conclude that AGC loss is driven primarily by edge effects on large trees (i.e., those over 70 cm dbh; Slik et al, 2013; Figure 1b). Thus, decline in biomass and AGC occurred independent of any compositional shifts, as observed elsewhere (Silva et al, 2021), probably because large trees make up a small proportion of stems but make a disproportionately large contribution to biomass (Slik et al, 2013).…”

Functional consequences of animal community changes in managed grasslands: An application of the CAFE approach

“…A species can also influence greater ecosystem function change because of large per‐capita contributions or because of large population size, although the latter might be less likely to be locally extirpated. The species remaining in a community after compositional change can also shift ecosystem function contributions depending on interactions with lost or gained species (Ruesink & Srivastava, 2001), or environmental changes (Messina et al, 2021; Silva et al, 2021; Terry & Rowe, 2015). For example, removing individual species from a pollinator community reduced the floral fidelity of the remaining species, resulting in reduced seed production by a focal plant (Brosi & Briggs, 2013).…”

“…Finally, in situations where remaining species are numerically dominant or have weak interactions with other species, overall richness, and compositional changes might have little impact on function. In these cases, differences in the remaining species' contributions may be context‐dependent and a direct effect of environmental perturbations (such as logging, Messina et al, 2021) or variation along environmental gradients (like habitat edges, Silva et al, 2021).…”