Conservation with elevated elephant densities sequesters carbon in soils despite losses of woody biomass

Abstract

Nature conservation and restoration in terrestrial ecosystems is often focused on increasing the numbers of megafauna, expecting them to have positive impacts on ecological self‐regulation processes and biodiversity. In sub‐Saharan Africa, conservation efforts also aspire to protect and enhance biodiversity with particular focus on elephants. However, elephant browsing carries the risk of woody biomass losses. In this context, little is known about how increasing elephant numbers affects carbon stocks in soils, including the subsoils. We hypothesized that (1) increasing numbers of elephants reduce tree biomass, and thus the amount of C stored therein, resulting (2) in a loss of soil organic carbon (SOC). If true, a negative carbon footprint could limit the sustainability of elephant conservation from a global carbon perspective. To test these hypotheses, we selected plots of low, medium, and high elephant densities in two national parks and adjacent conservancies in the Namibian component of the Kavango Zambezi Transfrontier Area (KAZA), and quantified carbon storage in both woody vegetation and soils (1 m). Analyses were supplemented by the assessment of soil carbon isotopic composition. We found that increasing elephant densities resulted in a loss of tree carbon storage by 6.4 t ha−1. However, and in contrast to our second hypothesis, SOC stocks increased by 4.7 t ha−1 with increasing elephant densities. These higher SOC stocks were mainly found in the topsoil (0–30 cm) and were largely due to the formation of SOC from woody biomass. A second carbon input source into the soils was megaherbivore dung, which contributed with 0.02–0.323 t C ha−1 year−1 to ecosystem carbon storage in the low and high elephant density plots, respectively. Consequently, increasing elephant density does not necessarily lead to a negative C footprint, as soil carbon sequestration and transient C storage in dung almost compensate for losses in tree biomass.

Grazing in a megagrazer‐dominated savanna does not reduce soil carbon stocks, even at high intensities

“…A recent perspective highlights the importance of quantifying a diversity of carbon pools to better understand how herbivory impacts long‐term carbon storage (Kristensen et al 2022). As an example, Sandhage‐Hofmann et al (2021) demonstrated that elephants were able to compensate for most above ground carbon losses by increasing carbon stocks below ground. This is important, because it shows that research focusing solely on the above ground impacts may under or over‐estimate herbivore impacts on overall ecosystem carbon storage (Sandhage‐Hofman et al 2021, Kristensen et al 2022).…”

“…As an example, Sandhage‐Hofmann et al (2021) demonstrated that elephants were able to compensate for most above ground carbon losses by increasing carbon stocks below ground. This is important, because it shows that research focusing solely on the above ground impacts may under or over‐estimate herbivore impacts on overall ecosystem carbon storage (Sandhage‐Hofman et al 2021, Kristensen et al 2022). Moreover, studies are beginning to demonstrate that a focus on total SOC stocks alone fails to consider the effects of long‐term carbon storage, which also leads to under‐ or overestimation of climate change mitigation potential (Lehmann and Kleber 2015, Lehmann et al 2020).…”

Rotational grazing with cattle‐free zones supports the coexistence of cattle and wild herbivores in African rangelands

“…Furthermore, browsers such as giraffes and dik‐diks play an important role in suppressing shrub encroachment and maintaining open grasslands, with benefits for wild and domestic grazers alike (Augustine et al, 2011). The presence of elephants in the conservancies is also crucial for maintaining ecosystem functions due to their positive effects on soil carbon and nitrogen pools, which can potentially reverse savanna degradation (Sandhage‐Hofmann et al, 2021; Sitters et al, 2020). Thus, a diverse assemblage of wild herbivores is an integral part of African rangeland conservation and restoration.…”

Dataset on woody aboveground biomass, disturbance losses, and wood density from an African savanna ecosystem

“…Sampling was stratified into two vegetation types (savanna and woodland) and three disturbance classes with low, medium, and high elephant densities; see Sandhage-Hofmann et al. (2021) [2] for details on this study design. Sites with ‘medium’ and ‘high’ elephant densities were located in the national parks Mudumu and Bwabwata, and least-disturbed sites with ‘low’ elephant densities were located in the respective adjacent communal conservancies Wuparo and Mashi.…”