Stable isotope profiles of soil organic carbon in forested and grassland landscapes in the Lake Alaotra basin (Madagascar): insights in past vegetation changes

Abstract

Abstract. The extent to which the central highlands of Madagascar were once covered by forests is still a matter of debate: while reconstructing past environments is inherently difficult, the debate is further hampered by the fact that the evidence documenting land cover changes and their effects on carbon and sediment dynamics in Madagascar has hitherto mainly been derived from lake coring studies. Such studies provide an integrated view over relatively large areas but do not provide information on how land-use change affects hillslopes in terms of carbon and sediment dynamics. Such information would not only be complementary to lake inventories but may also help to correctly interpret lake sediment data. Carbon stable isotope ratios (δ13C) are particularly useful tracers to study the past dynamics of soil carbon over time spans ranging from years to millennia and thus to understand the consequences of land-use change over such time spans. We analysed soil profiles down to a depth of 2 m from pristine forests and grasslands in the Lake Alaotra region in central Madagascar. Along grassland hillslopes, soil organic carbon (SOC) content was low, from 0.4 % to 1.7 % in the top layer, and decreased rapidly to ca. 0.2 % below 100 cm depth. The current vegetation predominantly consists of C4 grasses (δ13C ∼ −13 ‰), yet topsoil δ13C-OC ranges between −23.0 ‰ and −15.8 ‰, and most profiles show a decrease in δ13C-OC with depth. This contrasts with our observations in the C3-dominated forest profiles, which show a typical profile whereby δ13C values increase slightly with depth. Moreover, the SOC stock of grasslands was ∼ 55.6 % lower than along the forested hillslopes for the upper 0–30 cm layer. δ13C values in grassland and forest profiles converge to similar values (within 2.0 ± 1.8 ‰) at depths below ∼ 80 cm, suggesting that the grasslands in the Lake Alaotra region have indeed developed on soils formerly covered by a tree vegetation dominated by C3 plants. We also observed that the percent of modern carbon (pMC) of the bulk OC in the top, middle and lower middle positions of grasslands was less than 85 % near the surface. This could reflect a combination of (i) the long residence time of forest OC in the soil, (ii) the slow replacement rate of grassland-derived OC (iii) and the substantial erosion of the top positions towards the valley position of grasslands. At the valley positions under grassland, the upper 80 cm contains higher amounts of recent grass-derived OC in comparison to the hillslope positions. This is likely to be related to the higher productivity of the grassland valleys (due to higher moisture and nutrient availability), and the deposition of OC that was eroded further upslope may also have contributed. The method we applied, which is based on the large difference in δ13C values between the two major photosynthetic pathways (C3 and C4) in (sub-)tropical terrestrial environments, provides a relatively straightforward approach to quantitatively determine changing vegetation cover, and we advocate for its broader application across Madagascar to better understand the island's vegetation history.

Fire incongruities can explain widespread landscape degradation in Madagascar's forests and grasslands

“…Large-scale tree removal followed by fires exacerbate soil runoff (Prosser & Williams, 1998), and deforestation, followed by human-lit fires in high-MAP regions, contributes to mass deposition of river sediment in estuaries and offshore deltas, which in turn can lower aquatic biodiversity, fishery productivity and even the function of ports (Raharimahefa & Kusky, 2010). The rapid escalation in erosion captured by remote imagery over mere decades supports findings of recent and rapid formation of lavaka in the Lac Alaotra region thought to have formed coeval with anthropogenic tree removal and cyclical burning of the remaining vegetation with the introduction of pastoralism 1000 CE (current era), resulting in widespread sediment loss (Broothaerts et al, 2022;Razanamahandry et al, 2022).…”

“…The arising agricultural lands lose productivity over short timeframes when abandoned, and soil erosion and nutrient loss ensue (Styger et al, 2007). The cycle of clearance and subsequent degradation likely began as far back as 1000 CE (Common Era), with anthropogenic deforestation, fire setting (Burns et al, 2016) and erosion (Broothaerts et al, 2022; Brosens, 2022; Brosens et al, 2022; Razanamahandry et al, 2022). The cycle persists today (Frappier‐Brinton & Lehman, 2022; Raharimahefa & Kusky, 2010), with not only loss of rangeland productivity but silting of lakes (Bakoariniaina et al, 2006), river systems and deltas (Raharimahefa & Kusky, 2010).…”

The grassy ecosystems of Madagascar in context: Ecology, evolution, and conservation

“…In Africa, Cerling et al (2011) used carbon isotopes from soil organic matter to detect changing proportions of trees and grass, and similar methods have been usefully applied at more local scales to explore the history of closed forest versus open grassy ecosystems (e.g., Gillson, 2015; Mariotti & Peterschmitt, 1994; West et al, 2000). Razanamahandry et al (2022) have recently reported the first such study in Madagascar in the Lake Aloatra basin on the edge of the eastern forest belt. They found the grasslands in this study area had likely replaced forests in many but not all sites.…”

“…Razanamahandry et al (2022) have recently reported the first such study in Madagascar in the Lake Aloatra basin on the edge of the eastern forest belt. They found the grasslands in this study area had likely…”

Carbon isotope values for grasses in Madagascar's Central Highlands establish baselines for historical and paleoecological research

“…With the exception of a few protected areas and high elevation sites, the majority of this ecoregion is currently covered by open grassland that is dominated by C 4 grasses and largely devoid of trees (reviewed in Lehmann et al, 2022). Yet numerous studies converge to suggest that forest cover was more widespread for much of the Holocene (Burney, 1987a, 1987b; Crowley, 2010; Crowley et al, 2021; Gasse & Van Campo, 1998; Gasse & Van Campo, 2001; Joseph & Seymour, 2020; Razanamahandry et al, 2022). These data include charcoal and pollen data from lake cores, as well as stable carbon isotope (δ 13 C) data from soils and animal remains.…”