Stable forest–savanna mosaic in north‐western Tanzania: local‐scale evidence from δ¹³C signatures and ¹⁴C ages of soil fractions

Abstract

Aim The spatio‐temporal dynamics of dry evergreen forest patches in the savanna biome of the Kagera region (north‐western Tanzania) are largely unknown owing to a lack of pollen and macrofossil evidence. Our aims were to reconstruct local‐scale shifts of the forest–savanna boundary in order to determine whether the forests have been expanding or retreating on a centennial and millennial time‐scale. Location The Kagera region of north‐western Tanzania, East Africa. Methods The vegetation reconstruction was based on analysing δ13C signatures in soils along a transect spanning both C4 open savanna and C3 forest vegetation. Furthermore, we fractionated soil organic matter (SOM) according to density and chemical stability to analyse δ13C values of soil fractions with distinct radiocarbon ages. Results We found sharp changes in δ13C signatures in bulk SOM from the forest to the savanna, within a few metres along the transect. The forest soil profiles carried a persistent C3‐dominated signature. Radiocarbon dating of the oldest, most recalcitrant forest soil fraction yielded a mean age of 5500 cal. yr bp, demonstrating that the forest has existed since at least the mid‐Holocene. The savanna sites showed a typical C4 isotopic signature in SOM of topsoils, but subsoils and more recalcitrant SOM fractions also contained signals of C3 plants. The dense soil fraction (ρ > 1.6 g cm−3) carrying a pure C4 label had a mean age of c. 1200 cal. yr bp, indicating the minimum duration of the dominance of grass vegetation on the savanna site. At the forest edge, the older C4 grass signature of SOM has steadily been replaced by the more negative δ13C fingerprint of the forest trees. As this replacement has occurred mainly in the 10‐m‐wide forest–savanna ecotone over the last c. 1200 years, the forest expansion must be very slow and is very likely less than 15 m century−1. Main conclusions Our results suggest that forest patches in the Kagera savanna landscape are very stable vegetation formations which have persisted for millennia. During the last millennium, they have been expanding very slowly into the surrounding savanna at a rate of less than 15 m century−1.

Edge Effects on Foliar Stable Isotope Values in a Madagascan Tropical Dry Forest

“…It is unclear how stable the savannah-forest landscape is in northwestern Madagascar. This landscape may be a stable mosaic ecosystem of relative antiquity [58] , [59] , [60] . Alternatively, the patchy nature of forest and grasses in and around Ankarafantsika may reflect recently established vegetation responding to frequent anthropogenic disturbance.…”

“…However, whether or not it persisted into the deeper past is unknown. Future examination of soil isotope values and radiocarbon ages both in the forest and the savannah will help to clarify the antiquity of the savannahs [59] .…”

Ancient origin of a rainforest in Gabon as revealed by carbon isotope data of vegetation and soil

“…Although the abundance and diversity of indicator species provide a regional localization of forest refugia (Achoundong, 2000;Leal, 2001;Rietkerk et al, 1995;Robbrecht, 1996;Sosef, 1996), their exact localization is still a matter of debate and requires direct palaeoevidence (Tchouto et al, 2009). Carbon isotope analysis of soil organic carbon (SOC), on the other hand, provides information on local scale vegetation history (Boutton et al, 1998;Kingston et al, 1994;Wiedemeier et al, 2012). SOC predominantly originates from plant material derived from ecosystem vegetation cover, that is, from decaying leaf litter and woody debris (from stems and roots) in forests, and decomposing grasses (leaves, roots) in the case of savannahs.…”

The origin of the forest-grassland mosaic of central Cameroon: What we learn from the isotopic geochemistry of soil organic matter

“…The radiocarbon measurements on the bulk OM represent a mixture of younger and older carbon pools whose residence time is increasing with depth and consequently do not correspond to the absolute age of soil organic matter (Bremond et al, 2017; Desjardins et al, 2013; Guillet et al, 2001; Schwartz et al, 1996b). As a consequence, the 14 C ages were not calibrated and the timing of past vegetation changes must be interpreted with caution (Boutton et al, 1998; Trumbore et al, 1995; Wiedemeier et al, 2012). For an easier comparison with earlier studies, 14 C values are expressed in 14 C years BP (apparent age according to Guillet et al, 2001).…”