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Keywords:

  • 13C;
  • C3 plants;
  • C4 plants;
  • dry evergreen forest;
  • East Africa;
  • ecosystem dynamics;
  • savanna;
  • SOM fractionation;
  • vegetation mosaic;
  • vegetation reconstruction

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 At the forest edge, the older C4 grass signature of SOM has steadily been replaced by the more negative δ13C fingerprint of the forest trees.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.