Premium
Stable forest–savanna mosaic in north‐western Tanzania: local‐scale evidence from δ 13 C signatures and 14 C ages of soil fractions
Author(s) -
Wiedemeier Daniel B.,
Bloesch Urs,
Hagedorn Frank
Publication year - 2012
Publication title -
journal of biogeography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 158
eISSN - 1365-2699
pISSN - 0305-0270
DOI - 10.1111/j.1365-2699.2011.02583.x
Subject(s) - transect , biome , holocene , geology , radiocarbon dating , vegetation (pathology) , dominance (genetics) , macrofossil , geography , ecology , physical geography , forestry , paleontology , ecosystem , biology , oceanography , medicine , biochemistry , pathology , gene
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 δ 13 C signatures in soils along a transect spanning both C 4 open savanna and C 3 forest vegetation. Furthermore, we fractionated soil organic matter (SOM) according to density and chemical stability to analyse δ 13 C values of soil fractions with distinct radiocarbon ages. Results We found sharp changes in δ 13 C signatures in bulk SOM from the forest to the savanna, within a few metres along the transect. The forest soil profiles carried a persistent C 3 ‐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 C 4 isotopic signature in SOM of topsoils, but subsoils and more recalcitrant SOM fractions also contained signals of C 3 plants. The dense soil fraction (ρ > 1.6 g cm −3 ) carrying a pure C 4 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 At the forest edge, the older C 4 grass signature of SOM has steadily been replaced by the more negative δ 13 C fingerprint of the forest trees.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 .