Abstract. Today, on the Sanetti Plateau in the Bale Mountains of Ethiopia, only
fragmented patches of Erica species can be found at high altitudes (between 3900
and 4200 m a.s.l.). However, it is hypothesized that during the later part of the last glacial period and the
early Holocene the plateau was extensively covered by Erica shrubs. Furthermore,
it is assumed that the vegetation was later heavily destroyed by
human-induced fire and/or climate change phenomena. The objective of this
study is to contribute to paleovegetation reconstructions of the Sanetti
Plateau by evaluating the potential of stable isotopes (δ13C
and δ15N) and sugar biomarkers for distinguishing the dominant
plant species, including Erica, and the soils below the plants. In a companion
paper (Lemma et al., 2019a) we address the same issue by evaluating
lignin-derived phenols and leaf-wax-derived n-alkane biomarkers. The stable carbon (δ13C) and nitrogen (δ15N)
isotope values of the plant samples range from −27.5 ‰ to −23.9 ‰ and −4.8 ‰ to 5.1 ‰, respectively.
We found no significant δ13C and δ15N differences
between the dominant plant species. Mineral topsoils (Ah horizons)
yielded more positive values than plant samples and organic layers
(O layers), which reflects mineralization processes. Moreover, the δ15N values became generally more negative at higher altitudes. This
likely indicates that the N cycle is more closed compared to lower
altitudes. δ15N maxima around 4000 m a.s.l. point to fire-induced
opening of the N cycle at the chosen study sites. Erica species
yielded the lowest overall total sugar concentration (ranging from 58 to 118 mg g−1), dominated by galactose (G) and mannose (M). By contrast,
Festuca species revealed much higher total sugar concentrations ranging from 104 to
253 mg g−1, dominated by the pentose sugars arabinose (A) and
xylose (X). Although a differentiation between Erica versus Festuca, Alchemilla and Helichrysum is possible based
on (G + M) ∕ (A + X) ratios, Erica cannot be unambiguously distinguished from all
other plant species occurring on the Sanetti Plateau. In addition,
plant-characteristic (G + M) ∕ (A + X) sugar patterns change during soil
organic matter formation in the Ah horizons. This can be likely
attributed to degradation effects and soil microbial build-up of galactose
and mannose. In conclusion, soil degradation processes seem to render sugar
biomarker proxies unusable for the reconstruction of the past extent of
Erica on the Sanetti Plateau, Bale Mountains, Ethiopia. This finding is of
relevance beyond our case study.
Variation in δ13C and δ15N within and among plant species in the alpine tundra
