Chemotaxonomic patterns of vegetation and soils along altitudinal transects of the Bale Mountains, Ethiopia, and implications for paleovegetation reconstructions – Part II: lignin-derived phenols and leaf-wax-derived <i>n</i>-alkanes
oleh: B. Lemma, B. Lemma, B. Mekonnen, B. Mekonnen, B. Glaser, W. Zech, S. Nemomissa, T. Bekele, L. Bittner, M. Zech, M. Zech
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2019-09-01 |
Deskripsi
<p><i>Erica</i> is a dominant vegetation type in many sub-afroalpine ecosystems, such as the Bale Mountains in Ethiopia. However, the past extent of <i>Erica</i> is not well known and climate versus anthropogenic influence on altitudinal shifts are difficult to assign unambiguously, especially during the Holocene. The main objective of the present study is to chemotaxonomically characterize the dominant plant species occurring in the Bale Mountains using lignin phenols and <span class="inline-formula"><i>n</i></span>-alkane biomarkers and to examine the potential of those biomarkers for reconstructing vegetation history. Fresh plant material, organic layer and mineral topsoil samples were collected along a northeastern and a southwestern altitudinal transect (4134–3870 and 4377–2550 m a.s.l., respectively). Lignin-derived vanillyl, syringyl and cinnamyl phenols were analyzed using the cupric oxide oxidation method. Leaf-wax-derived <span class="inline-formula"><i>n</i></span>-alkanes were extracted and purified using Soxhlet and aminopropyl columns. Individual lignin phenols and <span class="inline-formula"><i>n</i></span>-alkanes were separated by gas-chromatography and detected by mass spectrometry and flame ionization detection, respectively.</p> <p><span id="page190"/>We found that the relative contributions of vanillyl, syringyl and cinnamyl phenols allow us to chemotaxonomically distinguish contemporary plant species of the Bale Mountains. <i>Erica</i> in particular is characterized by relatively high cinnamyl contributions of <span class="inline-formula">>40</span> %. However, litter degradation strongly decreases the lignin phenol concentrations and completely changes the lignin phenol patterns. Relative cinnamyl contributions in soils under <i>Erica</i> were <span class="inline-formula"><40</span> %, while soils that developed under Poaceae (<i>Festuca abyssinica</i>) exhibited relative cinnamyl contributions of <span class="inline-formula">>40</span> %.</p> <p>Similarly, long-chain <span class="inline-formula"><i>n</i></span>-alkanes extracted from the leaf waxes allowed for differentiation between <i>Erica</i> versus <i>Festuca abyssinica</i> and <i>Alchemilla</i>, based on lower C<span class="inline-formula"><sub>31</sub></span> <span class="inline-formula">∕</span> C<span class="inline-formula"><sub>29</sub></span> ratios in <i>Erica</i>. However, this characteristic plant pattern was also lost due to degradation in the respective O layers and A<span class="inline-formula"><sub>h</sub></span> horizons. In conclusion, although in modern-day plant samples a chemotaxonomic differentiation is possible, soil degradation processes seem to render the proxies unusable for the reconstruction of the past extent of <i>Erica</i> on the Sanetti Plateau, Bale Mountains, Ethiopia. This finding is of high relevance beyond our case study.</p>