Spatial and temporal 2H and 18O isotope variation of contemporary precipitation in the Bale Mountains, Ethiopia.

East Africa is an underrepresented region in respect of monitoring the stable isotopic composition of precipitation (δ18Oprec and δ2Hprec). In 2017, we collected precipitation samples from ten weather stations located along an altitudinal transect ranging from 1304 to 4375 m a.s.l. The δ18Oprec and δ2Hprec values varied from -8.7 to +3.7 ‰ and -38 to +29 ‰, respectively. The local meteoric water line is characterised by a lower slope, a higher intercept and more positive d-excess values (δ2H = 5.3 ± 0.2 * δ18O + 14.9 ± 0.9) compared to the global meteoric water line. Both altitude and amount of precipitation clearly correlate with our isotope data. However, the δ18Oprec and δ2Hprec values show at the same time a seasonal pattern reflecting rainy versus dry season. More enriched isotope values prevailed shortly after the end of the dry season; more depleted isotope values coincided with high precipitation amounts recorded in May, August and September. Moreover, HYSPLIT trajectories reveal that during the dry season water vapour originates primarily from the Arabian Sea, whereas during the wet season it originates primarily from the Southern Indian Ocean. These findings challenge the traditional amount effect interpretation of paleoclimate isotope records from East Africa and rather point to a previously underestimated source effect.

Long-term fire resilience of the Ericaceous Belt, Bale Mountains, Ethiopia.

Fire is the most frequent disturbance in the Ericaceous Belt (ca 3000-4300 m.a.s.l.), one of the most important plant communities of tropical African mountains. Through resprouting after fire, Erica establishes a positive fire feedback under certain burning regimes. However, present-day human activity in the Bale Mountains of Ethiopia includes fire and grazing systems that may have a negative impact on the resilience of the ericaceous ecosystem. Current knowledge of Erica-fire relationships is based on studies of modern vegetation, lacking a longer time perspective that can shed light on baseline conditions for the fire feedback. We hypothesize that fire has influenced Erica communities in the Bale Mountains at millennial time-scales. To test this, we (1) identify the fire history of the Bale Mountains through a pollen and charcoal record from Garba Guracha, a lake at 3950 m.a.s.l., and (2) describe the long-term bidirectional feedback between wildfire and Erica, which may control the ecosystem's resilience. Our results support fire occurrence in the area since ca 14 000 years ago, with particularly intense burning during the early Holocene, 10.8-6.0 cal ka BP. We show that a positive feedback between Erica abundance and fire occurrence was in operation throughout the Lateglacial and Holocene, and interpret the Ericaceous Belt of the Ethiopian mountains as a long-term fire resilient ecosystem. We propose that controlled burning should be an integral part of landscape management in the Bale Mountains National Park.

Phenolic Compounds as Unambiguous Chemical Markers for the Identification of Keystone Plant Species in the Bale Mountains, Ethiopia.

: Despite the fact that the vegetation pattern and history of the Bale Mountains in Ethiopia were reconstructed using pollen, little is known about the former extent of Erica species. The main objective of the present study is to identify unambiguous chemical proxies from plant-derived phenolic compounds to characterize Erica and other keystone species. Mild alkaline CuO oxidation has been used to extract sixteen phenolic compounds. After removal of undesired impurities, individual phenols were separated by gas chromatography and were detected by mass spectrometry. While conventional phenol ratios such as syringyl vs. vanillyl and cinnamyl vs. vanillyl and hierarchical cluster analysis of phenols failed for unambiguous Erica identification, the relative abundance of coumaryl phenols (>0.20) and benzoic acids (0.05-0.12) can be used as a proxy to distinguish Erica from other plant species. Moreover, a Random Forest decision tree based on syringyl phenols, benzoic acids (>0.06), coumaryl phenols (<0.21), hydroxybenzoic acids, and vanillyl phenols (>0.3) could be established for unambiguous Erica identification. In conclusion, serious caution should be given before interpreting this calibration study in paleovegetation reconstruction in respect of degradation and underground inputs of soil organic matter.