Water‐isotope ecohydrology of Mount Kilimanjaro

Abstract

Notwithstanding its crucial importance for assessing the vulnerability of tropical mountain ecosystems to both local anthropogenic pressure and climate change, knowledge of the relationships between climate, hydrology, and vegetation on Mount Kilimanjaro is highly fragmentary. Unraveling the pathway of water from precipitation to transpiration can also help validate paleoclimate proxies derived from the hydrogen‐ and oxygen‐isotopic signatures of plant compounds, such as leaf waxes. We measured δ2H and δ18O in precipitation, stream, and soil water, and in xylem and leaf water of locally common plants, for four successive seasons along a 3,000‐m altitudinal transect covering lowland savannah, lower and upper montane forest, and the subalpine zone on Mount Kilimanjaro. Across all four bioclimatic zones, δ2H and δ18O in precipitation decreased with monthly precipitation amount at a rate of −8 ± 2‰ and −1.0 ± 0.3‰ per 100 mm, respectively. Our xylem‐water isotope data indicate that plants in general took up topsoil water during the northeasterly monsoon season and short dry season and gradually shifted to deeper soil water during the southeasterly monsoon season and the main dry season. Savannah trees and shrubs tapped from a water pool recharged by source water with a relatively constant isotopic signature. This observation conforms to the “two water worlds” hypothesis and implies a possible seasonal bias in plant‐derived paleohydrological proxies. Finally, our results show strong correlation between xylem‐to‐leaf water‐isotopic enrichment and ambient relative humidity, despite a bias in the savannah that we attribute to a leaf‐cooling mechanism in this warm, semi‐arid environment.

Climate-human-landscape interaction in the eastern foothills of Mt. Kilimanjaro (equatorial East Africa) during the last two millennia

“…With a mean annual precipitation of 740 mm year −1 (1979–2019; GPCP Precipitation data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, at https://psl.noaa.gov/; Adler et al, 2003) and evaporation reaching ca 1735 mm year −1 (Payne, 1970), the lake has a negative local water balance and must be maintained by substantial sub-surface inflows. These originate from precipitation falling at the transition between the montane forest and subalpine zones (Bodé et al, 2020). Lake Chala and the surrounding low-elevation landscape experiences a tropical semi-arid climate, with major inter-annual rainfall variability linked to the El Niño Southern Oscillation (ENSO) resulting in excessive rain and flooding during El Niño years and droughts in La Niña years (Goddard and Graham, 1999).…”

Dry season plant water sourcing in contrasting tropical ecosystems of Costa Rica

“…Transpired water by plants exhibits an isotopic enrichment as isotopic fractionation occurs in the leaves (Dawson & Ehleringer, 1993; Yakir & Sternberg, 2000). Thus, the isotopic composition of water from leaves differs from that absorbed through root water uptake (e.g., Bodé et al, 2020). Therefore, studies focusing on source water uptake are typically based on stem water sampling representing root water sources.…”

Reversed Holocene temperature–moisture relationship in the Horn of Africa

“…It fills a steep-sided volcanic caldera basin that probably formed during Mount Kilimanjaro’s most recent phase of activity, originally dated to 200–150 kyr ago 57 . Lake Chala has a surface area of 4.2 km 2 and a maximum water depth historically varying between 92 m and 98 m (1999–2017), and is maintained against a negative local water balance by subsurface inflow originating from rainfall onto the forested and subalpine slopes of Mount Kilimanjaro 58 , 59 . Surface inflow is limited to run-off from the inner crater slopes, except that heavy rains can activate a creek breaching the crater rim in its northwestern corner 60 .…”