Plant water resource partitioning and isotopic fractionation during transpiration in a seasonally dry tropical climate

Abstract

Abstract. Lake Chala (3 • 19 S, 37 • 42 E) is a steep-sided crater lake situated in equatorial East Africa, a tropical semiarid area with a bimodal rainfall pattern. Plants in this region are exposed to a prolonged dry season, and we investigated if (1) these plants show spatial variability and temporal shifts in their water source use; (2) seasonal differences in the isotopic composition of precipitation are reflected in xylem water; and (3) plant family, growth form, leaf phenology, habitat and season influence the xylem-to-leaf water deuterium enrichment. In this study, the δ 2 H and δ 18 O of precipitation, lake water, groundwater, plant xylem water and plant leaf water were measured across different plant species, seasons and plant habitats in the vicinity of Lake Chala. We found that plants rely mostly on water from the "short" rains falling from October to December (northeastern monsoon), as these recharge the soil after the long dry season. This plant-available, static water pool is only slightly replenished by the "long" rains falling from February to May (southeastern monsoon), in agreement with the "two water worlds" hypothesis, according to which plants rely on a static water pool while a mobile water pool recharges the groundwater. Spatial variability in water resource use exists in the study region, with plants at the lakeshore relying on a water source admixed with lake water. Leaf phenology does not affect water resource use. According to our results, plant species and their associated leaf phenology are the primary factors influencing the enrichment in deuterium from xylem water to leaf water (ε l/x ), with deciduous species giving the highest enrichment, while growth form and season have negligible effects. Our observations have important implications for the interpretation of δ 2 H of plant leaf wax n-alkanes (δ 2 H wax ) from paleohydrological records in tropical East Africa, given that the temporal variability in the isotopic composition of precipitation is not reflected in xylem water and that leaf water deuterium enrichment is a key factor in shaping δ 2 H wax . The large interspecies variability in xylem-leaf enrichment (24 ± 28 ‰) is potentially troublesome, taking into account the likelihood of changes in species assemblage with climate shifts.

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

“…However, in recent years parts of the dry crater forest have been destroyed by fires set by alcohol distillers occupying shallow caves surrounding the lake. The crater rim and the more gently sloping outer slopes are covered by open savanna woodland ('Obstgartensteppe'; Volkens, 1897), a highly diverse plant community of Acacia (but note that African acacias are now assigned to Vachellia and Senegalia; Boatwright et al, 2014), Acalypha, Combretum, Grewia, Rhus, Lannea, Terminalia, Vepris and Ximenia trees and shrubs interspersed by grassland with dominant grasses Themeda triandra and Enteropogon macrostylus (De Wispelaere et al, 2017) complemented by Bothriochloa insculpta, Eragrostis superba, Cyperus niveus and Indigofera volkensii (A. Hemp, personal Hemp, 2006b). (b) Aerial view of Lake Chala within its steep-sided crater basin, and of the surrounding vegetation in a partly cultural landscape, viewed from the south (photo credit: Wikimedia Commons).…”

“…Here, Acalypha is one of the more responsive taxa in the pollen diagram: it occupies a special position in the PCA species plot (Figure 3a) and its temporal trends contribute significantly to the pollen zonation ( Figure 2). Its occurrence both in the colline forest and woodland on the outer crater slopes (De Wispelaere et al, 2017) as in the dry forest inside the crater (A. Hemp, personal observation) renders interpretation of its temporal dynamics in the pollen record anything but straightforward. The fact that the temporal trends in Acalypha broadly follow those of Euphorbia would seem to indicate that a substantial part of Acalypha pollen in our record originated from inner crater forest, and yet in the PCA Acalypha does not group together with Euphorbia or other crater forest taxa (Figure 3a).…”

Tracing Water Sources and Fluxes in a Dynamic Tropical Environment: From Observations to Modeling

“…Studies on isotope ecohydrology across the tropics are emerging, and so far they have addressed: plant water uptake and partitioning (Meinzer et al, 1999;Evaristo et al, 2016;De Wispelaere et al, 2017;De Deurwaerder et al, 2018;Bodé et al, 2020;Jiménez-Rodríguez et al, 2019), soil water dynamics and mixing (Mosquera et al, 2020), and soil evaporative losses (Hasselquist et al, 2018). Most of these studies come from the dry tropics; however, research coming from mountain and wet rainforest environments, as well as highland Páramos, are particularly scarce.…”

“…Isotope ratios ( 18 O/ 16 O, 17 O/ 16 O, 2 H/ 1 H) and derived second order variables (d-excess, lc-excess, and 17 O-excess) (Dansgaard, 1964;Angert et al, 2004;Landwehr and Coplen, 2006) have been widely used to infer water origin, apparent age, and related fluxes across the soil-plant-atmosphere continuum (SPAC) (Sprenger et al, 2016;Nehemy et al, 2019;Sprenger et al, 2019). These applications are primarily concentrated in temperate environments (Hervé-Fernández et al, 2016;Volkmann et al, 2016;Brinkmann et al, 2018;Goldsmith et al, 2019); however, in the tropics, isotope ecohydrological studies are incipient (Goldsmith et al, 2012;Evaristo et al, 2016;De Wispelaere et al, 2017;De Deurwaerder et al, 2018;Hasselquist et al, 2018;Bodé et al, 2020;Mosquera et al, 2020).…”

Variability in tree water uptake determined with stable water isotopes in an African tropical montane forest

“…This will be useful to understand why certain species outperform other species under specific conditions, and which species will be particularly vulnerable to change in the climate regime. Such knowledge is specifically missing for data scarce regions like the African tropics (de Wispelaere et al, 2017; Wright et al, 2018).…”