Strategies trees use to overcome seasonal water limitation in an agroforestry system in semiarid West Africa

Abstract

Agroforestry parklands, in which annual crops are grown under scattered mature trees, constitute the most prevalent farming system in semiarid West Africa, covering vast areas of land. The most dominant tree species in these systems is Vitellaria paradoxa, an indigenous tree to West Africa. Despite the importance of this tree in the region, no study to our knowledge has examined its sources and patterns of water uptake. In this study, we used oxygen stable isotopes at natural abundance levels to investigate water sources used by V. paradoxa both in the dry and wet season in an agroforestry parkland in Burkina Faso. We found that during the wet season soil moisture was highest near the soil surface (<10 cm depth), yet during this time V. paradoxa preferentially accessed water from slightly deeper soil depths, obtaining ca. 90% of its water from 10 to 50 cm depth. In contrast, soil moisture in the upper soil layers was significantly lower during the dry season and as a result V. paradoxa shifted to deeper water sources, obtaining ca. 30% of its water from groundwater and ca. 50% from 30 to 600 cm depth. We also found a negative relationship between tree size and the contribution of groundwater during the dry season, whereas during the wet season V. paradoxa predominantly used water near the soil surface regardless of tree size. Knowledge about the sources and patterns of tree water uptake provides crucial information to better understand how trees influence the local water balance.

Partitioning of Water Between Differently Sized Shrubs and Potential Groundwater Recharge in a Semiarid Savanna in Namibia

“…A positive relation between shrub size and both δ 2 H and δ 18 O values in the xylem was observed during the first half of the rainy season, suggesting that larger A. mellifera shrubs rely less on upper and very deep soil water during times of seasonal rain events than smaller shrubs. This result is based on the soil depth isotopic pattern in Figure 2 and Table S1 and seems anomalous but is consistent with previous studies in other dryland ecosystems (Meinzer et al, 1999; Hasselquist et al, 2010; Bargués Tobella et al, 2017). It supports the hypothesis that in environments where soil moisture is highly variable in the upper soil layers, the early investment in a deep tap-root to exploit deeper, more reliable water sources could reduce the probability of mortality during the establishment phase (Ehleringer and Dawson, 1992).…”

Effect of Drought and Topographic Position on Depth of Soil Water Extraction of Pinus sylvestris L. var. mongolica Litv. Trees in a Semiarid Sandy Region, Northeast China

“…The isotopic values of twig xylem water and the results from the mixing IsoSource model indicate that the Mongolian pine trees growing at both the top and bottom of sand dunes took up water from deeper soil layers under drought condition, e.g., the Mongolian pine trees at the top and bottom of sand dunes mainly used 40-300 cm soil water and 40-300 cm soil water as well as possibly groundwater during the dry period, respectively, and switched their water sources to 0-100 cm soil water during the wet period (Figures 4 and 5), which supported our first hypothesis. These results of the present study are consistent with those of previous studies in arid and semiarid regions, in which trees with dimorphic root systems exhibited a shift in water sources from mainly shallow water during the wet season to deep water during the dry season [32,34,[51][52][53]. In the present study, the gravimetric SWC at both the top and bottom of the sand dunes during the dry period were significantly lower than those during the wet period ( Figure 2 and Table 2).…”

Depth distribution of soil water sourced by plants at the global scale: A new direct inference approach

“…Second, coexisting plant species may have different ecohydrological niches that we were not able to adequately represent in this study, other than simply separating angiosperms from gymnosperms. Similarly, different species, or even the same species but of different age, size, and/or growing in diverse environmental conditions, are likely characterized by different root depths that might reach different soil depths and access different water sources, therefore hampering an equal comparison in the analysis of plant water uptake (e.g., Bargués Tobella et al, ). However, the large sample size of our study may make the general pattern reasonably valid.…”