The partitioning of water uptake between growth forms in a Neotropical savanna: do herbs exploit a third water source niche?

Will trees or grasses profit from changing rainfall regimes in savannas?

Increasing rainfall variability is widely expected under future climate change scenarios. How will savanna trees and grasses be affected by growing season dry spells and altered seasonality and how tightly coupled are tree-grass phenologies with rainfall? We measured tree and grass responses to growing season dry spells and dry season rainfall. We also tested whether the phenologies of 17 deciduous woody species and the Soil Adjusted Vegetation Index of grasses were related to rainfall between 2019 and 2023. Tree and grass growth was significantly reduced during growing season dry spells. Tree growth was strongly related to growing season soil water potentials and limited to the wet season. Grasses can rapidly recover after growing season dry spells and grass evapotranspiration was significantly related to soil water potentials in both the wet and dry seasons. Tree leaf flushing commenced before the rainfall onset date with little subsequent leaf flushing. Grasses grew when moisture became available regardless of season. Our findings suggest that increased dry spell length and frequency in the growing season may slow down tree growth in some savannas, which together with longer growing seasons may allow grasses an advantage over C3 plants that are advantaged by rising CO2 levels.

African savanna grasses outperform trees across the full spectrum of soil moisture availability

Models of tree-grass coexistence in savannas make different assumptions about the relative performance of trees and grasses under wet vs dry conditions. We quantified transpiration and drought tolerance traits in 26 tree and 19 grass species from the African savanna biome across a gradient of soil water potentials to test for a trade-off between water use under wet conditions and drought tolerance. We measured whole-plant hourly transpiration in a growth chamber and quantified drought tolerance using leaf osmotic potential (Ψ_osm ). We also quantified whole-plant water-use efficiency (WUE) and relative growth rate (RGR) under well-watered conditions. Grasses transpired twice as much as trees on a leaf-mass basis across all soil water potentials. Grasses also had a lower Ψ_osm than trees, indicating higher drought tolerance in the former. Higher grass transpiration and WUE combined to largely explain the threefold RGR advantage in grasses. Our results suggest that grasses outperform trees under a wide range of conditions, and that there is no evidence for a trade-off in water-use patterns in wet vs dry soils. This work will help inform mechanistic models of water use in savanna ecosystems, providing much-needed whole-plant parameter estimates for African species.

Different Responses in Root Water Uptake of Summer Maize to Planting Density and Nitrogen Fertilization

Modifying farming practices combined with breeding has the potential to improve water and nutrient use efficiency by regulating root growth, but achieving this goal requires phenotyping the roots, including their architecture and ability to take up water and nutrients from different soil layers. This is challenging due to the difficulty of in situ root measurement and opaqueness of the soil. Using stable isotopes and soil coring, we calculated the change in root water uptake of summer maize in response to planting density and nitrogen fertilization in a 2-year field experiment. We periodically measured root-length density, soil moisture content, and stable isotopes δ¹⁸O and δD in the plant stem, soil water, and precipitation concurrently and calculated the root water uptake based on the mass balance of the isotopes and the Bayesian inference method coupled with the Markov Chain Monte Carlo simulation. The results show that the root water uptake increased asymptotically with root-length density and that nitrogen application affected the locations in soil from which the roots acquired water more significantly than planting density. In particular, we find that reducing nitrogen application promoted root penetration to access subsoil nutrients and consequently enhanced their water uptake from the subsoil, while increasing planting density benefited water uptake of the roots in the topsoil. These findings reveal that it is possible to manipulate plant density and fertilization to improve water and nutrient use efficiency of the summer maize and the results thus have imperative implications for agricultural production.

What Do Plants Leave after Summer on the Ground?—The Effect of Afforested Plants in Arid Environments

The implementation of afforestation programs in arid environments in northern China had modified the natural vegetation patterns. This increases the evaporation flux; however, the influence of these new covers on the soil water conditions is poorly understood. This work aims to describe the effect of Willow bushes (Salix psammophila C. Wang and Chang Y. Yang) and Willow trees (Salix matsudana Koidz.) on the soil water conditions after the summer. Two experimental plots located in the Hailiutu catchment (Shaanxi province, northwest China), and covered with plants of each species, were monitored during Autumn in 2010. The monitoring included the soil moisture, fine root distribution and transpiration fluxes that provided information about water availability, access and use by the plants. Meanwhile, the monitoring of stable water isotopes collected from precipitation, soil water, groundwater and xylem water linked the water paths. The presence of Willow trees and Willow bushes reduce the effect of soil evaporation after summer, increasing the soil moisture respect to bare soil conditions. Also, the presence of soil water with stable water isotope signatures close to groundwater reflect the hydraulic lift process. This is an indication of soil water redistribution carried out by both plant species.

Contrasting water sources and water-use efficiency in coexisting desert plants in two saline-sodic soils in northwest China

Soil degradation resulting from various types of salinity is a major environmental problem, especially in arid and semiarid regions. Exploring the water-related physiological traits of halophytes is useful for understanding the mechanisms of salt tolerance. This knowledge could be used to rehabilitate degraded arid lands. To investigate whether different types of salinity influence the water sources and water-use efficiency of desert plants (Karelinia caspia, Tamarix hohenackeri, Nitraria sibirica, Phragmites australis, Alhagi sparsifolia, Suaeda microphylla, Kalidium foliatum) in natural environments, we measured leaf gas exchange, leaf carbon and xylem oxygen isotope composition and soil oxygen isotope composition at neutral saline-sodic site (NSS) and alkaline saline-sodic site (ASS) in northwest China. The studied plants had different xylem water oxygen isotope compositions (δ¹⁸ O) and foliar carbon isotope compositions (δ¹³ C), indicating that desert plants coexist through differentiation in water use patterns. Compared to that at the NSS site, the stem water in K. caspia, A. sparsifolia and S. microphylla was depleted in ¹⁸ O at the ASS site, which indicates that plants can switch to obtain water from deeper soil layers when suffering environmental stress from both salinity and alkalinisation. Alhagi sparsifolia had higher δ¹³ C at the ASS site than at the NSS site, while K. caspia and S. microphylla had lower δ¹³ C, which may have resulted from interspecific differences in plant alkali and salt tolerance ability. Our results suggest that under severe salinity and alkalinity, plants may exploit deeper soil water to avoid ion toxicity resulting from high concentrations of soluble salts in the superficial soil layer. In managed lands, it is vital to select and cultivate different salt-tolerant or alkali-tolerant plant species in light of local conditions.

Seasonal changes of fructans in dimorphic roots of Ichthyothere terminalis (Spreng.) Blake (Asteraceae) growing in Cerrado

Cerrado is a floristically rich savanna in Brazil, whose vegetation consists of a physiognomic mosaic, influenced by rainfall seasonality. In the dry season rainfall is substantially lower and reduces soil water supply, mainly for herbs and subshrubs. Climatic seasonal variations may well define phenological shifts and induce fluctuations of plant reserve pools. Some Cerrado native species have thickened underground organs that bear buds and store reserves, as adaptive features to enable plant survival following environmental stresses. Asteraceae species accumulate fructans in storage organs, which are not only reserve, but also protecting compounds against the effects of cold and drought. Ichthyothere terminalis is one Asteraceae species abundant in cerrado rupestre, with underground organs consisting of thickened orthogravitropic and diagravitropic roots. The objectives of this study were to analyze how abiotic environmental factors and plant phenology influence fructan dynamics in field grown plants, and verify if fructan metabolism differs in both root types for one year. I. terminalis accumulates inulin-type fructans in 10-40% of the dry mass in both root types. Fructan dynamics have similar patterns described for other Asteraceae species, exhibiting a proportional increase of polysaccharides with the senescence of the aerial organs. Multivariate analyzes showed that, as rainfall decreased, environmental factors had a stronger influence on metabolite levels than phenological shifts in both root types. Only slight differences were found in fructan dynamics between orthogravitropic and diagravitropic roots, suggesting they may have similar fructan metabolism regulation. However, these small differences may reflect distinct microclimatic conditions in both root types and also represent the influence of sink strength.

Divergence in plant water-use strategies in semiarid woody species

Partitioning of water resources amongst plant species within a single climate envelope is possible if the species differ in key hydraulic traits. We examined 11 bivariate trait relationships across nine woody species found in the Ti-Tree basin of central Australia. We found that species with limited access to soil moisture, evidenced by low pre-dawn leaf water potential, displayed anisohydric behaviour (e.g. large seasonal fluctuations in minimum leaf water potential), had greater sapwood density and lower osmotic potential at full turgor. Osmotic potential at full turgor was positively correlated with the leaf water potential at turgor loss, which was, in turn, positively correlated with the water potential at incipient stomatal closure. We also observed divergent behaviour in two species of Mulga, a complex of closely related Acacia species which range from tall shrubs to low trees and dominate large areas of arid and semiarid Australia. These Mulga species had much lower minimum leaf water potentials and lower specific leaf area compared with the other seven species. Finally, one species, Hakea macrocarpa A.Cunn ex.R.Br., had traits that may allow it to tolerate seasonal dryness (through possession of small specific leaf area and cavitation resistant xylem) despite exhibiting cellular water relations that were similar to groundwater-dependent species. We conclude that traits related to water transport and leaf water status differ across species that experience differences in soil water availability and that this enables a diversity of species to exist in this low rainfall environment.

Expanding our understanding of leaf functional syndromes in savanna systems: the role of plant growth form

The assessment of leaf strategies has been a common theme in ecology, especially where multiple sources of environmental constraints (fire, seasonal drought, nutrient-poor soils) impose a strong selection pressure towards leaf functional diversity, leading to inevitable tradeoffs among leaf traits, and ultimately to niche segregation among coexisting species. As diversification on leaf functional strategies is dependent on integration at whole plant level, we hypothesized that regardless of phylogenetic relatedness, leaf trait functional syndromes in a multivariate space would be associated with the type of growth form. We measured traits related to leaf gas exchange, structure and nutrient status in 57 coexisting species encompassing all Angiosperms major clades, in a wide array of plant morphologies (trees, shrubs, sub-shrubs, herbs, grasses and palms) in a savanna of Central Brazil. Growth forms differed in mean values for the studied functional leaf traits. We extracted 4 groups of functional typologies: grasses (elevated leaf dark respiration, light-saturated photosynthesis on a leaf mass and area basis, lower values of leaf Ca and Mg), herbs (high values of SLA, leaf N and leaf Fe), palms (high values of stomatal conductance, leaf transpiration and leaf K) and woody eudicots (sub-shrubs, shrubs and trees; low SLA and high leaf Ca and Mg). Despite the large range of variation among species for each individual trait and the independent evolutionary trajectory of individual species, growth forms were strongly associated with particular leaf trait combinations, suggesting clear evolutionary constraints on leaf function for morphologically similar species in savanna ecosystems.

Irrigation depth far exceeds water uptake depth in an oasis cropland in the middle reaches of Heihe River Basin

Agricultural irrigation in the middle reaches of the Heihe River Basin consumes approximately 80% of the total river water. Whether the irrigation depth matches the water uptake depth of crops is one of the most important factors affecting the efficiency of irrigation water use. Our results indicated that the influence of plastic film on soil water δ¹⁸O was restricted to 0–30 cm soil depth. Based on a Bayesian model (MixSIR), we found that irrigated maize acquired water preferentially from 0–10 cm soil layer, with a median uptake proportion of 87 ± 15%. Additionally, maize utilised a mixture of irrigation and shallow soil water instead of absorbing the irrigation water directly. However, only 24.7 ± 5.5% of irrigation water remained in 0–10 cm soil layer, whereas 29.5 ± 2.8% and 38.4 ± 3.3% of the irrigation water infiltrated into 10–40 cm and 40–80 cm layers. During the 4 irrigation events, approximately 39% of the irrigation and rainwater infiltrated into soil layers below 80 cm. Reducing irrigation amount and developing water-saving irrigation methods will be important strategies for improving the efficiency of irrigation water use in this area.