Plant identity and shallow soil moisture are primary drivers of stomatal conductance in the savannas of Kruger National Park

A modern two-layer hypothesis helps resolve the 'savanna problem'

For over a century, deep roots have been assumed to allow trees to avoid competition with grasses (i.e., the two-layer hypothesis). Yet, in part because it remains difficult to measure water uptake in the field, there has been a shift in savanna ecology away from the two-layer hypothesis and towards alternative explanations of tree-grass coexistence. Here, we combine hydrologic tracer experiments and soil water flow models to demonstrate how the distribution of active roots affects water uptake across a range of savanna conditions. Grass roots were shallower and provided pre-emptive access to enough soil water to allow nearly continuous grass cover, but slightly deeper roots provided trees with more total water under most conditions. This 'some water now or more water later' tradeoff varied with precipitation amount, soil texture, and tree and grass relative root abundance in ways that helped explain tree and grass landscape abundance.

Small differences in root distributions allow resource niche partitioning

Deep roots have long been thought to allow trees to coexist with shallow-rooted grasses. However, data demonstrating how root distributions affect water uptake and niche partitioning are uncommon.We describe tree and grass root distributions using a depth-specific tracer experiment six times over two years in a subtropical savanna, Kruger National Park, South Africa. These point-in-time measurements were then used in a soil water flow model to simulate continuous water uptake by depth and plant growth form (trees and grasses) across two growing seasons. This allowed estimates of the total amount of water a root distribution could absorb as well as the amount of water a root distribution could absorb in excess of the other rooting distribution (i.e., unique hydrological niche).Most active tree and grass roots were in shallow soils: The mean depth of water uptake was 22 cm for trees and 17 cm for grasses. Slightly deeper rooting distributions provided trees with 5% more soil water than the grasses in a drier season, but 13% less water in a wetter season. Small differences also provided each rooting distribution (tree or grass) with unique hydrological niches of 4 to 13 mm water.The effect of rooting distributions has long been inferred. By quantifying the depth and timing of water uptake, we demonstrated how even small differences in rooting distributions can provide plants with resource niches that can contribute to species coexistence. Differences in total water uptake and unique hydrological niche sizes were small in this system, but they indicated that tradeoffs in rooting strategies can be expected to contribute to tree and grass coexistence because 1) competitive advantages change over time and 2) plant growth forms always have access to a soil resource pool that is not available to the other plant growth form.

Convergence in Maximum Stomatal Conductance of C3 Woody Angiosperms in Natural Ecosystems Across Bioclimatic Zones

Stomatal conductance (g s) in terrestrial vegetation regulates the uptake of atmospheric carbon dioxide for photosynthesis and water loss through transpiration, closely linking the biosphere and atmosphere and influencing climate. Yet, the range and pattern of g s in plants from natural ecosystems across broad geographic, climatic, and taxonomic ranges remains poorly quantified. Furthermore, attempts to characterize g s on such scales have predominantly relied upon meta-analyses compiling data from many different studies. This approach may be inherently problematic as it combines data collected using unstandardized protocols, sometimes over decadal time spans, and from different habitat groups. Using a standardized protocol, we measured leaf-level g s using porometry in 218 C3 woody angiosperm species in natural ecosystems representing seven bioclimatic zones. The resulting dataset of 4273 g s measurements, which we call STraits (Stomatal Traits), was used to determine patterns in maximum g s (g smax) across bioclimatic zones and whether there was similarity in the mean g smax of C3 woody angiosperms across ecosystem types. We also tested for differential g smax in two broadly defined habitat groups - open-canopy and understory-subcanopy - within and across bioclimatic zones. We found strong convergence in mean g smax of C3 woody angiosperms in the understory-subcanopy habitats across six bioclimatic zones, but not in open-canopy habitats. Mean g smax in open-canopy habitats (266 ± 100 mmol m-2 s-1) was significantly higher than in understory-subcanopy habitats (233 ± 86 mmol m-2 s-1). There was also a central tendency in the overall dataset to operate toward a g smax of ∼250 mmol m-2 s-1. We suggest that the observed convergence in mean g smax of C3 woody angiosperms in the understory-subcanopy is due to a buffering of g smax against macroclimate effects which will lead to differential response of C3 woody angiosperm vegetation in these two habitats to future global change. Therefore, it will be important for future studies of g smax to categorize vegetation according to habitat group.

Characteristics and correlations of leaf stomata in different Aleurites montana provenances

Stomata are important indexes in studies of plant origin, evolution, and classification and are important organs in plant phylogenetic relationship studies. Nine Aleurites montana provenances were used in this study to examine stomatal density, stomatal area, stomatal perimeter, long axis length, and short axis length. The correlation and cluster analyses were conducted among the morphological parameters of the pores. The results showed that there were significant differences in different A. Montana provenances in terms of stomatal morphology parameters. The average stomatal density, stomatal area, stomatal perimeter, stomatal long axis length, and stomatal short axis length of each provenance was between 224.16–307.10 stomata/mm², 80.42–99.76 μm, 36.39–41.32 μm, 15.89–18.44 μm, and 6.53–7.46 μm, respectively, and the coefficient of variation was between 9.18%–20.15%, 17.57%–33.77%, 9.31%–18.79%, 9.71%–18.48%, and 10.26%–21.57%, respectively. Correlation analysis shows that there was a significant negative correlation between stomatal density and stomatal shape parameters (stomatal area, stomatal perimeter, stomatal long axis, stomatal short axis) and there was a significant positive correlation between stomatal parameters. There was no significant correlation between geographical environment factors and stomatal characteristics. There is a close relationship between stomatal morphology and stomatal conductance whereby dense small stomata can quickly adapt to changes in the environment; considering the characteristics A. montana stomata in terms of stomatal stability and ability to adapt to the environment, the Jianyang, Zhenghe, Fuding, Shaxian provenances were identified as being more suitable for planting at different sites. This study provides a theoretical basis for the genetic improvement and breeding of high quality A. montana provenances.