Differences in vertical root distributions are often assumed to create resource uptake trade‐offs that determine plant growth and coexistence. Yet, most plant roots are in shallow soils, and data linking root distributions with resource uptake and plant abundances remain elusive. Here we used a tracer experiment to describe the vertical distribution of absorptive roots of dominant species in a shrub–steppe ecosystem. To describe how these different rooting distributions affected water uptake in wet and dry soils across a growing season, we used a soil water movement model. Root traits were then correlated with plant landscape abundances. Deeper root distributions extracted more soil water, had larger unique hydrological niches and were more abundant on the landscape. Though most (>50%) root biomass and tracer uptake occurred in shallow soils (0–32 cm), the depth of 50% of tracer uptake varied from 11 to 32 cm across species and species with deeper rooting distributions were more abundant on the landscape (
Models of ecosystem development and response to environmental variation must incorporate change in vertical soil space as well as over time. Insufficient measurement of subsurface soil properties represents a major observational bias in ecosystem studies. We address these changes in horizontal (time) and vertical (soil profile) space along a three‐million‐year, semi‐arid, piñon‐juniper woodland substrate age gradient with characteristic progressive and retrogressive ecosystem development phases and a shift from nitrogen (N) and water to phosphorus (P) limitation. We present a novel pedological approach using isotopic tracers and biogeochemical analyses to address fine root distribution, depth of plant uptake and relative nutrient availabilities. We show that (a) the quantity of fine roots remains constant with ecosystem development but their distribution in the soil profile becomes increasingly deeper and less concentrated in the surface soil; (b) mean depth of tree uptake becomes deeper with substrate age and follows the relative availability of P as P‐limitation develops and (c) soil P transformations in the oldest soil profiles resemble the theoretical changes with age to produce a depth gradient of relative N and P availability.
- NSF-PAR ID:
- 10453711
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 108
- Issue:
- 6
- ISSN:
- 0022-0477
- Page Range / eLocation ID:
- p. 2256-2269
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract R 2 = .95). The water flow model revealed that deeper rooting distributions should extract more soil water (i.e. a range of 60–113 mm of soil water) because shallow roots were often in dry soils. These potential water uptake values were tightly correlated with species’ abundances on the landscape (R 2 = .90). Finally, each species’ rooting distribution demonstrated a depth and time at which it could extract more soil water than any other rooting distribution, and the size of these unique hydrological niches indices was also well correlated with species’ abundances (R 2 = .89).Synthesis . Our results demonstrate not only a correlation between root distributions and species abundance, but also the mechanism through which differences in rooting distributions can determine resource uptake and niche partitioning, even when most roots are found in shallow soils. -
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Abstract In savannas, partitioning of below‐ground resources by depth could facilitate tree–grass coexistence and shape vegetation responses to changing rainfall patterns. However, most studies assessing tree versus grass root‐niche partitioning have focused on one or two sites, limiting generalization about how rainfall and soil conditions influence the degree of rooting overlap across environmental gradients.
We used two complementary stable isotope techniques to quantify variation (a) in water uptake depths and (b) in fine‐root biomass distributions among dominant trees and grasses at eight semi‐arid savanna sites in Kruger National Park, South Africa. Sites were located on contrasting soil textures (clayey basaltic soils vs. sandy granitic soils) and paired along a gradient of mean annual rainfall.
Soil texture predicted variation in mean water uptake depths and fine‐root allocation. While grasses maintained roots close to the surface and consistently used shallow water, trees on sandy soils distributed roots more evenly across soil depths and used deeper soil water, resulting in greater divergence between tree and grass rooting on sandy soils. Mean annual rainfall predicted some variation among sites in tree water uptake depth, but had a weaker influence on fine‐root allocation.
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Abstract Light is a key resource for tree performance and hence, tree species partition spatial and temporal gradients in light availability. Although light distribution drives tree performance and species replacement during secondary forest succession, we yet lack understanding how light distribution changes with tropical forest development.
This study aims to evaluate how changes in forest structure lead to changes in vertical and horizontal light heterogeneity during tropical forest succession.
We described successional patterns in light using a chronosequence approach in which we compared 14 Mexican secondary forest stands that differ in age (8–32 years) since agricultural abandonment. For each stand, we measured vertical light profiles in 16 grid cells, and structural parameters (diameter at breast height, height and crown dimensions) for each tree.
During succession, we found a rapid increase in stand size (basal area, crown area and length) and stand differentiation (i.e. a gradual leaf distribution along the forest profile), which leads to fast changes in light conditions and more light heterogeneity. The inflection points of the vertical light gradient (i.e. the absolute height at which 50% relative light intensity is attained) rapidly moved towards higher heights in the first 20 years, indicating that larger amounts of light are intercepted by canopy trees. Light attenuation rate (i.e. the rate of light extinction) decreased during succession due to slower accumulation of the crown area with height. Understorey light intensity and heterogeneity slightly decreased during succession because of an increase in crown size and a decrease in lateral gap frequency. Understorey relative light intensity was 1.56% at 32 years after abandonment.
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Addressing the knowledge gaps identified in this synthesis will bring insights into physical and biological processes driving biogeochemical cycles at the nest and ecosystem scale and will improve our understanding of ecosystem biogeochemical heterogeneity and larger scale ecological phenomena.
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