Coupling vegetation organization patterns to soil resource heterogeneity in a central Kenyan dryland using geophysical imagery

Abstract

In dryland ecosystems, understanding the effects of heterogeneity in soil moisture and geophysical properties on vegetation structure and dynamics poses a suite of challenging research questions. Heterogeneity in soil depth can affect resource availability and the subsequent organization of woody vegetation, while spatiotemporal variation in soil moisture can reveal important ecohydrological feedbacks that govern the outcome of anthropogenic activities on the organization of dryland vegetation. In this research we investigate two cases of soil resource heterogeneity that affect the organization of dryland vegetation patterns by expanding previous electromagnetic induction (EMI) imaging techniques. In the first case we examine the influence of soil depth as a control on soil resource availability on hillslopes in tree‐grass systems in central Kenya. Our results indicate that woody vegetation clumping occurs where soil depth changes, and the deeper rooted Acacia tortilis occurs on deep soils while the drought tolerant Acacia etbaica occurs on shallow soils. In the second case we examine daily patch–interpatch scale moisture dynamics following two different‐sized rain events in a degraded landscape. With the aid of a numerical subsurface flow model, EMI, and soil moisture data, we have identified a possible positive feedback mechanism (“soil moisture halo effect”) that we believe may have contributed to the proliferation and two‐phase pattern formation of a native succulent Sansevieria volkensii in degraded ecosystems of Kenya. By determining how different plants respond to, and modify, the soil environment, we can better understand resource capture and dynamics, which in the longterm will help to develop management strategies.

Spatial vegetation patterns and neighborhood competition among woody plants in an East African savanna

“…Basal stem size of neighbors better predicted RGR of focal plants compared to height and crown area metrics. This finding strongly points to belowground competition for water as the likely mechanism generating competitive suppression of growth rates, as has been suggested by other studies of woody plant spatial patterns in the region (Franz et al 2011(Franz et al , 2012. Studies of competition among trees in forests generally show increased performance of models utilizing crown area to describe tree size, reflective of size-asymmetric competition for light typical in wetter systems (e.g., von Oheimb et al 2011, Fraver et al 2014.…”

“…The clumping patterns we documented could arise from several mechanisms operating alone or in concert, including, but not limited to, (1) dispersal limitation (e.g., greater seed deposition beneath existing canopies), and (2) spatial heterogeneity in seed germination and survival associated with edaphic variability and surface run-off/run-on patterns influenced by plant clusters. In particular, we suggest that facilitative moderation of the abiotic environment by existing woody clumps that trap surface runoff (Franz et al 2011) and improve resource availability may be an important mechanism contributing to spatial patterns in woody plant distribution. Additional beneficial or detrimental effects of clumped spatial patterns may be realized in the presence of fire and/or herbivory, but our results in plots not subject to either fire or large mammalian herbivory suggest that (1) endogenous abiotic components of savanna systems combined with soil-plant feedbacks are sufficient to produce aggregated arrangements, and (2) both interspecific and conspecific competition become important determinants of growth rates within woody plant clusters.…”

“…Distinct spatial patterns consisting of vegetated patches alternating with areas of bare soil have been described for savanna ecosystems worldwide (Aguiar and Sala 1999, Tongway et al 2001, Franz et al 2011. Analytical models have shown that surface runoff, local plant density and water infiltration rates, both by themselves and in concert with each other, can create and maintain the irregular vegetation patterns that characterize savanna ecosystems (Klausmeier 1999, HilleRisLambers et al 2001, Pueyo et al 2008).…”

Measuring and modeling water-related soil–vegetation feedbacks in a fallow plot

“…Then the interaction continues above the soil surface where the shoots and the leaves shadow the soil and limit the evaporation while transpiration begins. Vegetation reduces the soil moisture content, particularly in the hot season, but it also enhances the soil hydraulic conductivity with its root apparatus (Gish and Jury, 2004;Zimmermann et al, 2006), thus helping to replenish the subsoil water storage and creating a positive feedback system (Franz et al, 2011). When a spontaneously growing species establishes itself on bare land, water-related soil-vegetation feedbacks are often invoked to motivate field-scale soil moisture and vegetation patterns, describe patterns related to eco-hydrological processes and evaluate the associated water budget.…”

Integration of hydrogeophysical datasets and empirical orthogonal functions for improved irrigation water management

“…A Hemisphere GPS XF101 DGPS (Juniper Systems, Inc., Logan, UT) unit recorded the location of each measurement. Following basic quality assurance and quality control of the raw ECa data (Franz et al 2011), a spatial map with 5 by 5 m resolution was created using an inverse-distance weighting procedure.…”