A quantitative description of the interspecies diversity of belowground structure in savanna woody plants

Abstract

2015. A quantitative description of the interspecies diversity of belowground structure in savanna woody plants. Ecosphere 6(9):154. http:// dx.Abstract. The relative importance of resource competition and disturbance in limiting woody cover is one of the most basic questions in savanna ecology. Modeling approaches that seek to address this question are limited in their ability to accurately represent resource competition, which occurs belowground, by the limited detail of existing data on root system structure. Using compressed air, we excavated individual trees and shrubs and mapped their coarse roots on a three-dimensional grid system up to 1.5 m depth. We excavated four woody savanna species at three sites spanning a climate gradient on the Kalahari Transect in southern Africa and determined functions to describe the distribution of root biomass within the root systems. Overall, most of the variation in both large-scale and small-scale characteristics of root system structure was related to species. The species excavated fell into two groups that coexisted across the climate gradient. Acacia mellifera and Terminalia sericea had straight roots in a laterally extensive system that was shallow relative to the aggregate root profile for woody plants at the sites. Boscia albitrunca and Ochna pulchra had sinuous roots that were mostly concentrated beneath the canopy and were more prevalent in deep than near-surface soil layers, departing from the conventional model of decreasing root abundance with depth. Based on our results, we suggest a reframing of the theoretical characterization of the root zone used in modeling approaches to consider diversity within the woody plant community.

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

“…But it is contrary to results that have been found in other species in semiarid regions, where members of the Combretum , Terminalia , Prosopis , and Acacia genera utilize groundwater (which is most likely only soil water of potential groundwater recharge) beside soil water to support the highest transpiration demand in summer (Evaristo and McDonnell, 2017; Beyer et al, 2018). Our results instead fit with the excavation study of O'Donnell et al, (2015) in the Kalahari, where the root system of A. mellifera did not appear to be large enough to reach the water table or saturated zones. It means that in a woody plant encroached savanna by A. mellifera , groundwater will most likely be affected indirectly through the impact of these plants on the process of groundwater recharge.…”

Soils and fire jointly determine vegetation structure in an African savanna

“…Although recent evidence suggests that trees do not exclusively use deeper water than grasses (see e.g. Seghieri, ; February & Higgins, ; O'Donnell et al ., for examples of shallow‐rooting trees), trees sometimes can have deeper roots and can access deeper water than grasses (Seghieri, ; Bhattachan et al ., ; Kulmatiski & Beard, ; Mazzacavallo & Kulmatiski, ). Infiltration might therefore alleviate water/nutrient limitation in the soil subsurface, thereby promoting faster growth rates of deeper rooted trees in sandy soils than clayey ones.…”

Prediction and scale in savanna ecosystems

“…Historically, these perspectives have represented a major tension in savanna ecology. On the one hand, the Walter hypothesis (Walter, ) suggests that trees and grasses co‐exist because they use resources from different soil layers – trees from deep layers and grasses from shallow – a prediction that is sometimes (Kulmatiski & Beard, ) but not always (O'Donnell et al ., ) borne out. Differentiation in temporal water use is another possible avenue for tree−grass co‐existence via resource partitioning; canonically, grasses are more opportunistic with respect to rainfall arrival within a season than trees are (Rodriguez‐Iturbe, ), although, again, this is not always the case (Ryan et al ., ).…”