Water sourcing by trees in a mesic savanna: Responses to severing deep and shallow roots

Revisiting the Two-Layer Hypothesis: Coexistence of Alternative Functional Rooting Strategies in Savannas

“…Many stable isotope-based studies have in fact shown clear differences in functional rooting profiles between trees and grasses and among functional groups, such as C 3 forbs and C 4 grasses [16], [17], [18], [54], [55], although it has been suggested that such partitioning tends to occur in systems where precipitation falls in the non-growing season, which is not the case in (for example) the extensive sub-Saharan African savannas [7]. The more limited amount of work conducted in African savannas does not appear to support the two-layer model [19], [20], [25], but upon closer scrutiny it appears that the approaches available may usually only be adequate for quantifying coarse differences in rooting profiles (a consequence of the difficulty of studying belowground patterns and processes). More painstaking studies have shown that trees and grasses may exhibit subtle rooting differences as a function of depth [29], [50], and that tree and grass profiles do differ, even when there is substantial overlap [19], [28].…”

“…Subsequent work somewhat simplified the niche differentiation model by focusing on rooting separation alone, and over time this vertical resource partitioning model has become a general hypothesis for explaining tree-grass coexistence [4], [6], [7], [14], [15]. The evidence for or against such vertical partitioning in savannas has been mixed [16], [17], [18], [19], [20], and this may explain why demographic explanations have become more dominant over the past decade or so [8], [12], [21]. Fire and herbivory are well-known to be strong drivers of tree cover change in many savanna ecosystems [21], [22], [23], [24], but it is still far from clear how important or general the vertical resource partitioning mechanism is for allowing coexistence and for determining tree-grass ratios in the absence of fire.…”

Transpiration dynamics support resource partitioning in African savanna trees and grasses

“…For example, a recent experiment conducted in Kruger National Park found that infrequent but large precipitation events tend to favor trees at the expense of grasses, while frequent, small events favor grasses (Kulmatiski and Beard 2013b), and the difference was attributable to the deeper infiltration (reaching deeper roots) that occurs in the former scenario. We note that not all work in this system supports the notion of niche partitioning (e.g., Verweij et al 2011, February et al 2013, suggesting that the conditions under which it occurs may well vary, and the causes of this variation require further investigation. We propose that a next…”

“…The argument against niche partitioning also highlights that most precipitation in tropical savannas falls during the warm growing season (unlike the case for many temperate tree-grass systems), and therefore little deep infiltration occurs (Rodrı´guez-Iturbe et al 1999). Manipulative experiments on tree-grass competitive interactions from tropical savannas (as opposed to rooting-depth studies) have also found strong competitive effects between grasses and trees (Riginos 2009, Verweij et al 2011, February et al 2013, although these effects may be context dependent (Knoop and Walker 1985). Holdo (2013) provided a framework for this context by showing theoretically that the extent to which rooting-depth differentiation leads to niche partitioning should depend both on soil and climate variables, and that where conditions are conducive to deep water infiltration, niche partitioning is possible and probably common.…”

A deuterium-based labeling technique for the investigation of rooting depths, water uptake dynamics and unsaturated zone water transport in semiarid environments

“…In the Amazonian forests, for example, the impact of deep roots has been shown to be significant (Nepstad et al, 1994;Oliveira et al, 2005). In a mesic savanna the effects of severing deep taproots were found to be rather small in recent studies (Verweij et al, 2011). The method presented here is not suitable for the investigation of deep tap roots, mainly because of the limitation to which holes can be dug using a hand auger.…”