Woody Plant Encroachment has a Larger Impact than Climate Change on Dryland Water Budgets

Abstract

Woody plant encroachment (WPE) into grasslands is a global phenomenon that is associated with land degradation via xerification, which replaces grasses with shrubs and bare soil patches. It remains uncertain how the global processes of WPE and climate change may combine to impact water availability for ecosystems. Using a process-based model constrained by watershed observations, our results suggest that both xerification and climate change augment groundwater recharge by increasing channel transmission losses at the expense of plant available water. Conversion from grasslands to shrublands without creating additional bare soil, however, reduces transmission losses. Model simulations considering both WPE and climate change are used to assess their relative roles in a late 21^st century condition. Results indicate that changes in focused channel recharge are determined primarily by the WPE pathway. As a result, WPE should be given consideration when assessing the vulnerability of groundwater aquifers to climate change.

Figure 1

Model scenarios in the context of continental scale WPE. (a) Study site location along with WPE pathways in the United States. Diamonds indicate states where woody plant encroachment, via xerification (yellow) or thicketization (purple), has been observed. (b) Current spatial pattern of grasses, shrubs, and bare soils (2013) at the watershed study site. Stacked bar plots illustrate WPE model scenarios for the xerification pathway (c) and the thicketization pathway (d).

Figure 2

Impacts of two woody plant encroachment pathways on transmission losses. Relation between monthly transmission losses (T_L) and precipitation (P) for (a) xerification and (b) thicketization pathways. Insets display annual T_L/P relation with percent grass cover (%).

Figure 3

Combined impacts of climate change and WPE on transmission losses. (a) The difference in the average annual T_L between a historical grassland and end-member shrubland states forced with historical climate (left) and grassland forced with climate change forcings (right). The X-path is the xerification pathway and the T-path is the thicketization pathway. (b) The difference in the average annual T_L between a historical grassland and two shrubland states forced with a historical climate based on NLDAS-2 data (‘Hist.’) or one of 3 climate change projections, CNRM-CM5 (‘CNR’), CSIRO Mk.3.6.0 (‘CSI’), or HadGEM2-ES (‘HGE’). (c) The difference in the average annual T_L between shrubland and grassland when both vegetation states use the same meteorological forcings for historical conditions (‘Hist.’) or one of the 3 climate change projections.