Quantifying a sponge: The additional water in restored thicket

Abstract

Restoration of degraded subtropical thicket in the Eastern Cape, South Africa, can result in the return of more than 30 tonnes of soil organic carbon per hectare. 1,2 Given that soil carbon is usually positively correlated with soil water-holding capacity 3-5 , we hypothesised that restoration of thicket would greatly increase the sponge effect of its soils. As a first step towards examining this hypothesis, we used a model that predicts how changes in soil texture and soil carbon affect soil water-holding capacity.In sandy and loamy soils, increases in soil water-holding capacity will tend to range from ~1% to ~8% for each per cent increase in soil organic carbon. 3,5-9 By contrast, within clayey soils, and within a particular range of soil carbon, an increase in soil organic matter can be expected to reduce, not increase, soil water-holding capacity. An increased sponge effect is consequently not a fait accompli in thicket restoration (see Figure 1). Figure 1: Modelled relationship between soil organic carbon (%) and water retention (%) using the equations of Rawls et al. 3The texture of soils across subtropical thicket varies greatly, but a large proportion are sandy (~80% sand content), with relatively small amounts of clay (~10%) and silt (~10%). 10 We consequently used a ratio of 80:10:10 sand:silt:clay to estimate how much water is likely to be stored when restoring thicket. Assuming that soil carbon increases from 2% to 5% in the top 30 cm of soil 11 , the model predicts that an extra ~255 thousand litres of extra water would be stored per hectare. Across a farm of, for example, 5000 hectares, the amount of extra water stored would be 1.28 billion litres, and across the ~1 million hectares of the subtropical thicket biome that is degraded 12 the amount would be 255 billion litres. In conclusion, restoring degraded subtropical thicket at the biome-scale is likely to result in the additional storage of more than 200 billion litres of water. To put this amount in perspective, Theewaterskloof Dam, Cape Town's main storage dam, holds ~400 billion litres when full.The additional water storage in subtropical thicket soils would result in myriad benefits for society, including greater productivity of the landscape for livestock and game (particularly during droughts), flood mitigation, and greater flow of water from groundwater into rivers. 13 It would be instructive for government and private landowners to have a hydrological model that shows how creating an underground dam of 200 billion litres would increase supply of water to farmers and towns across the Eastern Cape. Our hypothesis is that the economic returns from the additional water alone would be well worth the costs of restoring the 1 million hectares of degraded thicket.

Regeneration dynamics of Portulacaria afra in restored succulent thicket of South Africa

“…The decline in ecosystem condition is well described in degraded succulent thicket habitat. The loss of the P. afra canopy exposes soils to the elements; leading to the loss of organic content ( Lechmere-Oertel et al, 2008 ; Mills & Fey, 2004 ), reduced water holding capacity ( Mills & de Wet, 2019 ; van Luijk et al, 2013 ), increased light exposure, and extreme fluctuations in soil temperature ( Lechmere-Oertel et al, 2008 ). This could potentially eliminate suitable germination sites for P. afra , disrupting natural regeneration.…”

Restoring South African subtropical succulent thicket using Portulacaria afra: root growth of cuttings differs depending on the harvest site during a drought

“…This loss of plant cover, including P. afra , results in the disruption of various ecological processes. Exposed soils are especially prone to: erosion due to higher rates of water runoff ( Mills & de Wet, 2019 ; van Luijk et al, 2013 ; Cowling & Mills, 2011 ); loss of soil organic carbon ( Mills & de Wet, 2019 ; Lechmere-Oertel et al, 2008 ); and a disruption of soil microbial communities ( Schagen et al, 2021 ). Furthermore, the loss of the cool, damp understory microclimate required for germination ( Wilman et al, 2014 ; Sigwela et al, 2009 ) halts woody species recruitment in the degraded landscape ( Lechmere-Oertel, Cowling & Kerley, 2005 ).…”

Automated mapping of Portulacaria afra canopies for restoration monitoring with convolutional neural networks and heterogeneous unmanned aerial vehicle imagery

“…seed set and asexual reproduction via rooting of lateral branches) cannot overcome rates of canopy reduction and mortality ( Lechmere-Oertel et al, 2008 ). The environmental buffering effects of P. afra increase the amount of soil organic matter ( Lechmere-Oertel et al, 2008 ), improving water infiltration ( van Luijk et al, 2013 ) and water holding capacity ( Mills & de Wet, 2019 ). The canopy of P. afra creates the cool shaded microhabitat required for the recruitment of canopy tree species ( Sigwela et al, 2009 ; Wilman et al, 2014 ), thus facilitating community assembly processes.…”

“…The loss of P. afra cover triggers a series of feedback loops that steers the ecosystem towards an alternate state with limited ecological functioning. The lack of vegetation cover exposes the soils to erosion which results in the depletion of soil organic carbon ( Cowling & Mills, 2011 ; Lechmere-Oertel et al, 2008 ; Lechmere-Oertel, Kerley & Cowling, 2005 ; Mills & Fey, 2004 ), and disrupts hydrological processes such as water infiltration and subsequent retention ( Cowling & Mills, 2011 ; Mills & de Wet, 2019 ; van Luijk et al, 2013 ). This leads to further loss of ecological processes and ultimately, significant loss of biodiversity ( Fabricius, Burger & Hockey, 2003 ; Sigwela et al, 2009 ).…”