Restoring South African subtropical succulent thicket using Portulacaria afra: exploring the rooting window hypothesis

Abstract

Drought prone, arid and semi-arid ecosystems are challenging to restore once degraded due to low levels of natural recruitment and survival of reintroduced plants. This is evident in the restoration of degraded succulent thicket habitats in the Albany Subtropical Thicket Biome located in South Africa. The current restoration practice for this ecosystem focuses predominantly on reintroducing Portulacaria afra L. Jacq., which is naturally dominant in terms of cover and biomass, but largely absent in regions degraded by domestic livestock. This has been achieved by planting unrooted cuttings with limited consideration of soil water availability in a drought-prone ecosystem. This study tests the effects of the timing of water availability after planting on the root development of P. afra cuttings. Cuttings were harvested from seven individual plants and grown in a glasshouse setting. Eighty four cuttings were taken from each individual, twelve for each of the seven watering treatments per individual plant. The treatments represented a time-staggered initial watering after planting, including: on the day of planting, 4 days, 7 days, 14 days, 21 days, and 28 days after planting. After 32 days, all treatments were watered on a bi-weekly basis for two weeks; a control treatment with no watering throughout the experiment was included. The proportion of rooted cuttings per treatment and dry root mass were determined at the end of the experimental period (day 42). The early onset of watering was associated with a higher percentage of rooting (X2(5) = 11.352, p = 0.045) and had a weak, but non-significant, impact on the final dry root mass (F5,36 = 2.109, p = 0.0631). Importantly, no clear rooting window within 28 days was detected as the majority of cuttings exhibited root development (greater than 50% of cuttings rooted for each individual parent-plant); this suggests that watering at the time of planting P. afra cuttings in-field for restoration may not be necessary. An unexpected, but important, result was that parent-plant identity had a strong interaction with the accumulation of root mass (F36,460 = 5.026, p < 0.001; LR7 = 122.99, p < 0.001). The control treatment, which had no water throughout the experiment, had no root development. These findings suggest that water availability is required for the onset of rooting in P. afra cutting. However, the duration of the experiment was insufficient to detect the point at which P. afra cuttings could no longer initiate rooting once exposed to soil moisture, and thus no rooting window could be defined. Despite harvesting material from the same source population, parent-plant identity strongly impacted root development. Further work is required to characterise the rooting window, and to explore the effect of parent-plant condition on in-field and experimental restoration results; we urge that experiments using P. afra closely track the parent-source at the individual level as this may be a factor that may have a major impact on results.

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

“…The findings presented here ( i.e., that source location impacts plant condition and consequently root development in P. afra cuttings) have important implications for succulent thicket restoration initiatives. Cutting source location effects have not been considered in P. afra planting efforts to date, and as shown here (initially demonstrated in Galuszynski et al, 2023 ) may play an important role in unrooted cutting establishment and survival, particularly under field and nursery conditions. As previously noted, planting of unrooted P. afra cuttings is an unreliable restoration practice ( van der Vyver et al, 2021 ; Mills & Robson, 2017 ).…”

“…A biome-wide field experiment—consisting of 330 plots to test the viability of various planting treatments using unrooted P. afra cuttings—revealed that cutting survival was affected by stem diameter ( i.e., larger cuttings established more readily than small cuttings), uncontrolled herbivory, and planting into the incorrect habitat ( van der Vyver et al, 2021 ). However, the reported variability in survival may also be due to the condition of the source populations, which has the potential to override any treatment effects ( Galuszynski et al, 2023 ). While cutting size and planting habitat can be easily managed, there is currently no clear method for identifying optimal material to source cuttings for restoration.…”

“…We watered the trays (to the point of soil saturation) directly after shifting them to the common garden, and thereafter the trays were watered weekly if there were no rainfall events in the previous seven days—we considered this akin to inland field conditions during a wet phase over summer. We did not want soil moisture availability to be a limiting factor in root development in this experiment; dry conditions have been demonstrated to impede root development in P. afra ( Galuszynski et al, 2023 ). Each week, the trays were moved (prior to any watering) to new, randomly assigned positions within the grid layout.…”

“…However, this restoration has failed to produce consistent results, with large-scale planting initiatives in the Great Fish River Nature Reserve and Addo Elephant National Park (representing ∼21.3 million planted P. afra cuttings) reporting a mean survival of 28% with high variation across sites ( Mills & Robson, 2017 ). These low establishment rates may be a consequence of a host of (potentially interacting) factors, such as local soil properties ( Mills et al, 2011 ), moisture availability ( Galuszynski et al, 2023 ), landscape position effects ( Duker et al, 2020 ), and/or herbivory ( van der Vyver et al, 2021 ). However, all the cuttings used in these plantings were sourced from local stands of P. afra , likely encompassing a wide range of spatial and temporal environmental conditions.…”

Science is the fuel required for lifting ecosystem restoration into the orbit of hundreds of millions of hectares

“…Not watering cuttings for 28 days after planting had minor effects on their growth. However, differences in health and condition of individual parent plants had an overriding influence on root growth (Galuszynski et al, 2023).…”