Who dies first? Investigating canopy mortality in Succulent Karoo plant communities during drought

By Hana Petersen, DST-NRF intern based at Tierberg-LTER, Prince Albert Tweet

Intern Hana Petersen collects data along a line transect at a site near Prince Albert in the Western Cape Pteronia pallens individual with a high degree of canopy mortality and two dead succulents (Ruschia approximata)

While Southern Africa has demonstrated its vulnerability to variability in climate, South Africa’s arid zones are in a particularly vulnerable position, with the prospect of even greater weather extremes with which to contend.

Climate change scenarios predicted for the Karoo include an increase in the magnitude and variability of rainfall events, as well as an increase in the duration of dry spells. High variability of rainfall often results in drought, which may consequently alter the diversity, abundance and composition of plant communities through mortality.

Despite the Succulent Karoo Biome being recognised by the IUCN as a biodiversity hotspot, drought research in this region is sparse. An understanding of the degree to which the present drought affects the growth and survival of Succulent Karoo shrubs is important for evaluating the potential effects that future climate change may have on species composition, and the size and age structure of plant communities within this diverse biome.

SAEON study

To better understand the effect of the current drought on shrubs, a study was initiated by the SAEON Arid Lands Node. This study aimed to determine the degree of canopy mortality in Succulent Karoo plant species during the ongoing drought, under different land-use management practices.

Two key questions addressed in this study were: (1) do different growth forms (non-succulent vs. succulent shrubs) in Succulent Karoo plant communities experience different degrees of canopy mortality (or are they all suffering equally), and (2) does age (size class) influence the likelihood of mortality of a long-lived perennial dwarf shrub, Pteronia pallens?

All plants encountered along a line transect were categorised using an index of canopy mortality (5 = >90% living, 4 = 50-90% living, 3 = 5-49% living, 2 = <5% living, 1 = dead), and their canopy dimensions were measured. Additional basal diameter measurements were taken for P. pallens individuals, to be used as a proxy for age. For this study, only plains habitats were sampled.

Preliminary descriptive results based on a total of 96 line transects, each 20 metres long, at eight sites within the Prince Albert Succulent Karoo vegetation type (SKv13; Mucina et al., 2006) indicate that overall canopy mortality was quite low, with around 80% of canopies falling into the moderate, good and healthy categories across all sites. Dead and dying canopies were observed most frequently in succulents, particularly in short-lived species (e.g. Brownanthus ciliate and Malephora lutea), suggesting that non-succulent plants are coping with the effects of the drought better than their succulent neighbours.

As a long-lived perennial dwarf shrub commonly occurring throughout the Prince Albert Succulent Karoo vegetation type, P. pallens was an ideal candidate for comparing canopy mortality in different size classes – a proxy for different age classes – across all eight sites. Overall, P. pallens canopies were in good condition across all sites. Where mortality was observed, it was in relatively young individuals of size classes 1 and 2 (<5-9 cm basal diameter).

This has implications for future species composition, since many of the newest recruits have failed to survive the current drought. Being unpalatable and therefore not grazed by livestock, P. pallens often dominates in landscapes where grazing has reduced the more palatable forage plants.

Drought may therefore, ironically, lead to an increase in the relative abundance of palatable species if these are able to recruit in the gaps and survive where P. pallens recruits could not.

At present, there is no model that can adequately predict how succulent plants will respond to the changing climate. It is possible that if drought conditions persist and the Karoo climate becomes generally hotter and drier, we may begin to see plant communities dominated by older perennial non-succulent dwarf shrubs, with a diminished diversity of succulent species as a worst-case scenario for the Succulent Karoo biodiversity hotspot.

Further analysis incorporating long-term regional climate records, together with fine-scale local rainfall and livestock data, will be done to determine the relationships between canopy mortality and canopy dimensions, size and age demographics, land-use types (livestock farming vs. conservation) and various climatic variables (particularly rainfall).

The results of this study may be used as a baseline for monitoring post-drought recovery rates of various species and growth forms.

Further reading  

• Archer, E., Engelbrecht, F., Hänsler, A., Landman, W., Tadross, M., Helmschrot, J., 2018. Seasonal prediction and regional climate projections for southern Africa. In: Archer, E., Engelbrecht, F., Hänsler, A., Landman, W., Tadross, M., Helmschrot, J. (Eds.), Climate Change and Adaptive Land Management in Southern Africa – Assessments, Changes, Challenges, and Solutions. Klaus Hess Publishers, pp. 14–21.
• Davis-Reddy, C.L., Vincent, K., 2017. Climate Risk and Vulnerability: A Handbook for Southern Africa. Introduction to International Disaster Management.
• Hewitson, B.C., 1996. Analysis of Regional Precipitation Impacts from GCM-derived Regional Climate Change Scenarios. Unpublished progress report for the Climate Systems and Analysis Group, University of Cape Town. 
• Hewitson, B.C., Crane, R.G., 2006. Consensus between GCM climate change projections with empirical downscaling: Precipitation downscaling over South Africa. International Journal of Climatology 26, 1315–1337.
• Mason, S., Waylen, P.R., Mimmack, G.M., Rajaratnam, B., Harrison, J.M., 1999. Change in extreme rainfall events in South Africa. Climatic Change 41, 249–257.
• Mittermeier, R.A., Turner, W.R., Larsen, F.W., Brooks, T.M., Gascon, C., 2011. Global Biodiversity Conservation: The Critical Role of Hotspots. In: Zachos, F.E., Habe, J.C. (Eds.), Biodiversity Hotspots: Distribution and Protection of Conservation Priority Areas. Springer-Verlag Berlin Heidelberg, pp. 3–22.
• Mucina, L., Jürgens, N., Le Roux, A., Rutherford, M.C., Schmiedel, U., Esler, K.J., Powrie, L.W., Desmet, P.G., Milton, S.J., 2006. Succulent Karoo Biome. In: Mucina, L., Rutherford, M.C. (Eds.), The Vegetation of South Africa, Lesotho and Swaziland. South African National Biodiversity Institute, Pretoria, pp. 220–299.
• Myers, N., Mittermeier, R.A., Mittermeier, C.G., da Fonseca, G.A.B., Kent, J., 2000. Biodiversity hotspots for conservation priorities. Nature 403, 853–858.