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Collapse of an iconic conifer

14 Sep 2017 - 15:45

Collapse of an iconic conifer.

By White J, Hoffman MT, Jack S, Puttick J, Bonora, D and February E.

Conifer populations appear disproportionately threatened by global change. The particularly rapid decline of the charismatic southern African endemic, Widdringtonia cedarbergensis over the last century is representative of this global decline in conifers and has led the species to be listed as critically endangered under the current IUCN Red List of Plants. Widdringtonia cedarbergensis (Clanwilliam cedar) is endemic to the Cedarberg Wilderness Area 200 kms north west of Cape Town. Individuals of the species are usually associated with rocky cliffs, outcrops and east facing slopes at high elevations and are rarely found at lower elevations, on flatter ground and deeper soil. This association with higher-lying rocky sites has not only been attributed to temperature amelioration and protection from fire, but has also been suggested to give trees reliable access to available water trapped between bedding plane. This water supply is reliant on regular rainfall, and therefore any change in the amount or seasonality of rainfall that may lead to a decline in plant available water will adversely affect the trees. Given that the area is projected to be one of the first in South Africa to be affected by anthropogenic climate change the likely sensitivity of W. cedarbergensis to elevated temperatures and reduced rainfall is of some concern.

We use a large and long-term collection of historical photographs of W. cedarbergensis (ca. 1931–1951 and 1960–1987) which were relocated and retaken (2007– 2013), and represent the longest visual record of change in W. cedarbergensis populations to date. This dataset, together with environmental data and fire records forms the basis of our study designed to account for the decline of this critically endangered species. We fit a generalized linear mixed-effects model was fitted to determine the relative importance of environmental factors and fire-return interval on mortality for the species.

Our results show that W. cedarbergensis populations have declined significantly over the recorded period, with a pronounced decline in the last 30 years from an initial total of 1313 live trees in historical photographs, 74% had died and only 44 (3.4%) had recruited in the repeat photographs, leaving 387 live individuals. Juveniles (mature adults) had decreased (increased) from 27% (73%) to 8% (92%) over the intervening period. Our model demonstrates that individuals that established in open habitats at lower, hotter elevations and experienced a greater fire frequency appear to be more vulnerable to mortality than individuals growing within protected, rocky environments at higher, cooler locations with less frequent fires. Not even a significant increase in rainfall over the course of the 20th century at the more proximal climate station at Algeria appears to have halted or reversed the negative demographic trend. Climate models for the study area and globally show that temperatures will continue to increase through this century, suggesting that in future favourable habitat for the trees is only likely at higher elevations. Current populations of W. cedarbergensis are already at or near the highest locally attainable elevations. In addition, isolated peaks and ridges on which the trees typically occur are often separated from the nearest adjacent higher ground by lower-lying valleys dominated by fire-prone vegetation. If these climate predictions are realised, further declines in the species can be expected.

Despite the Cederberg being declared a Wilderness Area in 1973 and one of the management foci being the conservation of the threatened W. cedarbergensis, our results show that there has been a significant decline in the number of trees since that time. We suggest that this continued decline is probably due to both natural and anthropogenically mediated increases in fire frequency and temperature. This study is the first to demonstrate a possible causal link between anthropogenic change and the decline of a species for South Africa.

Originally published as;

White, J. D. M., Jack, S. L., Hoffman, M. T., Puttick, J., Bonora, D., Visser, V., & February, E. C. (2016). Collapse of an iconic conifer: long-term changes in the demography of Widdringtonia cedarbergensis using repeat photography. BMC ecology, 16(1), 53.