The local landscape boundary between the Albany subtropical thicket and Nama-Karoo shrubland is not influenced by edaphic factors

Becker, Carina; Coetsee, Corli; Cowling, Richard M.; Potts, Alastair J.

doi:10.1016/j.sajb.2014.12.003

Cited by 12 publications

(5 citation statements)

References 25 publications

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“…Spekboom plants that were planted underneath the protective canopy of a thicket-clump on the valley floor were also not noticeably damaged despite sub-zero temperatures below the canopy, whereas those in the open had signs of frost damage. Differences in edaphic factors between upslope and downslope sites cannot be used to explain the low survival on valley floors, as no consistent differences have been detected in soil texture, depth, infiltration and salinity between these and adjacent thicket-clad slopes, except for minor differences in pH (Becker et al, 2015). Furthermore, a soil effect was also excluded by retaining plants in bags in the topographic gradient cages.…”

Section: Discussionmentioning

confidence: 99%

“…Thicket vegetation is restricted to north-facing valley slopes (~410-650 m elevation) underlain by deep and relatively fertile shalederived soils; karoo shrubland dominates the valley floor (~385-410 m elevation; Fig. 2b) where soils are similarly fertile, differing from slope soils only by having slightly higher pH (Becker et al, 2015). The study site has a mean annual rainfall of 340 mm with a high degree of variability (coefficient of variation = 35%; 1948-2011).…”

Section: Study Sitementioning

confidence: 99%

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Frost, Portulacaria afra Jacq., and the boundary between the Albany Subtropical Thicket and Nama-Karoo biomes

Duker

Cowling

Preez

et al. 2015

South African Journal of Botany

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a b s t r a c t a r t i c l e i n f o Available online xxxx Edited by WJ BondNumerous shrublands exist in areas where soil moisture should support tree growth. In South Africa, the dwarf shrublands of the Nama-Karoo biome and tree-dominated vegetation of the Albany Subtropical Thicket biome share a boundary that is often abrupt. This boundary is not associated with edaphic or rainfall transitions. Field observations and leaf-level experiments suggest that the vulnerability of thicket species to frost damage is responsible for this thicket-shrubland boundary. We tested this hypothesis by establishing cuttings of Portulacaria afra (spekboom) -a dominant thicket succulent shrub that is a keystone species in arid forms of thicket -in two separate transplant experiments. Firstly across a topographic gradient from frost-free, thicket clad slopes to frostprone, karoo shrubland dominated valley floor; and secondly, inside and outside a thicket clump in the frostprone valley floor. We quantified the effects of frost on spekboom by measuring photosynthetic efficiency (F v / F m ), leaf number, estimating the percentage of healthy stem before and after frost events in June 2013. Frost-exposed spekboom cuttings rapidly underwent declines in photosynthetic efficiency, followed by severe leaf and stem necrosis; herbivores played no role in these declines. Those planted on the frost-free valley slopes or under the frost-protecting thicket canopy on the valley floor remained largely unaffected. This supports the hypothesis that frost-exposure is likely to be the main factor determining the growth rates, and ultimately survival of spekboom. These results suggest that frost occurrence is an important factor involved in determining the boundaries between the arid, and spekboom-rich subtypes of Albany Subtropical Thicket, and the frost tolerant shrublands of the Nama-Karoo.

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Section: Discussionmentioning

confidence: 99%

Section: Study Sitementioning

confidence: 99%

Frost, Portulacaria afra Jacq., and the boundary between the Albany Subtropical Thicket and Nama-Karoo biomes

Duker

Cowling

Preez

et al. 2015

South African Journal of Botany

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“…In these valleys, soil conditions (i.e. particle size, depth, infiltration rate, sodium and electrical conductivity) are consistent across the valley floors and shale-derived sections of the valley slopes (Becker et al, 2015). Nonetheless, Karoo shrubland replaces thicket on the valley floor-this is largely driven by frost (Duker et al, 2015a(Duker et al, , 2015b.…”

Section: Cold Air Pooling Model Calibrationmentioning

confidence: 85%

Site selection for subtropical thicket restoration: mapping cold-air pooling in the South African sub-escarpment lowlands

Duker¹,

Cowling²,

Vyver³

et al. 2020

PeerJ

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Restoration of subtropical thicket in South Africa using the plant Portulacaria afra (an ecosystem engineer) has been hampered, in part, by selecting sites that are frost prone—this species is intolerant of frost. Identifying parts of the landscape that are exposed to frost is often challenging. Our aim is to calibrate an existing cold-air pooling (CAP) model to predict where frost is likely to occur in the valleys along the sub-escarpment lowlands (of South Africa) where thicket is dominant. We calibrated this model using two valleys that have been monitored during frost events. To test the calibrated CAP model, model predictions of frost-occurrence for six additional valleys were assessed using a qualitative visual comparison of existing treelines in six valleys—we observe a strong visual match between the predicted frost and frost-free zones with the subtropical thicket (frost-intolerant) and Nama-Karoo shrubland (frost-tolerant) treelines. In addition, we tested the model output using previously established transplant experiments; ∼300 plots planted with P. afra (known as the Thicket-Wide Plots) were established across the landscape—without consideration of frost—to assess the potential factors influencing the survival and growth of P. afra. Here we use a filtered subset of these plots (n = 70), and find that net primary production of P. afra was significantly lower in plots that the model predicted to be within the frost zone. We suggest using this calibrated CAP model as part of the site selection process when restoring subtropical thicket in sites that lie within valleys—avoiding frost zones will greatly increase the likelihood of restoration success.

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“…Certain CFR environmental features, such as elevation and precipitation, may be biogeographically correlated, along with vegetation type; thus, our statistical model was designed to account for these associations to identify factors that may play independent roles in shaping population structure. For example, the observation that fynbos thicket is associated with reduced gene flow in the east is also coincident with seasonal winter‐rainfall showing a similar pattern; however, these variables are not always naturally associated (Becker et al, 2015; Potts et al, 2013). The subtropical thicket of southern Africa was hypothesized to have undergone great declines during colder glacial periods of the Pleistocene (Cowling et al, 2005; Vlok et al, 2003), which as noted previously, may have resulted in rapid species turnover and extreme bottlenecks in the east.…”

Section: Discussionmentioning

confidence: 99%

Evolutionary stability, landscape heterogeneity, and human land‐usage shape population genetic connectivity in the Cape Floristic Region biodiversity hotspot

Tassone

Miles

Dyer

et al. 2021

Evolutionary Applications

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The local landscape boundary between the Albany subtropical thicket and Nama-Karoo shrubland is not influenced by edaphic factors

Cited by 12 publications

References 25 publications

Frost, Portulacaria afra Jacq., and the boundary between the Albany Subtropical Thicket and Nama-Karoo biomes

Frost, Portulacaria afra Jacq., and the boundary between the Albany Subtropical Thicket and Nama-Karoo biomes

Site selection for subtropical thicket restoration: mapping cold-air pooling in the South African sub-escarpment lowlands

Evolutionary stability, landscape heterogeneity, and human land‐usage shape population genetic connectivity in the Cape Floristic Region biodiversity hotspot

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