In southern Africa long-term regional rainfall data sets are very limited such that the variability of rainfall across the region is poorly understood. With available climate records so limited the development of strong proxy records are vital to develop drought management plans. In our continuing efforts to develop such proxy records we present the results of an investigation into the dendrochronological potential of Widdringtonia nodiflora and Widdringtonia schwarzii. We sampled W. schwarzii, from the area it is endemic to, the Baviaanskloof wilderness area, in the Langkloof region of the Eastern Cape Province. Widdringtonia nodiflora samples were collected from the Grootvadersbosch Nature Reserve near Heidelberg in the Western Cape Province. The results indicate that inconsistencies in ring width combined with poorly defined ring boundaries and converging rings make cross-dating between different trees from the same locality an impossible task for both species using manageable sample numbers. Without cross-dating, chronology development is an equally impossible task for both W. nodiflora and W. schwarzii. As with W. cedarbergensis, W. schwarzii may avoid drought stress by accessing deep water. This may explain the many false rings and poorly defined ring boundaries apparent in this species. This study concludes the dendrochronological assessment of all of the Widdringtonia species with only two South African conifers not evaluated (Podocarpus henkelii and P. elongatus). Widdringtonia cedarbergensis is the only South African conifer that has been successfully cross- dated, however, there are no significant correlations between ring width indices and climate variables for this species.
Within southern Africa, the socio-economic consequences of droughts are often enormous. For an understanding of the future implications of these droughts an understanding of the variation of the range of possible climates is required. At present regional data sets do not extend back for more than 100 years. Since all climate projections are based on this very limited data set, there are obvious limits to the probable accuracy of estimates. It is therefore important to develop other methods for establishing longer records. Within South Africa, Podocarpus species have been at the centre of dendrochronological research for the past 20 years with many claims for a relationship between ring width and certain climate variables. In the present study, the dendrochronological potential of Podocarpus sp. is re-evaluated. The primary objective is to ascertain whether cross-dating and chronology development are possible for Podocarpus sp. Even though whole-trunk cross-sections were used in the analysis of the 14 trees examined, a combination of poorly defined, locally absent and converging rings, combined with lobate growth, makes cross-dating an impossible task. These results suggest that future dendrochronological research using Podocarpus species is not justifiable, especially since whole-trunk cross-sections have to be used in the analysis and suitable specimens of Podocarpus sp. are rare and endangered.
Abstract Major components of most southern African archaeological sites are stone, bone, and charcoal. A new technique for climate reconstruction utilizes measurements of vessel size and frequency in the cross-sectional xylem anatomy of archaeological charcoal from Collingham Shelter and Mhlwazini Cave in the Natal Drakensberg. Previous wood anatomical studies have shown that links exist among vessel diameter, vessel frequency and climate. The present study demonstrates that in relation to rainfall, vessel diameter in the species Protea caffra and Protea roupelliae correlated positively, whereas vessel frequency correlated negatively . In P. roupelliae , mean vessel diameter increases from 46 to 62 μm along a rainfall gradient ranging from 760 to 1665 mm. The significant correlations between rainfall and tangential vessel diameter for a charred sample of P. roupelliae suggest that such measures on an archaeological charcoal sample may be used to reconstruct rainfall patterns through time. Using nine assemblages of archaeological charcoal, generalized patterns of wetter and drier periods can be postulated. Comparison with contemporary values indicates that at 200 and 2400 yr B.P. the area near the archaeological sites was wetter than at present. A dry phase occurred between 1300 and 300 yr B.P. Values for the contemporary wood sample are the lowest observed, indicating that present conditions are much drier than those at any time within the last ca. 2000 yr. Dating resolution however, is insufficient to allow more-detailed interpretation of rainfall conditions over the past 2000 yr.
Recent findings point to plant root traits as potentially important for shaping the boundaries of biomes and for maintaining the plant communities within. We examined two hypotheses: 1) Thin-rooted plant strategies might be favored in biomes with low soil resources; and 2) these strategies may act, along with fire, to maintain the sharp boundary between the Fynbos and Afrotemperate Forest biomes in South Africa. These biomes differ in biodiversity, plant traits, and physiognomy, yet exist as alternative stable states on the same geological substrate and in the same climate conditions. We conducted a 4-y field experiment to examine the ability of Forest species to invade the Fynbos as a function of growth-limiting nutrients and belowground plant-plant competition. Our results support both hypotheses: First, we found marked biome differences in root traits, with Fynbos species exhibiting the thinnest roots reported from any biome worldwide. Second, our field manipulation demonstrated that intense belowground competition inhibits the ability of Forest species to invade Fynbos. Nitrogen was unexpectedly the resource that determined competitive outcome, despite the long-standing expectation that Fynbos is severely phosphorus constrained. These findings identify a trait-by-resource feedback mechanism, in which most species possess adaptive traits that modify soil resources in favor of their own survival while deterring invading species. Our findings challenge the long-held notion that biome boundaries depend primarily on external abiotic constraints and, instead, identify an internal biotic mechanism-a selective feedback among traits, plant-plant competition, and ecosystem conditions-that, along with contrasting fire regime, can act to maintain biome boundaries.
'%3e%3cpath fill='%23001489' d='M0 0v6h9V0z'/%3e%3cpath fill='%23e03c31' d='M0 0v3h9V0z'/%3e%3cg stroke='%23fff' stroke-width='2'%3e%3cpath id='d' d='m0 0 4.5 3L0 6m4.5-3H9'/%3e%3cuse xlink:href='%23b' stroke='%23ffb81c' clip-path='url(%23c)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='none' stroke='%23007749' stroke-width='1.2'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)