CO2-fertilisation enhances resilience to browsing in the recruitment phase of an encroaching savanna tree

CO2-fertilisation is implicated in the widespread and significant woody encroachment of savannas due to CO2-stimulated increases in below-ground reserves that enhance sapling regrowth after fire. However, the effect of CO2 concentration ([CO2]) on tree responses to the other major disturbance in savannas, herbivory, is poorly understood. Herbivory responses cannot be predicted from fire responses, as herbivore effects occur earlier during establishment and are moderated by plant palatability and defence rather than below-ground carbon accumulation. The relationship between herbivory and [CO2] is explored here using a widespread, strongly encroaching savanna tree, Vachellia karroo. Using greenhouse-grown seedlings under past—through to predicted future—[CO2] (180–1000 ppm) and field-grown seedlings under ambient [CO2], we assessed plant survival, growth, defence and palatability. Increasing [CO2] improves the tolerance of greenhouse-grown seedlings to herbivory by stimulating growth and allowing a critical size threshold associated with survival to be reached earlier, thereby decreasing the probability of fatal herbivory during the vulnerable recruitment phase. Elevated [CO2] also decreases the time taken to reach a second size threshold linked to accelerated recovery of field-grown seedlings following herbivory. Seedling growth responses to increasing [CO2] are nonlinear, suggesting that historic growth and survival enhancements are smaller than those predicted for the future. Increasing [CO2] is associated with greater resistance to herbivores (more branched shoot architecture) but not leaf palatability (C:N ratio) or defence (leaf tannins and spine density). Increasing V. karroo densities already constitute a major land management problem in southern African savannas. However, encroachment by this species, and likely other savanna tree species, may be greatly exacerbated under future [CO2], as tolerance to herbivory at the recruitment stage is further enhanced

Determinants of the Fynbos/Succulent Karoo biome boundary: Insights from a reciprocal transplant experiment

Boundaries between Fynbos and Succulent Karoo vegetation in the Greater Cape Floristic Region are frequently characterised by sharp transitions from sandy, dystrophic to loamy, mesotrophic soils, together with a more gradual climate transition from cooler, wetter conditions typical of Fynbos at higher elevations to warmer, drier conditions at lower elevations typical of Succulent Karoo. There is very high species turnover across these boundaries, providing an opportunity to disentangle the relative roles of climate and soil type in determining the biome boundary. A fully reciprocal transplant approach was employed here to investigate this question, using three species from each biome occurring naturally in close proximity at Jonaskop, Western Cape. Greenhouse-germinated and established seedlings of all species were planted into both sandy, dystrophic and loamy, mesotrophic soils typical of each biome at four sites along an elevational transect (elevations 545 m, 744 m, 953 m, 1303 m) at Jonaskop, and their growth and survivorship monitored for 7 months. At least one site on the elevation gradient represented annual climate rainfall and temperature conditions during the experimental period typical of the range edge for each of the selected species, this typically being the lowest elevation site (warm × dry) for Fynbos species (Protea humiflora, P. magnifica and P. amplexicaulis), and the highest elevation site (cool × wet) for Succulent Karoo species (Ruschia lineolata, Drosanthemum speciosum and Pteronia incana). Fynbos species showed significant adverse responses to loamy mesotrophic soil, with highly significant reductions in growth and high and rapid rates of mortality relative to their native soil. Fynbos species showed somewhat reduced growth and survival at the lowest elevation when grown in native soils, but demonstrated significant interaction between soil type and elevation with much lower growth and survival at the lowest elevation on the loamy soils. Surprisingly, all the Fynbos species showed reductions in growth and survival at the highest elevation, with significant reductions in two of the three species. Succulent Karoo species, by contrast, showed very few significant performance differences between soil types and few significant soil × elevation interactive effects, but did show significant growth and survival responses to elevation, with high growth and survival at mid-level elevations, far higher than their natural extent at the site. These results suggest that the selected Succulent Karoo species are neither edaphically nor climatically constrained from establishing and growing in sandy dystrophic soils and cool climates typical of the Fynbos vegetation along this elevation gradient, but that Fynbos species are strongly limited both edaphically and climatically from growing under conditions typical of the Succulent Karoo.We propose that Succulent Karoo elements may be excluded competitively or through disturbance from colonising sandy dystrophic soils at higher elevations in Fynbos vegetation, with fire regime most likely responsible for maintaining the sharply delineated boundaries between these biomes. This is because fire would strongly exclude non-fire adapted Succulent Karoo species at and above the biome boundary, while loamy soils and climate together would strongly exclude Fynbos species from the heavier soils of the Succulent Karoo. The relative climate and soil affinities of these biomes, accentuated by the role of fire, could therefore provide a coherent explanation for biome boundaries in the Greater Cape Floristic Region. We note however that the limited species selection in this study precludes a conclusive general result, and that several interesting questions remain about soil, climate and disturbance determinants across this biome boundary

Live fast, die young: Estimating size-age relations and mortality pattern of shrubs species in the semi-arid Karoo, South Africa

We present a technique for estimating size-age relations and size-dependent mortality patterns of long-lived plants. The technique requires two sets of size data of individual (non-marked) plants that should be collected with a time-lag of several years in the same area of a study site. The basic idea of our technique is to assume general (three parameter) families of size-dependent functions which describe growth and mortality that occurred between the two data gathering events. We apply these growth and mortality functions to the size data of the early data set and construct predicted size-class distributions to compare it, in a systematic way, to the size-class distribution of the later data set. In a next step we calculate the size-age relations from the resulting growth functions, which yield the smallest difference between observed and predicted size-class distribution. Applying this technique to size data of five dominant shrub species at the Tierberg study site in the semiarid Karoo, South Africa produced new insight into the biology of these species which otherwise cannot be obtained without frequent measurements of marked plants. We could relate characteristics of growth behavior and mortality, for certain subgroups of the five species, to the life-history attributes evergreen vs. deciduous, succulent vs. woody, and early reproductive vs. late reproductive. The results of our pilot-study suggest a broad applicability of our technique to other shrublands of the world. This requires at least one older record of (individual) shrub-size data and performance of resampling.Conference Pape

Live fast, die young: Estimating size-age relations and mortality pattern of shrubs species in the semi-arid Karoo, South Africa

We present a technique for estimating size-age relations and size-dependent mortality patterns of long-lived plants. The technique requires two sets of size data of individual (non-marked) plants that should be collected with a time-lag of several years in the same area of a study site. The basic idea of our technique is to assume general (three parameter) families of size-dependent functions which describe growth and mortality that occurred between the two data gathering events. We apply these growth and mortality functions to the size data of the early data set and construct predicted size-class distributions to compare it, in a systematic way, to the size-class distribution of the later data set. In a next step we calculate the size-age relations from the resulting growth functions, which yield the smallest difference between observed and predicted size-class distribution. Applying this technique to size data of five dominant shrub species at the Tierberg study site in the semiarid Karoo, South Africa produced new insight into the biology of these species which otherwise cannot be obtained without frequent measurements of marked plants. We could relate characteristics of growth behavior and mortality, for certain subgroups of the five species, to the life-history attributes evergreen vs. deciduous, succulent vs. woody, and early reproductive vs. late reproductive. The results of our pilot-study suggest a broad applicability of our technique to other shrublands of the world. This requires at least one older record of (individual) shrub-size data and performance of resampling.Conference Pape

Cross-language neighborhood effects in learners indicative of an integrated lexicon

This study examined how acquisition of novel words from an unknown language (L2) is influenced by their orthographic similarity with existing native language (L1) words in beginning adult learners. Participants were tested in a two-alternative forced-choice recognition task and a typing production task as they learned to associate 80 L2 (pseudo)words with pictures depicting their meanings. There was no effect of L1 orthographic neighborhood density on accuracy in the two-alternative forced-choice task, but typing accuracy was higher for L2 words with many L1 neighbors in the earliest stages of learning. ERPs recorded during a language decision task before and after learning also showed differences as a function of L1 neighborhood density. Across sessions, L2 words with many L1 neighbors elicited slower responses and larger N400s than words with fewer L1 neighbors, suggesting that L1 neighbors continued to influence processing of the L2 words after learning (though to a lesser extent). Finally, ERPs recorded during a typing task after learning also revealed an effect of L1 neighborhood that began about 700 msec after picture onset, suggesting that the cross-language neighborhood effects cannot solely be attributed to bottom-up activation of L1 neighbors. Together, these results demonstrate that strategic associations between novel L2 words and existing L1 neighbors scaffold learning and result in interactions among cross-language neighbors, suggestive of an integrated L1-L2 lexicon

Evolution of hydrological niches in Restionaceae: a project update

Plants are in general exquisitely sensitive to differences in soil moisture availability, particularly when competing with each other. We have previously found that species segregate along soil moisture gradients in English wet meadows (Silvertown et al. 1999) and we now have good evidence from several sites in the Western Cape that the same phenomenon occurs in fynbos communities. Our discovery of plants segregating into hydrological niches, defined by their location along soil moisture gradients, in phylogenetically completely different communities in the northern and southern hemispheres suggests that this form of niche separation is very general indeed. We are now using the Restionaceae family, one of the key components of fynbos vegetation, to test the hypothesis that the radiation of the Restionaceae clade in the Cape involved the occupation of novel niches in hydrological niche space. We have so far collected hydrological data on 39 species of Restionaceae from 6 sites and have a species-level phylogeny of the family (Hardy et al. 2008) which enables us to estimate the rate of evolutionary change in hydrological niche parameters during the radiation of the group

Ecological research and conservation manangement in the Cape Floristic Region between 1945 and 2015: History, current understanding and future challenges

In 1945, the Royal Society of South Africa published a wide-ranging report, prepared by a committee led by Dr C.L. Wicht, dealing with the preservation of the globally unique and highly diverse vegetation of the south-western Cape. The publication of the Wicht Committee’s report signalled the initiation of a research programme aimed at understanding, and ultimately protecting, the unique and diverse ecosystems of the Cape Floristic Region. This programme has continued for over 70 years, and it constitutes the longest history of concerted scientific endeavour aimed at the conservation of an entire region and its constituent biota. This monograph has been prepared to mark the 70th anniversary of the Wicht Committee report. It provides a detailed overview of the circumstances that led up to the Wicht Committee’s report, and the historical context within which it was written. It traces the development of new and substantial scientific understanding over the past 70 years, particularly with regard to catchment hydrology, fire ecology, invasive alien plant ecology, the harvesting of plant material and conservation planning. The Wicht Committee’s report also made recommendations about ecosystem management, particularly with regard to the use of fire and the control of invasive alien plants, as well as for the establishment of protected areas. Subsequently, a combination of changing conservation philosophies and scientific conservation planning led to the creation and expansion of a network of protected areas that now covers nearly 19% of the Cape Floristic Region. We also review aspects of climate change, most of which could not have been foreseen by the Wicht Committee. We conclude that those responsible for the conservation of these ecosystems will face many challenges in the 21st century. These will include finding ways for effectively managing invasive alien plants and fires, as foreseen by the Wicht Committee. While the protected area network has expanded beyond the modest targets proposed by the Wicht Committee, funding has not kept pace with this expansion, with consequences for the ability to effectively manage protected areas. The research environment has also shifted away from long-term research conducted by scientists embedded in management agencies, to short-term studies conducted largely by academic institutions. This has removed a significant benefit that was gained from the long-term partnership between research and management that characterised the modis operandus of the Department of Forestry. Growing levels of illegal resource use and a changing global climate also pose new challenges that were not foreseen by the Wicht Committee