Can hydraulically redistributed water assist surrounding seedlings during summer drought?

Plant interaction studies provide a good understanding of the roles of key species, which can assist restoration of natural ecosystems. Among the interactions, facilitation and competition are known to affect ecosystem structure and function. We investigated whether a deep-rooted species could positively affect surrounding seedlings through hydraulic redistribution during dry months. We conducted two experiments in which seedlings from two species were growing together or isolated from source plants (field experiment) and where plants were isolated from source plants that were connected to or separated from a water table (glasshouse experiment). Survival, growth, water relations and soil water content were measured. We also applied δ2H enriched water adjacent to, or into, the roots of source plants to track water movement between plants. Soil water content was higher in shallow layers where source plants could interact with seedlings (field) and when accessing water tables (glasshouse). Seedlings from all treatments had an increase in leaf δ2H. Seedlings of Banksia attenuata that were isolated from source plants had the highest survival, growth and stomatal conductance rates. Seedlings of Gompholobium tomentosum presented higher stomatal conductance rates when growing with source plants than when isolated from them during the first months, but this relationship reversed towards the end of summer. These results suggest that source plants and seedlings competed, but the influence of facilitation and competition might change during the year, at least for the shallow-rooted species. Therefore, competition for water and/or other limiting factors must be considered when planning ecological restoration in such areas.

Historical nitrogen content of bryophyte tissue as an indicator of increased nitrogen deposition in the Cape Metropolitan Area, South Africa.

Information on changes in precipitation chemistry in the rapidly expanding Cape Metropolitan Area (CMA) of South Africa is scarce. To obtain a long-term record of N deposition we investigated changes in moss foliar N, C:N ratios and nitrogen isotope values that might reflect precipitation chemistry. Tissue from 9 species was obtained from herbarium specimens collected between 1875 and 2000 while field samples were collected in 2001/2002. There is a strong trend of increasing foliar N content in all mosses collected over the past century (1.32-1.69 %N). Differences exist between ectohydric mosses which have higher foliar N than the mixohydric group. C:N ratios declined while foliar delta(15)N values showed no distinct pattern. From relationships between moss tissue N and N deposition rates we estimated an increase of 6-13 kgNha(-1)a(-1) since 1950. Enhanced N deposition rates of this magnitude could lead to biodiversity losses in native ecosystems.

Dry mass allocation, water use efficiency and delta(13)C in clones of Eucalyptus grandis, E. grandis x camaldulensis and E. grandis x nitens grown under two irrigation regimes.

Clonal variation in water use efficiency (WUE), dry mass accumulation and allocation, and stable carbon isotope ratio (delta(13)C) of crude leaf fiber extracts was determined in six clones of Eucalyptus grandis W. Hill ex Maiden. grown for 16 months in field lysimeters in two soil water regimes. The relationships between delta(13)C and WUE calculated on the basis of leaf, harvestable stem, shoot and whole-plant dry mass accumulation were investigated. There was no clonal variation in dry mass accumulation but clonal allocation to roots, harvestable stems, branches and leaves varied. Water use efficiencies (mass of plant or plant part/water used over 16 months) differed significantly between clones. The clonal ranking of WUE varied depending on the units of dry mass accumulation used. Significant relationships between delta(13)C values and instantaneous water use efficiencies and ratios of internal leaf to ambient CO(2) concentrations were found only in the high soil water treatment. There were no relationships between delta(13)C values and whole-plant, shoot and harvestable stem water use efficiencies and soil water availability. Values of delta(13)C were negatively correlated with dry mass accumulation in the low soil water treatment. At the whole-plant level, WUE was positively correlated with dry mass accumulation in the high soil water treatment. We found significant differences in delta(13)C values between clones and the clonal rankings in delta(13)C and WUE were maintained in both soil water treatments.

Desertification of the eastern Karoo, South Africa: Conflicting paleoecological, historical, and soil isotopic evidence.

The desertification debate in South Africa has benefitted greatly in recent years from the contributions of a wide range of disciplines. In this paper we review the conflicting and supporting evidence for degradation in the eastern Karoo as reported in recent archaeological, historical, and stable carbon isotope studies as it relates to three key aspects of the debate: the precolonial environment, the rate and nature of change, and the relative contributions of humans and climate to the process. First, all studies suggest a greater grassiness at some time in the past, but researchers disagree on the timing of the switch to more shrubby conditions in the eastern Karoo. Second, regional rainfall records for the past 2 decades reveal an above-average rainfall period, and numerous long-term surveys show an increase in grass cover over the same period. These findings question the expanding Karoo hypothesis as well as the argument that the Karoo's carrying capacity has decreased in recent years. Finally, the relative responsibilities of humans and climate in the degradation process remain poorly understood and generally have not formed the focus of investigation.

Nonstructural carbohydrate allocation following different frequencies of simulated browsing in three semi-arid shrubs.

Nonstructural carbohydrate allocation patterns in response to different frequencies of simulated browsing (leaf and twig removal) were studied in the following semi-arid shrubs: Osteospermum sinuatum, a dwarf deciduous shrub, Pteronia pallens, a dwarf evergreen shrub, and Ruschia spinosa, a dwarf leaf-succulent shrub. Simulated browsing at all frequencies resulted in the elevation, or had no effect, on total nonstructural carbohydrate (TNC) concentrations of O. sinuatum plant parts, and resulted in the decrease in TNC concentrations of R. spinosa plant parts. The responses of P. pallens were intermediate with elevations as well as declines in TNC concentrations of plant parts measured in response to various clipping frequencies. At the low frequency of simulated browsing (every 26 weeks) elevations in plant TNC content were measured in the two non-succulent shrubs O. sinuatum and P. pallens. It was concluded that the overcompensation with respect to TNC accumulation observed in the two non-succulent species represents one of the ways in which excess photosynthate is utilized by browsed shrubs with a limited regiowth potential. Simulated browsing was the least detrimental with respect to biomass production to the non-succulent O. sinuatum and P. pallens, and most injurious to the leaf-succulent shrub, R. spinosa. The observed TNC allocation patterns could not adequately explain the variation among species in the production of new growth and it was concluded that some factor(s) other than the carbon resource was limiting regrowth.

Impacts of invading N2-fixing Acacia species on patterns of nutrient cycling in two Cape ecosystems: evidence from soil incubation studies and 15N natural abundance values.

This study examines the impacts of woody, N2-fixing invasive Acacia spp. on the patterns of nutrient cycling in two invaded ecosystems of differing nutrient status in the Cape floristic region. Patterns of soil nutrient mineralization were measured by a field incubation method while the significance of the fixation process in altering nutrient cycling was assessed by the δ15N natural abundance technique. The results confirm earlier reports that invasion by woody shrubs results in organic matter and nutrient enrichment of surface soils of both ecosystems. However, patterns of nutrient availability (phosphorus and nitrogen) were not necessarily enhanced. In the more fertile strandveld both phosphorus and nitrogen (significant at P<0.10) showed trends towards enhanced annual mineralization rates upon invasion, while in the low nutrient fynbos system only phosphorus followed this trend. It is unclear whether this differential response is a consequence of plant- or soil-derived feedbacks on the decomposition processes in each system. The δ15N values of the soils from the invaded sites of both ecosystems indicated a strong influence of the alien species on the soil nitrogen component. However, as with other studies of natural ecosystems, the contribution of nitrogen from fixation could not be readily quantified with the δ15N natural abundance method because of problems in selecting suitable non-N2-fixing reference plants. A technique of disrupting nodule structure and function, by fumigation with O2, to obtain the δ15N value of a non-N2-fixing speciment of the study species was tried and found to overcome some of the problems associated with the lack of suitable reference plants. With this technique it was possible to detect the almost total dependence of A. saligna on N2-fixation in the fynbos soils with their low nitrogen mineralization rates. In the strandveld ecosystem with much higher soil nitrogen release rates A. cyclops was only partly dependent on fixation (about half) for its nitrogen. The nutrient enrichment of both ecosystems and trends towards enhanced rates of nutrient mineralization could have profound implications on the long-term success of alien invader clearing operations and the restoration of the indigenous flora at these sites.

Regrowth and tannin production in woody and succulent karoo shrubs in response to simulated browsing.

Allocation of carbon to chemical defences has often been suggested to be a direct response to browsing or grazing by herbivores. This study quantitatively compares total polyphenol and tannin production in response to simulated browsing of three karoo shrubs in order to test this induced defence hypothesis. The three species studied responded to browsing either by rapid regrowth or by increasing polyphenol production in the remaining tissues. The patterns did not follow any phylogenetic relationships but were weakly associated with the palatability of each species. The highly palatable deciduous species Osteospermum sinuatum, which is capable of rapid regrowth, showed no or very low levels of constitutive and browsing-induced total polyphenols, condensed tannins and protein-precipitating tannins. The evergreen sclerophyllous species Pteronia pallens showed a limited regrowth capacity and had intermediate levels of polyphenols, while the evergreen succulent species Ruschia spinosa showed almost no regrowth over the study period. R. spinosa contained the highest constitutive and browsing-induced levels of polyphenols, condensed tannins and protein-precipitating tannins. In two of the species more than one anti-herbivore defence feature co-occur. P. pallens foliage contains both hepatotoxins and polyphenols while R. spinosa has both structural (spines) and chemical defences. Responses of karoo shrubs to simulated browsing are interpreted as the result of passive alterations in plant chemistry rather than as an active defence response to herbivores.

Density dependent interactions between VA mycorrhizal fungi and even-aged seedlings of two perennial Fabaceae species.

The interaction of density and mycorrhizal effects on the growth, mineral nutrition and size distribution of seedlings of two perennial members of the Fabaceae was investigated in pot culture. Seedlings of Otholobium hirtum and Aspalathus linearis were grown at densities of 1, 4, 8 and 16 plants per 13-cm pot with or without vesicular-arbuscular (VA) mycorrhizal inoculum for 120 days. Plant mass, relative growth rates, height and leaf number all decreased with increasing plant density. This was ascribed to the decreasing availability of phosphorus per plant as density increased. O. hirtum was highly dependent on mycorrhizas for P uptake but both mycorrhizal and non-mycorrhizal A. linearis seedlings were able to extract soil P with equal ease. Plant size distribution as measured by the coefficient of variation (CV) of shoot mass was greater at higher densities. CVs of mycorrhizal O. hirtum plants were higher than those of non-mycorrhizal plants. CVs of the facultatively mycorrhizal A. linearis were similar for both mycorrhizal and non-mycorrhizal plants. Higher CVs are attributed to resource preemption by larger individuals. Individuals in populations with high CVs will probably survive stress which would result in the extinction of populations with low CVs. Mass of mycorrhizal plants of both species decreased more rapidly with increasing density than did non-mycorrhizal plant mass. It is concluded that the cost of being mycorrhizal increases as plant density increases, while the benefit decreases. The results suggest that mycorrhizas will influence density-dependent population processes of faculative and obligate mycorrhizal species.

The costs of leaving home: ants disperse myrmecochorous seeds to low nutrient sites.

Leucospermum conocarpodendron (L.) Buek (Proteaceae) seedlings were excavated several months after a fire in Cape fynbos. Seedlings under burnt parental skeletons had short hypocotyls (mean 25 mm) indicating passive dispersal whereas seedlings in the open were more deeply buried (mean 48 mm) by ants. Soil nutrient concentrations at the site of germination were negatively related to depth of burial and distance from parent. Ant dispersal resulted in seedlings emerging in soils with lower nutrient concentrations than passively dispersed seeds. Tissue analysis supported the soil results with lower P content in seedlings from open (ant dispersed) sites. Seedling survival in the first year of establishment was also lower in open sites, but not significantly so. However seedlings were slightly taller in the open. The results of this study, the first on naturally occurring intraspecific variation in myrmecochory, strongly contradict current explanations for the high incidence of myrmecochory in nutrient poor environments.

Seasonal allocation of dry mass and nitrogen in a fynbos endemic Restionaceae species Thamnochortus punctatus Pill.

The annual growth pattern of male plants of the endemic, dioecious fynbos species, Thamnochortus punctatus, revealed sequential growth of the various below-and above-ground organs. Root and rhizome development were favoured in the wet winter months while culm elongation occurred in the warmer but drier spring to summer period. The reproductive phase of development followed in the hot dry summer and autumn. The asynchronous pattern of growth is suggested to enable the plant to maximize utilization of scarce resources within the confines of the environmental controls determined by the mediterranean-climate. Maximum root and rhizome activity in the rainy season enabled the plant to absorb and store nitrogen in mature culms for later above ground organ development. As the absorption of nitrogen from senscing material is essential for this pattern of organ development to assist in alleviating nutrient scarcity the high degree of nitrogen reabsorption, 35 to 70% for branches and culms respectively, shows the potential importance of asynchronous organ growth in overcoming nitrogen limitation of plant growth. Comparisons of dry mass and nitrogen allocation patterns indicate that developmental strategies employed by evergreen, perennial plants in climatically and edaphically constrained environments cannot be deduced from biomass allocation patterns alone as they do not appear to reflect strategies evolved to overcome nutrient limitation.