Defoliation depletes the carbohydrate reserves of resprouting <Emphasis Type="Italic">Acacia</Emphasis> saplings in an African savanna

Over the past century there has been a global trend towards tree expansion and densification in rangelands and savannas. This phenomenon is commonly referred to as bush encroachment. In South Africa Acacia karroo is one of the key species responsible for bush encroachment. It has been suggested that the combination of fire and browsing might limit bush encroachment by A. karroo more effectively than either browsing or fire alone. We hypothesized that these repeated disturbances progressively deplete root carbohydrates and compromise resprouting ability. This was tested by burning and then manually defoliating A. karroo once a month for 1 year. Manual defoliation did not inhibit the rapid shoot elongation after topkill of A. karroo saplings. During this initial phase, the growth of the new shoots of A. karroo was dependent more on mobilised root reserves than on photoassimilates from the new shoots. Frequent manual defoliation of resprouting A. karroo saplings prevented the replenishment of starch reserves. We suggest a mechanism for how the interaction of browsing and fire can suppress and perhaps reverse bush encroachment in African savannas. Saplings that have reduced starch reserves at the end of dry season due to browsing will struggle to resprout if they are burnt. Even if they do not die, they will be less able to escape fire damage in the next fire than if they had been able to resprout unimpeded.     

Herbivory effects on saplings are influenced by nutrients and grass competition in a humid South African savanna

Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient status (through fertilization and trampling) and grass competition. Studies evaluating the effects of herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass competition, and their interactive effects on sapling survival and growth of four dominant humid and four dominant mesic savanna species. We conducted this experiment in a humid South African savanna. We found no effects on sapling survival by the treatments provided. However, clipped saplings of all species increased their investment in relative growth rate of stem length (RGR_L). Clipping had a greater negative impact on relative growth rate of more humid than mesic species in terms of stem diameter (RGR_D), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGR_L and sapling biomass of three mesic species. Positive effects of nutrients on RGR_L of one humid and two mesic species were observed in their clipped saplings only. Grass competition had a strong negative impact on all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGR_L with grass competition whereas intact saplings showed no significant response. After clipping, humid savanna species were more vulnerable to grass competition than mesic species, with reduced ability to use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid species being more susceptible than mesic species, indicating that woody-plant control strategies are more likely to be effective in humid savannas.     

Is the lack of leguminous savanna trees in grasslands of South Africa related to nutritional constraints?

As with many grasslands globally, the Highveld grasslands of South Africa are tree-less, despite having a climate that can support tree growth. Models predict that fire maintains these grasslands. The question arises as to why fire-tolerant savanna trees do not survive in these ecosystems? Savanna tree survival in mesic areas is restricted by demographic bottlenecks, specifically limitations to sapling-escape from fire. It was hypothesised that ancient highly leached soils from grassland areas would prevent saplings from growing fast enough to escape the fire-trap. Growth rates of savanna tree seedlings (Acacia karroo Hayne and Acacia sieberiana Burtt Davy) were measured in a common garden experiment using soils from ten sites collected along a savanna-grassland continuum. Soils from grassland sites were relatively nutrient-poor compared to those from savannas with lower pH, and associated cations. A. sieberiana growth rates responded to pH and these nutrients, whereas A. karroo growth was less strongly linked to specific nutrients. Even so, both species accumulated more biomass when grown in soils from savanna sites compared to grassland sites. An exception was a low elevation low nutrient savanna site that resulted in poor growth, yet sustains high tree biomass in situ. Differences between growth in grassland and savanna soils were small. They may contribute to, but are unlikely to explain, the treeless nature of these grasslands.     

N-fertilization does not alleviate grass competition induced reduction of growth of African savanna species

Background and aims

Below-ground grass competition limits woody establishment in savannas. N₂-fixing legumes may, however, have a nutritional advantage over broad-leaved species. We hypothesised that broad-leaved non-legume savanna thicket species would be more severely constrained by grass competition for N and consequently respond more to N-fertilization than the legume, Acacia karroo.

Methods

A. karroo and five non-legume thicket species (Maytenus senegalensis, M. heterophylla, Euclea divinorum, Ziziphus mucronata, Schotia brachypetala) were grown together in an irrigated competition experiment with clipped-, unclipped-grass and without grass with/without N-fertilizer. The biomass, foliar nutrient, δ¹³C and δ¹⁵N of grasses and woody species were determined.

Results

Growth of both A. karroo and the non-legume species was equally sensitive (c. 90 % reduction) to both clipped- and unclipped-grass competition, regardless of N-fertilization. With grass competition, however, foliar [N] increased and δ¹⁵N decreased in response to N-fertilization. Grass biomass accumulation was also unchanged by fertilisation, despite increases in foliar [N] and decreases in δ¹⁵N.

Conclusions

The N₂-fixation capacity of A. karroo provided no growth advantage over non-legumes. The lack of responsiveness of biomass accumulation by both the woody species and the grasses to N-fertilization, despite evidence that plants accessed the N-fertilizer, indicates limitation by other nutrients.     

Demographic implications of life-history stage characteristics in two African acacias at a Makeni savanna plot in Zambia

Aims In spite of the importance of African acacias in vegetation succession and provision of goods and services, little is known about life-history variations within and among species. Much of the work done on African acacias has focused on seed predation and germination and seedling establishment, especially of Acacia tortilis, Acacia nilotica and Acacia karroo. The primary aim of the present work is to investigate differences in the demography of Acacia polyacantha and Acacia sieberiana and the relationship between life-history characteristics and population size. A secondary objective is to assess how fire, an important ecological factor in savanna vegetation, might modify the growth and demographic dynamics of the two acacias.Methods The study was conducted at the Makeni savanna plot in central Zambia, southern Africa. Seedling emergence from both non-scarified and scarified seeds sown at different times in the wet season and the fate and growth of seedlings and saplings were monitored over a period of 4 years. Annual growth of permanently marked sample trees in annually burnt and fire-protected blocks was recorded over a 6-year period (2002–08) in order to assess inter-specific differences and how fire modifies tree growth patterns. Censuses of natural saplings and trees were conducted periodically in sample blocks to determine recruitment into these life-history stages.Important findings Seedling emergence and sapling survival rates were much higher in A. sieberiana than in A. polyacantha. However, both seedling and sapling growth rates were higher in A. polyacantha than in A. sieberiana but tree growth rates were similar in the two species. Under fire protection tree growth was significantly influenced by tree size and year while under annual burning only tree size significantly affected tree growth. The dominance of A. sieberiana over A. polyacantha at the study site was attributed to higher seedling emergence rate, higher sapling survival rate and a large sapling bank that forms a reliable source of tree recruitment. The life-history stage characteristics of A. polyacantha suggest that this is an early successional species.     

Andropogon gayanus (Gamba Grass) Invasion Increases Fire-mediated Nitrogen Losses in the Tropical Savannas of Northern Australia

Invasive alien grasses can substantially alter fuel loads and fire regimes which could have significant consequences for fire-mediated nutrient losses. The effects of the alien grass Andropogon gayanus Kunth. (Gamba grass) on fire-mediated nutrient losses was evaluated in Australia’s tropical savannas. Losses of macronutrients during fire were determined by comparing the nutrient pools contained in the fine fuel before fire and in the ash after fire. Pre-fire grass nutrient pools were significantly higher in A. gayanus plots than in native grass plots for all nutrients measured (N, P, K, S, Ca, and Mg). Nutrient losses were substantially higher in A. gayanus plots, with 113% higher losses for N, 80% for P, 56% for K, 63 for S, 355% for Ca, and 345% for Mg. However, only losses of N and Mg varied significantly between grass types. A simplified savanna ecosystem nutrient budget estimated that A. gayanus fires led to the net N loss of 20 kg ha⁻¹ y⁻¹. This is a conservative estimate because total fuel loads were relatively low (7.85 t ha⁻¹) for A. gayanus invaded plots leading to a relatively moderate intensity fire (6,408 kW m⁻¹). Higher A. gayanus fuel loads and fire intensities could potentially lead to losses of up to 61.5 kg N ha⁻¹ from the grass fuel. Over the long term, this is likely to lead to depletion of soil nutrients, particularly N, in the already low-fertility tropical savanna soils.     

Population differentiation in a purported ring species,Acacia karroo (Mimosoideae)

Acacia karroo Hayne (Mimosoideae; Fabaceae) is a highly polymorphic species, ranging in height from 1 m to more than 30 m, and with enormous variation in the architecture of adults. Some populations of A. karroo with different morphologies are situated less than 20 km apart. This species has been considered to be a ring species on the basis of allozyme variation. I wished to determine whether this was supported by sapling morphology, and by chemical and physical defences to herbivory. I raised four phenotypes from the restricted area of Zululand (South Africa) in a common garden with controls, and with water and nutrient supplementation. I found that each of the four phenotypes maintained their differences in spite of nutrient and water supplementation. There was no significant genotype by environment interaction. I also found that the coastal population was significantly larger than another phenotype that grows just 12 km inland from it, suggesting that there might be local adaptation of these genotypes to particular soil types. I confirm that A. karroo maintains morphological differentiation even when there is substantial alteration of water and nutrient availability. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 748–755.     

Regeneration properties of a <Emphasis Type="Italic">Populus euphratica</Emphasis> riparian forest located in the vicinity of the Ejina Oasis,Inner Mongolia,China

To explore the regeneration properties of a Populus euphratica Oliv. forest under unstable environmental conditions, we performed replicate censuses to determine the sapling dynamics of a P. euphratica population in the vicinity of the Ejina Oasis (Inner Mongolia). Even when the stands were established on riverside flatland, we detected slight variations in ground levels and salt concentrations. Due to leaching by flood water, the salinity of topsoil was lower in the riverbed than on the riverbank. Newly recruited saplings grew on riverbanks with high-salinity soil. Saplings were distributed over a wide area via fluctuating water levels, and those growing where salt levels were relatively low were able to grow more easily and become canopy trees. Small individuals comprise a sapling bank on the forest floor that is relatively stable. The dieback of larger saplings results in few reaching a height of more than 2 m. The growth of saplings (including new recruits) is balanced by the death of smaller saplings and the dieback of larger saplings. Individual saplings persist for about 4 years on the forest floor. In summary, hydrological events coupled with soil conditions may drive vegetation distribution patterns in riparian areas in arid regions. P. euphratica forests regenerate via a sapling bank rather than a seed bank. Yearly fluctuations in water flow facilitate the spread of the sapling bank, which guarantees regeneration of the forest. Dieback through partial defoliation is a mechanism used by saplings to escape adverse conditions, thereby maintaining a stable state in arid regions.     

The relationships between soil factors,grass nutrients and the foraging behaviour of wildebeest and zebra

Summary We examined the relationships between soil factors, nutrients in grasses and foraging behaviour of wildebeest (Connochaetes taurinus) and zebra (Equus burchelli) in a semi-arid nature reserve in South Africa. We tested the hypotheses that: (1) Soil nutrient levels determine the abundance and distribution of grass species; (2) nutrient levels within grass species are correlated with soil nutrient levels; (3) the spatial distribution and diet composition of ungulates is influenced by the nutrient availability in grasses. The distribution of soil factors in upper ground levels did explain the differential abundance of grass species in the reserve. Ordination of nutrient levels in grass species showed high levels of particular nutrients in certain species, but no one species showed uniformly high levels of all nutrients. Moreover, grasses on fertile soils did not necessarily accumulate higher nutrient levels than grasses on poor soils. Thus, nutrient levels in grasses were not correlated with soil nutrient levels. Wildebeest and zebra responded to monthly variations in the levels of N and P in grasses by moving seasonally to habitat types characterized by grass communities containing a high proportion of nutritional species, rather than by selecting particularly nutritious species within communities. We suggest that within semiarid savannas, areas with a higher diversity of grass communities will be more likely to have some of these communities containing high nutrient levels at any given season, than a lower diversity area. Therefore, the higher-diversity area would be likely to support more herbivores, and thus diversity would control carrying capacity.     

Seedling and Sapling Dynamics of Treefall Pits in Puerto Rico1

Seedling and sapling dynamics in a Puerto Rican rain forest were compared between forest understory and soil pits created by the uprooting of 27 trees during Hurricane Hugo. Soil N and P, organic matter, and soil moisture were lower and bulk densities were higher in the disturbed mineral soils of the pits than in undisturbed forest soils ten months after the hurricane. No differences in N and P levels were found in pit or forest soils under two trees with N–fixing symbionts (Inga laurina and Ormosia krugii) compared to soils under a tree species without N–fixing sym–bionts (Casearia arborea), but other soil variables (Al, Fe, K) did vary by tree species. Forest plots had greater species richness of seedlings (<10 cm tall) and saplings (10–100 cm tall) than plots in the soil pits (and greater sapling densities), but seedling densities were similar between plot types. Species richness and seedling densities did not vary among plots associated with the three tree species, but some saplings were more abundant under trees of the same species. Pit size did not affect species richness or seedling and sapling densities. Recruitment of young Cecropia schreberiana trees (>5 m tall) 45 months after the hurricane was entirely from the soil pits, with no tree recruitment from forest plots. Larger soil pits had more tree recruitment than smaller pits. Defoliation of the forest by the hurricane created a large but temporary increase in light availability. Recruitment of C. schreberiana to the canopy occurred in gaps created by the treefall pits that had lower soil nutrients but provided a longer–term increase in light availability. Treefall pits also significantly altered the recruitment and mortality of many understory species in the Puerto Rican rain forest but did not alter species richness.     

Effects of fire severity on plant nutrient uptake reinforce alternate pathways of succession in boreal forests

Fire activity in the North American boreal region is projected to increase under a warming climate and trigger changes in vegetation composition. In black spruce forests of interior Alaska, fire severity impacts residual organic layer depth which is strongly linked to the relative dominance of deciduous versus coniferous trees in early succession. These alternate successional pathways may be reinforced by biogeochemical processes that affect the relative ability of deciduous versus coniferous trees to acquire limiting nutrients. To test this hypothesis, we examined changes in soil inorganic nitrogen (N) supply and in situ ¹⁵N root uptake by aspen (Populus tremuloides) and black spruce (Picea mariana) saplings regenerating in lightly and severely burned sites, 16 years following fire. Fire severity did not impact the composition or magnitude of N supply, and nitrate represented nearly 40 % of total N supply. Both aspen and spruce took up more N in severely burned than in lightly burned sites. Spruce exhibited only a moderately lower rate of nitrate uptake, and a higher ammonium uptake rate than aspen in severely burned sites. At the stand level, differences in species nutrient uptake were magnified, with aspen taking up nearly an order-of-magnitude more N per m² in severely burned than in lightly burned sites. We suggest that differences in nutrient sinks (biomass) established early in succession and effects of post-fire organic layer depth on nutrient uptake, are key mechanisms reinforcing the opposing stand dominance patterns that have developed in response to variations in organic layer depth.     

Ecosystem Level Impacts of Invasive Acacia saligna in the South African Fynbos

Recent efforts to clear invasive plants from the fynbos of South Africa forces managers to think about how N₂‐fixing invasives have altered ecosystem processes and the implications of these changes for community development. This study investigated the changes in nitrogen (N) cycling regimes in fynbos with the invasion of Acacia saligna, the effects of clear‐cutting acacia stands on soil microclimate and N cycling, and how altered N resources affected the growth of a weedy grass species. Litterfall, litter quality, soil nutrient pools, and ion exchange resin (IER)‐available soil N were measured in uninvaded fynbos, intact acacia, and cleared acacia stands. In addition, a bioassay experiment was used to ascertain whether the changes in soil nutrient availability associated with acacia would enhance the success of a weedy grass species. Acacia plots had greater amounts of litterfall, which had higher concentrations of N. This led to larger quantities of organic matter, total N, and IER‐available N in the soil. Clearing acacia stands caused changes in soil moisture and temperature, but did not result in differences in IER‐available N. The alteration of N availability by acacias was shown to increase growth rates of the weedy grass Ehrharta calycina, suggesting that secondary invasions by nitrophilous weedy species may occur after clearing N₂‐fixing alien species in the fynbos. It is suggested that managers use controlled burns, the addition of mulch, and the addition of fynbos seed after clearing to lower the levels of available N in the soil and initiate the return of native vegetation.     

The effects of nutrient availability and removal of competing vegetation on resprouter capacity and nutrient accumulation in the shrub Erica multiflora

Nutrient availability is increasing in the Mediterranean Basin due to the great number and intensity of fires and higher levels of anthropomorphic pollution. In the experiment described in this paper, we aimed to determine the effects of N and P availability and of the removal of competing vegetation on resprouter capacity, biomass, and nutrient accumulation in Erica multiflora. Plants of the resprouter species E. multiflora were clipped to 0% of aerial biomass in a post-fire Mediterranean shrubland and fertilisation experiments and removal of competing vegetation were established in a factorial design. The resprouting of clipped plants was monitored during the first year after clipping and at the end of the year, all plant resprout populations were harvested and their resprout structure, biomass and N and P content measured. N fertilisation had no significant effect on leaf biomass either at plant level or on the total aerial biomass per stump unit area; however N concentration in resprout biomass did increased. P fertilisation slightly increased resprouting vigour and had a significant effect on P content of the leaf biomass. The removal of competing vegetation increased the ratio between leaf biomass and stem biomass, the lateral expansion of resprout, the hierarchy of resprouts branching, and the P content of stems, above all when P fertilisation was applied. These results show that as a response to decreased competition E. multiflora has the capacity to modify the relative proportions of the nutrients in the aerial biomass. All these characteristics allow E. multiflora to persist in increasingly disturbed Mediterranean ecosystems and contribute to the retention of nutrients in the ecosystem during early resprouting phases.     

Establishment of Pinus halepensis Mill. saplings following fire: effects of competition with shrub species

An early study analysing the effects of competition from Cistus monspeliensis-dominated shrub canopy on Pinus halepensis saplings, both colonising a recently burnt area, has been extended in order to test initial predictions. Inter-specific competition effects on P. halepensis were experimentally analysed by a shrub thinning-out treatment carried out 1 year after fire. The extension of the recorded period confirmed (i) a significant increase in height, and (ii) the lack of variation in density of P. halepensis saplings when the shrub layer was removed. In contrast, the increase in relative growth rate in height (RGRh) and the decrease in mortality recorded during early post-fire stages for treated units did not persist in subsequent years. These two treatment-induced effects disappeared 1 year after the shrub clearing (29 months after fire). It is hypothesised that this time should represent a culminating point in the inter-specific competition established between Cistus and Pinus saplings simultaneously colonising recently disturbed areas and be a critical period for pine sapling survival. After this time, a reduction in relatively short-lived Cistus populations and an increase in P. halepensis abundance should be expected in the community. It is concluded that a shrub-clearing treatment could be recommendable if the initial post-fire pine sapling density is not high enough to successfully face the early critical competitive period. Consequences of early shrub competition on forest productivity during mature phases are also discussed.     

Shoot development and extension of Quercus serrata saplings in response to insect damage and nutrient conditions

BACKGROUND AND AIMS: Plants have the ability to compensate for damage caused by herbivores. This is important to plant growth, because a plant cannot always avoid damage, even if it has developed defence mechanisms against herbivores. In previous work, we elucidated the herbivory-induced compensatory response of Quercus (at both the individual shoot and whole sapling levels) in both low- and high-nutrient conditions throughout one growing season. In this study, we determine how the compensatory growth of Quercus serrata saplings is achieved at different nutrient levels. METHODS: Quercus serrata saplings were grown under controlled conditions. Length, number of leaves and percentage of leaf area lost on all extension units (EUs) were measured. KEY RESULTS: Both the probability of flushing and the length of subsequent EUs significantly increased with an increase in the length of the parent EU. The probability of flushing increased with an increase in leaf damage of the parent EU, but the length of subsequent EUs decreased. This indicates that EU growth is fundamentally regulated at the individual EU level. The probabilities of a second and third flush were significantly higher in plants in high-nutrient soil than those in low-nutrient soil. The subsequent EUs of damaged saplings were also significantly longer at high-nutrient conditions. CONCLUSIONS: An increase in the probability of flushes in response to herbivore damage is important for damaged saplings to produce new EUs; further, shortening the length of EUs helps to effectively reproduce foliage lost by herbivory. The probability of flushing also varied according to soil nutrient levels, suggesting that the compensatory growth of individual EUs in response to local damage levels is affected by the nutrients available to the whole sapling.     

Effects of Prescribed Fires on Young Valley Oak Trees at a Research Restoration Site in the Central Valley of California

Woodland restoration sites planted with Quercus lobata (valley oak) often have serious invasions of nonnative annual grasses and thistles. Although prescribed fire can effectively control these exotics, restoration managers may be reluctant to use fire if it causes substantial mortality of recently planted saplings. We studied the effects of prescribed fires on the survival and subsequent growth of 5‐ and 6‐year‐old valley oak saplings at a research field near Davis, California. One set of blocks was burned in summer 2003 at a time that would control yellow star thistle, a second set of blocks was burned in spring 2004 at a time that would control annual grasses, and a third set was left unburned. Very few oaks died as a result of either fire (3–4%). Although a large proportion was top‐killed (66–72%), virtually all these were coppiced and most saplings over 300 cm tall escaped top‐kill. Tree height, fire temperature, and understory biomass were all predictive of the severity of sapling response to fire. Although the mean sapling height was initially reduced by the fires, the growth rates of burned saplings significantly exceeded the growth rates of unburned control trees for 2 years following the fires. By 2–3 years after the fires, the mean height of spring‐ and summer‐burned saplings was similar to that of the unburned control saplings. The presence of valley oak saplings does not appear to preclude the use of a single prescribed burn to control understory invasives, particularly if saplings are over 300 cm tall.     

Effects of fire on sandhill herbs: nutrients, mycorrhizae, and biomass allocation

Differences in growth responses, tissue and soil inorganic nutrients, and mycorrhizal relationships of four herbaceous species were studied on burned and unburned sandhill sites in south-central Florida, USA. Three species, (Aristida stricta, Liatris tenuifolia var. laevigata, and Pityopsis graminifolia) responded positively to conditions following the burn by increased vegetative growth and flowering. The fourth species, Balduina angustifolia, is a fire-sensitive biennial and its first-year rosettes were, with an occasional exception, unable to survive or resprout following fire. Availability of all soil inorganic nutrients examined (Ca, K, Mg, and P) was low, as were total nitrogen, soil organic matter, and pH. There was a slight nutrient pulse of phosphorus into the soil following burning. For two species (Aristida and Liatris), shoot tissue concentrations of several inorganic nutrients (especially N and P) were higher on the burned site than the unburned site following burning. These differences generally dissipated over time since burning. The high concentration of tissue nutrients postburn followed by a decline on the burned site may result from rapid nutrient uptake after fire and dilution of this concentration following restoration of plant mass. Despite low levels of soil inorganic nutrients, including phosphorus, mycotrophy was absent or weakly developed among the herbaceous species examined, except for the tap-rooted Balduina angustifolia. Colonization of host plants by vesicular mycorrhizal fungi was unaffected by burning. Mycorrhizal inoculum potentials of sandhill soil were extremely low, varying seasonally from (mean +/- 1 SE) 0.3 +/- 0.2 to 3.8 +/- 0.7%.     

Nutrient cycling in Pinus sylvestris stands in eastern Finland

Nutrient cycling within three Pinus sylvestris stands was studied in eastern Finland. The aim of the study was to determine annual fluxes and distribution of N, P, K, Ca, Mg, Zn, Fe, B, and Al in the research stands. Special emphasis was put on determining the importance of different fluxes, especially the internal cycle within the trees in satisfying the tree nutrient requirements for biomass production. The following nutrient fluxes were included, input; free precipitation and throughfall, output; percolation through soil profile, biological cycle; nutrient uptake from soil, retranslocation within trees, return to soil in litterfall, release by litter decomposition. The distribution of nutrients was determined in above- and belowground tree compartments, in ground and field vegetation, and in soil.The nitrogen use efficiencies were 181, 211 and 191 g of tree aboveground dry matter produced per g of N supplied by uptake and retranslocation in the sapling, pole stage and mature stands, respectively. Field vegetation was more efficient in nitrogen use than trees. Stand belowground/aboveground and fine root/coarse root biomass ratios decreased with tree age. With only slightly higher fine root biomass, almost three times more nitrogen had to be taken-up from soil for biomass production in the mature stand than in the sapling stand.The annual input-output balances of most nutrients were positive; throughfall contained more nutrients than was lost in mineral soil leachate. The sulphate flux contributed to the leaching of cations, especially magnesium, from soil in the mature stand.Retranslocation supplied 17–42% of the annual N, P and K requirements for tree aboveground biomass production. Precipitation and throughfall were important in transferring K and Mg, and also N in the sapling stand. Litterfall was an important pathway for N, Ca, Mg and micro nutrients, especially in the oldest stands.     

Pay it forward: refuse dump from leaf‐cutting ants promotes caterpillar digestive performance by increasing plant nitrogen content

1. The activity of soil‐disturbing animals that increase soil nutrients can affect the carbon : nitrogen (C : N) ratio of plants, which, in turn, may determine the transfer of energy and nutrients through higher trophic levels. However, the strength and sign of this indirect effect depend on whether enhanced nutrient substrates increase plant foliar nutrients and/or plant defensive traits. 2. We investigated how the nutrient‐rich refuse dumps of the leaf‐cutting ant Acromyrmex lobicornis, as a result of their direct effects on thistles, indirectly impact the growth rate and digestive performance of a generalist chewing herbivore. We also included the application of commercial fertilisers to test whether the enhanced soil nutrients comprises the mechanism behind the impact of refuse dumps on the upward cascade effects. 3. Thistles growing on nutrient‐rich substrates (i.e. ant refuse dumps and fertilised soil) had more and larger leaves, up to 80% lower C : N ratios, and higher physical defences compared to plants growing on steppe soil. Caterpillars showed an enhanced digestive performance and growth rate when feed on nutrient‐rich plants and were able to adjust the C : N ratio of their excretion to regulate the relative acquisition of nutrients. 4. The positive effect of nutrient‐rich substrates on caterpillar feeding efficiency suggest that the enhanced nutritional quality of the thistles could compensate for the negative effects of the increased physical defences. The results of the present study indicate how organisms that increase soil nutrient availability may diminish the stoichiometric constraints at the base of food chains, enhancing the development and growth rate of herbivores and, thus, indirectly mediating plant–herbivore interactions.     

Soil nutrients are not responsible for arrested succession in disturbed coastal dune forest

Coastal dune forest succession frequently proceeds via the Acacia karroo pathway, but may become arrested. We examine whether soil fertility arrests forest succession in A. karroo stands in coastal dune forest in KwaZulu-Natal province, South Africa. We examined soil fertility of A. karroo stands, the adjacent forest, and forested dune slacks at Cape Vidal, and four rehabilitating A. karroo stands (13- to 28-yr-old) at Richards Bay. The effect of nitrogen supplementation on growth of three tree species (a forest pioneer, a late successional forest species, and A. karroo) was compared between A. karroo stands and adjacent dune forest at Cape Vidal. Soil fertility in A. karroo stands and the adjacent forest at Cape Vidal was similar and neither total nor readily mineralisable nitrogen were limiting in either habitat. At Richards Bay, where the dunes were previously strip-mined, total nitrogen accumulated rapidly (2.1–8.0 g N m⁻² yr⁻¹) and the oldest rehabilitating A. karroo stands (26–28 yr) had similar total nitrogen and other soil nutrient levels as stands twice their age at Cape Vidal. Seedling growth was unaffected by nitrogen supplementation. All species grew fastest in A. karroo stands demonstrating that soil nutrient levels in disturbed forest colonised by A. karroo are suitable for the growth of forest tree species. Soil fertility, including available nitrogen, is not limiting secondary succession at Cape Vidal, yet forest species are not replacing A. karroo stands at this site. Post-emergence factors, such as herbivory, are likely responsible for the arrested succession of forest in A. karroo stands.