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.     

Understorey gaps influence regeneration dynamics in subtropical coastal dune forest

Dominant understorey species influence forest dynamics by preventing tree regeneration at the seedling stage. We examined factors driving the spatial distribution of the monocarpic species Isoglossa woodii, a dominant understorey herb in coastal dune forests, and the effect that its cover has on forest regeneration. We used line transects to quantify the area of the forest understorey with I. woodii cover and with gaps in the cover. Paired experimental plots were established in semi-permanent understorey gaps with I. woodii naturally absent and in adjacent areas with I. woodii present to compare plant community composition, soil, and light availability between the two habitats. Isoglossa woodii was widespread, covering 65–95% of the understorey, while gaps covered the remaining 5–35% of the area. The spatial distribution of this species was strongly related to tree canopy structure, with I.␣woodii excluded from sites with dense tree cover. Seedling establishment was inhibited by low light availability (<1% of PAR) beneath I.␣woodii. When present, I. woodii reduced the density and species richness of tree seedlings. The tree seedling community beneath I. woodii represented a subset of the seedling community in gaps. Some species that were found in gaps did not occur beneath I. woodii at all. There were no significant differences between the sapling and canopy tree communities in areas with I. woodii gaps and cover. In the coastal dune forest system, seedling survival under I. woodii is dependent on a species’ shade tolerance, its ability to grow quickly during I. woodii dieback, and/or the capacity to regenerate by re-sprouting and multi-stemming. We propose a general conceptual model of forest regeneration dynamics in which the abundant understorey species, I. woodii, limits local tree seedling establishment and survival but gaps in the understorey maintain tree species diversity on a landscape scale.     

Resprouting enhances seedling persistence in a subtropical coastal dune forest

In theory, resprouting enables species with low reproductive output (i.e., few seedlings) to persist. The advantage conferred by seedling sprouts on tree species persistence was evaluated in a subtropical coastal dune forest in South Africa. Species with a higher frequency of seedling sprouts demonstrated greater persistence as evidenced by a larger proportion of seedlings >1 year old and a larger seedling bank than species with few seedling sprouts. Resprouted seedlings had a larger basal diameter than true seedlings. Although resprouting resulted in the maintenance of multiple stems in some seedlings, the proportion of multi‐stemmed seedlings was low. Multi‐stemming was not a favoured form of seedling growth except in one species that occupied relatively open sites. Despite the apparent difference among species in resprouting ability, we found that seedling resprouting was not phylogenetically constrained. These results demonstrate that seedling sprouts form an important component of seedling banks in coastal dune forest.     

Secondary succession in coastal dune fynbos: variation due to site and disturbance

Succession after fire and bushcutting in coastal dune fynbos was monitored for two and a half years and comparisons were made with adjacent, mature (13 year-old) fynbos. Sixty-two to 68% of pre-disturbance species, including all the dominants, were found in the successional communities 1.5 yr after disturbance: the patterns thus fitted the initial floristic composition model. On the more mesic south facing slope, post-fire succession differed from the north-facing (burnt) and bushcut sites in that ordinations showed a clear separation between the mature and successional communities. This difference was due to the post-fire abundance on the southfacing slope site of short-and medium-lived species not present in the mature fynbos. One and a half years after disturbance, species richness and equitability had increased relative to mature vegetation. This increase was greatest for the south facing slope where short-and medium-lived species and juveniles of pre-disturbance dominants co-occurred. In general, successional patterns were consistent with those described for other fynbos and fire-prone mediterranean shrublands.     

Climate and coastal dune vegetation: disturbance, recovery, and succession

The sand dune habitats found on barrier islands and other coastal areas support a dynamic plant community while protecting areas further inland from waves and wind. Foredune, interdune, and backdune habitats common to most coastal dunes have very different vegetation, likely because of the interplay among plant succession, exposure, disturbance, and resource availability. However, surprisingly few long-term data are available describing dune vegetation patterns. A nine-year census of 294 plots on St. George Island, Florida suggests that the major climatic drivers of vegetation patterns vary with habitat. Community structure is correlated with the elevation, soil moisture, and percent soil ash of each 1 m² plot. Major storms reduce species richness in all three habitats. Principle coordinate analysis suggests that changes in the plant communities through time are caused by climatic events: changes in foredune vegetation are correlated with temperature and summer precipitation, interdune vegetation with storm surge, and backdune vegetation with precipitation and storm surge. We suggest that the plant communities in foredune, interdune, and backdune habitats tend to undergo succession toward particular compositions of species, with climatic disturbances pushing the communities away from these more deterministic trajectories.     

The role of canopy gaps in the regeneration of coastal dune forest

In regenerating coastal dune forest, the canopy consists almost exclusively of a single species, Acacia karroo. When these trees die, they create large canopy gaps. If this promotes the persistence of pioneer species to the detriment of other forest species, then the end goal of a restored coastal dune forest may be unobtainable. We wished to ascertain whether tree species composition and richness differed significantly between canopy gaps and intact canopy, and across a gradient of gap sizes. In three known‐age regenerating coastal dune forest sites, we measured 146 gaps, the species responsible for gap creation, the species most likely to reach the canopy and the composition of adults, seedlings and saplings. We paired each gap with an adjacent plot of the same area that was entirely under intact canopy and sampled in the same way. Most species (15 of 23) had higher abundance in canopy gaps. The probability of self‐replacement was low for A. karroo even in the largest gaps. Despite this predominance of shade‐intolerant species, regenerating dune forest appears to be in the first phase of succession with ‘forest pioneers’ replacing the dominant canopy species. The nature of these species should lead to successful regeneration of dune forest.     

Short-term responses of rehabilitating coastal dune forest ground vegetation to livestock grazing

We investigated the responses of the ground vegetation in a 17‐year‐old coastal dune forest plant community to four levels of experimentally applied livestock grazing (three grazing levels and one ungrazed control) from May 1994 to March 1996. The effects of grazing were apparently subordinate to site‐specific intrinsic vegetation change and there were some indications that rainfall interacted with grazing level. Grazing had some apparent but no significant effects on plant species composition, significantly affected plant species richness over time, and significantly increased the range of species richness and vegetation cover values as well as the relative abundance and numbers of plant species with erect growth forms. Vegetation cover changed significantly over time, independently of grazing. Our results point to two important, easily measured mechanisms for the conservation management of coastal dune forests – the interaction of disturbance type with plant growth form and the increase of variation in community structural variables under disturbance. These mechanisms, although they potentially have wide application and predictive power, have not been studied adequately.     

Decline of birds in a human modified coastal dune forest landscape in South Africa

Previous studies demonstrate that old-growth forest remnants and vegetation regenerating after anthropogenic disturbance provide habitat for birds in a human modified coastal dune forest landscape in northern KwaZulu-Natal, South Africa. However, occurrence does not ensure persistence. Based on a 13-year monitoring database we calculated population trends for 37 bird species and general trends in overall bird density in different vegetation types. We evaluated species' characteristics as covariates of population trend and assessed changes in rainfall and proportional area and survey coverage per vegetation type. 76% of species assessed have declined, 57% significantly so at an average rate of 13.9% per year. Overall, bird density has fallen at 12.2% per year across old-growth forest and woody regenerating vegetation types. Changes in proportional area and coverage per vegetation type may partly explain trends for a few species but are unlikely to account for most. Below average rainfall may have contributed to bird declines. However, other possibilities warrant further investigation. Species with larger range extents tended to decline more sharply than did others, and these species may be responding to environmental changes on a broader geographical scale. Our results cast doubt on the future persistence of birds in this human modified landscape. More research is needed to elucidate the mechanisms driving population decline in the study area and to investigate whether the declines identified here are more widespread across the region and perhaps the continent.     

Costs of resprouting are traded off against reproduction in subtropical coastal dune forest trees

Resprouting can be an important means of regeneration for forest tree species resulting in multi-stemmed architecture, especially at less productive or frequently disturbed sites. However, the cost of resprouting may be traded off against growth or reproduction. In subtropical coastal forest in South Africa, trees grow on steep, sandy dunes with unstable soils and low to moderate nutrient availability. These coastal forests experience seasonally strong anticyclonic winds from August through October. We examined the hypothesis that basal resprouting resulting in multiple stems causes lower rates of sexual reproduction and recruitment by individuals. We examined whether trees traded off resprouting against seed output, seed size, seedling abundance and recruitment by seedlings. Species were designated as good and poor resprouters based on their frequency of multi-stemmed individuals at Cape Vidal. Good resprouters had more stems, produced less seed and had lower seed mass than poor resprouters, and had lower seedling abundance and fewer individuals in small diameter classes than large diameter classes. Seedling abundance in good resprouters was not influenced by the availability of understorey gaps. Good resprouters were most abundant on dune crests and seaward slopes that were exposed to sea winds. Persistence of established individuals by producing multiple stems from basal resprouts is important where a chronic disturbance regime potentially reduces the survivorship of single-stemmed individuals and thereby their opportunities for reproduction. Good resprouters appear to trade-off recruitment of new individuals for multiple stems that increase the persistence of established ones against disturbance. We conclude that multi-stemming arising from basal resprouts has evolved to promote individual persistence under low to moderate intensity but pervasive wind stress.     

Fine-scale structure of populations of the ectomycorrhizal fungus Hebeloma cylindrosporum in coastal sand dune forest ecosystems

The basidiomycete mushroom Hebeloma cylindrosporum is a frequently found pioneer ectomycorrhizal species naturally associated with Pinus pinaster trees growing in coastal sand dune ecosystems along the Atlantic south-west coast of France. The genotypic diversity and spatial structure of three populations of this fungal species have been studied. At each site the basidiocarps were mapped, sampled and propagated as pure mycelial cultures. For each of the isolates, we have studied polymorphisms in the mitochondrial genome, polymorphisms at two different nuclear loci and also fingerprints produced with a multicopy DNA probe. The comparison of the different polymorphisms obtained, with each of the four molecular methods used, allowed the identification of several of the different genets present in each site. In two of the studied sites most of the basidiocarps, which often occurred as dense patches of 10–30 in 1 m² or less, were of a unique genotype, suggesting the below-ground mycelia to be of a small size (from 50 cm² to approx. 7 m² for the larger mycelia) and that the root system of a single Pinus tree can host several genets of the same symbiotic fungus. In the two sites, which were studied again after a 3-year interval, none of the genotypes identified in the first year of sampling was re-identified 3 years later. These results contrast with those reported for other species of soilborne homobasidiomycete species, either ectomycorrhizal, parasitic or saprophytic, showing mostly large clones resulting from the vegetative growth and from persistence of below-ground mycelia. Sexual reproduction through meiospore dispersal seems to play a key role in the structuring of the populations of H. cylindrosporum. Mycelia associated with the root systems seem to be replaced after 1 or a few years, during which basidiocarp differentiation takes place. As opposed to the few other studied ectomycorrhizal species, H. cylindrosporum has the characteristics of ruderal species, with a short life-span adapted to pioneer situations, e.g. to nutrient-poor and unstable sandy soils of coastal sand dunes.     

Changes in soil and vegetation during dune slack succession

Many rare plant species occur in Dutch wet dune slacks, particularly in the Junco baltici‐Schoenetum nigricantis. For nature management it is important to understand the processes controlling the presence of these basiphilous early successional communities, which is why we investigated vegetation and soil development during succession in coastal dune slacks. We compared 12 chronosequential stages of 0, 2, 4, 9, 10, 13, 25, 30, 43, 60, 70 and 85 yr in five different dune slack systems. In four of these locations turf had earlier been removed in order to restore the basiphilous pioneer stage. The main variation in the vegetation is related to the acidification/soil enrichment gradient and the salinity/maritime gradient. During succession, organic matter accumulates and acidification takes place. Maritime influence can buffer the soil and postpone the succession of basiphilous pioneer communities for many years. A significant correlation with age was found for 18 variables. Multiple regressions predicted changes in the vegetation (dependent variables: biomass, cover of Salix repens, Calamagrostis epigejos and Schoenus nigricans) as a function of acidification, organic matter accumulation, increase in available P and presence of Na in the soil. We conclude that natural ageing of the vegetation and the associated process of accumulation of biomass drive succession in this hydrosere. The underlying soil processes are acidification and organic matter accumulation. During succession dominance shifts from S. nigricans to S. repens or C. epigejos. Maintenance of the pioneer character of the habitat is only possible by local intervention or by natural or man‐induced dune forming. The effect of sod‐stripping depends on the environmental conditions and, in case of acidification, success is limited. Succession can be postponed by mowing.     

Seed bank assembly follows vegetation succession in dune slacks

Question: Is the seed bank in dune slacks during the whole successional range mainly composed of early successional species or does it vary according to the changing vegetation? Location: Belgium, western part of the coast. Methods: We investigated the soil seed bank composition using a seedling germination method in a chronosequence of 20 dune slacks, ranging in age from five to 55 yr. Results: Both seed density and species richness in the seed bank were very low in the first successional stages and increased with age, mainly as a result of increasing seed production. The similarity between seed bank and vegetation was higher in older slacks. A comparison of characteristics between seed bank and vegetation showed that the seed bank was, to a large extent, composed of later successional species. Occurrence patterns of individual species also showed that seeds become incorporated in the soil after the species has established in the vegetation. Conclusion: The seed bank is not likely to be the driving force for successional changes in the vegetation, and successional changes rely on dispersal. Some early successional species persist in the seed bank, but generally only in low numbers. The results also confirm that most typical dune slack species do not form persistent seeds, so that re‐establishment from the seed bank is not to be expected when the species has disappeared from the vegetation.     

Secondary dune succession on Inhaca Island,Mozambique

Old field succession was studied on coastal dunes supporting tropical evergreen forest on Inhaca Island, Mozambique. Plots of 10×10 m were sited in three early successional stages and in relatively undisturbed forest. Woody species increased in number during succession; leptophylls were most frequent in younger vegetation, whereas microphylls and mesophylls were most frequent in forest. Grasses, shrubs and forbs dominated initially following abandonment, and shrubs persisted as dominants in the three early successional stages. The initial floristic composition model was generally supported by the pattern of species sequences, with many forest species entering early in the succession. Of the few species conforming to the relay floristic model, many were grasses and forbs of the forest understorey. Similarity between plots of equivalent vegetation age indicated that, at least in early succession, there was linearity in the successional pathway; there was no evidence for divergence or multiple pathways. In early succession, no accumulation was detected in either soil organic matter or extractable nutrients, thus providing little support for the facilitation model of succession. It is stressed that the findings are probably scale-dependent.     

Facilitation of tree seedling establishment in a sand dune succession

Abstract. A nucleated pattern of establishment by pine seedlings of Pinus strobus and P. resinosa around pre-established oak trees of Quercus rubra in a sand dune succession in Ontario, Canada was examined using field observations, seed planting and habitat manipulation. Densities of young pines beneath oak canopies were approximately six times greater than in treeless areas, and densities on the north sides of the trees were significantly greater than on the south sides. However, oaks younger than 35 yr showed no preferential establishment beneath them, while the pine population structures beneath older oaks indicated single periods of successful recruitment. Pine seed planted beneath and beyond oak canopies of three sizes germinated primarily beneath the canopies of medium and large-sized oaks, but subsequent survivorship of seedlings over two growing seasons was poor. Several micro-environmental conditions were changed by oak canopies, but only shade showed a pattern closely corresponding to that of seedling establishment. A habitat manipulation experiment confirmed the primary role of shade in facilitating pine seedling establishment beneath oaks. Failure of pine to continue recruiting successfully beneath facilitating trees is tentatively attributed to intraspecific competition among pine individuals.     

Restoration of coastal dune slacks in the Netherlands

In order to stop the continuous decline of typical dune slack communities along the Dutch coast, restoration projects have been carried out since 1952. Restoration measures consisted of re-introducing traditional management techniques in dune slacks, such as mowing, grazing and sod removal, or constructing artificial dune slacks to compensate for lost biodiversity elsewhere. An analysis of successful and unsuccessful projects showed that constructing new dune slacks was not very successful for maintaining new populations of endangered dune slack species, since such projects were often carried out in areas where seed banks were depleted, while hydrological conditions and seed dispersal mechanisms were sub-optimal. The construction of sand dikes to prevent sea intrusion in large beach plains was, unintentionally, a temporary success for the establishment of many Red List species, although such measures often disrupted natural dune slack formation. Successful sites were all characterised by a regular discharge of calcareous groundwater provided by local or regional hydrological systems, where not very long ago populations of typical dune slack plants were present. Under such conditions, sod removal was a successful measure to create pioneer stages which were relatively stable, due to a very slow accumulation of organic matter in the topsoil. It is argued that new and more flexible coastal defence strategies can provide new opportunities for natural and relatively stable pioneer stages of dune slack formation, suitable for the long term preservation of endangered dune slack species.     

Patterns of tree community composition along a coastal dune chronosequence in lowland temperate rain forest in New Zealand

Soil chronosequences provide an opportunity to examine the influence of long-term pedogenesis on the biomass and composition of associated tree communities. We assessed variation in the species composition of trees, saplings, and seedlings, and the basal area of adult trees, in lowland temperate rain forest along the Haast chronosequence on the west coast of the South Island of New Zealand. The sequence consists of Holocene dune ridges formed following periodic earthquake disturbance and is characterized by rapid podzol development, including a marked decline in phosphorus concentrations, accumulation of a thick organic horizon, and formation of a cemented iron pan. Tree basal area increased for the first few hundred years and then declined in parallel with the decline in total soil phosphorus, consistent with the concept of forest retrogression. There were also marked changes in the composition of the tree community, from dominance by conifers on young soils to a mixed conifer?Cangiosperm forest on old soils. Although a variety of factors could account for these changes, partial Mantel tests revealed strong correlations between tree community composition and soil nutrients. The relationships differed among life history stages, however, because the adult tree community composition was correlated strongly with nutrients in the mineral soil, whereas the seedling community composition was correlated with nutrients in the organic horizon, presumably reflecting differences in rooting depth. The changes in the tree community at Haast are consistent with disturbance-related succession in conifer?Cangiosperm forests in the region, but the opposite of patterns along the nearby Franz Josef post-glacial chronosequence, where conifers are most abundant on old soils. The Haast chronosequence is therefore an important additional example of forest retrogression linked to long-term soil phosphorus depletion, and provides evidence for the role of soil nutrients in determining the distribution of tree species during long-term succession in lowland temperate rain forests in New Zealand.     

Soil organic matter accumulation and its implications for nitrogen mineralization and plant species composition during succession in coastal dune slacks

Vegetation and soil development during succession in coastal dune slacks on Terschelling island, the Netherlands, was investigated, by comparing neighbouring ecosystems on similar substrates that had been developing for 1, 5, 35 and 76 years since the vegetation and organic soil layer had been removed. In this successional sequence, soil organic matter accumulated rapidly due to the production of litter and dead roots. N mineralization was extremely low, increasing from 0.2 g m^-2 yr^-1 after 5 years to 0.8 g m^-2 yr^-1 after 76 years. It was accompanied by a decline in the pH (KCl) in the upper 10 cm of the soil from 6.8 to 4.4. Most of the above-ground biomass accumulated in the shrub species Oxycoccus macrocarpos and Salix repens. The 5- year-old plots harboured many plant species (18 species per 0.25 m²), but plant species diversity was much lower in the older plots. It is concluded that most changes in species composition and the decline in diversity occurred because early successional plant species were gradually outshaded by the thick litter layer and the accumulated shrub biomass.     

Multi‐stemmed trees in subtropical coastal dune forest: Survival strategy in response to chronic disturbance

Questions: Is multi‐stemming a survival strategy in response to chronic disturbance? Is multi‐stemming under phylogenetic control? What environmental factors are associated with multi‐stemming? When do trees initiate growth of multiple stems? Do multi‐stemmed trees have limited stature? Location: Subtropical Indian Ocean coastal dune forest at Cape Vidal in the Greater St. Lucia Wetland Park, northeastern South Africa. Methods: Tree physiognomy and environmental conditions were sampled in 20 transects that were 300 m long and 5 m wide. Results: 38.9 % of trees were multi‐stemmed, with no correlation between multi‐stemming and taxonomic grouping. The multi‐stemming trait was most associated with stem decumb‐ency and substrate erosion and multi‐stemmed individuals were less common in protected dune slacks. Trees at Cape Vidal appear to facultatively produce multiple stems from an early stage. There was no trade‐off between tree height and stem number. Conclusions: Coastal winds and the unstable dune substrate are important environmental correlates of multi‐stemming. Both short stature and the high incidence of multi‐stemming are related to the tree‐disturbance interaction. The taxonomically widespread phenotypic plasticity in this trait indicates the importance of multi‐stemming to tree survival even under low levels of disturbance.