Dune Forest Research Articles

The role of biotic interactions for the early establishment of oak seedlings in coastal dune forest communities

Although forest managers are aware of the need to adapt forest management to climate change, this task is still challenging due to the difficulty for predicting local species and communities responses to climate change. The aim of this paper is to focus on the adaptation of sylvicultural techniques to changes in interactions between tree recruitment and understory species due to climate change. A space-for-time design was used in coastal dune forest communities from the south west of France. The study area is a 240km-long sand strip covered by forest communities with minimal variation in soil conditions along a natural gradient of increasing water stress. We transplanted seedlings of three oak species of contrasting strategies at both the wet and dry ends of the climate gradient, both in forest and gap plots and with and without understory shrubs. We measured Vapor Pressure Deficit in all treatment conditions. We found strong canopy and climate conditions effects on interactions between understory shrubs and oak transplants. Competition was dominant in the forest plots of the wettest site and facilitation in the gap plots of the driest site. Oak survival without shrubs (but not with shrubs) was strongly related to VPD values, which suggests that the positive effect of shrubs in the most stressful conditions was due to decreased atmospheric stress below their canopy. In contrast, we found that understory shrubs/oak seedlings interactions were weakly affected by oak species functional strategies. Our results provide evidence that future oak regeneration management should take into account changes in interactions with understory shrubs due to climate change. In particular, we recommend conserving understory shrubs in the most stressful sites in order to maintain a sufficient oak regeneration for the long term dynamics of coastal oak forest communities under changing climate.

Coastal dune topography as a determinant of abiotic conditions and biological community restoration in northern KwaZulu-Natal, South Africa

Topography is rarely considered as an independent goal of restoration. However, topography determines microenvironmental conditions and hence living conditions for species. Restoring topography may therefore be an important first step in ecological restoration. We aimed at establishing the relative importance of topography where coastal dunes destroyed by mining are rebuilt as part of a rehabilitation program. We assessed the response of (1) microclimatic and soil conditions, and (2) woody plant and millipede species richness and density, to location-specific topographic profiles. We enumerated the topographic profile using variables of dune morphology (aspect, elevation, and gradient) as well as relative position on a dune (crest, slope, and valley). Temperature, relative humidity, and light intensity varied with aspect, elevation, gradient, and position. However, regeneration age was a better predictor of soil nutrient availability than these topographic variables. Age also interacted with topographic variables to explain tree canopy density and species richness, as well as millipede species richness. The density of keeled millipedes (forest specialists) was best explained by topographic variables alone. The transient nature of these new-growth coastal dune forests likely masks topography-related effects on communities because age-related succession (increasing structural complexity) drives the establishment and persistence of biological communities, not habitat conditions modulated by topography. However, our study has shown that the microhabitats associated with topographic variability influence specialist species more than generalists.

Summer drought and canopy opening increase the strength of the oak seedlings–shrub spatial association

The knowledge of how shrub–seedling interactions vary with summer drought, canopy opening, and tree species is crucial for adapting forest management to climate change. The aim of this study was to assess variation in shrub–oak recruitment associations along a south–north drought climate gradient and between two levels of canopy cover in coastal dune forest communities in a climate change-adapted forest management perspective. Mapped data of associational patterns of seedlings of three oak species with interspecific pooled shrubs were analyzed using a bivariate pair correlation function in 10 (0.315 ha) regeneration plots located in forest and recent gap sites along the climate gradient. An index of association strength was calculated in each plot and plotted against a summer moisture index. The association strength increased with increasing summer drought from wet south to dry north and from closed forests to gaps. Consistent with facilitation theory, our results suggest that climate change may shift associational patterns in coastal dune forest communities towards more positive associations, in particular in canopy gaps. In a perspective of climate change, foresters may need to conserve understory shrubs in gaps in order to promote oak species regeneration.

A survey of an ectotrophic sand dune forest in the northeast Brazil

A survey of an ectotrophic sand dune forest in the northeast Brazil

Is microclimate-driven turnover of dung beetle assemblage structure in regenerating coastal vegetation a precursor to re-establishment of a forest fauna?

We questioned the capability of post-mining rehabilitation and successional changes in coastal vegetation to achieve restoration of dune forest, dung beetle assemblages in the Maputaland Centre of Endemism, South Africa. A repeat 2010 study of structural turnover between dung beetle assemblages across a 33 year successional sere of rehabilitating vegetation and old-growth forest (>73 years) produced comparable results to an earlier study across the 23 year chronosequence of 2000. Despite overlap, three structural patterns along the 33 year chronosequence were associated with specific stages of vegetation succession and their characteristic microclimates as in 2000. Although species biased to unshaded habitat dominated the earliest succession, there was rapid re-establishment of dominance by shade-associated forest species. In concert with progression from unshaded, post-mining vegetation to strongly shaded, early successional, Acacia shrub-woodland, there was an initial increase in similarity of the dung beetle fauna (species-poor, low abundance) to that in strongly-shaded forest (also species-poor, low abundance). However, in concert with decreasing shade cover in late successional woodland, the dung beetle fauna became species-rich with high abundance so that the early successional trajectory of increasing similarity to forest fauna either levelled off to a plateau (species in 2000; abundance in 2010) or declined (species in 2010, abundance in 2000). It remains to be seen if gaps forming in the oldest Acacia woodland permit forest tree saplings of the exposed understorey to recreate a forest canopy that would be tracked by dung beetles to re-establish a typically species-poor, deep shade, forest assemblage with low abundance.

Is Succession-based Management of Coastal Dune Forest Restoration Valid?

Habitat restoration and the theory of ecological succession are linked intrinsically. However, restoration management does not always rely on successional principles. This separation between theory and practical application may stem from the failure of succession to achieve restoration targets. Here we test the predictions of succession in a restoration context to ascertain the validity of succession-based management. Specifically we answer the following 4 questions: 1) does the rate of species turnover decrease as coastal dune forest develops; 2) is there a sequence of changing species “types” from pioneer species adapted to harsh conditions to species adapted to high levels of competition; 3) is this sequence of types directional and the same across all sites with similar climatic conditions; and 4) does species diversity increase or decrease? Our study took place in 7 coastal dune forest sites of various ages regenerating after mining disturbance. We conducted tree surveys in 1999, 2001, 2005, and 2009 and herbaceous plant surveys in 1995, 1999, 2003, and 2005. We assessed trends in species turnover, composition, diversity, richness, and evenness to see if these were congruent with successional theories. Patterns in turnover for both taxa showed a decelerating decrease. Sites of a similar age shared similar species composition of coastal dune forest trees and herbaceous plants. As sites aged, they increased in the number and diversity of species. Succession-based management is a valid approach to dune forest rehabilitation as long as restoration managers recognize disturbance as an ecological reality.

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

AbstractIn 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.

Tree-ring based precipitation reconstruction for the forest–steppe ecotone in northern Inner Mongolia, China and its linkages to the Pacific Ocean variability

We present a precipitation reconstruction for the ecotone of Hulunbei'er steppe and Great Xing'an Mountain forest region, Inner Mongolia, based on tree rings of Mongolian pine (Pinus sylvestiris Linnaeus var. mongolica Litvinov) scattered in regional sandy dune forests. The ring-width chronology shows most significant correlation with precipitation from prior August to current June. Based on this relationship, a precipitation reconstruction (1806–2007A.D.) was developed. The reconstruction was verified with independent data, and accounts for 39.7% of the actual precipitation variance during their common period (1953–2007). The reconstruction shows a relatively wet early half of the 19th century, a reversal from generally dry conditions during the late-19th century to generally wet conditions during the early 20th century, and a drying trend in general since the mid-20th century. The driest decades are found in 1856–1865 and 1889–1898, and the wettest decades occurred in 1944–1953 and 1826–1835, respectively. Multi-decadal (≥56.8year) and inter-annual (2.3, 4.6, 4.8, 4.9, 5.7–6.0, 9.0 and 9.1year) cycles are also detected in our reconstruction. Our precipitation reconstruction is significantly correlated with the East Asian Monsoon and Pacific Ocean signals, such as the El Niño-Southern Oscillation and Pacific Decadal Oscillation.

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.

The resilience of the medicinal plant community of rehabilitating coastal dune forests, KwaZulu-Natal, South Africa

The resilience of the medicinal plant community of rehabilitating coastal dune forests, KwaZulu-Natal, South Africa

A phytosociological survey of the dune forests of the Pacific Northwest

We report on phytosociological survey of the forests dominated by Pinus contorta var. contorta and/or Picea sitchensis that thrive on the Pacific Northwest, between the Strait of Juan de Fuca in Washington and Cape Mendocino in northwestern California. Through tabular and average linkage cluster analyses, 198 relevés taken at 69 sites were classified. Diagnostic tables, climatic, edaphic, and biogeographical data were used to establish floristic affinities among the seven identified associations and to interpret their distributions in psammophilous and zonal gradients, as well as their syntaxonomic position in the Temperate North American class Tsugetea mertensiano-heterophyllae. Four associations are described here for the first time: Arctostaphylo uva-ursi-Pinetum contortae, Carici obnuptae-Pinetum contortae, Morello californicae-Piceetum sitchensis, and Pseudotsugo menziesii-Pinetum contortae.

Density Effects of a Dominant Understory Herb, Isoglossa woodii (Acanthaceae), on Tree Seedlings of a Subtropical Coastal Dune Forest

Suppression of tree seedlings by the understory is an important ecological filter with implications for tree diversity and dynamics. In a greenhouse competition experiment, we used seedlings of four canopy species from coastal dune forest (Diospyros natalensis, Euclea racemosa, Sideroxylon inerme and Apodytes dimidiata) to examine the relative competitive effects of the dominant understory herb Isoglossa woodii on seedling performance. We manipulated I. woodii density, light and nutrient levels and measured growth responses. Total seedling biomass decreased with density of I. woodii. The magnitude of biomass suppression with competitor density was similar among tree species. Consequently there was no discernable hierarchy of competitive ranking among tree species. The relative growth rate of seedlings decreased at higher densities of I. woodii and increased at higher nutrient levels but was unaffected by variation in light conditions. Aboveground biomass decreased at higher densities of I. woodii and at higher light levels but increased at higher nutrient levels. Size asymmetric competition for light and nutrients may be the major driver of aboveground interactions between tree seedling and I. woodii. While tree species showed no hierarchy of competitive ability their seedlings exhibited equivalent responses to competition from an understory dominant, permitting species coexistence and the maintenance of species diversity.

Sprouting, succession and tree species diversity in a South African coastal dune forest

Abstract:Sprouting may play a significant role in maintenance of plant diversity where prevailing disturbance frequency and severity allows. When disturbance frequency and severity decrease, strong sprouters may be outcompeted. As a result, species composition and diversity may change. This study was carried out to investigate the relationship between sprouting, succession and species diversity in a coastal dune forest that currently suffers from low-severity, chronic disturbance due to sea winds and loose sand substrate. Historically, the site was occupied by shifting cultivators who left the site about 80 y ago. Data on trees that were at least 1.3 m tall from 42 sample plots measuring 20 × 20 m were used. The plots were ranked in order of advancement of succession using the first axis of Principal Components Analysis of forest structural variables. Regeneration pattern was examined using analysis of stem diameter frequency distribution. Abundance and regeneration of strong basal sprouters, incidence of basal sprouting and species diversity decreased with advancement of succession. Only a few species could regenerate under the canopy of late-successional sites. Basal sprouts decreased with advancement of succession whereas trunk sprouts increased. These results suggest that maintenance of high species diversity may need a level of disturbance that allows regeneration and maintenance of strong basal sprouters.

Development strategies for a coastal resort in Southern Mozambique

As part of a long-term monitoring project studying coastal development, a number of rapid scan field surveys and project interventions were conducted in Ponta do Ouro, at the southern border of Mozambique. Here we report on four issues in support of sustainable development: 1. tourism, 2. water supplies, 3. biodiversity and 4. governance. In our tourism study, we interviewed tourists and managers, and placed a researcher as an intern in a beach camp to assess management and interactions. Tourism brings in about $3 Million in revenue annually. The seasonal variability in demand requires managers to adjust staffing levels to ensure viability. Local staff are insensitive to profit margins – so tend to resist. Tourists come for the beach and scuba-diving, and do not want development. In our water study; quality, supply and demand were quantified at source and community level. It was found that the local lake will meet the needs of the entire population including tourists in peak season, if a small pay-for-service can be developed. Currently, most of the community hand-carry water from open wells. In the biodiversity study: botanical and zoological species richness was assessed in three different habitats: dune forest, grassland, and wetland; all within close proximity of the resort. In addition fish species on the coral reefs were identified during a scuba dive. It is suggested that zoological richness at order level of classification is the most useful indicator of bio-diversity and the need for conservation. In the governance study, a plan for the development of a town services department was presented to key stakeholders, and reactions were gauged. Leasehold taxes are diverted to the district and national level, leaving little for infrastructure improvements and services. It was found that the rural community members strongly support a local services department, and indicated a willingness to pay. However, most business managers have water and other services; and believe that ‘development’ would disturb the tranquillity and lead to an influx of migrant workers. Hence there is little consensus on the need for governance to be localised. Inevitably, mass tourism projects will provide in-direct benefits and services, as this coastal resort develops. Key words: Mozambique, botanical and zoological species, tranquillity tourism, water supplies, biodiversity and governance, Ponta do Ouro, development, community.

Predator satiation and recruitment in a mast fruiting monocarpic forest herb

Cross-pollination and satiation of seed predators are often invoked to explain synchronous mast reproduction in long-lived plants. However, explanations for the synchronous death of parent plants are elusive. The roles of synchronous seeding and post-reproductive mortality were investigated in a perennial monocarpic herb (Isoglossa woodii) in coastal dune forest in South Africa. Pre-dispersal seed predation and seed production were assessed by measuring fruit and seed set of inflorescences sprayed with insecticide or water and with no spray treatments. Seed predation was measured at different densities of I. woodii plants by monitoring removal rates of seed from the forest floor. The influence of adult plants on establishment of I. woodii seedlings was assessed by monitoring growth and survivorship of seedlings in caged and uncaged 1 × 1 m plots in understorey gaps and thickets. Fruit and seed set were similar between spray treatments. An I. woodii stem produced 767·8 ± 160·8 seeds (mean ± s.e.) on dune crests and 1359·0 ± 234·4 seeds on the foredune. Seed rain was greater on the foredune than in other topographic locations. Seed predation rates were 32 and 54 % on dune crests and in dune slacks, respectively, and decreased with seed abundance, number of inflorescences per stem and plant height. Seedling recruitment was greater beneath synchronously dying adult plants than in natural understorey gaps (no I. woodii). However, seedling growth rate beneath I. woodii mid-way through its life-cycle was less than in gaps, although survivorship was similar. The selective advantage of masting in I. woodii derives from satiation of both pre- and post-dispersal seed predators. In addition, post-seeding mortality of adult plants facilitates seedling establishment. Satiation of seed predators and the benefits of seedling establishment are strong drivers of the evolution of synchronous monocarpy in I. woodii.

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.

Landscape Composition Influences the Restoration of Subtropical Coastal Dune Forest

Successional processes should increase habitat complexity, and increase resources available for forest‐associated species. However, according to the theory of Island Biogeography, the size, amount of edge, and isolation of a habitat patch will influence the probability of successful colonization. If this is true for restoring patches of coastal dune forest, then restoration managers need to mitigate for spatial characteristics.We used patch occupancy models to assess correlations between the probability of forest birds and trees being present in a patch and patch characteristics that measured age, area, isolation, and the amount of edge. We surveyed birds and trees in an unmined coastal dune forest, remnant patches within the mine lease, and regenerating patches, some of which were being rehabilitated by a mining company. Contrary to expectations patch age only explained the patch occupancy of 6 of 21 birds and 11 of 25 woody plant species. Landscape spatial parameters, measuring edge, isolation, and area explained the patch occupancy of the remaining 15 birds and 14 woody plant species. However, responses to patch characteristics were varied and idiosyncratic. These varied responses may be related to species habitat affinities, dispersal abilities, and establishment constraints. For restoration to succeed, managers need to consider the spatial configuration of the landscape to facilitate colonization of rehabilitating patches.

Regeneration niches and functional traits of three common species in subtropical dune forest

South African coastal dune forests are young, highly disturbed subtropical communities where conventional models of forest dynamics may be challenged. We tested predictions from the gap-phase regeneration model by comparing seedlings of three common species representing contrasting regeneration strategies: Acacia karroo as a ruderal, Celtis africana as a coloniser of forest gaps, and Diospyros natalensis as a late-successional species. We grew seedlings under contrasting light and nitrogen levels in a greenhouse and in the field for 1 year to compare their growth and survival rates, allocation and photosynthetic traits. Species’ growth rates generally followed the expected order: Acacia>Celtis>Diospyros, but Acacia responded strongly to light and Celtis responded strongly to nitrogen, leading to cross-overs in growth rates. The plasticity of allocation and photosynthesis did not clearly differentiate the strategies, although it was greater in the light-demanding species. Acacia and Celtis tended to survive better in Acacia stands than in forest plots. Leaf-level light compensation points (LCPs) were similar for the three species in most conditions, but auxiliary data suggest Diospyros has a lower whole-plant LCP than Acacia. Growth rates and LCPs were lower than most of those reported for primary-forest species in the literature, suggesting an unusual degree of shade-tolerance in this habitat. We discuss reasons why variation in shade-tolerance may be less important here than in the prevailing model for forest regeneration and suggest other biotic factors that may help differentiate regeneration niches.

Ecological filtering by a dominant herb selects for shade tolerance in the tree seedling community of coastal dune forest

The regeneration niche is commonly partitioned along a gradient from shade-tolerant to shade-intolerant species to explain plant community assembly in forests. We examined the shade tolerance of tree seedlings in a subtropical coastal forest to determine whether the ecological filtering effect of a dominant, synchronously monocarpic herb (Isoglossa woodii) selects for species at either end of the light response continuum during the herb's vegetative and reproductive phases. Photosynthetic characteristics of seedlings of 20 common tree species and the herb were measured. Seedlings were grown in the greenhouse at 12-14% irradiance, and their light compensation points measured using an open-flow gas exchange system. The light compensation points for the tree species were low, falling within a narrow range from 2.1 ± 0.8 μmol m(-2) s(-1) in Celtis africana to 6.4 ± 0.7 μmol m(-2) s(-1) in Allophylus natalensis, indicating general shade tolerance, consistent with a high and narrow range of apparent quantum yield among species (0.078 ± 0.002 mol CO(2) mol(-1) photon). Rates of dark respiration were significantly lower in a generalist pioneer species (Acacia karroo) than in a forest pioneer (C. africana), or in late successional phase forest species. We argue that the general shade tolerance, and phenotypic clustering of shade tolerance, in many tree species from several families in this system, is a result of ecological filtering by the prevailing low light levels beneath the I. woodii understorey, which excludes most light-demanding species from the seedling community.

Causes of arrested succession in coastal dune forest

Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip-mining. However, although A. karroo establishes naturally, it forms monospecific stands, arresting forest succession for decades. This casts doubt on the efficacy of this restoration pathway. We investigated the causes of arrested succession. The seed and seedling banks of A. karroo stands and of forest at Cape Vidal, and three A. karroo stands (7–27 years old) on rehabilitated strip-mined dunes at nearby Richards Bay were examined. The establishment and growth of seedlings at Cape Vidal were also considered. The seed bank was larger and more diverse in forest, but the seedling bank was larger in Acacia stands. At Richards Bay, the size of the seed bank increased and the seedling bank decreased with Acacia stand age. Excluding mammalian herbivores in Acacia stands at Cape Vidal resulted in greater species richness and survival of naturally established seedlings, as well as two experimentally planted species. Neither seed dispersal nor seedling establishment limited recruitment of tree species in Acacia stands. Herbivory arrested forest succession by causing the differential mortality of seedlings. In contrast, at Richards Bay where there were few mammalian herbivores, the advanced regeneration in A. karroo stands converged on the diversity of nearby forests 29 years after restoration. Controlling herbivore access and seeding Acacia stands with forest species are site-specific options for preventing arrested succession when using the A. karroo successional pathway.