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Dissertation (PhD(Agric))--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet

Thesis (PhD (Soil Science))--Univ ersity of Stellenbosch, 2008. / The (semi-)arid Namaqualand region on the west coast of South Africa is wellknown for its spring flower displays. Due to the aridity of the region, soils research has lagged behind that of the more agriculturally productive parts of South Africa. However, rehabilitation efforts after the hundred or so years of mining, coupled with the increasing ecology and biodiversity research, have prompted a recent interest in Namaqualand soils as a substrate for plant growth. The area is also notable for the abundance of heuweltjies. Much of the previous heuweltjie-work focussed on biogenic aspects such as their spacing, origin and age, but although heuweltjies are in fact a soil feature, there have been few published studies on the soil forming processes within heuweltjies. However, the depositional history of the sediments on the Namaqualand coastal plain is well constrained, which is in stark contrast to the paucity of data on their subsequent pedogenesis. Given that the regolith has been subaerially exposed in some parts for much of the Neogene, the soil formation forms an important part of the sediments’ history. The primary aim of this thesis, therefore, was to examine the soil features of the Namaqualand coastal plain to further the understanding of pedogenesis in the region. The regolith of the northern Namaqualand coastal plain, often ten or more metres deep, comprises successive late Tertiary marine packages, each deposited during sea-level regression. The surface soil horizons formed from an aeolian parent material. The relatively low CaCO3 in the aeolian sands dictated the pedogenic pathway in these deposits. The non-calcareous pathway lead to clay-rich, redder apedal horizons that show a stronger structure with depth, and generally rest directly on marine sands via a subtle discontinuity that suggests pedogenesis continues into the underlying marine facies. The calcareous pathway lead to similar clay-rich, redder apedal B horizons, but which differ in that they are calcareous, and rest on a calcrete horizon often via a stoneline of rounded pebbles. Deeper in the profile, there is generally a regular alteration of sedimentary units, with the upper shoreface facies showing reddening, and the lower shoreface sands remaining pale. This seems to be a function of the grain size, since the upper shoreface materials are coarser, and the redder parts of the lower shoreface are also associated with slightly coarser sands. In some strata the oxidation of glauconite-rich sediments resulted in an orange colour. In an area with abundant heuweltjies, a strongly-cemented calcretized nest was present about 2 m deep within a silica cemented, locally calcareous dorbank profile. Vertical termite burrows are present up to 12 m deep, and appear to have been conduits for preferential vertical flow. Soil formation and termite activity is at least as old as the Last Interglacial. E horizons may have formed in a wetter Last Interglacial paleoclimate, but they are still active in the present day. The Namaqualand coastal plain, with its extensive areas of calcrete development, is almost a textbook setting for calcrete development by inorganic processes. However, these calcretes also show microscale biogenic features. These include M rods, MA rods, and fungal filaments. Abiotic alpha-fabric seems dominant in mature calcrete horizons, and beta-fabric in calcareous nodules in a calcic B horizon above calcrete. The apparent absence of Mg-calcite and dolomite, and abundance of sepiolite in the calcretes of coastal Namaqualand suggests that these Mg-rich clay minerals are the main Mg-bearing phase. Deformation (pseudo-anticlines) in the calcrete appear to result primarily from the displacive effect of calcite crystallization. Although evidence of shrink/swell behaviour is present in the form of accommodating planes, it does not appear to be as volumetrically significant as displacive calcite. Indurated light-coloured horizons that resembled calcrete but are non- to mildly calcareous, break with a conchoidal fracture, resist slaking in both acid and alkali, turn methyl-orange purple, and show a bulk-soil sepiolite XRD peak are similar to palygorskite-cemented material (‘palycrete’) from Spain and Portugal, and so were tentatively named ‘sepiocrete’. Sepiolite and palygorskite are often reported from arid region soils but there has been no recorded cementation of soils by sepiolite. The degree of induration in some of these horizons suggest that amorphous silica could play a role in cementation, and so this thesis compares the two silica-cemented horizons encountered in Namaqualand (silcrete and dorbank (petroduric)) to these ‘sepiocrete’ horizons. Both silica and sepiolite are present in the matrix, although the degree to which silica and sepiolite dominate seems to vary even within same horizon. It seems most probable that both contribute to the structural properties of the horizon. Sepiolitic horizons do not form a diagnostic horizon in the World Reference Base, Soil Taxonomy, or the South African system. To fit the existing soil classification schemes, the terms ‘sepiolitic’ and ‘petrosepiolitic’ (in the same sense as ‘calcic’ and ‘petrocalcic’) would be appropriate. The term ‘sepiolitic’ should be used for horizons which: contain sepiolite in amounts great enough for it to be detected by XRD in the bulk soil, peds (a fractured surface and not just the cutan) cling strongly to the wetted tongue, and methyl orange turns from orange to purple-pink over most of a fragmented surface. The term can be easily be applied as a adjective to other hardpans where sepiolite is significant but not necessarily cementing, such as ‘sepiolitic’ petrocalcic/petroduric. If the horizon is in addition to the above criteria cemented to such a degree that it will slake neither in acid (so cannot be classified as petrocalcic) nor in alkali (and so cannot be classified as petroduric) then the term ‘petrosepiolitic’ would be appropriate. The ‘sepiolitic’ criteria distinguish the ‘petrosepiolitic’ horizon from a ‘silcrete’, a silica-cemented horizon which does not fit the definition of petroduric. Sepiolite is more prominent than palygorskite in the XRD traces. The <0.08 μm fraction is the only size fraction where palygorskite could be detected before acetate treatment. It is unlikely that these fibrous clay minerals are inherited from either the marine or aeolian parent materials, they appear to be pedogenic in origin. Sepiolite and palygorskite are associated with the presence of calcite in the soil profile. Trends in MgO, Al2O3 and SiO2 show that the soil clay fractions lie on a mixing line between sepiolite and mica end-members, with a contribution from smectite, and is consistent with the XRD and TEM results. There is a good correlation between Fe2O3 and TiO2, which can be attributed to the ubiquitously presence of mica. There was no TEM evidence of fibrous mineral degradation to sheet silicates, nor for the evolution of mica laterally to a fibrous mineral. SEM analyses show that much of the sepiolite/palygorskite occurs as fringed sheets, but higher magnification often revealed these sheets to be composed of fibres. These are found coating (rather than evolving from) mica/illite particles, as free-standing mats, and are common on the grain-side of cutans. Some of these textures suggest illuviation of the fibrous clay minerals, but another explanation may be that sites such as that immediately adjacent to silicate grains have the highest concentration of silica for their formation. There was no conclusive evidence for or against the presence of kerolite in the clay fraction, although it does not appear to be a dominant phase in the <2 μm fraction. The hypothesis was that the permeable upper horizons in Namaqualand soils constitute a shallow ephemeral aquifer, which can be considered the pedogenic analogue of the saline lake environments in which sepiolite typically forms. The chemical evolution of the soil solution and clay mineral genesis could therefore be considered in the same terms as the geochemical evolution of closed-basin brines. The Namaqualand coastal plain, like other maritime areas, shows a trend of decreasing pH, increasing Ca and increasing Mg with increasing evaporation. This can be explained by their seawater-influenced initial ratios, and is consistent with the ‘chemical divides’ of the Hardie-Eugster model of brine evolution. Halite remains undersaturated at all concentrations in the saturated paste extracts. At higher concentrations, gypsum reaches saturation, and sulfate is removed from solution. H4SiO4 activity remains unchanged for all levels of evaporation and pH. Calcite remains close to saturation, and is only dependent on the HCO−3 activity and pH for the range of Cl− activity encountered. Most of the soils for which there is a positive sepiolite identification show a positive sepiolite saturation index. The sepiolite saturation index is independent of Mg2+ and H4SiO4 and only increases with increasing pH. Evidence of the pH control on sepiolite saturation is that sepiolite is commonly associated with calcareous horizons. Sepiolite precipitation is therefore more likely to be triggered when a solution encounters a pH barrier than by the concentration of ions by evaporation. The effect of a pH change on the sepiolite saturation index is much greater than that of the effect on calcite. The marine-influenced high Mg coupled with the Hardie- Eugster model of brine evolution offers an explanation for sepiolite-dominance at the coast, and palygorskite-dominance inland. Coastal areas, unlike continental areas, have Mg>HCO−3 initially, which results in an increasing Mg trend with evaporation during the precipitation of sepiolite according to the Hardie-Eugster scheme. The result is that after sepiolite precipitation is initiated by a geochemical pH-barrier, Mg levels will rise causing the increasing (Mg+Si)/Al ratio to continue to favour sepiolite precipitation. This suggests that once sepiolite has begun to precipitate, the subsequent salinity with its accompanying Mg increase makes substantial palygorskite formation unlikely to follow. The hardpan horizons in heuweltjies commonly grade from a ‘sepiolitic’ petrocalcic in the centre through ‘sepiolitic’/‘petrosepiolitic’ to the petroduric horizon on the edges. Noteworthy sepiolite-related pedofeatures in the calcrete include ‘ooids’ with successive sepiolite (hydrophilic and therefore a precipitational substrate) and micrite/acicular calcite layers in the coatings; and limpid yellow nodules with pseudo-negative uniaxial interference figures. They superficially resemble the spherulites in the fresh termite frass. Their fibrous nature and low birefringence, together with the low Ca, high Mg, Si composition, and molar Mg/Si ratios consistent with sepiolite. The pedogenesis of the hardpans in the heuweltjie is proposed to be as follows: enrichment of cations such as Ca and Mg in the heuweltjie centre caused by termite foraging results in calcite and clay authigenesis in the centre of the heuweltjie, leaving the precipitation of pure silica to occur on the periphery. The decaying organic matter concentrated in the centre of the mound by the termites is sufficient to supply the components for calcite precipitation in the centre of the heuweltjie. Following calcite precipitation, the pH is suitable for sepiolite precipitation. The movement of the Mg-Si enriched water downslope, coupled with the decrease in HCO−3 and increase in Mg2+ due to sepiolite precipitation, allows for the precipitation of the ‘sepiolitic’ zone on the outer side of the calcrete, and extend beyond the calcrete in some heuweltjies. The Namaqualand coastal plain is well positioned for further work on its regolith, particularly because of the mining excavations which provide excellent exposures of well-defined layers of the regolith down to bedrock. Soil formation and termite activity is at least as old as the Last Interglacial, and so more detailed work would further the understanding of the subaerial alteration history in southern Africa, as well as providing better-constrained information on the Namaqualand soils that can be used by land-use management and biosphere studies.

Anthropogenic inputs of nutrients and sediment are a widespread problem in U.S. streams causing localized impairment and contributing to eutrophication of coastal habitats. Sediments and dissolved nutrients interact through diverse processes including ion exchange, sorption and biotic assimilation by particle-bound bacteria. This study examined the effects of sediment re-suspension on nutrient partitioning in lab microcosms using fine benthic matter collected from two Virginia Coastal Plain streams. Kimages Creek was recently restored following dam removal and was characterized by large deposits of legacy sediments. Courthouse Creek was characterized by sandy substrates typical of Coastal Plain streams. Sediment characteristics differed between the two sites and were influenced by discharge. Net nutrient release rates were similar between streams though reactivity of Courthouse Creek sediments was greater than that of Kimages Creek. Equilibrium Phosphate Concentrations calculated for each site show that fine sediments at Kimages Creek have resulted in higher phosphorus retention potential.

As restingas constituem um complexo de tipos vegetacionais que ocorrem sobre cordões arenosos ao longo da costa brasileira. Sua diversidade fisionômica, florística e estrutural contrasta com grande número de fatores ambientais limitantes, incluindo a disponibilidade de água e nutrientes. O objetivo deste trabalho foi comparar a dinâmica do nitrogênio, bem como os atributos foliares relacionados ao uso do nitrogênio e da água, em três formações vegetais inseridas no \"complexo vegetacional da restinga\", as quais ocorrem ao longo de um gradiente de precipitação que varia de cerca de 800 a 2.600 mm anuais. Para tanto, foram avaliados parâmetros relacionados à ciclagem do nitrogênio e alguns atributos foliares funcionais relacionados às estratégias de uso de água e nutrientes nas principais espécies botânicas que nelas se desenvolvem. O primeiro capítulo compara a variação na disponibilidade e na composição isotópica de nitrogênio (?15N) nestas três formações de restinga que ocorrem sob regimes hídricos distintos. Foram analisados e comparados parâmetros relacionados à disponibilidade e às taxas de transformação do nitrogênio nos solos e sua assimilação e uso pelas plantas. Os solos das três áreas, arenosos e ácidos, apresentam deficiência nutricional, sendo o nitrogênio amoniacal (N-NH4 +) a principal forma disponível. Suas taxas líquidas de transformação foram baixas e variáveis, com prevalência de imobilização, sobretudo nas restingas sob menor precipitação média anual (PMA). Na vegetação, o aumento na PMA refletiu em maiores concentrações foliares de nitrogênio e fósforo, juntamente com menores razões C/N e massas foliares por unidade de área (MFA). Os valores de ?15N foliar foram baixos e variáveis, ocorrendo enriquecimento gradual em 15N com o aumento na PMA. As concentrações foliares de nitrogênio e fósforo, bem como a MFA e a razão C/N mostraram-se diretamente relacionadas; já a MFA relacionou-se inversamente à concentração foliar de nitrogênio. O aumento da PMA ao longo do gradiente, juntamente com variações nas condições edáficas e nas comunidades bióticas, pareceu contribuir para o aumento da disponibilidade de nitrogênio, refletindo na redução gradual da conservação do mesmo. O segundo capítulo compara as restingas quanto aos atributos foliares de gêneros por elas compartilhados. As maiores diferenças foram observadas entre os gêneros das restingas seca e úmida, com a última apresentando maiores concentrações de nitrogênio e fósforo foliares e menor razão C/N. Seus gêneros ainda apresentaram menores MFA e composição isotópica de carbono (?13C), além de maior ?15N que aqueles das restingas seca e intermediária. As relações entre os atributos foliares dos pares congenéricos das restingas seca e úmida, assim como dos contrastes filogeneticamente independentes (CFIs), mostraram que a concentração de nitrogênio relaciona-se diretamente à concentração de fósforo e inversamente ao ?15N, enquanto que a concentração de fósforo relaciona-se diretamente com o ?15N e inversamente com a razão C/N. A grande diferença entre as restingas e a similaridade nas correlações - utilizando-se tanto os pares congenéricos e os CFIs - fortalece a distinção quanto à economia foliar da restinga úmida e as demais, enfatizando a importância das condições ambientais no ajuste dos atributos analisados, independente do grau de parentesco filogenético das espécies. / Restingas are consisted of a complex of vegetation types that occur on sand ridges along the Brazilian coast. Their physiognomic, floristic and structural diversity contrasts with a large number of environmental limiting factors, including the availability of water and nutrients. The aim of this study was to compare the dynamics of nitrogen, as well as leaf traits related to the use of nitrogen and water at three vegetation types inserted in the \"restinga vegetation complex\", and occur along a precipitation gradient ranging from about 800 to 2600 mm annually. Accordingly, parameters related to nitrogen cycling were assessed, as well as some functional leaf traits related to strategies for use of water and nutrients from the main plant species that occur therein. The first chapter compares the variation in nitrogen availability and its isotopic composition (?15N) at these three restinga formations that occur under different water regimes. It was analyzed and compared parameters regarding nitrogen availability and transformation rates in soils, and also its uptake and use by plants. The sandy and acidic soils of the three areas have high nutritional deficiency, and ammoniacal nitrogen (N-NH4 +) is the major available form. Its net rates of transformation were low and variable, with the prevalence of immobilization, especially at the restingas under lower mean annual precipitation (MAP). At the vegetation, MAP increase reflected in higher leaf nitrogen and phosphorus concentrations, along with lower C/N ratios and leaf mass per area (LMA). Leaf ?15N values were low and variable, observing gradual 15N-enrichment with MAP increase. Nitrogen and phosphorus leaf concentrations, as well as LMA and C/N ratio were directly related, whereas LMA was inversely related to leaf nitrogen concentration. The increase in MAP along the gradient, jointly to variations in soil conditions and biota, seemed to contribute to an increment on nitrogen availability, reflecting in the gradual reduction of its conservation. The second chapter compares the restingas in relation to leaf traits of genera shared by them. The largest differences were observed between genera from dry and wet restingas, with the latter having higher leaf nitrogen and phosphorus concentrations and lower C/N ratio. Their genera also had lower LMA and carbon isotopic composition (?13C), as well as greater ?15N than those from dry and intermediate restingas. Relationships between leaf traits of the congeneric pairs from dry and wet restingas, and also of the phylogenetically independent contrasts (PICs), showed that nitrogen concentration is directly related to phosphorus and inversely to ?15N, whilst phosphorus concentration is directly related to ?15N and inversely to C/N ratio. The bulky difference amongst restingas, and the similarity in correlations - using both congeneric pairs and PICs - reinforces the distinction regarding leaf economy between wet restinga and the others, emphasizing the importance of environmental conditions on the adjustment of the analyzed leaf traits, apart from of the degree of species phylogenetic relatedness.

Plants established next to other plants may have their performance positively or negatively affected by their neighbors, which characterizes interactions of facilitation and competiton, respectively. Facilitation and competition, however, may occur simultaneously and understanding which one predominates in pairwise interactions under different ecological contexts is important to comprehend the structure of plant communities. According to the stress-gradient hypothesis (SGH), facilitation is expected to prevail in more severe environments, but the outcome of interactions may change depending on features of interacting individuals, such as life form and ontogenetic stages. As harsh conditions and high diversity of plant life-forms are characteristic of coastal dunes, the amount of studies about plant interactions has been rapidly increasing in these ecosystems, with apparent divergent conclusions. However, until now, there is not a systematic and quantitative synthesis about the factors affecting the net outcome of facilitation and competition in these ecosystems. We conducted a meta-analysis to investigate the effects of environmental stress and the simultaneous influence of plant life form and ontogenetic stage on the outcome of facilitation and competition in coastal dunes around the world. We used four performance measures to estimate the outcome of interactions: abundance, survival, growth, and reproduction. Contrary to what was expected by SGH, we found that negative impacts of neighbors on plant reproduction increase towards more arid conditions, but this effect was not observed for other performance measures. Our results also indicate that woody neighbors facilitate the survival of woody seedlings and the reproduction of herbs, while herbaceous neighbors facilitate the growth of other herbaceous plants. Overall, the outcome of plant interactions in coastal dunes depends on the performance variable measured and on both environmental conditions and plant features, indicating an interaction between these factors. Such interaction and different mechanisms underlying facilitation and competition should be more investigated in the future. The global scale of our meta-analysis supports generalization of important processes of succession and conservation in coastal dunes. Benefits of woody neighbors to the survival of woody seedlings corroborate the concept of successional feedbacks in the beach-inland physiognomic gradient, and our results reinforce the use of nurse plants in coastal dunes as a valuable tool to restoration of these endangered ecoystems / Plantas estabelecidas nas proximidades de outras plantas podem ter sua performance afetada positiva ou negativamente por seus vizinhos, caracterizando, respectivamente, interações de facilitação e competição. Considerando que ambas as interações podem ocorrer de forma simultânea, compreender o predomínio de cada uma delas em diferentes contextos ecológicos é fundamental para o entendimento da estrutura de comunidades vegetais. De acordo com a hipótese do gradiente de estresse (HGE), a facilitação tende a predominar em ambientes mais severos, mas o balanço das interações depende ainda das características dos indivíduos envolvidos, como forma de vida e estágio ontogenético. Como condições ambientais severas e alta diversidade de formas de vida vegetais são características de planícies costeiras, o número de estudos investigando interações entre plantas tem aumentado rapidamente nesses ambientes, com conclusões aparentemente divergentes. No entanto, ainda não há uma síntese sistemática e quantitativa dos fatores que afetam o balanço entre facilitação e competição nesses ecossistemas. Nós realizamos uma meta-análise em escala global para investigar os efeitos do estresse ambiental e a influência simultânea da forma de vida e do estágio ontogenético das plantas sobre o balanço entre facilitação e competição em planícies costeiras. Utilizamos quatro variáveis de performance para estimar o balanço de interações: abundância, sobrevivência, crescimento e reprodução. Ao contrário do predito pela HGE, encontramos que impactos negativos de plantas vizinhas sobre a reprodução de outras plantas aumentam com a aridez do ambiente, mas que esse efeito não é observado para outras variáveis de performance. Nossos resultados também mostram que vizinhos lenhosos facilitam a sobrevivência de plântulas lenhosas e a reprodução de plantas herbáceas, enquanto vizinhos herbáceos facilitam o crescimento de outras herbáceas. De modo geral, o balanço das interações depende da variável de performance medida e tanto de condições ambientais quanto de características das plantas, indicando que esses fatores interagem. Tal interação e os diferentes mecanismos subjacentes à facilitação e à competição devem ser melhor investigados no futuro. A maior sobrevivência de plântulas lenhosas na presença de vizinhos lenhosos corrobora a ideia de retroalimentação positiva no processo sucessional que caracteriza o gradiente fisionômico da praia ao interior. Nossos resultados também reforçam o potencial do uso de plantas-berçários como ferramenta para restauração de planícies costeiras degradadas

As restingas constituem um complexo de tipos vegetacionais que ocorrem sobre cordões arenosos ao longo da costa brasileira. Sua diversidade fisionômica, florística e estrutural contrasta com grande número de fatores ambientais limitantes, incluindo a disponibilidade de água e nutrientes. O objetivo deste trabalho foi comparar a dinâmica do nitrogênio, bem como os atributos foliares relacionados ao uso do nitrogênio e da água, em três formações vegetais inseridas no \"complexo vegetacional da restinga\", as quais ocorrem ao longo de um gradiente de precipitação que varia de cerca de 800 a 2.600 mm anuais. Para tanto, foram avaliados parâmetros relacionados à ciclagem do nitrogênio e alguns atributos foliares funcionais relacionados às estratégias de uso de água e nutrientes nas principais espécies botânicas que nelas se desenvolvem. O primeiro capítulo compara a variação na disponibilidade e na composição isotópica de nitrogênio (?15N) nestas três formações de restinga que ocorrem sob regimes hídricos distintos. Foram analisados e comparados parâmetros relacionados à disponibilidade e às taxas de transformação do nitrogênio nos solos e sua assimilação e uso pelas plantas. Os solos das três áreas, arenosos e ácidos, apresentam deficiência nutricional, sendo o nitrogênio amoniacal (N-NH4 +) a principal forma disponível. Suas taxas líquidas de transformação foram baixas e variáveis, com prevalência de imobilização, sobretudo nas restingas sob menor precipitação média anual (PMA). Na vegetação, o aumento na PMA refletiu em maiores concentrações foliares de nitrogênio e fósforo, juntamente com menores razões C/N e massas foliares por unidade de área (MFA). Os valores de ?15N foliar foram baixos e variáveis, ocorrendo enriquecimento gradual em 15N com o aumento na PMA. As concentrações foliares de nitrogênio e fósforo, bem como a MFA e a razão C/N mostraram-se diretamente relacionadas; já a MFA relacionou-se inversamente à concentração foliar de nitrogênio. O aumento da PMA ao longo do gradiente, juntamente com variações nas condições edáficas e nas comunidades bióticas, pareceu contribuir para o aumento da disponibilidade de nitrogênio, refletindo na redução gradual da conservação do mesmo. O segundo capítulo compara as restingas quanto aos atributos foliares de gêneros por elas compartilhados. As maiores diferenças foram observadas entre os gêneros das restingas seca e úmida, com a última apresentando maiores concentrações de nitrogênio e fósforo foliares e menor razão C/N. Seus gêneros ainda apresentaram menores MFA e composição isotópica de carbono (?13C), além de maior ?15N que aqueles das restingas seca e intermediária. As relações entre os atributos foliares dos pares congenéricos das restingas seca e úmida, assim como dos contrastes filogeneticamente independentes (CFIs), mostraram que a concentração de nitrogênio relaciona-se diretamente à concentração de fósforo e inversamente ao ?15N, enquanto que a concentração de fósforo relaciona-se diretamente com o ?15N e inversamente com a razão C/N. A grande diferença entre as restingas e a similaridade nas correlações - utilizando-se tanto os pares congenéricos e os CFIs - fortalece a distinção quanto à economia foliar da restinga úmida e as demais, enfatizando a importância das condições ambientais no ajuste dos atributos analisados, independente do grau de parentesco filogenético das espécies. / Restingas are consisted of a complex of vegetation types that occur on sand ridges along the Brazilian coast. Their physiognomic, floristic and structural diversity contrasts with a large number of environmental limiting factors, including the availability of water and nutrients. The aim of this study was to compare the dynamics of nitrogen, as well as leaf traits related to the use of nitrogen and water at three vegetation types inserted in the \"restinga vegetation complex\", and occur along a precipitation gradient ranging from about 800 to 2600 mm annually. Accordingly, parameters related to nitrogen cycling were assessed, as well as some functional leaf traits related to strategies for use of water and nutrients from the main plant species that occur therein. The first chapter compares the variation in nitrogen availability and its isotopic composition (?15N) at these three restinga formations that occur under different water regimes. It was analyzed and compared parameters regarding nitrogen availability and transformation rates in soils, and also its uptake and use by plants. The sandy and acidic soils of the three areas have high nutritional deficiency, and ammoniacal nitrogen (N-NH4 +) is the major available form. Its net rates of transformation were low and variable, with the prevalence of immobilization, especially at the restingas under lower mean annual precipitation (MAP). At the vegetation, MAP increase reflected in higher leaf nitrogen and phosphorus concentrations, along with lower C/N ratios and leaf mass per area (LMA). Leaf ?15N values were low and variable, observing gradual 15N-enrichment with MAP increase. Nitrogen and phosphorus leaf concentrations, as well as LMA and C/N ratio were directly related, whereas LMA was inversely related to leaf nitrogen concentration. The increase in MAP along the gradient, jointly to variations in soil conditions and biota, seemed to contribute to an increment on nitrogen availability, reflecting in the gradual reduction of its conservation. The second chapter compares the restingas in relation to leaf traits of genera shared by them. The largest differences were observed between genera from dry and wet restingas, with the latter having higher leaf nitrogen and phosphorus concentrations and lower C/N ratio. Their genera also had lower LMA and carbon isotopic composition (?13C), as well as greater ?15N than those from dry and intermediate restingas. Relationships between leaf traits of the congeneric pairs from dry and wet restingas, and also of the phylogenetically independent contrasts (PICs), showed that nitrogen concentration is directly related to phosphorus and inversely to ?15N, whilst phosphorus concentration is directly related to ?15N and inversely to C/N ratio. The bulky difference amongst restingas, and the similarity in correlations - using both congeneric pairs and PICs - reinforces the distinction regarding leaf economy between wet restinga and the others, emphasizing the importance of environmental conditions on the adjustment of the analyzed leaf traits, apart from of the degree of species phylogenetic relatedness.

Plants established next to other plants may have their performance positively or negatively affected by their neighbors, which characterizes interactions of facilitation and competiton, respectively. Facilitation and competition, however, may occur simultaneously and understanding which one predominates in pairwise interactions under different ecological contexts is important to comprehend the structure of plant communities. According to the stress-gradient hypothesis (SGH), facilitation is expected to prevail in more severe environments, but the outcome of interactions may change depending on features of interacting individuals, such as life form and ontogenetic stages. As harsh conditions and high diversity of plant life-forms are characteristic of coastal dunes, the amount of studies about plant interactions has been rapidly increasing in these ecosystems, with apparent divergent conclusions. However, until now, there is not a systematic and quantitative synthesis about the factors affecting the net outcome of facilitation and competition in these ecosystems. We conducted a meta-analysis to investigate the effects of environmental stress and the simultaneous influence of plant life form and ontogenetic stage on the outcome of facilitation and competition in coastal dunes around the world. We used four performance measures to estimate the outcome of interactions: abundance, survival, growth, and reproduction. Contrary to what was expected by SGH, we found that negative impacts of neighbors on plant reproduction increase towards more arid conditions, but this effect was not observed for other performance measures. Our results also indicate that woody neighbors facilitate the survival of woody seedlings and the reproduction of herbs, while herbaceous neighbors facilitate the growth of other herbaceous plants. Overall, the outcome of plant interactions in coastal dunes depends on the performance variable measured and on both environmental conditions and plant features, indicating an interaction between these factors. Such interaction and different mechanisms underlying facilitation and competition should be more investigated in the future. The global scale of our meta-analysis supports generalization of important processes of succession and conservation in coastal dunes. Benefits of woody neighbors to the survival of woody seedlings corroborate the concept of successional feedbacks in the beach-inland physiognomic gradient, and our results reinforce the use of nurse plants in coastal dunes as a valuable tool to restoration of these endangered ecoystems / Plantas estabelecidas nas proximidades de outras plantas podem ter sua performance afetada positiva ou negativamente por seus vizinhos, caracterizando, respectivamente, interações de facilitação e competição. Considerando que ambas as interações podem ocorrer de forma simultânea, compreender o predomínio de cada uma delas em diferentes contextos ecológicos é fundamental para o entendimento da estrutura de comunidades vegetais. De acordo com a hipótese do gradiente de estresse (HGE), a facilitação tende a predominar em ambientes mais severos, mas o balanço das interações depende ainda das características dos indivíduos envolvidos, como forma de vida e estágio ontogenético. Como condições ambientais severas e alta diversidade de formas de vida vegetais são características de planícies costeiras, o número de estudos investigando interações entre plantas tem aumentado rapidamente nesses ambientes, com conclusões aparentemente divergentes. No entanto, ainda não há uma síntese sistemática e quantitativa dos fatores que afetam o balanço entre facilitação e competição nesses ecossistemas. Nós realizamos uma meta-análise em escala global para investigar os efeitos do estresse ambiental e a influência simultânea da forma de vida e do estágio ontogenético das plantas sobre o balanço entre facilitação e competição em planícies costeiras. Utilizamos quatro variáveis de performance para estimar o balanço de interações: abundância, sobrevivência, crescimento e reprodução. Ao contrário do predito pela HGE, encontramos que impactos negativos de plantas vizinhas sobre a reprodução de outras plantas aumentam com a aridez do ambiente, mas que esse efeito não é observado para outras variáveis de performance. Nossos resultados também mostram que vizinhos lenhosos facilitam a sobrevivência de plântulas lenhosas e a reprodução de plantas herbáceas, enquanto vizinhos herbáceos facilitam o crescimento de outras herbáceas. De modo geral, o balanço das interações depende da variável de performance medida e tanto de condições ambientais quanto de características das plantas, indicando que esses fatores interagem. Tal interação e os diferentes mecanismos subjacentes à facilitação e à competição devem ser melhor investigados no futuro. A maior sobrevivência de plântulas lenhosas na presença de vizinhos lenhosos corrobora a ideia de retroalimentação positiva no processo sucessional que caracteriza o gradiente fisionômico da praia ao interior. Nossos resultados também reforçam o potencial do uso de plantas-berçários como ferramenta para restauração de planícies costeiras degradadas