Dracaena cinnabari, the dominant endemic tree of Socotra Island (Yemen), is in serious decline. The effect this will have on the island's plant diversity remains unknown. This dissertation aimed to assess the possible role of Dracaena as a nurse plant by identifying plants associated with Dracaena understorey and by assessing the importance of Dracaena for maintaining plant diversity. In the first part of the dissertation, relevés sampled in Dracaena understorey and in open sites to record the number of individuals of vascular plants were analysed. Species richness and composition were compared between understorey and open sites, and species associated with each of these habitats were identified. Additionally, the effects of shading and leaf litter on species richness and abundance were analysed. The second part of dissertation focused on mature woody species composition of Dracaena stands and investigated spatial relations between Dracaenas and other mature woody plants. The last part of the dissertation aimed to assess the fine scale spatial relationships between stones and plants in Dracaena stands and evaluated the differences of these relationships between understorey and open sites. The results confirmed the Dracaena role as an important nurse plant. The tree enhances the environmental heterogeneity and its presence creates two distinct microhabitats (understorey and open sites). The understorey habitat, compared with open site, harbours higher number of species, including endemics. The species in Dracaena stands can be classified as either understorey specialists, open-site specialists or generalists. The decline of Dracaena will negatively affect plant diversity, will reduce abundance of rare endemic plants and will lead to homogenization of vegetation. As no other tree species exists which could replace the Dracaena, our findings underline the importance of conservation efforts to preserve Dracaena stands on Socotra and identify this species as a key umbrella species whose conservation would ensure also the protection of a number of other associated species.

Nas últimas duas décadas estudos empíricos têm demonstrado a importância da facilitação entre plantas, especialmente sob condições ambientais severas. Tal constatação inspirou a proposição da hipótese do gradiente de estresse (HGE), um modelo conceitual que prediz que a freqüência relativa entre facilitação e competição deve variar inversarmente ao longo de um gradiente de estresse, sendo a facilitação mais comum sob condições extremamente severas. As restingas (ou planícies costeiras arenosas) são ambientes caracterizados por condições ambientais limitantes para o desenvolvimento das plantas, fazendo-se então um ambiente propício para o predomínio de facilitação. Além disso, o gradiente de intensidade de estresse e perturbação relacionado à distância do mar faz ainda deste ecossistema um modelo ideal para testar a HGE. O objetivo geral desta tese foi estudar facilitação entre plantas de restinga através dos seguintes objetivos específicos: i) Apresentar uma revisão sistemática dos trabalhos já publicados sobre facilitação entre plantas de restinga em escala mundial, assim como investigar, através de meta-análise, que fatores influenciam a presença e intensidade da facilitação entre plantas; ii) Testar se os padrões de associação espacial entre árvores adultas isoladas e plantas de distintas formas de vida se alteram ao longo de um gradiente ambiental praia-interior, conforme esperado pela HGE; iii) Testar a HGE através de experimentos de campo envolvendo transplante da espécie potencialmente beneficiada, assim como manipulação de recurso limitante ao longo do gradiente praia-interior; iv) Testar se o sombreamento é um mecanismo de facilitação na restinga. Para os testes empíricos (objetivos ii-iv), as observações e experimentos foram realizados em um gradiente de restinga arbustiva do Parque Estadual da Ilha do Cardoso (PEIC) localizado em Cananéia, São Paulo, Brasil. Os resultados indicaram que: i) diversas condições e aspectos da facilitação entre plantas de planícies costeiras têm sido pouco exploradas, como por exemplo, há poucos estudos nas regiões tropicais e poucos estudos que consideraram estágios de vida de semente ou de adultos da espécie alvo da interação. Dentre os estudos publicados, fatores como precipitação, região geográfica e estágio de vida da planta alvo definitivamente influenciam a existência e magnitude da facilitação entre plantas de restinga; ii) para a maioria das formas de vida, o padrão de associação espacial com árvores adultas não se altera ao longo do gradiente ambiental conforme predito pela HGE. Apenas árvores jovens apresentam associação espacial positiva com árvores adultas, o que sugere efeito de planta-berçário entre árvores da restinga arbustiva; iii) tanto a intensidade quanto a importância da interação entre árvores foram negativamente relacionadas ao estresse, rejeitando então a proposição clássica da HGE. Além disso, o sinal da interação foi dependente do estágio de vida da espécie alvo, uma vez que a presença do adulto vizinho foi predominantemente negativa sobre o estágio de semente mas positiva sobre a sobrevivência das plântulas; iv) o efeito de planta-berçário entre árvores da restinga é, pelo menos parcialmente, explicado pelo sombreamento gerado pela copa da espécie facilitadora. Dessa forma, concluimos que a facilitação via sombreamento é uma interação relevante entre árvores adultas e jovens na restinga. Entretanto, ao contrário do previsto pela HGE, o efeito de planta-berçário tende a ser mais intenso em condições ambientais mais amenas. Estes resultados sugerem a existência de um ciclo de retroalimentação positiva gerado a partir do estabelecimento das primeiras árvores isoladas. Este ciclo positivo apresenta implicações importantes para a compreensão da organização estrutural da vegetação da restinga arbustiva, em que moitas de espécies lenhosas estão distribuídas em uma matriz predominantemente herbácea. Adicionalmente, a facilitação entre árvores e sua dependência em relação às condições ambientais locais devem ser mecanismos essenciais para explicar o gradiente de fisionomias vegetacionais sobre os cordões arenosos, a qual inicia-se em uma vegetação aberta e termina em uma floresta, assim como para entender a dinâmica temporal desta vegetação ao longo do tempo. Por fim, nossos resultados também apresentam implicações práticas, relacionadas ao uso potencial de plantas-berçário e mímicos de sombra como técnicas de manejo para aumentar o estabelecimento de mudas transplantadas em projetos de restauração de restingas arbustivo-arbóreas. / In the last two decades, empirical studies have shown the importance of plant interaction, especially under harsh environmental conditions. These observations led to the proposition of the stress gradient hypothesis (SGH), a conceptual model predicting that the relative frequency of facilitation and competition will vary inversely across gradients of stress, with facilitation being the dominant interaction under highly stressful conditions. Coastal dunes (known as restinga in Brazil) are environments with limiting conditions for plant performance, which makes them suitable for facilitation predominance among plant interactions. Furthermore, there is a gradient of stress and disturbance related to the presence of the sea, making this environment an ideal model to test the SGH. The general goal of this thesis was to study plant facilitation in coastal dunes with the following specific objectives: i) to perform a systematic review of published studies on plant facilitation in costal dunes around the world as well as to investigate, using a meta-analysis approach, factors that affect the existence and magnitude of plant facilitation; ii) to test if spatial association between isolated adult trees and understory plants of distinct life forms changes along a beach-to-inland gradient, as expected by the SGH; iii) to test the SGH through field experiments involving target species transplantation coupled with manipulation of limiting resource along a beach-to-inland gradient; iv) to test if shading is a facilitation mechanism in coastal dunes. The empirical tests (objectives ii-iv) were conducted in an open scrub coastal dune located at Ilha do Cardoso State Park, Cananéia, São Paulo, Brazil. Our results indicate that: i) several aspects and conditions of plant facilitation in coastal dunes have been little studied. For instance, few studies were conducted in tropical regions and few of them studied the effect of neighbor presence on seed and adult life stage of the target species. Precipitation, geographic region and target life stage affect the existence and magnitude of plant facilitation; ii) For most of the understory life forms, the spatial association between adult trees and understor plants do not change along the environmental gradient as predicted by the SGH. Among all life forms, only young trees were positively associated with adult trees, suggesting a nurse plant effect among trees in coastal dunes; iii) Both intensity and importance of the interaction among trees were negatively related to stress, rejecting the classic proposition of the SGH. Furthermore, the signal of the net interaction depends on the life stage of the target species since the adult neighbor had mostly negative effects on target species seeds but positive effects on seedlings survival; iv) The nurse plant effect among trees was, at least partly, due to the shading provided by the facilitator crown. Overall, we concluded that facilitation driven by shade is a relevant interaction between adult and young trees in the studied coastal dune. However, contrary to what was expected by the early propositions of the SGH, the observed nurse plant effect tends to be more intense under mild conditions. Our results suggest a positive feedback relationship started with the establishment of the first isolated trees. This positive feedback has important implications to understand the vegetation organization on open scrub vegetation, where patches of woody species are scattered in a matrix of herbaceous plants. Furthermore, the facilitation between trees and their dependency on local environmental conditions could be fundamental to explain the gradient of vegetation physiognomies from open vegetation to forest observed along the beach-to-inland gradient, as well as to understand the dynamics of this vegetation over time. Finally, our results also have practical implications, highlighting the potential use of nurse plants and artificial shade as techniques to restore shrubland or forest physiognomies in coastal dunes.

Nas últimas duas décadas estudos empíricos têm demonstrado a importância da facilitação entre plantas, especialmente sob condições ambientais severas. Tal constatação inspirou a proposição da hipótese do gradiente de estresse (HGE), um modelo conceitual que prediz que a freqüência relativa entre facilitação e competição deve variar inversarmente ao longo de um gradiente de estresse, sendo a facilitação mais comum sob condições extremamente severas. As restingas (ou planícies costeiras arenosas) são ambientes caracterizados por condições ambientais limitantes para o desenvolvimento das plantas, fazendo-se então um ambiente propício para o predomínio de facilitação. Além disso, o gradiente de intensidade de estresse e perturbação relacionado à distância do mar faz ainda deste ecossistema um modelo ideal para testar a HGE. O objetivo geral desta tese foi estudar facilitação entre plantas de restinga através dos seguintes objetivos específicos: i) Apresentar uma revisão sistemática dos trabalhos já publicados sobre facilitação entre plantas de restinga em escala mundial, assim como investigar, através de meta-análise, que fatores influenciam a presença e intensidade da facilitação entre plantas; ii) Testar se os padrões de associação espacial entre árvores adultas isoladas e plantas de distintas formas de vida se alteram ao longo de um gradiente ambiental praia-interior, conforme esperado pela HGE; iii) Testar a HGE através de experimentos de campo envolvendo transplante da espécie potencialmente beneficiada, assim como manipulação de recurso limitante ao longo do gradiente praia-interior; iv) Testar se o sombreamento é um mecanismo de facilitação na restinga. Para os testes empíricos (objetivos ii-iv), as observações e experimentos foram realizados em um gradiente de restinga arbustiva do Parque Estadual da Ilha do Cardoso (PEIC) localizado em Cananéia, São Paulo, Brasil. Os resultados indicaram que: i) diversas condições e aspectos da facilitação entre plantas de planícies costeiras têm sido pouco exploradas, como por exemplo, há poucos estudos nas regiões tropicais e poucos estudos que consideraram estágios de vida de semente ou de adultos da espécie alvo da interação. Dentre os estudos publicados, fatores como precipitação, região geográfica e estágio de vida da planta alvo definitivamente influenciam a existência e magnitude da facilitação entre plantas de restinga; ii) para a maioria das formas de vida, o padrão de associação espacial com árvores adultas não se altera ao longo do gradiente ambiental conforme predito pela HGE. Apenas árvores jovens apresentam associação espacial positiva com árvores adultas, o que sugere efeito de planta-berçário entre árvores da restinga arbustiva; iii) tanto a intensidade quanto a importância da interação entre árvores foram negativamente relacionadas ao estresse, rejeitando então a proposição clássica da HGE. Além disso, o sinal da interação foi dependente do estágio de vida da espécie alvo, uma vez que a presença do adulto vizinho foi predominantemente negativa sobre o estágio de semente mas positiva sobre a sobrevivência das plântulas; iv) o efeito de planta-berçário entre árvores da restinga é, pelo menos parcialmente, explicado pelo sombreamento gerado pela copa da espécie facilitadora. Dessa forma, concluimos que a facilitação via sombreamento é uma interação relevante entre árvores adultas e jovens na restinga. Entretanto, ao contrário do previsto pela HGE, o efeito de planta-berçário tende a ser mais intenso em condições ambientais mais amenas. Estes resultados sugerem a existência de um ciclo de retroalimentação positiva gerado a partir do estabelecimento das primeiras árvores isoladas. Este ciclo positivo apresenta implicações importantes para a compreensão da organização estrutural da vegetação da restinga arbustiva, em que moitas de espécies lenhosas estão distribuídas em uma matriz predominantemente herbácea. Adicionalmente, a facilitação entre árvores e sua dependência em relação às condições ambientais locais devem ser mecanismos essenciais para explicar o gradiente de fisionomias vegetacionais sobre os cordões arenosos, a qual inicia-se em uma vegetação aberta e termina em uma floresta, assim como para entender a dinâmica temporal desta vegetação ao longo do tempo. Por fim, nossos resultados também apresentam implicações práticas, relacionadas ao uso potencial de plantas-berçário e mímicos de sombra como técnicas de manejo para aumentar o estabelecimento de mudas transplantadas em projetos de restauração de restingas arbustivo-arbóreas. / In the last two decades, empirical studies have shown the importance of plant interaction, especially under harsh environmental conditions. These observations led to the proposition of the stress gradient hypothesis (SGH), a conceptual model predicting that the relative frequency of facilitation and competition will vary inversely across gradients of stress, with facilitation being the dominant interaction under highly stressful conditions. Coastal dunes (known as restinga in Brazil) are environments with limiting conditions for plant performance, which makes them suitable for facilitation predominance among plant interactions. Furthermore, there is a gradient of stress and disturbance related to the presence of the sea, making this environment an ideal model to test the SGH. The general goal of this thesis was to study plant facilitation in coastal dunes with the following specific objectives: i) to perform a systematic review of published studies on plant facilitation in costal dunes around the world as well as to investigate, using a meta-analysis approach, factors that affect the existence and magnitude of plant facilitation; ii) to test if spatial association between isolated adult trees and understory plants of distinct life forms changes along a beach-to-inland gradient, as expected by the SGH; iii) to test the SGH through field experiments involving target species transplantation coupled with manipulation of limiting resource along a beach-to-inland gradient; iv) to test if shading is a facilitation mechanism in coastal dunes. The empirical tests (objectives ii-iv) were conducted in an open scrub coastal dune located at Ilha do Cardoso State Park, Cananéia, São Paulo, Brazil. Our results indicate that: i) several aspects and conditions of plant facilitation in coastal dunes have been little studied. For instance, few studies were conducted in tropical regions and few of them studied the effect of neighbor presence on seed and adult life stage of the target species. Precipitation, geographic region and target life stage affect the existence and magnitude of plant facilitation; ii) For most of the understory life forms, the spatial association between adult trees and understor plants do not change along the environmental gradient as predicted by the SGH. Among all life forms, only young trees were positively associated with adult trees, suggesting a nurse plant effect among trees in coastal dunes; iii) Both intensity and importance of the interaction among trees were negatively related to stress, rejecting the classic proposition of the SGH. Furthermore, the signal of the net interaction depends on the life stage of the target species since the adult neighbor had mostly negative effects on target species seeds but positive effects on seedlings survival; iv) The nurse plant effect among trees was, at least partly, due to the shading provided by the facilitator crown. Overall, we concluded that facilitation driven by shade is a relevant interaction between adult and young trees in the studied coastal dune. However, contrary to what was expected by the early propositions of the SGH, the observed nurse plant effect tends to be more intense under mild conditions. Our results suggest a positive feedback relationship started with the establishment of the first isolated trees. This positive feedback has important implications to understand the vegetation organization on open scrub vegetation, where patches of woody species are scattered in a matrix of herbaceous plants. Furthermore, the facilitation between trees and their dependency on local environmental conditions could be fundamental to explain the gradient of vegetation physiognomies from open vegetation to forest observed along the beach-to-inland gradient, as well as to understand the dynamics of this vegetation over time. Finally, our results also have practical implications, highlighting the potential use of nurse plants and artificial shade as techniques to restore shrubland or forest physiognomies in coastal dunes.

A project involving shrub removal was undertaken by the United States Forest Service in the Spring Mountains National Recreation Area (SMNRA) to reduce accumulated woody fuels, which can pose risks to human communities. This research focuses on the interactions between shrubs and the perennial forb Palmer's penstemon (Penstemon palmeri), one of many nectar sources for the endemic Spring Mountains Acastus Checkerspot Butterfly (Chlosyne acastus ssp robusta). Initial observations revealed that P. palmeri (`penstemon') seemed to grow almost exclusively under shrubs. Such spatial associations are often indicative of a history of positive interactions between plants, and led to the research presented here. These studies investigate and discuss the interactions between this penstemon and shrubs in the arid shrublands of the SMNRA. Spatial patterns between populations of penstemon and shrubs were measured during November 2008-May 2011, and the consequences of shrub-association for individual penstemon emergence, survival, growth, and reproduction were documented (Chapter 2). The results suggest that although shrubs reduced penstemon emergence, they increased seedling survival (a seed-seedling conflict) resulting in a strong shift toward association between shrubs and penstemon over time. Further, while no differences in growth were detected between microhabitats, the results suggest that shrubs inhibited P. palmeri flowering but improved the successful maturation of fruits when flowering occurred. The mechanisms driving these patterns were elusive, but seed-sowing and seedling transplant experiments suggested that shrub soils, rather than their canopies, alter the nature of seed-seedling conflicts in a way that may promote seed-bank persistence in penstemon populations (Chapter 3). To provide a detailed description of the reproductive response of penstemon to shrubs, structural equation modeling was used to describe the importance of shrubs for penstemon seed production (Chapter 4). The results suggest that competition with shrubs reduced penstemon seed production, but that shrubs simultaneously facilitated penstemon water balance and altered the foraging behavior of its pollinators, indirectly increasing seed production. I conclude by discussing the importance of these studies, and studies of plant interactions in general, for helping land managers balance the objectives of fuel load reduction with protecting desirable species (Chapter 5).

Acacia spirorbis est une espèce endémique de la Nouvelle-Calédonie, capable de former des symbioses avec des microorganismes du sol, notamment des ectomycorhizes et de se développer sur une très large variété de sols. Afin de tester les capacités de cette espèce à permettre d’initier un processus de restauration écologique des écosystèmes dégradés par l’exploitation minière, un essai en pépinière et trois essais sur le terrain ont été mis en place. Ces essais ont permis de mettre en évidence la capacité d’A. spirorbis à jouer, grâce à la facilitation, un rôle de plante nurse pour des espèces cibles de la restauration écologique comme des espèces du genre Tristaniopsis. Ainsi, nous avons tout d’abord décrit la diversité des champignons ectomycorhiziens associés avec A. spirorbis puis, nous avons constatés leur capacité à s’associer sur le terrain aux Tristaniopsis grâce à la présence initiale d’A. spirorbis. Les capacités d’A. spirorbis à faciliter l’implantation d’autres espèces en améliorant leur survie, leur croissance et en leur permettant de disposer de partenaires fongiques ectomycorhiziens diversifiés sont décrites. Ces résultats on conduit à proposer un itinéraire technique de restauration écologique des maquis miniers dégradés, itinéraire fondés sur la facilitation entre espèce utilisant A. spirorbis comme plante nurse. / Acacia spirorbis is an endemic species of New Caledonia, capable of forming symbiosis with soil micro-organisms, including ectomycorrhizae and developing on a very wide variety of soils. In order to test the capacities of this species to initiate a process of ecological restoration of ecosystems degraded by mining activities, a nursery trial and three field trials were setted up. These trials highlight the ability of A. spirorbis to play, through facilitation, a nurse plant role for target species of the genus Tristaniopsis in the frame of ecological restoration processes. Thus, we first described the diversity of the ectomycorrhizal fungi associated with A. spirorbis and then we described their ability to associate in the field with Tristaniopsis thanks to the initial presence of A. spirorbis. The capacities of A. spirorbis to facilitate the implantation of other target species by improving their survival, growth and allowing them to access to a diversified range of ectomycorrhizal fungal partners are described. These results led to propose a technical itinerary for ecological restoration of degraded mining maquis, an itinerary based on facilitation between species using A. spirorbis as a nurse plant.