Mangroves and saltmarshes are ecologically important coastal ecosystems; unfortunately, these low-lying coastal ecosystems are vulnerable to global climate change. As sea-levels rise, mangroves are expected to shift their distribution landward towards higher elevation sites that are occupied by other plants, including saltmarsh taxa. Therefore, mangrove recruits at the leading edge of expansion may interact with diverse assemblages of saltmarsh plants, and these interactions could influence the success of mangrove encroachment into higher tidal-elevation areas. The purpose of the research presented here was to investigate empirically the ecological interactions that may influence the recruitment of the black mangrove, Avicennia germinans, into saltmarsh habitats. Saltmarsh plants frequently occurred at the landward boundary of mangrove forests at two sites selected for field studies along the west coast of Florida: Cannon Island and Upper Tampa Bay Park. On Cannon Island, two different field tests investigated mangrove propagule entrapment and dispersal within saltmarsh vegetation. In the first experiment, the entrapment of mangrove propagules within saltmarsh plants, exhibiting different growth forms, was examined during seasonal high tide events. Natural polyculture plots retained a mean (±;SE) 59.3% (±;11.0) of emplaced propagules. Monocultures varied in their propagule retention capacities with plots of S. virginicus retaining on average 65.7% (±;11.5) of transplanted propagules compared to 7.2% (±;1.8) by B. maritima and 5.0% (±;1.9) by S. portulacastrum. Monocultures of the salt marsh grass, Sporobolus virginicus, and natural saltmarsh polycultures containing S. virginicus retained significantly more propagules than either of two succulent plants (i.e., Batis maritima and Sesuvium portulacastrum). Using digital images, saltmarsh plant structure was quantified; the number of entrapped mangrove propagules displayed a significant and positive correlation (r2 = 0.6253, p = 0.00001) with the amount of structure provided by saltmarsh plants. Therefore, the first field study identified structural and functional differences between saltmarsh plants. A second field study employed marked propagules in order to further examine the dispersal patterns of propagules at saltmarsh boundaries comprised of plants with different growth forms (i.e., grass vs. succulent) during seasonal high tides. Saltmarsh plant boundaries erected by taxa with distinct growth forms differentially influenced the proportion of propagules that dispersed seaward and the distance propagules moved seaward. In fact, nearly twice as many propagules dispersed seaward at boundaries erected by succulent plants compared to boundaries composed of grass. The results of this field study support my previous findings that propagule dispersal is comparatively lower in saltmarsh grass than in succulent saltmarsh plants. The findings from these two field studies suggest that the permeability of boundaries formed by saltmarsh plants may modulate landward dispersal of A. germinans propagules. The final field study was conducted at Upper Tampa Bay Park, where a second species of saltmarsh grass, Monanthochloe littoralis, co-occurred with the grass, S. virginicus, and succulent saltmarsh plants to form a mosaic landscape of saltmarsh plant patches. Patches were weeded to create 3 saltmarsh treatments: 1) M. littoralis monoculture; 2) S. virginicus monoculture; and 3) polycultures containing both grasses and at least one other saltmarsh taxa. Propagules of A. germinans were emplaced into saltmarsh patches and followed for 11 weeks. On the last sampling date, the greatest number of A. germinans (n = 51) had successfully established as seedlings within the M. littoralis monoculture plots. In contrast, only 20 (22% of the propagules initially emplaced) A. germinans seedlings established in S. virginicus monoculture plots. These findings suggest that among grass taxa, species identity influences mangrove establishment success, which builds upon our previous findings that demonstrated that saltmarsh growth form (i.e., grass vs. succulent) influenced mangrove propagule dispersal. Combined the findings from these field studies indicate that interactions among the early life history stages of black mangroves and neighboring plants influence mangrove recruitment. Specifically, these field studies provide empirical evidence that the species composition of saltmarsh plants influences mangrove propagule dispersal and seedling establishment. The work presented here has implications for understanding the suite of ecological interactions that may influence mangrove encroachment into saltmarsh habitats at higher tidal elevations as sea-levels rise.

L’installation de plantules est une phase critique fortement influencée par les interactions avec la végétation établie, compétition ou facilitation. Dans cette thèse, nous montrons que l’effet du pin d’Alep ou d'arbustes sur l’installation d’espèces feuillues dépend de la densité du couvert, de la stratégie des espèces cibles et nurses et des conditions locales. Sous couverts très denses, la survie et la croissance des plantules sont drastiquement limitées par compétition lumineuse et hydrique, le stress hydrique étant renforcé par une faible allocation de biomasse aux racines. À l’autre extrémité du gradient, en milieu ouvert, la photoinhibition et la concurrence avec les herbacées peuvent limiter l'installation des plantules. Les espèces sclérophylles sempervirentes sont peu sensibles aux fortes irradiances, températures et demandes évaporatives et peuvent profiter de conditions favorables momentanées par polycyclisme en milieu ouvert. L’effet d’un couvert végétal sur ces espèces est principalement compétitif, tandis que les espèces décidues à fort SLA bénéficient d’un couvert modéré. En pépinière, la litière des principales espèces ligneuses modifie les propriétés physico-chimiques et microbiologiques du sol sous-jacent mais sans effet sur des plantules de chêne au bout de 2 ans, montrant un faible effet allélopathique. L’éclaircie des peuplements de pin d’Alep denses est une stratégie pour faciliter l’installation d’espèces feuillues et ainsi améliorer la diversité et la résilience au feu des forêts. L’optimum de couvert semble se déplacer vers des couverts plus denses dans des sites à conditions climatiques ou édaphiques plus sévères et pour les espèces décidues. / Seedling establishment is a critical demographic phase, strongly influenced by plant-plant interactions. This thesis shows that the effect of Aleppo pine and shrubs on broadleaved seedling establishment depends on vegetation cover density, target and nurse species strategies and local conditions. A dense vegetation cover strongly limits seedling establishment by light and water competition, seedlings water stress being worsened by a low biomass allocation to roots. At the other extreme, in the open, photoinhibition and competition with grasses can limit seedling establishment. Sclerophyllous species are poorly sensitive to high temperature, irradiance and evaporative demand and can take advantage of favorable conditions at any time of the year by polycyclism in the open: interactions with pine are thus mostly competitive. In contrast, deciduous species with high SLA are more sensitive to photo-inhibition and benefit from the protection of a moderate cover, under which they are able to grow faster. In a nursery experiment, pine and shrub litters modified soil chemical and microbiological properties but without feedback on oak seedlings, indicating a poor allelopathic effect. Pine thinning is a strategy to enhance broadleaved seedling establishment and increase Mediterranean forest diversity and fire-resilience. However, the optimum thinning intensity seems to decrease in harsher climatic or edaphic conditions and for deciduous species.