Advances in the Knowledge of the Vegetation of Hispaniola (Caribbean Central America)

The vegetation types and floristic diversity in the Dominican Republic are analysed, a territory with a tropical climate and ombrotypes that range from dry to humid-hyperhumid, due to the Atlantic winds and the phenomenon known as rain shadows. The presence of high mountains and different substrates have led to a rich flora, and as a result, a high diversity of habitats, among which two large forest types are particularly notable: (1) the dry forest with 81 endemic species, of which 10 are trees, 65 shrubs, 5 climbers and 1 herbaceous species, and an absence of epiphytes and (2) the cloud forest with 19 trees, 20 shrubs, 8 climbers, 4 epiphytes, and 6 herbaceous species. In all cases, these plant communities are regarded as endemic due to their high rate of endemic species. In spite of their importance for conservation, these habitats are highly deteriorated due to deforestation for agriculture, to obtain timber, and even to add to tourism infrastructures.


Introduction
The analysis of the vegetation of Hispaniola reveals different situations in terms of their state of conservation, the floristic diversity of the communities and habitat types.The process observed in the study territory resembles the reports by other authors for other countries: namely that agriculture and tourism are the primary factors affecting conservation.The island of Hispaniola has a wide floristic diversity with over 6000 endemic plants, and a high rate of plant communities.This is due to several factors such as orography, edaphology, and its strategic situation in the central Caribbean, which has meant that species have arrived through migratory routes from North and South America.The northernmost areas of the island, exposed to the Atlantic Ocean, are rainier than those in the Caribbean Sea (Figure 1).Some works on vegetation published in the local journal of the Santo Domingo Botanical Garden (Moscoso) have been of assistance in this research.Deforestation changes the structure of the cloud forest [1] and gives rise to a "calimetal" of Gleichenia bifia (Willd.)Spreng.and Dicranopteris flexuosa (Schrad.)Underw., whose vigorous growth and dense coverage hinder the regeneration of the forest's own dynamic stages of deforestation in tropical areas.
The deterioration of the forests on the island of Hispaniola is exacerbated by the Caribbean hurricanes, which have a serious impact on their structure [2].However, the mountains on the island are still home to a number of well-conserved areas, thanks to a close linear correlation between environmental factors and forest types [3], and to indigenous people who exploit the forests sustainably, using the wood to build houses and coal using knowledge acquired through tradition [4][5][6].The presence of 28 climbing species in the cloud forest versus 44 in the dry forest is due to environmental factors, vegetation structure [7], and deforestation caused by human.These forests are crucial to maintaining the biodiversity of this island-considered a biodiversity hotspot [8]-, and also for preserving the relation between forest stands and the water cycle, which is of such overriding importance to society [9][10][11].
The fundamental aim of this chapter is to explain the diversity of plant communities, their causes and their floristic diversity, and highlight their conservation status.

Materials and methods
A vegetation study was carried out on the island of Hispaniola, with particular reference to the forests in the Dominican Republic.Sixty-five ecologically and physiognomically homogeneous plots were selected, and the species present on the plots were noted and assigned a phytosociological abundance-dominance index; this index was subsequently converted to the Van der Maarel (1979) index [12].We used our previous work as a basis for the bioclimatic and biogeographical classification of the territory [13,14].
A statistical treatment was applied to obtain cluster and PCA.To separate the two major vegetation units (cloud forest and dry forest), we conducted a regression analysis (ANOVA) for the species Magnolia hamorii Howard (Maha), Sideroxylon salicifolium (L.) Sw. (Sisa), Metopium toxiferum (L.) Krug.& Urb.(Meto) and Sideroxylon foetidissimum Jacq.(Sifo); and a canonical correspondence analysis (CCA) between the Io (ombrothermic index) and ETP (potential evapotranspiration) of the 65 plots in the sample and the dominant species, located at altitudes below 1,200 m: Hyeronima domingensis Urb.(Hydo), Magnolia hamorii (Maha), Magnolia pallescens Urb.& Ekm.(Mapa) and Pinus caribaea Morelet.This last species forms mixed stands with the cloud forest and gives the name to the vegetation class Byrsonimo-Pinetea caribaea Samek and Borhidi in Borhidi et al. (1979), a native Caribbean species of subtropical and tropical areas [15].Outside its distribution area (Central America), this tropical plant acts as an exotic, and is a well adapted in other tropical areas such as southern China [16].

Results
The results of the ANOVA regression analysis reveal the close correlation between the species present in the forests and the potential evapotranspiration and Io index.The species in the cloud forest are closely correlated with the ombroclimatic index, and in the dry forest with potential evapotranspiration, as shown in Figures 2 and 3 where R2 always higher than 0.9.
The forests in group B are separated into two subgroups such as G1 and G2.G1 (R1-R18) represents the communities described for the subhumid and humid territories in which the substrate takes prevalence and allows the entry of dry forest species.G2 (R19-R39) is classified into four plant communities.(Figure 5).The type A forest group is located in the northern areas of the great mountains; Cordillera Central, Northern, Bahoruco, Oriental, Los Haitises, all of which have an ombrotype that oscillates between the subhumid and the hyperhumid, and a thermotropical to supratropical thermotype (Photos 2-4).

Subgroup G2
The territory of the Dominican Republic, including some small adjacent islands, covers 48,198 km 2 and accounts for over two-thirds of the territory of the island of Hispaniola, located between parallels 17 and 19°N, and part of the Greater Antilles.The main aim of this chapter is to determine the forest vegetation (cloud forest), floristic diversity, and state of conservation in areas with high rainfall in the Dominican Republic.Most of the botanical studies to date are floristic in nature, and include the studies on the Sierra de Bahoruco, which highlight the substantial rainfall of up to 4000 mm and the very high rate of endemic species.
Studies by various authors in the central, northern, and eastern mountain ranges containing the cloud forest [18][19][20][21][22][23][24], and which together with our own previous studies, form the basis of this present work [25][26][27][28][29][30][31][32][33][34][35].All the above-mentioned studies focus on the knowledge of the Vegetation flora, with passing references to the vegetation; herein lies the interest of this work.The floristic approach is important as these are biodiversity hotspots [36], with a 33% rate of endemics compared to the total flora.In the analysis of the diversity of the 2 forest types (dry and rain); in the dry forest, there are 61 tree species and 5 stipes, of which 10 are endemic; compared to 75 in the cloud forest and 4 stipes, of which 19 are endemic.There is more number of epiphytes in the cloud forest, with Vegetation 90 36 species, than in the dry forest, with 13 species (Table 1).Shannon's index gives values of 3384 and 3528 for the dry and cloud forest, respectively.

Discussion and conclusions
There are areas with dry forest plant cover in the Dominican Republic; these are located in the southwest of the island, specifically in Procurrente de Barahona, Azua-Valle de San Juan-Hoya Enriquillo and in its extension toward western Haiti, including Beata Island and the Cibao Valley, Monte Cristi.These territories have a high rate of endemics, therefore, the habitats where they grow should also be considered endemic.The cloud forest, however, is always found in the mountains as a consequence of the moisture-laden winds from the Atlantic.In both cases, the forest distribution responds to the environmental factors on the island, which has a high rate of endemics owing to the island phenomenon, making it a hotspot in the Caribbean.
The presence of two groups of forests due to the humidity gradient [37] in the Dominican Republic, which varies between semiarid and hyperhumid.This classification is based on the floristic composition and less on forest structure [38].The cloud forest in Ebony Green (Cordillera Central) has suffered frequent fires, causing an invasion of Gleichenia bifida and Dicranopteris pectinata, which prevent the germination and development of primary forest species and in turn leads to a loss of seeds and intensification in the rate of invasive species.This situation requires rapid human intervention [39] (Photo 5), which should be aimed at reinforcing the seed bank of species of interest to conservation in botanical gardens in the Caribbean  Advances in the Knowledge of the Vegetation of Hispaniola (Caribbean Central America) http://dx.doi.org/10.5772/intechopen.72090[40].The rain forests on the ridges and mountain ranges of Hispaniola are rich in Cecropia schreberiana Miq.[41] and species of the genera Magnolia, Prestoea, Didymopanax, Cyathea; the greatest botanical diversity is found in the oldest mountains, while the younger formations are less floristically diverse [42].The diversity of trees and epiphytes is lower in the dry forest than in the cloud forest, but there are more succulents from the Cactaceae and Agavaceae family; we found no interactions between these families and parasitic elements in the Dominican Republic as occur in Puerto Rico.This type of dry forest dominated by succulent species has a low rate of epiphytes (4%) compared to 15% for the cloud forest.Epiphytes are renowned for their abundance in dry forests: Tillandsia recurvata (L.) L, Broughtonia domingensis (Lindl.)Rolf, Tillandsia balbisiana Schultes, Tillandsia usneoides (L.) L, and Bromelia pinguin L.
Deforestation caused by fire or heavy machinery is causing changes in the landscape, leading to the loss of species and plant cover (Photo 6) in areas where there is insufficient knowledge of the flora and vegetation, despite the fact that, so far, we have described nine phytosociological classes and more than 20 associations and plant communities on the island.

1 Figure 4 .
Figure 4. Principal component analysis (PCA) for dry and cloud forest.

Figure 6 .
Figure 6.Cluster analysis.Diversity of forest types.

Table 1 .
Species diversity in dry and cloud forests in the Dominican Republic.