**3.3 Floristic characters**

One hundred and three (103) species, belonging to 46 families, were recorded in the fives formations, including 70 trees and shrubs (**Table 1**). The floristic composition of dry Forests shows sixty-five (65) species in 54 genera and 35 families. The most represented families are respectively Rubiaceae, Euphorbiaceae, Fabaceae, Apocynaceae and Erythroxylaceae. *Euclea racemosa subsp. schimperi, A. digitata, Alangium salviifolium, Phyllarthron comorense, Vitex doniana, Albizia glaberrima, Sorindeia madagascariensis, Pyrostria bibracteata* and *Tamarindus indica* are the most abundant speces. The density varies from 500 to 800 plants per hectare. The different forms of biological adaptation observed in forest are foliar deciduousness in *A. lebbeck*, *Turraea sericea, A. digitata, Commiphora arafy*, pachycaulia in *A. digitata*, epiphytism in Aeranthes sp. and *Acampe pachyglossa*, Geophytism in *Dioscorea comorensis* and *T. leontopetaloides* and crassulescence in *C. quadrangularis*. The biological spectrum is represented by Mesophanerophytes (55%), Microphanerophytes (25%) and Nanophanerophytes (15%).



#### **Table 1.**

*List of endemic and/or rare species observed in the dry vegetation of Ngazidja.*

Natural regeneration is good for the dry forests of Lac salé and Infoundihe-Chambouani (986.64% to 995.64%) and it is bad for the Ngouni-Hamboda dry forest (50.96%).

In the thicket, fifty-three (50) species in 45 genera and 30 families. The most represented families are Fabaceae and Asteraceae. At the generic level, Erythroxylum is the richest in species with three species. The density varies from 310 to 365 individuals per hectare. The different forms of biological adaptation observed in this dry thicket are pachycaulia in *A. digitata*, geophytism in *T. leontopetaloides*, epiphytism in *Angraecum eburneum*, *Acampe pachyglossa* and *Platycerium alcicorne*, and crassulescence in *C. quadrangularis*. The predominant biological types are microphanerophytes (50%) and nanophanerophytes (35%). Natural regeneration is good for thickets (315.18% to 383.67%).

*Phytoecological Study, Ethnobotanical and Dynamic of Dry Vegetation in the Ngazidja Island… DOI: http://dx.doi.org/10.5772/intechopen.107330*

## **3.4 Specific characteristics of the plant formations studied**

After analyzing the relationships between the five types of vegetation formations and their floristic procession were established. A similarity between the vegetation formations, according to their dry forest and forested physiognomy and dissimilarity according to their floristic composition (**Table 1**) could be identified. Thus, the following vegetation formations were distinguished:


#### **3.5 Pressures, threats and dynamic on dry vegetation**

The threats affecting forests and dry thickets in the Comoros fall into two categories: anthropogenic threats and natural threats (**Figure 10**). Anthropogenic threats are mainly caused by the misuse of different resources from different vegetation formations:

**Figure 7.** *Comoranthus obconicus (fruits).*

**Figure 8.** *Aloe alexandrei (flowers).*

**Figure 9.** *Dry forest of Ifoundihé-Chambouani.*


*Phytoecological Study, Ethnobotanical and Dynamic of Dry Vegetation in the Ngazidja Island… DOI: http://dx.doi.org/10.5772/intechopen.107330*

**Figure 10.** *Types of threats and their impacts.*

of the Comoros are practically poor in tree species, riparian populations still use the few trees present in these thickets (*Phyllarthron comorense, A. lebbeck, Albizia glaberima, Ochna ciliata, Phyllanthus comorensis*). These uses vary from one locality to another. Thus, the most popular species used by the local population from the dry forest are *Alangium salviifolium, Phyllarthron comorense, Terminalia ulexoides, Drypetes comorensis, Phyllanthus comoriensis, Phoenix reclinata, F. sycomorus and Ochna ciliata.* The observations and information gathered on the ground show that plants in urban areas are often subject to felling, either for competing of space, risk of uprooted trees falling, or for cultural reasons (related to popular beliefs that baobabs and tamarinds are hosts to evil spirits) in some places.


Threats of natural origin are volcanoes and cyclones. Only the habitats of Ngazidja are threatened by the Karthala volcano which is active. Many eruptions occurred and are noticed by long lava flows some of which are still very visible today. The last most devastating eruption was that of April 5, 1977 in Singani (Hambou), which destroyed many habitats. The Comoros Islands have experienced several cyclonic disturbances. The most recent cyclones affected in Ngazidja in 1984, 1985 and 2018–2019. They devastated homes, plantations and various dry ecosystems. These cyclones can cause windfall in vegetation formations, creating canopy openings [9]. All these actions have led to the reduction of almost 80% of the cover of dry plant formations in Ngazidja, in the last 40 years.

## **4. Discussion**

The analysis results showed that the dry plant formations studied were different from each other, in terms of physiognomy, with species that characterized each formation. In fact, it consists of the dry forests of Lac-salé, Ngouni-Hamboda and Infoundihé-Chambouani, stratified forests and the two dry thickets of Singani and Domoni-Ambouani-Hahaya.

In terms of climate, the dry forest of Ifoundihé-Chambouani has several deciduous species while the dry forests of Ngouni-Hamboda and Lac-salé have few species that lose their foliage. Most species in these two last forests are usually evergreen for part of the year. Deciduous leaves condition the biological rhythm of a plant during the dry season to a slower state of life and it is one of the distinguishing criteria of tree formations of these dry forests. It is present in most trees of the upper and middle strata. Only trees from the mid and upper slopes lose their leaves during the dry season while those from the bottom do not lose their leaves during this season. For the Lac-salé, the lowland trees are found around Lake and thus directly receive moisture from this lake. This result is similar to what happens around Lake "Dziani Boundouni", on the island of Mwali [18].

At the national level, the dry forest facie of Ngazidja is characteristic of the dry ecosystem facies of Comoros archipelago similar at the dry forests found in Africa, Madagascar and the islands of the Indian Ocean, with true stratification and deciduous or semi-deciduous leaves. Because these dry forests have a canopy that can reach 15 m high with 20 m for the emergents like at Madagascar [19]. The abundance of small-diameter creepers and large-diameter thickets in dry forests is typical of dry vegetation communities in the Comoros [6].

The floristic composition of the dry forests is similar and as it is also the case for the floristic composition of the two thickets. Due to their proximity, species exchanges can occur between the Comoros Islands at the flora level [20]. Of the five vegetation formations studied, 106 species were identified including 70 trees and shrubs, which is very close to the score of 70 species of trees and shrubs found in the dry vegetation formations of Mayotte by [21, 22]. The dry forest Ngazidja has many floristic affinities with the "subhumid" zone, semi-xerophile megatherm of Islands of Maoré (Mayotte) [23] and Mwali (Mohéli) [24] and particularly its particularly dry coastal fringe. Several species were found in the Maoré dry vegetation formations [25, 26] with over 90% of native species, some of which are endemic to the Comoros or Maoré (such as *Aloe mayottensis*).

Ethnobotanical surveys showed that trees and shrubs are also exploited for firewood. Generally, in the Comoros, wood remains the most used fuel as it is free: on Ngazidja, all ligneous plants are used as firewood although there are preferences for some tree species such as *Nuxia pseudodentata*. Currently, this case has gotten worse because of the high demography that this Island is experiencing. Thus, since forest species have become very difficult to find, it is the species of thicket (*A. lebbeck*, *Ochna ciliata*, *Pyrostria bibracteata*, *Euclea racemosa* subsp. *schimperi*) that are targeted.

Invasive species are also numerous in these formation: *A. lebbeck* (Fabaceae) lebbeck or lebbek tree or flea tree or frywood and *L. camara* (Verbenaceae) "common lantana"

*Phytoecological Study, Ethnobotanical and Dynamic of Dry Vegetation in the Ngazidja Island… DOI: http://dx.doi.org/10.5772/intechopen.107330*

are considered among the highly invasives woody species of the Comoros Archipelago in dry vegetation [27, 28]. *L. camara* is very abundant and forms clumps in thickets. In Mayotte, this species is among the most threatening plant pests that invade any dry zone and forms dense bushes preventing any other species from establishing themselves [22, 23, 25].

In addition, most species are also completely invaded by lianas such as *Leptadenia madagascariensis and C. quadrangularis* (Vitaceae) veldt grape or adamant creeper or devil's backbone.

In Madagascar, nearly 23% of the 1003,000 ha of dry dense forests and 32% of the 1,444,000 ha of southern thorny scrub are considered degraded or secondary [29].

## **5. Conclusion**

The study included Five types of dry vegetation formations in Ngazidja island, Comoros Archipelago: dry forests of Lac-salé, Ngouni-Hamboda and Infoundihé-Chambouani, and dry thickets of Singani and Domoni-Ambouani-Hahaya.

The physiognomy of the tree forests shows that they are well orgnized with deciduous leaves. These vegetation formations are different from each other with their own species:


In all the studied formations, the overall natural regeneration is good. Mesophantophytes, nanophanerophytes and lianas of small diameter are the predominant biological types. The most common biological traits are foliar deciduousness, pachycaulia and aphylia.

In these plant formations, one hundred and three (103) species are distributed in forty-six (46) families, the most abundant of which are Fabaceae, Rubiaceae and Euphorbiaceae. The most dominant species are *Phyllarthron comorense, Diospyros comorensis, A. digitata, Euclea racemosa subsp. Schimperi*. Several endemic species were identified during this study, in these vegetation formations such as *Dioscorea comorensis, Diospyros comorensis, Phyllarthron comorense, Euclea racemosa subsp. Schimperi, Pyrostria bibracteata and Phyllanthus comoriensis*.

Some have just been observed for the first time in Ngazidja and are very rare and therefore threatened with extinction on the island such as *Turraea virens*, *Turraea* 


**Table 2.**

*List of species observed for the first time on the island of Ngazidja.*

*sericea, Comoranthus obconicus, Sterculia madagascariensi*s, *Alangium salviifolium, Commiphora arafy, Ouratea humblotii* and *Ochna ciliata* (**Table 2**).

Many of these species are widely used by the local population in various fields. The formations are subject to various pressures and threats of two types: anthropic and natural. The anthropogenic threat is the most important.

This article therefore aims at providing objective elements that can support conservation actions that could include:

• the delimitation or introduction of dry forests and thickets, in the strategy for the development of protected areas;

*Phytoecological Study, Ethnobotanical and Dynamic of Dry Vegetation in the Ngazidja Island… DOI: http://dx.doi.org/10.5772/intechopen.107330*

