The dry forest is one of the most threatened ecosystems due to the ease of change associated with human activities.
In Manabí, this ecosystem is home to a large number of species whose populations have suffered some kind of modification as a result of traditional unsustainable extraction practices.
Through geographic information systems (GIS), 24 permanent circular sampling plots with an area of 500 m2 were randomly established, considering three altitudinal floors (200 - 250; 251 - 300 andandgt; 300 masl).
In these plots, data on diameter at breast height (DBH) and total height of individuals with DBHandgt; 5 cm were recorded. From these data, structural parameters (abundance, frequency, dominance and the importance value index) were calculated.
627 individuals were found, grouped into 11 families and 21 species. The forest under study had a tendency to homogeneity with dominance of species such as Ceiba trischistandra, Bursera graveolens, Cordia lutea, Eriotheca ruizii, Erythrina velutina and Cochlospermum vitifolium; these six species represented 74% of the species found. The families with the greatest ecological importance are Malvaceae, Burseraceae, Fabaceae, Boraginaceae and Bixaceae.
The altitude is significantly associated with the abundance of species. The forest presented intermediate successional states and although a considerable degree of intervention was evidenced, they maintained an important potential in ecosystem processes.
Since its origins, the human species has survived by taking advantage of the elements of its environment, being in some areas not only a sporadic extraction but an excessive use of resources (Ferrer-Paris et al., 2019), which led to deterioration and disappearance of species (Portillo-Quintero and Sánchez-Azofeifa, 2010).
Ecuador has a sixth of its territory covered by deciduous and semi-deciduous forests (MAE, 2016) with a national deforestation rate of approximately 30 km2. year-1 during 2008-2014 (MAE, 2015). Most of them are between 0 and 1000 meters above sea level. They develop in particular edaphological conditions; flat and rocky soils, high saline concentrations and clayey strata, occasionally sandy impermeable that do not allow moisture retention and areas with rainfall of between 400 and 600 mm, focused on 3 or 4 months (MAE, 2012).
Due to this dry period, these forests are highly deciduous and their components have a characteristic physiognomy. In this sense, the areas with the highest amount of dry forests in Ecuador are in the provinces of Esmeraldas, Manabí, Santa Elena, Guayas, El Oro and Loja (Aguirre, 2013). In the Province of Manabí, dry forest areas are abundant, being dominated by Ceiba trischistandra and other species of ecological and economic importance.
In general terms, the conservation of these ecosystems implies the knowledge and evaluation of the behavior of the species on the forest surface and the design of mechanisms to determine the possibilities of use, conservation and / or regulation. In this sense, the present study aimed to evaluate the most important structural characteristics of an area with dry forest through an altitude gradient.
Materials and methods Location
The study area is located in Joá, 6 kilometers from the city of Jipijapa and is graphically located at 1 ° 23'34.72 ”South latitude and 80 ° 35› 51.12 ”West longitude. It is the limit between the Tiján plain, which extends from South to North from Piñas de Julcuy and the stowage of the Chongón-Colonche Coastal Mountain Range. Its climate is of the Equatorial Dry Forest.
Specifically, an area of 50 hectares was selected as a study subject, which has a Comprehensive Management Plan (PMI No. 06308007502) and a Technical Inspection Report in the Wooded Zone of the Joá Commune of the Jipijapa Canton (No. 026-2019 -JC-BIODDPAM.MAE), as requirements for the declaration of the category “Protective Forest and Vegetation”.
24 permanent circular sampling plots of 500 m2 each were used, they were randomly arranged on the ground under study (50 ha) considering three elevational floors (200-250; 251-300 andandgt; 300 masl). Six plots were established for each floor and data on total height and diameter at breast height (DBH) of each of the species with DBHandgt; 5 cm were recorded. The sampling period began in August 2019 and ended in September of the same year, during the dry season. From the data recorded in the field, the structural study was carried out, which included the following parameters:
According to the results, the most abundant species is Cordia lutea (19.30%), followed by Bursera graveolens (18.02%); while the rarest species in this area is Jatropha curcas (0.16%). Simple correspondence analysis showed that there are significant associations between species abundance and elevational levels (p andlt;0.05). The resulting graph with a total inertia of 100% in two axes shows that the presence of Erythrina velutina, Prosopis sp., Colicodendron scabridum, Cochlospermum vitifolium, Caesalpinia glabrata and Croton rivinifolius have a greater association with lower altitudes (200-250 masl).
It is important to indicate that only two of the species (C. glabrata and C. rivinifolius) are specific to this altitudinal floor. In this same sense, Ficus sp., Jatropha curcas, Cynophalla sclerophylla, Capparis angulata and Acacia farnesiana are more associated at altitudes between 251 and 300 meters above sea level. and Cynophalla mollis, Cojoba arborea, Ziziphus thyrsiflora, Geoffroea spinosa together with Pouteria sp., were mostly at altitudes above 300 meters above sea level. Additionally, it was possible to observe that species such as Muntingia calabura, Bursera graveolens, Eriotheca ruizii, Ceiba trichistandra and Cordia lutea were not associated with any specific altitudinal floor, as it was present along the altitudinal gradient.
The most frequent species in this study were: C. trischistandra (11.80%), C. lutea (11.80%), B. graveolens (10.80%), E. ruizii (9.23%) and E velutina (8.21%), which represented little more than 50% of the species in the area. The species with the lowest frequency was J. curcas (0.51%). Regarding this aspect, Aguirre and Delgado (2005) mention that the high frequency of E. ruizii could be a characteristic of a young forest in the process of recovery.
For this study, the dominant family was Malvaceae, with 38.15% followed by Burseraceae with 12.62% and Fabaceae with 12.43%, which agrees with Herazo-Vitola et al., (2017), who report dominance of Fabaceae in seven locations in the tropical dry forest in northern Colombia. The least dominant family was the Euphorbiaceae with 0.57%. In general terms, the dominant species in the tree stratum were C. trischistandra, C. vitifolium, B. graveolens, E. ruizii and E. velutina. In this regard, Aguirre et al. (2001) recorded that the dominant stratum is represented by Ficus sp., C. trichistandra and E. ruizii and in the middle stratum there are species such as B. graveolens, C. vitiftolium, G. spinosa and Erythrina sp., Agreeing with the results obtained in this study.
On the other hand, Aguirre and Delgado (2005) mention that as a result of human activities and after the abandonment of the land, the types of forest that dominate the landscape are the dry thorn scrub, the dry scrub with sparse trees, the faical and in the best of the cases a very sparse forest dominated by C. trichistandra and E. ruizii.
Therefore, it is possible that the forest formations in the area of Joa, Jipijapa have developed after a period of exploitation and abandonment, thus causing the dominance of these two species. The behavior of the diameter distribution of the 627 individuals showed a typical trend curve in the shape of an "inverted J", which showed that a large number of trees were in the lower diameter classes and this amount decreased as the diameter of the tree increased. stem.
This fact could be an indication that the forest has good regeneration (Uslar et al., 2004) or that the plant community is developing towards more advanced growth and productivity phases (Arruda et al., 2011; Hernández-Stefanoni et al. ., 2011) and shows that the total population has a well-preserved structure (Encinas et al., 2011).
Table 2 presents the percentage of individuals in each diameter class considering the altitudinal floor, in general terms the behavior was the same, a large percentage of individuals were concentrated in lower diameter classes and this amount decreased as the diameter class increased. However, an unusual behavior could be observed in the diameter class "andgt; 115" cm in the first altitudinal floor (200-250), where three times more individuals were located than in the 251-300 floor and five times more than in the floorandgt; 300 masl.
The area under study has a large number of species in low diameter classes, which ensures the permanence of the forest over time due to the fact that these species will develop to enter high diameter classes, this behavior is an indication that the forest it is dynamic.
The species with the highest ecological value in the El Artesan EcuadorianHands Forest and protective vegetation are: C. trischistandra, B. graveolens, C. lutea and E. ruizii, because their abundance is not associated with any altitudinal floor, that is, they occur along the entire altitudinal gradient in similar proportions. There are species highly associated with a particular altitude floor, and cases such as C. glabrata and C. rivinifolius can be cited in the lower part; J. curcas and C. sclerophylla in the middle floor and C. arborea, Z. thyrsiflora and G. spinosa at altitudes greater than 300 meters above sea level.
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