Abstract
The Brazilian Caatinga is one of the most diverse dry forests on the planet. Half of its original coverage is degraded, and it is currently identified as one of the main areas undergoing desertification. Natural regeneration depends on climatic and edaphic conditions, as well as the adult stratum present in an area. Despite its importance, this process is little known in the Caatinga. Thus, we aimed to analyze how the anthropic disturbance, vegetation cover, and soil properties influence regenerating stratum under different Caatinga vegetation cover levels. Our study was executed in the driest region of Brazil. We classified our studied areas as Area I (less vegetation cover) and Area II (greater coverage). Six plots of 50 x 20 m were delimited for sampling the adult and regenerating stratum, and the soil in each area. Our results show interactions between soil characteristics and adult and regenerating stratum. Area II showed greater diversity and a greater number of exclusive species; in contrast, a dominance of species more resistant to limiting conditions was observed in Area I, such as Aspidosperma pyrifolium Mart. & Zucc. The C and N contents in the soil indicated a positive and significant correlation with the diversity of the regenerating stratum. The data revealed that the area with less vegetation cover, richness, and diversity presented indications of desertification.
Keywords : Semi-arid climate – Plant Communities – Vegetation cover – Chronic Anthropic Disturbance – Desertification – Regenerating Stratum.
Introduction
The increase in the human population and the demand for natural resources are the main causes of global biodiversity loss and changes in the structure of forests around the world (McKee, 2009). Habitat fragmentation (Ribeiro et al., 2015) and changes in soil quality (Ferreira et al., 2018) derived from anthropic pressure reduce tree species richness and alter structural parameters such as stem diameter and basal area (Sobrinho et al., 2016). Such changes over time can shape the desertification process in less resilient areas (Alves et al., 2009). Desertification is defined as a reduction or loss of the biological productivity of drylands, their ecological complexity, and/or their human values, manifested through a reduced provision of the sum of dryland ecosystem services (Verstraete et al., 2009; Reid et al., 2005; Scholes, 2009).
The natural regeneration process of disturbed forests varies according to the abiotic and biotic characteristics of the environment. Seedling establishment is related to climatic conditions of the environment, such as precipitation, temperature and seasonality, richness and composition of the adult plant community, and the history of land use (Ribeiro et al. 2015; Sobrinho et al., 2016). Thus, arid and semi-arid ecosystems are less tolerant to human disturbances, becoming more vulnerable to desertification than humid tropical systems. These ecosystems consequently have lower regeneration rates since seedling establishment, which is the most vulnerable stage of life, can be compromised by prolonged drought periods (Alves et al., 2010; Ferreira et al., 2018; Marinho et al., 2016; Sobrinho et al., 2016). On the other hand, a milder microclimate induced by adult stratum could favor ecosystem regeneration (Derroire et al., 2016a; Lebrija-Trejos et al., 2011).
Seasonally Dry Tropical Forests (SDTF) are currently one of the most threatened ecosystems in the world, with 97% of their territory at risk (Miles et al., 2006). These forests are characterized by high seasonality, long periods of drought, and rainfall ranging from 240 mm to 1,500 mm per year (Pennington et al., 2009). These environments are home to high biodiversity and are main endemism centers, with species marked by their morphophysiological adaptation mechanisms developed to tolerate local climatic conditions (Murphy and Lugo, 1986).
The Brazilian Caatinga consists of one of the largest and most diversified dry forests in the world (Costa et al., 2016; Prado et al., 2015), with an extension of 912,529 km² (Silva et al., 2017). Approximately 46% of the native Caatinga vegetation has been cleared due to agriculture and pastures developed since the 16th century and currently due to industrial and timber extraction (Lapola et al., 2014; Ribeiro et al., 2015; Sfair et al., 2018). In addition, it is expected that the climatic changes which have occurred will affect the maintenance of endemic species in the Caatinga, reducing the vegetation cover and areas of climatic conditions suitable for establishing communities (Silva et al., 2019).
Reduced vegetation cover makes the soils more susceptible to erosion and compaction processes, reducing the water infiltration rates and the Organic Carbon and Organic Nitrogen levels, in addition to making the microclimate of the environment more stressful due to greater sun exposure, compromising regeneration from the community (Derroire et al., 2016a; Sousa et al., 2012). These factors induce the biotic homogenization of degraded lands (Leal et al., 2005) by limiting the occurrence of sensitive species and favoring the occurrence of stress resistance (Lôbo et al. 2011; Ribeiro-Neto et al., 2016).
In this study, we hypothesized that regenerating stratum in areas with less vegetation cover would show lower: i) richness, ii) diversity indices and iii) individuals with lower height and average diameter than in areas with denser vegetation cover. This hypothesis is due to higher environmental stress caused by increased sun exposure (Derroire et al., 2016b), limited soil nutrients (Sousa et al., 2012), and higher pressure from herbivores on the vegetation as animal access is facilitated in more open areas (Marinho et al., 2016; Skarpe et al., 2007).
Material and Methods
The study area encompasses the region known as Cariri Paraibano, in Northeastern Brazil. The region is located in the Borborema Plateau, between 400 and 1,100 m of altitude. This region extends over 11,192 km². According to IBGE (2010), the region has a population of 185,235 inhabitants.
Cariri Paraibano is one of the driest regions in Brazil. The climate is hot semi-arid (BSh) according to the Koppen classification (Alvares et al., 2013), with precipitation between 300 and 600 mm year-1, constituting the lowest rainfall levels in the country. The rains are usually concentrated in consecutive months, generally from January to July, resulting in long drought periods for the rest of the year, with September being the driest month (Silva et al., 2018). The average annual temperature is 27 °C, with a minimum of 23 °C and a maximum of 41 °C. Potential evapotranspiration is up to four times greater than precipitation, and a marked water deficit predominates in the region (Souza et al., 2015b).
The regional economy has been based on extensive agriculture and livestock since the beginning of its occupation (Alves, 2009; Souza et al., 2015b). Climatic conditions together with this anthropogenic exploration regime have lead the region to become sensitie to the desertification process (Souza et al., 2015a). The study by Oyama and Nobre (2004) shows that this process generates changes in the hydrological cycle, affecting precipitation, evapotranspiration, atmospheric humidity, and decreased runoff, leading to the possibility of significant local and large-scale climate change if the degradation regime continues unchecked.
Mineral weathering is incipient due to the semi-arid climate. Luvisols (40.9 % of the total area), Leptosols (35.6 %) and Regosols (3.9 %) dominates the Cariri (Araújo Filho et al., 2017). These soils are dominantly loamy, shallow (<1 m deep), eutrophic, and have low water retention capacity (Ferreira et al., 2018; Giongo et al., 2011; Menezes et al., 2012; Rückamp et al., 2010).