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).