2. With socioeconomic changes throughout the tropics, movement from rural to urban areas
is increasing. Consequently, many agricultural areas are abandoned, frequently at steep slopes
and high elevations (Foster 569). This is most likely due to low soil fertility (Carreiras 22) and
regional socioeconomic changes instigated by globalization. Little effort has been made to help
guide the reforestation, and hence, many exotic pioneer species are colonizing the abandoned
areas (Grau 1163). Biodiversity can be found to be similar in particular areas decades after
agricultural abandonment (Gonzalez 85). However, studies show that deforestation, agriculture,
and abandonment may have a lasting negative effect on tropical rainforests due to the presence
of invasive species and homogeneity.
Postagricultural secondary forest regrowth varies by region. Most studies focus on an
average of 30 years for a secondary forest to fully develop (Chinea 201; Norden 386). Although
all of the areas examined tropical rainforests, this rate is contingent on what and where the
particular study is analyzing.
In Puerto Rico, species diversity and composition was similar between secondary and old
growth forests after 30 years (Chinea 201). In an abandoned agricultural area in Costa Rica,
there were observations of 21-29 years to fully recover species diversity and composition
(Norden 386). A different study claimed 14-19 years to have 59% of primary forest species
(Barlow 18557). In Jamaica there was still different species composition with more endemic
species in mature forests, after 150 years (Chai 113).
The rate of succession depends on many possible factors. In Southern Bahia in Brazil,
more than half of the species of the secondary forests were found in old-growth forests after 40
years. This result was attributed to low-intensity land use (Fiotto 270). In Puerto Rico, the small
size of farms and light use of fire helped accelerate forest recovery (Grau 1161). Soil fertility
3. and availability of nutrients could be an influencing factor in varying recovery rates (Martin
306). In parts of lowland Costa Rica, less intensive land use and nutrient-rich volcanic soils
most likely led to faster woody regrowth. When local soils are more fertile, higher turnover rates
and nitrogen pools contribute to soil fertility being restored (Reiners 375). Homogeneity in
secondary forests is not found ubiquitously, but subject to regional circumstances such as soil
fertility and previous land use (Martin 306; Guariguate 114; Grau 1161; Fiotto 270)
Species similarity is often an effect of pioneer exotic species dominating because of low
quality conditions for seeds and their ability to thrive (Zang 5). These pioneer species are
successful mainly because they can thrive in the conditions of the new ecosystem, due to habitat
filtering (Katabuchi 646). Similar conditions lead to a suite of species that succeed in the
secondary forest ecosystem (Cornwell 1465). These conditions could range from low canopy
density to high light intensity. In Puerto Rico, an influencing condition was low nitrogen in the
soil that gave introduced species a “competitive advantage” to thrive in the human altered, or
novel ecosystem, soil (Cusack 34). Productive exotic pioneer species, often early successional
species, can potentially generate broadscale homogenization by dominating the forest with their
success (Grau 1163). Humans create novel ecosystems in which native species cannot grow and
alien species are instead the main influence in forest regrowth (Lugo 157). Although not always
the case, most dominating pioneer species are exotic to the area, altering the overall ecosystem
processes such as soil nutrient availability and seed dispersal methods (Chinea 203; Grau 1163).
In Puerto Rico, common pioneer species include Spathodea campanulata (fountain tree), Albizia
procera (white siris), and Magnifera indica (mango tree), all exotic (Chinea 203). In Peru, the
Melastomataceae and Bignoniaceae families were prevalent in secondary forests (Gonzalez 83).
In Australia, Blepharocarya involucrigera (rose butternut) and Melicope elleryana (evodia) (Yeo
4. 211). In Costa Rica, there was an abundance of Pentaclethra macroloba (oil tree), which was
also common in surround old-growth forests (Guariguate 117). Generally, epiphytes and
arborescent ferns are more plentiful in old growth forests with more woody pioneer species and
vines in secondary forests (Martin 297). But it is possible for diverse understory herbs to grow
under the canopy of tree plantations (Rodrigues 1610). Although, constituent species are often
different in secondary forests (Letcher 614), differences exist depending on local conditions with
some areas fully recovering in species diversity and composition. This begs the question, why
do these differences regionally exist?
Depending on the conditions, various species will thrive. Potential limiting factors to be
further discussed for significance include soil fertility, seed availability, plant competition, and
light availability, often based on previous land use (Bu 399).
Although some believe that soil is a main deterrent in agriculture, soil did not have much
effect on species composition in these areas, only explaining 23.5% of the discrepancies across
sites in Northeastern Costa Rica (Letcher 612). As time since abandonment increases, soil
fertility increases with fallow age as soil has time to regenerate (Van Do 258). In Luquillo in
Puerto Rico, soil nutrient availability was similar in abandoned pastures as old growth forests in
similar tropical rainforests. The soils of secondary forests had similar bulk density, percent of
organic matter, carbon exchange capacity (CEC) (Guariguate 114). It did not seem to be a
major limiting factor in these areas (Aide 544). Similarly in Mexico, two different sites with
different land use durations both had suitable soil conditions (Roman-Danobeytia 381). Organic
matter, nitrogen, calcium, and magnesium were all in the medium to high level of tropical forests
(Holl 238). Therefore, soil fertility can be eliminated as a potential influence for the areas we are
studying.
5. Seed availability is the next potential factor. Seed dispersal is a key limiting factor in
pastures because of the lack of trees, hence perches. Pioneer species often are the only
successful ones because of their adaptability to wind dispersal (Zimmerman 356). Often, these
are generalist species that are easily scattered in secondary forests (Chai 116). Forest growth can
be lengthy because the dispersed seeds are not necessarily adapted to be in a tropical rainforest
(Duncan 1005). For example in a rainforest in Africa, the hemiepiphytic Ficus genus was found
in the ground but it cannot grow from the ground (Duncan 1005). Seed availability and dispersal
are a limiting factor in how well species are able to effectively colonize in the abandoned
agricultural land (Zimmerman 356; Chai 116).
Competition from other plants is not always detected in studies as an issue for native
species (Zimmerman 357). Although, in Mexico, native saplings in an abandoned cattle pasture
faired better after alien grasses were removed, showing that competition from nonnative species
was a consideration for this case (Roman-Danobeytia 200). Competition from non-native
species could be a factor contingent on the local conditions and availability of generalist species.
In Northeast Costa Rica, secondary forests had very similar composition to old growth forests,
accredited by the authors to a high abundance of generalist species, high levels of seed dispersal,
and old growth remnants. These generalist species, canopy palms including Euterpe precatoria,
Iriartea deltoidea, Socratea exhorriza, found in intermediate secondary sites were similar to
those found in surrounding mature forests (Norden 356). Limiting factors found to have
significant influence include seed dispersal and the presence of remnant trees. Homogeneity and
lasting negative farming effects do not always have to exist in secondary forest recovery,
depending on the particular conditions in each region. The differences in conditions are partially
dependent on differing land uses, prior to abandonment. Species composition can be
6. significantly altered by the historical land use, especially by the previous crops grown
influencing exotic species available in the secondary forest (Helmer 239).
In a study discussing prior abandonment land use (PALU) in northern Brazil, the authors
found significant differences between varying land uses. With low prior abandonment land use,
6 years or less, and only one slash and burn episode, there was high species richness dominated
by pioneer species, mainly flowering plant species in the Vismia genus (Uhl 384). However, as
the years of prior use increased, more exotic species and a less developed canopy were more
common (Uhl 386). Slash and burn, considered to be the harshest treatment, had high species
richness but was dominated by pioneer early successional species in northeastern Brazil (Rocha-
Santos 479). Bulldozing was also found to result in low species recovery because all biomass
was removed and there was little resprouting from previous inhabiting species (Chazdon 58).
However, even with intense land use, potential conservation methods can aid in forest
recovery (Sandor 3). In Las Tuxtlas, Mexico, an area with trees used as live fences used to
separate pastures had better forest regeneration (Ormeno-Orrillo 831). Remnant trees can act as
nuclei of the forest and aid in restoring species composition and diversity (Sandor 3). In Costa
Rica, rapid biomass accumulation was accredited to remnant trees because of the availability of
seed sources and perches to disperse the seeds (Letcher 614). Although biomass accumulation
not ensure original species composition, a study in northeast Costa Rica did find considerably
higher species similarity in secondary and mature old-growth forests where remnant trees were
left (Norden 391). Remnant trees can be a valuable aid in forest regeneration in many cases,
again showing that local differences can affect how well a forest is restored. Biodiversity and
native species heterogeneity can be promoted with these methods, mitigating the negative
forestry effects of human agriculture and subsequent abandonment (Norden 391).
7. Although the prognosis can appear dismal for secondary forests after agricultural
abandonment in certain areas, studies have proven there are ways that aid in restoring it to
original conditions (Norden 391). Regaining species diversity in a secondary forest does not
denote original species composition (Letcher 614). With 30-40 years of regrowth, particular
local areas can regain many native species (Chinea 201; Norden 386). Although differences
cannot always be predicted, it shows that there is still hope for areas that are abandoned after
agriculture with varying land use prior abandonment, seed dispersal, availability of generalist
species, and remnant trees assisting in healthy regrowth. Homogeneity in plant communities of
early successional species is dependent on these factors.
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