|dc.description.abstract||This project is supported by 20 years of research and previous projects that have been
carried out in the Osa Peninsula by researchers at the TEC Forestry School, all with the
common interest of generating baseline information for the management of the forests
in the Osa Peninsula. The main objective was to “determine the population
demographics of the most intensively harvested merchantable tree species in the Osa
Peninsula”. Three research sites within the Golfo Dulce Forest Reserve were selected.
Within each site four 1-ha permanent plots were established during the “Forest
Management Alternatives” project conducted in 1990 and 1993 (Castillo, 1990,
Demographic parameters are presented for five merchantable species in the Osa
Peninsula (Peltogyne purpuea, Caryocar costaricense, Copaifera camibar, Qualea
polychroma y Aspidosperma spruceanum). Growth rate, mortality and recruitment for
early and late successional species were determined using data collected in three
censuses within the permanent plots. Additionally, demographic matrices were built to
predict the size of the populations in 45 years and the effect of selectively logging 50%
of all trees was determined for a 15-year cutting cycle which is the norm for
management plans usually approved by the Osa Conservation Area.
Predictions of tree demographics after harvestings in which 50% of all trees are
selectively logged every 15 years show a rapid decrease in the late successional species
in the short term (<30 years), and would also cause depletion of important merchantable
species in all successional stages due to the low recruitment of advanced regeneration.
Particularly, this negatively affects P. purpurea, C. costaricense y C. Camibar and
results show that this management regime could cause the local extinction of these
species. On the contrary, A. spruceanum and Q. poylchroma were observed to recover
more quickly after harvestings and this is due to their higher growth rates and their
Overall, it is recommended that the current Forestry Law should be modified to
decrease the extraction rate of these tree species. Thus, cutting cycles should be
extended and harvesting intensities should be modified in order to increase the time
period in between harvestings.
This analysis was based on several silvicultural treatments implemented in the
simulations, which are based on three different silvicultural systems (Cordero and
Howard, 1990; Castillo, 1991). 1. Improved traditional system: it is the traditional
harvesting method of felling and skidding. The planning of forestry operations can thus
reduce damage to the forest. Because of this, directional felling should be practiced to
reduce the amount of damage to standing trees, the forest floor and extracted logs.
During the skidding process, the machinery should remain in the skid trails and all logs
must be extracted by using cables (i.e. winched) in order to reduce road distance and the
damaged cause by machinery in the forest. 2. Skidding with oxen: the oxen are
equipped with chains and a skidding wooden frame. The damaged cause by machinery
in the forest is thus reduced. 3. Traditional skidding system with tractors: most common method employed in the country which is highly damaging to the forest, and without
prior planning can cause great deterioration of the ecosystem. From the above described
skidding methods the different silvicultural systems are derived: Oxen skidding method
(SACB), Improved traditional system (SATM), Traditional skidding system with
tractors (SATT) and Improved skidding method with oxen and tractors.
Regarding tree density (N/ha), there is a tendency to increase with time, which suggests
a recovery of the forest after harvestings. In primary forests tree density was 435,6
trees/ha. Tree density also varied from 350 trees/ha in the SATT treatment to 435
trees/ha in the SACB treatment, with a difference between treatments of 85 trees/ha.
Nonetheless, 15 years after the first harvesting tree density changed to 520 trees/ha in
the SACB and 569 trees/ha in the SABT treatment, reducing the difference between
treatments by 50%.
There is an increase in the basal area (m2
/ha) 15 years after the harvesting, although
values similar to those in primary forests have not been reached yet. In primary forests
the average basal area was 30,12 m2
/ha and, specifically for 2007 it was 29,43 m2
Basal area (m2
/ha) was high regardless of its decrease due to the silvicultural treatments,
and when compared with other humid forests this site ranks high. For example, in
Northern Costa Rica primary forests have been reported to present 23,8 m2
/ha, and in
logged forests it may ranged from 17 to 21,2 m2
In the development curve analysis, it is important to notice the problem with the
abundance of individuals according to their DBH distribution. Not all species reach the
canopy or are very abundant so the strategies for their survival/permanence change. In
this way, the development curves vary dramatically and are related to the autoecology
of the species, which are in turn affected by forest management practices.
Tree growth is influenced by many factors, nonetheless, when forests are being assessed
for forest management the degree of disturbance caused by the harvestings becomes an
important factor since it modifies the standing vegetation and this is tailored according
to the particular growth rates of the species. In these experiments, current annual growth
(ICA) was 2,88- 3,21 cm/year and the highest current annual growth value of 3,1
cm/year was reported for the most severe treatment.
For a 12-year period, and related to the silvicultural treatments implemented, certain
tendencies in species’ behaviors have been observed: current annual growth tends to be
greater in the 30-39,9 cm at DBH and the 50-59,9 cm at DBH categories, but then
decreases considerably in larger categories; this trend is clear in all four treatments.
The species with the highest diameter increment by treatment are: Vochysia ferruginea
with 15,58 mm/year in treatment 1, Vochysia allenii with 18,17 mm/year, and 12,72
mm/year in treatments 2 and 4, respectively, and Tachigali versicolor with 13,3
mm/year in treatment 2. The proceeding demonstrates these species’ potential for
growth. Growth rates are high, Vocyhsia alleni was shown to have the highest growth
rate at 12,53 mm/year. This highlights the 12-year growth interval considered in this
analysis, which is highly significant.
Other equally important within this forest are: Brosium utile, Qualea polychroma,
Simarouba amara, Symphonia globurifera, Tapirira myriantha and Vochysia megalophylla were present in all four treatments and present high current annual growth
rates. At the other extreme, 67 species are only found in one of the treatments and
demonstrate current annual growth rates of 5mm per year. This demonstrates the high
variability in terms of growth rates that exists within tropical forests.
Growth curves of species with high importance or economic values are presented. These
species are Qualea polychroma, Peltogyne purpurea, Calophyllum brasiliense, Carapa
nicaraguensis, Brosimun utile, Symphonia globulifera. Species in different genera but
with similar ecological charactgeristics were grouped: Vochysia (V. allenii, V,
ferruginea, V. guatemalensis, V. megalophyla), Virola (V. koschny, V. sebifera), and at
the family level: Sapotaceae (géneros Pouteria, Elaeoluma, Micropholis, Manilkara),
and lastly the shade-intolerant and fast-growing species: Trattinnicka aspera, Laetia
procera, Apeiba membranaceae, Cecropia obtusifolia, C. insignis, Jacaratia spinosa,
Casearia arborea, Jacaranda caucana, and Castilla tunu. All of these present the
typical hyperbole curve in their distributions.
With respect to mortality among treatments, results indicate that treatment III SATT
presented the highest mortality rates, and treatment IV SABT presented the highest
recruitment rates. The current tendency is where disturbances were greater mortality
Ecological guilds are used in grouping species presenting similar autoecological
characteristics in relation to their shade-tolerance. From a silvicultural perspective, this
classification allows to assess forest growth after the treatments by means of DBH
From the disturbance viewpoint, treatment 4 was the most severe, and it is to be
expected that shade-intolerant species present the highest current annual growth rates.
This was confirmed and for the fast-growing shade-intolerant species and the pioneer
species which attained 4,22 – 4,29 mm/year of DBH growth per year, respectively. It
was also observed that the shade-intolerant group presents the highest current annual
growth in all treatments, which was also to be expected.
An important objective in this project was to translate demographic information into
practical recommendations for extraction rates and harvesting methods for the study
species. Being able to monitor species for 17 years provides very valuable information
that sheds light on the ecology of the tree species, on their growth rates, and
silviculturally by studying their response to different silvicultural regimes. This
information together is useful when designing sustainable forestry practices for the
forests in the Osa Peninsula.
After the publication of the Standards for Sustainable Natural Forest Management,
according to the Executive Decree 34559-MINAE, published in the Gaceta 115 on June
16th, 2008, the use of the Practices Code and the Manual of Procedures is enforced
thereon. This decree demands the application of reference values for two very important
variables: basal area (m2
/ha) and the ranges of absence/presence of species according to
their ecological guild.
Because this decree enforces its implementation in the country, data from two very
specific sites was used to establish these guidelines: Northern Costa Rica and the area of influence of FUNDECOR (Foundation for the Development of the Central Cordillera),
and there are differences in the variables discussed here between sites.
To have more specific parameters of forest dynamics in this region, the Osa
Conservation Area (ACOSA) has planned to generate new reference values for this
region, and for this reason this study’s results become very valuable, because it provides
up to date information which has been requested to the researchers working in the area
by ACOSA. Given the importance of this request, the dissemination of the findings in
this study can be readily distributed and incorporated into the different government
sectors. Meetings were carried out between the ACOSA officials and the researchers,
with the objective of designing a strategy that could be submitted to the National
Commission of Sustainable Forestry in order to modify the current Executive Decree
This project’s results suggest that the reference values set by the Executive Decree
34559-MINAE are small compared to the values reported in the Osa Peninsula’s forests.
This represents a disadvantage for these forests because they are harvestable according to the decree. This is mainly due to the fact that the Peninsula’s forests are more productive than most forests in the country.||es