Stand dynamics and basal area change in a tropical dry forest reserve in Nicaragua

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Abstract

Stand dynamics and basal area change were determined in deciduous and gallery forest types at the Chacocente Wildlife Reserve, Nicaragua. All stems ≥10 cm dbh in 4 ha were tagged and identified by species and measured in 1993 and 2000. In year 2000 totally 519 stems ha−1 with a basal area of 15.62 m2 ha−1 were recorded in the deciduous forest type and corresponding figures were 308 stems ha−1 and 23.13 m2 ha−1 for the gallery forest type. Comparison of stem diameter and basal area distribution during this study period revealed no changes. Both forests types had a reversed J-shape diameter distribution dominated (>80%) by small stems (<30 cm dbh). In the deciduous forest small stems contributed to more than half of the basal area, whereas in the gallery forest large stems (>70 cm dbh) contributed to almost half the basal area. Based on a logarithmic model the mortality and recruitment rates were calculated at 4.5 and 2.5% year−1, respectively, in the deciduous forest type and 4.2 and 4.0% in the gallery forest type. The decrease in stand density in the deciduous forest type was significant whereas it was not the case for the gallery forest type. There was also a significant decrease in basal area of 1.2% year−1 in the deciduous forest and no change in the gallery forest. The recorded median diameter (dbh) increment was 0.14 cm year−1 with a range of 1.21 cm year−1 in the deciduous forest type and corresponding figures for the gallery forest were 0.24 cm year−1 and 0.71 cm year−1. Three of the five most common species in the deciduous forest, Lonchocarpus minimiflorus, Gyrocarpus americanus and Stemmadenia ovovata had mortality rates above 9%. Although L. minimiflorus and S. ovovata had recruitment rates above average the net balance was negative. Among the five most common species only Tabebuia ochracea a timber species had an annual recruitment higher than its mortality rate. Non-timber species as a group had the largest calculated negative balance between mortality and recruitment as well as between loss and gain of basal area indicating a possible anthropogenic influence. In the gallery forest Capparis pachaca was the only species, out of the most common, with a positive annual balance. In both forest types there was a higher than average calculated recruitment and basal area growth for species with no local use.

Introduction

Central America has approximately 3.4 million ha of tropical dry forest (Sabogal, 1992) mainly along the Pacific coast and it has been reduced to less than 0.1% of its original extent and it is considered the most endangered ecosystems in the tropics (Janzen, 1988, Gillespie, 1999). The main reason for this deforestation is the conversion of forests to agricultural land. Degradation in existing unprotected forest is caused by accidental and intentional fires, wood collection, grazing by livestock and selective logging of valuables species (Gerhardt and Hytteborn, 1992, Murphy and Lugo, 1995).

Out of the 0.1 million ha of dry forest in Nicaragua about 60% was declared conservation area (Alves-Milho, 1996) and the remaining dry forest was managed for production of timber and woodfuel. Given the high population density of more than 100 inhabitants km−2 in the Pacific Region the demand for woodfuel from the dry forest is high (Roldan, 2001). Historically Nicaraguan dry forests have provided timber like Swietenia humilis, Cederia odorata, Bombacopsis quinatum, Dalbergia retusa and Guaiacum sanctum for the domestic market and for export (Sabogal, 1992). The selective logging was made without consideration of the species-specific regeneration requirements and the long-term consequences on the species composition. At present there is a limited experience in sustainable dry forest management, which has mainly been carried out in development projects in the 1990s (Anon., 2002). Management plans in these areas were based on a polycyclic silvicultural system in which each logging operation was limited to certain species or species groups (Lamprecht, 1990, Alves-Milho, 1996).

In general, information about the dry forest dynamics and growth of economically valuable species, or the entire tree population, was lacking or inferred from a single survey based on size-class description of the population structure. Studies on changes of stocking, basal area distribution and growth over a longer time period is necessary to provide the manager with knowledge about the magnitude of change in population size, i.e., in mortality and recruitment rates, growth and yield (Díaz et al., 2000).

To develop sound management systems for harvesting and/or conservation of forest resources, it is fundamental to understand the dynamics of the forest (Sokpon and Biaou, 2002, Obiri et al., 2002). As the forest ecosystem is not stable in time in terms of size and structure, long-term observations are one of the best ways to provide new insights (Takahashi et al., 2003). The measurement of recruitment, mortality and growth rates provides useful information to analyze factors affecting population dynamics. Studies on population dynamics and growth have been carried out in permanent sample plots of different size according to type of forest (Manokaran and Kochummen, 1987, Finegan and Camacho, 1999, Gray, 2003).

Research on tropical dry forest has been performed in Mexico and Costa Rica providing information on species ecology of the same tree species that grow in the Nicaraguan dry forest (Rico-Gray et al., 1988, Janzen, 1988, Gerhardt, 1996, Mizrahi-Perkulis et al., 1997). However, there are only few studies available on the country's dry forest resources and little information on species ecology, biology, silviculture and utilization. A few floristic surveys have been carried out, mainly for conservation purposes (Sabogal and Valerio, 1995, Gillespie et al., 2000).

The objective of this study was to assess the forest dynamics and growth parameters in the Chacocente Wildlife Reserve. These parameters were analyzed per deciduous and gallery forest types, at species level as well as for aggregation of species into use groups. Changes in use groups were considered as a proxy for the anthropogenic influence.

Section snippets

Study area

This study was carried out in the Chacocente Wildlife Reserve (11°36N–11°30N and 86°08′–86°15W) located on the Pacific Coast, in the department of Carazo. In the period 1981 to 2001, the annual precipitation and temperature was 1431 ± 369 mm and 26.6 ± 0.34 °C, respectively. The dry period spans over 5 months, from December to April, and the altitude varies from sea level to ca. 300 m.a.s.l. (Sabogal, 1992). The total area of the wildlife reserve was ca. 4650 ha of which two-third were dry deciduous

Stand dynamics and diameter distribution

In total 899 stems were recorded in year 2000 at 2 ha, which was 13% less than in 1993 and this corresponds to an annual loss of 11.5 trees per hectare. The decrease in stand density was significant (t = 4.75, d.f. = 49, p = 0.00). The negative balance was due to 282 dead or missing stems and 144 newly recruited stems corresponding to a mortality and recruitment rate of 4.5 and 2.5% year−1, respectively, yielding a net annual decrease of 2%. Both in year 1993 and 2000, the diameter distribution showed

Stand dynamics

Annual mortality rates reported for tropical forests vary considerable; 0.9% in a dry forest in Ghana (Swaine et al., 1990); 2.03% in a rain forest in Costa Rica (Lieberman and Lieberman, 1987); 2.91% in a mixed deciduous forest in Thailand (Marod et al., 1999); 1.06% in a humid forest in Panama (Swaine et al., 1987). In comparison the rates recorded in this study in Chacocente (4.5% in the dry deciduous forest and 4.2% in the gallery forest) were considerably higher. Mortality rates in our

Conclusions

In this study in the Chacocente Wildlife Reserve in Nicaragua we found that there were differences among species use groups with regard to stand dynamics and growth, which indicated that there was an anthropogenic influence. All species included the stand density and basal area decreased in the deciduous forest type whereas there was no change in the gallery forest type. Species with no local use increased at the expense of other use groups in both forest types. In the deciduous forest type the

Acknowledgements

I would like to thank Claudio Calero, Ali Water and Alvaro Noguera for their help during fieldwork. The Ministry of Natural Resources and Environment allowed us kindly to carry out this research at Chacocente Wildlife Reserve. We also wish to thank to Karin Gerhardt for comments on the manuscript. Financial support was provided by the Swedish International Development Agency.

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