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SOIL CHARACTERISTIC COMPARISONS OF A NEOTROPICAL RAIN FOREST – LA SELVA, COSTA

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Kathy DeWein, George Goff, Michael Graham, Irene Stone, Susan Hoffmire

Abstract

The chemistry of soil is complex partly because living things are involved and partly because soil has a capacity to store ions, and nutrients.  Soil can be either a fertile or infertile setting dependent on the surrounding environment.  Soils of a neotropical rain forest are continuously washed by rain as long as the soil has existed, yet it still contains nutrients for plant growth, allowing many areas to remain fertile grounds for vegetative growth.  This is possible because of most of the essential plant nutrients in soils are cations, and can therefore, be held by the mechanism of cation exchange capacity (CEC).

Introduction

Is there a correlation between soil fertility and soil types? Cation exchange capacity is associated with the colloids in soil including both clay and organic matter.  The amount of CEC is a measurement of the amount of hydrogen ions that can be stored in the soil.  Important as well for formation of fertile soil are the influences of the local climate, canopy cover, topographic position of the area, the amount of vegetative matter in the area, and most importantly, the parent soil material and age of the soil.

Nutrient acquisition has been dependent upon atmospheric input and fluvial deposition.  The wet deposition from abundant rainfall averages about 3,993 mm annually, and the dry deposition from the sediment deposition of dust, fine aerosols, and sorption of atmospheric gases and vapor.  Nutrients have also been acquired through nitrogen fixation; a process of conversion of nitrogen of certain plant roots into nitrates and nitrites, essential for good soil fertility. 

La Selva’s soils have been commonly classified into three different soil orders and three different soil types.  The soil taxonomy of the La Selva Biological Reserve composed of humid tropical soils are classified as Entisols, which is young mineral soil generally lacking a B horizon; Inceptisols soil which is youthful soil with a distinct B horizon and poorly developed lower horizons, and lastly, Ultisols, well-weathered soil mostly composed of clay in the B horizon.  Roughly one-third of the property at La Selva consists of comparatively fertile inceptisols and some of the recent entisols soil of alluvial origin located near the major rivers forming the northern and western borders of La Selva.  Most of the remainder of area in the steeper southern foothills contains relatively fertile, more acidic ultisols, which are the highly weathered soils having low indications of the basic cations necessary for fertile soil.

Overall, the soils of La Selva have been weathered from frequent and abundant precipitation and consist of very deep, low-density, clay-sized particles with relatively high organic matter content and porosity, and a stable aggregate structure.  Because of this consistency, these soils have been able to retain large amounts of water, yet have readily permitted water flow to leach into the soils.

The type of soil areas studied at La Selva were volcanic soils from three previous lava flows, indicative of Andesitic and Basaltic rocks, the alluvial terraces, and the marshy depressions containing clays and depositions from the V-shaped valleys.  The specific sites analyzed were Alluvial Terraces: Sites A1, A2, and A3; Marshy Depressions: Sites M1, M2, and the Volcanic Soils: Sites V1, V2, and V3.

Sampling Method

The soil analysis tests performed were soil pH, cation exchange capacity (CEC), and chemical tests for nitrogen, potassium, and phosphorus. 

Table 1

Lichen samples were taken at this belt-transect height.  A two centimeter wide band of bark was scraped clear of lichen growth.  The samples were identified by tree location and placed in a plastic bag for transport to the lab for analysis. 

Once in the lab the lichen samples were microscopically examined for their morphological features.  Lichen samples for trees in the same sample location were combined.  Each lichen sample in each set was first classified according to their morphological features as belonging to one of four types:  fruitiose, foliose, squamulose or crustose.  Then each sample within these four types was looked at for color or textural differences.  These differences were assumed to represent a different morpho-species.  Each morpho-species was recorded.  (See Table 2)

Table 2

Data Analysis

Although our samples show the highest abundance and diversity of morpho-species of lichen to be on trees measured at the artificial edge,  lichens measured in the primary forest showed no significant difference in diversity and abundance.  However, the lichens measured at the natural edge showed substantially lower abundance and diversity. (see Chart 1)

The lichen diversity and abundance at the artificial edge are only slightly higher than that within the forest.  Both areas showed species richness but the low number of overlapping species clearly indicates species sensitivity to the two microclimates.

Chart 1

Conclusions

These results were expected for the forest location.  The Wilson Forest is a primary forest, carefully protected from human impact. The artificial edge is a pasture that has not been grazed for almost 15 years.  Perhaps this absence of human impact  has allowed lichens most suitable to the increased light and temperature conditions of the pasture edge to become established and healthy.  The lack of lichen growth at the natural edge might reflect the increased humidity of the river’s ecosystem, encouraging competitive moss growth. Most of the trees at this edge were observed to be covered with moss.  These conditions may allow the mosses to out-compete the lichens with respect to nutrients and space.

These results complement a study done by the staff biologist at Wilson Botanical Gardens (Quiros 2001).  His study indicated greater lichen growth on trees located at the edge of the garden versus trees growing in the center of the garden. 

 Further studies of this phenomenon can include higher sampling populations, different tree species and different natural edges.