Reading N. 2: Soil Composition

Soil Composition

DRY FOREST

Soils consist primarily of volcanic material mixed with small amounts of marine strata. These are mostly sandy, clayey or formed from volcanic stones. Only locally in the south, contains both pre-Cretaceous metamorphic rock and granite of similar age. It is the natural eastern limit of the dry forest in the northern part of the ecoregion. The Chongón-Colonche range is the major mountain group of the ecoregion, and lies in a primarily southeast-northwest direction. These foothills form basins of small rivers that emerge primarily in the rainy reason.

An important source of moisture in these mountains is the sea mist known as Garúa that forms during the austral winter which condenses at higher elevations. West of the coastal range the dry tropical forest becomes tropical rainforest, without a clearly discernible transition zone. These topographical conditions create an altitudinal gradient that is reflected in the characteristics of the vegetation in the dry forest, which despite what might be expected has higher rates of regeneration and growth than the wet forests, a favorable aspect for its natural regeneration.

The southwest zone is more affected by the Humbolt current and precipitation does not exceed 1500 millimeters (mm) per year, while the coastal strip is drier and precipitation reaches 300 mm. The vegetation is characterized as deciduous or semi-deciduous due to seasonal drought and can be considered a homogeneous arboreal mass that is quite dense. The dry forest has trees that reach 20 meters (m) high, abundant bromeliads, mosses and epiphytes, while the undercanopy has many species of thorny shrubs and cactus and the understory is dominated by small herbaceae mainly from the Acanthaceae and Polypodiopsidae families.

Some of the principal endemic species of vegetation specific to this ecoregion are: Ceibos, mesquite, Cordia lutea, Cordia,  Moral Fino (Sota), Maclura tinctoria, guaiacan, Palo santo, Chanduy, Croton.  There are other species highly prized by the lumber industry and thus endangered such as golden trumpet, laurel, cedar, ebony, "madero negro" and "Colorado”. Other species are very valuable for the non-wood products they provide; this is true of the tagua palm  and "barbasco".

Rainforest

Many tropical soils have been under forest cover for millions of years. Over this period, and under high rainfall conditions, deep tropical soils have been formed from the underlying rock. Tropical soils are often several metres deep, but the soils are often washed out, or strongly leached, with large amounts of nutrients and minerals being removed from the subsoils and considerable thickness of rock broken down to produce soil. Over many millions of years this leaching has left most of the soils lacking many of the fundamental nutrients needed by the above ground vegetation.

So how does such a lush vegetation exist if the soils are so depleted of nutrients. The answer lies in the very thin topsoils, made up mainly of decaying vegetal and animal remains. An amazing cycle exists between the huge body of vegetation above ground and this thin topsoil. The rainforest depends for its nutrients on the constant recycling of its enormous biomass.

Plant remains fall to the ground, are consumed and broken down by the huge range of soil organisms in and on the soil, converted by these back into nutrients which can then be used by the dense vegetation above. It is a constant cycle. The thin layer of topsoil is the engine house for the food supply for the tropical forest and, together with climate, is responsible for the maintenance of the huge biomass. This is surely one of the most incredible cycles in nature - and it works.

The tropical rainforest is often in the news for various reasons. There are major concerns about deforestation of it and the consequent damage to the soil. From a soil point of view, cutting down of the rainforest disturbs the natural soil-plant cycle and makes the soils extremely vulnerable to soil erosion and loss of this vital topsoil. The topsoil also holds huge amounts of carbon which is now known to have a major potential influence on CO² levels in the atmosphere and hence a major potential influence on climate change.

MANGROVE

Soils are made up of sand, silt and clay in different combinations, and 'mud' actually refers to a mixture of silt and clay, both of which are rich in organic matter (detritus).