Abstract
Environment and seedling community under isolated trees in pastures are different from those in the open pasture. The effect of the pasture trees on the soil nutrients and on the seedling growth were investigated. Seven isolated trees and eight plots were selected in two pastures of 12-yr and 32-yr old derived from a lowland rain forest with nutrient-rich soil at Los Tuxtlas, Mexico. The soil concentrations of total N, P Bray, K+, Na+, Ca2+ and Mg2+, plus others physical and chemical characteristics, were compared between the pasture trees and the open-pasture. An experiment was done to test the hypothesis that soil from under the pasture trees was better for seedling growth than soil from the open pasture. Seedlings of two native tree species and two domesticated species were grown in soil from the two different sites in a shade-house. The dry weight of the shoot and root/shoot ratio were compared. Only total N, P and Na+ differed slightly in concentrations between the sites, but did not promote more seedling biomass. It seems that the soil at this location is sufficiently nutrient-rich even in the open pastures and over-ride any effect of the pasture trees on nutrient availability.
Highlights
The landscape at Los Tuxtlas, México is characterised by a mosaic of forest fragments, and pastures with isolated remnants of forest trees and tree lines as living fences for pasture boundaries
Higher surface-soil nutrient concentrations are expected under the trees than in the open pastures since tree crowns protect the soil from leaching, promote nutrient throughfall, and there is a redistribution of nutrients from lower soil horizons through litterfall (Baillie 1989, Grubb 1989)
No difference was found in seedling growth between the soil from the open pastures and the soil under the trees (Beans1, t = 0.98, p = 0.59; Beans2, t = 0.02, p = 0.94; Maiz1, t = 1.68, p = 0.11; Maiz2, t = 0.03, p = 0.87; I. sinacae, t = 1.55, p = 0.2; C. megalantha, t = 0.98, p = 0.68)
Summary
The landscape at Los Tuxtlas, México is characterised by a mosaic of forest fragments, and pastures with isolated remnants of forest trees and tree lines as living fences for pasture boundaries. Soils under pasture tree canopies have less light, more equable temperatures and higher fertility compared to the open savanna (Isichei and Moughalu 1992, Mordelet et al 1993, Belsky 1994). Are soil nutrient concentrations themselves responsible for more seedling growth under pasture trees than in open pasture? The soil nutrient concentrations under pasture trees were compared to those from the open pasture. It was hypothesised that soil nutrient concentrations would be higher under the pasture trees, which in turn would promote higher seedling growth. Mean annual rainfall is 4 487 mm (with a gap of three years). Months with lowest rainfall are February (261 mm), March (115 mm), April (97.5 mm), and May (105 mm) (Biological Station unpublished data)
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