PHYSICOCHEMICAL HETEROGENEITY OF SUPERFICIAL SOIL LAYERS IN CONIFER PLANTATIONS VERSUS ORIGINAL BROAD LEAF FORESTS IN ARIEGE (PYRENEES, FRANCE)

Changes in physico-chemical properties of superficial soil layers foll owing the substitution of broad leaf trees by conifers have been studi ed in four Pyrenean sites. Parameter values may change significantly f rom native to reforested plots, but sometimes in opposite directions. Initial conditions (historical, pedological, and climatic) were not th e same in each plot and the present A soil horizon is highly dependent on these. A lessening of organic matter degradation and humification conditions can be, however, inferred from direct humus observations, b ut differences between physico-chemical properties in native and refor ested plots are, nevertheless, quantitatively low, particularly in the pH range usually appropriate for conifer plantations.