Abstract

MARTINS, Afonso; LOUSADA, José; BRANCO, Isabel  and  CAETANO, Paula. Soil and Environmental Variables Associated with the Quercus suber Decline in Portugal: Attempt of Identification and Difficulties. Silva Lus. [online]. 2006, vol.14, n.2, pp.155-167. ISSN 0870-6352.

The Quercus suber decline is a serious problem on wide areas of cork oak stands in Portugal, and several studies have been developed in order to identify which factors are closely related to that disease in order to orient the application of adequate management solutions. Many of the studies indicate that soil properties can be related to the disease due to the restrictions they cause to plant development and vegetation, which can enhance pathogen aggressiveness, among them the Phytophthora cambivora and Phytophthora cinnamomi fungi. In the present study, a quantified set of site variables on ten experimental areas were selected in the northern and southern parts of the country, with a surface between 975 and 3150 m^2.and an attempt was made to look for the site characteristics more closely related to the severity of the disease, quantified through the degree of defoliation. For site characterization, the following variables were selected: aspect, slope, distance to the summit, mean annual rainfall, mean annual temperature, density and height of understory vegetation, soil depth, soil compacity, coarse material, sand, silt and clay contents in the soil, soil organic matter content, soil exchangeable bases, and exchangeable acidity. The selection of those variables was supported by a wide agreement, although it is recognized that it is difficult to quantify some of them. The obtained information in the field and the analysis of results, supported by the multiple regression analysis, allowed the following conclusions: (i) Soil compaction, and density and height of understory vegetation, were shown to be the most responsible variables for the disease severity in the southern area of the country, which corroborates previous studies; (ii) The application of the multiple regression analysis using the 18 variables lead to a very low explanation of the disease severity - 17, 25, and 21 percent, respectively, for all the areas, the northern and southern areas, separately; (iii) The analysis of the obtained results of all the sampled areas showed some relations between the independent and dependent variables which are difficult to explain, while the separated analysis for northern and southern areas showed some relations which make sense and corroborates the field observations, but the difficulties in explaining some of the obtained relations continue; (iv) Thus, the statistical analysis of the results do not allow a clear identification of the variables closely related to the severity of the disease, which is ascribed to the interaction among variables and the difficulty in quantifying some of them.

Keywords : plant pathology; site quality; soil quality indicators.

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