The Beauce region of Quebec has been relentlessly affected by sugar maple (Acer saccharum Marsh.) tree decline since the late 1970s. Nutrient disturbances are generally associated with maple decline, but the severity of decline symptoms can vary quite dramatically between individuals within a stand. Possible causes of this variability were investigated, including soil chemistry and endomycorrhization. Within 18 mature sugar maple stands, a comparative study of fine root colonization rate by endomycorrhizal fungi, and foliar, fine-roots, and soil-nutrient status between healthy and declining sugar maple trees was carried out. Three individuals showing a very low degree of decline symptoms (healthy) and three individuals in the vicinity exhibiting severe decline symptoms (declining) were selected in each stand. Although trees of both health classes were K and Ca deficient, the diagnosis revealed that those in the declining group were experiencing a more severe nutrient stress and lower stem radial growth than those in the healthy group. The percent colonization by endomycorrhizal fungi in fine roots of sugar maple varied from 8 to 40% among stands, with an average of 23%. However, the endomycorrhization rate was not related to tree health status. The frequency of endomycorrhization was positively correlated to soil pH and soil exchangeable Mg saturation, but negatively to the proportion of H + Al held on the soil exchange complex. Also, the rate of endomycorrhization was correlated positively to foliar and root Ca content, but negatively to foliar and root N content. Foliar N, P, K, Ca, Mg, and Mn contents were positively correlated to corresponding nutrient content in fine roots. Fine-root chemistry was only partly related to soil chemistry. Declining trees had a lower foliar K content and a lower P and Ca content in fine roots than healthy ones. The results do not support the hypothesis that sugar maple decline and its disturbed nutrient status is associated with lower colonization by endomycorrhizal fungi in fine roots. They suggest, however, that soil chemical properties, particularly the soil composition in cations, regulates fine-root colonization by endomycorrhizal fungi and sugar maple nutrition and health. These results can neither confirm nor invalidate the hypothesis according to which a deleterious microbial population may have colonized the soil under declining trees. Key words: Acer saccharum, soil cation saturation, forest decline, nutrient status, fine root, endomycorrhizae.
Nutrient Content and Substrate Effect on Fine Root Density and Size Distribution in a Nicaraguan Rain Forest1