Mortality of sugar maple (Acer saccharum Marsh.) has reached unusually high levels across northern Pennsylvania since the early to mid-1980s. We evaluated the influence of glaciation, topographic position, foliage chemistry, defoliation history, and stand characteristics (species composition, structure, density) on the health of sugar maple in 43 stands at 19 sites on the northern Allegheny Plateau. Using percent dead sugar maple basal area as the measure of health, we found that all moderately to severely declining stands were on unglaciated summits, shoulders, or upper backslopes. Stands on glaciated sites and unglaciated lower topographic positions were not declining. The most important factors associated with sugar maple health were foliar levels of Mg and Mn and defoliation history. The lowest foliar Mg, highest foliar Mn, and highest number and severity of insect defoliations were associated with unglaciated summits, shoulders, and upper backslopes. Declining stands had less than -700 mg·kg -1 Mg and two or more moderate to severe defoliations in the past 10 years; both conditions were associated with moderately to severely declining stands. The decline disease of sugar maple seems to result from an interaction between Mg (and perhaps Mn) nutrition and stress caused by defoliation.Résumé : La mortalité de l'érable à sucre (Acer saccharum Marsh.) a atteint des proportions anormalement élevées partout dans le nord de la Pennsylvanie depuis le début ou le milieu des années 1980. Nous avons étudié l'influence de la glaciation, de la topographie, de la chimie foliaire, des défoliations passées, et des caractéristiques du peuplement (composition, structure et densité) sur la santé de l'érable à sucre dans 43 peuplements établis sur 19 sites dans la partie nord du plateau alléghanien. En utilisant le pourcentage de surface terrière que représentent les érables à sucre morts comme mesure de l'état de santé, nous avons trouvé que tous les peuplements où le dépérissement était modéré à sévère étaient situés sur les sommets, les contreforts, ou le haut des revers qui n'avaient pas subi la glaciation. Les peuplements situés sur des sites exposés à la glaciation ou des sites plus bas qui n'avaient pas subi la glaciation ne dépérissaient pas. Les facteurs les plus importants associés à la santé de l'érable à sucre étaient les concentrations foliaires de Mg et Mn et les défoliations passées. La plus faible concentration foliaire de Mg, la plus forte concentration foliaire de Mn, la sévérité la plus élevée et le plus grand nombre de défoliations d'insectes étaient associés aux sommets, aux contreforts, ou au haut des revers qui n'avaient pas subi la glaciation. Les peuplements dépérissants avaient moins d'environ 700 mg·kg -1 de Mg et avaient subi au moins deux défoliations modérées à sévères au cours des 10 dernières années. Ces deux facteurs étaient associés à des peuplements où le dépérissement était modéré à sévère. Le dépérissement de l'érable à sucre semble être le résultat d'une interaction entre la nutri...

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atmospheric acid deposition is supported on theoretical grounds (Reuss, 1983), by laboratory experiments (Law-Numerous studies have investigated the potential depletion of rence et al., 1999), by observational mass balance studies available base cation pools from forest soils in regions impacted by acid deposition. However, these studies mostly used indirect methods.

A survey of soils associated with oak species was conducted in 2003 and 2004 in Indiana, Illinois, Maryland, Michigan, Minnesota, Pennsylvania, Ohio, West Virginia, and Wisconsin to investigate the occurrence of Phytophthora spp. Soils taken from around the base of healthy and declining oak trees were flooded with H2O and Quercus robur leaflets were used as bait for Phytophthora spp. From 829 soil samples collected near trees, 21% were positive for Phytophthora spp., with 55% of the 125 sites surveyed yielding a Phytophthora sp. Phytophthora cinnamomi was the most frequently isolated species, representing 69.4% of the Phytophthora-infested sites surveyed. Other species, in decreasing order of isolation frequency were Phytophthora sp. 2, P. citricola, P. europaea, P. cambivora, P. quercina-like isolates, and Phytophthora sp. 1. No significant association was found between the presence of Phytophthora organisms and site characteristics such as latitude, elevation, soil pH, or the crown condition of the trees. However, in P. cinnamomi-infested sites, a significant association was found with the deteriorating crown status of Q. alba and the presence of P. cinnamomi. The absence of P. cinnamomi above the 40°N latitude range also was noteworthy.