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By coupling synoptic data from a basin-wide assessment of streamwater chemistry with network-based geostatistical analysis, we show that spatial processes differentially affect biogeochemical condition and pattern across a headwater stream network. We analyzed a high-resolution dataset consisting of 664 water samples collected every 100 m throughout 32 tributaries in an entire fifth-order stream network. These samples were analyzed for an exhaustive suite of chemical constituents. The fine grain and broad extent of this study design allowed us to quantify spatial patterns over a range of scales by using empirical semivariograms that explicitly incorporated network topology. Here, we show that spatial structure, as determined by the characteristic shape of the semivariograms, differed both among chemical constituents and by spatial relationship (flow-connected, flowunconnected, or Euclidean). Spatial structure was apparent at either a single scale or at multiple nested scales, suggesting separate processes operating simultaneously within the stream network and surrounding terrestrial landscape. Expected patterns of spatial dependence for flow-connected relationships (e.g., increasing homogeneity with downstream distance) occurred for some chemical constituents (e.g., dissolved organic carbon, sulfate, and aluminum) but not for others (e.g., nitrate, sodium). By comparing semivariograms for the different chemical constituents and spatial relationships, we were able to separate effects on streamwater chemistry of (i) fine-scale versus broad-scale processes and (ii) in-stream processes versus landscape controls. These findings provide insight on the hierarchical scaling of local, longitudinal, and landscape processes that drive biogeochemical patterns in stream networks.biogeochemistry | hydrologic connectivity | watershed | autocorrelation | heterogeneity S patial heterogeneity of ecosystems has been a focus of landscape ecology for more than two decades, but the linkages between these patterns and underlying processes are still poorly understood (1-3). Quantifying these pattern-process links is largely a problem of scale. Specifically, it is difficult to perform experiments at the landscape scale and measure responses over the range of spatial and temporal scales commensurate with the processes of interest (4,5).This problem of scale limits our understanding of both terrestrial and freshwater ecosystems. Effects of landscape pattern on ecosystem response can be evaluated at stream outlets by using biogeochemical signals that integrate physical and biological conditions of the catchment (6, 7). However, the spatial complexity of biogeochemical patterns and processes within stream networks has not been fully investigated because it is difficult to quantify such patterns at a grain and extent sufficient for examining spatial heterogeneity and processes across scales (8). Quantifying this variability and linking fine-scale and broadscale patterns and processes within the branched topology of stream networks is essenti...

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...