The expansion of the pinyon-juniper (Pinus monophylla Torr. & Frém.-Juniperus osteosperma Torr.) woodland type in the Great Basin has been widely documented, but little is known concerning how topographic heterogeneity influences the temporal development of such vegetation changes. The goals of this study were to quantify the overall rates of pinyon-juniper expansion over the past 3 decades, and determine the landscape factors influencing patterns of expansion in central Nevada. Aerial panchromatic photos (1966-1995) were used to quantify changing distribution of pinyon-juniper woodland, over multiple spatial scales (0.002-, 0.02-, and 0.4-ha median patch sizes), and for discrete categories of elevation, slope aspect, slope steepness, hillslope position, and prior canopy cover class. An object-oriented multiscale segmentation and classification scheme, based on attributes of brightness, shape, homogeneity, and texture, was applied to classify vegetation. Over the 30-year period, the area of woodland has increased by 11% over coarse, ecotonal scales (0.4-ha scale) but by 33% over single-tree scales (20-m 2 scale). Woodland expansion has been dominated by infilling processes where small tree patches have established in openings between larger, denser patches. Infilling rates have been greatest at lower elevations, whereas migration of the woodland belt over coarser scales has proceeded in both upslope and downslope directions. Increases in woodland area were several times greater where terrain variables indicated more mesic conditions. Management treatments involving removal of trees should be viewed in a long-term context, because tree invasion is likely to proceed rapidly on productive sites. Resumen La expansió n del bosque de tipo ''Pinyon-Juniper'' (Pinus monophylla Torr. & Frém.-Juniperus osteosperma Torr.) en la Gran Cuenca ha sido ampliamente documentada, pero poco se sabe respecto a como la heterogeneidad topográfica influye en el desarrollo temporal de tales cambios de vegetació n. Las metas de este estudio fueron cuantificar las tasas generales de expansió n del boque de ''Pinyon-Juniper'' en las tres ú ltimas décadas y determinar los factores del paisaje que influyen en los patrones de expansió n en la parte central de Nevada. Se usaron fotografías aéreas pancromá ticas (1966-1995) para cuantificar la distribució n cambiante del bosque de ''Pinyon-Juniper'' en mú ltiples escalas espaciales (parches tamañ o medio de 0.002, 0.02, y 0.4 ha) y para categorías discretas de elevació n, aspecto de la pendiente, inclinació n de la pendiente, posició n en la pendiente, y clase de cobertura de copa anterior. Una segmentació n multiescala objeto-orientada y un esquema de clasificació n, basado en atributos de brillantez, forma, homogeneidad, y textura, se aplicaron para clasificar la vegetació n. En el periodo de 30 añ os el área de bosque se incrementó en 11% en las escalas ecotonales (escala de 0.4 ha), pero aumentó en 33% en la escala de á rbol (escala de 20 m 2). La expansió n del bosque ha estado dominada por...

Ecosystems that have low mercury (Hg) concentrations (i.e., not enriched or impacted by geologic or anthropogenic processes) cover most of the terrestrial surface area of the earth yet their role as a net source or sink for atmospheric Hg is uncertain. Here we use empirical data to develop a rule-based model implemented within a geographic information system framework to estimate the spatial and temporal patterns of Hg flux for semiarid deserts, grasslands, and deciduous forests representing 45% of the continental United States. This exercise provides an indication of whether these ecosystems are a net source or sink for atmospheric Hg as well as a basis for recommendation of data to collect in future field sampling campaigns. Results indicated that soil alone was a small net source of atmospheric Hg and that emitted Hg could be accounted for based on Hg input by wet deposition. When foliar assimilation and wet deposition are added to the area estimate of soil Hg flux these biomes are a sink for atmospheric Hg.