The results of recent correlations showing a negative impact of population growth on economic development in cross-country data for the 1980s, versus "nonsignificant" correlations widely found for the 1960s and 1970s, are examined with contemporaneous and lagged components of demographic change, convergence-type economic modeling, and several statistical frameworks. The separate impacts of births and deaths are found to be notable but offsetting in the earlier periods. In contrast, the short-run costs (benefits) of births (mortality reduction) increase (decrease) significantly in the 1980s, and the favorable labor-force impacts of past births are not fully offsetting.

Arctic landscapes have visually striking patterns of small polygons, circles, and hummocks. The linkages between the geophysical and biological components of these systems and their responses to climate changes are not well understood. The “Biocomplexity of Patterned Ground Ecosystems” project examined patterned‐ground features (PGFs) in all five Arctic bioclimate subzones along an 1800‐km trans‐Arctic temperature gradient in northern Alaska and northwestern Canada. This paper provides an overview of the transect to illustrate the trends in climate, PGFs, vegetation, n‐factors, soils, active‐layer depth, and frost heave along the climate gradient. We emphasize the thermal effects of the vegetation and snow on the heat and water fluxes within patterned‐ground systems. Four new modeling approaches build on the theme that vegetation controls microscale soil temperature differences between the centers and margins of the PGFs, and these in turn drive the movement of water, affect the formation of aggradation ice, promote differential soil heave, and regulate a host of system properties that affect the ability of plants to colonize the centers of these features. We conclude with an examination of the possible effects of a climate warming on patterned‐ground ecosystems.

Frost boils in northern Alaska vary from large, 2-3-m diameter, barren non-sorted circles to completely vegetated hummocks. Summer warmth increases southwards from the coast. Average thaw-layer thickness shows the opposite trend. Frost heave shows no trend along the climate gradient but is affected by soil texture. Heave is greatest on frost boils with fine-grained sediments. Biomass increases from 183 g m À2 at the coast to 813 g m À2 in the Arctic Foothills. An aggrading permafrost table is evident in most of the frost-boil soil profiles, indicating that, over time, accumulation of plant biomass leads to reduced thaw-layer thickness. A conceptual model suggests how vegetation affects the morphology of patterned ground forms. In the coldest parts of the High Arctic well-developed frost boils do not form and there is little vegetation on frost boils or the inter-boil areas. In the warmest parts of the Low Arctic, vegetation is usually sufficient to stabilize the frost boil soils. Frost boils play an important role in Arctic ecosystems functions, including the flux of trace gases to the atmosphere, flux of water and nutrients to streams, and the recycling of important nutrients to wildlife populations.

O11, J1, O4, Demography, convergence, growth,

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Field studies of atmospheric CO 2 effects on ecosystems usually include few levels of CO 2 and a single soil type, making it difficult to ascertain the shape of responses to increasing CO 2 or to generalize across soil types. The Lysimeter CO 2 Gradient (LYCOG) chambers were constructed to maintain a linear gradient of atmospheric CO 2 ($250 to 500 ll l -1 ) on grassland vegetation established on intact soil monoliths from three soil series. The chambers maintained a linear daytime CO 2 gradient from 263 ll l -1 at the subambient end of the gradient to 502 ll l -1 at the superambient end, as well as a linear nighttime CO 2 gradient. Temperature variation within the chambers affected aboveground biomass and evapotranspiration, but the effects of temperature were small compared to the expected effects of CO 2 . Aboveground biomass on Austin soils was 40% less than on Bastrop and Houston soils. Biomass differences between soils resulted from variation in biomass of Sorghastrum nutans, Bouteloua curtipendula, Schizachyrium scoparium (C 4 grasses), and Solidago canadensis (C 3 forb), suggesting the CO 2 sensitivity of these species may differ among soils. Evapotranspiration did not differ among the soils, but the CO 2 sensitivity of leaf-level photosynthesis and water use efficiency in S. canadensis was greater on Houston and Bastrop than on Austin soils, whereas the CO 2 sensitivity of soil CO 2 efflux was greater on Bastrop soils than on Austin or Houston soils. The effects of soil type on CO 2 sensitivity may be smaller for some processes that are tightly coupled to microclimate. LYCOG is useful for discerning the effects of soil type on the CO 2 sensitivity of ecosystem function in grasslands.

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