Biocomplexity in Coupled Natural–Human Systems: A Multidimensional Framework

As defined by Ascher, biocomplexity results from a “multiplicity of interconnected relationships and levels.” However, no integrative framework yet exists to facilitate the application of this concept to coupled human–natural systems. Indeed, the term “biocomplexity” is still used primarily as a creative and provocative metaphor. To help advance its utility, we present a framework that focuses on linkages among different disciplines that are often used in studies of coupled human–natural systems, including the ecological, physical, and socioeconomic sciences. The framework consists of three dimensions of complexity: spatial, organizational, and temporal. Spatial complexity increases as the focus changes from the type and number of the elements of spatial heterogeneity to an explicit configuration of the elements. Similarly, organizational complexity increases as the focus shifts from unconnected units to connectivity among functional units. Finally, temporal complexity increases as the current state of a system comes to rely more and more on past states, and therefore to reflect echoes, legacies, and evolving indirect effects of those states. This three-dimensional, conceptual volume of biocomplexity enables connections between models that derive from different disciplines to be drawn at an appropriate level of complexity for integration.     

Accumulation of Carbon and Nitrogen in Residential Soils with Different Land-Use Histories

Urban areas are growing in size and importance; however, we are only beginning to understand how the process of urbanization influences ecosystem dynamics. In particular, there have been few assessments of how the land-use history and age of residential soils influence carbon (C) and nitrogen (N) pools and fluxes, especially at depth. In this study, we used 1-m soil cores to evaluate soil profile characteristics and C and N pools in 32 residential home lawns that differed by previous land use and age, but had similar soil types. These were compared to soils from eight forested reference sites. Residential soils had significantly higher C and N densities than nearby forested soils of similar types (6.95 vs. 5.44 kg C/m² and 552 vs. 403 g N/m², P < 0.05). Results from our chronosequence suggest that soils at residential sites that were previously in agriculture have the potential to accumulate C (0.082 kg C/m²/y) and N (8.3 g N/m²/y) rapidly after residential development. Rates of N accumulation at these sites were similar in magnitude to estimated fertilizer N inputs, confirming a high capacity for N retention. Residential sites that were forested prior to development had higher C and N densities than present-day forests, but our chronosequence did not reveal a significant pattern of increasing C and N density over time in previously forested sites. These data suggest that soils in residential areas on former agricultural land have a significant capacity to sequester C and N. Given the large area of these soils, they are undoubtedly significant in regional C and N balances.     

Effects of plant invasions on the species richness of abandoned agricultural land

While exotic plant invasions are thought to lead to declines in native species, the long-term impacts of such invasions on community structure are poorly known. Furthermore, it is unknown how exotic plant invasions compare to invasions by native species. We present data from 40 yr of continuous vegetation sampling of 10 fields released from agriculture to examine the effects of invasions on species richness. The effects of both exotic and native species invasions on species richness were largely driven by variations among fields with most species not significantly affecting species richness. However, invasion and dominance by the exotics Agropyron repens, Lonicera japonica. Rosa multiflora. Trifolium pratense and the native Solidago canadensis were associated with declines in richness. Invasions by exotic and native species during old field succession have similar effects on species richness with dominance by species of either group being associated with loss of species richness. Exotic species invasions tended to have stronger effects on richness than native invasions. No evidence was found of residual effects of invasions because the impact of the invasion disappeared with the decline of the invading population. When pooled across species, heavy invasion by exotic species resulted in greater loss o species richness than invasion by native species. Studies of invasion that utilize multiple sites must account for variability among sites. In our study, had we no included field as a factor we would have incorrectly concluded that invasion consistently resulted in changes in species richness.     

Exotic plant invasions over 40 years of old field successions: community patterns and associations

While exotic plant species often come to dominate disturbed communities, long-term patterns of invasion are poorly known. Here we present data from 40 yr of continuous vegetation sampling, documenting the temporal distribution of exotic plant species in old field succession. The relative cover of exotic species decreased with time since abandonment, with significant declines occurring ≥20 yr post-abandonment. The number of exotic species per plot also declined with time since abandonment while field-scale richness of exotics did not change. This suggests displacement occurring at small spatial scales. Life history types changed from short-lived herbaceous species to long-lived woody species for both native and exotic plant species. However, shrubs and lianas dominated woody cover of exotic plants while trees dominated native woody cover. The species richness of exotic and native species was positively correlated at most times. In abandoned hay fields, however, the proportion of exotic plant cover per plot was inversely related to total species richness. This relationship suggests that it is not the presence, but the abundance of exotic species that may cause a reduction in community diversity. While the development of closed-canopy forest appears to limit most introduced plant species, several shade-adapted exotic species are increasing within the fields. These invasions may cause a reversal of the patterns seen in the first 40 yr of succession and may result in further impacts on community structure.     

Characterization of Households and its Implications for the Vegetation of Urban Ecosystems

Our understanding of the dynamics of urban ecosystems can be enhanced by examining the multidimensional social characteristics of households. To this end, we investigated the relative significance of three social theories of household structure—population, lifestyle behavior, and social stratification—to the distribution of vegetation cover in Baltimore, Maryland, USA. Our ability to assess the relative significance of these theories depended on fine-scale social and biophysical data. We distinguished among vegetation in three areas hypothesized to be differentially linked to these social theories: riparian areas, private lands, and public rights-of-way (PROWs). Using a multimodel inferential approach, we found that variation of vegetation cover in riparian areas was not explained by any of the three theories and that lifestyle behavior was the best predictor of vegetation cover on private lands. Surprisingly, lifestyle behavior was also the best predictor of vegetation cover in PROWs. The inclusion of a quadratic term for housing age significantly improved the models. Based on these research results, we question the exclusive use of income and education as the standard variables to explain variations in vegetation cover in urban ecological systems. We further suggest that the management of urban vegetation can be improved by developing environmental marketing strategies that address the underlying household motivations for and participation in local land management.     

Trees Grow on Money: Urban Tree Canopy Cover and Environmental Justice

This study examines the distributional equity of urban tree canopy (UTC) cover for Baltimore, MD, Los Angeles, CA, New York, NY, Philadelphia, PA, Raleigh, NC, Sacramento, CA, and Washington, D.C. using high spatial resolution land cover data and census data. Data are analyzed at the Census Block Group levels using Spearman’s correlation, ordinary least squares regression (OLS), and a spatial autoregressive model (SAR). Across all cities there is a strong positive correlation between UTC cover and median household income. Negative correlations between race and UTC cover exist in bivariate models for some cities, but they are generally not observed using multivariate regressions that include additional variables on income, education, and housing age. SAR models result in higher r-square values compared to the OLS models across all cities, suggesting that spatial autocorrelation is an important feature of our data. Similarities among cities can be found based on shared characteristics of climate, race/ethnicity, and size. Our findings suggest that a suite of variables, including income, contribute to the distribution of UTC cover. These findings can help target simultaneous strategies for UTC goals and environmental justice concerns.