Research PaperThe urban matrix matters: Quantifying the effects of surrounding urban vegetation on natural habitat remnants in Santiago de Chile
Introduction
Urbanization is among the most drastic human-driven land-use change processes (Foley et al., 2005, Seto et al., 2011, Sushinsky et al., 2013). Urbanization usually results in the loss, degradation and fragmentation of natural habitats; reduction of native biodiversity and increase of invasive species; loss of agriculture lands; pollution of streams, air and soils; and modification of energy flows and nutrient cycles (Alberti, 2005, Grimm et al., 2008, McKinney, 2008). These modifications have long-lasting effects on the structure and functioning of ecosystems. Once urbanization takes place, chances to restore urbanized lands to a previous natural stage are severely limited (Lindig-Cisneros and Zedler, 2000, Pavao-Zuckerman, 2008, Standish et al., 2013).
Urbanization is a dynamic process, often combining periods of slower inner densification and rapid outward expansion (Antrop, 2004, Dietzel et al., 2005). As urban areas expand, protected and non-urbanizable areas outside urban boundaries become progressively fragmented, often leading to a system of scattered natural remnants embedded in an heterogeneous urban matrix (Ramalho & Hobbs, 2012). Here we refer to these remnants as Urban Natural Remnants (UNRs), defining them as “natural areas that have been partially or completely isolated by an urban matrix, but that still retain compositional and structural characteristics of the original natural habitat”.
UNRs can provide multiple ecosystem services (ES) for urban areas, and different cities have promoted their conservation by including them as part of the urban green spaces system; e.g. San Diego-California Ravines (Soule, 1991), Phoenix-Arizona Mountain Preserves (Esbah, Cook, & Ewan, 2009), Santiago-Chile Island Hills (Forray et al., 2012). Depending on their size and ecological features, ES provided by UNRs may include carbon sequestration, oxygen provision, air pollution reduction, microclimate regulation, water infiltration, recreational and educational opportunities, increased city attractiveness, and provision of habitat for biodiversity (La Rosa and Privitera, 2013, Lovell and Taylor, 2013, Silva de Araújo and Bernard, 2016). Nevertheless, the provision of ES by UNRs cannot be taken for granted, as this will depend on the capacity of UNRs to retain the underlying ecosystem processes supporting the delivery of services (de Groot et al., 2002, de Groot et al., 2010), which can be severely affected by the urbanization process.
Ecosystem processes within UNRs could be affected by urbanization via two main pathways: abiotic and biotic effects. Abiotic conditions in urban areas often differ from those in natural and rural areas. Thus, as urban areas expand, UNRs become exposed to a urban matrix with different abiotic conditions, such as higher average temperatures, lower wind speeds, higher levels of air particulate matter, higher atmospheric concentration of inorganic and organic pollutants, and accumulation of these compounds and heavy metals in soils (Grimm et al., 2008, Pickett et al., 2011). Urbanization also modifies the species composition within UNRs as they become increasingly isolated by a contrasting urban matrix (Fernández and Simonetti, 2013, Gibb and Hochuli, 2002, Jellinek et al., 2004, Soulé et al., 1992). This change in species composition is coupled with modification of trophic interactions (Crooks and Soulé, 1999, Faeth et al., 2005), which together with the modification of abiotic conditions, may radically alter key ecosystem processes within UNRs, therefore threatening the capacity of UNRs to provide ES.
However, there is ample evidence that the composition and structure of the surrounding matrix play a key role mediating the biotic and abiotic effects of fragmentation on remaining natural habitats outside urban areas (Ricketts, 2001, Murphy and Lovett-Doust, 2004, Prevedello and Vieira, 2010, Watling et al., 2011, Ruffell et al., 2017). Therefore, similar mediating effects may be assumed for UNRs. In fact, previous studies have shown that less intensive land-use types and higher vegetation cover in the surrounding urban matrix are positively associated with richness of native birds species in UNRs of Stockholm, Seattle and Phoenix (Donnelly and Marzluff, 2006, Litteral and Wu, 2012, Mörtberg, 2001). Nevertheless, while the effects of the urban matrix on UNRs biodiversity have been increasingly covered, there is still a gap of knowledge regarding the effect of the urban matrix on UNRs ecosystem processes. This lack of knowledge not only limits our ecological understanding of the effects of urbanization on UNRs ecology, but may also be reducing our capacity for planning and managing the surrounding urban matrix of UNRs to promote the long-term delivery of ES by these remnants.
Particularly relevant from an urban planning perspective is to evaluate if vegetation in the urban matrix play a role enhancing or mitigating the effects of urbanization on UNRs ecosystem process. While there is ample knowledge on the fundamental role that urban vegetation plays for directly providing ES in urban areas (Gómez-Baggethun and Barton, 2013, Lovell and Taylor, 2013, Niemelä et al., 2010), studies analyzing an additional role of urban vegetation for supporting the provision of ES by UNRs seems to be missing. This information could be particularly relevant for cities with limited resources for managing UNRs and where increasing urban vegetation in residential areas is highly needed. Because if well planned, urban vegetation could provide ES at the local scale, and at the same time support the long-term provision of ES by UNRs at larger scales.
Understanding the effects of urban matrix vegetation on UNRs ecosystem processes is a challenging task. The urban matrix is highly heterogeneous in space and time, and therefore approaches that treats the matrix as a rather static homogenous element, such as those based in landscape fragmentation theories (Ewers and Didham, 2006, Fahrig, 2003), may fail to identify relevant spatio-temporal patterns affecting ecosystem processes within remnants (Ramalho and Hobbs, 2012, Ramalho et al., 2014). Furthermore, humans play a key role modifying vegetation patterns in urban areas (Faeth, Bang, & Saari, 2011), therefore their potential role on the observed temporal patterns of vegetation and UNRs ecosystem processes needs to be included in the analysis. In this regard, an urban landscape ecological approach may be particularly helpful for understanding and addressing these types of challenges. This approach emphasizes the interrelation between landscape patterns and ecological processes operating at multiple temporal and spatial scales, and encourages place-based research that integrates ecology with urban design and planning (Wu, He, Huang, & Yu, 2013).
In this work, we used a multi temporal and spatial scale approach to evaluate the potential role of vegetation spatial patterns of the urban matrix on the primary productivity of 10 UNRs located in the city of Santiago de Chile. Using a set of 6 remote sensing-derived vegetation indices data (years 1985–2010), we analyzed how the temporal changes in primary productivity within UNRs relates to changes of vegetation patterns of the surrounding urban matrix at different temporal and spatial scales. Because in Santiago urban matrix vegetation cover is strongly positively associated with neighborhoods socioeconomic level (de la Barrera et al., 2016, Fernández and Wu, 2016), we also evaluated if UNRs productivity trends could be explained by the diverse socioeconomic levels of neighborhoods surrounding the assessed UNRs. We aimed to answer the following questions: (1) Is primary productivity of UNRs affected by the amount and spatial pattern of vegetation cover in the urban matrix? (2) At what temporal and spatial scales do these effects take place? (3) What ecological and socioeconomic factors may be driving these effects at different scales?
Section snippets
Study area
The city of Santiago is located in the Maipo River Basin of Central Chile (33°26′15″S; 70°39′01″W), bounded on the east by the Andes Mountain and on the west by the Coastal Mountain Range (Fig. 1), covering a total built-up area of near 600 km2 (Banzhaf, Reyes-Paecke, Müller, & Kindler, 2013). Climate is Mediterranean, with a marked cold and rainy winter, and a dry and hot summer season. Original vegetation is represented mostly by summer drought tolerant species, which can form dense or sparse
Spatial association between UNRs ΔNDVI and matrix vegetation ΔSAVI
Pearson correlation analysis shows that UNRs ΔNDVI and matrix vegetation ΔSAVI tend to be spatially positively correlated for all assessed periods (i.e. years elapsed since the baseline year), except for the shortest period (Fig. 4). These associations become stronger in time, and weaker as distance from UNRs edge increases (i.e. increasing buffer size). Although most correlation values are not statistically significant (p < 0.05), the spatial and temporal trends are consistent, showing peak
Discussion
This is the first study aimed to analyze how the rapid urbanization process of Santiago have affected the temporal trends of primary productivity of Santiago’s UNRs, and particularly aimed to assess the role of urban matrix vegetation patterns on enhancing or mitigating the effects of urbanization on these UNRs. If well managed, UNRs can provide several ES to urban areas (Ramalho and Hobbs, 2012, Standish et al., 2013). Therefore, efforts to understand the potential role of vegetation at UNRs
Conclusions
Increasing the provision of ES within urban areas is crucial for developing more resilient and sustainable cities (Jansson, 2013, McPhearson et al., 2015, Wu, 2014). Nevertheless, while there is ample knowledge of the direct role of urban vegetation in providing urban ES (Gómez-Baggethun and Barton, 2013, Lovell and Taylor, 2013, Niemelä et al., 2010), there is scarce evidence on the additional role that urban vegetation may play in sustaining the provision of ES by UNRs. Most urban ecological
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