NDVI spatial pattern and the potential fragility of mixed forested areas in volcanic lake watersheds Maria Letizia Costantini a,⇑ , Nicola Zaccarelli b , Stefania Mandrone c , David Rossi d , Edoardo Calizza a , Loreto Rossi a a Department of Environmental Biology, Sapienza University of Rome, Via dei Sardi 70, I-00185 Rome, Italy b Department of Biological and Environmental Sciences and Technologies, University of Salento, Ecotekne, Prov.le Lecce-Monteroni, I-73100 Lecce, Italy c ISPRA – Institute for Environmental Protection and Research, Nature Conservation Department, Via V. Brancati 60, 00144 Rome, Italy d CNR-IRSA – Water Research Institute, Via Salaria Km 29.300, I-00016 Monterotondo, Rome, Italy article info Article history: Received 28 May 2012 Received in revised form 9 August 2012 Accepted 18 August 2012 Available online 12 September 2012 Keywords: Spatial heterogeneity Mixed forests Change detection Remote sensing Semivariogram abstract Upland forested areas of watersheds undergo changes due to many factors including ecological succes- sion, natural disturbances and human activity. The rate of natural and man-induced ecological changes in these landscapes is a function of the structural and functional characteristics of the component ecosys- tems. Analyzing spatial patterns and detecting fragile areas are thus crucial for making previsions about the chance and rate of disturbance propagation within and between the ecosystems. In this study we have tested the hypothesis of occurrence of a relationship between the extent of temporal change and spatial heterogeneity of mixed forested areas in the watershed of two Italian volcanic lakes by using remotely sensed data. Landsat images were acquired in summer 1987, 1992 and 2000, when Nature Reserves were established, and the temporal variation in the Normalized Difference Vegetation Index (NDVI) was determined by the change detection analysis. To analyze the spatial variability of NDVI, semi- variograms were calculated using data from five randomly chosen forested areas (10 km 2 -wide) per watershed. Results show that NDVI varied greatly across the two study sites and most of the variation was spatially structured. NDVI varied also over time. A linear positive relationship was observed between the number of pixels changing between dates and the semivariogram range, as the maximum distance of spatial dependence estimated from the starting NDVI image. Spatial homogeneity of NDVI is thus sug- gested as an indicator of intrinsic fragility (i.e. susceptibility to change) of mixed forests and the semivari- ogram range as a rapid estimator that can be considered by forest managers and agencies. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Upland forested areas of watersheds undergo environmental changes due to many factors including continuous slow forest development and succession, discontinuous natural disturbances and human activities (Hessburg et al., 2000; Linke et al., 2007). Anthropogenic disturbances, such as land-use activities, are widely considered the primary cause of diversity decline (Hansen et al., 2001). Monitoring of land cover and the use of indicators of the for- est susceptibility to change are thus crucial points to biodiversity conservation and sustainable forest management, and novel ap- proaches to forest disturbance mapping are required (Linke et al., 2007). Disturbance is any process or event that results in changes in the physical and biological characteristics of the ecosystems within the watershed, encompassing a very broad range of temporal and spatial scales, therefore causing adverse effects on some ecosystem elements, while benefiting others with new opportunities (U.S. Environmental Protection Agency, 2012). The rate and extent of natural and man-induced disturbances in forested watersheds are a function of the landscape structure and the structural and functional characteristics of the component ecosystems. Strong mutual interactions between disturbance regimes and the land- scape structure create complex dynamic patterns over time, which are difficult to predict (Lloret et al., 2002, and literature cited there- in). In particular, land use practices may affect disturbance regimes by reducing the effective size of ecosystems and their connected- ness, introducing unnatural shape complexity, or homogenizing natural patterns (Hessburg et al., 2000; Dale et al., 2000). It has been largely acknowledged that the spatial heterogeneity at multi- ple scales influences the spread of disturbances (e.g. pest invasion, drought, pollution, wind, fires, ungulate browsing or land cover conversion) as a function of the abundance and arrangement of disturbance-susceptible patches (Franklin and Forman, 1987; Turner, 1989; Turner et al., 1989, 2001). For example, fires may 0378-1127/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2012.08.029 ⇑ Corresponding author. Tel./fax: +39 06 4940800. E-mail address: marialetizia.costantini@uniroma1.it (M.L. Costantini). Forest Ecology and Management 285 (2012) 133–141 Contents lists available at SciVerse ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco