1. Introduction
Human exploitation of land considerably modifies the landscape, altering the Earth’s topography, the energy balance, and the biogeochemical cycles, which in turn affect the provision of ecosystem services [
1,
2,
3,
4]. Consequently, land use and land cover (LULC) maps production, processing, and employment are central themes for remote sensing as well as for environmental sciences and landscape planning, in particular in urban and periurban areas [
5,
6,
7].
Ecological and environmental processes are multi-scaled in nature and their evaluation requires input data fitting the scale of the investigated processes in order to avoid spurious relationships and/or erroneous results [
8,
9]. The scale of a LULC map is commonly defined by a spatial extension (the represented area), a spatial resolution, and a thematic resolution.
The spatial resolution is usually related to the cell size and minimum mapping unit for raster and vector maps, respectively. The thematic resolution, also called the class or categorical resolution, represents the level of detail of discrete (or qualitative) variables (LU/LC typologies) with known and definable boundaries [
10]. The smaller (or bigger) the raster cell or the minimum mapping unit are, the higher (or lower) the spatial resolution is. The higher (or lower) the number of LULC types mapped is, the higher (or lower) the thematic resolution is.
In general, if the spatial and thematic resolutions are high, the possibility of map application in environmental evaluations is high. However, environmental assessment procedures and related environmental modeling can require different spatial and thematic resolutions of the LULC map to be efficiently implemented in a study case. For example, in the case of modeling of LULC change, higher spatial and thematic resolutions of input data increase the complexity of the simulation, hence increasing model noise and decreasing model performance [
10]. Consequently, a lower resolution can be preferred to achieve better model validation scores even if it leads to simpler simulations.
The extension of analysis and the resolution of other variables included in an environmental evaluation play a fundamental role in the definition of the required LULC map resolution [
10]. Moreover, producing accurate LULC maps with both high spatial and thematic resolution require high-resolution remote sensing data, plus complex and time-demanding processing procedures based on an elevated number of ground truths and training data [
11]. In many cases, when the existing LULC maps are not adequate to the scopes of territorial study, specific LULC maps can be produced or the available LULC data can be updated. Spatial and thematic resolutions of LULC maps are primarily affected by the pixel size and the spectral bands of the sensor in the remote sensing device, and by the image processing and LULC classification method [
8,
11,
12]. Innovative methods for the production of accurate LULC maps from remote sensing data have been proposed using free images and tools [
13,
14] and in absence of field data [
15]. However, despite the complexity of these methods, the thematic resolution of maps still remains limited to few (5–7) LC classes. Indeed, the production of LULC maps with high thematic content remains a time and resource-demanding process.
While the impact of map spatial resolution on environmental assessments has been evaluated by several studies, the effect of the thematic resolution is still poorly investigated [
8,
16]. Indeed, the thematic resolution of the LULC map is usually a compromise among the available data and the specific requests of the adopted environmental evaluation procedure. Furthermore, the choice of the assessment procedure and of the environmental model can be influenced by the available thematic resolution of the LULC maps. Therefore, the potential effect of different thematic resolutions of the LULC map on the final evaluation can be relevant and it deserves to be further investigated.
The thematic resolution of a LULC map can be defined by a ruleset and criteria aimed at describing the relationships between the classes. A hierarchical classification scheme was originally proposed in [
17] to standardize LULC data following different levels of aggregation, from the more detailed categories to less detailed ones. This hierarchical aggregation scheme has been adopted by several projects on LC mapping, such as the CORINE Land Cover Programme (CLC) [
18,
19]. CLC characterizes land cover in general because it has been developed for large areas with an extremely diversified LULC [
20]. The CLC classification scheme has been adopted at the continental (e.g., European), national, and subnational scales. CLC categories are distinguished by five levels following a common classification scheme based on standardized codes ranging from the first thematic level characterized by the lower resolution, through to the fifth thematic level characterized by the higher resolution. While the third level of thematic resolution has been produced at the European level, and the more detailed fourth and fifth levels have been carried out at the national, regional, or sub-regional scales. The CLC classification scheme is also adopted in specific maps based on high spatial resolution (e.g., aerial photos), choosing the most appropriate level of classification for the available resources and time. In many cases, a first or second CLC level can be chosen as a reference for new LULC maps to support specific local plans (e.g., municipal, natural reserve plans) or environmental evaluations (e.g., hydrological, ecological).
In this paper, an environmental assessment procedure is carried out with different thematic resolutions of the CORINE system to evaluate their effects on the final assessment. In particular, the environmental evaluation regards the landscape connectivity, and it is conducted with the PANDORA 3.0 model [
21,
22]. Landscape connectivity, i.e., the ability of the landscape to facilitate or impede exchanges of energy, organisms, and materials among habitat patches [
21], is a key theme in land use planning and biodiversity conservation policies [
23,
24]. Indeed, the reduction of landscape connectivity (i.e., habitat loss and fragmentation) is recognized as a major cause of species decline [
25,
26], the decrease of socio-ecological resilience, and the disruption of ecosystem services [
21,
22,
24].
Landscape connectivity assessment is then proposed for an urbanized context in the Bari metropolitan area (southern Italy). The objectives of the paper are: (1) to assess the impact of CLC thematic resolution on landscape connectivity; (2) to define priority areas for conservation objectives and future sustainable urban expansion. Indications are given for supporting practitioners and researchers faced with thematic resolution issues in environmental assessment and land use planning. The manuscript is organized as follows.
Section 2 reports on the literature review and key concepts on landscape connectivity.
Section 3 presents the material and methods while the results, discussion, and conclusions can be found in
Section 4,
Section 5 and
Section 6, respectively.
2. Thematic Resolution and Landscape Connectivity
Biodiversity and landscape connectivity measures are strongly scale-dependent. This means that assessment results can greatly vary with the extension and resolution of input data. The effects of the spatial resolution of LULC data on fragmentation and landscape connectivity have been largely recognized [
23,
27,
28], as well as the effects of varying the extension of the study area [
21]. Major efforts are required for the analysis of thematic resolution impact on landscape connectivity. Indeed, only a few studies have faced this issue and the results appear sometimes discordant.
A higher thematic resolution of LULC data seems to provide a more accurate representation of habitat suitability for bumblebee in Belgium [
29]. Similar results have been obtained in other studies. Seoane et al. (2004) [
30] demonstrated that a higher thematic resolution resulted in a better predictive performance of bird species distribution models. Moreover, they showed that general-purpose LULC maps (e.g., CORINE) can be a satisfactory alternative to more detailed vegetation maps obtained from satellite data. Cushman and Landguth (2010) [
31] proved that appropriate specification of the thematic resolution dominates the effects of spatial resolution and extent in the assessment of landscape genetic pattern–process relationships. Zeller et al. (2017) [
32] showed that pumas distribution in southern California responds more strongly to topographic variables and human development (i.e., roads and settlements) than to other characteristics related to the thematic resolution of LULC. Moreover, since equivocal results have been reported in the literature, the authors call for further research on the thematic resolution effect on the model performance and the study of habitat and movement relationships [
32]. Bailey et al. (2007) [
16] found that an intermediate level of thematic resolution (14 LULC classes) is sufficient to well correlate landscape metrics with the diversity of most species groups at the European scale. Simpkins et al. (2017) [
33] underlined that determining the optimal thematic resolution for landscape connectivity evaluation often involves expert opinion, or it is imposed by the use of LULC maps developed for other purposes. Consequently, the selection of thematic resolution presents levels of uncertainty difficult to quantify [
33]. Kallimanis and Koutsias (2013) [
27] underlined that many studies of landscape ecology and environmental assessment use few LC classes (10 or fewer) and several evaluations of landscape connectivity are based on only two classes. Indeed, several species have a reduced areal with few relevant LULC classes [
34]. In contrast to traditional conservation management approaches, land-use planning focuses on the sustainable development of multi-functional socio-ecological systems [
24]. In this view, the administrative boundaries usually define the relevant spatial extension and higher LULC thematic resolutions are used in landscape connectivity evaluations [
21]. In this context, Kallimanis and Koutsias (2013) [
27] showed the correlation between spatial and thematic resolutions in diversity patterns across Europe, using different Corine thematic levels. Their results indicated that a low thematic resolution conveys a significant portion of information that can be used in combination with high spatial resolution. However, by combining low spatial and thematic resolutions, even the spatial pattern properties change, as well as the geographic location of diversity peaks and troughs [
27].
Definitely, the effect of thematic resolution in landscape connectivity assessment appears scarcely studied and, consequently, a generalizable assumption is not possible. Indeed, depending on the objective of the study and the considered species, the optimal thematic level to be used in the assessment can differ, as well as the choice between the use of an available LULC map and a more detailed one to be produced.
In this perspective, we propose to investigate the effect of the different spatial distribution of LULC classes as a predictor of the impact of thematic resolution on landscape connectivity. The conceptual scheme of
Figure 1 reports a graphical synthesis of the assumed hypothesis. The scheme assumes a fixed spatial resolution to focus only on the possible impact of the thematic resolution on the connectivity measures. In general, landscape connectivity studies employ LULC data in habitat maps and/or cost surfaces, i.e., representations of the difficulty for an organism to traverse landscapes [
33,
35]. So, habitat or cost values are assigned to each LULC patch based on a range of species-specific factors that influence presence and movement. It is noteworthy that true values are not always available, and expert opinion can be employed [
33]. The scheme of
Figure 1 reports some scenarios of such value attribution to a LULC map with different levels of thematic resolution. The scheme displays some of the types of combinations that can lead to connectivity evaluation changes among CORINE levels. The values in the example refer to the Biological Territorial Capacity (BTC) index, an index of vegetational metabolism used in the PANDORA model (see following
Section 3.1 and
Appendix A) to define the bioenergy connectivity among landscape units. In general, the greater the BTC index in a landscape unit, the higher its ecological value and the potential bioenergy exchange among adjacent landscape units.
The six scenarios of
Figure 1 show that depending on the types of LC present in a landscape unit, the measures of bioenergy and length of the perimeter can vary across the CORINE level both in urban and natural scenarios: higher values of BTC can be revealed at the fourth, third or second CORINE thematic levels. To understand the relative impact on landscape connectivity of this hypothesis and, in general, of the change in thematic resolution of the LULC map, we propose to compare four thematic resolutions in a real study case using the PANDORA model (see
Section 3).
6. Conclusions
Which is the best LULC thematic resolution for environmental assessment and land use planning? The present paper aims at addressing this issue, evaluating the possible impact of different thematic resolutions on landscape connectivity assessment, a crucial environmental aspect for biodiversity conservation. Answering the question is not easy, because several variables play a role in the final decision. The modeling approaches, the considered species, the availability of data and resources to produce LULC maps with a suitable spatial resolution are some of the factors that surely affect the choice of the thematic resolution of the map. The present manuscript presents the landscape connectivity assessment of four scenarios with increasing thematic resolution (namely, LEV1, LEV2, LEV3, and LEV4) corresponding to the four CORINE levels in an urban context of southern Italy. The PANDORA 3.0 model was used to evaluate Bio-Energy Landscape Connectivity (BELC) based on bio-energy fluxes among landscape units. Scenarios comparison was investigated through the indicators of landscape connectivity and ecosystem services working at three scales: the largest (whole system), the middle (Bio-Energy Landscape Unit), and the smallest one (land cover patch). The results show that with a fixed spatial resolution:
The thematic resolution has a potential impact on the BELC but a direct relation between the number of LC classes and landscape connectivity measures does not exist.
The higher thematic resolution is not always related to the higher measure of landscape connectivity, but LEV1 strongly differs from the other more detailed levels.
The spatial distribution of LC classes can affect connectivity evaluation more than the change in thematic resolution.
The changes in thematic resolution of the LULC map can determine hotspots of landscape connectivity changes.
The changes in thematic resolution can determine errors in the identification of priority of intervention.
The proposed index of ecosystem services provides a more stable ranking of conservation priority among different thematic resolutions.
In conclusion, we demonstrated that the thematic resolution of the LULC map impacts the landscape connectivity evaluation due to the spatial pattern of the LULC classes. Researchers and practitioners, when choosing thematic resolution, should be aware of the possible misleading assessment that is synthetically aforementioned. Moreover, measures of ecosystem services that integrate connectivity index with other ecological features could be preferred to reduce the erroneous evaluation of priority ranking for conservation objectives. Further efforts are required to investigate the impact of thematic resolution and LC classification types (e.g., fuzzy map) on different approaches to landscape connectivity (e.g., functional connectivity), and on different environmental processes (e.g., hydrological and hydraulic modeling).