Zonation of Belgrade from the perspective of wild bee studies

The city of Belgrade: general features

Belgrade is the capital of the Republic of Serbia, and one of the largest cities in southeastern Europe. According to general planning regulation (2003: The 2021 Belgrade Masterplan), core administrative-urbanistic unit (Belgrade "proper") is about 776 km2 (max. S–N and E–W extent roughly within the 35×36 km rectangle), with population of >1.5 million. It is positioned at the confluence of two large European rivers, mostly navigable and representing important natural and commercial traffic corridors: the Danube River (the second longest in Europe, running from Alps in Germany through much of central Europe, and draining most of SE Europe into the Black Sea), and the Sava River (the second longest within SE Europe, running from Alps in Slovenia roughly west-eastwards to its mouth in Belgrade). The Danube River also marks the northern border of the Balkans east of Belgrade, and the Sava River marks most of the northern Balkans' border to the west. Therefore, Belgrade is situated in the border zone between two large, and quite different geographic units of southeastern Europe: the predominantly hilly to mountainous Balkan Peninsula to the south, and the vast lowlands of the Pannonian Plain to the north.

Its area is also climatically transitional, between temperate-continental and more steppic regime, and its relief is spanning the altitude range of 65–506 m. Biogeographically, the area harbours a varied mixture of elements from three principal provenances: temperate central-European (or, in wider sense, Euro-Siberian), thermophilic southern-European (sub-Mediterranean, or even broadly Mediterranean), and xerothermic-continental-steppic eastern-European (mostly wooded-steppic Pannonian, or in wider sense, Ponto-Pannonian); in more local regionalization, its position may be referred to as southeast-peri-Pannonian, and it is quite remote from higher mountain areas of the Balkans.

The appropriate environmental scope for both urban ecology and landscape ecology studies imply that natural context is largely impacted by socio-economic history and current dynamics, principally affecting land-use pattern. Our wider study area, the urbanistic Belgrade "proper" (Fig. S2.1), encompasses more than 50% of varied agricultural habitats as a matrix, with embedded mosaic of urban, suburban and rural habitats and land use types, showing contrasting patterns in the two distinct regions.

From the initial city core (situated just south of Danube-Sava confluence point, roughly: 44.798–44.825°N, 20.451–20.483°E), Belgrade experienced remarkable spread in all directions, particularly since the mid twentieth century, and gradually merged with different smaller neighbouring settlements. The processes followed somewhat different urbanistic rules and context-dependent dynamics in varying combinations of topography, development history and prevailing land-use, hence resulted in several distinct urban, mixed, and rural landscape and land-use types, with transitions forming dissimilar patterns in different sections and directions.

Figure S2.1. Main land-use types of Belgrade "proper" area, according to "The 2021 Belgrade Masterplan" (2003). The dominant pale-yellow colour denotes agriculture, whitish-grey and pale-violet denote dominantly built areas (largely with impervious fabric), and wide spectre of green/greenish/green-bluish/green-brownish denote a variety of "urban and peri-urban green surfaces". (The state from 2001/2002; slight changes that have happened since that period are negligible for our context.)

From the perspective of wild bee studies, various urbanistic sectors are characterized by differing types, extent and relative share of suitable habitats, from managed "urban green" through those in varied residential and agricultural regimes, to semi-natural and wild ecosystems, along different gradients of urbanized environment.

The available official maps (Fig. S2.1) from 2003 (The 2021 Belgrade Masterplan) mostly follow the standard, relatively simplified urbanistic classification of urban green types, which leaves a relatively large share of potentially important bee habitats hidden under other land-use categories (e.g. several types of residential settlements are remarkably "greenish").

There were attempts to improve the classification and mapping approach, most notably through the project "Green regulation of Belgrade" (2006–2008; link), with elaborate re-classification of urban habitats for assessing their "value for biodiversity", but due to various limitations, this system is also mostly unsuitable for evaluation of bee habitats. 

Available alternative is to reduce quality assessment to CORINE land-cover classes (Corine Land Cover (CLC) 2018, Version 2020_20u1 or Urban Atlas LCLU 2018, which is currently too simplified and of inappropriate resolution for this region (often reduced to measures of "impervious surfaces" vs. "green surfaces"). Until a more detailed habitat mapping is available, we use our own "tailored approach" specifically suited for diverse wild bee studies. It is based on landscape scale characterization of available elements of physiography (topographic, pedological, etc.), land-cover, urbanistic, and other features (variously combined), with focus on management regime within relevant/key urban habitat classes and respective resource availability. We defined a basic set of wider "urbanistic landscape zones", characterized with broadly predictable generalized patterns of positive vs. negative impacts on bee populations and communities regarding various aspects and gradients of urban environments. The concept allows to be conveniently fine-tuned to relevant taxon-specific or habitat-specific situations, for each particular study.