Zonation of Belgrade, version_01: M. sculpturalis 2019 survey

The zonation emphasizes the differences between two main sections (Balkan and Pannonian), within the framework of usual urban-rural gradients.

The Balkan section of Belgrade "proper" as the northernmost portion of Šumadija region of central Serbia, consists of predominantly hilly terrain (mostly within 100–250 m of altitude, with very limited area in 300–500 m range) of varied geology and pedology, while some sectors of river banks are relatively flat (65–90 m) and wide, hence with high water table and occasionally flooded in springtime (alluvial). The Balkan section shows relatively clear gradient of urbanization, principally along NW–SE axis, while also less markedly to the east and southwest.

Figure 2. Landscape/urbanistic zonation of the study area in Serbia (18x11 km, white rectangle), within Belgrade "proper" (light blue outline): BUC – Balkan Urban Core; BMP – Balkan Mixed Periphery; PUC – Pannonian Urban Core; PSU – Pannonian Semi-Urban; PPU – Pannonian Peri-Urban.

The small highly urbanized and populated "old downtown" core (about 10 km2), with largely impervious land cover and dense fabric of commercial, residential, infrastructural facilities, gradually transforms into a wide, more loosely structured peripheral zone, with variety of suburban, peri-urban and/or rural settlements of less intensive or countryside lifestyles, more greenish areas and recreational assets, but also some extensive industrial areas and transportation facilities. A more prominent presence of semi-natural habitats and near-natural remnants of original forest ecosystems emerges from about 2–4 km distance from the urban core, mixed with growing share of agricultural land cover further outwards. Beyond the distance of 5–7 km from the core area to the south and east, agricultural and semi/near-natural habitats are represented in much larger continuous tracts, intermixed with more agriculturally oriented settlements, resulting in variously marked transition from sub-urban outskirts to "rural belt", roughly semi-circularly positioned around Balkan urban core of Belgrade. Within this complex "greenish matrix", old peri-urban villages and small towns are progressively being inter-dispersed with variety of newly created modern settlements, along main traffic corridors (particularly after 1960s), and eventually became merged with more central city zones, occasionally representing sub/urban sprawl situations. Agriculture of the Balkan section is mostly of small scale to moderately intensive type, with widespread abandonment in several tracts, particularly closer to the sub-urban margins.

The Pannonian section consists of two regions, delimited by major rivers: the Syrmia Region (between Sava and Danube rivers) and Banat Region (north of the Danube). It is markedly flat and lowland, either alluvial, originally marshy and frequently flooded (in Banat and part of Syrmia Region, 67–80 m), or somewhat raised loess formations (represented in large part of Syrmia Region 80–105 m). Most of the marshlands were channeled and drained long ago, and the whole area was largely converted into a highly productive agriculture (often of intensive type).

In the Syrmia Region, older settlements of traditional Pannonian type (villages or small towns) were situated mostly in loess areas – less prone to flooding, separated by marshy area about 3–4 km wide from the Belgrade old core. Since 1950s, an entirely new peri-urban settlement was developed in between, built upon thick layers of riverine sand brought to raise the ground level (and lower the water table). This modern town (named Novi Beograd = "New Belgrade") of Le Corbusier architectural style is composed of spaced multi-story buildings and ample urban green areas, varying from strictly managed park-like blocks to spontaneous vegetation (in different successional stages), following mostly regular spatial pattern – much unlike Balkan section. Started as vast and populous residential unit, over decades it evolved into multi-functional municipality with extensive commercial, infrastructural and some industrial features. The expansion of new city unit gradually merged peri-urban Pannonian settlements along loess ridge with core of the Balkan section (across the Sava River). Therefore, within comparatively more uniform environmental settings, at least three distinctive urban landscape patterns can be recognized: (a) the Pannonian "new urban" core (about 16 km2) of distinctive fabric, without marked gradient pattern in urbanistic regime (except for the age of establishment of various blocks), (b) the mixed semi-urban area, now peripheral to "new core", representing the merged Pannonian settlements (from highly impervious center of Zemun municipality to extensively rural ones) with its own distinctive pattern of modern development, comprising a variety of habitat types (including: quite heterogeneous residential units, some small-scale agriculture, industry, infrastructure, and semi-natural habitats), and (c) peri-urban settlements loosely connected to the peripheral zone, urbanistically transitional from village to town state, but still strongly oriented to agriculture, and embedded into vast and intensive agricultural matrix (representing wide "rural belt" of the Pannonian section, which does not have a circular pattern).

The Banat Region is largely similar in basic environmental conditions, with somewhat different urbanization history and landscape configuration of settlements vs. agricultural assets. It is currently outside the scope of this study.

Survey design and processing of geo-spatial framework

The five broad landscape/urbanistic units of "Belgrade proper" represent the zonation framework suitable for setting the broad environmental context of this study. For the spatial scope of this paper, we are showing zonation within 18x11 km square frame (Fig. 2), which includes all records of M. sculpturalis (maximal distance among end-points about 15 km east-west, and 7 km north-south, respectively), and all locations surveyed for its principal host-plant, Styphnolobium japonicum (spanning 16x9 km square). Areas and/or landscape types not covered by the study are excluded from zonation mapping (extensive agricultural area in Syrmia region, the whole area of Banat region, parts of Sava River wider coastal zone not fitting into the above urbanistic definitions, etc.), while some are entirely beyond the map extent (e.g. rural belt of Balkan section). Apart from two centrally positioned urban cores (old and new), all other zones are present also beyond the defined frame (not shown). For the consistent delimitation of the urbanistically quite uniform Balkan old core, the heterogeneous peripheral zone of Balkan section is extended to include also wider coastal area west and north of old core, which deviates topographically from the above elaboration (but is justified regarding its overall habitat composition). It is inevitable, however, that exact and entirely consistent delimitation can not be realized in all situations, particularly in cases where the transition of principal urbanistic features are essentially gradual between otherwise distinct zones (e.g. part of southeastern delimitation between Balkan core and periphery).

All surveyed locations were primarily georeferenced in Google Earth Pro ver. (Google Inc. 2020), and further prepared as distribution maps in QGIS ver. 3.4 (QGIS Development Team 2018). To deal with uneven/patchy distribution of available sampling sites, within the limitation of the sampling approach, we quantified trees, their blooming status, and the presence of bees, at two spatial scales. Following the rationale from landscape studies on wild bees, we defined a framework of circular non-overlapping sectors of 250 m radius (hereafter: S250), as the smallest meaningful spatial scale for bees. To include all pinpointed trees, we selected the position of each sector, so that (a) it entirely or largely fits into one urbanistic zone, and (b) the closest neighbouring trees of different sectors are spaced more than 250 m, but taking care that (c) if observation points of neighboring M. sculpturalis records are spaced less than 300 m, they would fit into a single sector, while if more than 300 m apart, they are included in different sectors. Since larger-bodied bees may forage at much larger distances, we also added the framework of circular sectors of 500 m radius (hereafter: S500), to check if a coarser scale would provide any difference in relationship of bee vs. plant distribution. To define S500 sectors we could follow only the first criterion (included tree and bee records being within the same urbanistic zone), while merging contents from 2 or 3 S250 sectors wherever closely adjacent. As a result, the primary record sites were arranged into two series of standardised circular sectors: 40 locations S250 (roughly of 0.2 km2), and 23 locations S500 (roughly of 0.8 km2):

Figure 3. A and B: Distribution of surveyed locations for Styphnolobium japonicum in August 2019, defined as series of non-overlapping circular sectors in two-scale framework

(A) S250 (r=250 m, ca. 0.2 km2), and

(B) S500 (r=500 m, ca. 0.8 km2), respectively, assigned to defined landscape/urbanistic zones of the study area:

BUC – Balkan Urban Core: 11/6;

BMP – Balkan Mixed Periphery: 11/7;

PUC – Pannonian Urban Core: 11/6;

PSU – Pannonian Semi-Urban: 5/3;

PPU – Pannonian Peri-Urban: 2/1;

Few sectors are positioned across the adjacent zone borders, hence assigned to the zone where the surveyed plant micro/habitats predominantly belong).