The Structure And Formation Of The Goat Track Landscape In Semi-arid Hilly Loess Plateau, China

Grazing areas cover more than 25% of global land surface and have a huge influence on ecology and socio-economy of the planet. Striking networks of livestock tracks, or terracettes, molded to the contours are a common feature on hilly rangelands in the arid and semi-arid regions as well as in humid and alpine regions. Goat tracks are also a common feature on semi-arid hilly rangelands of the Loess Plateau, one of the regions with a long history of livestock grazing and the most severe soil erosion in the world. This paper quantified(1) the spatial pattern of topo-edaphic and biological structures of basic unit of the terracette landscape in the Loess Plateau,(2) the structures of the goat track network,(3) the behaviors of goats within the network. For the spatial pattern of topo-edaphic and biological structures, a basic unit of a terracette landscape can be considered as the area including a track and the area between this and the next track downslope. The profile of a basic unit, from upslope to downslope, includes three segments: a track segment, a shoulder segment, and a inter-slope segment. We measured the microtopography, plant and soil properties for each of the segments. For the network structure, by photogrammetric survey, we acquired the network, the structures were then explored from a perspective of graph theory. For the behaviors of goats, we measured the route selection and forage behaviors by photogrammetry. Based on these measurements above, we found:1. The goat track landscapes on the sloping rangelands of Loess Plateau have strong spatial heterogeneity in topo-edaphic and biological structure, and lead to banded pattern of vegetation and soil nutrient content. The banded pattern is organized with three segments of a basic landscape unit – track, shoulder and inter-slope. The track segment has gentler slope, compacted soil with mechanical crusts, and low cover of plants and biological soil crusts. The shoulder segment has steep slope, lowest bulk density, highest aboveground plant biomass and soil nutrient content. The inter-slope segment has the steepest slope, intermediate bulk density, lowest soil nutrient content, over 80% surface of this segment was covered by biological soil crusts. There is a corresponding pattern of water redistribution, the track segment has the greatest wetting depth, the inter-slope segment has the lowest wetting depth, and the shoulder segment has intermediate wetting depth. The track segment serves uniquely dual functions in facilitating the plant growth in the shoulder segment by providing water through surface runoff as well as belowground storage.2. The network of goat tracks on sloping rangelands of semi-arid Loess Plateau fits well with foraging behaviors of goats. There are considerable vertices within the network, the vertices density is 0.3/m2, among which about 30% are 2-degree, 60% are 3-degree, 10% are 4-degree and no vertex whose degree is bigger than four was found. The tracks are consisted of parallel edges that nearly paralleled the contour with departure angle(departure from an imaginary horizontal line) 8.0°, conjunctive edges whose departure angle are 27.4°, and serve as connector of parallel tracks. A vertex and edges that connect to the vertex is defined as a basic connect unit of a network. 14 types of basic connect units were found, the structures for each of these types were proposed. Polygons of the network are elongated with horizontal/vertical length ratio over 4, horizontal length 3 ~ 8 m, and vertical length 0.6 ~ 1.6 m.3. Tracks improve energy efficiency of foraging goats. The lower incline and firmer surface with high bulk density and mechanical crusts and the low plant cover of tracks likely facilitate more even footing and stable steps and reduce the traveling distance by the goats, which likely reduces energy expenditure. The tracks used by goats have relative small departure angle about 11° and small turn angle about 16°, both of which are relative energy expenditure saving. Conjunctive tracks may have a high vertical movement energy efficiency. Nearly 93% foragings observed were on tracks, indicating the tracks are not only traffic route, but also daily forage route of goats. There is a specific foraging posture for goats on the tracks, all the foraging positions were forward, about 93.4% were up-forward. The forage positions seemed like a circle on the up-ward with a radius about 80 cm, this radius means the foraging accessable percent was 88.0%. So goats could access most of the landscape without leaving the tracks, which is energy efficiency.4. There appears a positive feedback loop in the maintenance of the track structure: trampling by goats reduces the slope and increases the bulk density and mechanical crusts on the tracks, which help form the belt of fertility and concentrated vegetation growth in the shoulder segment. The tracks enabled the goats to travel and graze in ways that reduce energy expenditure and increase foraging efficiency, which leads to preferential use of the tracks. There are signs of dynamics in the terracette landscape as well, sediments captured by the(concave) lower inter-slope, the track and shoulder segments will wide the track inward, which can possibly result in upslope movement of the terracettes on these landscapes.In summary, there are important functional differences between landscapes with and without networks of livestock tracks, in regulating water runoff and soil erosion, forage production, and soil carbon storage. These functional differences may have significant implications to land use policies and practices aiming at soil and water conservation and socio-ecological sustainability of the Loess Plateau.