Anthropic erosion of mountain land in Canterbury
- New Zealand Geology Survey, University of Canterbury, Christchurch, New Zealand
- Division Land and Soil Science, DSIR, P.O. Box 29-199, Christchurch, New Zealand
Anthropic, or man-induced erosion can be recognized on Canterbury's mountain land. Almost all of it is identical in form and mode of origin to other erosion types, and may be indistinguishable unless obviously associated with roads, tracks, or fences. In particular, where vegetation is destroyed or impaired by fire or grazing, it is impossible to separately identify anthropic and natural components of the erosion. Early surveys reported significant anthropic erosion throughout the region. Subsequent work has revealed some errors in methodology, but more importantly it allows the human influence to be viewed in context with the more significant and more extensive natural erosion. The highest sediment yields and erosion rates in Canterbury are within the wetter, and in some places well-forested, areas near the main divide, rather than in the drier, depleted eastern ranges. Some barren screes have changed little over centuries, and even very active unstable screes may be old landforms. Soil stratigraphy reveals a long history of episodic stability and instability. Dated charcoals provide a fire history spanning more than 40,000 yr. More frequent fires between 500 and 1000 yr ago brought widespread deforestation. There probably were regional increases in erosion while forest soils adjusted to loss of tree-root strength. Early pastoralists further increased the frequency of fires and grazed sheep, greatly modifying the grass- and shrub-land vegetation in both stature and composition. Erosion rates would have increased because sheet erosion is more than 10 times greater from bare soils than from those with intact tussock, scrub, or scree cover. Repeat photographs near Porters Pass, however, show both increases and decreases in bare ground rather than a general trend over the last 90 yr. Widely distributed grassland transects also show little consistent change in bare ground in the last 10 to 35 yr. Harsh climate and soil infertility slow or inhibit plant re-establishment on bared ground for decades and probably centuries in the drier mountain lands of Canterbury.