Aspect influences on pasture microclimate at Coopers Creek, North Canterbury

Abstract

Rainfall, soil moisture, and soil and air temperatures were measured over five years on north and south aspects (about 25°slope) of a grazed pasture and on the exposed intervening ridge crest at Coopers Creek, North Canterbury (43°IS'S) at about 380 m a.s.l. Solar radiation and wind were measured only on the ridge, and radiation received on north and south slopes was derived from ridge measurements. Potential evapotranspiration on all aspects was calculated from solar radiation and air temperature according to the Priestley and Taylor formula. Daily mean solar radiation on the ridge averaged 3321angleys per day with a maximum of 5911angleys per day in December and a minimum of 1181angleysper day inJune. Over a year the north aspect received 80% more radiation than the south. Northwest winds prevailed; windrun at 2.1 m above ground was usually over 200 km.d" and occasionally was over 2000 krn.d". Total potential evapotranspiration on the ridge averaged 1093 mm.a" with a maximum of 5.9 mm.d" in December and a minimum of 1.0 mm.d" in June. Over a year, potential evapotranspiration on the north aspect was almost double that on the south aspect. Average annual rainfall was about 1400 mm and was distributed evenly over all aspects, although the catch from individual rain periods varied by up to 80% among aspects. The north aspect soil was always drier than the south aspect soil through the growing season and sometimes approached wilting point between December and March. Moisture content in the ridge soil was intermediate between north and south aspects. Mean annual Stevenson screen air temperatures (1.2 m above ground) were measured as 1O.5°c on the ridge aspect and calculated as 13.7°c and 9.9°c on the north and south aspects respectively. These are equivalent to a latitudinal displacement of about 9°within New Zealand. Soil temperatures on the north aspect were always warmer than on south or ridge aspects under both clear and overcast skies. Ridge temperatures were similar to those on the south aspect. Mean annual temperatures at 10ern depth were 7.7°c on the ridge aspect and 13.8°c and 8.2°c on north and south aspects respectively. On the north aspect, soil maximum temperatures under short grass (2-4 ern) were higher than those under tussock (30-40 em) and soil minima were lower. On the ridge, soil maxima and minima under short grass were lower than those beneath tussock.

Influence of slope and aspect on long‐term vegetation change in British chalk grasslands

“…Perring (1959) showed that the species composition of chalk grassland at many sites in Britain was strongly influenced by slope and aspect. Two distinct topographic gradients lie behind the observed distribution patterns, namely (i) the tendency of slopes facing the equator (and hence intercepting more direct solar radiation) to be both drier and warmer than those facing away from the equator (Geiger 1965;Påhlsson 1974;Radcliffe & Lefever 1981;Rorison et al . 1986a) and (ii) the tendency for soils on relatively steep (> c .…”

Introduction, production, and persistence of five grass species in dry hill country

“…Although these constraints are not severe compared to elsewhere in the world, this hill country is among the driest area of New Zealand, and suffers production loss attributable to water deficit (Radcliffe 1979).…”

Grassland responses to shelter — a review

“…Tussocks may also ameliorate extreme temperatures in their vicinity and reduce soil evaporation, so altering the micro-climate for plants (O'Connor & Lambrechtsen 1964;Radcliffe 1974;Radcliffe & Lefever 1981;Scott 1962;Sewell 1952;Zotov 1938) and influencing the zonation of plant species (Scott 1%1). Shelter from tussocks may also promote growth of introduced cocksfoot (Dactylis glomerata, Radcliffe 1974), and survival of oversown grasses and clovers (Scott & Archie 1976).…”