Standing crop and yield estimates for a hill country catchment in Central Otago

Abstract

Central Otago has extensive semi-arid areas where low rainfall, relatively high rates of evaporation, strong dry winds during the growing season, hot summers and cold winters limit plant growth. Further reductions in photosynthetic yield have been caused by fire, rabbits and inappropriate systems of land management during the past 130 years. Formerly dominant species populations have been depleted and numbers of fire tolerant, exotic and often unpalatable plants adapted to low soil moisture and fertility (O'Connor, 1982) have increased. Even areas of hill country grazed for only a few weeks a year, and which have been neither burned nor oversown with exotic pasture plants nor topdressed with mineral fertiliser in recent times, are in poor estate. As Cuff (1977) observed, the formerly dense swards of snowgrass served as sponges to absorb moisture after rain or snowmelt, ensuring water for plant growth during dry summer months. Their depletion has led to reduction in stored water and an increase in evaporation during late spring and summer when plant growth rates should be high. The results reported in this paper come from field assessments of (1) standing crop, and (2) yield of above-ground tissues of herbaceous plants, as well as (3) losses due to browsing by sheep and rabits in a 3.2ha catchment at 500m on the eastern flanks of the Carrick Range at Bannockburn near Cromwell, Central Otago. The period of fieldwork bracketed the 1984-85 drought and the research concerns a set of interrelated questions about the dynamics of disturbed low montane grassland, with particular reference to the influences of aspect, the seasons, available water, sheep and rabbits, on photosynthetic yield.