Alpine Phytomass and Primary Productivity in Central Otago, New Zealand

Phytomass was measured and net potential productivity estimated for normal climatic conditions on the Otago rainshadow mountains. Seven tall snow tussock grasslands were compared with lower-statured cushion, herbfield and short grassland communities. Total phytomass ranged from c. 5-9 Kg m (22 000-40000 Kcal m) in tall tussock and c. 2-4 Kg m (8100-16500 Kcal m) in cushion, herbfield or short grassland vegetation. Root: shoot ratios of less than unity, in all communities, probably reflect the oceanic nature of the climate. Total annual potential net production ranges from c. 0.7-1.0 Kg m (3200-4600 Kcal m) in tall tussock, and c. 0.4-0.7 Kg m-2 (1700-3400 Kcal m-2) in cushion or herbfield and is 1.0 Kg m (4200 Kcal m) in an induced mid-altitude Poa colensoi turf grassland. Growing season efficiency ratings (0.5-1.2%) tend to increase with altitude, suggesting an adaptation to the short, climatically erratic summers. The c. 150-200 day growing seasons are nevertheless considerably longer than for continental tundras abroad. Turnover times are typically c. 11 years, but may be up to 20 years at high altitudes. Growth is suppressed, presumably by cool. short growing seasons at high altitudes and perhaps by soil moisture stress at the base of the alpine zone. The Otago alpine cushion and herbfield communities achieve phytomass and productivity values within the range for world tundras. The larger values for tall tussock stands are, however, not unusual when compared with other natural large-statured herbaceous or shrub vegetation types.