Forecasting rehabilitation outcomes for degraded New Zealand pastoral streams

Abstract

To understand the timescales and magnitude of responses that can be expected following catchment and riparian rehabilitation, we forecast changes to selected stream ecosystem attributes following tree planting in a pastoral catchment. All planting scenarios were predicted to lead to decreases in daily maximum water temperature after 15-20 years to levels that would be suitable for sensitive invertebrate species. Cooling and reheating were rapid so that most benefits to water temperature along the mainstem were forecast to accrue from shading all of the stream channel network. All planting scenarios were predicted to increase sediment yields over the status quo over the 25-year timeframe examined, with maximal sediment yield occurring about 15 years after planting due to expected erosion of the streambanks under the developing forest shade. Sediment yield was greatest for full catchment planting over 25 years, although sediment yield would be lowest with this scenario over longer timescales. A macroinvertebrate biotic index was predicted to increase by 25% over 15 years if whole catchment afforestation were implemented, compared to 9% if only the 4th order mainstem were planted with riparian trees. The use of ecological forecasting to predict likely outcomes for a range of scenarios should prove useful for prioritising rehabilitation actions.