Potential forest cover of New Zealand as determined by an ecosystem process model

A forest biome map for New Zealand is presented, based on the ecosystem process model LINKNZ. Climate surfaces, landforms, slope, and soil types defined 88933 homogeneous landscape units covering the North and South islands (264000 km(2)). Forest development (2000 years) was simulated on each unit with 78 individually parameterised species selected by ecological importance. Forest biomes for the units were assigned by the relative biomass predicted for 21 plant functional types, categorised from the available species. An assessment of model performance against a systematic sample of 559 measured forest plots was satisfactory (dissimilarity index = 0.25). Direct validation was not possible over most of the landscape where native forest has been removed, but performance was sensible against data from 136 pre-deforestation pollen sites (dissimilarity index = 0.20). The modelled biome map reproduced the main characteristics of the current forest distribution in New Zealand and departures from the observed forest distribution were generally explained by omission of whole-stand disturbance effects from simulations. The model reproduced lowland areas of the striking beech gap in the west-central zone of the Southern Alps, but not the distribution of Nothofagus species in montane areas of this zone. The most likely explanation supported previous conclusions that the 'absence was due to a slow reinvasion of Nothofagus after exclusion during the Last Glaciation. Predictions of Nothofagus across some regions of the south-eastern South Island where it was nearly absent before settlement suggested that the ecological knowledge of some competing conifer species was incomplete.