Effects on New Zealand Vegetation of Late Holocene Erosion and Alluvial Sedimentation

During the last 1800 years there have been eight periods of increased erosion and alluvial sedimentation in New Zealand, which have generally decreased in magnitude towards the present. Throughout New Zealand, alluvium of all erosion periods contains abundant remains of plants as evidence of widespread destruction of vegetation during erosion periods. Indices of the relative magnitude of alluviation, and estimates of the damage to vegetation in the current Waipawa Period (since 1950), are applied to estimate the impact of earlier erosion periods. In 13 basins on the Ruahine Range, 2.6% of the 1950 vegetated area was destoyed by slope erosion during 1950-1986; on the Pouakai Range (Taranaki) it was 3.2%. On these ranges the areas of vegetation probably destroyed or damaged by erosion, alluviation and other factors are estimated as 17-18% in the Wakarara Period (180-150 years BP), 26% for Matawhero (450-330 years BP), 52% for Waihirere (680-600 years BP) and >52% for the pre-Kaharoa Period (1300-900 years BP). The vegetation of other areas, in both North and South Islands, would have been affected to greater or lesser extents. Large areas of vegetation along downstream channels, and on flood plains, were destroyed by alluviation in each previous erosion period. On alluvial sites the vegetation is closely linked to the history of the site it may grow entirely on the new surface or be a mixture of survivors and new colonists. The accelerated erosion, alluviation and consequent vegetation destruction in erosion periods, were associated with atmospheric warming and increases of major rainstorms and floods; they did not coincide with either the colonisation or activities of humans or mammals. Since 1950, the impact of rainstorms and floods has increased, while mammal populations have been reduced. Local fauna is seriously affected when vegetation is destroyed, and some species may have become extinct, or very reduced because of erosion periods. The present national forest estate is dominated by relatively young forests which are still developing towards some condition of quasi-equilibrium. Even in the absence of humans and mammals this vegetation would be in a dynamic state of imbalance and change.