Postglacial history of New Zealand wetlands and implications for their conservation
- Landcare Research, PO Box 40, Lincoln 7640, New Zealand
Most New Zealand wetlands formed at or after the end of the last glaciation (c. 18 000 cal yrs BP). Those associated with major rivers and close to the coast tend to be young as erosive processes both destroy and initiate wetlands. However, there is a strong linear trend in initiations since 14 000 cal yrs BP, which suggests that geomorphic processes such as soil deterioration, landslides, sand dune movement and river course changes are constantly adding new, permanent wetlands. Most wetlands began as herbaceous fens but usually transitioned to shrub- or forest-covered bog–fen systems, in particular after the beginning of the Holocene (11 500 cal yrs BP). Raised bogs formed from fens during the late-glacial and early Holocene, when river down-cutting isolated them from groundwater inflow. As climates warmed through the late-glacial and early Holocene, wooded wetlands spread and over 75% of lowland peat profiles preserve wood layers. Large basins with high water inflow often contain lakes or lagoons and have maintained herbaceous swamps, whereas those with limited catchments have become almost entirely covered with forest or shrub. Wetlands in drier districts tend to have been initiated during the mid- and late Holocene as the climate cooled and rain-bearing systems penetrated more often. Ombrogenous montane and alpine bogs may have been initiated by the same climate change. Natural fires frequently burnt some wetlands, particularly within the vast bog complexes of the Waikato Basin, but many wetlands record occasional fire episodes. By the time Mâori arrived in the 13th century, about 1% of the landscape was covered with some form of wetland and most of that wetland was under woody cover. Mâori firing of the landscape began the process of removing the woody cover, which induced wetter, more herbaceous systems and initiated new wetlands. Deforestation of catchments in drier districts increased water yield that may in turn have created lowland fens and lagoons. European logging, fire and draining destroyed both pristine forested wetlands and fire-transformed systems from the Mâori settlement era. The loss of wetlands is now largely a crisis of continued degradation through draining, weed invasion and fire in already human-altered systems in productive landscapes. Wetland history can help assess values and inform goals for conservation of wetlands, but transformation of the lowland landscape has been so complete that an historically authentic endpoint is unrealistic for most wetlands. The major conservation emphasis should be on larger wetlands that provide a range of ecosystem services.