As any gardener or farmer knows, cleared soil in New Zealand soon sprouts fast-growing weeds. In time the weeds are overtaken by woody shrubs. These in turn are gradually replaced by long-lived trees.
Primary and secondary succession
When one group of plants gradually replaces another, it is called plant succession.
- A primary succession is when plants colonise bare ground that has never developed a soil – for instance sand dunes, volcanic ashes, exposed river gravels or glacial surfaces.
- Secondary successions occur on soil that built up under earlier vegetation. They start after forest is cleared by an event like a storm, a landslide or a fire.
Primary succession after an eruption
When Mt Tarawera erupted in 1886, it buried vegetation under a thick layer of ash and scoria. For about 10 years the area was bare. Then toetoe (Cortaderia fulvida), bracken fern (Pteridium esculentum) and tree tutu (Coriaria arborea) appeared on the lower slopes. Above 600 metres, the first plants were Racomitrium moss, lichens and mat-forming daisies (Raoulia species).
Thirty years after the eruption, there was a young forest of pōhutukawa (Metrosideros excelsa), rewarewa (Knightia excelsa) and kāmahi (Weinmannia racemosa) on the lower slopes. Tree tutu was spreading up the volcano’s sides.
By the 1990s, kāmahi forest clothed the lower slopes. Some kāmahi was growing in tree tutu shrublands on the upper slopes. Tree tutu had spread upwards onto the flat area of the summit and was becoming a closed canopy over the low-growing mats of mosses, lichens and daisies.
Primary succession in glacial areas
At the end of the last Ice Age, glaciers retreated up their valleys and left moraines – glacial dumps of rubble – behind. On the West Coast, at Franz Josef, various plant communities can be seen growing on moraines of different ages. These have developed over more than 10,000 years. They show the different stages of a succession on moraine.
The first plants to grow on the fine gravels of moraine are similar to those on volcanic surfaces – mat-forming daisies, willow herbs (Epilobium species), lichens, and Racomitrium moss. Within 10–20 years a shrubland develops, dominated by tree tutu and tree broom (Carmichaelia arborea).
Seedlings of kāmahi and southern rātā (Metrosideros umbellata) establish themselves early, but in their first years, faster-growing shrubs overtop them. In time, the kāmahi and southern rātā – longer-lived and taller than shrubs – overtake the shrubs. They then dominate the forest for 300–400 years.
These first-generation kāmahi and rātā eventually die. After this, rātā are unable to germinate in the shade of the forest that has emerged. They are replaced by conifers: rimu (Dacrydium cupressinum), miro (Prumnopitys ferruginea) and Hall’s tōtara (Podocarpus hallii). Kāmahi is shade-tolerant, so it establishes a second generation. Rimu–kāmahi forest has lasted for at least 11,500 years on the oldest moraines.