A little over a year ago New Zealand Geographic featured the kauri tree, providing a broad sampling of information about Agathis, its history and current standing.
Recently, Dr Neil Mitchell of the Botany Department, Auckland University, has been using the kauri as a model species to study a potential menace to our flora: changing climate. If current predictions of global warming over the next century come to pass, many of our native plants might be wiped out.
The science of biogeoclimatography, the correlation of life forms with specific areas based on their climate profile, has its main application in agriculture. For instance, by knowing the optimum growth conditions for wheat in the US, Dr Mitchell has been able to predict the most suitable areas for cultivation in Australia, based on knowledge of the local climates. Now he has extended the work to natural plant communities, particularly kauri.
The technique works in the following way. Climatic information gained from meteorological stations around New Zealand is entered into a computer database. Basic data includes solar radiation, rainfall and temperature readings. Altitude is also specified, not just for the met stations, but for a meshwork of points over the whole country. Using computer programmes developed in Australia, Dr Mitchell is able to extrapolate from his limited number of weather stations and predict climate for any point in the country.
Of the model’s 17,000 North Island grid points, kauri are known to be present at 1500. By analysing the climate of these 1500 sites, the computer is able to develop its own ideas about what conditions are suitable for kauri, and predict other sites where kauri should be present.
The computer predicted kauri in an area between Te Kaha and Hicks Bay, at the eastern extremity of the Bay of Plenty, plus a smattering of sites north of Gisborne—well beyond the kauri’s present natural range. When Dr Mitchell made enquiries about these sites, he found old reports of kauri gum being found there, and even a place named Kauri, near Gisborne. Clearly, kauri was once present on the East Cape, but has long since vanished.
Such results are probably of only mild interest. It is when climate change is included in the equation that the alarm bells start ringing. Latest UN reports suggest global warming of 1°C by 2010 and 3°C by 2100AD. Entering a 1.5°C rise together with a conservative 10 per cent increase in rainfall into the computer model produces the startling result that only 23 per cent of the present 1500 sites would then be suitable for kauri. No kauri would grow north of Kaikohe, but an additional area would become appropriate inland from Gisborne. Since there are no naturally occurring trees there to seed a population, should conservationists start planting kauri in the Raukumaras now to replace Warawara, Omahuta and Puketi?
This is no idle question; the predicted rate of temperature increase allows us little time for complacency. According to climatologists, the most rapid climate change followed the last ice age, 14,000 years ago, when temperatures rose 3° in 2000 years. We are now contemplating a 3° rise in 100 years!
The survival of many trees may depend on how fast they can move. This may sound ludicrous, but we are not talking about individual specimens, but the migration of a species. In North America spruce has sprinted at up to 200km per century, but most other species waddle at a more sedate 40km per century. In New Zealand, natural movement is likely to be considerably slower, as pockets of bush are surrounded by inhospitable farmland habitat. Hoof and tooth destroy young trees.
Birds such as pigeon, tui and bellbird are major distributors of seed, but experiments on Tiritiri Island suggest that pigeons don’t carry seed much further than 500m. The birds themselves are not necessarily that common either — when did you last see a bellbird north of Auckland? (Kauri seed is wind-dispersed, but that method provides an even slower spread.)
Apart from the problems of dispersal, young trees stand little chance against the goats, possums and deer that are ravaging our forests.
Although kauri has been used as the example, many plants with restricted distributions will be affected. Some habitats may disappear altogether, such as alpine outposts on Moehau and the Raukumaras.
And what happens on islands when the climate changes? Where do the plants go then? For instance, Xeronema, the Poor Knights lily, lives only on high rocky outcrops on the Poor Knights and Hen and Chickens Islands. As the climate warms it will want to go higher, but there are no higher peaks. Exit Xeronema and a great company of other treasures.