Quick Dips

Chilly Pine Trees

The effect of increased atmospheric carbon dioxide on the cold-hardiness of pine trees is under scrutiny. The study has important implications for forestry as carbon dioxide levels in the atmosphere continue to rise, possibly causing global warming and reducing the frost tolerance of trees.

US Forest Service plant physiologist Dr Richard Tinus and HortResearch plant physiologist Dr Dennis Greer are growing seedling radiata and ponderosa pines, and Douglas firs, under carefully controlled climatic conditions at HortResearch's National Climate Laboratory in Palmerston North.

Half the trees are being grown under conditions of elevated carbon dioxide --  700 parts per million, compared to the current global levels of about 345 parts per million. All the trees are then exposed to varying levels of cold and their frost tolerance observed.

"We know that high CO₂ has many effects on plants -- rapidity of growth, photosynthesis and water use efficiency -- but the effect on cold acclimation and deacclimation has not been explored," says Tinus.

If carbon dioxide retarded cold acclimation, making trees more vulnerable to frost, it would reduce the range of areas in which they could be grown. Pine trees are so long-lived -- 30 years rotation for radiata and 50-80 years for ponderosa pines and Douglas firs -- that it is becoming important to know how they will react to increased carbon dioxide levels so decisions can be made about future plantings, Tinus says. In parts of North America there was a demand for retaining old growth forests with rotations of up to 300 years to ensure the preservation of a species of spotted owl.

"When we've managed forests in the past we've always assumed that the climate for the next rotation would be the same as the climate in the last rotation, but now we have reason to believe that's no longer true."

Even if the climate did not change, the effects of elevated levels of carbon dioxide in the atmosphere on the frost tolerance of trees could mean that they could no longer be grown in areas where they currently thrive, he said.

The trees being used in the experiment were raised from seed in a greenhouse then transferred as seedlings to environmentally controlled conditions which simulate autumn, winter and spring at either normal or elevated levels of carbon dioxide. The trees will be subjected to controlled temperature drops in two different ways.

One group of trees will undergo a "whole plant freeze test" by placing them in low temperature rooms in the National Climate Laboratory and introducing a controlled decline in temperature.

The use of high pressure discharge lamps above a thermal barrier of flowing water in the laboratory provides the plants with more light than the low-heat fluorescent tubes used in similar facilities elsewhere. The thermal barrier reduces the problem of heat build up, while the temperature and night/day cycle is accurately controlled by microprocessors.

The second means of testing the frost tolerance of the two groups of trees is by an electrolyte leakage test. Foliage samples are cut up and placed in water in test tubes. They are then frozen in an alcohol bath to a series of successively lower temperatures. After an incubation period, the conductivity of the water in which the foliage samples have been submerged is measured.

"If the foliage is damaged, the electrolytes in the pine needles will leak out. The amount that leaks out will be proportional to the damage," says Tinus.

The frost rooms at the climate laboratory can go down to -20^oC, but Douglas fir and ponderosa pine can withstand much lower temperatures. The use of an alcohol bath in the electrolyte leakage test allows the temperature to be dropped as low as -70^oC.

The experiment is scheduled to run for 24 weeks and the research team hope to have useful results by August.