Weather: Why are our trees shedding their leaves later?

Thursday, January 31, 2008

You might think that this is just another symptom of a warming world. But an expert study suggests that rising carbon dioxide concentrations in the atmosphere, not rising temperatures, may be the primary cause

by HENRY HENGEVELD

This past fall, our annual fall leaf clean-up began in mid-October, when the sunburst locust began to shed. Two weeks later, the chestnut tree suddenly dropped its leaves. However, at the time, the leaves on the maple and Chinese elms were still green. It wasn't until the second half of November that these began to fall, and we could do our final yard clean-up.

I suppose that shouldn't have come as a surprise. September and October were more than 4 C above normal in the Greater Toronto Area, and the first hard frost didn't come until mid November.

However, the delayed leaf-drop this fall wasn't really an exception. It seems that the end of the annual growing season in Ontario has been getting progressively later and later, and the experts agree. In fact, in its fourth science assessment report, the Intergovernmental Panel on Climate Change report on observational trends in growing seasons around the world, based on satellite measurements of photosynthetic activity as well as on-site measurements at various on-land locations, noted that, for North America, the growing season has lengthened by an average two days each decade since about 1950. That is a full 10 days so far. The number for Europe has been even greater.

While the experts point out that earlier onset of plant activity in the spring is a significant factor - perhaps the dominant factor in Europe - they note that continued leaf growth later into the fall season may be the most important factor in North America. A recent study by researchers in France indicates that date of senescence of leaves in North American vegetation may now be more than six days later than in 1980. One reason may be the delay in killing frosts due to warmer nights in the fall season.

Just one more symptom of a warming world - or is it?

Last November, a large team of researchers from Europe, Canada and the United States, led by University of Southampton biologist Gail Taylor, claims that it is not. In a recent article published in the scientific journal Global Change Biology, they argue that rising carbon dioxide concentrations in the atmosphere, not rising temperatures, may be the primary cause for the delay in change of leaf colour and eventual leaf drop in the autumn.

Their conclusions are based on two sets of experiments they undertook with poplar trees - one in Wisconsin and the other in Italy. Each experiment involved two plots of trees. In the "control plot," the grove of poplars was exposed to ambient air, which has carbon dioxide levels that are currently about 380 parts per million. Plants were allowed to grow under these conditions from initial planting of seedlings until maturity.

In the second plot, a similar grove was exposed to air which had been constantly fumigated with extra carbon dioxide, so that its average concentrations in the air during the multi-year experiment was artificially enhanced to 550 parts per million - about the level expected globally by 2050.

In both experiments, sensors were installed to measure the differences in growth behaviour of the two groves. The scientists found that the trees grown under higher CO2 conditions stayed green and retained their leaves longer than did those in the control plot. Leaf level photosynthetic activity and carbon uptake during the senescence period was also enhanced under the higher CO2 conditions.

The researchers note that an enriched CO2 environment allows trees to generate extra carbon compounds within the leaves which prolong their life span.

However, since these leaves now continue to photosynthesize later into the autumn, the trees also end up delaying the budding process for the next year's crop of leaves. That makes them more vulnerable to winter damage, particularly due to frost.

Taylor notes that this risk will need to be considered in managing the impacts of future environmental change on forests.

Current atmospheric CO2 concentrations are already significantly above those of the past - an increase of about 13.5 per cent in the last 30 years alone. Hence, the scientists involved in the experiment argue that the delay in fall colouring and final drop of tree leaves, which we are already experiencing today, may at least be partly due to the photosynthetic response to CO2 enrichment.

Their argument is strengthened by other studies. In general, these show good agreement between earlier spring onset of growing seasons and spring temperature rises, but find that the correlation between delayed end of the growing season and autumn temperature rises is rather weak.

Experts also note that longer growing seasons don't necessarily mean higher net growth yield of plants.

The warmer temperatures which have accompanied prolonged growing seasons have also extended the season for soil and plant respiration. Hence, the net ecosystem productivity appears to have changed little.

East come, easy go - or something like that! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.