Transplants of beech, Fagus sylvatica, L. were planted in 1 m plastic tubes containing a mixture of peat and grit and sunk into the ground inside open-top chambers at a site in southern Britain. Chambers were ventilated with either charcoal-filtered or unfiltered air and water was withheld from half of the trees within each chamber. The influence of air quality on the growth and development of roots and interactions between air quality and drought were investigated. During May and June 1988 the growth of fine (diameter < 1.5 mm) and large roots (diameter > 1.5 m) was not significantly altered by air quality. A destructive harvest conducted in July revealed that trees exposed to ambient pollution had significantly longer roots (P < 0.05) compared with trees grown in filtered air, despite slightly less root dry weight (not statistically significant). Specific root length (SRL, root length per unit root dry weight) was also significantly larger (P < 0.05) for trees grown in the unfiltered chambers and exposed to pollution, suggesting a root system which was not only longer but also consisted of thinner roots, compared with the roots of trees grown in clean air. The effects of pollution were more pronounced for roots in the lower half of the 1 m profile.
During August 1988, water was withheld from half of the trees within each chamber. Well-watered trees exposed to unfiltered air had significantly less root dry weight (P < 0.05) compared with well-watered trees grown in filtered air, although root length and SRL were both greater for trees grown in unfiltered air compared with those grown in filtered air. Again the effects were more pronounced lower in the root profile. Withholding water resulted in less root length being produced irrespective of air quality treatment, suggesting that trees exposed to pollution would not necessarily be more susceptible to drought-stress. Root length was also measured non-destructively in 1989 and again trees grown in unfiltered air had longer fine and large roots, compared with those grown in filtered air.
These results illustrate the importance of measuring root length to complement estimates of dry weight, which may not be the best indicator of root function. The production of longer, thinner roots may be an important mechanism to compensate for reduced carbon allocation to, and dry matter accumulation by roots of trees exposed to pollution.