Annals of Botany

Sugar maple succumbs to strength-sapping stress

On my first visit to the USA many years ago, I was amazed to see that many of the trees in the areas of Virginia close to Washington DC were harbouring structures that looked like small, woven tents. A local resident informed me that, indeed, these were the homes of ‘tent caterpillars’ and that the infestation was particularly bad. I have seen these tents on some of my subsequent trips but certainly not at the frequency I observed on that first visit. Questions obviously arise about the amount of damage done by these herbivores. Are there any long-term effects on the life of the tree? Based on the careful and thorough analysis carried out by Hartmann and Messier (Montréal; pp. 377–387) the answer to the latter question is clearly ‘yes’. They have compared past growth rates (as indicated by growth-ring analysis) in living and recently (post-1993) dead sugar maple (Acer saccharum) trees. Growth rates were related to known previous infestations by forest tent caterpillars (larvae of Malacosoma distria) and to soil/root disturbance caused by a thinning out (‘partial harvest’) that affected some trees 10–11 years prior to the growth-ring measurements. It was very clear that an infestation by tent caterpillars strongly reduced both growth and vigour. A further infestation, even if it occurred more than 10 years later, led to a further decline in vigour. Trees most badly affected by previous infestations were very likely to die if they suffered another defoliation. Furthermore, this decline was accelerated in those trees that had been disturbed by the partial harvest. Interestingly, the partial harvest itself, in the absence of tent caterpillar infestation, did not cause any significant decline in growth or vigour. Overall, the authors conclude that the data are consistent with Manion’s tree disease model [Manion P. 1981. Tree disease concepts. Englewood Cliffs, NJ: Prentice Hall] where decline and death are ‘driven by an interaction between predisposing and inciting stresses’.