An experimental field study of different levels of insect herbivory induced by Formica rufa predation on sycamore (Acer pseudoplatanus). III. Effects on tree growth.
Feeding damage by Lepidoptera larvae, aphids and typhlocybid leafhoppers on sycamore (Acer pseudoplatanus) was measured on ant-foraged (Formica rufa) and ant-free mature sycamores in North Lancashire, UK. Some trees were switched from low to normal levels of herbivory by excluding predatory ants using grease bands on trunks. Mean total loss of leaf area in ant-foraged parts of the woods was 1-1.6% and in unforaged areas was 6-10%. Phloem sap removal was approximately 2 to 3 times as great in unforaged as in foraged areas and reached 5-600 mg dry weight per shoot. Leaf area loss and phloem sap removal in banded trees was mid-way between foraged and unforaged trees. Year to year fluctuations in total sycamore leaf fall at each site were small. In the dry summer of 1983, leaf fall was earlier from the trees with normal herbivore levels than from trees with low herbivore damage. Fewer viable seeds were produced from trees with normal herbivore levels than from those with low herbivore damage. Mean radial growth of mature unforaged sycamore trees carrying normal herbivore populations was approximately 35% less than that of ant-foraged trees (low herbivore numbers) in 1973-83 and about 44% less in 1981-83. The differences were greater (43 and 47%, respectively) if actively growing trees alone were compared. Banded trees showed significantly smaller radial growth after banding than in the period immediately before banding. It is concluded that moderate levels of herbivory on sycamore can significantly reduce tree performance and this is reflected in substantially reduced timber increments.