High abscission rates of damaged expanding leaves: Field evidence from seedlings of a Bornean rain forest tree

Herbivore damage is known to cause the premature loss of mature leaves. How ever, the effects of herbivory on abscission during the early stages of lea f development remain unexplored, even though herbivores frequently prefer u nlignified. immature leaves. In a field experiment, we removed 50% of the t issue from leaves at various stages of development on seedlings of Shorea h opeifolia (Dipterocarpaceae), a dominant rain forest tree in Indonesian Bor neo. Four weeks following simulated herbivory, >88% of unlignified expandin g leaves had been abscised, compared to only 20% of fully expanded, unligni fied leaves and 0% of recently lignified, mature leaves. In a separate expe riment over 9 wk. simulated herbivory did not increase abscission rates of mature leaves, even when 75% of leaf tissue was removed. Because most (58%) of S. hopeifolia seedlings under natural conditions had lost 1% or less of the tissue from their mature leaves, herbivore damage probably has little effect on the abscission of mature leaves. In contrast, the tendency for da maged expanding leaves to abscise may explain why 49% of S. hopeifolia seed lings had already lost their youngest leaf. If similar patterns occur in ot her species, herbivore attacks on developing leaves may contribute substant ially to both leaf loss and the cumulative impact of herbivory on the growt h and survival of whole plants.