The causes and consequences of synchronous flushing in a dry tropical forest

The seasonal pattern of leaf production and fate of leaf cohorts were assessed in 59 species of woody plants over a period of 28 months in a West African dry forest (mean annual rainfall 1100 mm). In all species, leaves were produced in synchronous flushes. Flushing patterns varied somewhat between species and between individuals within species, although all flushing activity was restricted to wet periods. Damage to leaves by folivorous insects was largely confined to newly flushed leaves. Insects inflicted severe damage to 13 percent of new leaf cohorts. Species having potential deterrents to folivore attack (hairy leaves or secondary compounds) were attacked less frequently than species without apparent protection. Prolonged flushing activity brought about an added risk of insect attack in species lacking such protection. Drought is considered to be the most important single factor affecting the growth, distribution, and phenology of woody species in this dry forest. The restriction of leaf production to short, synchronous flushing periods is adequately explained in terms of physiological constraints relating to the avoidance of moisture stress. A consequence of this rapid flushing, however, may be escape from folivory. FLUSHING, OR THE SYNCHRONOUS PRODUCTION OF A NEW LEAF CROP, is characteristic of both temperate and tropical woody species (Richards 1952, Kramer and Kozlowski 1979). The pattern of leaf production in individual shoots, plants, and populations is generally periodic in the tropics (Alvim 1964, Longman and Jenik 1974, Leigh and Smythe 1978, Reich and Borchert 1982), but it is only in seasonally dry tropical environments that large numbers of shoots, individuals, and species flush in synchrony (Njoku 1963, Hopkins 1970, Daubenmire 1972, Frankie et al. 1974, Borchert 1980, Lieberman 1982, Reich and Borchert 1982, 1984). Rapid, synchronous flushing may be viewed as an adaptation to exploit scarce and intermittent water supplies and to avoid producing young, uncutinized foliage when the danger of desiccation is great. Young leaves are particularly vulnerable to folivore attack due to their lack of mechanical protection and their high nutrient and water content (Feeny 1970, Rockwood 1974, Scriber and Slansky 1981). McKey (1974) and Feeny (1976) suggest that the production of leaves in rapid, synchronous flushes may contribute to escape from folivory. They hypothesize that flushing occurs when folivores are at low density or are inactive, and that the majority of new leaves may therefore have the opportunity to mature and harden prior to an increase in the folivore population. Water conservation and escape from folivory are suggested explanations for the phenomenon of rapid synchronous flushing, but no study has been reported in which both possibilities are considered. In this paper we describe patterns of flushing phenology and folivory in a dry tropical forest in Ghana. In the context of flushing, we examine the incidence of folivory in relation to plant chemical and morphological traits, and ask whether rapid, synchronous production of leaves provides a significant means of folivore escape. We also evaluate the relative importance of folivory and drought in the evolution of leaf flushing patterns in species of this dry tropical forest.