The Cost of Being Dormant in the Tropics

The roots, stems, and twigs of Jacquinia ptangens were found to lose as much as one-half of their stored carbohydrate reserves when leafless during the rainy season. This finding implies that dormancy can be an expensive way to avoid an inimical environment. Jacquinia pungens A. Gray (Theophrastaceae) is an understory shrubby tree in Central American deciduous forests. In Costa Rica, it produces leaves at the beginning of the dry season (December) and drops them about six months later at the beginning of the rainy season (May-June) (Janzen 1970). In this paper we examine the hypothesis that the rainy (dormant) season is a period of substantial stress to the plant, as measured by metabolic expenditures at this time. The cost to J. pungens of rainy season dormancy is a special case of the general problem of understanding the cost of being dormant at high tropical temperatures. This dormancy contrasts strikingly with the apparent low metabolic cost of dormancy for a poikilothermic tree during the cold season in temperate zones. We chose J. pungens for this study because of its abundance, small size at reproductive maturity, conspicuous starch storage, lack of sexual activity while leafless, and potential for suggesting minimal maintenance costs for a woody plant during the rainy (coolest) time of year in the lowland tropics. METHODS AND STUDY AREA Twig, main stem, and root segments were collected from each of five 1.5-2.0 m tall J. pungens shrubs on seven dates (table 1). Twig segments were from the previous year's growth; main stem segments were 2-4 cm diameter and from about 30 to 80 cm up the trunk; and the root segments were from the tap root at a point 10-20 cm below the ground and the same diameter as the main stem segments. J. pungens is exceptional among tropical deciduous forest understory shrubs in that it has a very deep and large tap root (Janzen 1970). The shrubs were sacrificed when sampled. The woody segments (including the thin bark) TABLE 1. Total available carbohydrate (TAC) in mg per g of Jacquinia pungens twig, stem and root segments. Each value is the average of five samples and the same five shrubs were used for twig, stern and root segments on a given date. See text for calculation of TAC for entire shrub.