A Negative Correlation Between Growth and Rainfall in a Tropical Liana

Abstract

The vegetative phenology of Passiflora pittieri, a liana, was monitored for nine months in Corcovado National Park, Costa Rica. Corcovado is tropical premontane wet forest with a dry season from January to April. A time-series analysis of shoot production and rainfall revealed a significant seasonal trend in shoot production, high during the dry season and low during the wet season, and a strong negative correlation between shoot production and current rainfall even after removal of the seasonal trend from the data. Apparently, P. pittieri was able to respond immediately to some factor associated with dry spells. Evidence is also presented for an endogenous inhibition of shoot initiation following shoot production. Possible factors that regulate shoot production are also discussed. MOST PHENOLOGICAL STUDIES IN THE TROPICS have been primarily concerned with flowering and fruiting (Fournier and Salas 1966, Nevling 1971, Opler et al. 1976, Stiles 1978, Augspurger 1983). However, vegetative phenology may underlie many aspects of plant reproduction. Borchert (1983) stated . . the timing of flower induction and its possible controls are virtually unknown for all tropical trees and can only be inferred from an analysis of seasonal shoot growth patterns in conjunction with tree morphology. In fact, vegetative phenology can determine seasonal changes in net primary productivity (Lieth 1970) and can influence many attributes of forest structure and regeneration. Frankie et al. (1974) classified trees in a tropical wet forest as having either continuous leafing, discontinuous production of new leaves, or marked leaf drop and flushing. Many studies of tropical vegetative phenology are of trees that have marked leaf drop and flushing; studies of dry forest trees predominate (Janzen 1967, Daubenmire 1972, Reich and Borchert 1984). Frankie et al. (1974) and Opler et al. (1980) found that 28 percent of the overstory tree species, 40 percent of the understory tree species, and about half' of the shrub and treelet species in a tropical wet forest exhibited continuous leaf production. Yet very few studies investigate in more detail the temporal variation of growth in such plants. Trees have been the focus of almost all phenological studies. Trees are large populations of meristems connected in very complicated ways. Putz (1979) emphasized the difficulty of identifying phenological events in many Malaysian trees because of variable shoot behavior within trees. The phenology of lianas, important components of many tropical forests (Putz 1983, 1984), has been ignored, even though they are excellent candidates for such studies. Their greatly reduced number of meristems and simpler architecture is conducive to detailed investigations of vegetative growth. As part of a study on plant-herbivore interactions, I investigated the temporal variation of shoot production and its relationship to rainfall in a woody liana, Passiflora pittieri, in a wet forest in Costa Rica. Time-series analysis, a method that could be applied more frequently in phenological studies (see Putz 1979), was used to characterize rainfall and shoot production by the population of lianas. Analysis of shoot production by individual plants also allowed investigation of endogenous control of growth. The genus Passiflora contains largely herbaceous neotropical vines, but one subgenus, Astrophea, contains many species that are woody lianas and small trees (Killip 1938). Passiflora pittieri is a species of Astrophea widespread in Costa Rica and Panama (Longino 1984). In contrast to most other Passiflora, P. pittieri is a shade-tolerant species found most often in the understory of wet forest. It is a woody liana with a single, strong taproot. Mature leaves are dark green, glabrous, coriaceous, elliptic to oblanceolate, and vary in size from 2 to 30 cm long. Unsupported plants in the undergrowth have a shrubby aspect. On all but the largest plants, sympodial shoots are produced one at a time, often with a month or more between successive shoots. Large plants may produce more than one shoot simultaneously. Shoots rapidly elongate for 2 to 3 wk before growth ceases and leaves harden. Very small plants produce short shoots with one to three small leaves. Shrubby plants, the majority in this study, produce shoots that are 50 to 150 cm long with 10 to 20 leaves. Additional information on the biology of P. pittieri is presented in Longino (1984).