Reproductive biology and population structures of Podocarpus falcatus and P. latifolius in southern Cape forests

Abstract

This study concerns morphological differences of the fruit types of two Podocarpus species in relation to their different population recruitment rates. First, sampling data from the mixed evergreen forests of Gouna and Groenkop in the southern Cape, South Africa, were used to describe the population structures of the two species. Podocarpus falcatus is widely scattered at low density and has a low recruitment rate. Podocarpus latifolius is widespread and common at high density and has a high recruitment rate. Second, the morphology of their fruits and seeds was studied in relation to seed dispersal, predation and germination. The species represent the two basic 'fruit' types of the genus. Podocarpus falcatus produces a heavy fruit, but invests mostly in dispersal (with a yellow flesh pulp or epimatium enclosing the seed) and protective tissue (stony shell or sclerotesta) and very little into endosperm food reserves for the embryo. Podocarpus latifolius produces a light fruit which is composed mainly of dispersal tissue (blackish-purple fleshy receptacle) with protective tissue (thin leathery epimatium without woody sclerotesta) and food reserves for the embryo. The structure of the P. falcatus fruit is related to adaptation for protection during seed dispersal by bats. The stony sclerotesta delays germination for about one year and thereby causes the exposure of the seed to high post-dispersal predation by rodents and bushpig. Seedling recruitment is low. In contrast birds and baboons eat the receptacle of the P. latifolius fruit and discard the seed undamaged. Less protective tissue is required. Seeds germinate within two to four months and recruitment is high. The structural differences of the two fruit types also have predictive value for the relative success of the different sections of the genus. Section Afrocarpus (to which P. falcatus belongs) and related sections have relatively restricted geographic distributions. Section Eupodocarpus (to which P. latifolius belongs) and related sections have wide distributions.