Impacts of logging on density-dependent predation of dipterocarp seeds in a South East Asian rainforest

Abstract

Much of the forest remaining in South East Asia has been selectively logged. The processes promoting species coexistence may be the key to the recovery and maintenance of diversity in these forests. One such process is the Janzen-Connell mechanism, where specialized natural enemies such as seed predators maintain diversity by inhibiting regeneration near conspecifics. In Neotropical forests, anthropogenic disturbance can disrupt the Janzen-Connell mechanism, but similar data are unavailable for South East Asia. We investigated the effects of conspecific density (two spatial scales) and distance from fruiting trees on seed and seedling survival of the canopy tree Parashorea malaanonan in unlogged and logged forests in Sabah, Malaysia. The production of mature seeds was higher in unlogged forest, perhaps because high adult densities facilitate pollination or satiate pre-dispersal predators. In both forest types, post-dispersal survival was reduced by small-scale (1 m(2)) conspecific density, but not by proximity to the nearest fruiting tree. Large-scale conspecific density (seeds per fruiting tree) reduced predation, probably by satiating predators. Higher seed production in unlogged forest, in combination with slightly higher survival, meant that recruitment was almost entirely limited to unlogged forest. Thus, while logging might not affect the Janzen-Connell mechanism at this site, it may influence the recruitment of particular species.

Figure 1.

The relationship between P. malaanonan seedfall and distance from the nearest fruiting tree in (a) unlogged and (b) logged forest. Points are the mean number of seeds falling into 1 m² seed traps located at 2, 10, 20 and 30 m from 10 trees in each forest type. Solid lines represent the number of seeds predicted to fall at each distance by the model fitted to the data. Dotted lines represent the predictions for each of the 10 trees in each forest type. (c) The majority of seeds fell in the first six weeks of the study. Points are the mean number of seeds that fell in seed traps placed 2 m away from each tree and lines are the model predictions. Black line, unlogged; grey line, logged.

Figure 2.

(a) Survival of P. malaanonan seeds and seedlings in both unlogged and logged forest. Seed and seedling survival were not significantly affected by either forest type or distance from the nearest fruiting trees. Open circles, 2 m; filled circles, 30 m. (b) The effect of seed production at each tree on seed and seedling survival. Data are the mean survival rates (±s.e.), taking into account the variation between plots and focal trees. Black line, unlogged; grey line, logged.

Figure 3.

Survival of P. malaanonan seedlings declined with conspecific seed and seedling density in both (a) unlogged and (b) logged forest. Distance from nearest fruiting tree did not affect survival. Points are the observed proportion of seeds that survived through 4 day census intervals in 1 m² plots at each density at 2 and 30 m from the focal trees. Lines represent the expectations of the model fitted to the data and error bars represent the standard errors. Black dashed line, 2 m; grey solid line, 30 m.

Figure 4.

The causes of mortality of P. malaanonan seeds and seedlings in unlogged and logged forest. Data are the mean (±s.e.) proportion of deaths in which the mortality agent was implicated. Note that more than one agent could contribute to death; so the proportions can add up to greater than 1. Open bars, vertebrate predators; light grey bars, vertebrate browsers; dark grey bars, fungi; black bars, insects.

Figure 5.

(a) Insect predation on P. malaanonan seeds collected in traps or from the forest floor nearby was higher in logged forest than unlogged forest and at trees that produced fewer seeds. Lines are the predictions of the models fitted to the data (±s.e.). Black line, unlogged; grey line, logged. (b) The proportion of predated seeds attacked by different predators. Bars represent the mean (±s.e.) proportion of seed predators within three different categories and the proportion of predators that had been parasitized. Dark grey bars, moths; mid-grey bars, weevils; light grey bars, parasitoids.