Survival of pioneer plant seeds in tropical forest soils – results of a 20-year study
To understand how persistence in the soil seed bank influences species composition in regenerating forest patches and how it mediates the fitness consequences of dispersal by different animals, we investigated rates and sources of seed mortality in six representative pioneer species in cloud forest at Monteverde, Costa Rica. Starting in 1993, we used selective exclosures to expose seeds of Urera elata, Cecropia polyphlebia, Witheringia meiantha, Bocconia frutescens, Phytolacca rivinoides, and Guettarda poasanato microbes only, to both microbes and animals, or to neither threat. Seeds exposed to both microbes and animals were marked and buried in intact forest soils. Seeds exposed to microbial attack only were protected from animals with a combination of wire and nylon mesh, but were otherwise in direct contact with soil. Seeds from which both animals and microbes were excluded were protected within plastic vials that prevented direct contact with soil but allowed gas exchange and maintenance of ambient soil temperature and humidity. Subsets of seeds in each of the three treatments were recovered at intervals and assessed for presence/absence (animal exposure treatment) or viability (microbial exposure and total protection) through 2013.
Seeds exposed to animals were removed from intact soil at rates from 6.7 to 50% per year (50 to 93.3% annual survival). Survival of those exposed to microbial attack only ranged from 40 to 100% per year, as did survival of those protected from both kinds of biological threats. Despite the broad similarity in survivorship of seeds exposed to microbes and those protected from them, microbial attack was an especially important source of mortality in two of the species. In almost all cases, survivorship data fit exponential decay curves, suggesting constant mortality probability, but at least some seeds of all six focal species survived to 20 years. Survival rates in our experiments are consistent with the ratios of seed bank to annual seed rain densities that we have measured previously. Modeling of plant reproductive success incorporating seed survival, the dispersal patterns produced by birds, and the forest disturbance rates measured at Monteverde show that seed predators and pathogens affect not only plant fitness but the consequences of seed dispersal for it, as well as the composition of the seed bank available to colonize forest patches following natural or anthropogenic disturbance.