Dispersal spectra, diaspore size and the importance of endozoochory in the equatorial Andean montane forests

Abstract

The dispersal syndrome hypothesis states that plant diaspores show morphological features that are the results of adaptation for dispersal by a particular vector. This can enable to identify the relative importance of dispersal agents within plant communities. Nevertheless, there is still little information about seed dispersal spectra and diaspore traits related to different dispersal agents in the equatorial montane forests, despite their high biodiversity and important ecosystem services as watersheds for human communities. Due to an increase in environmental stress at high elevations a reduction in the prevalence of endozoochory, and a reduction in the size of endozoochorous diaspores in plant assemblages could be expected. We reviewed published data from 64 Andean cloud forest plots to assess the dispersal spectra, the incidence of different traits related to seed dispersal, and the distribution of dispersal syndromes within cloud forests of northern South America. We then evaluated two questions related to seed dispersal in these forests: (1) Does the number and percentage of endozoochorous species in woody plant assemblages decrease at higher elevation? and (2) Does the mean diaspore size of endozoochorously dispersed tree assemblages decrease with elevation?Endozoochory was the main dispersal syndrome of the woody plant species evaluated. Associated diaspore traits, such as single seeded, small size and black or purple fruits, were the most common, probably because they are attractive to bird seed dispersers. The most common fruit types in cloud forests were berries, drupes and dehiscent capsules. On the other hand, the percentage of endozoochorous species in cloud forests was correlated to the mean annual temperature, rather than annual rainfall. We also found a distinct increase in anemochory with elevation. Furthermore, and as predicted, we found that the mean diaspore length of endozoochorous assemblages decreased with altitude. Our results suggest that in the equatorial montane forests, endozoochory is the most common dispersal syndrome although environmental stress in high elevation zones promotes an increase in the prevalence of anemochorous species. Anemochorous species produce dry fruits and small diaspores, probably favored at high elevations due to the low temperatures and nutrients availability and the high costs involved in the production of suitable pulp for animal dispersers.