Are plant populations seed-limited? A review of seed sowing experiments

We define seed limitation to be an increase in population size following se ed addition. Here, we briefly consider how theoretical models deal with see d limitation and how seed sowing experiments can be used to unravel the ext ent of seed limitation in natural systems. We review two types of seed addi tion experiments: seed augmentation studies where seeds are added to existi ng populations: and seed introductions where seeds are sown in unoccupied s ites. Overall, approximately 50% of seed augmentation experiments show evid ence of seed limitation. These studies show that seed limitation tends to o ccur more commonly in early successional habitats and in early successional species. Most of the studies have concentrated on simply categorising popu lations as seed- or microsite-limited, but we believe that seed sowing expe riments could be used to reveal much more about community structure, and we discuss possible future directions. In 53% of introduction studies (where seeds were sown at sites from which t he species was known to be absent) the introduced species was recorded in a t least one of the experimental sites following sowing. However, of the sub set of studies where both seedlings and adult plants were recorded, 64% of sites contained seedlings while only 23% contained adults. This implies tha t. for many species, conditions for establishment are more stringent than c onditions for germination. The successful establishment of plants in unoccu pied parches indicates the potential for immigration to enhance local diver sity (the spatial mass effect). Few studies continued monitoring for long e nough to determine whether or not self-sustaining populations were successf ully established, and no study attempted to link introduction sites to a pu tative natural sourer of propagules, or considered the dynamics of the meta population as a whole.