Seed rain dynamics reveals dispersal limitation, strategies of reducing risks and response to climate

Background/Question/Methods It has long been recognized that species coexistence may be achieved by limited dispersal due to the reducing chance of interspecific competition. Much less mentioned is the timing of releasing seeds, which also provides clue for species coexistence because asynchronous reproduction alleviates interspecific competition for dispersal agents while synchronous reproduction facilitates the overall seed dispersal. Besides these two, climate change is believed to have fundermental impacts on the species diversity in forest systems. Under this scenario, our main questions are: How strong seed dispersal limitation is? How do coexisting species release their seeds, synchronously or asynchronously? What is relationship between seed production and main meteorological measures? Knowledge of long term seed rain dynamics is necessary in order to answer such questions. Here, we answered these questions by calculating the Jaccard coefficients between seed rain composition and the neighboring adult tree composition, and analyzing the long term seed rain dynamics for both whole community and different species and regressing seed rain density with meteorological measures by autoregressive models based on four-year data of seed rain collected from 150 seed traps set up in a 25 hm² plot in a temperate forest in Northeast China.

Results/Conclusions We found that the Jaccard coefficient drops sharply first as neighborhood radius increases and becomes steady when the radius reaches about 10 m, indicating severe dispersal limitation. Most species release their mature seeds synchronously in October. However, some species release their seeds asynchronously with other species, e.g. the seed density of a wind dispersed species reaches a peak in June when trees are leafless and an animal dispersed species releases seeds in late August when animal acitivity is most frequent. The seed rain dynamics of some other species forms a relatively flat curve with no obvious peaks. In unfavorable environment, gradually shedding seeds helps to reduce the risk of local extinction. Both rising in temperature and precipitation are well indicative of rise in seed rain density with two months’ time lag. In general, our study reveals that both dispersal limitation and the timing of releasing seed by the coexisting species may contribute to the diversity maintenance of this forest. We also observed that variations in temperature and precipition alter the seed rain density considerably, long term study is highly necessary in order to check the forest’s response to ongoing climate change.