Decadal survival of tropical pioneer seeds in the soil seedbank is accompanied by fungal infection and dormancy release

Pioneer trees require high-light environments for successful seedling
establishment. Consequently, seeds of these species often persist in the
soil seed bank (SSB) for periods ranging from several weeks to decades.
How they survive despite extensive pressure from seed predators and
soilborne pathogens remains an intriguing question. This study aims to
test the hypotheses that decades-old seeds collected from the SSB in a
lowland tropical forest remain viable by i) escaping infection by fungi,
which are major drivers of seed mortality in tropical soils, and/or ii)
maintaining high levels of seed dormancy and seed coat integrity when
compared to inviable seeds. We collected seeds of Trema micrantha
and Zanthoxylum ekmanii at Barro Colorado Island, Panama, from sites where
adult trees previously occurred in the past 30 years. We used carbon
dating to measure seed age and characterized seed coat integrity, seed
dormancy, and fungal communities. Viable seeds from the SSB ranged in age
from 9 to 30 years for T. micrantha, and 5 to 33 years for Z.
ekmanii. We found no evidence that decades-old seeds maintain high levels
of seed dormancy or seed coat integrity. Fungi were rarely detected in
fresh seeds (no soil contact), but phylogenetically diverse fungi were
detected often in seeds from the SSB. Although fungal infections were more
commonly detected in inviable seeds than in viable seeds, a lack of
differences in fungal diversity and community composition between viable
and inviable seeds suggested that viable seeds are not simply excluding
fungal species to survive long periods in the SSB. Synthesis: Our findings
reveal the importance of a previously understudied aspect of seed
survival, where the impact of seed-microbial interactions may be critical
to understand long-term persistence in the SSB.