Interaction between algae and seed germination in tropical dune slack species: a facilitation process

The importance of algae dune slack stabilisation has been extensively analysed, but little has been done regarding their potentially favourable role in germination and establishment of phanerogams. Dune slack is a seasonal freshwater habitat that depends on water table level fluctuation. Our hypothesis is that in these highly unpredictable systems, algae maintain a certain degree of humidity in the soil and, thus, permit phanerogam germination and establishment. The objective of this study was to compare the germination of four species characteristic of tropical dune systems: Cyperus articulatus L., Fimbristylis cymosa R. Br., Fuirena simplex Vahl and Rhynchospora colorata (L.) Pfeiffer, in different substrates and degrees of humidity. For germination trials, seeds of the four species were placed each on three substrates: in sand (control), filamentous algae on sand and cotton on sand. Two irrigation protocols were followed: continual watering (wet treatment) and weekly watering (dry treatment). Each week during a 21-week period, the number of germinated seeds from each species was recorded. The highest percentage of germination for the four species was observed during wet treatment. C. articulatus had the highest germination rate of the four species in algae (70%), and cotton (51%) on sand. Highest germination for F. simplex, F. cymosa and R. colorata took place in algae, although the percentages were very low. Differences in germination between the three substrates for these species were statistically significant (P<0.05). During dry treatment, C. articulatus was the only species that exhibited a significant germination, principally in algae and cotton. The results suggest that algae maintain substrate humidity and favour germination of the four species. This could be important in sand dune slacks where marked fluctuations in temperature and humidity occur.