Ready for change: seed traits contribute to the high adaptability of mudflat species to their unpredictable habitat

Shyam S. Phartyal, Sergey Rosbakh, Christian Ritz & Peter Poschlod

Recently drained fish ponds making a typical muddy habitat for mudflat plants in southern Germany. Photos credit: Peter Poschlod

Do we fully understand what role regeneration traits play in plant community assembly? A species can neither remain a part of an established community for a longer time nor can take part in a newly establishing community without optimizing its regenerative traits to its habitat. The lack of specific adaptations in seed dispersal, persistence, germination and establishment, therefore, can provoke the environment to act as a filter and eliminate species from local species pool.

In our study, we asked whether seeds of freshwater mudflat species have specific adaptions to match with their very specific habitat conditions. Mudflats are typical wetland habitats of recently exposed muddy sediments along the margin of rivers, streams, backwaters, oxbow lakes, and ponds. In temperate regions, these moist and nutrient-rich habitats are often flooded during winter and dry during summer supporting quick growth of annual plants. Freshwater mudflats are temporal, unpredictable and unstable in nature, often exist only for several weeks but in some extreme cases remain flooded for decades depending upon hydroperiod gradient.

To answer our question, we collected seeds of 30 central European mudflat plants and investigated functional traits of dispersal, persistence, and germination and establishment – three crucial functions for early life-stage. Dispersal traits (seed production or seed buoyancy) determine how many and how far seeds can spread, and persistence traits (desiccation tolerance, seed mass or ability to remain viable in soil) determine how seeds interact and survive temporally unfavorable environments. Germination and establishment traits (seed germination timing, speed or niche width) determine when environmental (temperature, light, aerobic/hypoxic) conditions become most favorable to trigger seed germination. Overall, 15 seed traits were monitored and subjected to cluster and phylogenetic component analysis, to define potential mudflat plant functional groups (if any) based on variation in seed ecological traits.

The lead author, Shyam S. Phartyal, collecting seeds of one of the study species (Cyperus glomeratus) from a comparatively dry fish pond in Croatia. Photo credit: Peter Poschlod

Due to the specific nature of mudflats, we expected that mudflat plants possess a number of specific adaptations: (i) produce high numbers of buoyant seeds to increase probability of long-distance dispersal by water, (ii) produce small-sized, desiccation-tolerant seeds with high persistence potential in soil, and (iii) produce fast germinating seeds with a specific and narrow germination niche.

Seed buoyancy and germination experiments. Photo credit: Shyam Phartyal

We detected a high variation in seed production and seed mass, whereas variation in seed buoyancy was low. All mudflat plants produced desiccation-tolerant seed with long-term persistent seed banks with a comparatively narrow germination niche. Being a complex group, some of mudflat plants grow well in comparatively dryer end of the gradient whereas others towards the wetter end. Thus, the specific micro-environments at two contrasting ends might act differently to different species and govern their adaptability as per variation in seed trait functions at the sub-community level. Our results support this argument as it grouped species into three functional types: (i) true mudflat species (producing many small-sized, high buoyant seeds with narrow germination niche – typically border by terrestrial and shallow-water communities at two contrasting ends), (ii) facultative mudflat species (producing many, large-sized, moderately buoyant seeds with broader germination niche – occurring towards terrestrial end), and (iii) flood-resistant mudflat species (producing few, moderate-sized, average buoyant seeds with broader germination niche – occurring towards the shallow-water end).

Our study highlights the importance of environmental filters for local species pool and also highlights how plant community assembly depends on trait-values of participating species.  

This is a plain-language summary for the paper of Phartyal et al. published in the Journal of Vegetation Science