Seed germination traits shape community assembly along a hydroperiod gradient

Abstract

Background and Aims Hydroperiod drives plant community composition in wetlands, resulting in distinct zonation patterns. Here, we explored the role of seed germination traits in shaping wetland community assembly along a hydroperiod gradient. Specifically, we tested the hypothesis that seeds of reed, mudflat, swamp, shallow- and deep-water communities only germinate under a specific set of environmental factors characterized by the community-specific optimal conditions for seedling survival and growth. Methods In a three-factorial experiment, we tested the seed germination response of 50 species typical for temperate wetlands of Europe to temperature fluctuations (constant vs. fluctuating temperature), illumination (light vs. darkness) and oxygen availability (aerobic vs. hypoxia). Phylogenetic principal component analysis, cluster analysis and phylogenetic linear regressions were used to confirm the community-specific seed germination niches. Key Results Our study revealed the presence of five distinct, community-specific seed germination niches that reflect adaptations made by the study communities to decreasing light intensity, temperature fluctuations and oxygen availability along the hydroperiod gradient. Light as a germination trigger was found to be important in mudflats, swamps and shallow water, whereas the seeds of reed and deep-water species were able to germinate in darkness. A fluctuating temperature is only required for seed germination in mudflat species. Germination of species in the communities at the higher end of the hydroperiod gradient (reed and mudflat) demonstrated a strict requirement for oxygen, whereas swamp, shallow- and deep-water species also germinated under hypoxia. Conclusions Our study supports the recent argument that the inclusion of seed germination traits in community ecology adds significant insights to community response to the abiotic and biotic environment. Furthermore, the close relationship between seed germination adaptations and community assembly could help reach a better understanding of the existing patterns of wetland plant distribution at local scales and wetland vegetation dynamics, as well as facilitate nature conservation measures and aquatic habitat restoration.

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

“…Thus, these “true mudflat species” border typical terrestrial communities at one end and swamp or shallow water communities at the other end, resulting in two additional clusters, i.e., “facultative mudflat species” and “flood‐resistant mudflat species.” The comparatively small group of “facultative mudflat species,” such as Persicaria lapathifolia or Pulicaria vulgaris, are terrestrial species (often ruderal) that demonstrate a broader germination niche width ( J ‐value 0.68), a comparatively low seed buoyancy of 31 days and produce large seeds (1.0 mg) in high number (3,262 seed/ramet), while the “flood‐resistant mudflat species” ( Elatine hydropiper, Eleocharis ovata, Schoenoplectus supinus ) occurring towards the shallow water ends also demonstrate a broader germination niche width ( J ‐value 0.55), have a moderate seed mass (0.18 mg) and seed buoyancy (37 days) but low seed production (1,546 seed/ramet) compared to the other two groups (Table ). The (dis)similarity to some extent in seed traits, especially dispersal and germination and establishment traits, might play a deceptive role in ecological optima and in the fine distinction among “true,” “facultative” and “flood‐resistant” mudflat species according to the variation in hydroperiod gradient at the micro‐scale level, which is the most fundamental driver governing wetland plant community structure and composition (Keddy, ; Rosbakh, Phartyal, & Poschlod, ). Nevertheless, despite the fact that these communities may coexist in various ways, they retain their distinct zonation pattern along a small hydroperiod gradient and can also occur independently elsewhere (Sculthorpe, 1967; Valdez, Hartig, Fennel & Poschlod, 2019).…”

“…Consequently, the inclusion of seed traits can provide significant information about how communities respond to the abiotic and biotic environment (Rosbakh & Poschlod, ; Tudela‐Isanta et al, ). For instance, in a species pool of a wetland ecosystem with a fluctuating water table, if a species lacks the ability to germinate under submerged water (at constant temperature under low light and hypoxia conditions) and requires the water to recede to germinate (at fluctuating temperature under illuminated and aerobic conditions), then it will be filtered out from the local community assemblage (Keddy, ; Rosbakh, Phartyal, & Poschlod, ; van der Valk, ). Further, a species in a mudflat community will not survive long‐term if it does not have the ability to build a persistent seed bank (Poschlod & Rosbakh, ).…”

Need to Seed? Ecological, Genetic, and Evolutionary Keys to Seed-Based Wetland Restoration

“…Hydrology is a defining feature of wetland ecosystems and has an overriding influence on the germination and establishment of native plants, as well as the structure and function of wetlands (Weiher and Keddy, 1995;Cronk and Fennessy, 2001;Doherty et al, 2014;Moor et al, 2017;Daniel et al, 2019;Rosbakh et al, 2020). The hydrology of many wetland systems has been altered via methods such as tile drainage, disconnection of floodplain wetlands through channelization, fragmentation, and increased demands upstream for agriculture and urbanization that reduce inputs to wetland systems (Galatowitsch and van der Valk, 1994;Turner and Lewis, 1996;López-Merino et al, 2011;Downard et al, 2014;Donnelly et al, 2020).…”

“…Hydrologic factors to consider prior to seeding are frequency of flooding, duration of flooding, timing and seasonality of flooding, and depth of flooding (Casanova and Brock, 2000;Perillo, 2009;Webb et al, 2012;Mitsch and Gosselink, 2015). Many wetland seedlings are particularly vulnerable to standing water and the associated declining light levels with water depths and the low (or no) oxygen conditions that can occur (i.e., hypoxia or anoxia; Fraser and Karnezis, 2005;Mitsch and Gosselink, 2015;Rosbakh et al, 2020). Increasing depth, duration, and frequency of inundation generally results in decreased establishment, growth, and plant community diversity (Casanova and Brock, 2000;Webb et al, 2012;Shoemaker and Ervin, 2020).…”

“…Increasing depth, duration, and frequency of inundation generally results in decreased establishment, growth, and plant community diversity (Casanova and Brock, 2000;Webb et al, 2012;Shoemaker and Ervin, 2020). The ideal hydrologic conditions for target native species should be maintained at the site during and at least 1 year after restoration to minimize mortality (Budelsky and Galatowitsch, 2000;Rosbakh et al, 2020).…”