5 Seed functions in plant communities

5.1 The role of seed functional diversity in buffering plant communities from species colonization

Del Vecchio, S.1, Mattana, E.2, Buffa, G.1

1Ca’ Foscari University of Venice, Italy; 2Royal Botanic Gardens, Kew, UK

Species traits considerably contribute to species assemblage in plant communities. However, species traits may differ between adult and reproductive phase, thus different rules can drive species assemblage during each phase of plant life cycle. We analyzed if the colonization of a foredune plant community depended on the functional diversity of resident species, measured on both seed traits (reproductive phase) and vegetative traits (adult phase). We also accounted for phylogenetic and taxonomic diversity. In a dataset of 84 plots x 19 species, we classified species as “residents” or “colonizers”, according to their fidelity (Phi coefficient) and frequency values for the foredune plant community. We associated to each species seed traits, namely the germination percentage under different temperature and light regimes, seed mass and shape, and vegetative traits, i.e. leaf traits (SLA, LDMC, LA), life form, and clonality. For each plot, we measured the Functional Diversity (FDis index) of residents, on both seed traits (“FDseed”) and vegetative traits (“FDveg”). We also measured phylogenetic (“PD”) and taxonomic diversity (“TD”). Through GLM we investigated if the cover percentage of colonizers per plot depended on FDseed, FDveg, and PD of residents. TD was excluded because it was correlated to FD. The cover percentage of colonizers increased at increasing FDseed, FDveg and PD, although significantly only for FDveg. Foredune plant communities with high FD and PD did not counteract species colonization, suggesting that the functional trait space was not saturated. FD during reproductive phase can have a weaker influence on species colonization than FD of adult species, suggesting that colonizing species may take advantage from entering plant communities during early phases of plant life cycle. We therefore underline the importance to analyze patterns of both seed and vegetative functional diversity in plant communities to thoroughly understand the process of community assembly.

5.2 Easy to grow: assessing the role of regeneration traits in global urban trees selection

Phartyal, S.1, Prouty, H.1, Yadav, H.1, Rosbakh, S.2, Chen, S.C.3, Poschlod, P.2

1Nalanda University, India; 2University of Regensburg, Germany; 3Royal Botanic Gardens, Kew, UK

Urban trees provide important ecosystem services to city dwellers and are considered one of the efficient nature-based solutions for mitigating the adverse impacts of rapid urbanization. The 3-30-300 rule for urban ecosystem suggests everyone should see at least 3 trees from their home, live in a locality with 30% canopy cover, and be within 300m proximity of ample green space. Similarly, to achieve high tree diversity, the 10-20-30 rule advocates that the composition of one species, genus, and family in an urban ecosystem should not exceed 10, 20, & 30%, respectively. However, implementing these thumb rules and using urban trees as a nature-based solution may fail if human’s selection of tree species is based solely on the species’ aesthetic, functional utilities, and ease of establishment. Here, we quantified how regeneration traits influence the selection of urban tree species. Specifically, we asked, are the current global urban tree selection skewed towards easily reproducible species? We compiled data on seed storage behavior, dormancy, and clonality for 4734 species (c. 8% of the globally known tree taxa) listed currently in the Global Urban Tree Inventory. Results indicate that seeds of most (75%) urban tree species have a low probability of recalcitrance, signifying their orthodox storage behavior. Seed dormancy information was available for 12% (593) species, of which 77% produce dormant (mainly physiological) seeds. Clonality data for 11% (547) species indicate that 92% of them can reproduce through vegetative means. Surprisingly, most species with dormancy also had clonality traits. In addition to aesthetics and utilities, the ease of handling species regeneration (through vegetative propagation, desiccation tolerance, or dormancy-alleviation during storage) tends to be another driving force for urban tree selection. It will be worth studying more regeneration traits of the urban trees to know how human choices have shaped community assembly in urban ecosystems.

5.3 Experimental germination responses to different temperature and light regimes and species spatial distributions in a guild of Western Australian annuals

Da Silva, I.A.1, Erickson, T.E.2, Mayfield, M.M.3, Merritt, D.J.2, Dwyer, J.M.1

1University of Queensland, Australia; 2Kings Park Science, Australia; 3University of Melbourne, Australia

Annual species have evolved sets of germination cues that are presumably predictive of the post-germination environment. In naturally patchy environments, germination microsites often vary considerably in the amount of light they receive and in the diurnal temperature fluctuations they experience. It is therefore possible that species’ differential germination responses to temperature and light may be related to their spatial patterns of occurrence, but this remains largely untested. We investigated if species’ germination responses to experimental temperature and light treatments are associated to their occurrences along local gradients of overstory and litter cover in Western Australian winter annual communities. We first surveyed species’ occurrences in 150 quadrats across gradients of overstory and litter cover. Nineteen species recorded in this survey were then included in a germination experiment that manipulated (1) Light vs. Dark (12h light or 24h dark) approximating seeds near the soil surface versus those covered by litter and (2) Cold vs. Warm temperature regimes (7/18ºC and 7/24ºC) approximating diurnal fluctuations experienced in shaded versus sun-exposed microsites, respectively. In binomial models of occurrences in the field, one species was positively, and one was negatively associated with overstory cover. For litter cover three species were positively and 4 negatively associated. In the germination experiment, six species had highest germination probabilities in the Light treatment (regardless of temperature), five in Cold + Light, one in Warm + Light, two were indifferent to the treatments and four did not germinate at all. Overall, species’ experimental germination responses did not correspond with their spatial distributions along overstory and litter cover gradients. Our results suggest that either our experimental treatments did not accurately mimic microsite differences in the field, or that patchy germination is associated with factors other than light and temperature regimes, such as soil moisture, pH and salinity.

5.4 Inferring community assembly processes from functional seed trait variation along a temperature gradient

Rosbakh, S.1, Chalmandrier, L.1,2, Phartyal, S.1,3, Poschlod, P.1

1University of Regensburg, Germany; 2University of Canterbury, New Zealand; 3Nalanda University, India

Assembly of plant communities has long been scrutinized through the lens of trait-based ecology. Studies generally analyze functional traits related to the vegetative growth, survival and resource acquisition and thus ignore how ecological processes may affect plants at other stages of their lifecycle, particularly when seeds disperse, persist in soil and germinate. Here, we analyzed an extensive data set of 16 traits for 167 species measured in-situ in 36 grasslands located along an elevational gradient and compared the impact of abiotic filtering, biotic interactions and dispersal on traits reflecting different trait categories: plant vegetative growth, germination, dispersal, and seed morphology. Abiotic filtering impacted mostly the vegetative traits and to a lesser extent on seed germination and morphological traits. Increasing low-temperature stress towards colder sites selected for short-stature, slow-growing and frost-tolerant species that produce small quantity of smaller seeds with higher degree of dormancy, high temperature requirements for germination and comparatively low germination speed. Biotic interactions also filtered certain functional traits in the study communities. The benign climate in lowlands promoted plant with competitive strategies including fast growth and resource acquisition (vegetative growth traits) and early and fast germination (germination traits), whereas the effects of facilitation on the vegetative and germination traits were cancelled out by the strong abiotic filtering. The changes in the main dispersal vector from zoochory to anemochory along the gradient strongly affected the dispersal and the seed morphological trait structure of the communities. Stronger vertical turbulence and moderate warm-upwinds combined with low grazing intensity selected for light and non-round shaped seeds with lower terminal velocity and endozoochorous potential. We clearly demonstrate that, in addition to vegetation traits, seed traits can substantially contribute to functional structuring of plant communities along environmental gradients. Thus, the “hard” seed traits are critical to detect multiple, complex community assembly rules.

5.5 Seed germination responses to microclimatic conditions in alpine communities

Espinosa del Alba, C.1, Fernández-Pascual, E.1, Jiménez-Alfaro, B.1

1University of Oviedo, Spain

Topographic roughness of alpine landscapes creates a mosaic of microhabitats ranging from open areas subjected to freeze-thaw cycles (fellfields) to long-term snow-covered sites (snowbeds). Although such microclimatic heterogeneity is a key driver of plant distributions in alpine ecosystems, it is largely unknown how the seed regeneration niche of co-occurring species differ in the same community. We conducted a move-along seed germination experiment mimicking fellfield and snowbed environments. Experimental conditions were based on field measurements of soil temperatures and they were set as weekly temperature regimes with monthly photoperiod modifications but without water limitation. In total, we tested the germination response of 54 co-occurring species in two alpine communities, calcareous and siliceous, of the Cantabrian Mountains (Spain). Both study systems showed different dormancy patterns, with 50% and 80% of species germinating before winter in calcareous and siliceous bedrocks, respectively. At the local scale, we found that germination responses varied greatly between microhabitats, with the same species germinating better (higher and faster rates) in fellfield than in snowbed conditions. Nevertheless, we also observed intraspecific variation with germination differences between populations ranging from 0 to 65%, on average they differed more in fellfield (14.78%) than in snowbed (9.85%), calculated from 35 species. These results suggest specific germination adaptations of alpine plants to microhabitat conditions at both regional and local scales. While climate warming is expected to accelerate germination rates and to lengthen the growing season, the shrinkage of snowbeds will likely have negative effects on the species adapted to this microhabitat. The combination of higher temperatures and autumn precipitations could trigger early germination, letting the seedlings vulnerable to winter temperatures. Nevertheless, further studies are needed to test the role of water availability on regulating germination traits in water-limited alpine communities, where low summer precipitation could prevent autumn germination and avoid winter’s adverse conditions.

5.6 Identifying seed functional traits contributing to seed survival and plant regeneration in fire-prone ecosystems

Tangney, R.1, Merritt, D.J.2, Miller, B.P.3, Ooi, M.K.J.3

1University of New South Wales, Australia; 2Kings Park Science, Australia; 3University of Western Australia, Australia

Regeneration from seed is a primary means of plant population persistence for many species in fire-prone environments. For plants to re-establish successfully following fire, their seeds must survive the fire event, be at a depth within the soil profile that allows for their emergence (i.e., not too deep) and be situated in a location that provides suitable access to light, water, and temperature for germination. Over the past seven years, we have been examining how seed traits influence seed survival and seedling emergence behavior in species from across Australia. Here we synthesize the results from our experimental fires in the field, and our controlled glasshouse and laboratory experiments. Seed lethal temperatures are strongly influenced by seed moisture content during exposure, and that seed size influences seed survival and seedling emergence. We have also identified three distinct functional responses based on lethal temperature thresholds and maximum seedling emergence depth that describe the varying sensitivity of species to fire severity. 1) Seeds that can emerge despite high soil temperatures. 2) Seeds that can only emerge because they are able to emerge from deep within the soil. 3) Seeds that have limited emergence depth but also die at low soil temperatures. Our current research is focused on the understanding critical thresholds in seeds that possess physical dormancy (PY). Collating data from 50 species from across Australia we have quantified the response of seeds to exposure to temperatures between 40ºC and 150ºC. From this we can understand the traits that seeds possess to survive through fire, germinate, and emerge, which can provide a deeper understanding of post-fire recovery processes and inform how changing climate and fire regimes may influence post-fire vegetation composition. Future work will examine the biochemical mechanisms that underpin seed survival including small heat-shock proteins and the role of fatty acids in seed coats.

5.7 Cracking the code: seed coat chemistry in physically dormant seeds

McInnes, S.J.1, Tangney, R.1,2,3, Thordarson, P.1, Ooi, M.K.J.1

1University of New South Wales, Australia; 2Kings Park Science, Australia; 3University of Western Australia, Australia

Plant species in fire-prone ecosystems maintain dormant seeds in the soil which germinate in response to fire cues, such as heat shock, thus ensuring seeds are available to germinate post-fire to replace killed individuals. Seeds of physically dormant species in fire-prone ecosystems have varying temperature thresholds to overcome dormancy, ranging from 40ºC to 120ºC. Variation in dormancy temperature thresholds both between and within species is likely to be an important driver of community assemblage, as fire intensity and severity will determine which seeds germinate and thus define the post-fire diversity. However, the mechanism behind this variation is not clear, despite the importance of seed dormancy in timing germination and ensuring seedling recruitment. The seed chemistry of plants in fire-prone regions is an understudied subject that may help in understanding what controls these dormancy mechanisms. This study tested the hypothesis that different lipid compositions in Faboideae seed coats are correlated with dormancy-breaking thresholds through applying analytical chemistry techniques to plant species from fire-prone and fire-free ecosystems. Seed coat lipids were identified through gas chromatography/mass spectrometry, and a positive correlation between lipid melting points and dormancy temperature thresholds was found. Additionally, overall lipid compositions were found to be distinct between species from fire-prone and fire-free habitats, establishing that the chemistry of the seed coat is seemingly under selection pressure in fire-prone systems. These findings contribute to our understanding of what drives the variation in dormancy-breaking thresholds, giving us better insight into how different species regenerate after fire. As future environmental conditions under climate change will result in more intense and frequent fires, understanding the processes driving plant population dynamics in fire-prone regions is imperative.

5.8 Burned fruits and heated seeds: are seeds still able to germinate?

Fidelis, A.1; Martins, R.G.1; Zirondi, H.L.1

1São Paulo State University, Brazil

In several flammable ecosystems, fire is responsible for triggering germination in different species. Smoke can stimulate germination in species with permeable seed coats, while heat shock can break dormancy of hard-coated seeds. However, less is known about what happens to seeds that are inside the fruits during fires: will the fruits be enough to protect seeds from being damaged by the flames? Will the heat stimulate seeds to germinate? Thus, we investigated if seeds inside the fruits during fire can germinate. We collected seeds from species with both permeable and impermeable seeds. Seeds were sampled before and after fire, from four Cerrado species and put them to germinate (27ºC, 12/12/ hs light, 30 days). Germination and viability percentages, and mean germination time was evaluated for all species. Although viability decreased from 98% to 54%, germination percentages were higher in seeds sampled in burned than in unburned fruits (57% and 11%, respectively) for Mimosa leiocephala (physical dormancy). Species with permeable seed coats showed different results: Jacaranda decurrens showed similar germination and viability percentages in both treatments, while Kielmeyera rubriflora showed a decrease in germination and viability for burned fruits. Our results showed that germination of seeds inside fruits during fire events may vary depending on seed coat permeability: species with physically dormant seeds were less damaged by fire, with an increase in germination percentages, while seeds with permeable coats had a decrease in both germination and viability. Thus, fire affect germination not only when seeds are in the seed bank but also before dispersion, showing the complexity of fire-responses to germination of Cerrado species.

5.10 Testa extensions in orchid seeds: mechanisms to attach the bark of trees

Ortúñez, E.1, Gamarra, R.1, Valdelvira, G.1

1Autonomous University of Madrid, Spain

In Orchidaceae, testa extensions have been observed in obligate epiphytes, usually the so-called “twig orchids”. These species are restricted to grow on the ultimate branches of their hosts, characterized by a short diameter (2,5 cm or less), with a limited source of epiphytic nutrients, such as decomposition of bark, debris from the host, and animal excreta. However, other taxa are common on the bark of trunks and thick branches. To-date, the testa extensions have been found in three subtribes within the subfamily Epidendroideae (Oncidiinae, Aeridinae, Angraecinae), the first distantly related to the other two. During our research on seed micromorphology in tropical orchids, we have analyzed the qualitative characters in the seed coat, checking an important variability in the traits, such as the testa extensions. To check the presence of testa extensions in orchid seeds, we obtained samples from mature capsules of specimens housed in the herbaria. An average of 10 seeds was analyzed under scanning electron microscope and 30 under light microscope. The aims of this research are to describe the typology of testa extensions and its relationship with the seed shape, and to check if these extensions are related with the type of branches in which the orchids grow.

5.11 Alien and native seed germination performance might shape the species assembly of temperate grasslands under a global warming scenario

Trotta, G.1,2, Vuerich, M.1, Boscutti, F.1, Manzoni, L.1, Petrussa, E.1

1University of Udine, Italy; 2University of Trieste, Italy

Global warming is rapidly altering our ecosystem in terms of biodiversity, functions and services loss. The rising temperatures have often shown the ability to enhance biological invasion by facilitating spread and growth of alien species in the new lands. The success of plant invasion requires the overcome of multiple biological barriers. Among the crucial life stages, seed germination greatly contributes to the final species assembly of plant community. Several studies have already suggested that alien plant success is likely linked to their high seed germination rates and longevity. We hypothesized these traits to be further enhanced by future temperature rise. Among terrestrial ecosystems, temperate dry grasslands are considered important hotspot of biodiversity, but also among the most prone ecosystems to biological invasions. In these habitats, Asteraceae is one of the most represented family, in terms of abundance and number of species, for both alien and native plants. In this scenario, we designed a germination test experiment at two temperatures (i.e., 22ºC and 28ºC) including common Asteraceae grassland species, considering native and alien species (split into archaeophyte and neophyte). Our aim was to test the relationship between seed germination, temperature, and their interaction with alien status. The test was performed on both separated and mixed pools of species by using a full-factorial orthogonal design. We expect to find a decrease/delay in native species germination compared with alien seeds with the increasing temperature. We also suppose alien species germination to affect the final species assembly of mixed germination plates, by inhibiting local species germination, also in the light of plausible allelopathic interactions. These results will give important information on future management actions aimed at curbing alien plant invasions, by improving our knowledge on seed-bank response and interactions mechanisms of common species occurring in natural disturbed areas or restoration sites.