Our basic understanding of plant litter decomposition informs the assumptions underlying widely a... more Our basic understanding of plant litter decomposition informs the assumptions underlying widely applied soil biogeochemical models, including those embedded in Earth system models. Confidence in projected carbon cycle-climate feedbacks therefore depends on accurate knowledge about the controls regulating the rate at which plant biomass is decomposed into products such as CO2. Here we test underlying assumptions of the dominant conceptual model of litter decomposition. The model posits that a primary control on the rate of decomposition at regional to global scales is climate (temperature and moisture), with the controlling effects of decomposers negligible at such broad spatial scales. Using a regional-scale litter decomposition experiment at six sites spanning from northern Sweden to southern France-and capturing both within and among site variation in putative controls-we find that contrary to predictions from the hierarchical model, decomposer (microbial) biomass strongly regulat...
Charophyte oospore banks have been studied in different habitat types, whereas only one detailed ... more Charophyte oospore banks have been studied in different habitat types, whereas only one detailed study of bulbil banks has been recorded from the literature. The density of oospore banks generally appears to be very high, dominating the propagule bank in many wetlands. Oospore banks are characterised by high spatial variability within and between the sites. Bulbils and oospores are not uniformly distributed with sediment depth, oospores being generally more numerous in the top few centimetres. Animal activities can mix the sediment, bury propagules and/or homogenise their distribution. Burial deeper than 2 cm limits oospore germination, but burial to a depth of 5 mm can improve their germination capacities. Oospore banks are highly variable between years. They are generally perennial, cumulating several generations of oospores, and buffering the risk of reproductive failure with time. The limited germination rates recorded for oospores from the bank could be related to this perennial strategy, whereas up to 100% of freshly harvested oospores may germinate. Germination rates and patterns and the conditions required for germination vary greatly from one study to another as a result of the large range of species and habitats investigated. Certain water birds consume charophytes and could be efficient dispersal agents for oospores. Various abiotic factors can affect the charophyte biomass and therefore the oospore production which then in turn could affect the propagule bank. The oospore bank density appears to influence charophyte abundance in the field, but above a threshold, its abundance is not related to the number of oospores in the soil. Below a threshold of density, charophytes may be unable to establish dense stands of vegetation.
Flooding duration in wet grasslands represents a crucial indicator for agricultural and biodivers... more Flooding duration in wet grasslands represents a crucial indicator for agricultural and biodiversity targets. However, flooding duration in wet grasslands is not an easy variable to observe by itself, all the more so because it is really heterogeneous in space, due to subtle topographic relief. Moreover, actual plant diversity, composition and community structure are the synthetic results of the hydro-meteorological history in the past. Wet grasslands are thus truly ungauged with respect to flooding duration, which has to be simulated in a robust manner both over the spatial territory and back over the past many years. A water-balance modelling with a daily time step, based on available meteorological data and on a precise topographic characterization, was proposed to assess the flooding duration at any location of a studied area situated along the west coast of France and over a period of six years. The simulated (calculated) flooding durations were used in the plant species distribution modelling based on 350 sampled vegetation quadrats. From fieldbased milestone observations of water level, the hydrological modelling was shown to predict well the annual cumulative flooding duration at any spot. A strong and consistent effect of flooding duration was evident for 29 species. Plant diversity appeared to reach a maximum at an intermediate flooding duration. The hydrology-vegetation modelling approach could thus lead to a robust and versatile tool to predict the consequences of changes in flooding regime on vegetation patterns.
Plant community functional structure may drive ecosystem functions in relation with (i) the trait... more Plant community functional structure may drive ecosystem functions in relation with (i) the trait values characterizing dominant species, according to the "biomass ratio hypothesis" proposed by Grime, and (ii) thanks to trait dissimilarity among species, according to the "diversity hypothesis" proposed by Tilman. Both propositions have already yielded support but their relative importance and how they impact biomass production in field situations is still not well known. This study therefore tested (i) whether or not there was a close relationship between biomass production and the community-weighted mean trait values (CWM), as expected from the "biomass ratio hypothesis", and (ii) the impact of the functional diversity (FD Q) on biomass production, which is expected to be positive according to the "diversity-hypothesis". The study considered a range of plant assemblages occurring in a wet grassland and five above-ground and four below-ground plant traits were measured to characterize their functional structure. The effects of species diversity, soil water content, soil nitrogen availability and grazing intensity on biomass production were also determined. We showed that biomass production was not related either to species richness and diversity or to any of the resource and disturbance parameters considered. Conversely, the functional structure was found to explain up to 55% of the variability of the biomass production. The results obtained clearly supported the "biomass-ratio hypothesis". Functional diversity was mainly found to negatively impact biomass production with only poor support to the "diversity hypothesis". We suggest that such a dilution effect of increasing FD Q on community primary production may be typical of fertile habitats. In order to significantly improve our understanding of the relationship between functional diversity and ecosystem processes, further studies should consider plant assemblages that have been shaped over the long term and habitats across a wide range of productivity.
Charophyte oospore banks have been studied in different habitat types, whereas only one detailed ... more Charophyte oospore banks have been studied in different habitat types, whereas only one detailed study of bulbil banks has been recorded from the literature. The density of oospore banks generally appears to be very high, dominating the propagule bank in many wetlands. Oospore banks are characterised by high spatial variability within and between the sites. Bulbils and oospores are not uniformly distributed with sediment depth, oospores being generally more numerous in the top few centimetres. Animal activities can mix the sediment, bury propagules and/or homogenise their distribution. Burial deeper than 2 cm limits oospore germination, but burial to a depth of 5 mm can improve their germination capacities. Oospore banks are highly variable between years. They are generally perennial, cumulating several generations of oospores, and buffering the risk of reproductive failure with time. The limited germination rates recorded for oospores from the bank could be related to this perennial strategy, whereas up to 100% of freshly harvested oospores may germinate. Germination rates and patterns and the conditions required for germination vary greatly from one study to another as a result of the large range of species and habitats investigated. Certain water birds consume charophytes and could be efficient dispersal agents for oospores. Various abiotic factors can affect the charophyte biomass and therefore the oospore production which then in turn could affect the propagule bank. The oospore bank density appears to influence charophyte abundance in the field, but above a threshold, its abundance is not related to the number of oospores in the soil. Below a threshold of density, charophytes may be unable to establish dense stands of vegetation.
The aim of this study was to determine whether the spatial heterogeneity of grassland vegetation ... more The aim of this study was to determine whether the spatial heterogeneity of grassland vegetation structure would lead to spatial heterogeneity in the net nitrogen mineralisation process in the soil and therefore in the quantity of mineral nitrogen available for the plants. The net nitrogen mineralisation in the soil was compared between different vegetation patches generated by grazing, on two different types of plant communities: mesophilous and meso-hygrophilous.
Herbivores influence nutrient cycling either through direct effects (e.g. excreta) or through ind... more Herbivores influence nutrient cycling either through direct effects (e.g. excreta) or through indirect effects such as a modification of plantesoil feedbacks. This work investigated if grazing-enhanced net N mineralization rates were related to (1) enhanced quality of plant litter and/or (2) reduced plant litter inputs. Rates of net N mineralization in soil and both the quantity and quality of litters were characterized in various plant patches occurring within a grazed grassland. Soil incubations were performed in controlled conditions to assess the respective role of litter quantity and quality on N mineralization.
Question: The vegetation in a polder after partial tidal restoration does not resemble the target... more Question: The vegetation in a polder after partial tidal restoration does not resemble the targeted salt-marsh vegetation. Is this difference in vegetation due to lack of dispersal or unsuitable abiotic conditions ? What could be done for a better restoration of the site? Location: Northwestern France. Methods: Seeds were trapped at the single inlet of the polder with a 200-µm mesh net to estimate inputs of seeds from the bay. In parallel, seed dispersal was studied in the polder by placing Astroturf® seed traps on the surface of the sediment at three different elevations in three distinct areas. Abiotic conditions such as flooding frequency, water table level and soil salinity were monitored. Results: All but one species from the adjacent salt marshes were trapped at the inlet. Not all of these species were on the seed traps inside the polder. Seed dispersal was not homogeneous in the polder and seed trap content mostly discriminated in function of their elevation. Salinity and water logging at the bottom of the slope were very high compared to tolerance of most halophytes but decreased rapidly higher up the slope. Conclusions: The development of salt marsh target species is highly restricted by limited hydrochory inside the polder but also by unfavourable soil conditions induced by the actual hydrological regime. Halophytes are excluded at the bottom of the slope by abiotic conditions and out-competed by subhalophytes higher up. In order to restore salt marsh vegetation inside the polder, a larger opening should be induced in order to increase the flooded surface, and diminish water logging and flooding frequencies.
Grazing impacts the structure and functional properties of vegetation through floristic changes (... more Grazing impacts the structure and functional properties of vegetation through floristic changes (i.e., long-term effect) and current defoliation (i.e., short-term effect). The aim of this study was to assess the relative importance of these two grazing effects on productivity (ANPP) and plant quality (C/N ratio) among plant patches submitted to a variety of grazing intensity for several years. Longterm grazing effect was measured by comparing ANPP and C/N ratio among plant patches with contrasting floristic composition. Short-term impact of grazing was measured by comparing ANPP and C/N in plant patches, with and without defoliation. Floristic contrasts led to a lower ANPP in highly grazed patches than in lightly grazed ones. This result may be related to the increasing proportion of grazing-tolerant and grazing-avoiding species with increasing grazing intensity. Vegetation C/N contrasts were recorded among grazed patches but did not linearly relate to grazing intensity. Short-term effect of current-year defoliation on ANPP was limited as vegetation compensated for biomass removal. No evidence for grazing-enhancement of ANPP was found even at moderate grazing intensity. Long-term floristic changes with grazing thus appeared to be the main driving factor of variations in ANPP. In contrast, C/N ratio showed no general and consistent variation along the grazing gradient but varied consistently depending on the community investigated, thus suggesting an effect of the species pool available.
Clonal architecture may enable plants to effectively respond to environmental constraints but its... more Clonal architecture may enable plants to effectively respond to environmental constraints but its role in plant tolerance to defoliation remains poorly documented. In several non-clonal species, modifications of plant architecture have been reported as a mechanism of plant tolerance to defoliation, yet this has been little studied in clonal plants. In a glasshouse experiment, five rhizomatous and five stoloniferous species of grazed pastures were subjected to three frequencies of defoliation in order to test two hypotheses. (1) We expected plant clonal response to defoliation to be either a more compact architecture (low clonal propagation, but high Electronic supplementary material The online version of this article (
Clonal architecture is involved in performance of clonal fragments, as it determines spatial dist... more Clonal architecture is involved in performance of clonal fragments, as it determines spatial distribution of ramets. It is expected to rely on the species-specific expression of several architectural traits (structural blue-print). However, in contrasting environments, realized clonal architectures may differ, due to phenotypic plasticity. In this paper, we compared clonal architectures between two rhizomatous ecologically close Cyperaceae (Carex divisa and Eleocharis palustris) in non-defoliated and defoliated conditions. Two questions were addressed. (1) How much do the structural blue-print and resulting colonization and occupation of space differ between both species? (2) Does the structural blue-print constrain plastic responses of clonal architecture to defoliation? Traits related to performance, spatial pattern, architecture and biomass allocation of clonal fragments were monitored through an original non-destructive mapping method. In nondefoliated conditions, both species showed similar biomass but contrasting architectures and patterns of biomass allocation to rhizomes that resulted in different spatial patterns. The rhizome network of C. divisa, which consisted in only two primary rhizomes but several branches, was involved in resource storage rather than in spatial colonization. Conversely, E. palustris produced on average six primary rhizomes that grew in the whole horizontal plane, maximizing both occupation and colonization of space. These differences in structural constraints coupled with allometric relationships, resulted in differential responses to defoliation. In C. divisa, the costs associated to defoliation caused a decrease in branching, limiting the area occupied and number of ramets produced by clonal fragments, but increasing ramet density. Conversely, the weakly branched rhizome network of E. palustris was not affected by defoliation. Both spatial strategies (consolidation vs. colonization) are likely to provide ecological advantages allowing their coexistence in grazed meadows.
Grazing resistance in plants, which can be defined as the ability to grow and reproduce under gra... more Grazing resistance in plants, which can be defined as the ability to grow and reproduce under grazed conditions, is either associated to defoliation avoidance or tolerance. Clonal traits are often neglected when studying functional responses to grazing, despite frequent occurrence in grassland vegetation. We investigated whether clonal traits and response to defoliation were associated to increased grazing resistance. First, grazing resistance was estimated for eight clonal species using abundance patterns in a long-term field study. We then analysed its correlation with traits in undisturbed conditions and responses to defoliation in a garden experiment. A few traits were correlated to grazing resistance, though only one was a clonal trait (belowground clonal biomass). Grazing resistance was negatively correlated to shoot height and belowground clonal biomass and positively correlated to inflorescence biomass, suggesting that tall rhizomatous species investing little in sexual reproduction were at a disadvantage under grazed conditions. Both shoot height and belowground clonal biomass were negatively affected by defoliation but their decrease was significantly less for species that expressed the greatest grazing resistance in the field. Our findings show that incorporating clonal traits slightly improved predictions about field grazing resistance in the eight investigated species.
The aim of the study is to investigate the relative importance of plant-plant interactions with r... more The aim of the study is to investigate the relative importance of plant-plant interactions with regard to flooding and drought effect on perennial plant performances in wetlands. Flooding is expected to be the major driver and, accordingly, the importance of drought is hardly if ever taken into account. Focusing on five widespread species, the growth, the survival and the competitive ability of plants were monitored on permanent plots spread along two elevation gradients. Flooding duration and drought intensity were found to vary substantially along the~0.5 meter range elevation gradient. Flooding and drought alternate over the hydrological year and the pin-point surveys were thus conducted over the course of one year. The data were modeled taking into account survival, recruitment and competitive growth throughout flooding and drying out periods. Flooding and drought both directly impacted the plant performances and their competitive effect, with the effect of drought being much more general among species and of higher magnitude than flooding. The importance of competition was found to be high for all species, particularly during the drying out period. It varied more along the flooding gradient than along the drought gradient. The higher flooding tolerance shown by the studied species compared to drought may be related to species specific growth timing together with efficient response traits. These results offer new insights into the filters operating over the species pools. This suggests that the drying out period and drought conditions may be even more important for species' relative success and the importance of competition than the flooding pattern. The general applicability of this result, obtained in mild Atlantic climate and fertile wetlands, remains to be studied.
Our basic understanding of plant litter decomposition informs the assumptions underlying widely a... more Our basic understanding of plant litter decomposition informs the assumptions underlying widely applied soil biogeochemical models, including those embedded in Earth system models. Confidence in projected carbon cycle-climate feedbacks therefore depends on accurate knowledge about the controls regulating the rate at which plant biomass is decomposed into products such as CO2. Here we test underlying assumptions of the dominant conceptual model of litter decomposition. The model posits that a primary control on the rate of decomposition at regional to global scales is climate (temperature and moisture), with the controlling effects of decomposers negligible at such broad spatial scales. Using a regional-scale litter decomposition experiment at six sites spanning from northern Sweden to southern France-and capturing both within and among site variation in putative controls-we find that contrary to predictions from the hierarchical model, decomposer (microbial) biomass strongly regulat...
Charophyte oospore banks have been studied in different habitat types, whereas only one detailed ... more Charophyte oospore banks have been studied in different habitat types, whereas only one detailed study of bulbil banks has been recorded from the literature. The density of oospore banks generally appears to be very high, dominating the propagule bank in many wetlands. Oospore banks are characterised by high spatial variability within and between the sites. Bulbils and oospores are not uniformly distributed with sediment depth, oospores being generally more numerous in the top few centimetres. Animal activities can mix the sediment, bury propagules and/or homogenise their distribution. Burial deeper than 2 cm limits oospore germination, but burial to a depth of 5 mm can improve their germination capacities. Oospore banks are highly variable between years. They are generally perennial, cumulating several generations of oospores, and buffering the risk of reproductive failure with time. The limited germination rates recorded for oospores from the bank could be related to this perennial strategy, whereas up to 100% of freshly harvested oospores may germinate. Germination rates and patterns and the conditions required for germination vary greatly from one study to another as a result of the large range of species and habitats investigated. Certain water birds consume charophytes and could be efficient dispersal agents for oospores. Various abiotic factors can affect the charophyte biomass and therefore the oospore production which then in turn could affect the propagule bank. The oospore bank density appears to influence charophyte abundance in the field, but above a threshold, its abundance is not related to the number of oospores in the soil. Below a threshold of density, charophytes may be unable to establish dense stands of vegetation.
Flooding duration in wet grasslands represents a crucial indicator for agricultural and biodivers... more Flooding duration in wet grasslands represents a crucial indicator for agricultural and biodiversity targets. However, flooding duration in wet grasslands is not an easy variable to observe by itself, all the more so because it is really heterogeneous in space, due to subtle topographic relief. Moreover, actual plant diversity, composition and community structure are the synthetic results of the hydro-meteorological history in the past. Wet grasslands are thus truly ungauged with respect to flooding duration, which has to be simulated in a robust manner both over the spatial territory and back over the past many years. A water-balance modelling with a daily time step, based on available meteorological data and on a precise topographic characterization, was proposed to assess the flooding duration at any location of a studied area situated along the west coast of France and over a period of six years. The simulated (calculated) flooding durations were used in the plant species distribution modelling based on 350 sampled vegetation quadrats. From fieldbased milestone observations of water level, the hydrological modelling was shown to predict well the annual cumulative flooding duration at any spot. A strong and consistent effect of flooding duration was evident for 29 species. Plant diversity appeared to reach a maximum at an intermediate flooding duration. The hydrology-vegetation modelling approach could thus lead to a robust and versatile tool to predict the consequences of changes in flooding regime on vegetation patterns.
Plant community functional structure may drive ecosystem functions in relation with (i) the trait... more Plant community functional structure may drive ecosystem functions in relation with (i) the trait values characterizing dominant species, according to the "biomass ratio hypothesis" proposed by Grime, and (ii) thanks to trait dissimilarity among species, according to the "diversity hypothesis" proposed by Tilman. Both propositions have already yielded support but their relative importance and how they impact biomass production in field situations is still not well known. This study therefore tested (i) whether or not there was a close relationship between biomass production and the community-weighted mean trait values (CWM), as expected from the "biomass ratio hypothesis", and (ii) the impact of the functional diversity (FD Q) on biomass production, which is expected to be positive according to the "diversity-hypothesis". The study considered a range of plant assemblages occurring in a wet grassland and five above-ground and four below-ground plant traits were measured to characterize their functional structure. The effects of species diversity, soil water content, soil nitrogen availability and grazing intensity on biomass production were also determined. We showed that biomass production was not related either to species richness and diversity or to any of the resource and disturbance parameters considered. Conversely, the functional structure was found to explain up to 55% of the variability of the biomass production. The results obtained clearly supported the "biomass-ratio hypothesis". Functional diversity was mainly found to negatively impact biomass production with only poor support to the "diversity hypothesis". We suggest that such a dilution effect of increasing FD Q on community primary production may be typical of fertile habitats. In order to significantly improve our understanding of the relationship between functional diversity and ecosystem processes, further studies should consider plant assemblages that have been shaped over the long term and habitats across a wide range of productivity.
Charophyte oospore banks have been studied in different habitat types, whereas only one detailed ... more Charophyte oospore banks have been studied in different habitat types, whereas only one detailed study of bulbil banks has been recorded from the literature. The density of oospore banks generally appears to be very high, dominating the propagule bank in many wetlands. Oospore banks are characterised by high spatial variability within and between the sites. Bulbils and oospores are not uniformly distributed with sediment depth, oospores being generally more numerous in the top few centimetres. Animal activities can mix the sediment, bury propagules and/or homogenise their distribution. Burial deeper than 2 cm limits oospore germination, but burial to a depth of 5 mm can improve their germination capacities. Oospore banks are highly variable between years. They are generally perennial, cumulating several generations of oospores, and buffering the risk of reproductive failure with time. The limited germination rates recorded for oospores from the bank could be related to this perennial strategy, whereas up to 100% of freshly harvested oospores may germinate. Germination rates and patterns and the conditions required for germination vary greatly from one study to another as a result of the large range of species and habitats investigated. Certain water birds consume charophytes and could be efficient dispersal agents for oospores. Various abiotic factors can affect the charophyte biomass and therefore the oospore production which then in turn could affect the propagule bank. The oospore bank density appears to influence charophyte abundance in the field, but above a threshold, its abundance is not related to the number of oospores in the soil. Below a threshold of density, charophytes may be unable to establish dense stands of vegetation.
The aim of this study was to determine whether the spatial heterogeneity of grassland vegetation ... more The aim of this study was to determine whether the spatial heterogeneity of grassland vegetation structure would lead to spatial heterogeneity in the net nitrogen mineralisation process in the soil and therefore in the quantity of mineral nitrogen available for the plants. The net nitrogen mineralisation in the soil was compared between different vegetation patches generated by grazing, on two different types of plant communities: mesophilous and meso-hygrophilous.
Herbivores influence nutrient cycling either through direct effects (e.g. excreta) or through ind... more Herbivores influence nutrient cycling either through direct effects (e.g. excreta) or through indirect effects such as a modification of plantesoil feedbacks. This work investigated if grazing-enhanced net N mineralization rates were related to (1) enhanced quality of plant litter and/or (2) reduced plant litter inputs. Rates of net N mineralization in soil and both the quantity and quality of litters were characterized in various plant patches occurring within a grazed grassland. Soil incubations were performed in controlled conditions to assess the respective role of litter quantity and quality on N mineralization.
Question: The vegetation in a polder after partial tidal restoration does not resemble the target... more Question: The vegetation in a polder after partial tidal restoration does not resemble the targeted salt-marsh vegetation. Is this difference in vegetation due to lack of dispersal or unsuitable abiotic conditions ? What could be done for a better restoration of the site? Location: Northwestern France. Methods: Seeds were trapped at the single inlet of the polder with a 200-µm mesh net to estimate inputs of seeds from the bay. In parallel, seed dispersal was studied in the polder by placing Astroturf® seed traps on the surface of the sediment at three different elevations in three distinct areas. Abiotic conditions such as flooding frequency, water table level and soil salinity were monitored. Results: All but one species from the adjacent salt marshes were trapped at the inlet. Not all of these species were on the seed traps inside the polder. Seed dispersal was not homogeneous in the polder and seed trap content mostly discriminated in function of their elevation. Salinity and water logging at the bottom of the slope were very high compared to tolerance of most halophytes but decreased rapidly higher up the slope. Conclusions: The development of salt marsh target species is highly restricted by limited hydrochory inside the polder but also by unfavourable soil conditions induced by the actual hydrological regime. Halophytes are excluded at the bottom of the slope by abiotic conditions and out-competed by subhalophytes higher up. In order to restore salt marsh vegetation inside the polder, a larger opening should be induced in order to increase the flooded surface, and diminish water logging and flooding frequencies.
Grazing impacts the structure and functional properties of vegetation through floristic changes (... more Grazing impacts the structure and functional properties of vegetation through floristic changes (i.e., long-term effect) and current defoliation (i.e., short-term effect). The aim of this study was to assess the relative importance of these two grazing effects on productivity (ANPP) and plant quality (C/N ratio) among plant patches submitted to a variety of grazing intensity for several years. Longterm grazing effect was measured by comparing ANPP and C/N ratio among plant patches with contrasting floristic composition. Short-term impact of grazing was measured by comparing ANPP and C/N in plant patches, with and without defoliation. Floristic contrasts led to a lower ANPP in highly grazed patches than in lightly grazed ones. This result may be related to the increasing proportion of grazing-tolerant and grazing-avoiding species with increasing grazing intensity. Vegetation C/N contrasts were recorded among grazed patches but did not linearly relate to grazing intensity. Short-term effect of current-year defoliation on ANPP was limited as vegetation compensated for biomass removal. No evidence for grazing-enhancement of ANPP was found even at moderate grazing intensity. Long-term floristic changes with grazing thus appeared to be the main driving factor of variations in ANPP. In contrast, C/N ratio showed no general and consistent variation along the grazing gradient but varied consistently depending on the community investigated, thus suggesting an effect of the species pool available.
Clonal architecture may enable plants to effectively respond to environmental constraints but its... more Clonal architecture may enable plants to effectively respond to environmental constraints but its role in plant tolerance to defoliation remains poorly documented. In several non-clonal species, modifications of plant architecture have been reported as a mechanism of plant tolerance to defoliation, yet this has been little studied in clonal plants. In a glasshouse experiment, five rhizomatous and five stoloniferous species of grazed pastures were subjected to three frequencies of defoliation in order to test two hypotheses. (1) We expected plant clonal response to defoliation to be either a more compact architecture (low clonal propagation, but high Electronic supplementary material The online version of this article (
Clonal architecture is involved in performance of clonal fragments, as it determines spatial dist... more Clonal architecture is involved in performance of clonal fragments, as it determines spatial distribution of ramets. It is expected to rely on the species-specific expression of several architectural traits (structural blue-print). However, in contrasting environments, realized clonal architectures may differ, due to phenotypic plasticity. In this paper, we compared clonal architectures between two rhizomatous ecologically close Cyperaceae (Carex divisa and Eleocharis palustris) in non-defoliated and defoliated conditions. Two questions were addressed. (1) How much do the structural blue-print and resulting colonization and occupation of space differ between both species? (2) Does the structural blue-print constrain plastic responses of clonal architecture to defoliation? Traits related to performance, spatial pattern, architecture and biomass allocation of clonal fragments were monitored through an original non-destructive mapping method. In nondefoliated conditions, both species showed similar biomass but contrasting architectures and patterns of biomass allocation to rhizomes that resulted in different spatial patterns. The rhizome network of C. divisa, which consisted in only two primary rhizomes but several branches, was involved in resource storage rather than in spatial colonization. Conversely, E. palustris produced on average six primary rhizomes that grew in the whole horizontal plane, maximizing both occupation and colonization of space. These differences in structural constraints coupled with allometric relationships, resulted in differential responses to defoliation. In C. divisa, the costs associated to defoliation caused a decrease in branching, limiting the area occupied and number of ramets produced by clonal fragments, but increasing ramet density. Conversely, the weakly branched rhizome network of E. palustris was not affected by defoliation. Both spatial strategies (consolidation vs. colonization) are likely to provide ecological advantages allowing their coexistence in grazed meadows.
Grazing resistance in plants, which can be defined as the ability to grow and reproduce under gra... more Grazing resistance in plants, which can be defined as the ability to grow and reproduce under grazed conditions, is either associated to defoliation avoidance or tolerance. Clonal traits are often neglected when studying functional responses to grazing, despite frequent occurrence in grassland vegetation. We investigated whether clonal traits and response to defoliation were associated to increased grazing resistance. First, grazing resistance was estimated for eight clonal species using abundance patterns in a long-term field study. We then analysed its correlation with traits in undisturbed conditions and responses to defoliation in a garden experiment. A few traits were correlated to grazing resistance, though only one was a clonal trait (belowground clonal biomass). Grazing resistance was negatively correlated to shoot height and belowground clonal biomass and positively correlated to inflorescence biomass, suggesting that tall rhizomatous species investing little in sexual reproduction were at a disadvantage under grazed conditions. Both shoot height and belowground clonal biomass were negatively affected by defoliation but their decrease was significantly less for species that expressed the greatest grazing resistance in the field. Our findings show that incorporating clonal traits slightly improved predictions about field grazing resistance in the eight investigated species.
The aim of the study is to investigate the relative importance of plant-plant interactions with r... more The aim of the study is to investigate the relative importance of plant-plant interactions with regard to flooding and drought effect on perennial plant performances in wetlands. Flooding is expected to be the major driver and, accordingly, the importance of drought is hardly if ever taken into account. Focusing on five widespread species, the growth, the survival and the competitive ability of plants were monitored on permanent plots spread along two elevation gradients. Flooding duration and drought intensity were found to vary substantially along the~0.5 meter range elevation gradient. Flooding and drought alternate over the hydrological year and the pin-point surveys were thus conducted over the course of one year. The data were modeled taking into account survival, recruitment and competitive growth throughout flooding and drying out periods. Flooding and drought both directly impacted the plant performances and their competitive effect, with the effect of drought being much more general among species and of higher magnitude than flooding. The importance of competition was found to be high for all species, particularly during the drying out period. It varied more along the flooding gradient than along the drought gradient. The higher flooding tolerance shown by the studied species compared to drought may be related to species specific growth timing together with efficient response traits. These results offer new insights into the filters operating over the species pools. This suggests that the drying out period and drought conditions may be even more important for species' relative success and the importance of competition than the flooding pattern. The general applicability of this result, obtained in mild Atlantic climate and fertile wetlands, remains to be studied.
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