Phenotypic plasticity versus ecotypic differentiation under recurrent summer drought in two drought-tolerant pine species

Despite worldwide reports of high tree mortality, growing evidence indicates that many tree species are well adapted to survive repeated dry spells. The drought resilience of trees is related to their phenotypic plasticity and ecotypic differentiation. Whether these two mechanisms act at the same organisational level of a tree and involve similar plant traits is still unknown. We assessed phenotypic plasticity and ecotypic differentiation across four populations of Pinus sylvestris and Pinus nigra seedlings grown for 3 years under a recurrent summer drought treatment or well-watered control conditions in a common garden. We measured the response to the summer drought treatment of a total of 26 traits including shoot and needle morphology, needle anatomy and foliar macronutrients, and related the traits to the growing season water deficit (GSWD) at the location of the seed origin. Foliar phenotypic plasticity in response to recurrent summer drought was surprisingly low, with the needle length and the fraction of mesophyll and phloem tissue adjusting to some extent. In comparison, shoot morphological traits were much more plastic in both species with predominant responses to the summer drought stress including shorter and less numerous apical and lateral shoots. These three traits were also correlated with GSWD at the seed origin, indicating local adaptation. In contrast, between-population variation of foliar morphological and anatomical traits, and macronutrients were mostly unrelated to the GSWD at the seed origin. Consequently, phenotypic plasticity and ecotypic differentiation occurred at the same level of organisation and in the same plant traits, that is, shoot morphology. This combination of plasticity and ecotypic differentiation allowed P. sylvestris and P. nigra seedlings to rapidly acclimate to recurrent and long-lasting dry spells. Synthesis. Pinus sylvestris and P. nigra seedlings showed considerable ecotypic differentiation and phenotypic plasticity of shoot morphological traits, and not foliar traits. Acclimation to recurrent severe summer drought was achieved by reducing shoot growth.

Resistance of plant communities to invasive species

Aurélie Thébault

Invasive species are nowadays considered as one of the most important threat to biodiversity. By displacing native species, modifying ecosystem functioning and causing substantial losses to agricultural production, they represent a menace to natural and managed ecosystems. Although ecology of invasions has become an important research topic since the last decades, the mechanisms that determine why a given species may invade a given ecosystem and why some biomes are less resistant to invasion are still not clarified. Ecology of invasions is divided into 2 main topics: invasiveness and invasibility. While invasiveness refers to species ability to invade a community, invasibility focuses on the resistance of a community to invasion. Invasiveness may be the result of ecological processes, such as release from biotic constraints or human alteration of the environment (disturbance, stress…) or the consequence of evolutionary processes, such as hybridization or polyploidization that may increase genetic variation and therefore, enhance niche breadth. Invasibility has been said to be influenced by disturbance and biotic factors such as community diversity, dominant species identity, biotic interactions or community compositional stability. The invasion success is the consequence of the interaction between species invasiveness and community invasibility. Most studies in ecological invasions have focused on either invasiveness or invasibility, but hardly both together. By working at the same time and in the same conditions with native and introduced genotypes and by comparing their ecological performances, this thesis aims at a better understanding of both invasiveness and invasibility mechanisms. Two worldwide invasive species, Centaurea maculosa and Senecio inaequidens, were used in several experiments (pot, microcosm, field) to disentangle the importance of invasiveness and community invasibility in their invasion success. Both species encountered polyploidisation in their native range, leading to the presence of diploid and tetraploid populations, whereas only tetraploid populations have been found in the introduced range. Using native diploid, native tetraploid and introduced tetraploid genotypes of the two model-species, allows assessing the effects of genetic variation (diploid vs. tetraploid genotypes) and environmental variation (genotype from native vs. introduced range) on species phenotypic traits variations and consequently on fitness variation and invasiveness. In the community context, studying response of different genotypes to experimental factors and community change gives information on the interaction between invasiveness and invasibility. Plants were grown in pot, in field or in artificially built communities where (i) the management treatment, (ii) the community diversity and (iii) the spatial organisation of resident species were manipulated. In addition, (iv) community species composition, (v) community competitive ability and (vi) compositional and functional stability of the community were monitored. According to the experiments, data were gathered on survival, morphological traits (vegetative height, lateral spread, shoot and root biomass), leaf traits (specific leaf area, leaf dry matter content) and reproductive traits (probability of flowering, capitulum production) of the genotypes (native diploid, native tetraploid and introduced tetraploid) of both model species. Through the use of statistical models and multivariate approaches, the effects of management and biotic factors on survival, growth and reproduction of native and introduced genotypes of these two worldwide invasive species were assessed. Invasion strategy of the two model species was investigated through a growth experiment in optimal conditions in a pot experiment. High investment in seed production could explain invasive success of S. inaequidens through high propagule pressure, whereas C. maculosa's strategy seemed to be oriented towards interactions with belowground communities, as shown by the shift in rhizosphere bacterial communities between genotypes. For both species, polyploidisation in the native range could be linked to a specialisation towards higher competitive ability, which could have allowed the first step of invasion. Introduction in the new range could be related to a loss of specialisation through selection of traits allowing coping with various or changing environments, improving successful spread. Survival of both model species was highly affected by community spatial pattern, management and neighbouring competition. Responses of growth and reproductive output of native and introduced genotypes to management, community spatial pattern and community diversity were species-specific. Growth and reproductive output of both genotypes of S. inaequidens were affected by experimental factors whereas introduced genotypes of C. maculosa were less affected than native ones. Comparison of response of native and introduced genotypes to experimental factors allowed defining two strategies of invasion based on phenotypic plasticity. Centaurea maculosa was able to maintain fitness in all kinds of environments, either favourable or stressful ("Jack-of-all-trades" invader). Senecio inaequidens was able to deal with all kinds of environments and was also able to increase its fitness in favourable conditions ("Jack-and-Master" invader). The combination of the effects of management, community spatial pattern and community diversity on species genotypes allowed defining, for both invasive species, two invasion phases which were impacted by different factors. The introduction phase corresponds to the survival of seedlings and their ability to deal with neighbouring competition. If seedlings manage to survive despite neighbouring competition, they grow and reproduced in order to spread, which corresponds to the establishment phase. In terms of management perspectives, regular mowing or use of highly covering species could limit invasive success of S. inaequidens and C. maculosa respectively. The synthesis of all the experiments conducted in this thesis with C. maculosa and S. inaequidens highlights (1) the importance of polyploidisation in the invasion process as well as (2) the species-specific invasion strategies and consequently (3) the species-specific response of invasive species to abiotic and biotic factors. It also emphasizes on (4) the temporal evolution of the interaction between invasiveness and invasibility since the community factors that affected invasive species fitness changed according to the invasion stage (introduction vs. establishment phase) of the invader. The provided insights into the importance of the interaction between species invasiveness and community invasibility will contribute to improve knowledge in ecology of invasions, in addition to supply some clues for management efficiency.

EPFL2009

Drought-induced shifts in plants traits, yields and nutritive value under realistic grazing and mowing managements in a mountain grassland

Alexandre Buttler, Claire Emilie Deléglise, Constant Signarbieux, Thomas Spiegelberger

Extreme drought events can dramatically impact grassland ecosystems, causing potential loss of forage production for livestock in temperate grasslands. However, knowledge of drought effects on forage production, nutritive value and plant community stability in the real context of farming management is scarce. For this purpose, the effect of a simulated summer drought was studied under two realistic management types on a semi-natural grassland in the Swiss Jura mountains. The first management ("grazing") consisted in six consecutive utilizations by animals over the growing season, representing a rotational grazing system as regionally conducted. The second management ("mowing") consisted of three harvests, corresponding to the usual mowing frequency of hay meadows. In both managements, drought caused minor short-term modifications of species composition and almost no persistent effects were observed. Besides, important short-term changes were observed in community weighted mean of several key functional traits, reflecting a strong decline in community growth during the drought followed by a partial recovery two months later. Forage yields, and to a lesser extent its nutritive value, thus declined during the drought period. Both were still affected in the following months, but had recovered in spring of the following year. Forage loss was twofold higher in the grazing management, but recovery as well as nutritive value was slightly improved in this management. The results suggest that rotational grazing can amplify negative drought impacts, compared to traditional mowing, and highlight the need to adapt such management in dry years. They also demonstrate that, even under a fairly intensive management, resilience of such mountain grasslands after one extreme drought event can be expected to be high, with no persistent changes in species and trait compositions. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier2015

Warming may extend tree growing seasons and compensate for reduced carbon uptake during dry periods

Christoph Bachofen, Charlotte Grossiord, Eugénie Isabelle Mas, Yann Vitasse

Warming and drought alter plant phenology, photosynthesis and growth with important consequences for the global carbon cycle and the earth’s climate. Yet, few studies have attempted to tease apart their effects on tree phenology, particularly leaf senescence, and on source and sink activity. 2. We experimentally assessed the single and combined effects of warming and reduced soil moisture on the phenology (leaf-out and senescence date, growing season length) and aboveground sink (height and diameter growth, leaf area and Huber values) and source activity (net photosynthesis, photosynthetic efficiency, chlorophyll concentration and total carbon (C) uptake) of two tree species with distinct strategies to deal with drought: European beech and pubescent oak. 3. Warming advanced leaf-out, irrespective of soil moisture levels, particularly in oak and to a lower extent in beech, leading to a prolonged growing season in oak but not beech. No impacts of warming on senescence timing were found for both species. Reduced moisture had little impact on the phenology of both species. Warming-induced advances in phenology and higher photosynthetic efficiency increased the annual C uptake for oak and compensated for the reduced photosynthetic activity in the presence of reduced moisture. Conversely, for beech, source activity, including yearly C uptake, was lower in all treatments than the control, indicating no compensation of the C budget by phenological shifts. 4. Synthesis. Our results demonstrate that a warming-driven earlier activity and higher photosynthetic efficiency compensates for reduced photosynthesis during hot and dry periods, but only for pubescent oak, which is a rather drought tolerant species. Current predictions of warming-induced mitigation effects through extended C uptake seem incorrect for beech.

2022