Intraspecific variability of leaf form and function across habitat types.

Trait-based ecology has already revealed main independent axes of trait variation defining trait spaces that summarize plant adaptive strategies, but often ignoring intraspecific trait variability (ITV). By using empirical ITV-level data for two independent dimensions of leaf form and function and 167 species across five habitat types (coastal dunes, forests, grasslands, heathlands, wetlands) in the Italian peninsula, we found that ITV: (i) rotated the axes of trait variation that define the trait space; (ii) increased the variance explained by these axes and (iii) affected the functional structure of the target trait space. However, the magnitude of these effects was rather small and depended on the trait and habitat type. Our results reinforce the idea that ITV is context-dependent, calling for careful extrapolations of ITV patterns across traits and spatial scales. Importantly, our study provides a framework that can be used to start integrating ITV into trait space analyses.

Phenotypic dissimilarity index: Correcting for intra- and interindividual variability when quantifying phenotypic variation.

In trait-based ecology, phenotypic variation (PVar) is often quantified with measures expressing average differences between populations standardized in the range 0-1. However, these measures disregard the within-population trait variability. In addition, some of them cannot be partitioned between populations. These aspects can either alter their interpretation or limit their applicability. To overcome these problems, we propose a new measure, the phenotypic dissimilarity (PhD) index, to quantify PVar between populations in scenarios of varying within-population interindividual trait variability. PhD can also quantify within-population PVar while accounting for intraindividual trait variability. Using simulated and real data, we show that using the PhD index becomes important when the within-population trait variability is not negligible, as in all ecological studies. By accounting for within-population trait variability, the PhD index does not overestimate PVar across an environmental gradient compared to other estimators. Traits inherently vary within species. Accounting for such variability is essential to understanding species' phenotypic responses to environmental cues. The proposed PhD index will provide ecologists with a tool for quantifying PVar within species and compare it between species at different levels of biological organization. We provide an R function to calculate the PhD index.

Daily Temperature Effect on Seedling Growth Dynamic of Three Invasive Alien Species.

A greater relative growth rate (RGR) is positively correlated with a species' ability to deploy a larger leaf area either due to a greater total number of leaves (LN) in the canopy or due to an average size of individual leaves (LA). This study aimed to analyze and compare, (1) the temporal (i.e., daily) RGR, leaf production rate (LPR), and leaf area production rate (LAPR) changes during the early growth stages of three among the most invasive species in the world, namely, Ailanthus altissima, Phytolacca americana, and Robinia pseudoacacia. (2) the interspecific differences in the relationship between RGR, LPR, LAPR, and mean daily air temperature. Our results show that growth dynamics as a function of temperature differ between invasive alien species (IAS). While these differences are partly explained by differences due to the growth form of the investigated species, the three IAS have a different behavior to adjust RGR, LPR, and LAPR with air temperature changes even within the same growth form, and in agreement with species habitat requirements in their native range. In conclusion, the results help disentangle the relative role of RGR, LPR, and LAPR in defining non-native species growth responses to mean daily air temperature also in relation to a species' growth form.

Global patterns of biomass allocation in woody species with different tolerances of shade and drought: evidence for multiple strategies.

The optimal partitioning theory predicts that plants of a given species acclimate to different environments by allocating a larger proportion of biomass to the organs acquiring the most limiting resource. Are similar patterns found across species adapted to environments with contrasting levels of abiotic stress? We tested the optimal partitioning theory by analysing how fractional biomass allocation to leaves, stems and roots differed between woody species with different tolerances of shade and drought in plants of different age and size (seedlings to mature trees) using a global dataset including 604 species. No overarching biomass allocation patterns at different tolerance values across species were found. Biomass allocation varied among functional types as a result of phenological (deciduous vs evergreen broad-leaved species) and broad phylogenetical (angiosperms vs gymnosperms) differences. Furthermore, the direction of biomass allocation responses between tolerant and intolerant species was often opposite to that predicted by the optimal partitioning theory. We conclude that plant functional type is the major determinant of biomass allocation in woody species. We propose that interactions between plant functional type, ontogeny and species-specific stress tolerance adaptations allow woody species with different shade and drought tolerances to display multiple biomass partitioning strategies.

The triangular space of abiotic stress tolerance in woody species: a unified trade-off model.

Tolerance of abiotic stress in woody plants is known to be constrained by biological trade-offs between different forms of stress, especially shade and drought. However, there is still considerable uncertainty on the relationship between tolerances and the limits on tolerance combinations. Using the most extensive database available on shade, drought, waterlogging and cold tolerance for 799 northern hemisphere woody species, we determined the number of dimensions needed to summarise their tolerance combinations, and the best trade-off model among those currently available, for description of the interdependence between tolerances. Two principal component analysis (PCA) dimensions summarised stress tolerance combinations. They defined a triangular stress tolerance space (STS). The first STS dimension reflected segregation between drought-tolerant and waterlogging-tolerant species. The second reflected shade tolerance, which is independent of the other tolerances. Cold tolerance scaled weakly with both dimensions. Tolerance combinations across the species in the database were limited by boundary-line trade-offs. The STS reconciles all major theories about trade-offs between abiotic stress tolerances, providing a unified trade-off model and a set of coordinates that can be used to examine how other aspects of plant biology, such as plant functional traits, change within the limits of abiotic stress tolerance.

Inherent variation of functional traits in winter and summer leaves of Mediterranean seasonal dimorphic species: evidence of a 'within leaf cohort' spectrum.

The covariation pattern among leaf functional traits involved in resource acquisition has been successfully provided by the leaf economic spectrum (LES). Nevertheless, some aspects such as how the leaf trait variation sources affect LES predictions are still little investigated. Accordingly, the aim of this paper was to test whether leaf trait variations within different leaf cohorts could alter LES. Improving this knowledge can extend the potential of trait-based approaches in simulating future climate effects on ecosystems. A database on leaf morphological and physiological traits from different leaf cohorts of Cistus spp. was built by collecting data from literature. These species are seasonal dimorphic shrubs with two well-defined leaf cohorts during a year: summer leaves (SL) and winter leaves (WL). Traits included: leaf mass area (LMA), leaf thickness (LT), leaf tissue density (LTD), net photosynthetic rate on area (Aa) and mass (Am) base, nitrogen content on area (Na) and mass (Nm) base. The obtained patterns were analysed by standardized major axis regression and then compared with the global spectrum of evergreens and deciduous species. Climatic variable effect on leaf traits was also tested. Winter leaves and SL showed a great inherent variability for all the considered traits. Nevertheless, some relationships differed in terms of slopes or intercepts between SL and WL and between leaf cohorts and the global spectrum of evergreens and deciduous. Moreover, climatic variables differently affected leaf traits in SL and WL. The results show the existence of a 'within leaf cohort' spectrum, providing the first evidence on the role of leaf cohorts as LES source of variation. In fact, WL showed a high return strategy as they tended to maximize, in a short time, resource acquisition with a lower dry mass investment, while SL were characterized by a low return strategy.

Highlighting the differential role of leaf paraheliotropism in two Mediterranean Cistus species under drought stress and well-watered conditions.

The differential degree by which paraheliotropism may counterbalance the deleterious impact of high irradiance between congeneric species in relation to different water availabilities has been poorly investigated. We followed the evolution of gas exchange, quenching analysis and OJIP parameters in restrained (R) and free (F) to move leaves of Cistus monspeliensis (CM) and Cistus salvifolius (CS) under drought stress (WS) and well-watered conditions (WW). Concerning gas exchange parameters, leaf restriction effect was overall not significant in CM except in apparent carboxylation efficiency (Ce) under WS, while CS showed a significant sensitivity of maximum net photosynthetic rate (Amax), stomatal conductance (gs) and Ce even under WW. The recovery analysis highlighted also a faster gs recovery in F leaves. Furthermore, in both the species, restriction affected photon allocation pathways especially in terms of light-regulated and light-independent constitutive non-photochemical energy dissipation under WW, ultimately affecting electron transport rate (ETR). Nevertheless, the OJIP analysis provided us evidences that CM was characterized by a down-regulation of ETR while an impairment occurs in CS. In CM this was due to its ability to modify a certain fraction of reaction centers thus resulting in a higher capability for dissipation of excess light energy under well-watered conditions, not affecting electron transport efficiency. This response was not observed in CS. Overall, we demonstrated that congeneric species, even mostly sharing the same physiological targets, differ in the degree by which leaf movements help to counterbalance the negative effect of the high irradiance in relation with the amount of water available.

Short-term physiological plasticity: Trade-off between drought and recovery responses in three Mediterranean Cistus species.

Short-term physiological plasticity allows plants to thrive in highly variable environments such as the Mediterranean ecosystems. In such context, plants that maximize physiological performance under favorable conditions, such as Cistus spp., are generally reported to have a great cost in terms of plasticity (i.e., a high short-term physiological plasticity) due to the severe reduction of physiological performance when stress factors occur. However, Cistus spp. also show a noticeable resilience ability in response to stress factors. We hypothesized that in Cistus species the short-term physiological response to stress and that to subsequent recovery can show a positive trade-off to offset the costs of the photosynthetic decline under drought. Gas exchange, chlorophyll fluorescence, and water relations were measured in C. salvifolius, C. monspeliensis, and C. creticus subsp. eriocephalus during an imposed experimental drought and subsequent recovery. Plants were grown outdoor in common garden conditions from seeds of different provenances. The short-term physiological response to stress and that to recovery were quantified via phenotypic plasticity index (PIstress and PIrecovery, respectively). A linear regression analysis was used to identify the hypothesized trade-off PIstress-PIrecovery. Accordingly, we found a positive trade-off between PIstress and PIrecovery, which was consistent across species and provenances. This result contributes in explaining the profit, more than the cost, of a higher physiological plasticity in response to short-term stress imposition for Cistus spp because the costs of a higher PIstress are payed back by an as much higher PIrecovery. The absence of leaf shedding during short-term drought supports this view. The trade-off well described the relative variations of gas exchange and water relation parameters. Moreover, the results were in accordance with the ecology of this species and provide the first evidence of a consistent trade-off between the short-term physiological responses to drought and recovery phases in Mediterranean species.

Cistus creticus subsp. eriocephalus as a Model for Studying Plant Physiological and Metabolic Responses to Environmental Stress Factors.

Variations in physiology and metabolic products of Cistus creticus subsp. eriocephalus along an altitudinal gradient (350-750 m.a.s.l.) within the Monti Lucretili Regional Natural Park (central Italy) were studied. The results showed that the phenol production was in relationship with the net photosynthetic rates and the chlorophyll content. In particular, the increasing caffeic acid (CA) content with altitude suggested its role in providing an additional photo-protection mechanism, by its ability to consume photochemical reducing power and acting as an alternative C-atom sink under high light conditions. The metabolic production was tested by high performance thin layer chromatography (HPTLC) fingerprint analysis, highlighting the potential of this technique in biologic studies.