Annabel J. Porté

Xylem embolism threshold for catastrophic hydraulic failure in angiosperm trees.

Hydraulic failure is one of the main causes of tree mortality in conditions of severe drought. Resistance to cavitation is known to be strongly related to drought tolerance and species survival in conifers, but the threshold of water-stress-induced embolism leading to catastrophic xylem dysfunction in angiosperms has been little studied. We investigated the link between drought tolerance, survival and xylem cavitation resistance in five angiosperm tree species known to have contrasting desiccation resistance thresholds. We exposed seedlings in a greenhouse to severe drought to generate extreme water stress. We monitored leaf water potential, total plant water loss rate, leaf transpiration, stomatal conductance and CO2 assimilation rate during drought exposure and after rewatering (recovery phase). The time required for the recovery of 50% of the maximum value of a given ecophysiological variable after rewatering was used to determine the critical water potential corresponding to the threshold beyond which the plant failed to recover. We also investigated the relationship between this potential and stem xylem cavitation resistance, as assessed from vulnerability curves. This minimum recoverable water potential was consistent between ecophysiological variables and varied considerably between species, from -3.4 to -6.0 MPa. This minimum recoverable water potential was strongly correlated with P50 and P88, the pressures inducing 50 and 88% losses of stem hydraulic conductance, respectively. Moreover, the embolism threshold leading to irreversible drought damage was found to be close to 88%, rather than the 50% previously reported for conifers. Hydraulic failure leading to irreversible drought-induced global dysfunction in angiosperm tree species occurred at a very high level of xylem embolism, possibly reflecting the physiological characteristics of their stem water-transport system.

Allometric relationships for branch and tree woody biomass of Maritime pine (Pinus pinaster Aı̈t.)

Modelling biomass repartition in a tree is either done using theories regarding carbon transfer and allocation or through empirical repartition coefficients. The latter can be derived from the study of the allometric relationships inside a tree, which reflect the equilibrium between tree structure and biomass. In order to quantify the biomasses of the main aerial compartments (needles, stem wood, stem bark, branch wood and buds) of a Maritime pine tree (Pinus pinaster Ait.) and to assess their relationships with tree structure, we undertook some destructive measurements of architecture and biomass. The study of leaf area was presented in a specific paper [Porte et al., Ann. For. Sci. 57 (1) (2000) 73], and the present paper is dealing with the woody compartments (branch wood, stem bark and wood). We collected biomass samples on thirty 5-year-old, sixteen 26-year-old and ten 32-year-old Maritime pines. Allometric equations were developed per site to estimate branch wood biomass. It depended only on the branch basal diameter and the models were very satisfying. Using these equations, we estimated the total branch wood biomass of each sampled tree. A single relationship for all sites was found to model crown or trunk biomass. A power function of tree diameter at breast height (DBH) and the inverse of tree age was fitted to the branch wood data. A power function of DBH and tree age was used for the stem wood and bark models, which takes into account the differences in vitality with different ages. All models performed quite well. Input variables were easy to measure so that the models could be applied to estimate the aerial biomass of a whole stand, per compartment, over a 20-year-long period. The allometric relationships presented here can be derived to be used as biomass repartition laws, for a 5–30-year-old Maritime pine stand in humid Lande.

Statistical mechanics unifies different ecological patterns.

Recently there has been growing interest in the use of maximum relative entropy (MaxREnt) as a tool for statistical inference in ecology. In contrast, here we propose MaxREnt as a tool for applying statistical mechanics to ecology. We use MaxREnt to explain and predict species abundance patterns in ecological communities in terms of the most probable behaviour under given environmental constraints, in the same way that statistical mechanics explains and predicts the behaviour of thermodynamic systems. We show that MaxREnt unifies a number of different ecological patterns: (i) at relatively local scales a unimodal biodiversity-productivity relationship is predicted in good agreement with published data on grassland communities, (ii) the predicted relative frequency of rare vs. abundant species is very similar to the empirical lognormal distribution, (iii) both neutral and non-neutral species abundance patterns are explained, (iv) on larger scales a monotonic biodiversity-productivity relationship is predicted in agreement with the species-energy law, (v) energetic equivalence and power law self-thinning behaviour are predicted in resource-rich communities. We identify mathematical similarities between these ecological patterns and the behaviour of thermodynamic systems, and conclude that the explanation of ecological patterns is not unique to ecology but rather reflects the generic statistical behaviour of complex systems with many degrees of freedom under very general types of environmental constraints.

Conflicting values: ecosystem services and invasive tree management

Abstract Tree species have been planted widely beyond their native ranges to provide or enhance ecosystem services such as timber and fibre production, erosion control, and aesthetic or amenity benefits. At the same time, non-native tree species can have strongly negative impacts on ecosystem services when they naturalize and subsequently become invasive and disrupt or transform communities and ecosystems. The dichotomy between positive and negative effects on ecosystem services has led to significant conflicts over the removal of non-native invasive tree species worldwide. These conflicts are often viewed in only a local context but we suggest that a global synthesis sheds important light on the dimensions of the phenomenon. We collated examples of conflict surrounding the control or management of tree invasions where conflict has caused delay, increased cost, or cessation of projects aimed at invasive tree removal. We found that conflicts span a diverse range of taxa, systems and countries, and that most conflicts emerge around three areas: urban and near-urban trees; trees that provide direct economic benefits; and invasive trees that are used by native species for habitat or food. We suggest that such conflict should be seen as a normal occurrence in invasive tree removal. Assessing both positive and negative effects of invasive species on multiple ecosystem services may provide a useful framework for the resolution of conflicts.

Improving models of forest nutrient export with equations that predict the nutrient concentration of tree compartments

Abstract• The objective of this study was to explore the distribution of major nutrients (N, P, K, Ca and Mg) in the aboveground compartments of an intensively managed tree species (Pinus pinaster Ait.). A total of 53 trees were cut down in even-aged stands respectively 8, 16, 26, 32 and 40 years old. The nutrient concentrations of the aboveground compartments were analysed.• Nutrient concentrations of foliage did not vary with any of the variables used, except needle age. Nutrient concentrations of living branches, stem bark, stem sapwood, stem heartwood, stemwood and stem decreased with increasing branch diameter, bark thickness, sapwood thickness and heartwood thickness, respectively. Beyond a certain value of the predictive variable (stem diameter ≈ 15 cm; branch diameter ≈ 2.5 cm), the concentration of all the nutrients stabilised.• A 50 year-old pine stand was used to obtain a validation dataset for nitrogen concentration. For this nutrient, the regression relationships gave satisfactory estimates for most compartments (mean error = 12–25%) and particularly for the stem.• A procedure is proposed to estimate the nutrient exports associated with harvests of Pinus pinaster biomass.Résumé• L’objectif de cette étude est d’explorer la distribution des éléments majeurs (N, P, K, Ca, Mg) dans les compartiments aériens d’une essence gérée de manière intensive. Au total, 53 pins maritimes (Pinus pinaster Ait.) ont été abattus parmi des peuplements équiennes de 8, 16, 26, 32 et 40 ans.• Les concentrations en nutriments du feuillage ne varient pas pour une classe d’âge d’aiguilles donnée. Les concentrations des branches vivantes, de l’écorce, de l’aubier et du duramen décroissent lorsque le diamètre ou l’épaisseur du compartiment considéré augmente. La concentration de l’ensemble des nutriments devient constante lorsque la variable prédictive (diamètre ou épaisseur) atteint une valeur plateau.• Un jeu de données de validation pour les concentrations en azote, provenant d’un peuplement équienne de pins de 50 ans, permet de confirmer les performances satisfaisantes des modèles construits (erreur moyenne = 12–25 %) et en particulier pour le tronc.• Une procédure d’estimation des exportations de nutriments associées aux récoltes de biomasse de Pinus pinaster est présentée.

Field Evidence of Colonisation by Holm Oak, at the Northern Margin of Its Distribution Range, during the Anthropocene Period

A major unknown in the context of current climate change is the extent to which populations of slowly migrating species, such as trees, will track shifting climates. Niche modelling generally predicts substantial northward shifts of suitable habitats. There is therefore an urgent need for field-based forest observations to corroborate these extensive model simulations. We used forest inventory data providing presence/absence information from just over a century (1880–2010) for a Mediterranean species (Quercus ilex) in forests located at the northern edge of its distribution. The main goals of the study were (i) to investigate whether this species has actually spread into new areas during the Anthropocene period and (ii) to provide a direct estimation of tree migration rate. We show that Q. ilex has colonised substantial new areas over the last century. However, the maximum rate of colonisation by this species (22 to 57 m/year) was much slower than predicted by the models and necessary to follow changes in habitat suitability since 1880. Our results suggest that the rates of tree dispersion and establishment may also be too low to track shifts in bioclimatic envelopes in the future. The inclusion of contemporary, rather than historical, migration rates into models should improve our understanding of the response of species to climate change.

Adaptive evolution and phenotypic plasticity during naturalization and spread of invasive species: implications for tree invasion biology.

Although the genetic aspects of biological invasions are receiving more attention in the scientific literature, analyses of phenotypic plasticity and genotype-by-environment interactions are still seldom considered in tree invasion biology. Previous studies have shown that invasions of tree species can be affected by intraspecific phenotypic plasticity, pre-adaptation, and post-introduction evolution, and we suggest there are opportunities for new developments in this field. Here, we present a description of the use of quantitative and molecular genetics in tree invasion biology, and propose an approach based on common garden experiments, quantitative and molecular genetic methods to investigate the role of adaptive evolution, phenotypic plasticity, and genotype-by-environment interactions in tree invasions, particularly at the infraspecific level. We illustrate the utility of this approach using examples from quantitative genetic studies of Pinus and an example from a classical reciprocal common garden experiment with Acer species. By using this approach, researchers can test hypotheses about the role and strength of genetic and environmental effects on population-level invasiveness and gain insights into evolutionary processes that occur during biological invasions. Moreover, knowledge of phenotypic plasticity and local adaption of tree populations may help researchers improve assessments of invasion risk.

Using cover measurements to estimate aboveground understorey biomass in Maritime pine stands.

Abstract• Understorey plays a major role in forest fluxes and stocks balances, however this compartment is generally poorly quantified. Our objectives were to establish models to estimate understorey biomass using vegetation cover measurements and to investigate upscaling methodologies from stand to regional level.• Understorey aboveground biomass measurements were undertaken in Maritime pine stands of mesohygric, mesic and dry moorlands in South West France.• Average biomass stock in this compartment was estimated to 3.50 t DM ha−1. The more abundant species groups varied with moorland types, with a higher relative contribution of herbaceous species (23.3%), bracken (59.2%) and mosses (31.6%) for mesohygric, mesic and dry moorlands, respectively. For each species group, we established significant relationships to estimate biomass using a volumetric index, based on cover and height measurements. No relationship between stand characteristics and understorey biomass was founded. We investigated the upscaling of these estimations to a several thousands hectare area using understorey cover measurements done along a regular spatial grid. The only significant correlation linked one satellite vegetation index to understorey biomass.• We successfully developed empirical relationships to estimate the understorey biomass at the stand level. Further investigations could focus on the analysis of understorey variability over a finer space grid and the potential use of satellite vegetation indexes.Résumé• Le sous-bois est un compartiment non négligeable dans les études de stocks et de flux des forêts; cependant il est encore mal quantifié. Les objectifs de notre étude étaient d’établir des relations permettant d’estimer la biomasse du sous-bois de peuplements et d’analyser les possibilités d’extrapolation à l’échelle du massif.• Des mesures de biomasse aérienne de sous-bois ont été réalisées sur une série de peuplements de pin maritime en Landes mésohygrophile, mésophile et sèche dans le Sud-Ouest de la France.• Nous avons estimé le stock moyen de biomasse dans ce compartiment à 3.50 t MS ha−1. Le groupe d’espèces le plus abondant diffère selon le type de landes : herbacées (23.3 %), fougères (59.2 %) et mousses (31.6 %) en landes mésohygrophile, mésophile et sèche, respectivement. Pour chaque groupe d’espèces, des relations significatives ont été mises en évidence entre un indice volumique et la biomasse de sous-bois. Aucune relation n’a été mise en évidence entre les caractéristiques du peuplement et la biomasse du sous-bois. Nous avons envisagé le calcul à l’échelle d’une zone atelier de plusieurs milliers d’hectares en utilisant une grille spatialisée de relevés de recouvrements. Seul un indice satellite de végétation a présenté une corrélation positive avec la biomasse du sous-bois.• Les relations que nous avons développées permettent d’estimer la biomasse du sous-bois à l’échelle du peuplement. L’analyse spatiale à une échelle plus fine et l’utilisation d’un indice de végétation pourraient être des pistes à explorer.

Invasive Acer negundo outperforms native species in non-limiting resource environments due to its higher phenotypic plasticity.

BackgroundTo identify the determinants of invasiveness, comparisons of traits of invasive and native species are commonly performed. Invasiveness is generally linked to higher values of reproductive, physiological and growth-related traits of the invasives relative to the natives in the introduced range. Phenotypic plasticity of these traits has also been cited to increase the success of invasive species but has been little studied in invasive tree species. In a greenhouse experiment, we compared ecophysiological traits between an invasive species to Europe, Acer negundo, and early- and late-successional co-occurring native species, under different light, nutrient availability and disturbance regimes. We also compared species of the same species groups insitu, in riparian forests.ResultsUnder non-limiting resources, A. negundo seedlings showed higher growth rates than the native species. However, A. negundo displayed equivalent or lower photosynthetic capacities and nitrogen content per unit leaf area compared to the native species; these findings were observed both on the seedlings in the greenhouse experiment and on adult trees in situ. These physiological traits were mostly conservative along the different light, nutrient and disturbance environments. Overall, under non-limiting light and nutrient conditions, specific leaf area and total leaf area of A. negundo were substantially larger. The invasive species presented a higher plasticity in allocation to foliage and therefore in growth with increasing nutrient and light availability relative to the native species.ConclusionsThe higher level of plasticity of the invasive species in foliage allocation in response to light and nutrient availability induced a better growth in non-limiting resource environments. These results give us more elements on the invasiveness of A. negundo and suggest that such behaviour could explain the ability of A. negundo to outperform native tree species, contributes to its spread in European resource-rich riparian forests and impedes its establishment under closed-canopy hardwood forests.

A Test for Pre-Adapted Phenotypic Plasticity in the Invasive Tree Acer negundo L

Phenotypic plasticity is a key mechanism associated with the spread of exotic plants and previous studies have found that invasive species are generally more plastic than co-occurring species. Comparatively, the evolution of phenotypic plasticity in plant invasion has received less attention, and in particular, the genetic basis of plasticity is largely unexamined. Native from North America, Acer negundo L. is aggressively impacting the riparian forests of southern and eastern Europe thanks to higher plasticity relative to co-occurring native species. We therefore tested here whether invasive populations have evolved increased plasticity since introduction. The performance of 1152 seedlings from 8 native and 8 invasive populations was compared in response to nutrient availability. Irrespective of nutrients, invasive populations had higher growth and greater allocation to above-ground biomass relative to their native conspecifics. More importantly, invasive genotypes did not show increased plasticity in any of the 20 traits examined. This result suggests that the high magnitude of plasticity to nutrient variation of invasive seedlings might be pre-adapted in the native range. Invasiveness of A. negundo could be explained by higher mean values of traits due to genetic differentiation rather than by evolution of increased plasticity.