Sonia Mediavilla

Relative growth rate of leaf biomass and leaf nitrogen content in several mediterranean woody species

In many plant species, herbivory is a major determinant of leaf mortality and it can cause a strong reduction in productive potential. Most predation occurs on young, expanding leaves. Thus, a rapid growth of the leaves can reduce the impact of predation. Furthermore, in cold Mediterranean climates the length of the growing season is constrained to a short period in spring and early summer owing both to low winter temperatures and drought stress in early summer. Therefore, a rapid deployment of leaf area and a high photosynthetic capacity during the spring and early summer might have important positive effects on the final carbon balance of the leaf population. Relative growth rates (RGR) of leaf biomass were measured in 19 woody species typical of Central Western Spain with deciduous and evergreen habits. Highly significant differences were detected in the leaf growth rate of the different species. The differences between species, however, did not correlate either with the mean leaf life-span of each of the species or with other leaf traits such as photosynthetic capacity, specific leaf area or nitrogen content. Leaf growth rate was positively correlated with time elapsed between leaf initiation and fruit maturation, so that species with fruit dispersal in spring and early summer in general had lower leaf growth rates than species with autumn fruit shedding. This relationship shows the effects of the concurrence between vegetative and reproductive organs for nutrients and other resources. Nitrogen concentration in the leaves was very high at the time of bud break, and declined during leaf expansion owing to the dilution associated with the increase in structural components. The rate of nitrogen dilution was, thus, positively related to the leaf growth rate. Relative growth rates calculated for nitrogen mass in leaves were very low compared to the growth in total mass. This suggests that most leaf nitrogen is translocated from the plant stores to the leaf biomass before the start of leaf expansion and that the contribution of root uptake during leaf expansion is comparatively low.

Ontogenetic changes in leaf phenology of two co-occurring Mediterranean oaks differing in leaf life span

Large differences in leaf physiology and morphology between ontogenetic stages of a single woody species have often been observed. Far less attention, however, has been devoted to studying the ontogenetic changes observed in leaf phenology patterns, despite the relevance of leaf phenology in determining the leaf carbon balance and leaf and plant mortality. Leaf emergence patterns and leaf longevity were studied in the saplings and mature trees of the evergreen Quercus ilex and the deciduous Quercus faginea. Our aim here was to analyze and interpret the possible tree-age related differences in these leaf traits. Unlike the adults, in which only one flush of leaf growth was observed, several leaf cohorts were produced within each year in the saplings. Sapling leaves showed a lower mean duration than those of the adults. However, Q. faginea saplings exhibited large plasticity in leaf longevity, which was not seen in the case of Q. ilex. The differences in leaf emergence patterns and in leaf longevity between growth stages seemed to be related to differences in resource availability for leaf production and in leaf mass per unit area, respectively. We propose that the sequential leaf development in saplings may be an important mechanism enabling tree species to cope with resource limitation in the early stages of life.

Leaf longevity and drought: avoidance of the costs and risks of early leaf abscission as inferred from the leaf carbon isotopic composition

Plant species with longer leaf longevity tend to maintain lower photosynthetic rates. Among other factors, differences in stomatal limitation have been proposed to explain the negative effects of leaf longevity on photosynthesis, although it is not yet clear why stomatal limitations should be stronger in species with longer leaf longevity. We measured carbon isotopic composition (δ13C) in the fresh leaf litter of several Mediterranean woody species to estimate the mean stomatal limitations during the photosynthetically active part of the leaf life. Interspecific differences in δ13C were best explained by a multiple regression including, as independent variables, the maximum leaf longevity and the annual water deficit. For a similar level of water availability, stomatal limitations were higher in species with longer leaf longevity. We hypothesise that stronger stomatal control of transpiration in longer-living leaves arose as a mechanism to reduce the risk of leaf desiccation and to avoid the high costs for the future C assimilation of anticipated leaf mortality in species with a long leaf life expectancy. This stronger sensitivity to drought should be added to the suite of traits accompanying long leaf longevity and contributes decisively to the overall limitations to C assimilation in long-lived leaves.

Estimation of chlorophyll in Quercus leaves using a portable chlorophyll meter: effects of species and leaf age

Abstract• The potential of a chlorophyll meter (CCM-200, Opti-Sciences, Tyngsboro, MA, USA) for the estimation of total leaf chlorophyll contents was evaluated in leaves of different developmental stages for Quercus pyrenaica Willd., Q. faginea Lam., Q. suber L. and Q. ilex L. subsp. ballota (Desf.) Samp.• For all species and leaf stages, a strong non-linear relationship (P < 0.0001) was found between the chlorophyll content index (CCI) and leaf chlorophyll content per unit area. In all species, the slope of the CCI-chlorophyll content relationship varied during leaf development, suggesting that caution should be exercised when using the CCM-200, since the interpretation of CCI readings should be limited to samples of similar leaf age. Also, the data indicate that the models should also be speciesspecific, owing to differences in the intercept and/or slope of the equations.• The addition of fresh LMA as a complementary parameter improved the accuracy of chlorophyll estimation in non-species-specific equations to a considerable extent when leaf samples of similar development were considered.• After due consideration, it may be concluded that the CCM-200 chlorophyll meter is a reliable method for acquiring an estimation of chlorophyll contents in Mediterranean Quercus species, with potential use as an effective tool in forest management.Résumé• Le potentiel d’un chlorophyllomètre (CCM-200, Opti-Sciences, Tyngsboro, MA, USA) pour l’estimation du contenu total des feuilles en chlorophylle a été évalué dans des feuilles à différents stades de développement chez Quercus pyrenaica Willd., Q. faginea Lamb. Q. suber L. et Q. ilex L. subsp. ballota (Desf.) Samp.• Pour toutes les espèces et les feuilles à différents stades, une forte relation non linéaire (P < 0,0001) a été trouvée entre l’index de teneur en chlorophylle (CCI) et la teneur en chlorophylle des feuilles par unité de surface. Chez toutes les espèces, la pente de la relation CCI-teneur en chlorophylle a varié avec le développement de la feuille, ce qui suggère que la prudence est de mise lors de l’utilisation du CCM-200,puisque l’interprétation des indications du CCI devraient être limitées à des échantillons de feuilles d’âge similaire. Aussi, les données indiquent que les modèles devraient également être spécifiques à l’espèce, en raison des différences dans l’interception et/ou la pente des équations.• L’ajout de nouvelles LMA en tant que paramètre complémentaire a amélioré la précision de l’estimation de la chlorophylle dans les équations non spécifiques de l’espèce, et ceci dans une large mesure lorsque des échantillons de feuilles de développement similaires ont été examinés.• Après mûre réflexion, il peut être conclu que le chlorophyllomètre CCM-200 est une méthode fiable pour l’acquisition d’une estimation de la teneur en chlorophylle des espèces méditerranéennes de Quercus, avec la possibilité de l’utiliser comme un outil efficace pour la gestion des forêts.

Differences in biomass allocation patterns between saplings of two co-occurring Mediterranean oaks as reflecting different strategies in the use of light and water.

Production and biomass allocation patterns, the growth rates of aboveground biomass, and crown traits were examined in saplings of the deciduous Quercus faginea and the evergreen Q. ilex to determine whether differences in these traits might account for the greater mortality during periods of drought undergone by Q. faginea. Strong differences were observed in almost all the traits analyzed, which suggests that the two species use different strategies to cope with the main limiting factors for woody seedling establishment in Mediterranean environments: excess light and low water availability. In Q. faginea, sapling design seems to be oriented to maximize light capture and, hence, leaf productivity during the short life span of the leaf biomass. Thus, the seedlings of Q. faginea showed crown traits that permit self-shading to be minimized: longer shoots with more spaced leaves that result in lower leaf area index than in Q. ilex. In addition, the larger area per unit leaf biomass in Q. faginea leads to a larger interceptive leaf area per unit plant mass and to higher light capture. These characteristics imply higher investments in woody tissues (SWR) that permit the plants to support a wide canopy and facilitate water transport to meet the strong transpiratory demands of a canopy with such characteristics. By contrast, in Q. ilex, saplings are apparently designed to guarantee leaf survival against temperature extremes and photoinhibition through avoidance of excessive radiation.

Morphological and chemical leaf composition of Mediterranean evergreen tree species according to leaf age

Changes in morphology [leaf dry mass per unit area (LMA), thickness and density] and chemical composition (macronutrients and fibres content) in different age leaves of eight evergreen Mediterranean woody species were investigated. LMA and leaf thickness increased with leaf age increasing. Young tissues possessed higher concentrations of N, P, K, and Mg and lower Ca concentrations on a dry mass basis. However, mineral content was independent of age on leaf area basis (except for Ca content) suggesting that the changes in mineral concentration with leaf ageing are due to dilution in the larger dry mass accumulated in the oldest leaves. Leaf tissue density (LTD) increased during the first year of the leaf life. Lignin and hemicellulose concentrations did not vary along leaf life and the cellulose concentration increased with leaf age in most species between the current-year and the one-year old leaves. Our results suggested that physical leaf reinforcement with a higher cellulose concentration and density might be a leaf response to the unfavourable climatic conditions during the first winter.

Patterns of leaf morphology and leaf N content in relation to winter temperatures in three evergreen tree species

The competitive equilibrium between deciduous and perennial species in a new scenario of climate change may depend closely on the productivity of leaves along the different seasons of the year and on the morphological and chemical adaptations required for leaf survival during the different seasons. The aim of the present work was to analyze such adaptations in the leaves of three evergreen species (Quercus ilex, Q. suber and Pinus pinaster) and their responses to between-site differences in the intensity of winter harshness. We explore the hypothesis that the harshness of winter would contribute to enhancing the leaf traits that allow them to persist under conditions of stress. The results revealed that as winter harshness increases a decrease in leaf size occurs in all three species, together with an increase in the content of nitrogen per unit leaf area and a greater leaf mass per unit area, which seems to be achieved only through increased thickness, with no associated changes in density. P. pinaster was the species with the most intense response to the harshening of winter conditions, undergoing a more marked thickening of its needles than the two Quercus species. Our findings thus suggest that lower winter temperatures involve an increase in the cost of leaf production of evergreen species, which must be taken into account in the estimation of the final cost and benefit balance of evergreens. Such cost increases would be more pronounced for those species that, like P. pinaster, show a stronger response to the winter cold.

Ash and Mineral Contents in Leaves of Woody Species: Analysis by Near‐Infrared Reflectance Spectroscopy

Abstract Near‐infrared reflectance spectroscopy (NIRS) was evaluated for its effectiveness to determine ash and mineral concentrations [potassium (K), magnesium (Mg), copper (Cu), iron (Fe), and zinc (Zn)], in a total of 182 leaf samples of 17 woody species located in the central‐western region of the Iberian Peninsula. Chemical analysis revealed great variability in all leaf mineral elements. This variability was mainly related to differences in leaf habit (deciduous versus evergreen) and to differences in mean leaf longevity and among leaf age classes within evergreen species. A set of samples including all 17 species and leaf age classes was used to develop the calibration equations using multiple linear regression (MLR) and partial‐least squares regression (PLSR). The set of samples that did not enter in the calibration was used for external validation. In general, the most satisfactory results were obtained using PLSR and derivative transformations. Despite the strong heterogeneity of the samples included in the study, the results showed that NIRS can be employed as an effective tool, alternative to the more time‐consuming standard methods. The best predictive model was obtained for ash content. Models with acceptable accuracy were obtained in the prediction of K and Mg contents. However, their applicability for the determination of trace elements was more limited.

Costs of leaf reinforcement in response to winter cold in evergreen species

The competitive equilibrium between deciduous and evergreen plant species to a large extent depends on the intensity of the reduction in carbon gain undergone by evergreen leaves, associated with the leaf traits that confer resistance to stressful conditions during the unfavourable part of the year. This study explores the effects of winter harshness on the resistance traits of evergreen leaves. Leaf mass per unit area (LMA), leaf thickness and the concentrations of fibre, nitrogen (N), phosphorus (P), soluble protein, chlorophyll and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) were determined in three evergreen and two deciduous species along a winter temperature gradient. In the evergreen species, LMA, thickness, and P and structural carbohydrate concentrations increased with the decrease in winter temperatures. Nitrogen and lignin concentrations did not show definite patterns in this regard. Chlorophyll, soluble proteins and Rubisco decreased with the increase in winter harshness. Our results suggest that an increase in LMA and in the concentration of structural carbohydrates would be a requirement for the leaves to cope with low winter temperatures. The evergreen habit would be associated with higher costs at cooler sites, because the cold resistance traits imply additional maintenance costs and reduced N allocation to the photosynthetic machinery, associated with structural reinforcement at colder sites.

Nitrogen resorption efficiency in mature trees and seedlings of four tree species co-occurring in a Mediterranean environment

Background and aimsMany studies have revealed marked changes in the patterns of nitrogen use along ontogeny in tree species. However, there are no reports of the ontogenetic changes in resorption efficiency, despite the fact that nitrogen resorption from senescing leaves is one of the most important plant mechanisms to conserve and optimize the use of this nutrient. Our objective is to analyze the differences in nitrogen resorption between seedlings and adults and to elucidate which of the leaf traits changing along ontogeny determine these possible differences.MethodsGreen and senescent leaves were sampled from five seedlings and mature specimens of four tree species. N resorption efficiencies were calculated from the differences between N contents in green and shed leaves collected from the same specimens. Minimum N concentrations in leaf litter were used as an estimation of resorption proficiency.ResultsN concentration in green leaves and N resorption efficiency were higher in adult specimens for all species. However, with the exception of Pinus pinea, there were no significant differences in proficiency.ConclusionsContrary to our expectations, the species studied seem to have a lower potential for N resorption during the initial stages of their life cycle. The higher N concentrations seen in adult leaves could explain the higher percentage they are able to withdraw during senescence, probably because they have higher amounts of readily translocatable soluble molecules.