Alfonso Escudero

Effects of leaf longevity and retranslocation efficiency on the retention time of nutrients in the leaf biomass of different woody species.

SummaryA study was made of the retention times of N and P in the leaf biomass and their relationship with the retranslocation percentages and the leaf longevities in some woody species in Central Spain. The retention times of both nutrients were strongly related to the nutrient status of each species. These results suggest that a prolonged retention time is a way of increasing nutrient use efficiency in conditions of low nutrient availability. Plants can increase the retention time of nutrients in their leaf biomass by means of an increase in leaf longevity and/or by means of an increase in retranslocation efficiency. However, the effect of the retranslocation efficiency on retention times was almost negligible compared with the effect of leaf longevity. This suggests that an increase in leaf longevity is probably the best adaptation for increasing efficiency in the use of nutrients.

Ecological significance of the phenology of leaf abscission

The phenology of leaf abscission in different woody species with deciduous, marcescent and evergreen habits was studied over a complete annual cycle. Statistical treatment of the data permit the characterization of each species as a function of abscission time and of the length of the abscission period (estimation of the greater or smaller degree of gradualness of leaf fall). Abscission time seems to be related to moisture stress; the sooner the water deficit becomes apparent, the earlier the leaf fall. The length of the abscission period does not depend on the deciduous, marcescent or evergreen nature of the species; neither is it apparently related to the phylogenetic affinity of the different species. The latter finding means that a prolonged abscission period could indeed be an adaptive trait of the species better suited to more infertile soils.

The efficiency of nitrogen retranslocation from leaf biomass in Quercus ilex ecosystems

Nitrogen retranslocation from senescing leaves represents a crucial adaptation by tree species towards a more efficient use of this nutrient. As a result, this part of the nitrogen cycle has received increasing attention in recent years. However, there remain strong discrepancies with respect to the factors responsible for interspecific differences in the efficiency of this process.

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.

Coupling N cycling and N productivity in relation to seasonal stress in Quercus pyrenaica Willd. saplings

The N economy of saplings of the Mediterranean Quercus pyrenaica Willd. species was analysed at whole-plant level. The amount and timing of N cycling were determined by strong seasonality in N productivity, which peaked in late spring. Most leaf N was allocated to a main first flush of leaves in early spring, and a minor second flush during early summer or autumn. After peak of N productivity, the allocation of biomass production and N resources was mainly switched to lignotuber and coarse roots. Q. pyrenaica displayed a non-conservative pattern of N use as compared with other co-occurring Quercus species. Potential leaf N-resorption efficiency was moderate, but N proficiency was low, which contributed to high losses of N throughout leaf litter and to short mean residence times of N (MRTN). Additionally, summer water stress reduced foliar area, producing leaf litter with a high N concentration due to almost negligible resorption efficiency, and in some cases, the loss of the whole above-ground fraction. A large investment of plant resources to the root system appeared as a key trait in the N economy and plant fitness of young Q. pyrenaica saplings, allowing them: (1) to counteract N losses and to maintain a positive N balance, (2) to access deep soil resources (water and nutrients), and (3) to ‘safe’-store N for coupling supply and demand processes during spring growth or sprouting after a strong summer drought.

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.