Field patterns of leaf plasticity in adults of the long-lived evergreen Quercus coccifera.

BACKGROUND AND AIMS: Quercus coccifera, as a long-lived sprouter, responds plastically to environmental variation. In this study, the role of foliar plasticity as a mechanism of habitat selection and modification within the canopy and across contrasted habitats was characterized. An examination was made of the differential contribution of inner and outer canopy layers to the crown plasticity expressed in the field by adult individuals and its dependence on environmental and genetic factors. METHODS: Within-crown variation in eight foliar traits was examined in nine populations dominated by Q. coccifera. The difference between mean trait values at the inner and outer canopy layers was used as a proxy for crown plasticity to light. Correlations between geographic distances, environmental differences (climatic and edaphic) and phenotypic divergence (means and plasticities) were assessed by partial Mantel tests. A subset of field measurements was compared with data from a previous common garden experiment. KEY RESULTS: Phenotypic adjustment of sun leaves contributed significantly to the field variation in crown plasticity. Plasticity in leaf angle, lobation, xanthophyll cycle pigments and beta-carotene content was expressed in sun and shade leaves concurrently and in opposite directions. Phenotypic plasticity was more strongly correlated with environmental variation than mean trait values. Populations of taller plants with larger, thinner (higher specific leaf area) and less spiny leaves exhibited greater plasticity. In these populations, the midday light environment was more uniform at the inner than at the outer canopy layers. Field and common garden data ranked populations in the same order of plasticity. CONCLUSIONS: The expression of leaf plasticity resulted in a phenotypic differentiation that suggests a mechanism of habitat selection through division of labour across canopy layers. Signs of plasticity-mediated habitat modification were found only in the most plastic populations. Intracanopy plasticity was sensitive to environmental variation but also exhibited a strong genetic component.

Plasticity, instability and canalization: is the phenotypic variation in seedlings of sclerophyll oaks consistent with the environmental unpredictability of Mediterranean ecosystems?

• Evergreen oaks from the Mediterranean basin exhibit a conservative resource-use strategy based on a reduced expression of phenotypic variation (i.e. canalization). We hypothesized that genetic variation across closely related species is more canalized than the response to environmental variation. • Seedlings of Quercus ilex and Q. coccifera, two important oak species from the Mediterranean basin that belong to the same subgenus and section, were grown in contrasted light and nutrient environments following a factorial design. Phenotypic variation was explored in a total of 75 variables including photosynthetic capacity, nutrient allocation, allometric relationships and crown architecture. • Path analysis showed that phenotypic variation was not significantly affected by differences between species but by those between and within environments, which are argued to be primarily linked to phenotypic plasticity and developmental instability, respectively. This finding is interpreted as evidence of genetic canalization across species. • The similar importance of plasticity and instability as sources of phenotypic variation and the high degree of genetic canalization are consistent with the expected role of the environmental unpredictability of Mediterranean ecosystems in shaping the developmental patterns of these two species.

Low leaf-level response to light and nutrients in Mediterranean evergreen oaks: a conservative resource-use strategy?

We have explored leaf-level plastic response to light and nutrients of Quercus ilex and Q. coccifera, two closely related Mediterranean evergreen sclerophylls, in a factorial experiment with seedlings. Leaf phenotypic plasticity, assessed by a relative index (PI = (maximum value - minimum)/maximum) in combination with the significance of the difference among means, was studied in 37 morphological and physiological variables. Light had significant effects on most variables relating to photosynthetic pigments, chlorophyll fluorescence and gas exchange, whereas nutrient treatment had a significant effect in only 10% of the variables. Chlorophyll content was higher in the shade whereas carotenoid content and nonphotochemical quenching increased with light. Nutrient limitations increased the xanthophyll-cycle pool but only at high light intensities, and the same interaction between light and nutrients was observed for lutein. Predawn photochemical efficiency of PSII was not affected by either light or nutrients, although midday photochemical efficiency of PSII was lower at high light intensities. Photosynthetic light compensation point and dark respiration on an area basis decreased with light, but photosynthetic capacity on a dry mass basis and photochemical quenching were higher in low light, which translated into a higher nitrogen use efficiency in the shade. We expected Q. ilex, the species of the widest ecological distribution, to be more plastic than Q. coccifera, but differences were minor: Q. ilex exhibited a significant response to light in 13% more of the variables than Q. coccifera, but mean PI was very similar in the two species. Both species tolerated full sunlight and moderate shade, but exhibited a reduced capacity to enhance photosynthetic utilization of high irradiance. When compared with evergreen shrubs from the tropical rainforest, leaf responsiveness of the two evergreen oaks was low. We suggest that the low leaf-level responsiveness found here is part of a conservative resource use strategy, which seems to be adaptive for evergreen woody plants in Mediterranean-type ecosystems.