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

Abstract

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.

Fig. 1.

Distribution of Kermes oak (Quercus coccifera) in Iberia, including geographic locations of the studied populations. 1, Aranjuez (ARJ); 2, Ainsa (AIN); 3, Cañada de Verich (CÑV); 4, Cardeña (CAR); 5, El Saler (SAL); 6, Gargallo (GAR); 7, Facinas (FAC); 8, Serra da Arrábida (ARR); 9, Tarazona (TAR).

Fig. 2.

Relationship between sun (open circles), mean (dashed line) and shade (filled circles) phenotypes vs. crown plasticity values. Plasticity was calculated as the difference between trait mean for the shade leaves and that for the sun leaves of each individual. The value of r² is given for each relationship together with the significance of the equivalent correlation coefficient r (*P < 0·05; **P < 0·01; ***P < 0·001). (A) Leaf angle; (B) leaf area; (C) lobation index; (D) spininess index; (E) specific leaf area; (F) β-carotene content on a total chlorophyll basis; (G) pool of xanthophyll cycle pigments on a total chlorophyll basis; (H) total chorophyll concentration on a leaf area basis.

Fig. 3.

Field (this study) vs. common garden (Balaguer et al., 2001) standardized plasticity values exhibited by three populations (CNV, GAR and ARR; Rock, Garrigue and Forest, respectively, in Balaguer et al., 2001) in response to light availability. Plasticity was computed as the difference between shade and sun values. The diagonal line indicates identical values for both studies. (A) Leaf area; (B) leaf angle; (C) specific leaf area; and (D) leaf perimeter/leaf area (ILB in Balaguer et al., 2001).