Background and AimsDespite long-held interest, knowledge on why leaf size varies widely among species is still incomplete. This study was conducted to assess whether abiotic factors, phylogenetic histories and multi-trait interactions act together to shape leaf size.MethodsFifty-seven pairs of altitudinal vicariant species were selected in northern Spain, and leaf area and a number of functionally related leaf, shoot and whole plant traits were measured for each pair. Structural equation modelling helped unravel trait interactions affecting leaf size, and Mantel tests weighed the relative relevance of phylogeny, environment and trait interactions to explain leaf size reduction with altitude.Key ResultsLeaves of highland vicariants were generally smaller than those of lowlands. However, the extent of leaf size reduction with increasing altitude was widely variable among genera: from approx. 700 cm 2 reduction (96 in Polystichum) to approx. 30 cm2 increase (37 in Sorbus). This was partially explained by shifts in leaf, shoot and whole plant traits (3564 of explained variance, depending on models), with size/number trade-offs more influential than shifts in leaf form and leaf economics. Shifts in traits were more important than phylogenetic distances or site-specific environmental variation in explaining the degree of leaf size reduction with altitude.ConclusionsEcological filters, constraints due to phylogenetic history (albeit modest in the study system), and phenotypic integration contribute jointly to shape single-trait evolution. Here, it was found that phenotypic change was far more important than shared ancestry to explaine leaf size differences of closely related species segregated along altitudes.
Bibliographical noteFunding Information:
We thank Mar Triguero and Luis Giménez-Benavides for help during field work, Oscar Godoy, Charles Price, David Ackerly, Lonnie Aarssen, James Grace and anonymous referees for their useful comments on early stages of this manuscript, and Matthew Bowker for statistical advice on the usage of path analysis of Mantel tests. We also thank Herminio Nava, Sara Robinson and Amparo Mora, who helped to locate study populations and also to select species, the Departamento de Biologia de Organismos y Sistemas (Universidad de Oviedo, Spain) for letting us use their facilities to process field samples, Melchor Maestro (IPE-CSIC, Zaragoza, Spain) for kindly analysing leaf nitrogen, and the officials of the ‘Parque Nacional Picos de Europa’ for granting permission to carry out the study. RM was supported by the Minisiterio de Educación y Ciencia (Spain).
- Leaf size evolution
- indirect selection
- leaf economics
- morphological correlates
- structural equation models