Leaf functional traits and insular colonization: subtropical islands as a melting pot of trait diversity in a widespread plant lineage

Aim: One of the main goals of functional biogeography is to examine
distribution patterns of trait diversity, and islands provide excellent
study cases for this emerging field. We tested the hypothesis that
multiple dispersals from a common mainland pool would promote functional
similarity among island systems when environmental conditions are similar,
but also novel phenotypic traits related to colonization history and
exploitation of new habitats. Location: Mediterranean Basin and
Macaronesian islands Methods: We used the well-known
biogeographical history of a woody plant complex (Periploca laevigata
s.l.) to examine trait variation and how it relates to climatic conditions
of mainland and subtropical island settings. In a common garden
experiment, we measured a suite of leaf physiological and anatomical
traits tightly related to plant performance in 320 seedlings representing
21 populations of five sublineages: the oldest (2.6 my) island
colonization (western Canary Islands) as a reference, three sublineages
stemming from independent events of island colonization in the last 0.5 my
from NW Africa (Cape Verde, Fuerteventura, Lanzarote), and their
widespread Mediterranean mainland counterpart. Results: We observed strong
phenotypic divergence between island and mainland sublineages linked to
contrasting climatic conditions. Mediterranean mainland populations
displayed a very specialized leaf phenotype characteristic of arid plants
(i.e. small leaves, amphistomaty, isobilateral mesophyll, high
photosynthetic rates). In turn, low seasonality on islands was linked to
the recurrent expression of a phenotype characterized by larger leaves and
lower photosynthetic rates. Our analyses showed that the high investment
in secondary compounds (i.e. tannins) on islands decouples photosynthesis
from growth rates. Despite this pattern of parallel differentiation, each
island sublineage displayed a distinctive phenotype, with some traits
related to colonization time, which resulted in a mosaic of functional
variation across island systems. Main conclusions: Our data suggest that
the studied subtropical islands promote expression of traits specific to
certain sublineages and other common traits that are no untypeset proof
Page 2 of 50 Journal of Biogeography longer adaptive in the original
mainland pool due to Pleistocene climatic shifts. These findings
ultimately extend the role of islands as biodiversity refugia and hotspots
of plant functional diversity.