Abstract
Ulmus laevis Pallas is a rare and endangered tree species in the northern part of Belgium, restricted to a minimal amount of natural populations that mainly consist of a strongly reduced number of individuals. Probably isolated for several generations, random phenotypic variation in the strongly declined populations can be hypothesised. We analysed U. laevis trees grown in a field trial with a single-tree-plot design (completely randomised). The plants were vegetatively propagated through cuttings from nearly all known natural relict populations in Flanders. We analysed three short-shoot leaves from different ramets from different genotypes from eleven natural populations. Leaf size and shape variables were computed using landmark and procruste methods. We visualised the variability in leaf morphology in a principal component analysis (PCA) including shape, size, and categorical morphological variables. The variance structure of the morphological variables was studied applying mixed modelling methods. The overall PCA distinguished two deviating natural populations. The Zandhoven population showed a deviating simple leaf margin toothing, correlated with larger numbers of split secondary veins. The Halle population is characterised by absence of pubescence at the lower side of the leaves. Both relict populations with deviating morphological leaf characteristics may point to an ecotype evolution putatively due to isolation combined with greatly diminished population sizes. The variance structure analysis of individual leaf variables showed that the deviating categorical leaf characters are under stronger genetic control (specialist characters) compared to the more plastic variables.
Highlights
Habitat reduction and fragmentation have become major threats to many native species in Western Europe, reducing their adaptive and evolutionary potential in the face of climate change (Aguilar et al 2006; Leimu et al 2010)
The hierarchical structure was tested for each morphological variable, including the principle component scores of the axes explaining the larger part of the variation, applying ANOVA
We studied the leaf shape variability of the sampled U. laevis trees applying landmark and procruste methods
Summary
Habitat reduction and fragmentation have become major threats to many native species in Western Europe, reducing their adaptive and evolutionary potential in the face of climate change (Aguilar et al 2006; Leimu et al 2010). We focus on Ulmus laevis Pallas (white elm), a European riparian tree species that is wind pollinated and selfincompatible. The centre of its distribution area is limited to Central and Eastern Europe, whereas populations in Western Europe have probably been less abundant naturally and have been further reduced by habitat loss. In other parts of western Europe relict populations suffer from varying human pressures making them more prone to climate change impacts (Hemery et al 2010) and urging conservation programs (Goodall-Copestake et al 2005; FuentesUtrilla et al 2014). Several molecular studies have detected low genetic diversities within natural populations together with high population differentiations (Vakkari et al 2009; Venturas et al 2013) and Central European populations show a demographic signature of recent bottlenecks (Nielsen and Kjaer 2010)
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