Japanese red maple, Acer pycnanthum K. Koch, is the disjunct sister species of the red maple, Acer rubrum L. Whereas A. rubrum is one of the most widely distributed and abundant species in eastern North America, A. pycnanthum is rare in central Honshu, Japan. Although its morphological similarity to A. rubrum is well known, little is known about the sites and communities where it occurs, its natural history characteristics, sexual and asexual regeneration, and the reasons for its restricted occurrence. We located and described all known sites, totaling <18 ha, which supported populations of three or more clones. Twenty-seven of the 30 ecosystems described are remnant natural populations, which are confined to lower slopes of three river basins where wetlands have persisted and recurrently formed for millions of years due to unique geological, topographic, and soil properties. Acer pycnanthum, an obligate species of forested wetlands, occurs on diverse seepage and floodplain ecosystems that are characterized by poor drainage; acid, wet soils; high understory and ground-cover diversity, and associated rare species. The area occupied by each population is typically very small, usually <0.5 ha and often <0.2 ha. Regeneration is limited to sites with high light irradiance following disturbance. Because of the rarity of natural wetland sites, its inability to colonize upland sites occupied by dense natural vegetative cover or by planted conifers, and the encroachment of agriculture and urbanization, the occurrence of A. pycnanthum is increasingly limited. Using an ecosystem approach, conserving whole ecosystems, including their total vegetative diversity, is a high priority.Key words: biodiversity, disjunct species, landscape ecology, rare species, red maple (Acer pycnanthum, Acer rubrum), wetlands.

Aim We analysed variation in chloroplast DNA (cpDNA) in red maple (Acer rubrum L.) and silver maple (Acer saccharinum L.) across a large part of their geographic ranges. Acer rubrum is one of the most common and morphologically variable deciduous trees of eastern North America, while its sister species A. saccharinum has a more restricted habitat distribution and displays markedly less morphological variation. Our objective was to infer the impact of biogeographic history on cpDNA diversity and phylogeographic structure in both species.Location Deciduous forests of eastern North America.Methods We sequenced 1289 to 1645 bp of non-coding cpDNA from A. rubrum (n = 258) and A. saccharinum (n = 83). Maximum parsimony networks and spatial analysis of molecular variance (SAMOVA) were used to analyse phylogeographic structure. Rarefaction analyses were used to compare genetic diversity.Results A total of 40 cpDNA haplotypes were recovered from A. rubrum (38 haplotypes) and A. saccharinum (7 haplotypes). Five of the seven A. saccharinum haplotypes were shared with nearby samples of A. rubrum. SAMOVA recovered four phylogeographic groups for A. rubrum in: (1) south-eastern USA, (2) the Gulf and south-eastern Coastal Plain, (3) the lower Mississippi River Valley, and (4) the central and northern regions of eastern North America. Acer saccharinum had significantly lower haplotype diversity than A. rubrum, and novel haplotypes in post-glaciated northern limits of its range were shared with A. rubrum.Main conclusions This is the first study of A. rubrum to report a distinct phylogeographic group centred on the lower Mississippi River, and the first to examine data comparatively with A. saccharinum. We hypothesized that A. rubrum would display stronger phylogeographic structure and greater haplotype diversity than A. saccharinum because of its greater geographic range, and ecological and morphological variation. This hypothesis was supported by the cpDNA analysis. The sharing of cpDNA and chloroplast simple sequence repeat (cpSSR) haplotypes in areas of geographic overlap provides evidence of introgression, which led to an increase in haplotype diversity in both species, and to novel phylogeographic structure in A. rubrum. We recommend that introgression be considered, along with other potential causes, as an explanation for the phylogeographic structure of cpDNA in plants.

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