Polyploids, organisms that have multiple sets of chromosomes, are common in certain plant and animal taxa. However, there are only a few reports of intraspecific ploidy variation within the genus Quercus. The aim of the study was to investigate the suspected ploidy level of two oaks that have unusual microsatellite banding patterns. Polyploidy was investigated by using microsatellite analysis, stomata length measurements and nuclear DNA content estimation by flow cytometry. Each putative triploid tree has patterns of microsatellite variation unexpected for diploid genomes, with up to three alleles at some loci, significantly longer stomata and 1.5 times more DNA per nucleus compared with diploids. To our knowledge, this report contains the first evidence for triploidy in Q. petraea and confirmation of this phenomenon in Q. robur. Regardless of the positive or negative aspects of the presence of triploid oaks in forest stands, it is of value to be able to screen for them. This study demonstrates that nuclear microsatellites and estimation of DNA content by flow cytometry can readily be used for this purpose.

Background and aims Di-nitrogen (N₂)-fixing plants are absent in the pioneer stages of glacial forefields in the European Alps despite low amounts of nitrogen (N) in the soil. We aimed to evaluate whether symbiotic N₂-fixation is needed to meet the N demand of plants during the early stages of soil formation, and how phosphorus (P) availability affects plant establishment. Methods We measured total and available N and P in soil and N and P in the vegetation along the 137 year chronosequence in front of the Damma glacier (Switzerland). Results Available N as determined by in situ resin N decreased from the pioneer stage (<16 years since deglaciation) to the intermediate (57–80 years), likely resulting from increased plant N uptake and reduced funnelling of N by rocks. N concentrations in the vegetation were positively correlated to in situ resin N, and the N:P-ratio of the vegetation was negatively correlated with P concentrations in the vegetation. Conclusions The N requirement of plants in the pioneer stage of soil development in the forefield is fulfilled by atmospheric N deposition being funnelled between rocks. N₂-fixation is not needed as the vegetation is often limited by P, or co-limited by N and P.