Aspen (Populus tremula) is a pioneer tree, distributed in cool climatic zones throughout northern Eurasia. The species exhibits vast natural genetic variation in growth, morphology, phenology, phytochemistry and biotic interactions, and is an ideal tool to examine the basis of complex traits and their genotype x environment interactions.
The genus Populus, to which aspen belongs, is a model in forest tree research with a mature genome sequence and rich genetic resources. High heritability in morphological, biochemical and herbivore community traits has been identified in aspen. I am researching the genetic basis of variation in a wide range of complex traits, i.e. those that are controlled by multiple genetic loci.
Aspen is a common tree on the Swedish landsape. Its leaves have a characteristic tremble owing to biomechanical traits including flattened petioles that enable them to flutter in the slightest breeze. In terms of reproductive mechanisms, aspen is able to self-propagate vegetatively by means of root suckers, enabling the formation of genetically identical clonal stands. Aspen also reproduces sexually; pollen can travel hundreds of kilometers and gives rise to high genetic diversity within the species. The seeds have a cotton-like appearance and are transported by wind and have a short viability. Aspen has desirable properties for wood products, including potential as a bioenergy crop. Aspen has further potential in phytoremediation and landscape restoration projects.
As a keystone species, aspen forms part of the diet of large and small herbivores alike, from moose to mites. Its decaying wood is host to a diverse fauna and fungi/epiflora, and its living canopy sustains many morphs of herbivorous arthropods. The leaves and bark contain high concentrations of specialised metabolites including salicinoid phenolic glycosides (SPGs) and condensed tannins. Although these chemicals deter many generalist herbivores from ovipositing on and consuming the foliage, some specialist herbivores have evolved tolerance of these compounds and are able, instead of finding the SPGs toxic, to use them in their own defence against predators. Other aspen defences against herbivores include leaf trichomes and extra-floral nectaries at the leaf-petiole junction that exude nectar to encourage ants to guard the tree against herbivores such as aphids. Arthropods feeding on aspen have devised different uses of the leaf tissue as camouflage, shelters for larvae and safe homes for eggs; thus it is common to see single or multiple leaves rolled into cigar-shapes or cone-shapes, toughened gall structures on the leaf or petiole, or serpentine mines formed under the epidermis by insect larvae. Larger herbivores include voles, hares, and numerous cervids.
My primary study tool is the Swedish Aspen (SwAsp) Collection, a collection of 116 aspen genotypes collected from 12 populations across Sweden from latitudes between approximately 56°N and 66°N. I collect data from the original wild trees comprising this collection but my main focus is to study these genotypes two common gardens, where each genotype has been clonally replicated. Using genome-wide association mapping, metabolomics and detailed greenhouse experiments, I seek to examine genetic loci underlying complex phenotypic traits ranging from spring phenology to leaf anatomy, from stem growth to secondary metabolism, from individual arthropod herbivores to the herbivore community, and fungal diseases such as leaf rust and stem blight.
A further tool is the Umeå Aspen collection (UmAsp), a collection of wild aspens growing within an hour's drive of Umeå, now cloned and growing in two replicated field trials, one near the coast and one inland. Through the study of UmAsp I aim to identify causal variants for many phenotypes occurring in wild trees without the constraints of geographic influences, particularly the latitudinal gradients driving important traits such as growth rates, seasonal canopy duration and autumn senescence.
I am also investigating biogeographical variation in aspen leaf morphology and specialised metabolites using a new collection of genetic material from northern Europe and Asia.
12. Rae AM & Street NR, Robinson KM, Harris N, Taylor G (2009) Five QTL hostspots for yield in short rotation coppice bioenergy poplar: The Poplar Biomass Loci. BMC Plant Biology 9:23 doi:10.1186/1471-2229-9-23
13. Robinson KM, Karp A, Taylor G (2004) Defining leaf and canopy traits linked to high yield in short rotation coppice willow. Biomass and Bioenergy 26: 417- 431 doi.org/10.1016/j.biombioe.2003.08.012
14. Rae AM, Robinson KM, Street NR, Taylor G (2004) Morphological and physiological traits influencing biomass productivity in short-rotation coppice poplar. Canadian Journal of Forest Research 34: 1488-1498 doi: 10.1139/x04-033
15. Ferris R, Long L, Robinson KM, Bunn SM, Bradshaw HD, Rae AM, Taylor G (2002) Leaf stomatal and cell development: the identification of putative QTL in relation to elevated CO2 in poplar. Tree Physiology 22: 633-640 doi: 10.1093/treephys/22.9.633
Non-peer reviewed publications/ Popular science
• Robinson KM, Closset M, Albrectsen BR (2009) Young Chrysomela larvae prefer lower phenolics in their diet (Coleoptera, Chrysomelidae). Skörvnöpparn: Insekter I Norr 1: 32-34
• Taylor G, Beckett KP, Robinson KM, Stiles K, Rae AM (2001) Identifying QTL for yield in biomass poplar. Aspects of Applied Biology 65, Biomass and energy crops II: 173-182
Interesting aspen links
Poplar longhorn beetle Saperda carcharias on aspen in the SwAsp Collection. Photo: K. Robinson
• Aspen description at the Swedish Museum of Natural History.
• Aspen is a focal species of Trees for Life, a charity restoring the Caledonian Forest in Scotland.
• Eadha's projects in Scotland actively promote aspen use and woodland restoration.
• A very nice suite of descriptions of insects on aspen from Tommi Nyman, at Joensuu University, Finland.