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. 

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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. 

bud.flushSpring bud flush date is a highly heritable trait in aspen.                      Photo: N. Streetant aphid domeAnts and aphids on a leaf contorted by aphid damage.    Photo: K. RobinsonC.petioli. smallThe midge larva Contarinia petioli induces characteristic galls on the petiole.                  Photo: K. Robinson
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. 

Publication list

  1. A single gene underlies the dynamic evolution of poplar sex determination
    Nat Plants. 2020 Jun 1 [Epub ahead of print]
  2. Inferring the Genomic Landscape of Recombination Rate Variation in European Aspen (Populus tremula)
    G3-GENES GENOMES GENETICS 2020, 10(1):299-309
  3. Functional and evolutionary genomic inferences in Populus through genome and population sequencing of American and European aspen
    Proc Natl Acad Sci U S A. 2018, 115(46):E10970-E10978
  4. A major locus controls local adaptation and adaptive life history variation in a perennial plant
    Genome Biol. 2018 Jun 4;19(1):72
  5. Storage lipid accumulation is controlled by photoperiodic signal acting via regulators of growth cessation and dormancy in hybrid aspen
    New Phytol. 2018, 219 (2):619-630
  6. Both plant genotype and herbivory shape aspen endophyte communities
    Oecologia 2018, 187 (2):535-545
  7. Autumn senescence in aspen is not triggered by day length
    Physiol Plant. 2018, 162(1):123-134
  8. Relative impacts of environmental variation and evolutionary history on the nestedness and modularity of tree–herbivore networks
    Ecology and Evolution 2015; 5(14): 2898–2915
  9. Comparative physiology of allopatric Populus species: geographic clines in photosynthesis, height growth, and carbon isotope discrimination in common gardens
    Front Plant Sci. 2015, 6:528
  10. Populus tremula (European aspen) shows no evidence of sexual dimorphism
    BMC Plant Biol. 2014; 14(1):276
  11. Geographic structure in metabolome and herbivore community co-occurs with genetic structure in plant defence genes
    Ecology Letters 2013, 16 (6):791-798
  12. Robinson KM, Ingvarsson PK, Jansson S, Albrectsen BR
    Genetic Variation in Functional Traits Influences Arthropod Community Composition in Aspen (Populus tremula L.)
    PLoS ONE 2012; 7(5):e37679
  13. Albrectsen BR, Witzell J, Robinson KM, Wulff S, Luquez VMC, Ågren R, Jansson S
    Large scale geographic clines of parasite damage to Populus tremula L.
    Ecography: 2010 33:483-493
14. 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

15. 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

16. 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

17. 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

ceruraThe larva of notodontid moth Cerura vinula is a voracious feeder on aspen.      Photo: K. Robinson

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
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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.