Passive adsorption of neighbouring plant volatiles linked to associational susceptibility in a subarctic ecosystem

Abstract. Neighbouring plants may affect volatile compound emissions of a focal plant and confer associational resistance or susceptibility. Associational resistance has been reported as a result of adsorption of neighbouring plant volatile and semivolatile compounds on focal plant foliage in field experiments. However, these associational effects in a natural ecosystem remain largely unknown. The effects of the presence and density of Rhododendron tomentosum (Rt) understorey on the volatile profile and herbivore density of mountain birch, Betula pubescens ssp. czerepanovii (MB) was investigated in a subarctic forest site. The monoterpene β-myrcene, sesquiterpene aromadendrene and sesquiterpene alcohols, palustrol and ledol were recovered from the foliage of MB trees that had Rt growing in the understorey. The number of Rt shoots growing directly under the MB trees correlated positively with the rate of recovery of adhered compounds and negatively with total MB emissions. Palustrol and β-myrcene recovery from MB leaves showed the highest positive correlation with Rt density. Recovery of adhered compounds was higher at lower sampling temperatures. Herbivory was at very low levels both in control and Rt plots. The proportion of foliage infected by a gall mites (Acalitus spp.) was positively correlated with the recovery of the adhered ledol and palustrol from MB foliage. These results indicate that understorey plant volatiles, both sesquiterpene and highly volatile monoterpenes, may adhere onto and be subsequently re-released from MB foliage at low temperatures during the subarctic growing season. The Rt density also plays an important role in the adherence and re-release rates of neighboring plant volatiles and may induce a response in MB volatile emission. Presence of Rt volatiles on MB foliage may make them more susceptible to gall mite infestation suggesting that high Rt density in the subarctic ecosystem may confer associational susceptibility to herbivores on MB.