Understorey Rhododendron tomentosum and Leaf Trichome Density Affect Mountain Birch VOC Emissions in the Subarctic

Abstract

Subarctic vegetation is composed of mountain birch [Betula pubescens ssp. czerepanovii (MB)] forests with shrubs and other species growing in the understorey. The effects of the presence and density of one understorey shrub, Rhododendron tomentosum (RT), on the volatile emissions of MB, were investigated in a Finnish subarctic forest site in early and late growing season. Only MB trees with an RT-understorey emitted the RT-specific sesquiterpenoids, palustrol, ledol and aromadendrene. Myrcene, which is the most abundant RT-monoterpene was also emitted in higher quantities by MB trees with an RT-understorey. The effect of RT understorey density on the recovery of RT compounds from MB branches was evident only during the late season when sampling temperature, as well as RT emissions, were higher. MB sesquiterpene and total emission rates decreased from early season to late season, while monoterpene emission rate increased. Both RT and MB terpenoid emission rates were linked to density of foliar glandular trichomes, which deteriorated over the season on MB leaves and emerged with new leaves in the late season in RT. We show that sesquiterpene and monoterpene compounds emitted by understorey vegetation are adsorbed and re-released by MB, strongly affecting the MB volatile emission profile.

Foliar behaviour of biogenic semi-volatiles: potential applications in sustainable pest management

“…Deposition flux of MTs and SQTs typical of nearby conifer forest trees was detected in mountain grassland (Bamberger et al 2011). Substantial recovery of MTs, SQTs and SQT alcohols typical of the understory shrub Rhododendron tomentosum Harmaja (Ericaceae) (Jesionek et al 2017) was made from mountain birch foliage in the subarctic (Mofikoya et al 2018b). Earlier, Himanen et al (2010) reported recovery of R. tomentosum SQT alcohols from the foliage of Betula pendula Roth and B. pubescens Ehrh.…”

“…One of the evolutionary reasons for the location of GTs on plant surfaces is the phytotoxicity of oily MTs in high concentrations (Tissier et al 2017;Synowiec et al 2017). To avoid phytotoxicity for the terpene secretory tissues, GTs have special compartments (Tissier et al 2017;Mofikoya et al 2018b) filled with essential oils, i.e. MTs mixed with SQTs and other compounds mentioned above.…”

“…High temperature dependence of volatility of VOCs may lead mixtures of MTs and SQTs to escape directly to the atmosphere by passive diffusion through the GT cuticle during warmer temperatures (Niinemets et al 2014;Tissier et al 2017). Released SQTs (Mofikoya et al 2018b), SQT alcohols (Himanen et al 2010;Mofikoya et al 2018b) and monoterpenes (Mofikoya et al 2017(Mofikoya et al , 2018b have been observed to deposit on neighbouring plant foliage in field conditions. Re-emission of these deposited SVOCs from neighbour foliage has been shown to be highest in the morning (Himanen et al 2010).…”

“…For example, the MT β-myrcene and the SQT β-farnesene are both emitted by Betula species and R. tomentosum shrubs (Himanen et al 2010), so their origin on Betula leaves could be from either sources, the emitter or the neighbour. Highly species specific sVOCs of an emitter plant are the best indicators of VOC deposition on neighbouring plants in nature (Mofikoya et al 2018b).…”

Unravelling the functions of biogenic volatiles in boreal and temperate forest ecosystems

“…In addition to green foliage, wood, phloem and bark of trunk and branches of living trees act as important pools of stored BVOCs such as oleoresin MTs in conifer forests (Taipale et al 2011;Ghimire et al 2016). Furthermore, some plant species such as woody shrubs (Himanen et al 2010;Mofikoya et al 2018) growing in the understory can act as significant emission sources. On the forest floor, important BVOC emission sources include leaf, needle and wood litter Mäki et al 2017;Kivimäenpää et al 2018) and the root systems of living (Lin et al 2007;Rasheed et al 2017) and dead trees (Haapanala et al 2012;Kivimäenpää et al 2012).…”