Abstract
Trees are suffering mortality across the globe as a result of drought, warming, and biotic attacks. The combined effects of warming and drought on in situ tree chemical defenses against herbivory have not been studied to date. To address this, we transplanted mature piñon pine trees—a well-studied species that has undergone extensive drought and herbivore-related mortality—within their native woodland habitat and also to a hotter-drier habitat and measured monoterpene emissions and concentrations across the growing season. We hypothesized that greater needle temperatures in the hotter-drier site would increase monoterpene emission rates and consequently lower needle monoterpene concentrations, and that this temperature effect would dominate the seasonal pattern of monoterpene concentrations regardless of drought. In support of our hypothesis, needle monoterpene concentrations were lower across all seasons in trees transplanted to the hotter-drier site. Contrary to our hypothesis, basal emission rates (emission rates normalized to 30 °C and a radiative flux of 1000 μmol m−2 s−1) did not differ between sites. This is because an increase in emissions at the hotter-drier site from a 1.5 °C average temperature increase was offset by decreased emissions from greater plant water stress. High emission rates were frequently observed during June, which were not related to plant physiological or environmental factors but did not occur below pre-dawn leaf water potentials of −2 MPa, the approximate zero carbon assimilation point in piñon pine. Emission rates were also not under environmental or plant physiological control when pre-dawn leaf water potential was less than −2 MPa. Our results suggest that drought may override the effects of temperature on monoterpene emissions and tissue concentrations, and that the influence of drought may occur through metabolic processes sensitive to the overall needle carbon balance.
Highlights
Trees are suffering mortality related to drought, warming and biotic attacks from pests and pathogens across the globe (Allen et al 2010, 2015, Hartmann et al 2018)
We focused our study on the piñon pine Pinus edulis, a widespread, well-studied species that has undergone extensive drought-herbivore-related mortality (Breshears et al 2005, 2018)
Our results demonstrate the need to further study different states of plant water relations and physiological function—including those that increase the likelihood of resin exposure—on biogenic volatile organic compound (BVOC) emissions from piñon pine shoots and needles
Summary
Trees are suffering mortality related to drought, warming and biotic attacks from pests and pathogens across the globe (Allen et al 2010, 2015, Hartmann et al 2018). Despite the acknowledged importance of monoterpenes in affecting higher trophic level interactions and contributing to tree resistance, the effect of warming with drought on their synthesis and emissions has not been evaluated despite observed and predicted increases in temperature and drought conditions. The internal concentration of monoterpenes in the needles—and seasonal variations therein—generally increase under drought stress (Blanch et al 2009), but this can vary with drought severity (Niinemets 2015) and may not reflect the composition of released volatiles (Llusià and Peñuelas 1998, Trowbridge et al 2014). An increase in monoterpene emissions under warming could mitigate drought-related decreases in emissions with implications for plant defense and herbivore success in a changing climate. We lack a basic understanding of how these two variables interact to affect monoterpene production and emission rate in mature conifers in the field
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