Drought supersedes warming in determining volatile and tissue defenses of piñon pine (Pinus edulis)

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.

The many dimensions of phytochemical diversity: linking theory to practice

“…Although we know of no published examples, beta‐diversity metrics can also describe temporal variation. For example, we applied Jaccard dissimilarity ( d ) to data from Trowbridge et al () to quantify seasonal variation in volatile terpenes in piñon pine ( Pinus edulis ). We calculated mean dissimilarity across sequential samples (up to nine per season) for 36 individuals and found that within‐individual temporal beta diversity in terpene emission was substantial (mean d = 0.57; range d = 0.41–0.71), nearly equivalent to beta diversity across individuals within a single period (mean d = 0.61; range d = 0.56–0.72).…”

Hotter droughts alter resource allocation to chemical defenses in piñon pine

“…S1B). Because fresh and dry weight monoterpene data for both tissue types provided the same statistical results, we present all monoterpene concentration data here on a fresh weight (FW) basis (Trowbridge et al 2014(Trowbridge et al , 2019. This also avoids introducing unnecessary error by converting all values to a dry weight using a conversion factor when chemistry was originally extracted from fresh mass.…”

“…Elevated temperature alone has also been shown to modify terpene metabolism (Penuelas and Munne-Bosch 2005 ). However, recent studies have provided evidence that drought can override the effects of heat in controlling monoterpene production and emissions (e.g., Trowbridge et al 2014 , 2019 ), but the observational nature of these studies made it impossible to tease apart the relative contribution of heat and drought. Thus, an enhanced understanding of carbon allocation to defense compounds in response to heat and drought stress, both separately and in concert, would provide crucial insight into mechanisms governing defense production in mature trees.…”

Growth, Chemistry, and Genetic Profiles of Whitebark Pine Forests Affected by Climate-Driven Mountain Pine Beetle Outbreaks

“…However, how this may translate to effects on beetle choice of individual trees is unclear. In piñon pine, Trowbridge et al (2019) found that increased emission rates of terpenoids during warm periods were offset by decreases in production resulting in no net change in emission rates overall. This may generally be the case, but more work is needed to understand how variability in precipitation, whether it involves drought or above average rainfall, influences attraction or repulsion of beetles to particular trees.…”