Differential controls by climate and physiology over the emission rates of biogenic volatile organic compounds from mature trees in a semi-arid pine forest

Abstract

Drought has the potential to influence the emission of biogenic volatile organic compounds (BVOCs) from forests and thus affect the oxidative capacity of the atmosphere. Our understanding of these influences is limited, in part, by a lack of field observations on mature trees and the small number of BVOCs monitored. We studied 50- to 60-year-old Pinus ponderosa trees in a semi-arid forest that experience early summer drought followed by late-summer monsoon rains, and observed emissions for five BVOCs-monoterpenes, methylbutenol, methanol, acetaldehyde and acetone. We also constructed a throughfall-interception experiment to create "wetter" and "drier" plots. Generally, trees in drier plots exhibited reduced sap flow, photosynthesis, and stomatal conductances, while BVOC emission rates were unaffected by the artificial drought treatments. During the natural, early summer drought, a physiological threshold appeared to be crossed when photosynthesis ≅2 μmol m(-2) s(-1) and conductance ≅0.02 mol m(-2) s(-1). Below this threshold, BVOC emissions are correlated with leaf physiology (photosynthesis and conductance) while BVOC emissions are not correlated with other physicochemical factors (e.g., compound volatility and tissue BVOC concentration) that have been shown in past studies to influence emissions. The proportional loss of C to BVOC emission was highest during the drought primarily due to reduced CO2 assimilation. It appears that seasonal drought changes the relations among BVOC emissions, photosynthesis and conductance. When drought is relaxed, BVOC emission rates are explained mostly by seasonal temperature, but when seasonal drought is maximal, photosynthesis and conductance-the physiological processes which best explain BVOC emission rates-decline, possibly indicating a more direct role of physiology in controlling BVOC emission.

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

“…Combined, these results suggest that emission rate is controlled by measured physiological and environmental factors above the −2 MPa Ψthreshold, but not below. These results contrast the findings of Eller et al (2016), who found that BVOC emissions are correlated with leaf physiology below a plant physiological threshold, namely a net CO 2 assimilation rate A<ca. 2 μmol m −2 s −1 and stomatal conductance g s <0.02 mol m −2 s −1 in the needles of mature P. ponderosa trees.…”

“…Combined, our observations are in contrast to other studies and atmospheric models that have derived algorithms based solely on the effects of foliar monoterpene concentrations and temperature for estimating monoterpene emission rate. Our observations are consistent with empirical and modeling studies demonstrating that monoterpene emissions are not under stomatal control when averaged over longer periods of time (Harley 2013, Grote et al 2013, Eller et al 2016; g s rarely entered models of emission rate (tables 1 and 2). As noted, when Ψwas above −2 MPa, a model with T a , C i (a surrogate for A apart from g s ), needle monoterpene concentration, and experimental water treatment were necessary to explain observed emission rate variability.…”

“…Past studies, in addition to citing temperature as a primary driver of emission rate from conifers, have also shown monoterpene emission rate to be a function of foliar monoterpene concentrations through a Henry's Law relationship (Lerdau et al 1994, Lerdau et al 1997. Other studies have found no relationship between monoterpene concentrations and emission rate (Constable et al 1999, Trowbridge et al 2014, Eller et al 2016. In addition to our inability to find the expected positive temperature-emissions relationship in this study, we also failed to verify a relationship between monoterpene concentrations and emission rate across most time periods studied except for June (table 1).…”

“…The interactive effect of these stressors is expressed through shifts in monoterpene synthesis and emission (or volatilization). Monoterpene emission rate is generally unaffected by mild drought (Staudt et al 2002b, Lavoir et al 2009, Niinemets 2010, Eller et al 2016 and decreases only during more severe droughts (Llusià and Peñuelas 1998, Staudt et al 2002a, Lavoir et al 2009, Trowbridge et al 2014. 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 Peñuelas 1998, Trowbridge et al 2014).…”

Phenotypic plasticity of floral volatiles in response to increasing drought stress

“…Most research on environmental sensitivity of VOC emissions has focused not on flowers but on leaf emissions by trees, and often in the context of atmospheric processes (e.g. Niinemets, 2010;Trowbridge et al, 2014;Seco et al, 2015;Eller et al, 2016). Floral VOCs could also potentially respond to abiotic conditions such as temperature, soil nutrients or soil water availability (Majetic et al, 2009;Yuan et al, 2009;Burkle and Runyon, 2016).…”