Temperature sensitivities of MeOH and AA emissions and
AA/MeOH emission ratios from physiologically active trees, detached
stems and leaves, hydrated AIR, and whole ecosystems
To better understand the role of temperature in potentially enhancing
AA/MeOH emission ratios, the air temperature sensitivities of MeOH and
AA emission rates were characterized from branches of well-watered
poplar trees, detached stems and leaves, and whole ecosystems.
Well-watered poplar trees were individually placed in a growth chamber
with diurnally changing air temperature (Figure 5 ). At night in
the dark (20:00-6:00), considerable branch transpiration was observed
together with relatively high MeOH emissions, and low to undetectable AA
emissions. Under constant daytime (6:00-20:00) light conditions, net
positive CO2 assimilation occurred. As observed at the
leaf level, branch transpiration together with MeOH and AA emissions
were strongly coupled to the diurnal pattern of air temperature,
reaching maximum fluxes during the early afternoon peak in air
temperature of 27 ºC at 14:00. MeOH emissions were greater than AA
emissions at all air temperatures by roughly a factor of 10, except for
1 hour following light to dark transitions where a short burst in AA
emissions were observed. Outside of this light-dark period, branch
AA/MeOH emission ratios remained low and increased as a function of air
temperature from 0-12% (Figure 5 ). A high temperature
sensitivity of MeOH and AA emissions from detached poplar branch
segments were also observed. Emissions of MeOH and AA and water loss
from detached stem segments in the dark increased with temperature from
30-50 °C (Supplementary Figure S7 ). Similar to physiologically
active leaves and branches, emissions were dominated by MeOH, resulting
in AA/MeOH emission ratios below 12%. Similar temperature sensitivities
of MeOH and AA emissions were also obtained from hydrated whole leaf
cell wall preparations (alcohol insoluble residue, AIR). Gas-exchange
analysis under controlled temperature with hydrated AIR in porous Teflon
tubes showed rapid equilibration of MeOH and AA emissions at each
chamber temperature in the dark (Figure 6 ). MeOH and AA steady
state emissions from hydrated AIR samples increased as a function of
temperature from 30-50 ºC and were completely dependent on the presence
of liquid water interacting with AIR. Similar to physiologically active
leaves (Figure 4 ), branches (Figure 5 ), and detached
stems (Supplementary Figure S7 ), emissions from hydrated AIR
were dominated by MeOH with AA/MeOH emission ratios increasing slightly
with temperature but remaining below 30%.
In contrast to AA/MeOH emission ratios from physiologically active
leaves, branches, detached stems, and hydrated leaf AIR samples which
remained less than 30%, AA/MeOH emission ratios from drought stressed
poplar branches were high ranging from 400-3,000% (Figures 2-3,
Supplementary Figures S2-S5 ). Similarly, detached poplar leaves placed
into the temperature-controlled chamber in the dark in a dry air stream,
showed a similar pattern of suppressed MeOH emissions together with
temperature stimulated emissions of the fermentation volatiles
acetaldehyde, ethanol, acetic acid, and acetone (Figure 7 ).
Acetaldehyde emissions peaked at 42.5 ºC, AA emissions peaked at 47.5
ºC, and the AA/MeOH emission ratio reached a maximum of 2,500% at 45
ºC.
We extended our analysis of the temperature sensitivities of AA and MeOH
leaf emissions at the ecosystem scale at the Lochristi poplar plantation
in Belgium and a mixed hardwood forest in Alabama. Likely due to the
very low ambient temperatures at night in Belgium during the growing
season (averaging around 15 °C), no significant night time AA
concentration or vertical flux was observed (Figure 8a,b) . In
contrast, night-time MeOH emissions of 10 µmol ha-1day-1 and ambient concentrations of 0.5 ppb were
detected. Similar to emissions from physiologically active poplar leaves
(Figure 4 ) and branches during the day (Figure 5 ),
average ecosystem emission rates of AA and MeOH during the 2015 growing
season showed a clear diurnal pattern reaching maximum values in the
afternoon together with air temperature. AA emissions and ambient
concentrations closely followed diurnal increases in air temperature
with both reaching a maximum in the afternoon (16:30). In contrast, MeOH
emissions peaked around midday (12:00), and this pattern resulted in the
AA/MeOH emission and concentration ratios peaking in the early evening
(17:30). Ecosystem AA/MeOH emission ratios increased as a function of
air temperature from a low of 28% in the early morning to 158% in the
afternoon (Figure 8b ). Likewise, AA/MeOH concentration ratios
also increased as a function of air temperature from a low value of 41%
in the early morning to 188% in the afternoon (Figure 8a ).
We also analyzed ambient concentration time series of AA and MeOH as a
function of ambient temperature above a mixed forest canopy in Alabama,
USA and a citrus grove in California. Similar to the poplar plantation
in Belgium, ambient concentrations of AA and MeOH above the mixed
forested canopy in Alabama and citrus grove in California followed
strong diurnal patterns tightly coupled with air temperature, with air
temperature having a positive impact on AA/meOH ambient concentration
ratios (Figure 9 ). While ambient concentrations increased
during the day relative to the night in the Alabama site (Figure
9a ) in a similar way to the Belgium site (Figure 8a ), ambient
concentrations decreased during the day at the California site
(Figure 9b ). This may be related to a shallow boundary layer at
night where emissions are concentrated, followed by a rapid increase in
the boundary layer height during the day due to increased turbulent
mixing. In addition, the site had the highest ambient concentrations of
AA (3.6-5.6 ppb) when compared with the Belgium and Alabama sites.
Despite the high ambient backgrounds and dilution effect due to
turbulent mixing during the day, the MeOH/AA concentration ratio at the
California grove increased linearly with air temperature from a low of
19% at night to 29% in the afternoon. This is comparable to the
pattern at the Alabama mixed hardwood forest site where MeOH/AA
concentration ratios also increased linearly with air temperature from
< 5% during early mornings below 20 ºC, to high values of
25% during afternoon air temperatures up to 32 ºC. Given that the
MeOH/AA concentration ratios increased with temperature, but remained
below 30%, these values are comparable to AA/MeOH temperature
sensitivities observed in the hydrated AIR, leaf, and branch poplar
studies.