i Effect of Termination of Long-term Free Air CO2 Enrichment on Physiology and Carbon Allocation

172 Pages·2016·1.98 MB·English

by  Dohyoung Kim

Effect of Termination of Long-term Free Air CO Enrichment
2
on Physiology and Carbon Allocation in a Loblolly Pine Dominated Forest
by
Do Hyoung Kim
Environment
Duke University

Date:_______________________
Approved:

___________________________
Ram Oren, Supervisor

___________________________
Gabriel G. Katul

___________________________
James S. Clark

___________________________
Sari Palmroth

___________________________
Jean-Christophe Domec

Dissertation submitted in partial fulfillment of
the requirements for the degree of Doctor of Philosophy in
Environment in the Graduate School of
Duke University

2016

i
v
ABSTRACT
Effect of Termination of Long-term Free Air CO Enrichment
2
on Physiology and Carbon Allocation in a Loblolly Pine Dominated Forest
by
Do Hyoung Kim
Environment
Duke University

Date:_______________________
Approved:

___________________________
Ram Oren, Supervisor

___________________________
Gabriel G. Katul

___________________________
James S. Clark

___________________________
Sari Palmroth

___________________________
Jean-Christophe Domec

An abstract of a dissertation submitted in partial fulfillment of
the requirements for the degree of Doctor of Philosophy in
Environment in the Graduate School of
Duke University

2016
Copyright by
Do Hyoung Kim
2016
Abstract
The work described here focuses on the last period of enrichment at the Duke
FACE site, as well as the following two years post-enrichment, as the forest acclimates to
the decreased availability of CO . The first chapter, Response to CO enrichment of
2 2
understory vegetation in the shade of forests (Kim et al. 2016) states that “Responses
of forest ecosystems to increased atmospheric CO concentration have been studied in
2
few free-air CO enrichment (FACE) experiments during last two decades. Most studies
2
focused principally on the overstory trees with little attention given to understory
vegetation. Despite its small contribution to total productivity of an ecosystem,
understory vegetation plays an important role in predicting successional dynamics and
future plant community composition. Thus, the response of understory vegetation in
Pinus taeda plantation at the Duke Forest FACE site after 15-17 years of exposure to
elevated CO , 6-13 of which with nitrogen (N) amendment, were examined.” Previous
2
work at the Duke FACE site concluded that Toxicodendron radicans (poison ivy) will
thrive in a future with CO -enriched atmosphere (Mohan et al. 2006) and that among the
2
subcanopy species, shade-tolerant species will be able to take advantage of eCO , thus
2
perform better than shade intolerant species (Kerstiens 2001; Ellsworth et al. 2012). From
Kim et al. (2016) “aboveground biomass and density of the understory decreased across
all treatments with increasing overstory leaf area index (L). However, the CO and N
2
treatments had no effect on aboveground biomass, tree density, community composition
and the fraction of shade-tolerant species.” Indeed, none of the vine species, including

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poison ivy, showed a treatment-induced enhancement of biomass or density. “The
increases of overstory L (~28%) under elevated CO resulted in a reduction of light
2
available to the understory (~18%) sufficient to nullify the expected growth-enhancing
effect of elevated CO on understory vegetation.” Alternatively, L of the untreated stand
2
was already so high, that the increase of overstory L with CO caused only a small
2
change in light availability below the overstory canopy. Under such deep shade, it is
possible that light was too limiting for any species to respond to CO , with or without N
2
addition.
The discrepancy between earlier results (Mohan et al. 2007) and these may reflect
the difference in the size of the sample plots. Previous studies investigated relatively
small plots (1.44 m2), small enough to be affected by local variation in L and, thus, light.
The small sample size, may have biased the results, especially because these studies were
done over the period of time before the canopy recovered from the damage it endured
during the December 2002 ice storm (McCarthy et al. 2006). The current study used data
from harvest of 40% of the area in each plot, representing >100 m2 for each CO × N
2
treatment combination, and the stand canopy completely recovered following eight years
without disturbance. In conclusion, there is no indication that elevated atmospheric CO ,
2
in forests on N-poor or rich soils, will affect the understory density, biomass or
composition under intact canopy.
The following investigation, on the effect of eCO termination on
2
belowground C allocation, required to first assess what impact it had on C uptake in

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photosynthesis. To do so, it is possible to use the tested assumption that c/c is unaffected
i a
by eCO (Ellsworth et al. 2012) and, because c following termination was as in aCO2
2 a
plots, differences in photosynthesis would be proportional to difference in mean canopy
stomatal conductance multiplied by L (i.e., total canopy conductance). Mean canopy
conductance of each species is calculated from scaled sap-flux, and the simplest approach
to the calculation is to assume that leaves are well coupled to the atmosphere, an
assumption that was verified for P. taeda, but not for the broadleaved species in the
stand. A more complete data set from an adjacent stand was used to test the assumption,
as captured in Kim et al. (2014) Sensitivity of stand transpiration to wind velocity in a
mixed broadleaved deciduous forest. “Wind velocity (U) within and above forest
canopies can alter the coupling between the vapor-saturated sub-stomatal airspace and the
drier atmosphere aloft, thereby influencing transpiration rates. In practice, however, the
actual increase in transpiration with increasing U depends on the aerodynamic resistance
(R ) to vapor transfer compared to canopy resistance to water vapor flux out of leaves
A
(R , dominated by stomatal resistance, R ), and the rate at which R decreases with
C stom A
increasing U. We investigated the effect of U on transpiration at the canopy scale using
filtered meteorological data and sap flux measurements gathered from six diverse species
of a mature broadleaved deciduous forest. Only under high light conditions, stand
transpiration (E ) increased slightly (6.5%) with increasing U ranging from ~0.7 to ~4.7
C
m s-1. Under other conditions, sap flux density (J ) and E responded weakly or did not
s C
change with U. R , estimated from Monin-Obukhov similarity theory, decreased with
A

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increasing U, but this decline was offset by increasing R , estimated from a rearranged
C
Penman-Monteith equation, due to a concurrent increase in vapor pressure deficit (D).
The increase of R with D over the observed range of U was consistent with increased
C
R by ~40% based on hydraulic theory. Except for very rare half-hourly values, the
stom
proportion of R to total resistance (R ) remained < 15% over the observed range of
A T
conditions. These results suggest that in similar forests and conditions, the direct effect of
U reducing R and thus increasing transpiration is negligible. However, the observed U-
A
D relationship and its effect on R must be considered when modeling canopy
stom
photosynthesis.” Because, in the FACE site, D was measured within the canopy, and was
not affected by CO , one can proceed to calculate mean canopy stomatal conductance
2
from sap flux, L and D.
Before termination of eCO , mean canopy stomatal conductance was lower under
2
eCO than aCO . Indeed, canopy conductance was occasionally lower, even accounting
2 2
for the higher L under eCO (Chapter III. Response of stomatal conductance to
2
termination of long-term CO enrichment). Following termination of enrichment, the
2
increase in mean canopy stomatal conductance more than compensated for the reduction
in L, resulting in occasionally higher total canopy conductance in plots previously
subjected to eCO . Given the likely similarity of c/c , it seems safe to assume that canopy
2 i a
photosynthesis was higher in these plots than plots subjected to aCO all along, or at least
2
that canopy photosynthesis is not different. Thus, changes in soil CO efflux (F ) must
2 CO2

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reflect changes in C allocation belowground, and the consequences to belowground
constituents.
Armed with this information, it is possible to interpret the effect of termination of
eCO , as presented in Chapter IV (Dynamics of soil CO efflux under varying
2 2
atmospheric CO concentrations reveal dominance of slow processes). In this chapter,
2
we evaluated the effect on soil respiration (R ) of sudden changes of photosynthetic rates
S
by altering CO enrichment in plots subjected to +200 ppmv for 15 years. Five-day
2
intervals ranging 1.0 - 1.8 × ambient did not affect F . Only ~160 days following eCO
CO2 2
termination did F decrease, longer than the 10 days observed for experimental
CO2
blocking of C flow to belowground, but shorter than >400 days it took for increase of
F following initiation of CO enrichment. The reduction of F upon termination of
CO2 2 CO2
enrichment (~35% in native and N-fertilized soils) cannot be explain by the reduction in
leaf area (~15%) and associated carbohydrate production and allocation, suggesting a
disproportional contraction of the belowground ecosystem components; this was
consistent with the reductions in base respiration and F -temperature sensitivity. These
CO2
asymmetric responses pose a tractable challenge to process based models attempting to
isolate the effect of individual processes on R .
S

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Contents
Abstract ......................................................................................................................................... iv
List of Tables ................................................................................................................................. xi
List of Figures ............................................................................................................................ xiii
1. Introduction ............................................................................................................................... 1
2. Response to CO enrichment of understory vegetation in the shade of forests ............... 7
2
2.1 Introduction ....................................................................................................................... 7
2.2 Materials and Methods .................................................................................................. 13
2.2.1 Site description .......................................................................................................... 13
2.2.2 Data ............................................................................................................................. 14
2.2.3 Statistical analysis ...................................................................................................... 16
2.3 Results .............................................................................................................................. 19
2.4 Discussion ....................................................................................................................... 24
3. Sensitivity of stand transpiration to wind velocity in a mixed broadleaved deciduous
forest ............................................................................................................................................. 36
3.1 Introduction ..................................................................................................................... 36
3.2 Materials and Methods .................................................................................................. 40
3.2.1 Site description and data collection ........................................................................ 40
3.2.2 Data preparation ........................................................................................................ 42
3.3 Results and Discussion .................................................................................................. 46
3.3.1 Sap flux density ......................................................................................................... 46
3.3.2 Total, aerodynamic, and canopy resistance ........................................................... 48

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3.3.3 Vapor pressure deficit and its effect on resistances .............................................. 50
3.4 Conclusion ....................................................................................................................... 53
4. Response of stomatal conductance to termination of Long-term CO enrichment ....... 64
2
4.1 Introduction ..................................................................................................................... 64
4.2 Materials and Methods .................................................................................................. 66
4.2.1 Site description .......................................................................................................... 66
4.2.2 Data collection ............................................................................................................ 67
4.2.3 Model structure ......................................................................................................... 68
4.3 Results and Discussion .................................................................................................. 69
4.3.1 Environmental variables and leaf area index ........................................................ 69
4.3.2 Response of stomatal conductance ......................................................................... 70
4.3.3 Response of transpiration and canopy conductance ............................................ 72
5. Dynamics of soil CO efflux under varying atmospheric CO concentrations reveal
2 2
dominance of slow processes .................................................................................................... 81
5.1 Introduction ..................................................................................................................... 81
5.2 Materials and Methods .................................................................................................. 86
5.2.1 Site description .......................................................................................................... 86
5.2.2 Data collection ............................................................................................................ 88
5.3 Results .............................................................................................................................. 90
5.4 Discussion ........................................................................................................................ 94
Appendix A................................................................................................................................ 108
Appendix B ................................................................................................................................ 123

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