A comment on ``Appropriate experimental ecosystem warming

Agricultural and Forest Meteorology 150 (2010) 497–498
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Agricultural and Forest Meteorology
journal homepage: www.elsevier.com/locate/agrformet
A comment on ‘‘Appropriate experimental ecosystem warming methods by
ecosystem, objective, and practicality’’ by Aronson and McNulty§
Jeffrey S. Amthor a,*, Paul J. Hanson b, Richard J. Norby b, Stan D. Wullschleger b
U.S. Department of Energy, SC-23/Germantown Building, 1000 Independence Ave., SW, Washington, DC 20585-1290, United States
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422, United States
Article history:
Accepted 24 November 2009
The expected magnitude of climatic-change-induced temperature increases in terrestrial ecosystems during the coming 50–100
years dictates that a thoughtful program of scientific research on
potential ecological effects of global warming be executed. A key
component of this program is experimental manipulation of
temperature in field studies. That temperature manipulation can
be accomplished with myriad types of chambers or with various
configurations of overhead infrared (IR) radiation emitting lamps.
A number of criteria—financial, logistic, and scientific—can be used
to choose or recommend a particular method of ecosystem
warming. Any realistic approach will warm both aboveground
and belowground ecosystem components.
A recent article in this journal by Aronson and McNulty (2009;
hereafter ‘‘A&McN’’) claimed that the ‘‘method. . .that is most true to
climate change predictions, is IR heating lamp[s]’’ (p. 1791). This
conclusion—which has also been drawn elsewhere—is based on a
misconception of the mechanisms of global warming. While a
complete description of the mechanisms of global warming is
lengthy and complex, a useful summary is (a) increased greenhouse
gas concentrations increase absorption of upwelling longwave
radiation by the atmosphere, (b) the atmosphere therefore becomes
warmer (its kinetic energy increases), and (c) this increases the
temperature of plants and soils in contact with the warmer
atmosphere through convection and conduction. On the contrary,
A&McN (p. 1792–1793) stated that ‘‘natural ecosystem heating is in
the form of greater infrared (IR) radiation incidence on organisms
and the Earth surface from an atmosphere that is holding more of
this radiation with increased levels of greenhouse gasses in
comparison with pre-industrial levels. An ecosystem heating
This letter was written by a U.S. Government employee and U.S. Government
contractor employees. The views and opinions of the authors expressed herein do
not necessarily state or reflect those of the U.S. Government or any agency thereof.
* Corresponding author. Tel.: +1 301 903 2507; fax: +1 301 903 8519.
E-mail addresses: jeff.amthor@science.doe.gov (J.S. Amthor), hansonpj@ornl.gov
(P.J. Hanson), norbyrj@ornl.gov (R.J. Norby), wullschlegsd@ornl.gov
(S.D. Wullschleger).
0168-1923/$ – see front matter . Published by Elsevier B.V.
method that replicates nature would ideally use the same form of
heat (i.e., radiation as opposed to conduction or convection)’’ [italics
added]. Although global warming causes a small increase in
downwelling longwave radiation because the atmosphere is
warmer, the predominant cause of global-warming-induced plant
and soil warming is contact with warmer air (note that IR lamps used
in ecosystem warming experiments do not simulate the increase in
downwelling longwave radiation associated with global warming in
either spectral distribution or energy flux density). Thus, the claim in
A&McN (p. 1796) that ‘‘infrared lamps are the best method for
replicating natural. . .ecosystem warming conditions [because as
warming continues] there will be an increase in IR from the
atmosphere’’ is inconsistent with the main mechanisms of ecosystem warming from climatic changes caused by increasing atmospheric greenhouse gas concentrations.
A&McN also exaggerated a similarity between actual global
warming and experimental warming methods involving ‘‘passive
nighttime warming.’’ A&McN stated that ‘‘passive nighttime
warming usually involves an IR reflective nighttime [curtain]
covering the soil and low-lying vegetation to trap the IR that has
accumulated inside the soil and on the soil surface over the course
of the daylight hours’’ (p. 1793). The notion of IR radiation
accumulating inside the soil and on the soil surface is questionable,
and the A&McN statement that the ‘‘replication of natural heating
from [the method of passive nighttime warming] is quite high, in
terms of only using radiative heat transfer’’ (p. 1793) is inaccurate.
Not only do passive nighttime warming experimental methods
involve a mechanism inconsistent with actual ecosystem warming,
the premise of nighttime-only warming can also be challenged.
That premise was based on a decrease in the so-called diurnal
temperature range (DTR)—the difference between nighttime
minimum temperature and daytime maximum temperature.
DTR was observed to decrease at the global scale from 1950 to
1980, but since then the global trend in DTR disappeared, albeit
significant geographic variability in DTR trends remains (Trenberth
et al., 2007). Also, general circulation model (GCM) projections
indicate both future increases and decreases in geographically
specific DTR by the end of this century (Meehl et al., 2007).
Nonetheless, earlier reports of general declines in DTR led
Luxmoore et al. (1998; as cited by A&McN) to propose that
nighttime-only warming was an important process worthy of
study as an isolated factor. The more recent analyses of observed
and projected DTRs cited above, however, indicate that nighttime-
J.S. Amthor et al. / Agricultural and Forest Meteorology 150 (2010) 497–498
only warming experiments would probably be of limited value to
understanding effects of future warming on terrestrial ecosystems.
An important methodological issue not mentioned by A&McN is
the physical effect of warming on atmospheric humidity and the
difference in water vapor pressure (VPD) between substomatal
cavities and the local atmosphere. As Kimball (2005) noted, ‘‘one
problem that has been identified for infrared heating of
experimental plots is that the vapor pressure gradients. . .from
inside the leaves to the air outside would not be the same as would
be expected if the warming were performed by heating the air
everywhere (i.e. by global warming).’’ The same problem could
occur with any method of warming if humidity is not also
controlled. For example, an experimental increase of air temperature from 25 8C to 29 8C, with 50% relative humidity (RH) in the
25 8C atmosphere, would cause a drop in RH to a value of less than
40% in the warmed ecosystem. This in turn would increase VPD
from about 1.1 kPa to 1.8 kPa if leaf temperature is 3 8C less than air
temperature, meaning that an experimental plot-based temperature manipulation without concomitant control of humidity can
result in a parallel moisture manipulation. Although it remains
unclear how humidity will be affected by future global warming—
and it will likely depend on local and regional conditions and vary
temporally—we assume as a first approximation, based on GCMs,
that RH (as contrasted with vapor pressure per se) will be
approximately conserved. Warming will, in any case, affect some
aspect(s) of atmospheric water vapor (content, RH, VPD, etc.) so an
objective of warming experiments should be to account for
changes to atmospheric water vapor in a way that improves
understanding of integrated ecosystem responses to future global
warming. Humidity control via addition of infrastructure to add
water vapor to warmed experimental plots could therefore be
needed. This may be difficult to achieve with an IR-lamp system (to
our knowledge humidity control in an IR-lamp experiment has not
yet been implemented), and might nullify one advantage of the IRlamp approach; the relative lack of experimental infrastructure
that might alter micrometeorology. (Non-chambered systems
have other potential advantages, including relatively unencumbered movement of flora and fauna into and out of experimental
plots.) On the other hand, well-designed chamber-based warming
experiments can accommodate parallel control of humidity (e.g.,
Bronson et al., 2009). This will become more important as field
experiments explore the full range of temperature and humidity
changes possible during the coming decades.
While we maintain that several methods of experimental
temperature manipulation in terrestrial ecosystems (including IRlamp systems) can be of value and produce important scientific
insights, the choice of method should be based on sound science,
not a misunderstanding of the physics of climatic change. The
physically ‘‘realistic’’ approach is to warm the air enveloping an
ecosystem. Furthermore, because the time scale of ecosystem
experiments is short relative to the time scale of ongoing global
warming, active warming of the soil will generally be needed
because the rate of future atmospheric warming will be
accompanied by a parallel rate of soil warming. Although multiple
experimental methods can be of value, we emphasize that active
warming approaches (compared with passive warming
approaches) allow a wider range of scientific questions to be
answered with appropriate quantitative control of the temperature treatments. We note that A&McN gave short shrift to active
warming in chambers by lumping that approach in with passivewarming chambers. Moreover, approaches that can increase
temperature at least 4 8C above ambient temperature, and that
use a number of distinct controlled-temperature levels (e.g.,
ambient temperature, +1 8C, +2 8C, +3 8C, +4 8C, etc.), have the
potential to provide insights about nonlinear and threshold
responses of ecosystems to warming. These responses may be
key to understanding future effects of global warming on the
biosphere. In short, the A&McN conclusion that ‘‘the most effective
ecosystem warming methods, in terms of realistic global warming
impacts, are IR lamps and passive night warming’’ (p. 1796) is
unjustified and incorrect. Future terrestrial ecosystem warming
studies should carefully consider the implications of selecting an
experimental methodology so that the best science is produced to
inform critical policy decisions.
Aronson, E.L., McNulty, S.G., 2009. Appropriate experimental ecosystem warming
methods by ecosystem, objective, and practicality. Agricultural and Forest
Meteorology 149, 1791–1799.
Bronson, D.R., Gower, S.T., Tanner, M., Van Herk, I., 2009. Effect of ecosystem
warming on boreal black spruce bud burst and shoot growth. Global Change
Biology 15, 1534–1543.
Kimball, B.A., 2005. Theory and performance of an infrared heater for ecosystem
warming. Global Change Biology 11, 2041–2056.
Meehl, G.A., Stocker, T.F., Collins, W.D., Friedlingstein, P., Gaye, A.T., Gregory, J.M.,
Kitoh, A., Knutti, R., Murphy, J.M., Noda, A., Raper, S.C.B., Watterson, I.G.,
Weaver, A.J., Zhao, Z.-C., 2007. Global climate projections. In: Solomon, S.,
Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller,
H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Cambridge
University Press, Cambridge (U.K.), pp. 747–845.
Trenberth, K.E., Jones, P.D., Ambenje, P., Bojariu, R., Easterling, D., Klein Tank, A.,
Parker, D., Rahimzadeh, F., Renwick, J.A., Rusticucci, M., Soden, B., Zhai, P.,
2007. Observations: surface and atmospheric climate change. In: Solomon, S.,
Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller,
H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Cambridge
University Press, Cambridge (U.K.), pp. 235–336.