Agricultural and Forest Meteorology 150 (2010) 497–498 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Discussion 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 a 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 A R T I C L E I N F O 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: firstname.lastname@example.org (J.S. Amthor), email@example.com (P.J. Hanson), firstname.lastname@example.org (R.J. Norby), email@example.com (S.D. Wullschleger). 0168-1923/$ – see front matter . Published by Elsevier B.V. doi:10.1016/j.agrformet.2009.11.020 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- 498 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. References 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. 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