Reply to comment on ''Appropriate experimental ecosystem warming methods by ecosystem, objective, and practicality'' by Aronson and McNulty

The letter by Amthor et al. (in this issue) on our paper(Aronson and McNulty, 2009) raised some interesting questionsand noted an error in our original text. InAronson and McNulty(2009), we misstated that anthropogenic-induced global warm-ing would result from increased infrared radiation (IR), when thesentence should have read increased convection and conduction.The authors do not believe that increased greenhouse gasemissions will increase solar radiation output, or signiﬁcantlyincrease IR to the Earth’s surface. We thank Amthor et al. for thatcorrection.Theassertion that IRlamps mimictheaction of global warmingdue to increased longwave radiation on the land surface was citedfrom Harte et al. (1995). However there have been advances in thestudy of global warmingheat transfer, whichwere not considered.This assertion was erroneously repeated in Aronson and McNulty(2009) and we thank Amthor et al. for this correction as well.In our paper we made recommendations for ecosystemwarming study methods based on the literature available onsimulated ecosystem warming. Of the active warming methods inthe literature, only two have been published on extensively, andthereforelendthemselvestoreview:heat-resistancecablesburiedin or on the soil surface and IR lamps. Of these, only IR lampstransfer heat to the soil and air above the surface without directcontactofaheatingelementonthesoil.GiventhatIRlampsdonotcontact the soil surface directly, and therefore minimize in situdisturbance, we stand by our assessment of these methods.However, any new technologies which can actively heat the soiland surrounding air effectively and better mimic the action bywhich global warming will warm the soil, would be welcomed bythe scientiﬁc community.Ourdiscussionofpassivenighttimewarmingisconsistentwiththe current IPCC assessment, in that changes in downwardlongwave radiation affect nighttime or winter surface tempera-tures, as stated by Denman et al. (2007) ‘‘... the nighttimetemperature change to be a result of increased nighttimecloudiness, and hence downward longwave radiation connectedto the increase in aerosols [in the atmosphere]’’. As Amthor et al.state, this phenomenon is constrained to speciﬁc geographic (i.e.colder) regions. We asserted that study sites also need to havesigniﬁcant radiation input for this method to be viable. Thecombinationofthesefactorsmaybewhythismethodismostoftenused at the mid-latitudes (Aronson and McNulty, 2009). We didnot advocate the expansion of this experimental practice, butrather that of chamber-based methods for passive warmingexperiments.The notation by Amthor et al. on the effect of various warmingmethods on water vapor pressure differences (VPD) is indeed animportant (and confounding) impact of ecosystem response toglobal warming.However, we couldnot locatea sufﬁcient numberof published studies that regulated (or adequately measured)relative humidity (RH) in conjunction with ecosystem warming toevaluate the practicality of alternative warming methods onregulating RH. While we agree with Amthor et al. that some formsof ecosystem warming should lend themselves more easily to themanipulation of RH in conjunction with warming, there was littledata to support the practical application of this theory at the timeour article was written. Given that data was lacking to assess therelative effectiveness of various passive or active warmingtreatments, it was not possible to factor RH into an overallassessment of warming method applicability. We agree withAmthor et al. that this issue deserves attention in future warmingexperiments.Althoughsomeformsofactivewarming(e.g.activechambers)may well be conceptually or scientiﬁcally more robust methodsfor ecosystem warming studies as stated by Amthor et al., themost extreme warming is predicted to occur in the far northernlatitudes, where less economic development limits the distribu-tion of electrical power with which to perform active warming.There is also a deﬁcit of experimental warming research intropical regions, due to similar concerns. It would not beadvantageous to the overall study of climate change impacts toonly focus on those areas of the planet with sufﬁcient infrastruc-ture to permit active warming experiments. In recognition of