Microclimatic Buffering in Forests of the Future: the Role of Local Water Balance

Ecology in a human-dominated World - - 1

C and 1.1 kPa lower on average, respectively, compared to areas to areas compared respectively, on average, lower C and 1.1 kPa ° 1–11 –––––––––––––––––––––––––––––––––––––––– Oikos Society Nordic Ecography © 2018 Authors. © 2018 The without canopy cover. Canopy buffering of temperature and vapor pressure deficit was vapor pressure and buffering of temperature Canopy without canopy cover. with water balance, implying that and varied of canopy cover, at higher levels greater in the project changes We hydrology. are subject to changes in local buffering effects the such changes may impact water balance for the mid-21st century how and predict some suggest that results Our climate extremes. buffer to forests US of western ability as sites become increasingly will lose their capacity to buffer climate extremes forests combined with accelerating canopy losses water limited. Changes in water balance potentially of disturbance will create and severity in the frequency due to increases US forests. conditions of western non-linear changes in the microclimate Keywords: climate extreme, microclimate buffering, water balance microclimate climate extreme, Keywords: Introduction all of 80% comprising planet the on ecosystem terrestrial dominant the are Forests et al. 2013). This plant biomass and harboring the majority of species on Earth (Pan (Ricklefs trees by created microclimates of part in due is diversity the to biodiversity et al. 2000, Cardelús 1998, Chen et al. 1999, Gehlhausen and Briones 1977, Hietz long have microclimates While forest et al. 2006). 2005, Grimbacher and Chazdon et al. 2003), the implications of microclimates been studied (Chen et al. 1999, Geiger vapor pressure deficit (VPD), with biologically meaningful effect sizes. For example, deficit (VPD), with biologically meaningful effect sizes. pressure vapor 50% for VPD under at least and season, maximum temperature during the growing 5.3 est canopy were Forest canopies buffer climate extremes and promote microclimates that may function microclimates and promote canopies buffer climate extremes Forest the biophysical for understoryas refugia species under changing climate. However, buffering and its stability through and maintain microclimatic conditions that promote buffering is sensi microclimatic posited that forest We largely unresolved. time are during the this effect measured and we and canopy cover, to local water balance tive States of the northwestern United a climate gradient in forests season across growing and temperature of maximum canopies buffer extremes found that forest We (US). 42

Editor-in-Chief: Miguel Araújo Miguel Editor-in-Chief: 2018 7 June Accepted 42: 1–11, 2019 doi: 10.1111/ecog.03836 Normand Signe Editor: Subject Ecography K. T. Davis (http://orcid.org/0000-0001-9727-374X) ([email protected]) and P. E. Higuera, Dept of Ecosystem and Conservation of Ecosystem Sciences, Dept E. Higuera, and P. ([email protected]) (http://orcid.org/0000-0001-9727-374X) Davis T. K. USA. – Z. A. MT, Missoula, of Montana, Univ. Management, of Forest and KTD, Dept – S. Z. Dobrowski USA. MT, Missoula, of Montana, Univ. ID, USA. Moscow, of Idaho, Univ. of Geography, Dept Abatzoglou, T. USA. – J. MT, 1, Missoula, Service Region Forest U.S. Holden, Kimberley T. Davis, Solomon Z. Dobrowski, Zachary A. Holden, Philip E. Higuera and John T. Abatzoglou and John Philip E. Higuera A. Holden, Zachary Solomon Z. Dobrowski, T. Davis, Kimberley the role of local water balance of local water the role Microclimatic buffering in forests of the future: of in forests buffering Microclimatic Research www.ecography.org ECOGRAPHY doi: 10.1111/ecog.03836 Ecology in a human-dominated World and watertheirtiming (i.e.actualevapotranspiration microclimatic bufferingbecausethe interactionofenergy 1990). Awater-balanceframework isusefulforassessing availability ofenergyanduseablewater for plants(Stephenson of thelocalwaterbalancewhich describestheconcomitant Consequently, we examinethese processes through thelens flux, which requires evapotranspiration in order tooccur. conversion ofthisenergytolatentasopposedsensibleheat radiationand tion andattenuationofincidentshortwave and Dearing 2014,Locosselli etal.2016,Lenoir2017). from regional conditionstobeconsidered ‘decoupled’ (Varner strong this effectmustbeandhow longasitemustbeisolated of regional influences, althoughthere isambiguityabouthow dynamics are driven by processes thatare largelyindependent and Dearing 2014).Under theseconditions, localclimate climatic conditions, for example within talus slopes (Varner a microclimate atasiteiseffectively isolatedfrom macro regional climatetrends. In contrast,decouplingoccurswhen cit). However, microclimates inbuffered areas maystilltrack (e.g.dailytemperature orvapor pressurethe understory defi matic buffering is the moderation of extreme conditions in related phenomenon(Lenoir etal.2017).Forest microcli ing toconfusiononthetopic. We theseasseparatebut view the terms‘buffering’ and ‘decoupling’ loosely, add further on biotaare largelyunresolved. Research inthisarea uses matic bufferingforunderstandingclimatechangeimpacts in a changing climate, and the implications of microcli (Hannah etal. 2015,McLaughlin etal. 2017). atsitesbecomingwarmeranddrier be transient,particularly ing isephemeral,thenrefugia created by forest canopieswill Frey etal. 2016,Kovacs etal. 2017).If microclimatic buffer 1998, Suggitt etal. 2011,Ashcroft andGollan 2013, (1–3 yr)collectionsofmeteorological data(Breshears etal. stood, asmoststudiesare term descriptive andbasedonshort throughcapacity offorests timeispoorlyunder mayvary (Lenoir etal. 2017).However, ifandhow thebuffering create conditionsthatare decoupled from regional warming that forest canopies,incombinationwithtopography, can canopies onregional warming.Further, ithasbeensuggested was presumed tobeduethemoderatingeffectofforest sites withlower canopycover (De Frenne etal. 2013).This of compositionalshiftstowards warmadaptedtaxathan communities indensetemperateforests showed lessevidence McLaughlin etal. 2017).For plant example,understory (Dobrowski 2011,Keppel etal. 2012,Hylander etal. 2015, mate conditionsamidstunfavorable regional conditions as microrefugia, locationsthatprovide favorable localcli This bufferingmay promote microclimates thatfunction Suggitt etal. 2011,von etal. 2013,Frey Arx etal. 2016). environments from climateextremes (Chen etal.1999, and naturalcauses. rapid warminganddeforestation driven by anthropogenic given concernsabout ingly garneringattention,particularly for understandingclimatechangeimpactsonbiotaisincreas 2 Forest microclimatic buffering isdue largely to intercep The magnitudeofcanopybufferingeffects,theirstability It is well known to that buffertrees understory serve ------The twositesin Oregon are too closetodifferentiate onthemap. Figure Field methods Material andmethods to bufferclimateextremes inthefuture? 3) What are theimplicationsofthisforabilityforests withlocalwaterbalance? does thisbufferingeffect vary pressure deficitextremes duringthegrowing season?2) How what extentdoforest canopiesbuffertemperature and vapor ity of forests to buffer climate extremes? Specifically, 1)to time. Here we ask:how willclimatechangeaffecttheabil our understanding of the stability of microclimates through balance andmicroclimatic bufferinginforests mayimprove Therefore, understandingthe relationship between water and Gollan 2013,von etal.Holden Arx etal. 2016). ering varies by regional hydrological conditions(Ashcroft at a site. Indeed, evidence suggests that microclimatic buff turn, shouldinfluencetheamountofmicroclimatic buffering deficit) drives theratio oflatenttosensibleheatfluxandin and unmetatmosphericdemandforwater–climatic ground, within radiation shields that perform ground,similarly to within radiation shields that perform to aPVC poleat2m(‘high’) and 10 cm (‘low’) above the LogTag Recorders, Auckland, New Zealand) were attached sensors (accurate to 0.1 At eachsubplot,LogTag temperature andrelative humidity tion, logisticalproblems resulted inatotaloffive subplots. minimize edge effects. At one site, Lubrecht-high eleva mum distance from the control site ( maintained similarslopeandaspect,amini cally placedinawaythatcaptured arangeofcanopy covers, Surrounding eachclearing,sixsubplotswere opportunisti diameter, asareference whichserved fornocanopycover. for atotalofsixsites(Table 1). established twosites,oneatlow andoneathighelevation, Forest inGreenough, Montana. At eachlocationwe Idaho; and the Univ. of Montana Lubrecht Experimental Forest, National Oregon; theClearwater Forest innorthern U.S. (Fig. 1):theOregon State Univ. Dunn Experimental three locations across a climate gradient in the northwestern We measured microclimatic bufferingofforest canopiesat Sites were centered onaforest clearingatleast100min 1. Map of the study sites USA. (triangles) in the northwest ° C and 0.1% RH; model HAXO-8, ~ 100 m) in order to - - - - - Ecology in a human-dominated World

------3 is ’ C) ° Elev. ‘ 7.42 7.50 12.26 13.81 10.42 10.85 MNT ( C) ° 27.64 25.67 26.72 23.49 27.97 23.66 MXT ( 137 325 383 353 270 434 CWD (mm) 504 469 483 444 378 389 AET (mm) We modeled solar radiation, average wind speed, and and wind speed, modeled solar radiation, average We with daily 4-km resolution precipitation data described described data precipitation resolution 4-km daily with (2013) to run a simple soil water-balance Abatzoglou by et al. (2013) to run at Dobrowski model adapted from near-surfaceestimate better soil mois To daily time steps. and its effect on sensible heating, soil water holding ture precipitation at 100 mm, such that any capacity was fixed that capacity was assumed to be runoff or lost to exceeding deeper soil layers. the high sensor in the open (delta low-to-high maximum maximum the high sensor in the open (delta low-to-high minimum delta low-to-high (ΔMXT.LH); temperature VPD (ΔVPD. delta low-to-high (ΔMNT.LH); temperature cli standard between LH)), to help quantify the differences the above m 2 stations weather from (derived products mate conditions that understory and microclimate ground) plants experience. of daily for each subplot to use as predictors soil moisture total solar radiation, and ΔVPD. Daily ΔMNT, ΔMXT, extracted at each subplot data were and air temperature grids described by (~ 250 m) resolution 8 arc-second from the northwest et al. (2016) and extended to cover Holden speed data at each site was Wind ern US study domain. mesoscale analysis (RTMA; the real-time from retrieved the wind speed approximate To et al. 2011). Pondeca De used we at the time of maximum daytime temperature, RTMA The 2200Z (3 pm local time). from the value model data with surfaceblends weather observations from no locations with in exclusively almost sited stations not meant to are modeled values Our canopy cover. level values at the ground the actual wind speed reflect site instead meant to capture at each subplot but are associated with landscape position and differences level represent They to dominant wind directions. exposure used were These data an index of windiness for each day. Data were then aggregated to daily maximum and mini to daily maximum then aggregated were Data deficit, and pressure maximum vapor mum temperatures, subplot with varying each For humidity. minimum relative daily buffering (delta maxi calculated we canopy cover, (ΔMXT);mum temperature delta minimum temperature subtracting VPD (ΔVPD)) by (ΔMNT); delta maximum with sensor post the reference from at each sensor the values did this for each day at each site and for we no canopy cover; also calcu We and high). each sensor post height (i.e. low and sensors in the forest the low between lated the difference

- - 595 1880 1708 1350 1394 1261 RH Elev. (m) Elev. C) and C) ° ) with the fol 0.87). Canopy air 12 . P =

2 243 –114.548 –113.323 –123.316 –123.300 –114.683 –113.443 Longitude + T () ) × is temperature (in temperature is T 62 Latitude 46.5462 46.9125 44.7058 44.6809 46.5546 46.9028 17 () ( () 0.84; Supplementary material Appendix 1 Appendix material 0.84; Supplementary p

100 = / ex

2 RH 6112 at .. 0 =× =× ) and effective water pressure of the air ( water pressure ) and effective Vapor pressure deficit was calculated for each sensor at deficit was calculated pressure Vapor t rs sat ai sa P PP PT Clw high high Lubrecht Arx et al. 2013): ( where equations, lowing 2008, von and Unsworth Monteith humidity (cf. is relative sensors were removed from the analysis. from removed sensors were saturated between each time step (30 min) as the difference from individual sensors within each site that were greater greater individual sensors within each site that were from site mean the overall deviations from standard than three total, In visually inspected for potential sensor failures. were low high sensors and 0% of data from 2.8% of the data from All climate data were both visually and quantitatively checked both visually and quantitatively All climate data were within a site values temperature Mean for potential errors. and observations calculated for each hour of each day, were for dates that included data from all sites. for dates that included data from Analysis Sensors were deployed over three years: from August through through August from years: three over deployed were Sensors (May–June) snowmelt following in 2014 and October in 2015 and 2016. Analysis was conducted October through cover was also highly correlated between the point and between was also highly correlated cover stand scales (R further used for analysis. were measurements A1). Point Fig. graph and the ‘CanopyApp’. Measurements with this app Measurements graph and the ‘CanopyApp’. a spherical from to point measurements compared were A2) 1 Fig. material Appendix densiometer (Supplementary (R correlated strongly and the two were number of points with canopy divided by the total number by number of points with canopy divided post, sensor the of point the at Second, measured. points of facing photo with an upward was measured canopy cover First, at the ‘stand’ scale, a densitometer was used to note densitometer scale, a ‘stand’ at the First, direction points in each cardinal of canopy at five presence each measurement 2 m between the sensor post with from the from was derived cover canopy (total 20 points). Percent al. 2013). The sensors 2013). et al. conditions (Holden forested and humidity every 30 min. and relative temperature recorded at two scales. at each subplot was measured Canopy cover commercially available non-aspirated Gill shields in open shields in open non-aspirated Gill available commercially Clw low low Lubrecht Site Dunn low Dunn high Table 1. Location and climate conditions of each study site. Actual annual evapotranspiration (AET) and climatic water deficit (CWD) are deficit (CWD) and climatic water (AET) Actual annual evapotranspiration site. study each climate conditions of 1. Location and Table (Dobrowski et al. normals from 1980–2009 30 yr climate mean minimum temperature (MXT) and temperature 2013). Mean maximum site. at each at 2 m in a clearing as measured (June–September) 2015 and 2016 seasons for the growing averaged (MNT) are elevation. Ecology in a human-dominated World at thereference site,whilenegative values (blue)indicate moisterconditionsthanreference conditions. microclimatic bufferingacross therangeofconditions experiencedatthesite. Positive values (pink)indicate overall drierconditionsthan than thereference sensor. We propose tousethearea between these two linesasameasure of‘bufferingcapacity’ (BC)becauseit represents higher orlower VPD thanthereference sensor dependingonthereference sensoralwayshaslowerVPD. At site(B)theunderstory VPD line represents the 1:1 relationship. The slope of the two fitted lines are similar, sensorcan havehowever either at site(A) theunderstory sured outintheopenat reference sensorfortwosensorsatdifferent sites. Fitted lines(solid)are derived from alinearmodel.Thedashed Figure 2. Examples oftherelationship between maximum VPD measured andmaximum underaforestVPD mea canopy(understory) lation wasthree days;thus,tominimize potentialeffectsof found that theaverage lag periodforsignificantautocorre a temporallylagged5-foldcross validation procedure. We model whileminimizingserialautocorrelation, we employed autocorrelation withacorrelogram. To selectanoptimal models for each response variable were examinedforserial be overfit andwouldbetransferable.The residualsofthebest of selectingmodelstoensure thatourfinalmodelswouldnot repeated foreachsite. We approach chosethisconservative ing five sites was used for calibration, and the process was was leftoutandusedforvalidation, datafrom theremain pendent six-fold cross validation whereby data from one site on root meansquared error (RMSE)from aspatiallyinde sensors. Thefinalmodelforeach responsewasselectedbased models toaccountforrepeated measurements from thesame the effectsofsubplotnestedwithinsite were includedinall resent patternsintheresponse. Randomintercept termsfor included incandidatemodelstomaximize ourabilitytorep variable anduptothree-way interactionswereexplanatory modeled separately. Second order polynomialtermsforeach the subplot.Thedatafrom thehighandlow sensors were daily timestepandcontributetothelocalwaterbalanceof (temperature responses atthe only).Thesepredictors vary solar radiation,average windspeed,andminimumdailyRH models asafunctionofpercent canopycover, soilmoisture, eled ΔMXT, ΔMNT, andΔVPDwithlinearmixed effects the forests andclearingsduringthegrowing season,we mod To assessdailydifferences intemperature and VPDbetween Daily microclimatic differences 4 (A) Understory maximum VPD (kPa) 0 1 2 3 4 5 012345 Slope =0.68 Area =−0.03 Ref erence maximumVPD(k Pa ) - - - - - (B) between thisslopeandbiophysicalfactors,butwe foundthat citations therein). We initiallyexplored therelationship sensor represents free airconditions(Lenoir etal.2017and described asameasure of‘decoupling’ whenthereference asameasure ofmicroclimateserves variability andhasbeen during eachgrowing season(Fig. 2).Theslopeofthisline measurement madeatreference sensorsatthe same height perature sensor and the same or VPD for eachunderstory linear modelbetween dailymeasurements ofmaximumtem canopies across arangeofbiophysicalconditions,we fita To summarize themicroclimatic bufferingeffectofforest Buffering capacity chosen basedonlowest Bayesian informationcriterionvalue. within 0.01 the five folds. If candidatemodelshadaverage RMSE values this procedure atotaloffive times,averaging RMSEacross and assessedmodelskillofcandidatemodels,repeated time period resulting in 1178 daily observations). We then fit 5-d timestep(onedailyvalue wasretained foreachfive-day serial autocorrelation, we subsampledeachtimeseriesata effect ofthedecouplingandbuffering processes. We referto reference sites (Fig. 2). This metric captures the combined sitesare warmer/drierorcooler/wetter than which understory the fittedand1:1linewhichdiscernsbetween conditionsin of bufferingcapacitybasedonthesummedarea between conditions (Fig. 2).Therefore, we developed anovel metric ferent amountsofmicroclimatic bufferingacross different sites withsimilarslopescanactuallyhave substantiallydif Understory maximum VPD (kPa) 0 1 2 3 4 5 012345 Slope =0.67 Area =−5.49 Ref erence maximumVPD(k ° C or0.01kPa ofeachother, thetopmodelwas Pa ) - - - Ecology in a human-dominated World ------5 http:// < C lower when lower C C). Minimum C). Minimum ° cm (low) above above (low) cm ° C and –1.7 kPa, kPa, –1.7 and C ° levels. Additionally, Additionally, levels. 2 C across all levels of levels all C across ° et al. 2018). (Davis > ) was –7.2 was ) –2 C lower than reference condi than reference C lower ° C with canopy cover greater than 50%. greater C with canopy cover ° The differences between the understory and reference sites the understory between reference The differences and were less pronounced at 2 m (high) than at 10 m (high) than at 2 less pronounced were in maximum reduction The mean 3D–F). (Fig. the ground at high sensors was 0.69 temperatures and 1.4 canopy cover 0.24 kPa VPD at high sensors was on average Maximum and in the understory cover of canopy all levels lower across than 50%. ΔMXT greater with canopy cover lower 0.31 kPa soil best explained by and ΔVPD for high sensors were levels of canopy cover and 1.1 kPa (38%) lower with canopy (38%) lower and 1.1 kPa of canopy cover levels at and ΔVPD ΔMXT in Variation 50%. than greater cover the interaction between sensors was best explained by the low with a smaller effect of solar radiation and canopy cover, 1 material Appendix wind speed (Supplementary average cor negatively A2–A4). Both ΔMXT and ΔVPD were Table magnitude of the effect but the cover, with canopy related can with low was contingent on solar radiation. Specifically, as VPD decreased and temperature buffering of opy cover, at high canopy cover, contrast, In solar radiation increased. as solar radia VPD increased and buffering of temperature ΔMXT and ΔVPD 3A, C). The average (Fig. tion increased and high solar radiation in a site with 80% canopy cover m watts 310 percentile; (90th wind speed wind speeds). Increased (at average respectively maximum temperature was associated with less buffering in understoryand dif Daily sites. reference between VPD and and the forest between in minimum temperature ferences RH, minimum best explained by clearing (ΔMNT) were wind speed, and their interaction (Supplementary average was not A2–A4). Canopy cover Table 1 material Appendix of canopy cover at all levels related to ΔMNT; significantly tended to be higher in the forest the minimum temperature ΔMNT was 3.1 than in the clearing (average and clearing the forest less between differed temperature 3B). high (Fig. humidity and wind speed were when relative changing climatic conditions, we do not make adjustments in do not make adjustments we climatic conditions, changing rising CO calculations for water balance time, so will change over cover canopy how it is unknown conditions climate for BC under future made predictions we canopy cover, in a 10% increase cover, canopy with present cover. in canopy decrease and a 10% Data deposition Repository: the Dryad Digital from available Data dx.doi.org/10.5061/dryad.406ms4g Results differences microclimatic Daily in the under sensors, maximum temperatures all low Across 3.4 story on average were 5.3 and cover, canopy of levels all across tions VPD was on than 50%. Maximum was greater canopy cover in the understory all across (27%) lower 0.73 kPa average ------35) as a function of canopy cover and canopy cover 35) as a function of

= al. 2007) may partially offset this effect. Given Given et al. 2007) may partially offset this effect. (Piao 2 To examine how buffering capacity varies spatially across across varies spatially capacity buffering examine how To increased plant productivity and leaf area index with elevated index with elevated leaf area and plant productivity increased CO under the uncertainty changes in ecohydrology surrounding fer increases in evapotranspiration demands with increased demands with increased in evapotranspiration fer increases net radia deficit and increased pressure atmospheric vapor et al. 2016). However, et al. 2010, Swann tion (Donohue al. (2013) for compatibility between current current et al. (2013) for compatibility between Dobrowski water use efficiency with Increased predictions. and future buf to argued been has concentrations dioxide carbon rising (Abatzoglou 2013) using the multivariate adaptive con adaptive multivariate 2013) using the (Abatzoglou 2012). and Brown structed (Abatzoglou analogs approach from in water balance simulations applied differences We (1980–2009) estimates of to baseline projections downscaled data from 20 global climate models (Supplementary material models (Supplementary climate global 20 from data to a statistically downscaled that were A1) Table 1 Appendix ~4-km spatial grain with observational gridMET data from et al. (Dobrowski 800 m resolution 1980 to 2009 at from ratio. AET-to-deficit 2013) was used to calculate the current calculated were ratio the AET-to-deficit of projections Future 2040 to 2069 with for the years using climate projections maximum temperature. Forest canopy cover was obtained canopy cover Forest maximum temperature. 2017) and left (Dimiceli resolution MODIS at 250 m from balance data Water for predictions. resolution at its native focused on maximum VPD because of its relevance for plant VPD because of its relevance focused on maximum et al. et al. 2013, Restaino and survival (Breshears growth very for similar and patterns were 2016) and because results Using the models described above, we predicted BC for predicted we the models described above, Using of the northwestern areas forested VPD across maximum (2040–2069) time (1980–2009) and future U.S. for current We ratio. and AET-to-deficit periods based on canopy cover Extrapolating to northwestern U.S. forests U.S. to northwestern Extrapolating were linear. We included site as a random effect in all models, We linear. were estimated with Kenward–Roger were of freedom and degrees at different et al. 2014). Sensors (Kuznetsova approximations modeled separately. heights were across broader spatial scales and to improve compatibility spatial scales and to improve broader across tested for non-linear rela We climate datasets. with future models, but all patterns additive tionships with generalized 2008) with supplemental radiation and winds bilinearly inter 2008) with supplemental used a monthly (rather We (2013). Abatzoglou polated from for predictions balance model to allow than a daily) water the average AET-to-deficit ratio (AET/climatic water deficit) ratio (AET/climatic water deficit) AET-to-deficit the average ratio was models. AET-to-deficit effects with linear mixed at each subplot study period (2014–2016) the calculated over et al. (Daly PRISM climate data from with 800-m monthly we posited would covary with the water balance and canopy with the water balance posited would covary we met buffering average 3-yr this modeled We site. the of cover ric for each sensor (n biophysical gradients we calculated the average growing sea growing the average calculated gradients we biophysical which each sensor, seasons at growing the three over BC son this summed area as a ‘buffering capacity’ (BC) metric, where where (BC) metric, as a ‘buffering capacity’ area this summed conditions cooler and/or moister indicate BC values negative BC site and positive to the reference compared in the forest the opposite. indicate values Ecology in a human-dominated World maximum temperature ateachsubplotwaslarger (indicated For low sensors,the3-yraverage bufferingcapacity(BC) for Buffering capacity canopy cover wassmall(0.61 the predicted difference in minimumtemperature at80% sensors, althoughthemarginalR relationship between canopycover andΔMNTforthe2-m centile soilmoisture). There wasastatisticallysignificant and ΔVPD when soilmoisture washigh(Fig. 3D,F;ΔMXT decreased withincreasing canopycover, andata faster rate material Appendix 1 Table A2–A4). Both ΔMXT and ΔVPD moisture, canopycover, and theirinteraction(Supplementary Appendix 1Fig. A3. ences inkPa. Results for comparisonsbetween theground intheforest and2mintheopencanbefoundSupplemenatry material high sensors(2mabove ground; (D,E,F))from thelinearmixed effectsmodels. Temperature differences are indegrees Cand VPDdiffer mum sensorsandreferenceVPD (ΔVPD;(CandF))between understory sensorsintheopenforlow (10 cmabove ground; (A,B,C))and Figure 6 3. Fitted dailydifferences in maximum temperature (ΔMXT; (AandD)), minimumtemperature (ΔMNT; (BandE)),maxi (D) (A)

= Solar radiation (mean watts m–2)

Soil moisture (%) –0.60 kPa with80%canopycover, 90thper 100 200 300 20 40 0 Canop Canop 25 25 y co y co 50 50 ° C; Fig. 3E). ve ve 2 r (%) r (%) 75 75 ofthemodelwas0.03and (B) (E) Soil moisture (%) Wind (m s–1) 20 40 10 15 20 0 0 5 = 02 Canop –2.5 Minimum RH(% 25 55 ° C y co - 50 07 ve and higherAET-to-deficit ratios(F by more negative BCvalues) atsiteswithhighercanopycover Idaho) duetolow AET-to-deficit ratiosandlow canopycover. was limitedindrierforests (e.g.easternOregon andcentral western U.S.buffered maximum VPD(Fig. 5A)butthiseffect and F respectively; Fig. 4)andhighsensors(F the low (F high canopycover andhigherAET-to-deficit ratios,forboth larger (indicatedby more negative BCvalues) atsiteswith respectively; Fig. 4). Average BC for maximum VPD was nounced (F withthehighsensors,itwaslesspro tern wasalsoobserved F 1,4 r (%) 75 5 Under 1980–2009conditions, mostforests in thenorth

= ) 100 7.31, p 7.31, 1,4

= 9.30, p 9.30, 1,31 1,29 (C) (F)

= =

–2 =

16.11, p 16.11, Solar radiation (mean watts m ) 0.054, respectively; Fig. 4).Althoughthispat

30.72, p 30.72, Soil moisture (%) 100 200 300 = 20 40 0

0.041, respectively; Fig. 4). Cano Cano < 25 25 < 0.001 and F 0.001andF py py co co 50 50 ve ve 1,30 r (%) r (%) 75 75

= 1,32 1,4 39.61, p 39.61, 1,4

= 17.76, p 17.76, 15.08, p 15.08, 4.96, p 4.96, ∆ Te ∆ VPD mp < −8 −4 0 4 −2 −1 0 1 0.001and = = < 0.016, 0.087, 0.001 - - - - - Ecology in a human-dominated World - - - - - 7 r ve co py 20 50 80 Cano

Max temperature Max VPD 2.5 .0 w 1.52 Lo Climate extremes can have profound consequences for profound can have Climate extremes .0 for the distribution of understory regenera species, tree 1998, Spies and primaryand tion, (Ohmann productivity et al. 2006, Arnone et al. et al. 2003, Grimbacher Geiger et al. 2013), even Boeck et al. 2011, Dingman 2008, De variabil and microclimate subtle changes in microclimate ity could alter plant species composition and carbon cycling areas. broad across season, with a large enough effect to have biological implica season, with a large enough effect to have also highlight that microclimatic our results tions. However, which balance, water on dependent strongly is buffering over will change implies that the buffering capacity of forests independent of time with changes in biophysical variables, itself. canopy cover changes in forest is expected to increase biological systems and climate change conditions (Jentsch and intensity of extreme the frequency maxi 2011). Extreme 2008, Smith and Beierkuhnlein damaging by plants directly can affect temperatures mum photosynthetic apparatus (Berry 1980) and and Björkman and Robberecht 1990, Kolb other plant tissues (Helgerson et al. (Adams exacerbating drought by 1996) or indirectly et al. 2013). Plant– Williams Boeck et al. 2011, 2009, De to changes in climate especially sensitive are water relations particularly and consequent increases heat waves extremes, transpi plant for demand atmospheric increase that VPD in extremes et al. 2013). Although temperature ration (Reyer VPD of studied, the critical role frequently been more have and survival recognized is increasingly for plant growth et al. 2016). et al. 2013, Restaino Will et al. 2013, (Breshears the importance and climate extremes of microclimate Given

- - - - 0.51 deficit 2.5 AET: .0 al. 2015). Our Our 2015). al. 1.52 et High .0 0.51 0 0 −5

100 −10 −15

−100 −200 −300

) (BC capacity ing er Buff |), the eastern Cascades and forests in |), the eastern Cascades and forests present ΔBC/|BC = results suggest that forest canopies strongly buffer extremes extremes buffer strongly canopies forest that suggest results the growing VPD throughout and in maximum temperature organisms, 2) buffering effects must be large enough to be and 3) the effects must be temporally biologically relevant, the long-term persis for them to promote stable in order understoryof tence (Hylander organisms microclimatic buffering to ameliorate the impacts of climate buffering to ameliorate the impacts of climate microclimatic canopies must conditions must be met: 1) forest change, three variables that affect understory of biophysical buffer extremes Ample evidence indicates that forest canopies buffer cli Ample evidence indicates that forest this buffering capacity affects but how mate extremes, poorly understood. For to warming remains biotic response Discussion Fig. A6, A7), but the majority of the landscape still lost Fig. climate changes in future buffering capacity due to projected conditions. proportion of their buffering capacity (Fig. 5C). Increasing Increasing 5C). proportion of their buffering capacity (Fig. the loss of buffer 10% reduced by canopy cover future 1 Appendix material (Supplementary slightly capacity ing study sites (Supplementary material Appendix 1 Fig. A5). 1 Fig. material Appendix study sites (Supplementary buffering capacity (i.e. to current As a proportion relative ΔBCP to lose a greater predicted are Washington and eastern Oregon ests in Oregon and Washington), are predicted to lose the lose to predicted are Washington), and Oregon in ests Fig. 5B), (2040–2069; most buffering capacity in the future than those of our wetter are although some of these forests cm above ground) is more negative with higher canopy cover and AET-to-deficit ratio (AET:deficit). Negative BC values BC Negative ratio (AET:deficit). and AET-to-deficit cover with higher canopy negative is more ground) (10 cm above sensors low indicate that it is cooler/moister in the understory than in the open. wettest for (e.g. the buffer microclimate that strongly Areas 4. The buffering capacity (BC) for maximum (max) temperature and vapor pressure deficit (VPD) at high (2 m above ground) and above deficitpressure (VPD) at high (2 m vapor and (max) temperature 4. The buffering capacity (BC) for maximum Figure Ecology in a human-dominated World future (B,C).Projections are madeinareas withatleast 20% canopycover whichwe defineasforested. (B, C).Red indicatesitisdrierintheforest (positive BCvalues) currently (A)orashifttowards reduced microclimatic bufferinginthe the future (B,C). Yellow indicatesnodifference between forest and reference conditions(A)ornochangeinbufferingcapacity over time the landscape. Blue indicates microclimatic buffering or a shift to greaterby forests buffering capacity in ((A); moister in the understory) the ground wasashigh16 face where many organisms live. The buffering effect near nearthegroundto buffermicroclimates, sur particularly between current andfuture predictions (ΔBCP absolute difference between current andfuture (2040–2069)predicted difference bufferingcapacity(ΔBC;(B));andthe proportional Figure 5. Microclimatic formaximum bufferingcapacity neartheground surface VPDunder current conditions(1980–2009;(A));the 8 Our results clearlydemonstratethatforest canopiesserve ° C and5.0kPa atdailytime = ΔBC/|BC present |; (C)).Histograms totherightofplotsdisplaydistributioncellson - mum temperature and VPD was greater under warmer or cover was 5.3 scales. On average, maximumtemperatures and VPD were ° C and1.1kPa lower, respectively, where forest canopy ≥ 50%. While the absolute difference in maxi - Ecology in a human-dominated World ------9 al. 2017). While these While 2017). al. et al. 2014, Lenoir 2014, al. et Our analyses come with three important limitations, three analyses come with Our If forest microclimatic conditions change over time, this time, conditions change over microclimatic forest If resolution. Within a cell this size there is variability in canopy is variability there a cell this size Within resolution. in buffering capac in finer scale differences that results cover as an average should be interpreted Thus our predictions ity. varypotential buffering capacity that will with small-scale Second, and hydrology. in both canopy cover site differences out with AET-deficit-ratios our analysis extrapolated to areas side of the range of our study sites, particularly in very wet northwestern forests and for the distribution of understorynorthwestern forests are biota on impacts change climate of Predictions species. microhabitat and incorporating microclimate increasingly (Slavich effects those based on coarser-scale over improvements models are are suggest that these microclimates gridded data, our results the furthershould work explore Future transient. be to likely conditions and the implications stability of microclimatic for understanding climate microclimates of changing forest change impacts (Lenoir et al. 2017). changes impact our ability to anticipate future which directly the spatial scale of our anal First, microclimates. in forest this large, an area Within ysis, ~800 m, has constraints. due soil moisture, in available can be large variability there paths and plant access to groundwater. flow to hydrologic permanent or long-term access to communities with Plant their likely to retain are (e.g. riparian areas) groundwater temperatures as even extremes, capacity to buffer climate et al. 2018). These et al. 2017, Klos (McLaughlin increase in the analysis not captured local moderating effects are in incident radia differences Furthermore, here. presented surfacewith interacting topography, mountainous in tion with in temperature in large differences result soil moisture et al. 2016) that cannot be resolved aspect position (Holden made at predictions coarse data. Additionally, using relatively data at 250 m based on canopy cover scale were the regional without a commensurate increase in AET. For example, sites example, sites For in AET. commensurate increase without a as such canopies, open and ratios AET-to-deficit low with in central forests Cascades or lower-elevation the eastern condi current capacity under minimal buffering have Idaho, that show predictions 5A). Future Fig. (i.e. 1980–2009; tions and wetter cooler across AET will likely increase although also pre of the northwestern U.S., water deficits are areas AET-to-deficit leading to a net decline in dicted to increase A4). Future 1 Fig. Appendix material ratios (Supplementary declines in buff (i.e. 2040–2069) suggest large predictions Although Northwest forests. Pacific capacity in wetter ering microclimate buffer to capacity the maintain will forests these conditions in the under microclimatic to some extent, the as buffering capac variable story likely to become more are also suggest that proportionally, projections ity declines. Our in buffering capacity will occur in low- the largest changes limited more have or dryelevation currently which forests, buff regions, microclimatic In these drier buffering capacity. important in a microsites canopies may create forest ering by system. moisture-limited important of will have implications for the biodiversity ------) - - reference al. 2017), but but 2017), al. et Kovacs 2016, al. et C), spanning different vegetation types vegetation different C), spanning ° C to 5 ° The role of water balance in determining forest buffer role of water balance in determining forest The The microclimatic buffering capacity of northwestern buffering The microclimatic increased temperatures or changes in precipitation lead to or changes in precipitation temperatures increased deficit, water climatic higher and water availability decreased strong relationship we found to water balance suggests that to water balance suggests found we relationship strong time. The ability of for varyover buffering capacity will also where may diminish in areas ests to buffer climate extremes ing capacity has important implications for the ability of under changing climate condi to act as microrefugia forests et al. 2016). Although we et al. 2013, Frey Frenne tions (De space, the in buffering capacity across examined differences lower VPD have been linked to reduced temporal variability temporal variability been linked to reduced have VPD lower 2013). and Gollan (Ashcroft of soil and air temperatures capacity. Other work has also suggested that wetter micro has also suggested that wetter work Other capacity. level at the ground likely to buffer temperatures more sites are humidity and higher relative 2009). Additionally, (Fridley contributing to the buffering effect. Average buffering capac contributing to the buffering effect. with water balance met seasons also varied growing ity across subplots with higher of canopy cover, level a given rics. For greater buffering ratios of AET to deficit had significantly forests have a frictional effect that decreases wind speeds in a frictional effect that decreases have forests (Raynor 1971, Chen et al. 1995), open areas versus forested furtherand balance, water local the of components altering result in large temperature differences between open areas open areas between differences in large temperature result et al. 2012). 2009, Keppel and those under a canopy (Fridley buffering, associated with more wind speeds were Lower here, mixing. Although not measured likely due to decreased al. 2013). At the ground level, level, Arx the ground (von et al. 2013). At temperate forests solar radiation was important sensible because it can drive stable boundaryand under heating, can this conditions, layer humidity, all components of the local water balance. Higher local water balance. Higher of the all components humidity, in temper differences to greater was also related soil moisture European in sites forested and open VPD between and ature ering. At daily timescales, microclimate buffering was most daily timescales, microclimate ering. At with other studies consistent to canopy cover, related strongly Arx Frey et al. 2013, (von wind, and solar radiation, moisture, with soil it also varied forests. Sites with higher moisture availability are better able are availability with higher moisture Sites forests. fluxes heat sensible to opposed as latent to energy translate to buff microclimatic greater et al. 1999) and thus provide (Dai forests will likely varyforests climate change, independent of with local water balance plays Specifically, changes in canopy cover. the buffering capacity of these in determining role a pivotal in temperature and/or VPD of similar magnitudes directly directly magnitudes similar of VPD and/or temperature in al. et reproduction (Will and affect plant survival, growth, et al. 2017). et al. 2015, Larson 2013, Rother et al. 2016, Lenoir et al. 2017). These et al. 2016, Holden Frey biologically VPD, are and in both temperature differences, that increases shown studies have significant, as experimental we documented are consistent with those reported in other reported consistent with those are documented we studies (1.5 Arx et al. 2011, von et al. 2013, (Suggitt structure and forest drier conditions, proportionallydrier conditions, ΔMXT/MXT (e.g. con the understorybetween reference the difference and the maximum reference constant across fairly ditions remained that buffering effects VPDs. The average and temperatures Ecology in a human-dominated World Allen, C. D. Allen, C.D.andBreshears, D.1998.Drought-induced Adams, H.D. etal.2009. Temperature sensitivityofdrought- Abatzoglou, J. T. andBrown, T. J.2012.Acomparisonofstatistical Abatzoglou, J. T. 2013. Development meteoro of gridded surface References (agreement numberNNH11ZDA001N-FIRES). provided by aNASAApplied Wildland Fire Applications award Stennis was project [1012438].Additional forZAH support and theUSDANational Inst. ofFood andAgriculture, McIntire 1461576. SZD received from JFSP additional 15-1-03-20 support were alsofundedby theNational ScienceFoundation grantBCS Science Program project # 16-1-01-15. KTD and SZD (JFSP) Funding impacts where forest canopiesare lost. forest microclimatic bufferingmayamplifyclimatechange expected tochangewithclimatewarming.Rapidlossesin dependent onthelocalwaterbalance,whichinturnis that current microclimates maybetransientandstrongly ourresultsinteraction. Despite theseuncertainties, suggest on dynamicchangesinvegetation, waterbalance,andtheir climate change;however, theultimatepatternswilldepend buffering capacityismostvulnerabletothedirect impactsof isms. Our analysisisafirststepinanticipatingwhere forests’ organ microclimate conditions experienced by understory the potentialtocausedramatic,non-linearchangesin Disturbances thatresult incompletecanopyremoval have outbreaks(Weed andwildfires etal. 2013, Westerling 2016). indirectly affectcanopycover viadisturbancessuchasinsect Breshears 1998,Allen etal.2010).Climatechangeswillalso cations andviadrought-induced forest (Allenand mortality ological changesinabove ground tobelow ground plantallo may directly result incanopycover declinesthrough physi responses toclimatewarming.For example,drierconditions complex wayduetohumanactivitiesanddirect andindirect narios; however, canopy cover willlikelychangeinamore examined three simplifiedpotentialfuture canopycover sce (Supplementary materialAppendix 1Fig. A5).Third, we ested area undercurrent andfuture conditionsrespectively U.S.for sites accountedfor75%and86%ofnorthwestern areas. 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