CatherineL. Rose,USDA Forest Service, Paciflc Northwest Research Stat on Bend Oregon97701 ForestCanopy-Atmosphere Interactions Abstract [xch.rnges ol nuteri.ris (sases.nutrents. lvrter pollutants.rnd energy)bei\\'een forest canopiesand thc armosphcrcdrile inpor lxnl cco!)stcln processestrrd ll]tluencenlrln)_ meteorological phenomena. Tfees mrstexchange !l ater,carbon dioxidc. and cncrgy r!ilh lhc rlrnosphcrc to sur!ivc. As ncw irstrunents .rnd quutitative t(xts eDrergefor mersufing rbfest and atnrosphericcondi tion\. lhc conrplexi!ic! ol crnopy almosphcrcinlcrrclions can be nore uccuratelyunder\lmd rnd nndeled. Improved kno\lledgc of ca. opv atmosph erc inlcraction s i\ bccoming norc inpor hnr as hunar uctivities .rlterbofi dre struclLueand function of ibrcn c,lnopies.as $cll as fic chcnicll and phlsicrl prcperlies of the atnosphere. The increaseclscieniific focus on slructural and functional altribuLcsof lbrest crnopies in fecent ]ears pf(nni\es to ]ield nex insights into the eitects of human dinurbrncc on environmeDtsm tbrest canopies nd the rtmosphere. Impofi.Lni opics for future researchin carropr-almosphcrc inteftcrions inclLrde:(1) the mfluence ofele\ rtion. fore\t edge.and canop! roughnesson armosphcricdcposition ofpolluunts;(2) the dyn m- ics of cr$oD sequestrarionin forest bidrass in felation to foreri managernenrpracticcs and olhcr dislurbanccsi(l) the efiects of anthropogenicpollut.rnts on fbrest functioning and rnnospheric feedbacksrand (,1)the functional changcsin lbllrn canopies associ.rted$ith structuralchanges. aDd eonsequences for \\'atefshedh)drology and nutrieni cycling. Introduction physiology,function. and structure,with cmpha- sison conileroustbrests. An oven'iew ofcanop),- The forest canopy fonns a major inlert'acewilh amrosphericfunctions and interactions is provided. the atorosphereover much of Earth's surlace. including the interception.retention, modifica- Canopy-nediatedcxchan,ecs of massand ener-ey tion, andconductanca ofenergy (light, heat), gases largely detineecosystem productivity. Key inter- (pdmarily carbon dioxide). water, and nutrients actioDsbetween the canop),and lhc atmosphcrc (Figure1). Canopyinteractions with wind and areinlluenced by environmentalconditions such pollutantsalso are briefly discussed. as climate. moislurc.and nutricnt availability (Mooneyand Gulmon t982.VanCleveetal. 1983). Anthropogenicdisturbances such as defores- In turn. u,ind velocity, energy budgets,raintall. tation, stratosphericozone depletion.pollution, andothcr cnvironmental conditions are variably biomassburning, water shoftages, and other lac modified by the canopyand can affect forest hy tors associatedwith expanding human popula- drology (Cavclicrand Goldstcin1989). Forest tionsand industrialization pose a growingthreat canopiesmodity climates locally, regionally. and to tbrest ecosystems.A better understandingof interactionsbctween ti)rest canopiesand the at -ulob:rl1y(Avisstrr and Pielke1989. Shukla et al. 1990).Changes in the pool sizesand fluxesof mosphereis urgentlyneeded to accuratelyassess. lruffients. water, and carbon through torcst eco nritigute.und perhaps prcvcnl negative inrprcr. systemshrve importantconsequences not only on ccosystemhealth tiom human disturbance. for thcplospcrit5 o[ humancconomie ').tenr.. but alsotir the sustainabilityol lif'eon Earrh. TraceGases As a consequenceofcomplex mctcorological Trace-qases in the atmospher.erange from highly and biological processes,resources and micro reactivespecies with lifetimcsin sccondsto hours. clirnatesvithin forestcanopies are spatially and to \tahlecornpound. uith lifetime.in 'iriches," learr,rr tcmporallydivcrsc. Within thcsccanopy centuries.The sources,silks. and dynanics of specializedorganisms have evolved that alter the thesegases are imponantfor severalreasons. First, environmentti)r otherspecies and iteratively fbnn many gasesa11ect the chemistry and physics of a trophic chain or "food rveb." Complexity and the atmosphereand thcrcby can affect various diversityal onc levelnu urc complcxityand di- naturill processes,such as concentrationsof tlo versity at yet higher levels. This paper explores posphedcoxidants, ultraviolct absorption in the some()f the impoftantinteractions between the stratosphere,and the energybudget ofEarlh. Sec- atnosphereand forestcanopies in ten'nsofcarropy ond. trace gasesor their reiiction productsinter- NorthwestScicncc, Vrl.70, SpecialIssue. 1996 NHs A, TI - ' r-f -72. VxFlx^ ^\'L - ,l {.4 r\,. r -HrO' o"-V I -/' d -, ,a-- trYx - tl EF Fisure L Clonplcx and intcracting proccssesinfluence the exchangeof materialsand energy betweenforeir canopicsand thc atmosphcrc. ucl\\ lthIhe hrrrrr r, ' ciru\cJ mull ll uLlc of clf(cl. in concentration,and seasonaldistribution pat ranging from increasedproductivity to heavy terns are better characterized.Long-term trends rn0rtrlitr.I hrrJ.rhc protluctron or con\umplion in concentrationshave been documentedin icc ofthese gasesser-r,'es as an importantindicator of bubblesfromAntarctic ice cores(Rasmussen and <co\)\temlunctioning. The li,restr.rn,'p1 is .r Kialil 1984,Pearman et al. l986). This group of majorbiologic entity intcractingwith the atmo- gasesincludes carbon dioxide (CO.). methane sphereto influencethe concentrationsand lluxes 'CH,r.anJ nitr,'ur or.iJe rN_O r. three.t..rble bio- of tracegases. Consequently, the mcchanisms of genic gascsthat are increasingglobally. this intemctionare important to understandingthe d1'namicsof tcrrcstrialecosyslens in relationto Carbon Dioxide the atmosphere. Energyto drive most processesin forest ecosys- Excludingthe anthropogenicchlorotluorocar- tems is derived initially by the fixation of atmo- bons,the morestable gases are generally higher sphericCO. into sugarsthrough photosynthesis Rose in lcaves.In photosynthesis.CO, diftirsesthrough tween the ecosystenand the atmospherewas small pores or stomates,into foliage where it is determinedfiom the sum of thc eddy tlux above bi,\chemic"ll)r'educerl t,' rugrrsusinp enercl in the lbrest and the changein the meanconccntra- the form of adenosinetriphosphate (ATP) pnr- tion of CO. in the fbrest air below the canopy. duccdby thecapture ofsunlightby leilfpigments. Thc CO, exihangepattems were similar in ntany In respiration,the breakdown of sugarsto yield ways to pattems observedat the leaf lcvei. Sea- cnergytbr plant growth and metabolismproduces sonal changesin the relation of CO, uptakc and CO, as an end product. RespiredCO, then dif- dark respirationvaried however,with other en- lu.e\ backoul ol Ihe leul\tomilles into lhertm(F vironnental and physicalchamcteristics that had sphere. liftle impofianceat thelcaflevel. such as the above- The uptakeof CO. bv forest ecosystemsdur- canopy spatial distribution of PPFD and turbu ing the daytime is controlled primarily by the re- lent transport. sponseoI photosyntheticrate to incrcasinglight This andotherrccent studieshave demonstrated (photosyntheticallyactive photon tlux densit\,.or lhrt in modelinpCO erchangc.lolerr crnupies PPFD) (Hollinger et al. 1994b).The sarurating cannotbe treatedas sinrplednalogs of leavcs.At responscofleafphotosynthesis to light is explained the canopylevel. gas exchange is controllednot by the fact that carbon uptake is limited at low only by biological processesin the lcaf, but also PPFD by insutlicient electon tnnspon and pho- by canopy architecture,temporal changes in the tophosphorylation,and at high PPFD by insulfi- enrironmenl. .ilr ehar-rcleri\tic\.rnd en\ir. 'nmen- cientcarboxylation capacity (Farquhar ct al. 1980. tal gndients that vary spatially around vegeta- Kirchbaumand Farquhar1984). In responseto tion (Norman1993). Modeling effons that attempt temperature.photosynthesis in manyspecies has to scaleatmospheric intcractions to the level of beenlbund to reacha maximumbetween l5 - 30 wholecanopies iue described by Baldocchi(1993), 'C. This resultsftom the differentialeffect of tem- Norman( 1993).and Schimel et al. ( 1993.).Mod- pcratureon the kiletics of RuP. carboxylase- eling of gasexchange in conifercanopies is de- oxygelasefor carbondioxide andbxygen (Berry scribedby Priceand Biack (1990) and Wang and and Raison 1981).Other factorssuch as vapor Jarvis( 1990). pressure saturationdsficit and water stressalso Approachesto modeling whole-canopypho candirectly inhibit photosyntheticmechanisms, tosynthesisare reviewed by Norman(1993). The and indircctly inhibit photosynthesisby reduc- most popular approachesto estimating canopy ing stomatalaperturc (Morison 1987). Similarly. gas exchangeuse the lbllowing variables:(l) thc functional responseof nighttirne respiration absorbed,photosynthctictlly active radiation and production to temperaturedoninates the of car- leaf photosyntheticefiiciency; (2) averageillu bon dioxideby leaves.Respiration during dark- mination and Ieaf areaindex; (3) photosynthetic ness rncrcasesapproximately exponentially with rate and total leaf area; (4) photosyntheticrate the inverseof the absolutetemperature. and leaf area of sun and shadeibliage; and (5) Forestcanopies differ in thcir productiveca leaf energybalance and environmental gradients. pacities and their rates of exchangescasonally Particularllinrprrlxnl 1c'te'aJ'.h topic. in canopl andin rclationto tempcratureand the availablility gas exchangeinclude dynarnicvadations in tur- ofwater andnutrients. For example.humid tropical bulenceand physidogy. fbrestscomprise only 6.6 pcrcent of Earth's sur face area.but accomplish l7 percentof the total l\,4ethane net pdmary production. Anether generalization Troposphericnethane is increasingat