Forest Canopy-Atmosphere Interactions

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-

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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

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 about I .1 is that young tbrestsare a morc efficient sink 1br pcrcentper year.a substantiallygreater percent- CO.l howcver.older tbrcstscontain largcr stores age increasethan fbr CO, Methane is produced of carbol. by specializedanaerobic bacteria that reduce COr. A recentstudv by Hollingcret al. ( 1994b)ex Primary sourcesof methaneinclude anaerobic ploledho* environmentalmoditications by aforcst cnvirenments.such as wctlands.digestive sys- canopypotentially alter basic pattcrns of ecosystems of cattle and termites.and mining of fossil ten CO. exchangeconpi:Lred to the cufient un- fuels (Mooneyet al. 1987).Methane may have derstandingof leaf gas exchange.Net llux be- an indirect eflect on foresl canopiesthfough its

Canopy-AtmosphereInteractions inlluence on clirrate anclrtmospheric chemistry. primarilyfrom combustiolrand. to a lesserex A strong greenhousegas. methaneabsorbs out- tent.from biologicalprocesses ofnitrification and going infrared radiation within the tropospltere denitrificationin soils.The role ol NO inatmo 20 times morc eftectivelyper moleculcthan does sphericchemistry is complex- undervaryingcon- CO..In addition,by rcactingwith hydroxylmdi ditions,it can catalyzeboth the productionand cals, mcthane can produce compoundsharrnful destructionof ozonc. It also is leadily converted to canopyfoliagc. suchas carbon nonoxide (CO) to nitric i,rcidthat fonns a nlajor component of andozone (O..). acidprecipitation dowlwind of industrialareas. As a fertilizer N deryrsitionto N deficient ti)r- Nitrogen estsmay increase plant productivity or stimulate unhealthychanges in vegetationgrowth pattcrns Although dinitrogen(N.) is the most abundant andnutrition (Aber and Melillo 1991). clcmentin the atnosphere.it is alsothc element Although present lhalmo:l lrIn il\ lh.,l,,\)nlhe\i. and prirnar) pro- in the atmosphcrcat low ductionin manytenest al ccosystems.This pata concentations,ammonia sources have been aug- cloxis explainedby the fact thatN. in the atmo- mentedby irtensive agricultureand livestock spherecan bc c()nveftedto biologically availablc famin-q.Atrnospheric ammonia can bc dissolved fbrms by only a ltrv spccicsof prokarlotes. and or adsorbedby aerosolsor precipitation. or be takcn up beccusvegetation.ln aerosolsor geticallyexpensive. Foresl canopics are impor- cloud droplets,anmonia can protonateto reducc tant sourcesofnitr()gen fixation b1'both freeJiv- the acidity of precipitation.tsut if depositied ing and symbioticbacteria on vegetationand ammonium is retaincd by vegetationor lost as uprphrte.. \nuther lbrnr ol nitrogcn.nilrou. o\- nitrate,soil acidificationrcsults (Binkley and ide.N,O. is increasingglobally due primarily to Richrer1987). combustion.Nitfous oxide nlay a1]'ectfi)rest cano Nlturulterre.trill eco\yst(m: ernil rn eren piesby contributinga smallamount towards thc widerarrayof sulfurcompounds. mostly in anaerc- breakdoun of the stratosphericozone layer and bic conditions,including hydrogen sulfide (H,S). to global warming in the next severaldecades dimelh)I .ultide.melh) | mercrplrn-. arhon dis (Mooneyet al. 1987.1. ulfide, and carbonylsulfide. All but the last com- poundrrc 'hort lrrcd.{r'r,rrtling lo curentcr i- Feactive Gases dence.the flux of H.S is muchhigher in tropical thantempcrate fbrests. Sullur ftom anthropogenic Reactivcgases are produced biologically and b1, fiel combustionconfibutes to acidilled precipi- combustion.but aremorc difficult to quantity due tation and altcrednutrient cycling in lbrcsts. to their ephcmeral nature.The effects of thesc gasesalso are problematic becausc thev canin- Ozone in the troposphereis a strong oxidant teraclrcgionallv or locallyto causethe produc- formed by a complex set of rcactionsinvolving tion or destruclionof other gaseshavilg a major NO. gases,hydrocarbons, oxygen. and sunlight. inrpact on atmosphedccherristry and terrestrial Ozonecan injure trcc canopiesby damagingicaf birl1a.Major classes of reactivegases include the membranes and inhibiLing photosynthesis non-methanehydrocarbons (NMHC) andCO. The (Bytnerowiczand Crulke 1992).ln lact.on r re- NMHC, includingisoprene and tetpenes. are linkcd gional scale,ozone is the only air pollutantknown k) the destructionof ozoneand productionof at- to bephytotoxic at ambientlevels in westemNorth mospheric CO. There is no evidence,howevcr, Amcrica. The literatureon ozonc eft'ectson lbr thatatmospheric conccntrations ofNMHC d ectly estedecosystems in the westeruUnited States is afTectphysiological or biochcnricalcharacteris rlnthcsizcdin Ol*'n et ul.rl()92r. Ozonc rn,'ni- tics of loliage. Carbon ntonoxide has no dircct toring data fbr the Pacific Northwest have been bi,'lo!icaleifecl\ Jl cur|cnlllnrtlsnheric concen sumnarizedby Eilerset al. (l99,1). trations.but it interactswith nitrogenoxides and hvdloxyl to producetropospheric ozone. a phy- Radiation and Heat totoxiccompound. Radiationabsorption in theatmosphere is a tunction Reaclivcnitrogen gases include oxides of ni- ofthe pathlengththrough the atmosphereand the trogen (N). refered to as NO,. which arederived contentof absorbirgcompounds, pluticulally water l0 Rose vapor In addition to shofiwave radiation tiom longwavcradiation and its partitioninginto scn- the sun and sky, an impoftant contributionto thc sibleand latentheat exchaDge. When consider- ndiation balanceofcanopies is madeby Jongwave ing encrgy partitioning of the whole canopy,ad- radiation.Longwave radiation is emittedby the vectioD,and soil and canopyheat storagealso gases(especially water vapor and CO.) present must be takeninto account(Baldocchi l993). in thc L'werulmo.phcre. The pru'enceol cloud. Importanttonote loo, is thatheat tEnspon in fbrests increasesthc downward tlux of longwave radia- can move up the gfadient, contrary to expecta- tionbecau.c cloud' rre m,rreenecli\c emilter). tions.This is becausesporadic turbulence at scales Radiationlosses ftotrr the forest canopy include much larger than the local gradientcan produce "up-gradienC' themal radiationemitted, as well as any incident mixing. mdiationthat is reflected or transmitted(Jones l992). Photosyntheticpigments in thc ti)restcanopy Water absorbincident light rudiationmainly in the vis- Less than I pcrcentofEarth's watcr is contailed iblewavelengths (Salisbury and Ross 1985). Flux in the atmosphere,soil. lakes.and rivers.The at- densitvof radiationin a canopyat a grvcnwave- mospherecontains only 0.001percent ofEarth's length is the sum of the direct beam and difluse water,which would anount to an averagedepth sky radiation that penelratcsti)liage gaps and is ovcrthe planet'ssurfacc of 20-30mm (Mooney modified by scatteringon Ieaf surfaces.Conse- et ill. lgR7r.Nererlheles.. ualer rr ln imporlanl quently,properties oflight at anypoint in acanopv controllingfactor in vegelationprocesses and in are dependenton conditions in the atmosphcre. troposphericlemperature structurc through eflects andthe physical, chemical, and structural attributes on both solar and terrcstrialradiatiotr. of the canopy(Baldocchi 1993). Detailed intbr- mrlion on phl:icrl rttrihute'ol th( clnop) i. Water vapor fanst'er dependson thc tlows of nccdedto assessthe diffusivc source-sinkgradi energy that convefi liquid to vapor and on mo- ent and the transmissionofradiation directly.The lecular and turbulentditlirsion that transf'erswa total leaf areaof a canopy.spccies compositions. ter vapor moleculesbetween the leafand the frcc leaf spatialand elevational distribution, and an- air.As with CO, exchange.water vapor cxchange Bularorienlirliun all rlfect thc |ldirrrr,'n enr iron from lrec canopiesis dependenton all biological ment of forest cilnopies.Various stresscs to veg- and structuralattributes that influencc local con etation, ilcluding nitrogen delicielcies and ccntrationsin thecanopy. such as radiation, tem- moisture deficit. can affect thc light absolption perature.turbulence, humidiLy, and leaf physiologi- andreflectancc propefties of foliagedue to changes cal condition. in ccllular structureand pigmcnt tirnctioning of Actual evapotranspiration(AET) is thc sum tbliage. of all water lost b the atmospherefiom the soil A uselll approximationof radiationbehavior or vcgetationsurfaces (evaporation) and through in forest canopiesis that a constantfractioD is plant tissucs(transpiration). Potcntial evapotmns- absorbedper unit distalce into tlte canopy.This pirutiontPETr i. Iheumounl olerrp,'trrn.pira producesexponentiai light decaywith incrcas- tion that$'ould occur with unlinited watcr sup- ing cumulativeleaf area.The dccay of radiative ply. In drier climates.AET is lower than PET. energywith distanceinto thecanopy causes hearing Evapetranspirationincreases directlv with watel- ofthe biomassand air thatproduccs a stableden- availability and with higher evaporativedemand sity stratillcationin the canopyof rvarner lighter at thc lerf surface(higher temperatuleand lower air above,and coolerdenser air below and par humidit),)(Aber and Melillo 1991). tially isolatcsthe lower canopy from the atmo- Forcst canopiesilteract both directly and in- sphere.This effect is strongesttbr canopieswith directly with the hydrologic cycle. Leaves, thick foliage near the top. At night, the canopy branches.and stens intcrceptand redirect water interfereswith thcloss 0flongrvave radiation under to the soil in throughtallor backto the atmosphere the canopy.whereas substantial radiative cool- via evaporationaDd transpiration. Anluallv. as ing occursabove the canopy,and leadsto strong muchas a third ofthc inconringprecipitation may stablelayers near the top ofthe canopy.Tempera- be intcrceptedby forest canopiesranging from ture and humidity at the leaf surface arc dcter- tropical to borealtypes. Stluctural aspectsof the minedby thebalance ol incomingshofiwave and canopy influencc the anount of interceptedra

CanopyAtmosphere lntelactions 11 diation. boundaryJayer charactcristics,and the aerosols.During fires,large quantities of nutri- amountofu,ater availablelbr evaporation.These ents also are lost in the gasesand smokc fiom influcncesare evident when fbrest cover over an ibrcst canopies(Waring and Schlesinger1985). entire watershedis renoved and streamflow in- Ecosystemcarbon and nutrient cycles rre creasessubstantially. mutually linked becauscthe chemical composi- Water movementfrom the soil through the lbrest tion andquantity oflitter suppiiedby thecanopy canopyto the atmosphereis driven by a seriesof rrodulrtc the proce.sesol' Jecotnpo:iliLln.min interrelatcd.interdependent proceses. For cxample, eralization.and nitrification (Fogel andCromack the rateofwater absorptionby the soil is affected 1977,Mccntemeyer 1978. Melillo et al. 191J2. both by the ratc of water loss from the leavesto Pastorand Post 1986).Canopy-atmosphere in the atmosphereand by thc rale at which water teractionsinfluence theseprocesses through can movc from the soil to the root surf'acc.Water changesin the carbonand nutrient ratios of foli- loss and interceptionby trce canopiesalso is in- rge mcdiatedby differencesin the availability of fluencedby boLhthe amountand durationoflcaf photosynthateand nutrients.Atmosphedc condi area. For exan]ple.pine fbrestslose more water tion. lJ\ur.rhlet,' photo.lnthe'i.m11 increu.e to the atmospherethan do hardwoods.owlng to the foliar carbon nutdent mtio, whereascondi- higher leafareaand to the maintenanceofneedles tions limiting gas exchangewould have the op yeal lound. Species-specificdifl'erences m wa- positeefftct, dependingalso on sitenutrient avail- ter loss alsoer.ist. Species with deeproots that ability.Furthermore, the canopy in{luences nutdent can accessabundant $'ater supplies.typically cycling by alteringmoisture and temperature con- sustainhighcr lates of evapotranspiration(War- ditions at the tbrest floor. ing andSchlesinger 1985). The quantity and distribution of water vapor Windand Turbulence conffolsboth ecosystem and atmospheric dyneunics Air in the lower atmosphereis never completely directly.Changes in Earth'senergy balance in still. There is usually a net horizontal nrotion or responseto greenhouscwam-fng areexpected to wincl,with random movementsof smallerair dranaticallyalter rcgional water budgets (Mooney pockets.In tiee convection.air movementsare et al. 1987).Hollingcr et al. ( 199:la)recently fbund causedby changesin air density.as resultstiom rapid transmittanceof hydraulic pulsesfrom the hertingor cuoling. Tn lbrced conr ectiun..tir m,'r e- top of a Nothofagustree to a mid stcm and basal nent is deternined by an external pressuregm stemposition, respcctively. These pulses were dientcausing wind. Forced convection may lead induccdby changesin atmospherichunlidity as to the generationof eddiesor turbulenccas a re- air of variousmoisture content passed over the sult oftiictional forcesthat develop as wind flovs canopy. These lindings indicatc that the water over an object. Characteristicsof the eddiesand column is undcr tensiol'rgoverned by physical turbulencc over a forest canopy combined with tbrccs and that water storageplays little role. attdbutesof the canopy boundarylayet control Additionalresearch is needeclto clarify the mecha- many aspectsof massand encrgy exchangewith nisns governingcvapotranspiration by forest the atmospherc. cilnoples. Turbulent motion has a controlling influence Nutrients on the distributionand rcdistributionofmass and encrgythrough forest ecosystems.Many aspects Thc forest canopy is a major repositoryof plant of canopy-atmosphereintcractions are influenced andecosystem nutrient capital. Processes within by thetact that windflows and turbulent exchanges the caropy influencc|he retentiol'r,uptake, leach- aboveforestcanopies are poorly describedby flat- ing, and redistributionof nutrientswithin the planeboundary layer models.In addition,physi- canop,vand among thc canopy.the soil. and the iul .rnJbi,'logical influences on canoplmicro- atmosphere.ln addition to the uptake of nutri climate are ofien complementary.For example, ents depositiedin various forms ftom the atmo- seedand pollen dispersaJdepend on a turbulent sphere to the lbrest canopy. canopies also are wind tield. which in turn is affectedby the biom emittersof variouscornpounds, including hydrogen ass profilc. Diumal changesin canopy relative sulfide,ammonia, and K, S . and P-containin-q humidityaffect the movements ofinsects and other

12 Rose organisms.(Murlis et al. 1992, Fitzjarrald and Coniteroustbrests with fine needlesare highly Moore 1995). efficient at collectingwind drivencloud droplets (Unsworthand Wilshaw 1989).Previously de- Consistentwith meanwind protiles,turbulent positedpollutants on leaf surfacescan dissolve, transport of horizontal lnomeDtumdecays rap- more solutions than the idly below the canopy surtace;howevcr. the de creating concentratcd incidentcloudwater. Furthermore, m0istened leaf greeto which the canopy is coupledto the atmo- of particles by re sphercchanges diulnally, with strongercoupling surlacesincrease deposition resuspension.and by pro duringthe day than night (Fitzjarrald et al. 1988). ducing bouncedrought may afford someprotection fiom ozone pendsto a large degrcc on the foughnessof the period.Dr1 \ir minule surt'rce.When canopiesare sparse,each tree is duringthis Jeplsition particlesis negligible. although fine sulfatepar- considereda roughnesselcment. ticles may enter stomates. Propertiesof the plant cuticle determinethe PollutantDeposition intcraction'uith uet depo.ition.Precipitrtron The processescontrolling pollution depositionto may be both a sourceand a sink for pollutants. tirrcstcanopies deserve special mention here. As polluted watcr evaporatesfrom the leaf sur- Atmosphericdeposition occurs thrcugh thrce sepa- face, ion exchangemay occur betweenthe leaf rate processes:(1) wet depositionin precipita- irterior and surl'ace.With rapid evaporation.acid tion; (2) dry depositionin particlesand gases;and may reach damaginglevels on leaf surfaces. (3) clouddeposition in droplets.Wet and dry depo S()lublegases also may diffuseinto the waterfilm. sition are ubiquitous, whcreascloud deposition The most important canopy transfbrmationsof is conrmonin coastaland mountainous areas. Wet incident precipitation are the direct assinilation depositionis generallycontrolled by theconcen- of nutrientsby foliage, hydrogen ion bullering tration.size, and solubilityof particlesor gases, in the canopy. and metal cation leaching gas reactivity, and the meteorologicalprocesses (Bredemeier191i9). affectingprecipitation. Dry depositionis controlled Although beyond the scopeof this paper,at- by many factors,including atmosphericchemis- mosphericprocesses have imponant effects on try andmeteorology; canop)' structurei andphysi- the transportand dispersionof atmosphericpoJ- ological.chemical. and structuralpropefiies of lutants.These processes bave major consequenccs foliage.Cloud fog depositionis controlledby lbr regionalpatlern: .'l'p,'llulanl deposilion in factorssimilar to wet deposition,though proper- mountainousareas. See B6hm (1992)lbr a re- tie.,'i thc rc(cplur\urlace ffe moreilnpllnirnl view of regionaldeposition in relationto atmo- (Lovett 1994). sphericprocesscs.

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