Light interception and its relation to structural differences in three Australian rainforest canopies M. D. LOWMAN continuous throughout the year. Within several Zoologj3 Departrnent, University of h'c~' Australian rainforests, there are distinct England, Arrnidale, NSW 2351, Australia seasonal peaks in leaf-fall (Webb et al. 1969), without concurrent replacement of canopy by Abstract leaf emergence (Brasell et al. 1980; Lowman 1982). In particular, the cool temperate rain- Relative diflerences in the seasonality of light forest sheds approximately half of its canopy penetrating the canopies of three Australian duringautumn, but this is not replaced until the i.ainforests were tt~easured directly n,ith a following spring. Both warm temperate and quanturn rneter and indirectbl ~rsing subtropical rainforests exhibit less significant hemispherical photographs. The tnossy (but nonetheless pronounced) leaf-falls in early rnicrophyll~fern.forest had higher incident light spring, with new leaf replacement occurring levels (100-200 pEinstcin r,12 per s) and tnore gradually throughout late spring and summer. sky visible (21.8%) at ground level than the Because these rainforests exhibit degrees of sitnple (5-20 pEinsteiti nli' per s with 11.2°0 seasonality in leaf-fall and leaf replacement, it slkylight) and cornpl~~~(5- 10 pEinstein m-2 per is postulated that the amount of sky visible s with 10.8% skylight) notophyll vine ,forests. from the forest floor up through the canopy will There brvre nosignificant changes in atnounts of exhibit temporal fluctuations relative to the shy visible through the canopy over tirne (10 proportion of leaf loss. In addition, the rnonths) .for anjl canopy, despite the ,fact that incidence of l~ght through the canopy is Icafl,fall ~vasa seasonal event. Profile diagratns predicted to vary according to canopy density ~vercconstructed at each site to illustrate the and structure. To examine these hypotheses, depth and stratification of the canopy, and the two types of light measurements were con- differences in these structural features were ducted in three rainforest formations. First, related to the variation in light attenuation. hemispherical photographs (Evans & Coombe Ecological aspects of'light interception through 1959; Anderson 1964; Horn 1971) were taken rainforest canopies are discussed, including tree monthly during 1 year to indicate the relative branch growth patterns and leafing phenologies, changes in the leafiness of the canopy by and seedling regeneration on the,forcst ,/loor. measuring the proportion of sky visible at ground level, and to indicate the canopy struc- Introduction ture by comparing the amounts of sun flecks with light gaps. Second, direct light readings Light penetration through a forest canopy were made at different levels within the three maintains understorey vegetation and deter- canopies to compare available light for leaves mines the degree of suppression or vigour of its under different canopy conditions. Profile growth. In deciduous forests, the annual loss of diagrams were constructed to illustrate the leaves from the canopy results in a large structural features of each canopy, and were ex- seasonal change in light influx to the forest floor amined in relation to light interception through (Horn 1971), and many herbaceous under- the canopies. Light was defined as any portion storey species leafout early in the spring, before of unobscured sky visible through the tree the new canopy in the overstorey flushes and canopy. This study utilized light to measure intercepts light (Jackson 1966). indirectly the density and deflective capacity of In evergreen rainforests, a complete seasonal three leafy canopies. The measurement of light loss of canopy foliage never occurs: leaf-fall is in its true physical sense was not attempted. Methods All woody plants greater than 1 m in height within a 70 X 8 m transect were included. Tree heights and canopy depth were calculated with Structural descriptions of the canopies the aid of a hypsometer, and drawn to scale. Profile diagrams of forest transects were Canopy depth was defined as the cumulative constructed according to standard methods vertical length intersecting elements of canopy described by Davis and Richards (1933) and taken from the top of the uppermost canopy Richards (1952). One site (containing at least down through the lowest-level foliage (Fig l), one of the photographic locations) was selected and mean canopy depth was calculated from 15 as representative of a mature, undisturbed replicate measurements taken at 5 m intervals canopy in each of the three rainforest along the profile diagrams. formations. The locations of the three rain- forests and their respective classifications according to Webb (1 959, 1968) are: Light measurements (1) Cool temperate or microphyll fern forest (MFF) at 1400 m elevation on a slight eastern Direct light readings Light readings were aspect in the New England National Park taken in different regions of the canopy using a (32" 30's); Lambda quantum meter. Access to the upper (2) Warm temperate or simple notophyll canopy was attained using ropes and technical vine forest (SNVF) at 60 m elevation on the climbing apparatus (Lowman 1982, 1984). In east-sloping bank of a tributary of the Hacking most cases, an emergent tree was climbed in River in the Royal National Park (34" 10's); order to reach across to the upper canopy of a and neighbouring tree. The same sites were (3) Subtropical or complex notophyll vine measured under clear, sunny conditions and forest (CNVF) at 800 m elevation in the repeated in cloudy conditions. At least five Dorrigo National Park (30" 20tS), on the measurements were taken in each region. eastern slopes of the New England plateau. Canopy regions were delineated as follows: FIG. 1. Profile diagram of a cool temperate rainforest, New England National Park, NSW. Speciesabbreviations: Am Acacra ~i~clatio~~lon;Ar ..llbx~a rusc!lolla; Co Cordj~lineslrlda; Cq C'oprosma quadrihda; Cs Cryplocarjri sp.; Da Drcksonra anlarclrca: Ds I1or~phor.a.sas.sufia.s; Eh Elacocarpus holopc~alus;Nm Nolhofagus moorcl; Oe Orilcs c,~cclsa;TI Trochocarpa laurina; Ts ksfnania slip~lala. (1) Upper canopy: measured, and instead sky visibility was used as Above - over the uppermost canopy leaves; an indicator of canopy leafiness, corrections for exposed to direct light. zenith and time of year were not necessary Within - approximately 10 cm below the (Horn 197 1). The spherical photographs were uppermost canopy leaves; light diffused with projected onto a horizontal screen to a standard only one leaf layer overhead. size, and areas of light marked with black ink on (2) Lower canopy: acetate sheets. The light was separated into 'sun Sun fleck - leaf surface within 2 m ofground flecks', defined as any small spherical image of level that received a transient beam of direct light; and 'gaps', any larger or less symmetrical light. area of light. An area meter (Lambda Shade - leaf surface within 2 m of ground Instruments, Model 300) was used to calculate level that received no direct light. areas of total light. gaps, and sun flecks; and the Gap - area wider than 5 m on the forest floor amounts were expressed as percentages of the with direct light. area of the circular image. This method does not quantify the physical amount of light Measurement of light through the canopies reaching the forest floor (since it varies with Hemispherical photography was used in an cloud cover. and position of sun on a daily and unorthodox manner to measure relative seasonal basis), rather, it provides a relative changes in rainforest canopy structure as index of canopy cover and density as indicated indicated by amounts of unobscured sky visible by light infiltration. at ground level (Horn 1971). Ten initial light readings were made at random within each forest to determine mean light penetration Results levels, and three permanent photographic sites were chosen in each forest that best represented Structural characteristics of the canopies mean light levels. Photographs were taken every month for I year, at appoximately Figures 1-3 illustrate the profile diagrams 1000 h. Because light quantity was not being constructed in representative sites of each rain- 10 20 3 0 L 0 50 60 70 Distance (m) FIG. 2. Profile diagram ofa warm lemperale rainforest. Royal National Park, NSW. Spec~esabbreviations: As 4cmcnu smilh~i; Ca C'rratopclal~imaprlalu~n; Da Diploglo~trsa~tslralrs; Ds Dorl~phorasassafras; Fc Ficus coronata; La Ln,istona auslralis; Lr L~lsrarr~icula~a; PC Polvosma cunningharnii; Pu Pi//osporu~nu~rd~tlatutn; Sa Sloanea australi,: Sc S,~n:gr~r~prc.oolmtnlanum: Sg Svncarpru ,~/o~nul~lcra:TI Trtslanra laurina: Wh I47lckru h~rc~gc~lrana.Vines: Piper novae-hollandiar; I'lss~tss/ercull<foolia. Distance (m) FIG. 3. Prolile d~agramof a subtropical rainforest, Dorrigo National Park, NSW. Species abbreviations: Aa Alec/r,~on s~rhcino.r~u~;An Asplrniu~~rnidus; Be Bo~isloaruodirfi)rrrrrs; Ba Brach.vchi/on act~rifoliuttr:Cm Cryptocary mt7issnmr; Da Diploglottrs austra1i.s; De Dendrocnidc3excclsa; Ds Doryphora su.vsafias: Em Endiandra inuellr~ri,Fw Frcus watkrn~iana;Gb (;ca~ccoishcnfha~~rii; Ha Ir'eritirra actinophi~ffa;Lm Lirtospadi.r ~nonosrachi~u;Nd Nc~uf~tst~udmlhutu; Oe Ori/t~sc..rcr,lsa: Sf Si<g:si~r~i~,jruntitii; So Sr,hr:ornrria ovatu; Sw Sloanra ~vooflsii.Vines: Cm Calamus mur~llcr~vine sp. forest formation. More detailed information on The CNVF is the most diverse rainforest the sites, including leaf biomass and leaf area canopy in New South Wales, composed of indices, is given elsewhere (Lowman 1982). species such as sassafras (D.sassafras), giant The canopy of the MFF is dominated by one stinging tree (Dendrocnide excelsa), yellow species, antarctic beech (Nothofagus moorei) carabeen (Sloanea ~~oolsii), booyong with an understorey of 8-10 species including (Argyrodendron actinophyllurn), flame tree sassafras (Doryphora sassafras), possumwood (Brachychiton acertifolium) and many others.