Aspri r nger Vertical Profile and Canopy Organization in a Mixed Deciduous Forest Author(s): Geoffrey G. Parker, John P. O'Neill and Daniel Higman Reviewed work(s): Source: Vegetatio, Vol. 85, No. 1/2 (Dec. 15, 1989), pp. 1-11 Published by: Springer Stable URL: http://www.jstor.org/stable/20038549 Accessed: 08/05/2012 09:04 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http ://www.j stor.org/page/info/about/policies/terms .j sp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Springer is collaborating with JSTOR to digitize, preserve and extend access to Vegetatio. STOR http: //www .j stor.org Vegetatio 85: 1-11, 1989. ? 1989 Kluwer Academic Publishers. Printed in Belgium. Vertical profile and canopy organization in a mixed deciduous forest Geoffrey G. Parker, John P. O'Neill & Daniel Higman Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, MD 21037-0028, USA Accepted 4.6.1989 Keywords: Foliage-height profile, LAI, Leaf community, Litter phenology, Stratification, Vertical struc- ture Abstract A combination of optical measurements of leaf heights and observations on litterfall provided a vertical and temporal description of the leaf community structure in a tall, Liriodendron forest on the Maryland coastal plain. Leaf area, mass, and number were bimodally distributed with height. Median leaf number occurs far below (7-8 m) and median leaf mass far above (22-23 m) the median leaf area (18-19 m). Tree species exhibited leaf stratification into 3 height levels: understory (0-10 m), mid canopy (10-25 m), and overstory (25-37 m). Species leaf area in litterfall was related to the species basal area, although representation of leaf number in litterfall was not correlated with stem numbers for species in the stand. Species also showed a clear phenological sequence of leaf fall. Abbreviations: LAD = Leaf Area Duration; LAI = Leaf Area Index; SLA = Specific Leaf Area. Nomenclature: Radford, A. E, Ahles, H. E. & Bell, C. R. 1968. Manual of the vascular flora of the Carolinas. University of North Carolina Press, Chapel Hill, N.C. Introduction The forest canopy is the interface between the atmosphere and the forest; leaves are the plat- forms of interaction between forest and atmos- phere. However, different approaches treat the canopy in strikingly different ways and at various scales. Forest micro-meteorological studies tend to regard canopies as continuous porous media with bulk properties, such as roughness and zero- plane displacement (Campbell 1977; Lee 1978). Physiologists often focus on small numbers of foliar elements and successfully describe pro- cesses, such as gas exchange, for isolated pieces of a canopy (Larcher 1980). The relation between behaviors of constituent elements and that of the whole canopy is rarely dealt with. Furthermore, models of light interception (e.g. Anderson 1966; Miller 1967), photosynthetic capacity (Schulze etal. 1977), and evaporation (e.g. Lindroth & Halldin 1986) require information on the number and arrangement of canopy elements as inputs. Detailed description of the disposition of canopy elements would also aid in bridging the scales between approaches. Where leaves are the chief elements of interest, the explicit vertical organi- zation of foliage (as recognized by Smith 1973) should be emphasized, not soley the distribution of individual stems or species (e.g. Smith 1973; Popma etal. 1988). This paper reports such dis- tributions for a forested area on the Maryland coastal plain (USA). Objectives of this study, part of a larger analysis of forest canopy-atmosphere interaction within the Washington-Baltimore metropolitan area, were 1) to characterize the community of canopy leaves by area, mass, number, and species; 2) to indicate the vertical and temporal distribution of these components; and 3)to examine the re- lationship between the communities of stems and leaves. scattered patches of Tipularia discolor. Ground cover includes Lonicera japonica and Lycopodium sp. Stem community All stems in the study area greater than or equal to 2.5 cm dbh were identified, measured and mapped to within 1.0 m in the fall of 1987. Census results were summarized by species with respect to both stem density and basal area. Materials and methods The study site is a small forested area within the Muddy Creek Basin at the Smithsonian Environ- mental Research Center (SERC), located about 15 km south of Annapolis, MD (38?53'N, 76? 33' W) within the inner mid-Atlantic Coastal Plain on the Chesapeake Bay's western shore (Correll 1977). The study area surrounded a tall (48 m) scaffolding tower for meteorological and dry deposition studies. The tower is situated within a natural gap in the canopy; cutting of branches during installation of guy wires was minimal. The catchment mean slope is 4% (Chirlin & Schaffner 1977) with deep, fine sandy loam soils (Pierce 1982), with an underlying clay aquiclude near sea level (Correll 1977). The forest is tall and deciduous, similar to the 'tulip poplar association' described by Brush et al. (1980) and the 'yellow poplar' cover type of Eyre (1980). The stand appears to have originated from pasture abandoned after the Civil War (Higman, mscr.) and there has been no cutting on this parcel since at least 1915. The overstory consists of tulip poplar (Liriodendron tulipifera), four hickories (Carya spp.), five oaks (Quercus spp.), beech (Fagus grandifolia) and sweetgum (Liquidambar styraciflua). The understory is composed of flowering dogwood {Cornus florida), ironwood (Carpinus caroliniana), red maple (Acer rubrum), and holly (Ilex opaca). Shrubs include black haw (Viburnum dentatum) and spice bush (Lindera benzoin). Prominant herbs are Claytonia virginica, Dentaria laciniata and Orchis spectabilis, with Leaf area profile Height distribution of leaf area was determined with Aber's (1979) modification of the Mac Arthur & Horn (1969) camera method for vertical foliage profiles. Vertical sightings of the distance to the nearest leaf were made optically on a tripod-based (1 m) camera with a 200-mm telephoto lens calibrated to measure distances up to 40 m (+ 3%). Distances were recorded for each of 15 intersections on a gridded focusing screen at each of 80 sampling locations (10 around each of 8 positions at major compass directions near the meteorological tower, see Fig. 1). In the meter below the camera level, absolute leaf area index was estimated from the mean number of leaf con- tacts with a plumb line, at a total of 191 locations. Leaf species were recorded for each height obser- vation. Equations developed by Mac Arthur & Horn (1969) were used to estimate the distribu- tion of leaf area fractions with height. The impor- tance of each species at a given height was esti- mated from the fraction of observations within each 1-m height increment. Litterfall Litterfall was recovered weekly in the autumn of 1987 at each of 24 bushel baskets arranged in triangles at 8 compass directions from the tower (see Fig. 1). Leaf litter was sorted by species; the number of leaves per collection basket and date 20 meters Fig. 1. Map of the study site. Positions of lives stems 2.5 to 20.0 cm dbh (points), 20.1 to 40 (small circles), greater than 40 (large circles). The tower position is indicated with a square and the litterbaskets, with triangles. Boundaries of large canopy gaps are outlined. was recorded. Non-leaf litter was sorted into 3 categories: 1)twigs and branches, 2)fruits and seeds, and 3) bark and lichens. Samples from all collection dates were then composited by collec- tion basket and dried to constant weight at 60 ? C. These observations provided the mass of litter by type, species, and basket as well as the number of leaves by species, basket, and date. Relationships between leaf area and mass were obtained for tree species in the plot from individual leaves (21-55 per species). Leaf area was measured with a Li-Cor model 3100 Area Meter and mass obtained after drying to constant weight at 60 ?C. Regressions of area on weight gave correlation coefficients exceding 0.95 for each species (SAS Institute Inc. 1985). Area/ mass ratios (specific leaf area, SLA) tended to decline with increasing leaf mass and could not be regarded as constant. Accordingly, equations for predicting the area of groups of leaves of each species were produced from these data on individual leaves. The data on areas and mass of n individual leaves per species were randomly sampled in groups of from 1 to n - 1 (10 repli- cates per group) and the total area and total mass of the reconstituted sample were recorded. Linear regressions on these composited data yielded higher correlation values than for the individual leaves, with no evidence of curvilinearity. Leaf areas were calculated from the mass of leaf samples using these latter equations. Results Tree stem community 17 species of woody plants with dbh > 2.5 cm were found. Total density and basal area of those stems was 597 and 17.5 m2 for the 0.503 ha study area (Table 1). Numerous small trees of under- story species dominated stem density, but basal area was dominated by few, large overstory individuals. Carpinus caroliniana, Comus florida, and Fagus grandifolia accounted for nearly 80 % of the stems; Liriodendron tulipifera, Carya species and 4 species of Quercus dominated the basal area (>80% of the total). Canopy cover (fraction of points without open sky above) was 93.3 %; most of the open sky area was in several canopy gaps (see Fig. 1). Foliar area-mass relation Individual species differed markedly in mean leaf area, mass, and specific leaf area (SLA, cm2 leaf area per g of dry leaf, Table 2). Relative varia- bilities (coefficients of variation) of leaf area and mass for all species ranged from 30-75% and 40-85% for leaf area and mass, respectively. Specific leaf areas showed much less variation (13-23%). SLA was inversely correlated with leaf mass, both within and between species. Larger leaves had proportionately less surface area per unit mass than did small leaves. Also, large-leaved species had lower SLA than did small-leaved species. Equations for estimating areas from masses of Table 1. Total density and basal area of live stems greater than 2.5 cm dbh by species as estimated from stem census in the 0.503 ha plot. Percentage of total density and basal area given in parentheses. Species Density (%) Basal area, m2 (%) Importance * Liriodendron tulipifera 46 (7.71) 9.34 (53.49) 61.20 Carpinus caroliniana 209 (35.01) 0.41 (2.33) 37.34 Fagus grandifolia 124 (20.77) 1.42 (8.13) 28.90 Cornus florida 142 (23.79) 0.54 (3.08) 26.87 Carya spp.** 27 (4.52) 1.83 (10.50) 15.02 Quercus alba 7 (1.17) 1.47 (8.43) 9.60 Liquidambar styraciflua 15 (251) 0.75 (4.31) 6.82 Quercus velutina 5 (0.84) 0.91 (5.22) 6.06 Quercus coccinea 2 (0.34) 0.44 (2.52) 2.86 Nyssa sylvatica 6 (1.01) 0.11 (0.66) 1.67 Lindera benzoin 6 (1.01) 0.006 (0.04) 1.05 Juglans nigra 1 (0.17) 0.09 (0.51) 0.68 Quercus rubra 1 (0.17) 0.06 (0.33) 0.50 Acer rubrum 2 (0.34) 0.03 (0.15) 0.49 Ulmus americana 1 (0.17) 0.05 (0.27) 0.44 Ilex opaca 2 (0.34) 0.002(0.01) 0.35 Fraxinus americana 1 (0.17) 0.004 (0.02) 0.19 Sums 597 17.46 200 Sum of percentage density and basal area. C. tomentosa, C. glabra, C. cordiformis, and C. ovata. samples of multiple leaves (Table 3) had higher correlation coefficients than did similar relations for individual leaves, for all species. In this stand L. tulipifera dominated all cate- gories in the leaf community, including the total number (36.6%), mass (41.2%) and area (43.8%) of leaves (Table 4). Nearly 80% of the total num- ber of leaves was accounted for by L. tulipifera, Fagus grandifolia and Cornus florida. For leaf mass and area, codominants were Carya species and F. grandifolia. The total number of leaves was esti- mated at 1776 per sq.m. The leaf area index (LAI) and leaf mass, from autumnal litterfall observa- tions, were 5.26 m2m~2and 382.7 gm"2respec- tively. Leaves accounted for 87% of the total litterfall of 440.0 gm2. Fruits and seeds, pre- Table 2. Mean leaf area, dry mass and specific leaf area (SLA) by species. Parenthetical values are 95% confidence intervals. Species Area (cm2) Mass (g) SLA (cm2 g"1) Acer rubrum Carpinus caroliniana Carya spp.* Cornus florida Fagus grandifolia Liquidambar styraciflua Liriodendron tulipifera Quercus alba Quercus coccinea Quercus velutina 49.8 (9.4) 19.0 (2.6) 164.9 (35.9) 40.0 (6.1) 24.1 (5.1) 49.0(13.3) 51.7 (9.7) 44.3 (6.8) 73.5(16.5) 114.6(12.5) 0.234(0.051) 0.076 (0.013) 1.450 (0.254) 0.176(0.031) 0.155(0.027) 0.370(0.115) 0.352(0.071) 0.374 (0.048) 0.794 (0.165) 0.854(0.116) 219.5(15.0) 261.7(16.8) 108.6 (8.3) 237.6 (12.8) 156.2(14.0) 142.8(11.9) 152.8 (7.2) 117.7 (4.9) 93.4 (4.9) 140.8 (5.5) Mostly Carya tomentosa. Table 3. Coefficients of linear equations (y = mx + b) for estimating leaf area (cm2) from dry mass (g) of samples of multiple leaves (see text for method). Correlation coefficient (r) also given. Species b m r Acer rubrum 11.5 208.9 0.993 Carpinus caroliniana 2.0* 249.3 0.993 Carya spp. -29.9* 115.2 0.992 Cornus florida 2.8* 226.5 0.996 Fagus grandifolia 26* 154.6 0.979 Liquidambar styraciflua 17.5 129.2 0.990 Liriodendron tulipifera 10.2 145.8 0.997 Quercus alba -1.9* 118.6 0.996 Quercus coccinea -3.6* 929 0.995 Quercus velutina 14.5 133.4 0.998 Coefficient not significantly different from zero (p > 0.05). Foliage - height profile Total leaf area showed a bimodal distribution with height, with a distinct understory maximum at 5-6 m and a more diffuse peak between 25 and 30 m (Fig. 2, left panel). The corresponding mini- ma were at 0-3 m (distinct) and 11-16 m (diffuse). The highest leaves observed, at 37 m above the ground, were about 5 m above the aver- age canopy height. From the top of the canopy the cumulative leaf area downward (Fig. 2, right panel) reaches leaf area index of unity at about 29 m and LAI = 2 at 25 m above the ground. On the other hand, cumu- lative leaf area from the bottom of the canopy upward reaches a LAI = 1 (canopy closure) between 6 and 7 m above the ground. dominantly L. tulipifera samaras, contributed 5.7%; twigs, stems and branches, 6.9%; and bark and lichen fragments, 0.4%. The study stand was completely deciduous; trees of even normally marcescent species such as F. grandifolia and Quercus coccinea lost nearly every leaf. Stratification of leaf area by species Optical observations of leaf identity during the profile study indicated each species' contribution to the total leaf area within each vertical interval. Table 4. Total number, mass, and area of leaves by species as estimated from observation on litterfall. Percentage of total numbers, mass, and area given by species (in parentheses). Species Number, m~2 (%) Mass, gm"2(%) Area, m2m"2 (%) Importance * Liriodendron tulipifera 649.13 (36.55) 157.61 (41.19) 2.305 (43.82) 121.56 Fagus grandifolia 371.03 (20.89) 33.80 (8.83) 0.524 (9.96) 39.68 Carya tomentosa 58.76** (3.31) 65.13 (17.02) 0.731 (13.90) 34.23 Quercus coccinea 82.97 (4.67) 44.75 (11.69) 0.414 (7.87) 24.23 Cornus florida 170.83 (9.62) 13.76 (3.60) 0.313 (5.95) 19.17 Carpinus caroliniana 250.36 (14.10) 6.84 (179) 0.172 (3.27) 19.16 Liquidambar styraciflua 93.06 (5J24) 21.68 (5.67) 0.292 (5.55) 16.46 Quercus alba 69.55 (3.92) 19.96 (5.22) 0.235 (4.47) 13.61 Quercus velutina 19.36 (109) 11.03 (2.88) 0.157 (298) 645 Acer rubrum 8.41 (0.47) 1.35 (0 35) 0.036 (0.68) 1.50 Quercus falcata 1.93 (OH) 0.74 (019) 0.005 (0.10) 0.40 Quercus rubra 0.81 (0.05) 0.33 (0.09) 0.001 (0.02) 0.16 Unidentified fragments - - 568 (148) 0.077 (1.46) 2.94 Sums 1776.20 (100.04) 382.66 (100.00) 5.260 (100.03) * Sum of percentage numbers, mass, and area. ** Calculated as the quotient of the estimated number of leaflets (407.8 m-2) and the average (6.94 + 0.89; n = 36). 300.05 number of leaflets per leaf 35 - 30 - r_J?' 25 - "r-^3 H20 ? g x 15 - ^ Z3 10 - 'u_ 5 - I 0 - cT^ LAI PER m INTERVAL 12 3 4 5 CUMULATIVE LAI lative (right Vertical distribution of LAI (left panel) and its cumu- distribution downward from the top of the canopy panel). Fig. 3 shows the stratification of tree foliage with height for all the canopy species, from those re- stricted to the understory {Carpinus and Cornus, toward the left side) to exclusively overstory species (Q. coccinea, right side). Several species intergrade between the understory (< 10 m), mid- canopy (10-25 m), and overstory (>25m). F. grandifolia, though with several large trees (dbh to over 40 cm), holds its leaves predominantly in the understory and lower mid-canopy. Leaves of Liquidambar styraciflua, Q. alba and Q. velutina are found in the mid-canopy. Carya species (mostly C. tomentosa) and L. tulipifera, the tallest species in this stand, have leaves over a wide range (>25 m) of the mid and upper canopies; Carya favors the mid-canopy, and L. tulipifera is the dominant of the overstory. The leaves in the highest light environment in this stand are L. tulipifera, Carya spp., and Q. coccinea. Leaf species richness peaked in the 7-11 m stratum (7 species) and in the 20-25 m stratum (6 species). 35- 30- 25- 20 O LU I 15- percent of observations [?i |?i i I I I I l I 0 50 100 Liriodendron tulipifera Fagus ll grandifolia Carpinus caroliniana Cornus florida Acer rubrum LE o-1 Fig. 3. Vertical stratification of tree foliage by species, indicated by the fraction of all leaves for each species observed within each interval of height. 7 Stratification of leaf number, area, and mass The vertical distribution of leaf number and mass may be obtained from that of leaf area. The leaf number at each height interval, N{h), is estimated as the sum, across all species, of the product of leaf area, LA(h) (Fig. 2), the species fractional contribution, A(sp) (Fig. 3), and the species number/area ratio, NAR{sp) (from Table 4). That is, N(h) = ? LA(h) *A{sp) * NAR(sp). (1) Leaf mass distribution (M(h)) is obtained similar- ly, except that the species mass/area ratio (MAR(sp)) is substituted for the NAR(sp): sp M(h) =YJLA(h)*A (sp) * MAR (sp). (2) Vertical distributions are distinctly different for area (Fig. 4, left panel), number (center panel) and mass (right panel). Profiles for number and mass are also bimodal but with peak concentrations at different heights. The under story mode of leaf numbers is 4 to 5 times larger than the overstory mode. Peak leaf biomass is in the overstory, but peak leaf area and number are in the understory. The height corresponding to the median leaf area is at approximately 19 m, whereas for leaf number it is at 7 m, and for leaf mass, at 22 m. Half the leaf area is at half the canopy height, but median leaf number is at one fifth and median leaf mass at two-thirds of the full height of the forest. The ratio of leaf area and leaf number distributions indicates that average leaf areas differ consider- ably between canopy strata. Mean leaf areas in the mid-canopy and overstory are large (averaging 50 cm2) but those in the understory are distinctly smaller (20-30 cm2). Leaf duration Sequential sampling of collectors throughout the litterfall season provided an estimate of the timing of leaf abscission by species. When the cumulative number of leaves collected over all sampling lo- cations is considered (Fig. 5), a litterfall sequence is evident by species. Understory species C. caro- liniana and C. florida lose leaves first, followed by L. styraciflua, A. rubrum, L. tulipifera, and F. grandifolia (Fig. 5). Last to fall are leaves of Quercus and Carya species. The median litterfall dates for the first and last species to fall are 17 days apart, 8% of the growing season length AREA 35- 30 25- \ 3 ?20- a UJ = 15- i r ^ 10- 5- 01 NUMBER MASS 0 2 4 6 8 10 0 PERCENTAGE OF TOTAL PER m INTERVAL Fig. 4. Comparison of the vertical foliar distribution by leaf area (left panel), number (middle), and mass (right). 270 280 300 330 Day of the year Fig. 5. Cumulative percentage of total number of fallen leaves by species through the 1987 litterfall season (C.c. = Carpinus caroliniana, C.f. = Cornus florida, L.s. = Liquidambar styraciflua, F.g. = Fagus grandifolia, L.t. = Liriodendron tulipifera, C. spp. = Carya spp., Q. spp = Quercus spp.). (frost-free period averages 210 days according to Lull 1968). If the phenology of leaf expansion at the beginning of the growing season is similar to that of abscission then some species may have a significantly longer lifespan in the canopy than others, to the extent that expansion and ab- scission coincide with the beginning and end of the functional period of leaves (Lechowicz 1984). The community of stems and the community of leaves Relative abundances of stems differ markedly from those of leaves. Leaves of some species (e.g. Carpinus, Cornus) are far less numerous than the stem values would suggest; other species (e.g. Q. coccinea) have proportionately far more leaves than expected on the basis of stems. The correla- tion between % of total numbers of stems and % of numbers of leaves for the 11 species compared was 0.441 (p > 0.05), but between percent area of leaves and basal area of stems, 0.977 (p < 0.01) (Fig. 6). Number Area Relative stem abundance Relative basal area Fig. 6. Relation between relative number of stems and number of leaves by species (left panel) and between relative basal area of stems and leaf area by species (right panel). Solid lines mark equivalence of leaf and stem attributes. Discussion Canopy folliar structure has components due to vertical level, seasonal duration, and leaf species. In a deciduous forest, the importance of these separate factors may be assessed non-destruc- tively, with a program of ground-level observa- tions. The foliage community in this stand is dominated in all aspects by L. tulipifera, particu- larly at the primary atmosphere-canopy interface in the upper third of the layers. This species thus intercepts light, precipitation, momentum and large particles first. Lower layers receive throughfall, sunfleck and diffuse radiation, and particle and gas loads that are substantially modi- fied by L. tulipifera. By considering the vertical distribution of leaf area by species we have also estimated the vertical distribution of number, mass, and average area of leaves. This information is useful beyond micro- meteorological and leaf-physiological studies. The distribution of leaf mass can help in estimating wind loading and susceptibility to stem breakage (Strong 1977). The corresponding dis- tribution of leaf and branch area can yield the distribution of intercepted water (Lindroth & Halldin 1986) and insights into the chemistry of throughfall precipitation (Carroll 1980). The stra- tification of leaf number may be of importance to folivores and foliage-gleaning birds, for whom the number of leaves may be more important than their combined area (Jackson 1979). The leaf area index in this stand was deter- mined from foliage abscised following the litterfall season. Because of unmeasured losses due to early leaf drop and consumption by canopy herbivores, the LAI is an underestimate. Earlier studies in deciduous temperate forests have shown that autumnal litterfall accounts for almost all of annual leaf fall (Bray 1964; Bray & Gorham 1964; Carlisle etal. 1966; Hughes 1971; Dixon 1976). The effects of folivores is variable and can range from a few percent to complete defoliation; 5 to 15% of leaf area is commonly removed (Schowalter etal. 1986). However, early ab- scission and canopy herbivory are low in forests similar to the one studied: average annual leaf area lost over three years in a Tennessee Lirioden- dron forest was 5-10%, which included both her- bivory and the expansion of leaf holes (Reichle et al. 1973). If 10% of leaf area were lost then the maximum LAI of this stand would be 5.84. The foliage-height distribution might be altered by leaf consumption since folivory may be specific to physiological types of leaves (e.g., shade vs. sun) or to different leaf species, which are in turn differ- entially distributed with height. The estimation of vertical distribution of leaf mass and leaf number from that of leaf area as- sumes that the relation between number and area or mass and area is constant with height. Yet understory (shade) leaves of canopy trees often have higher leaf area and lower SLA than do overstory (sun) leaves of the same species (Jackson 1967; Larcher 1980). Thus the size of the overstory mode of leaf mass distribution (Fig. 4) may be overestimated and the understory one underestimated. Similarly, the extent of the overstory mode of leaf number is likely to have been underestimated and the understory mode overestimated. Spatial variability of canopy attributes The spatially averaged canopy leaf area, number, and mass may be assessed with observations on litterfall such as we have presented. However, the variability or confidence intervals of these canopy attributes are not equivalent to those obtained from ground-based observations. The coefficient of variation (CV) of mean LAI estimated with litterfall observations from 24 collectors is 10.7%. The corresponding 95 % confidence limits for the mean LAI are narrow, from 5.02 to 5.50. Leaf mass and number have similarly low variability (CV = 11.0 and 15.1%, respectively), also with narrow confidence bounds (364.9 to 400.4 g m " 2 and 1767 to 1794 m"2 respectively). In the canopy however, leaf area, number, and mass are not so evenly distributed horizontally. For example, the mean percentage of points with open sky at each sighting position had a coefficient of variation of 211.1% over the eight sectors. The spatial distribution of leaves within 10 the canopy is likely to be at least as variable as that of unobstructed vertical channels. Leaves form disjunct, monospecific clusters within the canopy. With increasing height in the canopy, single crowns become fewer in number and farther apart (e.g. Ishizuka 1984; Bongers etal. 1988; Popma et al. 1988) and the areas of canopy gaps larger (Hubbel & Foster 1986). This mosaic pattern is considerably averaged by the litterfall process (Ferguson 1985) such that a given area of forest floor contains mixtures of leaves from a large, poorly defined volume of canopy above it. Quantification of spatial variability of elements within tree crowns will require methods which provide appropriate resolution in all three spatial dimensions within the canopy. It would be useful to describe the structural importance of canopy elements not only by the total amount of those components (e.g. leaf area) but also by their distribution by species, height and duration. The importance of a particular species in the canopy over the growing season (leaf area duration, LAD) would be phenologi- cally weighted, i.e. 6XAr = 6X,/0<&, (3) Where LAsp(t) gives the species leaf area with time and te and ta are times of emergence and abscis- sion, respectively. Hypothetically, differences in time of emergence and abscission could offset or exacerbate variations in relative leaf area. Extrapolation of functional attributes from single elements to the full canopy will be aided by the sort of measurements described here. Several effects contribute to the disparity between the importance values of leaves and stems. First, some stems contributing to litterfall were not censused (e.g. Q.falcata); leaves were never collected in litter or counted in the canopy for some of the rarer species of stems censused {Nyssa sylvatica, Ulmus americana, Lindera benzoin, Juglans nigra, and Ilex opaca). Differ- ences between species representation in leaves and stems would likely remain even if sampling were ideal. A stem's size reflects the history of its functional success, whereas its leaf area relates closely to its current function (Waring 1983). Sapwood area, the amount of conducting tissue, is more closely related to foliage area than is total basal area (Shinozaki etal. 1964a, b). This result suggests that leaf assemblages pro- vide potentially more information about the species composition of local basal area than about that of the number of stems in the vicinity of the sample. Thus inferences concerning the quantity or species importances of leaves derived from information on stem distributions must be made cautiously. Conclusions The extrapolation of measurements on functions taken at fine scales to that at the scale of the whole forest canopy requires intermediate detail. This study reports on the distribution of canopy leaves by position, type, and timing in a mature Lirioden- dron forest on the Maryland coastal plain. These data should prove useful in paleoecological studies for inferring plant species composition from leaf assemblages, for inferring species effects in canopy processing of materials and energy, as well as for the extrapolation of process measure- ments taken at small scales to larger scales. Ground-based measurements can yield much detail about canopy structure, but they are often limited to providing spatial averages. Other approaches, ultimately requiring controlled ac- cess within the canopy, will be necessary for pro- viding details on its spatial variation. Acknowledgements We thank Liza Remenapp and Robyn Burnham for help in the field, Kathy Beebe for management of the data, and Robyn Burnham, Dave Correll, Peter Curtis, and Bert Drake for discussions and useful comments on previous versions of this pa- per. 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