Functional E C O ~ O1989, ~ 3, 53-62 Canopy organization and foliage photosynthetic capacity in a broad-leaved evergreen montane forest D . Y . HOLLINGER Forest Research Institute, New Zealand Ministry of Forestry, PO Box 31-01I , Christchurch, New Zealand Abstract. The spatial and physical characteristics of foliage varied systematically with canopy height in a broad-leaved evergreen forest of Nothofagus solandri var. clifforfioides (H0ok.f.) Poole (mountain beech) in the Craigieburn Range, South Island, New Zealand. Leaf inclination angle, leaf weight per unit area, leaf nitrogen concentration, leaf photosynthetic capacity and leaf reflectance of photosynthetically active radiation all decreased with depth in the canopy. These changes will result in a more uniform distribution of photosynthetically active radiation through the canopy and a greater canopy carbon gain than possible in canopies composed of randomly oriented leaves with constant physical properties. The mean leaf area index of the Nothofagus solandri var. cliffortioides foliage was 6.7 and mean branch silhouette area index was 1.4. Mistletoe (Alepisflavida [Hook.f.] Tiegh) was present in the canopy and had a leaf area index of 0.14, and a stem silhouette area index of 0.02. Total beech and mistletoe foliage biomass was 1183 and 36g m-2 respectively and total beech and mistletoe foliage nitrogen mass was 12.3 and 0.37g m-'. Key-words Leaf orientation, leaf area ~ n d e x ,leaf nitrogen, Nothofagus, photosynthetic capacity, mistletoe Introduction How are the canopies of trees and other plants organized? The null hypothesis is that there is no organization; that canopies consist of uniform leaves randomly displayed in angle and orientation. Although many researchers implicitly reject this hypothesis, the null hypothesis has not been rigorously tested in broadleaf trees. Instead, in many simulation models, physical or physiological properties are constant but leaf angles and orientation follow some a priori distribution such as that on the surface of a sphere. A third hypothesis is that plant canopies are in some way optimal. Many researchers have considered how canopies may display foliage to maximize space filling, light interception, or photosynthetic carbon gain (e.g. Monsi & Saeki, 1953; Verhaggen, Wilson & Britten, 1963; de Wit, 1965; Duncan, 1971; Horn, 1971; Fisher & Honda, 1979; Miller & Stoner, 1979; Oker-Blom & Kellomaki, 1982; Oker-Blom, 1984). These analyses are hindered by the enormous complexity of calculating the canopy radiation field (Ross, 1981) and the unknown significance of the simplifying assumptions. In canopies composed of leaves with constant physical properties, photosynthetic gain is maximized when all leaves are receiving an identical photon flux (Verhaggen et al., 1963). Thus carbon assimilation by a canopy is increased by characteristics which lead to a uniform light distribution through the canopy by enhancing light penetration at the top of the canopy and increasing interception at lower levels. Examples include vertically orientated leaves at the top of dense canopies with horizontal leaves lower down (Duncan, 1971) and small leaves at the top of multilayered canopies (Horn. 1971; Oker-Blom, 1984). Similarly. other analyses have investigated how nitrogen, a resource known to be directly related to leaf photosynthetic capacity (e.g. Field & Mooney, 1986). might be allocated amongst foliage in different microsites to maximize canopy carbon gain (Field, 1983: Hirose & Werger. 1987). Results from Field (1983) suggest that canopy carbon gain is increased when greater amounts of nitrogen are allocated to leaves in brighter microsites. Together, these studies suggest that canopy carbon gain of an individual plant will be increased if leaf spatial and physical properties vary through the canopy so that (1) light will be more uniformly intercepted through the canopy than would be expected with a random arrangement of foliage and (2) leaf nitrogen will be continuously apportioned with the greatest amount in the brightest canopy sites (i.e, the top). 54 A final alternative considers the competitive D. Y. Hollinger environment of plants, and suggests that the canopy architecture of a plant must represent an evolutionarily stable strategy (ESS) according to the criteria of Maynard Smith (1976). To be an ESS there must exist no other alternative architecture that increases fitness over the common architecture. A tree architecture representing an ESS, for example, may not maximize light interception or carbon gain in isolation but will (assuming these are related to fitness) i n the competitive forest environment. It is unlikely that Duncan's optimum architecture of vertical leaves at the top of a canopy and horizontal leaves at the bottom is evolutionarily stable (Duncan, 1971). A competitor could arrange its foliage more horizontally high in the canopy and increase its light interception and carbon gain to the detriment of a n adjacent individual with Duncan's architecture. This study considers these hypotheses concerning canopy organization with reference to the architecture of undisturbed, monospecific Nothofagus solandri var. cliffortioides (H0ok.f.) Poole forest, although data from other forest types are also discussed. The Nothofagus solandri var, cliffortioides system is suited to investigations on whether a canopy might be organized to increase carbon gain because the crown of each tree generally extends from the top to the bottom of the canopy (important as any trade-off at one level can be recovered at a lower one) and because the canopy is monospecific; there being a high probability that neighbouring trees will be related (important in any inclusive fitness arguments). Data on the size, location, and optical properties of leaves, and orientation of leaves and stems in a forest canopy are also critical prerequisites in efforts to model the canopy radiation regime or carbon gain. Data presented here may cause modellers to question some of the assumptions they build into their models and provide a starting point for further modelling efforts of our own to assess the consequences of canopy organization on carbon gain and fitness in N. solandri var. cliffortioides. Materials and methods Components of the canopy architecture were measured in a stand of monospecific New Zealand mountain beech (Nothofagus solandri var. cliffortioides [Hook.f.] Poole) growing on a level site at an elevation of 900m in the Craigieburn Range (Lat. 43O08'S, Long. 171°41'E), South Island, New Zealand. Basal area of the stand was approximately 72mZ ha-' and trees were relatively even aged, most being 70-90 years old. Canopy height ranged between 15 and 17m. Mean annual rainfall at this montane maritime site is approximately 1450mm. and the mean annual air temperature (1964-1980) is 8.0°C (McCracken, 1980). Daytime temperatures rarely exceed 25°C or remain below zero. Frosts are common throughout the year, with the average frost-free period at this site being 41 days. Mountain beech retains much of its foliage into a second year. Foliage loss occurs gradually during the second summer after budbreak (Wardle, 1970). The canopy was sampled in mid-March (1987) after the current foliage had fully expanded and much of the previous season's growth had been lost. Samples of foliage for determining the leaf area distribution were obtained from 18 randomly located square columns (0-5m on a side) that extended from the ground to the top of the canopy within the 42 m2 confines of a 21 m tall scaffolding tower. Areas where such a vertical column would meet a tower walkway or ladder were not sampled. Columns were delineated by stretching four nylon cords from a frame fixed at the top of the tower to one located just above ground level. All of the foliage and the branches with a diameter <I cm within the column were removed in l m increments from the ground up by hand clipping, giving (with a maximum tree height of 17m) 17 samples, each 0.25m3 volume, for each column. About one-half of the sample volumes did not contain foliage. Leaf sub-samples (approximately 100 leaves) were taken from each sample for nitrogen analysis and for area and weight determinations. These sub-samples were sealed in plastic bags and kept cool until the area determinations were made (within 3 h of sampling). The area of the fresh foliage and twig samples was determined with a video leaf-area meter (model AMS-Basic, Delta-T Devices, Cambridge, England). Nitrogen concentrations were determined by automated colorimetric analysis of Kjeldahl digests of ground (20 mesh) leaf material. Accuracy of the digestions and system were checked by analysis of standard foliage (Pinus radiata D. Don needles, batch FRI 2976). Harvested twigs were dried at 70°C for 24 h in a forced draft oven, leaves were separated from stems, redried and then weighed. Leaf areas for each sample were calculated based on the arealdry weight relationship of each fresh sub-sample and the total leaf dry weight of the associated sample. 55 Canopy organization The projected area of branches was measured in eight of the sample columns. For branches and twigs with diameters larger than l c m in each sample volume, measurements were made of branch length and diameter. For twigs with diameters < I cm, projected area was estimated from the area to weight ratio of a bulked twig sub-sample and the dry weight of twigs in each increment. Leaf inclination angle from the horizontal and azimuth angle were measured with an inclinometer and compass at the top of the canopy (15-16m), mid-canopy (11-12m) and at the bottom of the canopy (7-8m). All the leaves on a number of randomly selected twigs were measured, giving a total of approximately 175 leaves for each canopy level. N. solandri var. cliffortioides leaves are predominantly planar and few problems were encountered with the inclinometer measurements. The azimuth data were averaged according to Batshelet (1981). Photosynthetic measurements were made on mature foliage in the spring prior to growth of new foliage with the system described by Schulze et al. (1982). Each twig sample was illuminated with a photosynthetically active photon flux density l s-' supplied by a (PFD) of 1000-1500 ~ m o mP2 400-watt metal arc lamp. Projected leaf areas were measured with a video leaf area meter (model AMS-Basic, Delta-T Devices, Cambridge, England). Reflectance and transmittance characteristics were measured for seven leaves randomly selected from each of the three canopy levels in midsummer 1986. Measurements of leaf optical properties between 400 and 1100nm were made with an external integrating sphere (model 1800-12, Li-Cor, Inc., Lincoln, Nebraska, USA) and scanning spectroradiometer (model Li-1800, Li-Cor). Adaxial and abaxial surfaces were measured separately. The average of two scans of each leaf surface (2nm wavelength intervals) was used for the calculations of leaf transmittance and reflectance. Results Foliage distribution and leaf characteristics The mean leaf area index (LAI) of the N. solandri var. cliffortioides foliage was 6.7 f 1.2 (values in text given as mean and 95% confidence interval). The silhouette area index of twigs and branches was 1.4 f 0.7. Foliage was concentrated at the top of the canopy (15-16m), with a second apparent peak in the mid-canopy (10m) (Fig. 1). Little Nothofagus solandrl var. clltfortlo1d.s I 1 Leaf area index by tier Fig. 1. Leaf area index by 1m tier and relative height (A) in a Nothofagus solandri var. cliffortioides stand. Error bars signify one standard error. Sample size for each tier shown on figure. Total LA1 = 6.7for beech, 0.14 for mistletoe. On Figs 1-5, the verticaI positions of the observations are given both as (left ordinate) height above the ground an? (right ordinate) in the dimensionless form f i where H = 1-HIHwhere H = height above ground and H = maximum tree height in the stand, to facilitate comparison with results from other forests (e.g. Rauner, 1976). + foliage was found in the 2-5m strata; foliage at the l m height was N. solandri var, cliffortioides seedlings. Two species of mistletoe, Peraxilla tetrapetala (Murr.) Tiegh. and Alepis flavida (H0ok.f.)Tiegh., grew in the canopy, although only Alepis flavida was encountered in the actual harvests (present in 8.6% [13] of the 151 samples containing foliage). The mean LA1 of Alepis flavida was 0.14 0.17 and the mean stem silhouette area index (SAI) was 0.02. Foliage biomass of N. solandri var. cliffortioides was 1183 241g m-2 and of A. flavida 36 54g m-'. Foliage occurred in fewer of the sample volumes at the top of the canopy than at lower layers (P < 0.01, X2 = 9.5, d.f. = 2). Foliage in the top third of the canopy by LA1 (>=15m), was found in 48% of the samples in this region. In the middle third of the canopy (11-15 m), foliage was present in 61% of the samples, and in the bottom third (7-llm), 75%. The foliage area densities (calculated only from samples containing foliage) for the top, middle and bottom third of the green canopy were 1.6, 1.0 and 0.6 m2 m-3. The foliage area densities calculated from all samples at these levels were 0.8, 0.6 and 0.4m2 m-3. + + + 56 higher than in any of the canopy above (Fig. 4). Because of the concurrent decrease in leaf weight per unit area, the decrease in leaf N per unit area (g mP2)with depth into the canopy was greater than the decrease in N concentration (Fig. 4). The N weight per unit area of foliage at the bottom of the canopy at 1.05g m-2 was about 41% of the canopy top value of 2.54g m-'. Beech and mistletoe foliage N concentrations were not significantly different. There was no trend of N concentration or N weight per unit area with height in the mistletoe. N per unit area was high in the mistletoe at about 2.7g m-'. Total canopy nitrogen mass for the beech and mistletoe was 12.3 2.0 and 0.37 f 0.48g m-'. Leaf photosynthetic capacity (photosynthesis at 18-23 O C , leaf-air vapour concentration difference 5-10 mmol H20mol-' air, PFD 1000-1500 kmol m-2 s -1 ) declined with depth into the canopy (Fig. D. Y. Hollinger + Mean leaf size (ern2) Fig. 2. Mean leaf size as a function of canopy position. Error bars signify -t one standard error, sample size of each tier shown on Fig. 1. 5), reflecting the decline in leaf nitrogen. Photosynthetic capacity varied with leaf N concentration and hence location in the canopy. Leaf N content or weight N per unit leaf area accounted for most of the variance (r2 > 0.85) in photosynthetic capacity. Photosynthetic capacity at the top of the canopy at approximately 5 kmol mP2 s-' was 2-3 times greater than at the bottom of the Mean leaf size in N. solandri var. cliffortioides reached a maximum of 0.83 cmZin the mid-canopy (Fig. 2). Leaves at the 10-11m level of the canopy were about 36% larger than those at the top of the canopy and 93O/0 larger than those at the bottom. Mean leaf size in A. flavida was 3.0cmZ and did not vary with height (data not shown). The mean leaf weight per unit area of beech foliage decreased continuously from the top of the canopy (Fig. 3) and values at the bottom of the canopy (6m) were only about half of those at the top (16m). The leaf weight per unit area of the seedling (1m) strata was only about 25% of that at the top of the canopy. Mean mistletoe leaf weight per unit area at 263 k 31g m-2 was similar to that of beech leaves from the top of the canopy, but did not vary systematically with height. Beech A Nitrogen distribution and leaf photosynthetic capacity Leaf nitrogen concentrations were significantly higher in the mid- and upper canopy than at the bottom (P < 0-01, F test), where the mean concentration was about 85% of that at the top (Fig. 4). However, seedling foliage N concentration was 0 1 , loo Mislk10~ I 300 200 Leaf weight per unit area (g II-~) Fig. 3. Leaf weight per unit area of Nothofagus solandri var. cliffortioides and Alepis flovida as a function of canopy position. Error bars signi@ f one standard error, sample size of each tier shown on Fig. 1. Canopy position: Bottom Middle D. Y. Hollinger Inclination angle class (degrees) Fig. 6. Leaf inclination angle distributions at three heights within a Nothofagus solandri var. cliffortioides canopy. 12.6% for the random distribution, 11.8% for the best fit elliptical distribution (ratio of horizontal to vertical semi-axes of 2.1) and 26.2% for the spherical distribution. The 95% confidence intervals for 'G' values calculated from these distributions on the summer solstice are 12.6% (random), 13.6% (elliptical 2.1) and 32.5O/0 (spherical) of the 'G' value calculated for the N. solandri var. cliffortioides canopy. 0-001, F test). PAR reflectance from the adaxial surface of leaves was relatively constant with canopy depth at about 0.06 (Table 2 and Fig. 9) but that of the abaxial surface was 2-4 times this value and decreased significantly at the bottom of the canopy (P < 0.001, F test). The abaxial surfaces of leaves at the top of the canopy were densely pubescent. This pubescence decreased at lower levels in the canopy. The spectral reflectance of the abaxial (pubescent) surfaces in the PAR band decreased below about 450nm but was otherwise quite uniform (Fig. 9). The PAR reflected from both leaf surfaces at the bottom of the canopy was enhanced in the 500-600nm region. The reflectance of NIR from both leaf surfaces and at all levels was relatively constant at about 45% (Table 2). The pubescence on the abaxial leaf surface had no significant effect on NIR reflectance. Discussion The LA1 of 6.7 for this stand (beech and mistletoe) is between that of two N. solandri var. cliffortioides pole stands measured by a different Optical characteristics of leaves The mean leaf transmittance of photosynthetically active radiation (PAR) increased significantly down from the top of the canopy (P < 0.001, F test) and was not significantly different for adaxial or abaxial surfaces (Table 2). Most of the increased transmittance of PAR with depth was in the 500-600nm region (Fig. 8). Leaves at the bottom of the canopy also transmitted significantly more near infrared radiation (NIR) than those higher up in the canopy (P< 0.001, F test). The transmittance of NIR is not symmetrical; more NIR was transmitted through the leaf from the abaxial than from the adaxial side (P < 0-001, F test). The reflectance of PAR from the adaxial and abaxial surfaces was significantly different (P < ........ - - -01: I 0800 N. solandrl var. c1111or11e1dos R.",om Sph.rlc.1 Elllp8otdml (2.1) I I I I I 1 0800 1000 1200 1400 1800 1800 Solar time Fig. 7. Projection of foliage area in the direction of the solar beam ('G')of Nothofagus solandri var. cliffortioides foliage from top of canopy and three hypothetical foliage distributions at the spring and autumn equinox. Table 1. Mean leaf angles at different levels in a Nothofagus solandri var, cliffortioides canopy (mean and standard deviation). Canopy level Inclination (O) Orientation (0) Top (15-16m) Middle (11-12 m) Lower (7-8m) 43.3 (22.4)a 22.1 (15.6)" 17.0 (14.3)a 345 (77)b 355 (74)C 315 (75Ib -- - " Significantly different from a spherical distribution ( P< 0.01,Kolmogorov-Smirnov one-sample test. Not significantly different from a mean direction of 360" ( P< 0.05,V test, Batschelet, 1981). Not significantly different from a mean direction of 360" ( P< 0.005,V test). 59 Canopy organization ;60 . . S - Bottom -. .... . ... _ ..................... Middle .' . L. Wavelength (nm) Middle Fig. 8. Mean Nothofagus solandri var. cliffortioides foliage transmittance (adaxial surface) at three heights within the canopy. Error bars represent the maximum standard error in the 400-700nm wave band (n = 7). + "'a abaxial S 20 0400 technique (Nordmeyer, 1980a) and is typical of values from broad-leaved temperate evergreen forests (e.g. Kira, Shinozaki & Hozumi, 1969). Values from deciduous forests tend to range somewhat lower, from about 3-6 (Miller, 1967; Ford & Newbould, 1971; Miller & Lin, 1985; Hutchison et al., 1986). The distribution of foliage area within the canopy observed here is bimodal. Hutchison et al. (1986) reported a bi- or even tri-modal distribution of foliage in a deciduous oak-hickory forest. The mean N concentrations of N. solandri var. cliffortioides and A. flavida foliage are very low. However, the Craigieburn data are consistent with 530 600 adaxlal 800 700 900 1000 1100 lL!dx 40 abaxial 20 0400 500 adaxial 600 Wavelength 700 800 (nm) 900 lo00 1100 Fig. 9. Mean reflectance of abaxial and adaxial surfaces of Nothofagus solandri var. cliffortioides foliage at three heights within the canopy. Error bars represent 2 the maximum standard error in the 400-700nm wave band. Table 2. Mean leaf optical properties by height in a mountain beech canopy (mean and standard deviation). Transmittance - Canopy level - - 400-7OOnm - 702-1100 nm adaxial abaxial adaxial abaxial Top (15-16m) 0.029 (0.008) 0.026 (0.005) 0.281 (0.030) 0.314 (0.034) Middle (11-1 2 m) 0.031 (0.009) 0.033 (0.008) 0.287 (0.045) 0.349 (0.027) Bottom (7-8m) 0.056 (0.008) 0.060 (0.006) 0.416 (0.025) 0.465 (0.024) Tukey's critical difference (P < 0.05) 0.011 0.051 Reflectance Canopy level 400-700 nm 702-1100 nm adaxial abaxial adaxial abaxial Top (15-16m) 0.060 (0.006) 0.233 (0.036) 0.442 (0.071) 0.495 (0.060) Middle (11-12m) 0.059 (0.005) 0.199 (0.027) 0.427 (0.069) 0.477 (0.050) Bottom (7-8m) 0.066 (0.006) 0.127 (0.018) 0.428 (0.026) 0.427 (0.052) Tukey's critical difference (P < 0.05) 0.032 0.091 60 previously reported values for Nothofagus sol- D.Y. Hollinger andri var. cliffortioides (Adams, 1976; Nordmeyer, 1980b). Trends i n spatial and physical properties of leaves Many of the trends found in this study seem to be generally characteristic of forests. Leaf weight per unit area decreased with increasing depth, as also found by Miller (1967), Kira et al. (1969), Ford & Newbould (1971) and Hutchison et al. (1986) and is probably a general feature of plants which produce leaves simultaneously throughout a canopy. The pattern of a mid-canopy maximum in mean leaf size is identical to that reported for a deciduous forest canopy (Hutchison et aJ., 1986). The small size of N. solandri var. cliffortioides leaves and the windy but otherwise moderate habitat suggest that the size trends observed in the Nothofagus solandri var. cliffortioides canopy are not associated with thermoregulation or water-useefficiency (e.g. Parkhurst & Loucks, 1972; Givnish, 1979). Leaf inclination angle seems to decrease with increasing depth in the canopy in many forests and has been observed in aspen, oak, pines and oakhickory canopies (Miller, 1967; Ford & Newbould, 1971; Norman & Jarvis, 1974; Hutchison et al., 1986). Horizontally inclined leaves will be more efficient collectors of the isotropic diffuse radiation that makes up anincreasing proportion of the total radiation with increasing depth into the canopy. The general model of a spherical foliage angle distribution that has proved useful for many crop species (e.g. Ross, 1981)may be inappropriate for broad-leaved forest canopies. Either random or ellipsoidal foliage angle distributions better model the true distribution at the top of the canopy and increasingly horizontal distributions are necessary lower in the canopy. Most observers have assumed that foliage orientation angles are random. However, the subtle non-random azimuthal orientation observed in the N. solandri var. cliffortioides canopy may be widespread. Miller & Lin (1985) also presented data showing forest leaf orientation angles biased toward the equator. The azimuthal asymmetries in a canopy may perhaps be best addressed in simulation models by directly calculating the 'G' value (or k, the related extinction coefficient where k = Gfsin 0 , 8 = the solar altitude) from measured foliage orientations by the method of Lemeur (1973). The high leaf reflectance at the top of the canopy attributed to leaf abaxial surface pubescence will increase photosynthetically active photon flux densities in the lower canopy. The increased reflectivity at the top of the canopy will tend to re-direct light reflected from the lower layers of the canopy. Alternative hypotheses for the change in reflectance with height including more favourable energy balance eonsiderations associated with either increased reflectance or an increased boundary layer, are not supported by the constant low reflectance of the adaxial surfaces, the moderate temperatures at the site, or the broad photosynthetic temperature optima in N. solandri var. cliffortioides (Hollinger, unpublished data). Similar profiles in leaf reflectance might also be expected in other dense canopies where light intensities at the top of the canopy are well above saturating levels for leaf photosynthesis. The change in many leaf characteristics (e.g. leaf weight per unit area, N concentration, photosynthetic capacity) through the canopy suggests that any separation into 'sun' and 'shade' foliage classes is arbitrary. Instead, foliage characteristics appear to change continuously - presumably in response to a continuous change in the light micro-environment. Canopy architecture - moving towards optimal carbon gain We can reject the hypothesis that leaf angles in N. solandri var. cliffortioides and several other tree species are random and that leaf physical properties are constant. We can also reject the hypothesis that leaf orientations are consistent with a spherical model. However, are the observed leaf angles and characteristics consistent with an optimization of some factor, or does the canopy architecture represent an evolutionarily stable strategy? Leaf angles in the canopy were not consistent with maximizing carbon gain according to the results of Duncan (1971), although the deviations from random (e.g. more horizontal angles lower in the canopy) are in the direction that would increase rather than decrease carbon gain. Other deviations in leaf properties from constant values were also in the direction that would increase carbon gain. Smaller leaves at the top of the canopy result in greater penumbral effects (Horn, 1971; Oker-Blom, 1984) and leaf absorptance (l-reflectance-transmittance) is lowest at the top of the canopy. Both of these changes serve to increase photosynthetically active photon flux densities in the lower canopy which increases carbon gain in 61 Canopy organization canopies consisting of leaves with constant physiological properties (Verhaggen et al., 1963). Nitrogen (and consequently additional photosynthetic capacity) in N. solandri var, cliffortioides is also aUocated in the direction that enhances overall canopy carbon gain (e.g. more at the top) (Field, 1983). Consistent with a hypothesis that these variations maximize canopy carbon gain and are not just a result of canopy position, leaf size and N content in mistletoe show no variation with height in the canopy. The data presented here show that canopy properties are not constant with depth, so that the results of Verhaggen et al. (1963) and Duncan (1971) may not apply. Explicitly to specify the optimization function with the additional freedom of varying foliage is far born trivial. The 'optimum' architecture for carbon gain will depend upon a myriad of factors, only some of which are presented in this study. Some of the more important additional factors are the photosynthesis-light and photosynthesis-nitrogen relationships, general climate of the site (relating both to the ratio of direct to diffuse radiation and the timing of favourable photosynthetic periods) as well as any costs (e.g. herbivory) associated with various architectures. To consider the evolutionary stability of an architecture requires additional data about how architecture of adjacent individuals may interact, the relatedness of these individuals and about the heritability of architecture. Simulation models provide a powerful method of testing some of these ideas. Natural systems provide other avenues of investigation. Both the natural variation within a species and that existing between species can be used to test these hypotheses. For example, the investigations of Miller (1967), Ford & Newbould (1971), Hutchison et al. (1986) and this current study show a very consistent pattern in broadleaved tree architecture. Is this because it is an ESS? How does the architecture differ between species in a mixed species forest? An ESS may result in canopy architecture converging when species exist as stable mixtures but diverging where one species competitively replaces another. Conclusions Many canopy models have, in the interests of simplicity and economy, considered randomly located and orientated leaves of constant physical and physiological properties. The data from Nothofagus solandri var. cliffortioides, the oakhickory forest of Hutchison et al. (1986) and other previous studies of canopy architecture suggest that these simplifications are unrealistic. The field data instead lend support to a hypothesis that canopy architecture and canopy nitrogen distribution are optimized for maximum carbon gain in the particular habitat of the plant. Additional modelling should give us a better understanding of the trade-offs between leaf and canopy characteristics for maximizing carbon gain both with and without competitors. However, for simulations of actual stand production a renewed emphasis on measurements of canopy architecture would probably be appropriate. Acknowledgments Thanks to J. Hunt, A. Allan, J. Byers, T. McSeveny and G. Rogers for expert technical assistance and A. Greene for the loan of the spectroradiometer. My appreciation also to J. Hunt, U. Benecke and A. Nordmeyer for their helpful discussions and to U. Benecke and J. Orwin for their comments on the manuscript. References Adams, J.A. (1976) Nutrient requirements of four Nothofagus species in aorth Westland, New Zealand, as shown by foliar analysis. 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