Light climate and energy flow in the seagrass canopy of Amphibolis griffithii (J.M. Black) den Hartog

 Absorption of light and radiation use efficiency (RUE) were measured in a dense stand of the seagrass Amphibolis griffithii in Warnbro Sound, a temperate marine embayment in southern Western Australia. Total light intercepted by the canopy was measured and compared with dry weight leaf production, under both summer and winter conditions. RUE was found to be higher in winter (1.56 g MJ^–1) than summer (1.01 g MJ^–1). These values are very similar to values measured for annual crop plants and emphasise the value of applying theory developed for terrestrial crop plants to seagrasses. Canopy extinction coefficients were 0.93 m^–1 in winter and 0.44 m^–1 in summer. There were large differences in hours above saturating irradiance (H _sat) between the top (H_sat = 5 h 14 min) and base (18 min) of the canopy in winter. Energy flows in A. griffithii suggest that this species is highly susceptible to short-term perturbations in incident irradience during the winter period as the energy stored within the rhizomes is small relative to daily respiratory demands. Received: 5 October 1995 / Accepted: 14 August 1996     

Angle distribution of foliage in individual Chamaecyparis obtusa canopies and effect of angle on diffuse light penetration

 The vertical distribution of foliage angle and area of three Chamaecyparis obtusa trees was determined by the triangle method, which calculates foliage geometry using measured coordinates of the leaf ”corners”, in a 43-year-old plantation in central Japan. Vertical distribution patterns of leaf area were different depending on tree size, but the boundary heights, which divide the canopy into sunlit and shaded parts, were similar in the three sample trees. The value of the average foliage angle [I(Z)] at a given depth (Z) from the tip of the stem decreased continually from the upper to lower layers within the canopy. The vertical patterns of changes in I(Z) were different among the three trees, but could be expressed by the following allometric equation as a function of depth.

Influence of elevated CO2 on canopy development and red:far-red ratios in two-storied stands ofRicinus communis

Vertical structure of plant stands and canopies may change under conditions of elevated CO₂ due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO₂ on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO₂-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands ofRicinus communis exposed to ambient (340 μl l⁻¹) and elevated (610 μl l⁻¹) CO₂. Plants in each stand were grown in a common soil as closed “artificial ecosystems” with a ground area of 6.7 m². LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO₂ treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO₂ the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO₂. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO₂. CO₂ enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO₂ at the stand-level. We suggest that, under elevated CO₂, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction.     

Leaf orientation and distribution in a Phaseolus vulgaris L. crop and their relation to light microclimate

Changes in canopy structure parameters (leaflet orientation, leaflet inclination and leaf area index) were measured in crops of beans (Phaseolus vulgaris L.) in the field as the canopy developed between July and October. These changes were compared with the corresponding changes in seasonal light transmission. The beans showed clear heliotropic behaviour, with preferential orientation of leaflets towards the sun’s beam, especially on sunny days. Nevertheless a significant proportion of the leaves pointed in other directions, with as much as 20% oriented towards the north. The highest proportion of leaf inclinations was in the range 30–40° on cloudy days and between 40° and 50° on sunny days. Two methods were compared for assessing changes in light transmission: (a) the use of a Sunfleck Ceptometer and (b) the use of continuous records obtained with sensors installed in the canopy. Over the growth period studied, the total of the leaf plus stem area indices (L _S) increased from 0.26 to 5.2 with the transmission coefficient (τ) for photosynthetic photon flux density (PPFD), obtained using the Ceptometer, correspondingly decreasing from 0.72 to 0.05, and the canopy extinction coefficient decreasing from 1.4 to 0.62. The continuous records of light transmission gave generally similar estimates of τ. Some contrasting leaf angle distribution functions were compared for estimation of L _S from the light measurements. The best leaf angle function to predict L _S from the observed light transmission was a conical function corrected by the degree of heliotropism. Received: 27 January 1999 / Accepted: 11 June 1999     

Partitioning of biologically active radiation in plant canopies

 Plant germination, growth, maturation, and productivity are heavily influenced by the quality and quantity of the light in its environment. The light environment has traditionally been quantified in terms of radiant heat energy and available photosynthetic radiation (PAR), but detailed spectral irradiance or photon flux distributions have rarely been studied. This information is needed to translate the research that plant photobiologists and photochemists have been conducting with regard to understanding the light controls on plant physiology in the field environment of plant canopies. More interest has recently been generated as the potential impacts of global climate changes on intensively managed and natural terrestrial ecosystems are identified and evaluated. Linkages between the identified impacts of various wavelengths of light on plant physiology and the light environment of the plant canopy are identified, with detailed discussion concerning the impacts of plant canopy structure on the plant light response. Solar radiation in the ultraviolet-B (280–320 nm), ultraviolet-A and blue (350–500 nm), PAR (400–700 nm), blue (400–500 nm), green (500–600 nm) red (600–700 nm), far red (700–800 nm) and near infrared (800–1100 nm) is followed from the top of the plant canopy to the photoreceptor at the cellular level within the plant phytoelement.     

Variation in crown light utilization characteristics among tropical canopy trees

BACKGROUND AND AIMS: Light extinction through crowns of canopy trees determines light availability at lower levels within forests. The goal of this paper is the exploration of foliage distribution and light extinction in crowns of five canopy tree species in relation to their shoot architecture, leaf traits (mean leaf angle, life span, photosynthetic characteristics) and successional status (from pioneers to persistent). METHODS: Light extinction was examined at three hierarchical levels of foliage organization, the whole crown, the outermost canopy and the individual shoots, in a tropical moist forest with direct canopy access with a tower crane. Photon flux density and cumulative leaf area index (LAI) were measured at intervals of 0.25-1 m along multiple vertical transects through three to five mature tree crowns of each species to estimate light extinction coefficients (K). RESULTS: Cecropia longipes, a pioneer species with the shortest leaf life span, had crown LAI <0.5. Among the remaining four species, crown LAI ranged from 2 to 8, and species with orthotropic terminal shoots exhibited lower light extinction coefficients (0.35) than those with plagiotropic shoots (0.53-0.80). Within each type, later successional species exhibited greater maximum LAI and total light extinction. A dense layer of leaves at the outermost crown of a late successional species resulted in an average light extinction of 61% within 0.5 m from the surface. In late successional species, leaf position within individual shoots does not predict the light availability at the individual leaf surface, which may explain their slow decline of photosynthetic capacity with leaf age and weak differentiation of sun and shade leaves. CONCLUSION: Later-successional tree crowns, especially those with orthotropic branches, exhibit lower light extinction coefficients, but greater total LAI and total light extinction, which contribute to their efficient use of light and competitive dominance.     

Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

We report effects of elevated atmospheric CO₂ concentration (C_a) on leaf area index (LAI) of a Florida scrub‐oak ecosystem, which had regenerated after fire for between three and five years in open‐top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated C_a, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated C_a. For this scrub‐oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated C_a. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated C_a. These findings have important implications for biogeochemical cycles of C, N and H₂O in woody ecosystems regenerating from disturbance in elevated C_a.     

Visual behaviour of neonata larvae of the light brown apple moth

When neonate larvae of a leafroller moth,Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae) were released into the middle of a circular arena with blue paper on one side of the arena and apple foliage on the other side, more larvae walked towards the apple foliage. These oriented responses were enhanced, in terms of the number of larvae responding, by increasing the amount of light reflected from or transmitted through apple foliage. Larvae also responded to painted targets, and specifically to targets reflecting light in the region of 470–570 nm (green-yellow region to the human eye). When the amount of 470–570 nm reflected from targets was reduced, numbers of larvae responding to targets decreased. The addition of 400–500 nm reflected light to 470–570 nm reflected light also resulted in a reduction of oriented responses to targets. Tests using neutral colours (white, black, and a series of greys) indicated that, in the absence of targets reflecting primarily in the 470–570 nm region, larvae oriented towards targets with low levels of reflectance. When the orientation of walking larvae was measured at various distances from targets of different colours or diameters, targets subtending 5–8 degrees elicited responses from 50% of all larvae. Behaviour other than walking was also influenced by visual stimuli: fewer larvae spun down on a silken thread when blue paper was placed beneath a walking platform than when brown or green papers were present.     

Light Spatial Distribution in the Canopy and Crop Development in Cotton

The partitioning of light is very difficult to assess, especially in discontinuous or irregular canopies. The aim of the present study was to analyze the spatial distribution of photosynthetically active radiation (PAR) in a heterogeneous cotton canopy based on a geo-statistical sampling method. Field experiments were conducted in 2011 and 2012 in Anyang, Henan, China. Field plots were arranged in a randomized block design with the main plot factor representing the plant density. There were 3 replications and 6 densities used in every replicate. The six plant density treatments were 15,000, 33,000, 51,000, 69,000, 87,000 and 105,000 plants ha⁻¹. The following results were observed: 1) transmission within the canopy decreased with increasing density and significantly decreased from the top to the bottom of the canopy, but the greatest decreases were observed in the middle layers of the canopy on the vertical axis and closing to the rows along the horizontal axis; 2) the transmitted PAR (TPAR) of 6 different cotton populations decreased slowly and then increased slightly as the leaves matured, the TPAR values were approximately 52.6–84.9% (2011) and 42.7–78.8% (2012) during the early cotton developmental stage, and were 33.9–60.0% (2011) and 34.5–61.8% (2012) during the flowering stage; 3) the Leaf area index (LAI) was highly significant exponentially correlated (R² = 0.90 in 2011, R² = 0.91 in 2012) with the intercepted PAR (IPAR) within the canopy; 4) and a highly significant linear correlation (R² = 0.92 in 2011, R² = 0.96 in 2012) was observed between the accumulated IPAR and the biomass. Our findings will aid researchers to improve radiation-use efficiency by optimizing the ideotype for cotton canopy architecture based on light spatial distribution characteristics.     

Growth analysis ofQuercus serrata seedlings withinMiscanthus sinensis grass canopies differing in light availability

The effects of different light regimes on the survival, growth and morphology ofQuercus serrata seedlings were studied in canopies ofMiscanthus sinensis. The seedlings of various ages (0–3 yr) were grown in three light regimes: under a denseM. sinensis canopy (TG plot) receiving 2.5%–8.7% of full sunlight, under a relatively sparse canopy (SG plot) receiving 3.8%–16.1% of light and in an adjacent open site (NG plot). There was a little difference in the survival ofQ. serrata seedlings among the three plots. Height and diameter of stem and total leaf area of the seedlings were significantly lower in the shadier plots. However, the first (bottom) flush of the stem was significantly longer in the TG plot than in the NG and SG plots. Total dry weights of individual 1- and 2-yr-oldQ. serrata seedlings in the TG plot were reduced to about one-twelfth of those in the NG plot. Although the relative proportion in dry weight of each organ did not differ significantly among the plots, leaf area ratio, specific leaf area and stem height per unit dry weight were significantly higher in shadier plots. The leaf area per unit stem height was increased considerably in the sunnier plots.     

Herbivore-induced expansion of generalist species as a driver of homogenization in post-disturbance plant communities

We used a controlled experiment to investigate how disturbance scale (canopy gap area) and herbivory influence post-disturbance plant community dynamics. Twenty canopy gaps were installed in a temperate hemlock-hardwood forest during the winter of 2002–2003: seven small gaps (50–150 m²), seven medium gaps (151–250 m²), and six large gaps (251–450 m²). Within each gap, we established 4–12 sample plots (depending on gap size); 1–3 of which were enclosed with wire mesh white-tailed deer (Odocoileus virginianus) exclosures in 2005. Gaps were revisited and intensively sampled in 2007. After five growing seasons, ground-layer plant communities in non-exclosed plots were more similar compositionally than exclosure plots. Non-exclosed plots in small and medium gaps were more similar to non-exclosed plots in large gaps than they were to exclosed plots in their respective gap size class. Shade-tolerant forbs and trees were less common outside exclosures, while generalist species associated with higher understory light levels and exotics were more prevalent outside exclosures. Our results suggest that even in forests with relatively low deer densities (6.5–9.3 deer km⁻²), white-tailed deer herbivory may influence the developmental trajectory of post-disturbance plant communities and be a mechanism for decreasing β-diversity along environmental gradients.     

Modelling environmental limits to light use efficiency for a canopy of two broad-leaved tree species with contrasting leaf habit

We used outputs from a model of canopy carbon uptake [Dungan et al. (2004) Functional Ecology 18: 34–42] and measurements of irradiance (PAR, 400–700 nm) intercepted by the canopy to investigate the effect of daily changes in environmental conditions on daily light use efficiency, ε, for a canopy comprising two broadleaved New Zealand tree species with contrasting leaf habit. Irradiance absorbed by the canopy was 93% of the incident irradiance, and seasonal changes in the proportion of this absorbed by leaves of each species was estimated with a detailed model of leaf area phenology. Over the year, ε for semi-deciduous wineberry (Aristotelia serrata) was 0.43 g C MJ-1 PAR, with maximum and minimum values of 0.80 g C MJ-1 PAR and 0.07 g C MJ-1 PAR in summer and winter respectively. In contrast annual ε was 0.60 g C MJ-1 PAR for winter deciduous fuchsia, with a maximum value of 0.92 g C MJ-1 PAR in spring. The most important environmental regulator of ε for both species was τ, atmospheric transmissivity. Maximum values for ε were estimated on days when τ ≈0.2, on cloudy days in mid-summer. Limits to photosynthesis from restricted root-zone water availability were also important, showing that drought limitations can restrict ε even at a field site with annual rainfall of 4800 mm. Environmental limits to photosynthesis and ε have been investigated for only a few canopy tree species. Uncertainty in models of the national carbon budget required for reporting purposes would be reduced by considering the environmental regulation of ε for a wider range of tree species.     

Estimating absolute pollen productivity for some European Tertiary-relict taxa

Tertiary-relict plants are survivors from the pre-Quaternary periods. Today, most European Tertiary relicts are confined to small, isolated stands distributed in the Mediterranean and Black Sea regions. In the past, however, the fossil record indicates that these species were probably distributed over large parts of the European continent and may have been important constituents of the vegetation. Little is known about their pollen representation, which limits our ability to reconstruct this past vegetation with any accuracy. This paper draws on the results of pollen trapping experiments in Bulgaria and Georgia, where relict stands of Aesculus hippocastanum, Cercis siliquastrum, Fagus orientalis, Juglans regia and Pterocarya fraxinifolia are still in existence. We compared average pollen accumulation rates (PAR) to vegetation data from around the trapping locations to derive estimates of absolute pollen productivity using various pollen dispersal functions. Composite dispersal functions that model pollen components carried above the vegetation canopy and falling as rain provided better relationships between PAR and plant abundance than functions that consider only a single component or the ‘trunk-space’ component carried under the canopy. A composite dispersal function with a simple model for regional pollen and the best overall correlation statistics gave the following estimates of absolute pollen productivity (grains cm⁻² yr⁻¹ with 1 SE intervals): Carpinus betulus 19,000–28,700; Fagus orientalis 15,600–20,400; Juglans regia 27,200–36,200; Pterocarya fraxinifolia 182,000–192,600; Quercus spp. 21,700–24,800; Tilia begoniifolia 51,600–68,300; and T. tomentosa 14,700–18,200. These estimates were applied to fossil data from the Black Sea coast to reconstruct palaeovegetation using absolute and relative methods.     

Response of clonal plasticity of Fargesia nitida to different canopy conditions of subalpine coniferous forest

The aim of this study is to explore the effects of canopy conditions on clump and culm numbers, and the morphological plasticity and biomass distribution patterns of the dwarf bamboo species Fargesia nitida. Specifically, we investigated the effects of canopy conditions on the growth and morphological characteristics of F. nitida, and the adaptive responses of F. nitida to different canopy conditions and its ecological senses. The results indicate that forest canopy had a significant effect on the genet density and culm number per clump, while it did not affect the ramet density. Clumps tended to be few and large in gaps and forest edge plots, and small under forest understory plots. The ramets showed an even distribution under the closed canopy, and cluster distribution under gaps and forest edge plots. The forest canopy had a significant effect on both the ramets’ biomass and biomass allocation. Favourable light conditions promoted ramet growth and biomass accumulation. Greater amounts of biomass in gaps and forest edge plots were shown by the higher number of culms per clump and the diameter of these culms. Under closed canopy, the bamboos increased their branching angle, leaf biomass allocation, specific leaf area and leaf area ratio to exploit more favourable light conditions in these locations. The spacer length, specific spacer length and spacer branching angles all showed significant differences between gaps and closed canopy conditions. The larger specific spacer length and spacer branching angle were beneficial for bamboo growth, scattering the ramets and exploiting more favourable light conditions. In summary, this study shows that to varying degrees, F. nitida exhibits both a wide ecological amplitude and high degree of morphological plasticity in response to differing forest canopy conditions. Moreover, the changes in plasticity enable the plants to optimize their light usage efficiency to promote growth and increase access to resources available in heterogeneous light environments. __________ Translated from Acta Ecologica Sinica, 2006, 26(12): 4019–4026 [译自: 生态学报]     

Light Interception and Photosynthetic Efficiency in Some Glasshouse Crops

Productivity of glasshouse crops is strongly limited by lightreceipt, and efficient interception and use of light in photosynthesisis correspondingly important. Mature row crop canopies of cucumberand tomato intercepted about 76% of the light incident on theirupper surfaces; about 18% was lost through gaps between therows. Light transmitted through the entire depth of the canopywas reflected back by white plastic on the ground, so that thelower surface of the canopy received approximately 13% of thelight incident on the upper surface. The light flux incidenton the sides of these canopies (c. 2m tall and 6m x 16m in area)amounted to some 20–30% of that incident on the uppersurface. About 32% of daylight falling on the glasshouse (c.9m x 18m in area) was intercepted by the glasshouse structureand glazing; of the 68% entering the house, some fell on headlandsoccupying 35% of the glasshouse area. The loss of light to headlands,and the gain from canopy side-lighting, would be relativelysmaller for larger glasshouses. At near-ambient CO₂ concentrations, net photosynthetic ratesof the cucumber canopy were comparable to those of closed canopiesof other glasshouse and field crops which have maximum lightconversion efficiencies of 5–8µg CO₂ J^–1 at50–200 W m^–2 incident light flux density. Efficiencydecreases only slightly with stronger light. Glasshouse cropswith CO₂ enrichment to 1200 vpm achieve conversion efficienciesof 7–10µg CO₂ J^–1. Efficiencies of utilizationof intercepted light, on an energy basis, reach 6–10%in various field and glasshouse crops with near-ambient CO₂,and reached an exceptional 11% for the cucumber canopy. Glasshousecrops with CO₂ enrichment achieve maximum efficiency of lightenergy utilization between 12% and 13%. Key words: Glasshouse cucumber and tomato, light interception and utilization, photosynthetic efficiency, row crops     

Leaf δ13C in Pinus resinosa trees and understory plants: variation associated with light and CO2 gradients

 Our objective was to evaluate the relative importance of gradients in light intensity and the isotopic composition of atmospheric CO₂ for variation in leaf carbon isotope ratios within a Pinus resinosa forest. In addition, we measured photosynthetic gas exchange and leaf carbon isotope ratios on four understory species (Dryopteris carthusiana, Epipactus helleborine, Hieracium floribundum, Rhamnus frangula), in order to estimate the consequence of the variation in the understory light microclimate for carbon gain in these plants. During midday, CO₂ concentration was relatively constant at vertical positions ranging from 15 m to 3 m above ground. Only at positions below 3 m was CO₂ concentration significantly elevated above that measured at 15 m. Based on the strong linear relationship between changes in CO₂ concentration and δ¹³C values for air samples collected during a diurnal cycle, we calculated the expected vertical profile for the carbon isotope ratio of atmospheric CO₂ within the forest. These calculations indicated that leaves at 3 m height and above were exposed to CO₂ of approximately the same isotopic composition during daylight periods. There was no significant difference between the daily mean δ¹³C values at 15 m (–7.77‰) and 3 m (–7.89‰), but atmospheric CO₂ was significantly depleted in ¹³C closer to the ground surface, with daily average δ¹³C values of –8.85‰ at 5 cm above ground. The light intensity gradient in the forest was substantial, with average photosynthetically active radiation (PAR) on the forest floor approximately 6% of that received at the top of the canopy. In contrast, there were only minor changes in air temperature, and so it is likely that the leaf-air vapour pressure difference was relatively constant from the top of the canopy to the forest floor. For red pine and elm tree samples, there was a significant correlation between leaf δ¹³C value and the height at which the leaf sample was collected. Leaf tissue sampled near the forest floor, on average, had lower δ¹³C values than samples collected near the top of the canopy. We suggest that the average light intensity gradient through the canopy was the major factor influencing vertical changes in tree leaf δ¹³C values. In addition, there was a wide range of variation (greater than 4‰) among the four understory plant species for average leaf δ¹³C values. Measurements of leaf gas exchange, under natural light conditions and with supplemental light, were used to estimate the influence of the light microclimate on the observed variation in leaf carbon isotope ratios in the understory plants. Our data suggest that one species, Epipactus helleborine, gained a substantial fraction of carbon during sunflecks. Received: 21 March 1996 / Accepted: 13 August 1996     

Seasonal changes in canopy photosynthesis and foliage respiration in a Rhizophora stylosa stand at the northern limit of its natural distribution

Gross photosynthesis and respiration rates of leaves at different canopy heights in a Rhizophora stylosa Griff. stand were measured monthly over 1 year at Manko Wetland, Okinawa Island, Japan, which is the northern limit of its distribution. The light-saturated net photosynthesis rate for the leaves at the top of the canopy showed a maximum value of 17 μmol CO₂ m⁻² s⁻¹ in warm season and a minimum value of 6 μmol CO₂ m⁻² s⁻¹ in cold season. The light-saturated gross photosynthesis and dark respiration rates of the leaves existing at the top of the canopy were 2−7 times and 3–16 times, respectively, those of leaves at the bottom of the canopy throughout the year. The light compensation point of leaves showed maximum and minimum peaks in warm season and cold season, respectively. The annual canopy gross photosynthesis, foliage respiration, and surplus production were estimated as 117, 49, and 68 t CO₂ ha⁻¹ year⁻¹, respectively. The energy efficiency of the annual canopy gross photosynthesis was 2.5%. The gross primary production GPP fell near the regression curve of GPP on the product of leaf area index and warmth index, the regression curve which was established for forests in the Western Pacific with humid climates.     

Leaf construction cost, nutrient concentration, and net CO2 assimilation of native and invasive species in Hawaii

To examine the predictability of leaf physiology and biochemistry from light gradients within canopies, we measured photosynthetic light-response curves, leaf mass per area (LMA) and concentrations of nitrogen, phosphorus and chlorophyll at 15–20 positions within canopies of three conifer species with increasing shade tolerance, ponderosa pine [Pinus ponderosa (Laws.)], Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], and western hemlock [Tsuga heterophylla (Raf.) Sarg.]. Adjacent to each sampling position, we continuously monitored photosynthetically active photon flux density (PPFD) over a 5-week period using quantum sensors. From these measurements we calculated FPAR: integrated PPFD at each sampling point as a fraction of full sun. From the shadiest to the brightest canopy positions, LMA increased by about 50% in ponderosa pine and 100% in western hemlock; Douglas fir was intermediate. Canopy-average LMA increased with decreasing shade tolerance. Most foliage properties showed more variability within and between canopies when expressed on a leaf area basis than on a leaf mass basis, although the reverse was true for chlorophyll. Where foliage biochemistry or physiology was correlated with FPAR, the relationships were non-linear, tending to reach a plateau at about 50% of full sunlight. Slopes of response functions relating physiology and biochemistry to ln(FPAR) were not significantly different among species except for the light compensation point, which did not vary in response to light in ponderosa pine, but did in the other two species. We used the physiological measurements for Douglas fir in a model to simulate canopy photosynthetic potential (daily net carbon gain limited only by PPFD) and tested the hypothesis that allocation of carbon and nitrogen is optimized relative to PPFD gradients. Simulated photosynthetic potential for the whole canopy was slightly higher (<10%) using the measured allocation of C and N within the canopy compared with no stratification (i.e., all foliage identical). However, there was no evidence that the actual allocation pattern was optimized on the basis of PPFD gradients alone; simulated net carbon assimilation increased still further when even more N and C were allocated to high-light environments at the canopy top. Received: 12 August 1998 / Accepted: 25 March 1999     

Drought alters the canopy architecture and micro-climate of Hevea brasiliensis trees

 In this study a comparison of the canopy architecture and the growth and distribution of roots was made in 10-year-old trees of Hevea brasiliensis grown in a severely drought-prone area on the west coast of India under rainfed and irrigated conditions. LAI and light interception increased significantly in the irrigated compared to the rainfed trees. Girth and height of the tree were 29 and 19% more while width and height of the canopy were 19 and 20% more in the irrigated than rainfed trees. There were 22% more primary branches which had 26% more diameter in the irrigated trees than rainfed trees. The branches were inserted on the main trunk at an angle of 58.36° in the irrigated and 44.22° in rainfed trees. The above changes led to more light penetration which altered the light distribution inside the rainfed trees during summer and inhibited leaf photosynthesis particularly in the top canopy leaves. In the rainfed trees most of the growth occurred during the short favorable season immediately after the monsoon between June and October and no growth or even shrinking of the trunk was seen during summer. In the irrigated trees a higher growth was seen throughout the year and summer had no adverse effect. Although there was some difference in the root distribution pattern, the total root density per unit soil volume did not vary between the irrigated and rainfed trees. Key words  Hevea brasiliensis· Drought · Crown architecture · Micro-climate · Root growth Received: 8 May 1998 / Accepted 8 October 1998