Drag and reconfiguration of freshwater macrophytes

SUMMARY 1. Submerged freshwater macrophytes face large hydrodynamic forces in flowing waters in streams and on wave‐swept lake shores and require morphological adaptations to reduce the drag and the physical damage. This experiment studied five species of freshwater macrophytes and strap‐formed plastic leaves to test the predictions that: (i) increasing flexibility leads to greater reconfiguration and lower drag coefficients, (ii) flexible plants experience a steeper decline of drag coefficients with increasing water velocity than unflexible plants and (iii) plants mounted vertically on a horizontal substratum bend over in fast flow attaining a shielded position of low drag. 2. The results confirmed all three predictions. In fast flow, plants mounted upright on a horizontal platform gradually approached a position aligned with the flow, depending on their flexibility. In the range 8–50 cm s^?1 the deflection followed an interspecific negative linear relationship between log (tangent Φ) and velocity, where Φ represents the shoot angle normal to the horizontal level. Above 50 cm s^?1, further deflection was reduced perhaps by a combination of the elasticity and packing of shoots and the increasing lift generated by fast flow. 3. Drag coefficients of plants ranged between 0.01 and 0.1, typical of moderately to very streamlined objects. Drag coefficients declined log‐log linearly at increasing velocity, following negative slopes between ?0.67 and ?1.24 (median: ?1.0) because of reconfiguration and formation of a shielding canopy. Drag coefficients declined much less (median: ?0.55) for plants floating freely in the streaming water and which were capable of changing their shape but unable to form a shielding canopy. Drag coefficients declined even less for relatively unflexible plastic leaves (?0.30 to ?0.40), and they remained constant for stiff, bluff objects. 4. The experiments suggest that flow resistance of flexible, submerged macrophytes in natural streams may increase in direct proportion to water velocity because they form a shielding submerged canopy, and high water stages at peak flow may result in greater proportions of the water passing unimpeded above the canopy. In contrast, stiff amphibious and emergent reed plants should experience an increase of flow resistance with at least the square of velocity as reconfiguration is small and former aerial plant surfaces come into contact with the streaming water at higher water stages. Field experiments to test these predictions are urgently needed.     

Morphological adaptations of emergent plants to water flow: a case study with Typha angustifolia, Zizania latifolia and Phragmites australis

1. Water velocity plays an important role in shaping plant community structure in flowing waters although few authors have yet attempted to explain the adaptation of plants to flow. 2. We aimed to test two hypotheses, that: (i) some emergent macrophytes reconfigure their shoot distribution in fast currents and form clumps, and (ii) the shape and morphology of such clumps minimises drag caused by the current. The study focuses on three emergent macrophytes that co‐occur along a gradient of water velocity. 3. The species showed a clear zonation in response to water depth and current velocity. Phragmites australis occupied shallower and more slowly flowing water than Typha angustifolia and Zizania latifolia, which had similar preferences. 4. Both T. angustifolia and Z. latifolia shoots were more clumped at high velocity, whereas they were more randomly distributed at low flow or in stagnant water. Because of the low shoot density, water flowed more easily through T. angustifolia clumps, whereas Z. latifolia clumps had a high shoot density and large amounts of trapped litter, causing stagnant water in the centre of the clump. The clumps of Z. latifolia with a high density of shoots were longer and narrower than T. angustifolia clumps. Phragmites australis was less tolerant of flow than the other two species and large amounts of litter trapped in the clumps impaired flow. 5. The shoot distribution of both T. angustifolia and Z. latifolia is reconfigured at high flow and this minimises drag on the clumps.     

Biomechanical factors contributing to self-organization in seagrass landscapes

Field observations have revealed that when water flow is consistently from one direction, seagrass shoots align in rows perpendicular to the primary axis of flow direction. In this study, live Zostera marina shoots were arranged either randomly or in rows perpendicular to the flow direction and tested in a seawater flume under unidirectional flow and waves to determine if shoot arrangement: a) influenced flow-induced force on individual shoots, b) differentially altered water flow through the canopy, and c) influenced light interception by the canopy. In addition, blade breaking strength was compared with flow-induced force to determine if changes in shoot arrangement might reduce the potential for damage to shoots.

Under unidirectional flow, both current velocity in the canopy and force on shoots were significantly decreased when shoots were arranged in rows as compared to randomly. However, force on shoots was nearly constant with downstream distance, arising from the trade-off of shoot bending and in-canopy flow reduction. The coefficient of drag was higher for randomly-arranged shoots at low velocities (< 30 cm s^− 1) but converged rapidly among the two shoot arrangements at higher velocities. Shoots arranged in rows tended to intercept slightly more light than those arranged randomly. Effects of shoot arrangement under waves were less clear, potentially because we did not achieve the proper plant size–row spacing ratio. At this point, we may only suggest that water motion, as opposed to light capture, is the dominant physical mechanism responsible for these shoot arrangements. Following a computation of the Environmental Stress Factor, we concluded that even photosynthetically active blades may be damaged or broken under frequently encountered storm conditions, irrespective of shoot arrangement.

We hypothesize that when flow is generally from one direction, seagrass bed patterns over multiple scales of consideration may arise as a cumulative effect of individual shoot self-organization driven by reduced force and drag on the shoots and somewhat improved light capture.     

江西常绿阔叶林木本植物不同冠层高度当年生小枝茎构型对叶生物量的影响

以江西阳际峰自然保区69种木本植物为对象,采用标准化主轴回归(SMA)的方法,对不同冠层高度当年生小枝的构件生物量(叶生物量、茎生物量、小枝生物量)与茎构型特征(茎直径、茎长度、茎宽长比、茎体积和茎密度)进行了分析,研究当年生小枝茎构型对叶生物量的影响.结果表明:不同冠层高度及生活型间叶生物量、茎生物量、小枝生物量、茎直径、茎长度、茎宽长比及茎体积均无显著差异,而茎密度差异显著.不同冠层高度及生活型间,叶生物量与茎生物量及总生物量之间均呈显著等速生长关系.小枝叶生物量分别与茎直径、茎体积呈显著异速生长关系,且该异速生长指数在不同冠层高度无显著差异.茎长度、茎宽长比及茎密度对当年生小枝叶生物量变异的解释力较小(<24%).冠层高度和生活型对小枝叶-茎生物量的等速分配关系影响不显著.小枝茎构型中,相比于茎长度、茎宽长比以及茎密度,茎的直径与体积对当年生小枝叶生物量的影响更大,冠层高度对当年生小枝茎构型与叶生物量间的异速分配模式无显著影响.     

Velocity gradients and turbulence around macrophyte stands in streams

1. Submerged macrophytes strongly modify water flow in small lowland streams. The present study investigated turbulence and vertical velocity gradients using small hot-wire anemometers in the vicinity and within the canopies of four macrophyte species with the objective of evaluating: (a) how plant canopies influence velocity gradients and shear force on the surfaces of the plants and the stream bed; and (b) how the presence and morphology of plants influence the intensity of turbulence. 2. Water velocity was often relatively constant with water depth both outside and inside the plant canopies, but the velocity declined steeply immediately above the unvegetated stream bed. Steep vertical velocity profiles were also observed in the transition to the surface of the macrophyte canopy of three of the plant species forming a dense shielding structure of high biomass. Less steep vertical profiles were observed at the open canopy surface of the fourth plant species, growing from a basal meristem and having the biomass more homogeneously distributed with depth. The complex distribution of hydraulic roughness between the stream bed, the banks and the plants resulted in velocity profiles which often fitted better to a linear than to a logarithmic function of distance above the sediment and canopy surfaces. 3. Turbulence increased in proportion to the mean flow velocity, but the slope of the relationships differed in a predictable manner among positions outside and inside the canopies of the different species, suggesting that their morphology and movements influenced the intensity of turbulence. Turbulence was maintained in the attenuated flow inside the plant canopies, despite estimates of low Reynolds numbers, demonstrating that reliable evaluation of flow patterns requires direct measurements. The mean velocity inside plant canopies mostly exceeded 2 cm s^??1 and turbulence intensity remained above 0.2 cm s^??1, which should be sufficient to prevent carbon limitation of photosynthesis in CO₂-rich streams, while plant growth may benefit from the reduced physical disturbance and the retention of nutrient-rich sediment particles. 4. Flow patterns were highly reproducible within canopies of the individual species despite differences in stand size and location among streams. We propose that individual plant stands are suitable functional units for analysing the influence of submerged macrophytes on flow patterns, retention of particles and biological communities in lowland streams.     

Above-ground competition does not alter biomass allocated to roots in Abutilon theophrasti

We tested whether plants allocate proportionately less biomass to roots in response to above-ground competition as predicted by optimal partitioning theory. Two population densities of Abutilon theophrasti were achieved by planting one individual per pot and varying spacing among pots so that plants in the two densities experienced the same soil volume but different degrees of canopy overlap. Density did not affect root:shoot ratio, the partitioning of biomass between fine roots and storage roots, fine root length, or root specific length. Plants growing in high density exhibited typical above-ground responses to neighbours, having higher ratios of stem to leaf biomass and greater leaf specific area than those growing in low density. Total root biomass and shoot biomass were highly correlated. However, storage root biomass was more strongly correlated with shoot biomass than was fine-root biomass. Fine-root length was correlated with above-ground biomass only for the small subcanopy plants in crowded populations. Because leaf surface area increased with biomass, the ratio between absorptive root surface area and transpirational leaf surface area declined with plant size, a relationship that could make larger plants more susceptible to drought. We conclude that A. theophrasti does not reallocate biomass from roots to shoots in response to above-ground competition even though much root biomass is apparently involved in storage and not in resource acquisition.     

Predicting the hydraulic forces on submerged macrophytes from current velocity, biomass and morphology

Aquatic macrophytes are important in stabilising moderately eutrophic, shallow freshwater lakes in the clear-water state. The failure of macrophyte recovery in lakes with very soft, highly organic sediments that have been restored to clear water by biomanipulation (e.g. in the Norfolk Broads, UK) has suggested that the physical stability of the sediment may limit plant establishment. Hydraulic forces from water currents may be sufficient to break or remove plants. Our aim was to develop a simple model that could predict these forces from plant biomass, current velocity and plant form. We used an experimental flume to measure the hydraulic forces acting on shoots of 18 species of aquatic macrophyte of varying size and morphology. The hydraulic drag on the shoots was regressed on a theoretically derived predictor (shoot biomass × current velocity^1.5). Such linear regressions proved to be highly significant for most species. The slopes of these lines represent species-specific, hydraulic roughness factors that are analogous to classical drag coefficients. Shoot architecture parameters describing leaf and shoot shape had significant effects on the hydraulic roughness factor. Leaf width and shoot stiffness individually did not have a significant influence, but in combination with shoot shape they were significant. This hydraulic model was validated for a subset of species using measurements from an independent set of shoots. When measured and predicted hydraulic forces were compared, the fit was generally very good, except for two species with morphological variations. This simple model, together with the plant-specific factors, provides a basis for predicting the hydraulic forces acting on the root systems of macrophytes under field conditions. This information should allow prediction of the physical stability of individual plants, as an aid to shallow-lake management. Received: 11 March 1999 / Accepted: 18 January 2000     

Positive diversity effects on productivity in mixtures of arable weed species as related to density-size relationships

Aims Diversity–productivity relationships among herbaceous species have mostly been studied in grasslands, while less is known about diversity effects among weedy species with a short life cycle.Methods We studied diversity–productivity relationships, shoot density, size and allometry in experimental communities of different species richness (one, three, six, and nine species), functional group number (one to three functional groups: grasses, small herbs and tall herbs) and functional group evenness (even and uneven number of species per functional group) based on a pool of nine arable weed species with a short life cycle in a 2-year experiment.Important findings Higher species richness increased above- and belowground biomass production in both years of the experiment. Additive partitioning showed that positive selection effects increased with increasing species richness and functional group number, while positive complementarity effects were greater when tall herbs were present. Relative yield totals were larger than 1 across all species richness levels but did not increase with species richness, which is consistent with constant positive complementarity effects. Community biomass production and diversity effects increased in the second year of the experiment, when communities achieved greater shoot densities and average shoot sizes. At the community level, varying productivity was mainly attributable to variation in mean shoot sizes. Tall herbs reached greater observed/expected relative yields (=overyielding) due to increased shoot sizes, underyielding of small herbs was mainly attributable to decreased shoot sizes, while grasses partly compensated for reduced shoot sizes by increasing densities. Shifts in community-level density–size relationships and changes in shoot allometry in favour of greater height growth indicated that a greater biomass at a given density was due to increased dimensions of occupied canopy space. We conclude that diversity effects are also possible among short-lived arable weed species, but selection effects through sizes differences among species are key for positive species richness–productivity relationships.     

The effect of sewage sludge flooding on growth and morphometric parameters of Phragmites australis (Cav.) Trin. ex Steudel

Phragmites australis is the most frequently used species in constructed wetlands (CW) for dewatering and mineralization of sludge. Such CW create a very hostile environment for plants because their roots are exposed to permanently anaerobic conditions. Our working hypothesis assumed a simple relationship between the success of reed in colonising sludge flood stands and maximalisation of shoot oxygen production and transport to rhizomes and roots. Morphometric parameters and density of common reed shoots growing in reed bed systems inundated with sewage sludge were investigated. It was proved that flooding with sewage sludge significantly influenced characteristics of common reed populations. In comparison with natural stands, on stands treated with sewage sludge, maximal density of shoots was higher, biomass was twice as high, shoot diameter was significantly greater, and leaf weight ratio of an individual shoot was also significantly higher. Tendency to shorten shoots was noted. Growing period was longer for reed shoots treated with sewage sludge than on natural stands.     

Streamlining of plant patches in streams

1. Plants in shallow streams often grow in well‐defined monospecific patches experiencing a predictable unidirectional flow, though of temporally variable velocity. During maximum patch development in summer we studied: (i) the shape and streamlining of 59 patches of Callitriche cophocarpa, (ii) allometric relationships between canopy size and sediment area colonized by roots (root area) and (iii) fine‐scale flow gradients for a representative patch exposed to a range of velocities to evaluate relationships between patch shape and physical impact. 2. Canopy and root area viewed from above were elongated and streamlined in the flow direction, while uniform vegetative growth in all directions from a single colonizing shoot would have generated a circular form. Canopies were slightly wider in the upstream part than in the gradually tapering part downstream and the maximum height to length ratio averaged 0.25. The canopy and root area of the patches were more elongate and slender in sites with shallow water, where currents accelerate alongside patches and restrict lateral expansion, compared to deeper sites where currents can pass above the canopy. Similarly, the frontal area relative to planform area or root area was significantly lower in shallow water . Canopy shape and indices of streamlining did not change significantly with approach velocity (0.02–0.40 m s^?1), either because canopy shape is not sensitive to approach velocity or summer velocities were too low to induce such changes. 3. Sediment elevation within patches (avg. 4.1 cm) increased significantly with patch length, but did not differ between unstable sand or more stable coarse sediment for the same patch length. Shape of canopy and root area did not change significantly with sediment type. 4. Pressure drag on the canopy as a whole is probably reduced by its rounded front, restricted height and overall slender form with a low frontal area, while the downstream overhanging canopy increases drag compared to an ideal streamlined object. Across a 100‐fold range of root areas from 0.01 to 1 m², the frontal area of the canopy increased 29 times, planform area increased 38 times and volume increased 76 times, suggesting a trade‐off between physical impact of flow, light interception and anchoring strength. 5. The canopy was compressed at high approach velocities, with low current velocity within the canopy while steep velocity gradients developed across the exposed outer surfaces as the diverted flow accelerated. Because drag processes are additive, and exist at different spatial scales and Reynolds numbers on the surface and inside of plant canopies, direct measurements on entire canopies under controlled conditions are needed to test the functional importance of their shape, size and porosity to flow.     

Shoot growth and tree architecture of saplings of the major canopy dominants in a warm-temperate rainforest

Sapling density, shoot growth, and sapling architecture were studied in five major canopy dominants both under closed canopy and gaps in a warm-temperate rainforest. The five species showed wide variations in distribution, shoot growth, and sapling architecture. Distylium racemosum and Quercus acuta had significantly higher sapling densities under closed canopy than in gaps. Castanopsis sieboldii and Machilus thunbergii had significantly higher sapling densities in gaps than under closed canopy. Quercus salicina showed no significant difference in sapling density between the two habitats. Under closed canopy, C. sieboldii and M. thunbergii had wider crowns than the other species. Distylium racemosum had the greatest number of terminal shoots among the species. Quercus acuta had a branchless small crown. Quercus salicina showed intermediate values in crown width, depth and the number of terminal shoots among the species. Distylium racemosum showed the greatest height-growth rate among the species under the closed canopy, but was the slowest in gaps. Castanopsis sieboldii and M. thunbergii showed the greatest height-growth rates among the species in gaps. Quercus salicina showed the slowest height-growth rates both under closed canopy and gaps. All of the five species showed low mortality under closed canopy. For the major canopy dominants: (i) sapling architecture may not be an important factor in determining mortality but it may be important for height-growth rate; and (ii) sprouting helps saplings to survive until gap formation and facilitates rapid growth in the gaps.     

Developmental changes in shoot N dynamics of lucerne (Medicago sativa L.) in relation to leaf growth dynamics as a function of plant density and hierarchical position within the canopy

Shoot N concentration in plants decreases as they get bigger, due to the fact that N accumulates less rapidly than dry matter in plants during the plant growth process, leading to an allometric relationship between shoot N content (N(sh)) and shoot mass (W(sh)): N(sh)=a(W(sh))b. The results obtained on lucerne plants growing either under controlled low density conditions or in dense stands under field conditions show that the value of the allometric coefficient b that represents the ratio between the relative N accumulation rate in shoots [dN(sh)/(N(sh)dt)] and the relative growth rate [dW(sh)/(W(sh)dt)], decreases from 0.88 for a low plant density to 0.72 for a dense stand. Therefore, the fractional increase of shoot N per unit of shoot dry matter is lower when plants are in competition for light in dense canopies. This decrease can be entirely explained by the parallel decline in the leaf area per unit of shoot mass. Thus, a remarkably constant linear relationship can be established between N(sh) and leaf area (LA): N(sh)=1.7 g m(-2) LA, regardless of the conditions (low versus high density, controlled versus field conditions). Moreover, in a field dense stand, the comparison of plants with contrasting positions between the top and the bottom of the canopy (dominant, intermediate or suppressed plants), also shows that the difference in N(sh) at similar shoot mass is explained by the proportion of leaf mass to shoot mass. These data support the idea that leaf growth drives the dynamics of shoot N accumulation. These results also indicate that competition for light among individual plants within a dense canopy induces developmental changes in plant morphology (leaf:stem ratio) that explain the differences observed in shoot N concentration. This last observation could be extrapolated to multi-specific plant stands. Therefore, the sharing of N resources among plant species could partially be the result of the sharing of light within the canopy.     

Effects of density on above and below ground biomass of the native alpine grass Poa fawcettiae and the environmental weed Achillea millefolium

Competition between Poa fawcettiae Vickery, the dominant native snowgrass, and the invasive herbaceous Achillea millefolium L., was examined in three glasshouse experiments. The first experiment investigated the potential for intra-specific competition in plants by growing them in pots with low and high density. The second experiment examined the potential for inter-specific competition at low, medium and high density. In the third experiment plants in pots where either roots or shoots of the species could not compete were compared to those where root and shoot competition was possible.
Achillea millefolium plants produced more than four times the biomass of P. fawcettiae plants. As a result the two species responded differently. In the A. millefolium monocultures both root and shoot biomass per plant declined at high density. By contrast, P. fawcettiae biomass was not affected. In mixed species pots, P. fawcettiae had no effect on the biomass of A . millefolium plants, while P. fawcettiae shoot and root biomass per plant decreased when grown with A. millefolium at all densities tested. Root competition from A. millefolium appears to be the main cause of the decrease in biomass of P. fawcettiae . The results imply that A. millefolium may have a competitive advantage over P. fawcettiae in the Australian Alps.     

Quantifying above- and below-ground growth responses of the western Australian oil mallee,Eucalyptus kochii subsp. plenissima,to contrasting decapitation regimes

Resprouting in the oil mallee, Eucalyptus kochii Maiden & Blakely subsp. plenissima Gardner (Brooker), involves generation of new shoots from preformed meristematic foci on the lignotuber. Numbers of such foci escalated from 200 per lignotuber in trees aged 1 year to 3,000 on 4- to 5-year-old trees. Removal of shoot biomass by decapitation 5 cm above ground in summer (February) or spring (October) resulted in initiation of 140-170 new shoots, but approx. 400 shoots were induced to form if crops of new shoots were successively removed until sprouting ceased and rootstocks senesced. Initially, the new shoot biomass of regenerating coppices increased slowly and the root biomass failed to increase appreciably until 1.7-2.5 years after cutting. Newly cut trees showed loss of fine root biomass, and structural roots failed to secondarily thicken to the extent shown by uncut trees. After 2 years, the biomass of shoots of coppiced plants was only one-third that of uncut control trees and shoot:root dry mass ratios of coppiced plants were still low (1.5-2.0) compared with those of the controls (average ratio of 3.1). Spring cutting promoted quicker and greater biomass recovery than summer cutting. Starch in below-ground biomass fell quickly following decapitation and remained low for a 12-18 month period. Utilization of starch reserves in naturally regenerating coppices was estimated to provide only a small proportion of the dry matter accumulated in new shoots. Results are discussed in relation to their impact on coppicing ability of the species under natural conditions or when successively coppiced for shoot biomass production.     

Effects of simulated herbivory in three old field Compositae with different inflorescence architectures

The effects of simulated herbivory (early or late defoliation and cutting of the flowering shoot) on the growth and reproduction of three species of monocarpic composite forbs (Crepis pulchra, Picris hieracioides and C. foetida) with different inflorescence architectures were studied in experimental plots. For the three species studied, early defoliation had no significant effect on subsequent growth. In contrast, late defoliation, occurring at the start of the season of drought, had a negative effect on growth and reproduction in the two Crepis species, particularly C. foetida, but had less effect on P. hieracioides. Sexual biomass was more clearly affected by late defoliation than was vegetative biomass, although the effects differed markedly among species possibly as a result of differences in phenology. Clipping the flowering shoot removed about 3 times less biomass than late defoliation and had little effect on vegetative biomass. It had much greater effects on the sexual biomass in P. hieracioides and C. pulchra, and resulted in the production of many shoots sprouting from the rosette, allowing the treated plants to regain a vegetative biomass close to that of control plants. Clipping did however lead to the production of shorter shoots and a reduction in the number of capitula formed. In C. foetida, much branching occurred even when the main shoot was not cut; the architecture of individual plants was therefore only slightly changed by clipping the apical bud and the sexual biomass of this species was not affected by ablation of the flowering shoot. Overcompensation was found in only two families of C. pulchra for vegetative biomass. No over-compensation was found for sexual biomass, despite an increase in the number of flowering shoots in C. pulchra and P. hieracioides following clipping. However situations close to compensation for the vegetative biomass in the three species and in P. hieracioides for the sexual biomass were recorded. The response of the three study species to simulated herbivory were related to their architecture and to the time of defoliation.     

Velocity and turbulence distribution around lotic macrophytes

Flow velocity and turbulence patterns were measured in and around a common lotic macrophyte, Ranunculus penicillatus subspecies pseudofluitans (stream water-crowfoot), using a two-dimensional electromagnetic current meter (EMCM). Due to the high shooting density of this species, there was a sharp velocity gradient at the plant boundary, with velocities dropping to a constantly low value after no more than 5 cm into the plant, thus forcing most of the flow over and around the macrophyte. There was a dead-water zone immediately downstream of the plant, beyond which the current moved in from the sides to allow flow under the trailing shoots. High turbulence intensities were recorded for both downstream and cross-stream velocity components at the lateral margins and downstream of the plant. Meanwhile, pulses of water upstream of the plant produced turbulence in the downstream component, but not in the cross-stream component.     

Plastic responses to light intensity and planting density in three Lamium species

In this survey plastic responses to light intensity and planting density were examined in three Lamium species (L. purpureum, L. album and L. maculatum). Low light intensity enhanced plant height, length and width of leaves, but reduced number of shoots and leaves, as well as root and shoot weights. Higher density resulted in smaller plants and leaves, but had significant effect on module number (shoots and leaves) only on older plants. The effect of light intensity on measured traits was greater than the effect of density, and consistent with predictions about plastic responses on light intensity variation. Generally, the three Lamium species differed in the magnitude but not in patterns of plasticity. However, associations of analyzed traits with fitness significantly differed among species as well as among light treatments.     

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.     

The drag and reconfiguration experienced by five macrophytes from a lowland river

Drag and flexibility of five macrophytes (fresh mass 3 g) collected from the same river were measured at velocities from 0 to 0.5 m s⁻¹ in a flume. Drag increased with increasing velocity for all five species examined. Sparganium emersum Rehmann, which has simple strap-like leaves experienced significantly less drag than the other, bushier species whilst there was no significant difference between the drag on Callitriche stagnalis Scop., Ranunculus penicillatus pseudofluitans (Syme) S.D. Webster, and Myriophyllum spicatum L. above 0.4 m s⁻¹. Potamogeton x zizii W.D.J. Koch ex Roth, which has large flat leaves, experienced significantly higher drag than all the other species. All the plants were very flexible but flexibility (as angle of bend) did not explain the drag experienced by the plants, e.g. S. emersum was the least flexible. The plants also changed shape and compressed (reconfigured) under increasing water velocity which reduced the rate at which drag increased. Reconfiguration capacity was assessed as E-values. There were no significant differences in E-values between species indicating that all the samples examined had a similar capacity to reconfigure. It is concluded that measurement of the drag experienced by plants is useful and may prove helpful in explaining the distribution of macrophytes in rivers.     

NUTRIENT AND LIGHT LIMITATION OF ALGAE IN TWO NORTHERN CALIFORNIA STREAMS1

Nutrient-diffusing subsrates were used to investigate nutrient limitation of attached algal assemblages in a shaded stream and an unshaded stream in northern California. Water from both streams contained low levels of nitrogen (< 14 μg.L^?1) and very low N:P ratios (< 2). After 31 days of colonization and growth, attached algal biomass on nitrate-diffusing substrates was significantly greater than on control substrates in the unshaded stream. Nitrate-diffusing substrates also supported larger numbers of grazing insects in the unshaded stream. The prostrate diatoms Achnanthes lanceolata Bréb. and Coconeis placentula Ehr. displayed the most consistent positive responses to nitrate enrichment. Nutrient enrichment did not increase the accrual of algal biomass in the shaded stream, but algal biomass was significantly greater at sites located under openings in the tree canopy, implicating light as a limiting factor in this stream. Several Navicula and Nitzschia species, and one unidentified Gomphonema species, were positively associated with higher light levels in the shaded stream. Shade appears to be the primary factor limiting algal growth in small northern California streams, but when its effect is reduced by logging, the inherently low levels of nitrogen in these streams can become limiting.