Differential response to shading in orthotropic and plagiotropic shoots of the clonal herb Glechoma hirsuta

Previous studies have shown that internodes and petioles of closely related erect and stoloniferous species show marked differences in their response to shading. Vertical structures show strong elongation responses while horizontal structures show significantly smaller elongation responses. This paper reports an experiment designed to test whether internodes and petioles on orthotropic (vertically oriented) and plagiotropic (horizontally oriented) shoots produced by the same plant, also show such differential responses. The study species, Glechoma hirsuta, produces plagiotropic shoots in its vegetative stage and orthotropic shoots during its generative stage. Shoots of G. hirsuta were grown either in full daylight or under simulated canopy shade. Internode and petiole elongation, biomass investment patterns and other growth-related parameters were measured on plants in each light treatment. In orthotropic shoots the length of internodes responded more strongly to shading than the length of petioles, while the opposite was true for plagiotropic shoots, confirming the hypothesis that vertical spacers have higher degrees of shade-induced plasticity than horizontally oriented spacers. Growth and development of horizontally oriented shoots was reduced by shading, whereas that of vertically oriented shoots was unaffected by light treatment. The results are discussed in terms of the differences in function of vertically and horizontally oriented spacers, and the probable benefits of plasticity in enhancing essential activities including photosynthesis, habitat exploration and seed dispersal. Received: 23 December 1996 / Accepted: 19 July 1997     

Recovery of deep Posidonia oceanica meadows degraded by trawling

The recovery capacity of meadows of the Mediterranean seagrass Posidonia oceanica (L.) Delile in an area affected by illegal trawling were assessed after protection by anti-trawling reefs. The differences in vegetative growth between two impacted and two undisturbed localities were tested using growth, shoot balance, aborted branches, and leaf and rhizome production of both plagiotropic and orthotropic rhizomes. The organic matter in sediments, silt clay fraction and light intensity incident on the bottom were also measured in order to evaluate the physical conditions. Environmental and plant variables were measured in three sites placed inside each locality. The vegetative growth was positive in both impacted and control meadows but growth rates were lower in impacted than in control meadows. Average growth, production and shoot balance were greater in plagiotropic rhizomes from undisturbed localities (40.7±1.75 vs. 28.4±1.34 mm/year, 1133±0.06 vs. 708±0.04 mg DW/shoot/year, 1.36±0.08 vs. 0.96±0.06 shoots/year, respectively). Significantly greater values were also found in undisturbed localities for orthotropic rhizomes in terms of shoot balance and rhizome production (0.07±0.01 vs. 0.01±0.003 shoots/shoot/year and 155 vs. 124 mg DW/shoot/year, respectively). Of the physical parameters measured, only light intensity differed significantly between impacted and undisturbed localities. This parameter was 15.5% to 67.6% lower in impacted localities than in undisturbed localities, and this is the factor that causes the retardation of vegetative growth. The results show that recovery of P. oceanica meadows is possible after eliminating the cause of the impact. However, the very low rates of vegetative growth may prolong the time to total recuperation to almost 100 years. Therefore, effective management of P. oceanica meadows should aim to prevent meadow loss.     

Partial shoot reiteration in Wollemia nobilis (Araucariaceae) does not arise from 'axillary meristems'

Background and Aims

Conifers are characterized by the paucity of axillary buds which in dicotyledonous trees usually occur at every node. To compensate, conifers also produce ‘axillary meristems’, which may be stimulated to late development. In juvenile material of Wollemia nobilis (Araucariaceae: Massart''s model) first-order (plagiotropic) branches lack both axillary buds and, seemingly, axillary meristems. This contrasts with orthotropic (trunk) axes, which produce branches, either within the terminal bud or as reiterated orthotropic axes originating from axillary meristems. However, plagiotropic axes do produce branches if they are decapitated. This study investigated how this can occur if axillary meristems are not the source.

Methods

The terminal buds of a series of plagiotropic branches on juvenile trees were decapitated in order to generate axillary shoots. Shoots were culled at about weekly intervals to obtain stages in lateral shoot development. Serial sections were cut with a sliding microtome from the distal end of each sample and scanned sequentially for evidence of axillary meristems and early bud development.

Key Results

Anatomical search produced no clear evidence of pre-existing axillary meristems but did reveal stages of bud initiation. Buds were initiated in a group of small starch-rich cortical cells. Further development involved de-differentiation of these small cells and the development of contrasting outer and inner regions. The outer part becomes meristematic and organizes the apex of the new branch. The inner part develops a callus-like tissue of vacuolated cells within which vascular cambia are developed. This kind of insertion of a branch on the parent axis seems not to have been described before.

Conclusions

Axillary meristems in Wollemia characterize the leaf axils of trunk axes so that the origin of reiterated shoots is clear. Plagiotropic axes seemingly lack axillary meristems but still produce axillary branches by distinctive developmental processes. These observations demonstrate limited understanding of branch initiation in trees generally.     

Thorn and hook ontogeny in Artabotrys hexapetalus (Annonaceae)

Artabotrys hexapetalus is widely planted in the tropics and is known as "climbing ylang-ylang," an ornamental liana or woody climber. New natural sprouts, or water shoots, and those induced by the damage of Hurricane Andrew (24 August 1992) were collected and fixed in formalin/acidic acid/alcohol. Seeds from these plants were planted and grown in a greenhouse where seedling morphology was observed and young material collected and fixed. The development of lateral plagiotropic and orthotropic shoots was studied using both epi-illumination light microscopy and scanning electron microscopy. A series of buds develops in the axils of leaves on the orthotropic shoot. At the lateral margins of the axillary shelf, plagiotropic shoots form that will develop into either vegetative shoots, or thorns, or sympodial shoots that bear hooks and flowers. In between the two marginal buds, a series of median vertical buds develop that either remain dormant or grow out as renewal orthotropic shoots. Previous work that suggested that the plagiotropic shoot buds were displaced out of the median vertical series of supernumerary buds is not supported. The sympodial development of plagiotropic branches as inflorescence hooks is documented.     

Anthesis effects on Posidonia oceanica (L.) Delile phenology in the Bay of Calvi (Corsica,Mediterranean Sea)

In Posidonia oceanica (L.) Delile, anthesis induces a decrease in the number of juvenile leaves resulting in a significant reduction in the number of leaves on the flowering shoots. All the leaves of the flowering shoots are narrower than the leaves of nonflowering shoots. A modification of the leaf growth also appears in flowering shoots: the oldest leaves are longer and the leaves induced during or after anthesis are shorter. At 10 m depth, in the Bay of Calvi, anthesis lasts roughly 3 months and the flowering is induced 7 months before anthesis.     

STRUCTURE AND DEVELOPMENT OF THE DIMORPHIC BRANCH SYSTEM OF THEOBROMA CACAO

The vegetative morphology of Theobroma cacao, the cacao tree, was studied in order to provide a foundation for further investigations on the morphogenesis of the cacao dimorphic shoot system. The seedling of cacao has a determinate orthotropic shoot with a (2+3) phyllotaxis. Branch dimorphism is initiated after 1 to 2 years of growth at which time the apical meristem of the orthotropic shoot aborts and a pseudowhorl of plagiotropic branches is initiated from axillary positions in the shoot tip. The plagiotropic branches are characterized by a distichous phyllotaxis and indeterminate growth. Subsequently an axillary bud below the pseudowhorl develops into a new orthotropic shoot. The apical meristem of this shoot eventually aborts and another pseudowhorl is formed. The apical anatomy of the two types of shoots is similar. The developmental potentiality of the orthotropic shoot axillary buds to form one or the other type of shoot was investigated. The phyllotaxis of the axillary buds of the orthotropic shoot is spiral and that of the axillary buds of the plagiotropic branch is distichous. Pruning and apical puncture experiments showed that the axillary buds of a plagiotropic branch, and of an orthotropic seedling shoot which has not yet formed a pseudowhorl, always give rise to the parent type of shoot. However, the axillary buds of an orthotropic shoot which already bears a pseudowhorl give rise to either type of shoot for several nodes below the point of origin of the pseudowhorl. The type of shoot has no influence on the form of branch which develops from an axillary bud grafted to it. This evidence supports the hypothesis that the axillary buds are initiated as one or the other type of shoot, i.e., once initiated they are predestined.     

Modular growth and functional heterophylly of the phreatophyte Ziziphus lotus: A trait-based study

The variation of plant functional traits, from the cell to the whole-plant level, is a central question in trait-based ecology with regard to understanding ecological strategies and adaptations that result from environmental drivers. Here, we analyzed whole-plant and leaf traits of the phreatophyte Ziziphus lotus (L.) Lam., a long-lived shrub that dominates one of the few terrestrial groundwater-dependent ecosystems (GDEs) in Mediterranean Basin drylands. We (a) assessed architectural traits and growth patterns, (b) analyzed leaf morpho-functional traits (specific leaf area [SLA] and stomata pore index [SPI]) and physiological traits (gas exchange rates), as well as their variations within individuals, and (c) evaluated temporal variations in modular growth (i.e., sequential iteration of structural units) between growing seasons and in leaf traits within seasons. Z. lotus' growth pattern was based on the repetition of modules composed of shoots (short and long) and branches (flowering and plagiotropic) that promoted a functional differentiation between vegetative and reproductive structures, respectively. We identified morpho-functionally distinct leaves (i.e., heterophylly) borne on different types of branches. Leaves on flowering branches had higher SLA and water use efficiency (WUEi), but lower SPI and transpiration rates than leaves on vegetative ones. We also observed trade-offs in the elongation of vegetative and flowering structures between growing seasons: the shorter the long shoots, the larger the flowering branches. The modular differentiation and heterophylly of Z. lotus might contribute to prioritizing the investment of resources of this phreatophyte, either for growth or reproduction, and could improve the efficiency in uptake and conservation of resources in drylands.     

Branching responses of a plagiotropic clonal herb to localised incidence of light simulating that reflected from vegetation

Plants sense the presence of neighbouring vegetation through phytochrome photoreceptors perceiving a lowered red to far-red ratio (R:FR) of light reflected from such vegetation. We hypothesised that it would be advantageous for the grassland clonal herb, Trifolium repens, to have an inhibitory branching response to perception by leaves of light reflected from neighbouring vegetation (i.e. light with lowered R:FR ratio) but have no response to interception of such light by the plagiotropic stem. We tested whether photoreception of reflected light by plagiotropic stems resulted in a different branching response to photoreception by leaves and whether leaf ontogeny influenced the response. To simulate light reflected from vegetation, FR light-emitting-diodes were used to supplement controlled environment room light so that the R:FR ratio, but not the photosynthetic photon fluence rate, of light incident at the stem or leaf of a phytomer of T. repens was lowered from 1.20 to 0.25. The plagiotropic stems were unresponsive to light simulating that reflected from vegetation. This response differs from that of stems of orthotropic species, indicating that plagiotropic stems have evolved an organ-specific photobiology. Treatment of the mature leaf with light of lowered FR ratio reduced phytomer production only of the branch in the axil of the treated leaf. Similar treatment of the immature leaf retarded, in addition, branching at basal phytomers on the same side of the primary stem axis. Thus the response to light simulating that reflected from neighbouring vegetation depended upon whether the light was incident at the stem or the leaf and on the stage of leaf development. We argue that such responses improve the performance and fitness of T. repens within grassland habitats by allowing axillary buds on plagiotropic stems to branch freely when stems are in receipt of light reflected from vegetation while leaves are in full light.     

LEAF DEVELOPMENT AND PRIMARY VASCULAR ORGANIZATION IN SHOOTS OF ANISOPHYLLEA DISTICH A

Anisophyllea disticha is characterized by strong shoot dimorphism. Orthotropic shoots with helically arranged scale leaves produce tiers of plagiotropic shoots, while plagiotropic shoots are anisophyllous and bear dorsal scale and ventral foliage leaves arranged in a unique tetrastichous system. In this study we compare the patterns of leaf development and primary vascular organization in the two types of shoots. Orthotropic shoots have an open vascular system with five sympodia. Expansion of orthotropic shoot scale leaves occurs from P1 to P10–12, and leaf tissues mature precociously. Plagiotropic shoots have a closed vascular system with six sympodia. Leaves in ventral and dorsal orthostichies do not differ significantly in size until ca. P15, but ventral leaves are distinct histologically from the second node in an orthostichy, P4–6. Ventral foliage leaves have a diffuse plate meristem, and leaf expansion continues until ca. P30. Differentiation of ventral and dorsal leaf trace procambium parallels the divergent patterns of leaf expansion. These observations demonstrate the strong correlation among shoot symmetry, leaf development, and vascular differentiation within dimorphic shoots of one species.     

MORPHOLOGICAL STUDIES OF THE NYMPHAEACEAE (SENSU LATO). XIII. CONTRIBUTIONS TO THE VEGETATIVE AND FLORAL STRUCTURE OF CABOMBA

Six species of Cabomba have been examined although the anatomy of the vegetative axes is based on the study of only C. caroliniana and C. palaeformis. A plant consists of an erect short shoot with decussate leaves which bears axillary flowering shoots and rhizomes. A rhizome bears decussate leaves and may also form axillary flowering shoots or turn upward and become a new short shoot. The phyllotaxies of the flowering shoots are proximately decussate or ternate (C. piauhyensis). The flowering shoots with decussate phyllotaxy change to 1/3 phyllotaxy distally; they bear axillary flowers proximally, and extra-axillary flowers distally. Flowering shoots with ternate phyllotaxy do not change distally but each produces first axillary and then extra-axillary flowers. Decussate vegetative axes and flowering shoots have four vascular bundles; ternate vegetative axes and flowering shoots have six vascular bundles, distantly paired into two or three vascular bundle-pairs, respectively. An elliptical vascular plexus occurs at each node. Each leaf receives one bundle-pair from one trace and each flower three bundle-pairs. A two-level receptacular vascular plexus occurs in flowers; the proximal, larger portion provides traces to perianth and stamens and the distal, smaller portion becomes carpellary traces. Each of the three sepals typically receives five branch traces from a basal principal trace, and each of the three petals receives, typically, three branch traces from a basal principal trace. Sepals and petals generally occur in a single, basally connate whorl. Each stamen receives one trace. Each stamen of three-stamen flowers is opposite a petal; each stamen of six-stamen flowers is aligned with an interval between a petal and adjacent sepal. Each staminal trace, which is just above the principal petal trace, in a three-petal flower, is frequently adnate to the latter trace. Each carpel receives one principal trace from the distal, small extension of the receptacular plexus, and each principal trace becomes three conventional veins of a carpel. Ovules may be borne directly over one of the veins or in any position between veins and are supplied by branches of the nearest vein or nearest two veins. All traces, ovular supply veins and the proximal portions of all veins are amphicribral. The several anatomical and morphological differences in vegetative axes and flowers between Cabomba and Brasenia suggest a greater taxonomic distance between the two genera than commonly supposed. It is suggested that extra-axillary flowers in 1/3 helical and ternate flowering shoots of Cabomba might be advantageous in preventing anthesis of flowers beneath peltate leaves. The aberrant position might be the initial evolutionary step toward what, in other nymphaeaceous genera, has shifted each flower to an adjacent helix. It is proposed that the zigzag stem accompanying the trigonal and sympodial flowering shoots may offer greater stability and floatability in water than the monopodial form. Several suggestions are offered for the variability of ovular positions: 1) the variability is a vestige of former laminar placentation in conduplicate carpels; 2) it is a vestige of a primitive condition antedating the current close association of ovules with ventral carpellary veins; 3) it is an early stage of evolution which might have terminated in laminar placentation and cantharophily, but which was replaced by a trend toward myophily.     

Genetic variability of Posidonia oceanica (L.) Delile in relation to local factors and biogeographic patterns

To evaluate genetic differences of Posidonia oceanica (L.) Delile both at smaller (within a meadow) and larger scale (Mediterranean basin), plants of P. oceanica were analyzed by PCR technique and compared using random amplified polymorphic DNA (RAPD) markers. Results were associated to known differences in phenology. At the small-scale level, P. oceanica shoots collected in the bay of Monterosso al Mare (Liguria, NW Mediterranean Sea) showed genetic differences among sampling stations, with a decrease in genetic diversity along an anthropogenic disturbance gradient. At basin level, genetic differences were detected among 11 P. oceanica shoots coming from different regions of the Mediterranean, and transplanted to the Port-Cros National Park (France) between 1989 and 1991: Izmir, Turkey; Athens, Greece; Taranto, Italy; Ischia Island, Italy; Lavezzi, France; Port-Cros, France; Banyuls, France; Palma de Majorca, Balearic Islands, Spain; Marsa Bay, Algiers. By cluster analysis two major Mediterranean groups were distinguished, the Eastern Mediterranean Group (EMG) and the Western Mediterranean Group (WMG). This suggests that eastern and western populations of P. oceanica have diverged during the colonization of the Mediterranean (after near extinction of the Mediterranean biota in the Messinian period, approximately 5.6 million years ago), and have experienced little gene flow between them. Cluster analysis also indicated that previously described phenological differences among P. oceanica populations in different sectors of the Mediterranean are not mere phenotypic responses to different climatic and hydrological conditions but may well have a genetic basis.     

Shoot age as a confounding factor on detecting the effect of human-induced disturbance on Posidonia oceanica growth performance

The response of orthotropic rhizome elongation and primary production of Posidonia oceanica to anthropogenic perturbations and potential confounding effects of shoot age were assessed using a Linear Multilevel Model (LMM). This model examined the confounding effect of age by comparing the estimates of impact and variance components obtained by excluding and including Age as an explanatory variable. Age had a negative effect on rhizome elongation and primary production with an annual decrease of 0.6 mm y^− 1 and 7 mg dw y^− 1 respectively. According to the LMM when age effect was omitted, the differences between disturbed and control locations in rhizome elongation and primary production were 2.62 mm y^− 1 and 0.044 g dw y^− 1 respectively. These effects were statistically not significant. On the contrary, when age effect was included in the statistical model, impacts became evident for both variables, with significant differences between disturbed and control locations of 5.85 mm y^− 1 and 0.081 g dw y^− 1 for rhizome elongation and primary production, respectively. Thus, particular attention should be paid to the potential confounding effect of shoots age in analyses of impacts on growth performance of P. oceanica.     

Growth and posture control strategies in Fagus sylvatica and Acer pseudoplatanus saplings in response to canopy disturbance

Background and Aims

Forest tree saplings that grow in the understorey undergo frequent changes in their light environment to which they must adapt to ensure their survival and growth. Crown architecture, which plays a critical role in light capture and mechanical stability, is a major component of sapling adaptation to canopy disturbance. Shade-adapted saplings typically have plagiotropic stems and branches. After canopy opening, they need to develop more erect shoots in order to exploit the new light conditions. The objective of this study was to test whether changes in sapling stem inclination occur after canopy opening, and to analyse the morphological changes associated with stem reorientation.

Methods

A 4-year canopy-opening field experiment with naturally regenerated Fagus sylvatica and Acer pseudoplatanus saplings was conducted. The appearance of new stem axes, stem basal diameter and inclination along the stem were recorded every year after canopy opening.

Key Results

Both species showed considerable stem reorientation resulting primarily from uprighting (more erect) shoot movements in Fagus, and from uprighting movements, shoot elongation and formation of relay shoots in Acer. In both species, the magnitude of shoot uprighting movements was primarily related to initial stem inclination. Both the basal part and the apical part of the stem contributed to uprighting movements. Stem movements did not appear to be limited by stem size or by stem growth.

Conclusions

Stem uprighting movements in shade-adapted Fagus and Acer saplings following canopy disturbance were considerable and rapid, suggesting that stem reorientation processes play a significant role in the growth strategy of the species.     

A Developmental Pattern of Flowering in Colored <Emphasis Type="Italic">Zantedeschia</Emphasis> spp.: Effects of Bud Position and Gibberellin

The restricted flowering of colored cultivars ofZantedeschia is a consequence of developmental constraints imposed by apical dominance of the primary bud on secondary buds in the tuber, and by the sympodial growth of individual shoots. GA₃ enhances flowering inZantedeschia by increasing the number of flowering shoots per tuber and inflorescences per shoot. The effects of gibberellin on the pattern of flowering and on the developmental fate of differentiated inflorescences along the tuber axis and individual shoot axes were studied in GA₃ and Uniconazole-treated tubers. Inflorescence primordia and fully developed (emerged) floral stems produced during tuber storage and the plant growth period were recorded. Days to flowering, percent of flowering shoots and floral stem length decreased basipetally along the shoot and tuber axes. GA₃ prolonged the flowering period and increased both the number of flowering shoots per tuber and the differentiated inflorescences per shoot. Activated buds were GA₃ responsive regardless of meristem size or age. Uniconazole did not inhibit inflorescence differentiation but inhibited floral stem elongation. The results suggest that GA₃ has a dual action in the flowering process: induction of inflorescence differentiation and promotion of floral stem elongation. The flowering pattern could be a result of a gradient in the distribution of endogenous factors involved in inflorescence differentialtion (possibly GAs) and in floral stem growth. This gradient along the tuber and shoot axes is probably controlled by apical dominance of the primary bud. Online publication: 7 April 2005     

Comparing shoot population dynamics methods on Posidonia oceanica meadows

Prediction capacity of three main shoot population dynamics methods (age structure, net shoot recruitment per plagiotropic rhizome and shoot census) have been tested for a period of four years (2002-2006) on a Posidonia oceanica meadow. Accuracy of each method was checked by comparing measured and predicted densities at the end of the study period. Predicted densities came from the evolution of initial densities (measured in 2002) by a basic exponential model of population growth. The exponential model used the different net shoot recruitment rate estimates by each population dynamics method on three depths (upper, medium and lower limit) and three localities at each depth. Predictions performed by shoot census and net shoot recruitment per plagiotropic rhizome methods matched with measured densities at the end of the study period. Conversely, age structure method underestimated shoot densities at each depth, indicating an unreal decrease of shoot population in the meadow.     

Effects of shoot position on shoot and leaf morphology of Avicennia marina in the hyperarid Red Sea coastal region of Egypt

We examined the effects of shoot position on shoot growth and morphology of Avicennia marina (Forssk.) Vierh. in the Red Sea coastal region of Egypt. To determine differences in morphological characteristics, we collected shoots from the upper and lower canopies of A. marina individuals in the wild and compared the morphological characteristics of these shoots. The study plot was established in an A. marina mangrove forest. Heights and diameters of individual trunks (n = 14) in the plot were measured at ground level. Then, five shoots with young but fully expanded leaves were collected from the upper and lower canopies of the individuals. We measured shoot length, and dry weight and also area, dry weight, thickness, and Soil Plant Analysis Development (SPAD) value of collected leaves. Our measurements showed that leaf area, dry weight, specific leaf area, and SPAD value of leaves from the upper canopy were smaller than those of lower-canopy leaves in most individuals. From the differences in traits between upper and lower leaves, we concluded that leaves in the upper canopy are typically adapted to high light levels, whereas leaves in the lower canopy exhibit adaptations to low light conditions. In addition, soil-water salinity at the study site was far higher than the optimum salinity for A. marina. Hence, it is also suggested the salinity level at this site may have influenced the reduced leaf size in the upper canopy.     

Spontaneous plant regeneration in transformed roots and calli from Tylophora indica: changes in morphological phenotype and tylophorine accumulation associated with transformation by Agrobacterium rhizogenes

We examined the effects of genetic transformation by Agrobacterium rhizogenes on the production of tylophorine, a phenanthroindolizidine alkaloid, in the Indian medicinal plant, Tylophora indica. Transformed roots induced by the bacterium grew in axenic culture and produced shoots or embryogenic calli in the absence of hormone treatments. However, hormonal treatment was required to regenerate shoots in root explants of wild type control plants. Transformed plants showed morphological features typically seen in transgenic plants produced by A. rhizogenes, which include, short internodes, small and wrinkled leaves, more branches and numerous plagiotropic roots. Plants regenerated from transformed roots showed increased biomass accumulation (350–510% in the roots and 200–320% in the whole plants) and augmented tylophorine content (20–60%) in the shoots, resulting in a 160–280% increase in tylophorine production in different clones grown in vitro.     

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 role of disturbance in promoting the spread of the invasive seaweed Caulerpa racemosa in seagrass meadows

Human disturbances, such as anchoring and dredging, can cause physical removal of seagrass rhizomes and shoots, leading to the fragmentation of meadows. The introduced green alga, Caulerpa racemosa, is widely spread in the North-West Mediterranean and, although it can establish in both degraded and pristine environments, its ability to become a dominant component of macroalgal assemblages seems greater in the former. The aim of this study was to estimate whether the spread of C. racemosa depends on the intensity of disturbance to the canopy structure of Posidonia oceanica. A field experiment was started in July 2010 when habitat complexity of a P. oceanica meadow was manipulated to simulate mechanical disturbances of different intensity: rhizome damage (High disturbance intensity = H), leaf removal (Low disturbance intensity = L), and undisturbed (Control = C). Disturbance was applied within plots of different size (40 × 40 cm and 80 × 80 cm), both inside and at the edge of the P. oceanica meadow, according to an orthogonal multifactorial design. In November 2011 (16 months after the start of the experiment), no C. racemosa was found inside the seagrass meadow, while, at the edge, the cover of the seaweed was dependent on disturbance intensity, being greater where the rhizomes had been damaged (H) than in leaf removal (L) or undisturbed (C) plots. The results of this study indicate that physical disturbance at the margin of seagrass meadows can promote the spread of C. racemosa.     

Branch geometry in Cornus kousa (Cornaceae): computer simulations

Computer simulations similar to actual trees were constructed using simple branching rules. Branch orientation with respect to the direction of gravity was a fundamental consideration. In Cornus kousa BUERG. ex HANCE, several types of branches develop from winter buds, varying from orthotropic shoots to plagiotropic ones. Based on actual observations and measurements of branching structures with a wide range of orientations, we made a flexible geometrical model consisting of five forking branches that varied in outgrowth depending on the direction of the shoot with respect to gravity. Repetition of the branching by computer generated a realistic tree pattern, which was close to the shape of a young C. kousa tree. Reproductive shoots seem to be under a branching rule that was a modification of vegetative branching, although the reproductive branch size was considerably smaller than the vegetative one, and reproductive branching was bifurcated instead of five-forked. We conclude that all branchings in orthotropic and plagiotropic shoots in the vegetative phase and shoots in the reproductive phase are formed under the same branching rule, but each has different parameter values.