Density-dependent growth responses in two clonal herbs: regulation of shoot density

Summary It has been shown in clonal perennial herbs that shoot natality decreases, and shoot mortality increases, in stands of increasing density. In a two-year garden experiment, we have tested Hutchings' (1979) hypothesis that these responses are the result of physiological integration, i.e. the exchange of resources and growth substances between shoots of a single clone. Dense monocultures of two rhizomatous graminoids, Brachypodium pinnatum and Carex flacca, were created that differed more than 10-fold in the density of clones (genets), but that had similar densities of shoots. A more effective shoot density control was expected in stands with the smaller clone densities (larger clones) due to more extensive clonal connections. Shoot turnover was evaluated by counting living and dead shoots at different times. In the summer of the second year, when shoot densities and stand structure were similar between treatments, shoot natality (the number of shoots born per plot) and shoot mortality (the number of shoots that died per plot) were usually unrelated to clone density in either species. If there was a significant treatment effect, it could be attributed to (small) differences in shoot density. Over the whole range of shoot densities, natality was negatively density-dependent. The number of shoots that died in a given growth period was proportional to the number of shoots present, suggesting that mortality rates were density independent. In Carex, however, there were some indications that mortality rate increased with increasing density. Our study confirms that clonal herbaceous species can effectively prevent an overproduction of shoots, but in contrast to Hutchings' (1979) propositions, we found no evidence that physiological integration may be the responsible mechanism. An alternative explanation for the observed patterns is proposed.     

中国亚热高大竹类植物毛竹竹笋克隆生长的密度调节（英文）

 对许多多年生克隆植物来说,大量的研究表明：当光是限制因子时,随着立地密度的不断增加,克隆分株的出生率逐渐减小、死亡率逐渐增加。本文观测了乔木状高大竹类植物毛竹竹笋的出生与存活过程,结果表明：竹笋的出生率,即每样方的出笋数,明显地随着成竹立竹度的增加而增加。更确切地说,竹笋的数量,不管是出笋数还是活笋数,都明显地随着带新叶(1龄叶)的成竹立竹度的增加而增加,而与带老叶(2龄叶)的成竹立竹度相关性不显著。并且竹笋的死亡率是非密度制约的。这可能是由于对毛竹来说,其立地总是比较开敞,而且,其竹笋的生长在很大程度上是不直接需光的。     

Growth and development of Potamogeton distinctus in an irrigation pond in SW Japan

Growth and development of a population of Potamogeton distinctus were studied over one vegetation period.
The morphological structure of the species is described. Maximum development of the size of shoot complexes occurred in August (up to 12 g dry weight/shoot complex). The plant exhibits a regular growth cycle with continuous horizontal and vertical growth that is illustrated by the development of horizontal shoots, vertical shoots and different plant parts. The lower horizontal shoots allow an extension up to 1.8 m from the point of germination. Before the rainy season in June the 1st to 5th generations of vertical shoots mainly contribute to the standing crop of the plants' stand, and after the rainy season the 5th to 9th generation shoots are predominant. Floating leaves make up the main proportion of biomass during the whole growth cycle. Almost 100% of the shoot complexes were found flowering and producing numerous seeds. Up to 16% of the standing crop are allocated to inflorescence and seeds in August. Turions failed to form that year because of rapid desiccation of the habitat. The observed average lifetime of a vertical shoot is appr. 65 days, the estimated turnover rate is between 2.15 and 2.27.     

Morphological Constraints of Shoot Demography of a Clonal Plant: Extra- and Intravaginal Tillers of Festuca rubra

Abstract Festuca rubra forms tillers in two different ways: extravaginally and intravaginally. Demography of these two tiller types was observed in seventeen selected tussocks of Festuca rubra s.s. over four growing seasons. Extravaginal tillers were bigger at birth and on the average produced twice as many daughter tillers per tiller. In general, the natality and mortality of extravaginal tillers were less regular than that of intravaginal tillers. Overall tillering rate per tiller was correlated with the density of the surrounding vegetation; mortality, natality and tiller life span were not. High density of the surrounding vegetation did not result in increased formation of extravaginal tillers. The proportion of the extravaginal tillers was not correlated with the density of the F. rubra tussocks. There is no evidence for foraging by extravaginal tillers, but they do act as founders of small clusters of tillers.     

Stand Density Effects on Branching in an Annual Legume (Senna obtusifolia)

Effects of proximity to nearest neighbours on shoot morphologyand branch growth were examined for Senna obtusifolia (L.) I.& B., an annual legume species that displays wide variationin branching. Periodic surveys described location and type ofgrowth at all nodes on plants in regularly spaced monocultures.Stands with interplant distances of 15-50 cm (51-5 plants m^-2)formed closed canopies with similar amounts of leaf area andbiomass. Number of lower primary branches, their degree of curvature,and location of branch apices relative to the main stem respondedto interplant distance relatively early in shoot growth, beforecanopy closure. The final effects of ten-fold differences inplanting density on the formation of these ascending lower brancheswere only two-fold differences in number of vertical shoot axeswithin the stand. An additional study examined response of lowerbranches to an adjacent gap in the stand. Initial number anddirection of branch growth were not affected by location ofthe gap. The first effect of crowding by neighbours was on number ofbranches initiated by developing shoots. Differences in branchingsubsequently increased through differentiaI survival and directionof branch growth. Such alterations principally affected lateralexpansion of individual shoots, with little effect on radialsymmetry or vertical distribution of leaves.Copyright 1994,1999 Academic Press Branch morphology, canopy, competition, light quality, Senna obtusifolia, shoot growth, sicklepod     

Physiological acclimation and growth response to partial shading in <Emphasis Type="Italic">Salix matsudana</Emphasis> in the Mu Us Sandland in China

In a controlled experiment, Salix matsudana plants were subjected to uniform nonshading (F-S), partial shading (P-S) and uniform shading (U-S). The shoots of the plants in the F-S and U-S treatments were referred to as H-H and L-L, respectively. The plants in the P-S treatment had two kinds of shoots: (1) shoots under the nonshading treatment that were connected to others under the shading treatment (H-L).(2) Shoots under the shading treatment that were connected to others under the nonshading treatment (L-H). The physiological acclimation and growth response of the species to the partial shading were examined. The partial shading had significant effects on photosynthetic dynamics, transpiration and stomatal conductance, but no effect on instantaneous water use efficiency and maximum quantum yield. Water saturation deficit and coefficient of water loss were significantly smaller in the H-L shoots than in the H-H shoots. Leaf natality, leaf mortality and leaf turnover were greater in the H-L shoots than in the H-H shoots. In contrast, these three parameters were smaller in the L-H shoots than in the L-L shoots. The H-L shoots had significantly larger branching ratio, total branch length and shoot biomass than the H-H shoots. The L-H shoots had smaller branching ratio, total branch length and shoot biomass than the L-L shoots. Total plant biomass in the treatments increased as follows: F-S相似文献   

Developmental responses of subdominant grassland species to current weather conditions and their relevance for annual vegetation changes

Year-to-year vegetation changes have been observed many times in temperate grasslands. Ordinarily, variation in annual weather patterns is considered responsible for these changes. However, the exact mechanisms of vegetation dynamics have seldom been studied. In order to analyse the direct response of plants to distinct weather situations, the shoot growth rates of three subdominant grassland species were determined during three growing seasons and tested for significant relationships to meteorological variables measured simultaneously in the study site. Half of the shoots grew in the natural community with competition from neighbouring plants. For the other half, above-ground interference was avoided by regularly clipping the surrounding vegetation. The results lead to the distinction of three different impact patterns of current weather conditions on the growth of subdominant grassland species: (i) As a consequence of extraordinary weather conditions, e.g. lasting periods of drought in the summer, plants die completely or partially or pass into secondary dormancy. Such weather situations may cause quantitative or even qualitative changes in species composition by altering the density and frequency of the species involved. (ii) Major divergences from average weather conditions, such as unusually warm or cold periods in the spring, affect the growth of subdominant species and may therefore lead to quantitative annual variation of the species involved in terms of cover or biomass. (iii) Average weather conditions with slight deviations from the long-term means of the weather variables do not produce detectable direct growth responses and therefore average weather conditions are not key factors for year-to-year variations in the quantitative or qualitative performance of subdominant species.     

An architectural model for Eleocharis: morphology and development of Eleocharis cellulosa (Cyperaceae)

Species of Eleocharis are prominent in aquatic and wetland habitats and serve as models for study of physiological adaptations to aquatic environments. The genus has an unusual morphology because the major photosynthetic organ is the stem. In order to define an architectural model for the genus to understand the evolution of this morphology, we examined mature morphology and development of E. cellulosa in living and fixed material using light and scanning electron microscopy. Eleocharis cellulosa has sympodial, vertical shoots that produce the photosynthetic culms and horizontal shoots that mix monopodial and sympodial development. Each sympodial unit produces three bracts, an elongated photosynthetic internode, then a fourth bract and an inflorescence that either aborts on vegetative culms or expands on reproductive culms. On each sympodium, the first bract subtends a precocious axillary bud that reiterates the sympodial unit; the second bract subtends a bud that develops the horizontal shoot. In both horizontal and vertical shoots, the internode below the second bract is produced by both the second bract and the renewal shoot. Sympodial growth is present in seedlings. In other species of Eleocharis, the structure of the sympodial unit is conserved but morphological diversity develops from variation in horizontal shoot growth.     

Drag forces on common plant species in temperate streams: consequences of morphology, velocity and biomass

Swift flow in streams may physically influence the morphology and distribution of plants. I quantified drag as a function of velocity, biomass and their interaction on the trailing canopy of seven European stream species in an experimental flume and evaluated its importance for species distribution. Drag increased at a power of 1.3–1.9 with velocity and 0.59–0.77 with biomass in 75% of the measurements. Velocity and biomass interacted because higher velocity causes reconfiguration and greater internal shelter to unimpeded flow and higher biomass enhances shelter among neighbouring shoots. Increase of drag with velocity did not differ systematically among inherently streamlined or non-streamlined species while increase of drag with biomass was smallest among non-streamlined shoots which provide greater mutual shelter. At low shoot density, inherently streamlined species usually experienced the lowest drag conducive to colonisation and growth in swift flow. At high shoot density, no systematic differences in drag existed between the two morphologies. No clear relationship existed between drag forces, morphology and field distribution of species as a function of current velocity probably because a variety of environmental conditions and plant traits influences distribution. Drag on the trailing canopy usually increased 15- to 35-fold for a 100-fold increase of biomass suggesting that an even distribution of plants at low density across the stream bed offers greater resistance to downstream flow than an uneven distribution with the same biomass confined to dense patches surrounded by open flow channels. Thus, management strategies to ensure a patchy plants distribution should be suitable for combining agricultural drainage and ecological stream quality. Handling editor: S. M. Thomaz     

No evidence of a generalized potential ‘cost’ of apical dominance for species that have strong apical dominance

尚无证据表明顶端优势强的物种存在广义顶端优势潜在“成本”     

Variation in plant performance in a grassland: Species‐specific and neighbouring root mass effects

Question: What characteristics of local biotic neighbourhood is the best proxy of competitive effects experienced by plants in a herbaceous community: (1) total above‐ground biomass, (2) root mass or (3) relative above‐ground abundance of selected species? Location: Grassland at ca. 1100 m a.s.l. in the Krkono?e Mts., northern Czech Republic. Methods: We implanted two phytometer species, Antho‐xanthum alpinum and Festuca rubra, into a mountain grassland, and examined their response to local variation in (1) total above‐ground biomass, (2) root mass at three soil depths, and (3) relative abundance of individual species above‐ground. Results : Performance of both phytometer species was determined much more consistently by the mass of neighbouring roots and by species composition of neighbours than by the total above‐ground biomass. The two phytometer species showed different responses to these parameters. The most important relationships were (1) negative relationship between performance of Anthoxanthum and mass of neighbouring roots at 0–3 cm, (2) positive relationship between performance of Festuca and mass of neighbouring roots at 3–6 cm, and (3) negative relationship between performance of Festuca and relative abundance of Festuca in the neighbourhood. Conclusions: Neighbouring root mass and above‐ground species composition are better determinants of biotic interactions than total above‐ground biomass of neighbours in the studied mountain grassland. However, the relationships found are not necessarily due to variation in competitive intensity but can be due to other hidden factors as well, e.g. local availability of resources.     

Responses of crown architecture in Betula pendula to competition are dependent on the species of neighbouring trees

We measured the growth responses of individual shoots and branches of Betula pendula when growing next to trees of the same species or Pinus sylvestris, Larix sibirica or Alnus glutinosa. We used the three-dimensionally digitized response variables and the size and distance of trees growing within a 5-m radius of the study trees to establish a relationship between tree performance and the effect of competing neighbouring tree species on crown architecture. B. pendula was able to modify its crown architecture and thus alter its strategy to compete with different neighbours. Trees of B. pendula growing beside species counterparts had the highest growth of new long shoots in relation to the already existing branch length [growth vigour (GV)], while GV was the lowest next to L. sibirica. With B. pendula or P. sylvestris as its main neighbour, B. pendula invested in short shoots by growing them rather densely in short branches with limited numbers, whereas with L. sibirica the number, length and angle of the branches were high. The competitive response was also strongly dependent on tree ontogeny and the shoot and branch characteristics were significantly affected by their location inside the crown. B. pendula was able to respond to the challenges posed by its neighbours, which was also reflected in the GV. The ability to maintain steady growth with alternative crown designs in different neighbourhoods reflects plasticity in the crown responses.     

Ramet size equalisation in a clonal plant,Phragmites australis

The influence of shading from older generations of dead culms (standing litter) on density, growth rate and development of size structure at the ramet level was investigated in a pure stand of Phragmites australis by experimental neutral shading of plots after removal of standing litter. Initial differences in height distribution between autumn and spring cohorts disappeared in the course of shoot growth. The Gini coefficients of shoot heights and estimated shoot weights indicated that the size structure of the shoots became more equal with increasing mean size in both shaded and unshaded plots. Relative growth rate for height (RHGR) and weight of individual shoots was negatively related to shoot size during the early and presumably storage-dependent growth period, suggesting a strong support for growth of smaller shoots. No etiolation was indicated by mean or maximum height in shaded and unshaded plots, or by the relationship between shoot height and weight. Mean shoot density was significantly lower in shaded than in unshaded plots in one of two shade treatment years. A regression model indicated a small but significant effect of shoot density on the approximately linear relationship between RHGR and the logarithm of height. The growth rate of small shoots was slightly larger at low than at high shoot density. Therefore, it is suggested that the shade from standing litter in P. australis stands can decrease shoot natality in the spring cohort, and thereby increase the support to fewer small shoots.     

Effects of shoot bending on ACC content,ethylene production,growth and flowering of bougainvillea

Several factors, such as environmental conditions, pruning, and plant growth regulators, affect the flowering of bougainvillea. However, information on the effect of shoot bending on growth and flowering of bougainvillea is scarce. In the natural environment, most of the bougainvillea flowering shoots are inclining whereas vertical shoots are not flowering shoots. Bougainvillea shoots are artificially grown vertically, horizontally and at an inclined orientation, to investigate the effect of these orientations on plant growth and the development of flower buds. The results of this indicate an effect of shoot bending on the growth rate of bougainvillea and the rate of flower bud formation. Additionally, our results suggest that vertical shoots have a higher growth rate, more prolific vegetation growth, and longer plastochrons (which are the intervals between the initiations of successive leaves). In contrast, horizontal and inclined shoots exhibited slower growth, a shorter time to reach flowering, and more flower buds. Inclined shoots had a higher endogenous ACC (1-aminocyclopropene-1-carboxylate) content and produced more ethylene than either horizontal or vertical shoots, indicating that more ACC in the inclined shoot is converted into ethylene, and the higher ethylene concentration in the inclined shoot causes it to mature earlier and flower sooner.     

Shoot demography in new England populations of Maianthemum canadense desf.

Summary The demography of shoots of eight populations of a herbaceous perennial exhibiting clonal growth, is presented. The study was done along an elevational gradient, from a more open secondary mixed forest to a denser, more mature stand. Most shoots lived one to three years on the average, but shoots as old as twelve years were found. Large variation in formation and mortality of shoots was observed among plots and years. Yearly trends in the mortality rates of site replicates showed a higher correlation than rates of shoot formation. Although the density of shoots was highest in the drier sites, the turnover of shoots was highly variable and apparently uncorrelated with site location. Age structures revealed a tendency of longer-lived and higher reproductive activities among shoots from more mesic sites. It is hypothesized that environmental rather than density controls are primary causes of the population dynamics observed in this species.     

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.     

Clonal integration and effects of simulated herbivory in old-field perennials

Summary We compared the growth, phenology and leaf demography of partly defoliated, connected shoots with that of partly defoliated, severed shoots in four old-field perennials (Solidago canadensis, S. altissima, S. gigantea, Aster lanceolatus) with differing genet architectures (rhizome systems), in a common garden and in the field. Our main hypothesis was that defoliation would have fewer negative effects on shoot performance if shoots were connected than if their rhizomes were severed. Since degree of clonal integration is related to differences in genet architecture, our second hypothesis was that the effects of defoliation would be less pronounced in more integrated than in less integrated clones. Removing about 50% of the total leaf area from shoots had different effects depending on plant species, shoot density, and in particular whether rhizome connections between shoots were left intact or severed. In agreement with our prediction, experimentally isolated shoots in the field or in high density clumps in the garden suffered the most from defoliation, while shoots with intact connections or in low density clumps suffered the least. Our second prediction was neither confirmed nor falsified in the present study. Solidago altissima showed overcompensation in response to simulated herbivory in the common garden, i.e. defoliated shoots grew faster and were larger at harvest than their non-defoliated neighbours.     

Dynamics of growth,carbon and nutrient translocation in Zizania latifolia

We studied the seasonal resource dynamics between organs of wild rice (Zizania latifolia (Griseb.) Turcz. ex Stapf.) to obtain a better understanding of its growth dynamics, carbon and nutrient translocation. The results of observation from January 2002 to February 2004 showed the shoot density markedly increased after emergence of shoots at the end of March until May (up to 800 ind/m²). However the shoot mortality due to self-thinning reduced the total new shoots by more than 70% by the end of July. Thereafter, the shoot density was nearly constant with the aboveground biomass peaking at the end of August. In the late winter, the rhizome biomass declined by respiration loss to about 25% of its peak value. Meanwhile the decline in rhizome reserves from January to the end of April was about 20%. This small reduction compared with other perennial emergent species implies that there is a lower contribution of rhizome reserves to support new shoot formation. The initial heterotrophic growth of new shoots based on the rhizome resources lasted for a short period, then switched to autotrophic growth at the end of April or the beginning of May. Thus, in most periods of foliage development, nutrients were obtained mostly from soil through uptake by roots, not through resource allocation of the rhizome. In autumn, the standing dead shoots retained most of the nutrients and carbohydrates without translocating downwards. This suggests that in practice, the plant can remove nutrients from sediment more efficiently than other emergent plants.     

Neighbour influence on the tiller demography of two perennial pampa grasses

Abstract. The aim of this study was to evaluate the possibility to use neighbour species composition to explain demographic variations in seasonal tillers in perennial grasses. Two warm-season species of the Argentina flooded pampa, Paspalum dilatatum and Sporobolus indicus, were used as models. Tiller structure and dynamics of target plants and the identity of their nearest neighbours were analyzed within a natural grassland community for an entire annual growth cycle. Canonical Correlation Analysis of tillering behaviour of target plants and neighbour species composition showed significant relationships for summer neighbourhood data. Community components affecting tiller demography were identified, and their spatial variation was described as different gradients of neighbourhood composition (NCG). NCG varied between target species, determining different spatial variation in neighbouring canopy dynamics and tiller dynamics. The tiller structure and dynamics of P. dilatatum plants were significantly correlated with the seasonal pattern of their neighbouring canopy density (NCD). Plants placed on the NCD-fluctuating extremes along NCG showed an unstable modular structure and short-lived tillers, as compared to those placed in the centre of the gradient. Relative density of the neighbouring canopy showed little variation along the NCG of S. indicus plants, which presented a tiller structure more stable and uniform than P. dilatatum plants did. Identification of NCG may be a promising approach to understanding changes in tiller dynamics of target species in relation to spatial and temporal changes in community structure.