Effect of simulated macroalgae on the fish assemblage associated with a temperate reef system

Increased habitat complexity is supposed to promote increased diversity, abundance and biomass. This study tested the effect of the macroalgal cover on temperate reef fishes by mimicking macroalgae on artificial reefs in NW Sicily (Mediterranean Sea). Macroalgal cover affected reef fishes in different ways and independently of intrinsic temporal trends. The fish assemblages of manipulated and control artificial reef units differed in the relative abundances of the associated species, but little in species composition. In line with studies in seagrass habitats, fishes were most abundant in reefs covered by artificial macroalgae. Three species (Boops boops, Serranus scriba and Symphodus ocellatus) exhibited consistently greater abundance on vegetated reef units than on control reef units. The total number of species and the abundance of three particular species (S. scriba, S. ocellatus and Thalassoma pavo) displayed temporal trends which were independent on short and large temporal scales. Only fish total biomass and one species (Spicara flexuosa) displayed strong effects of interaction among the experimental factors. Mechanisms to explain these findings are discussed from observational evidence on habitat use and interactions among multiple species. This study highlights that manipulative experiments involving repeated sampling of fish in artificial habitats appear to be a valid approach to study fish-habitat relationships in fluctuating environments. It is also concluded that macroalgae mimics may serve as a tool for restoring lost marine vegetated habitats when current human-induced conditions prevent the recovery of pristine macroalgal stands.     

Establishment of grassland species in monocultures: different strategies lead to success

The establishment pattern of monocultures of 61 species common to Central European semi-natural grasslands was analysed in a field experiment. The objectives were to identify key traits for successful establishment, defined in terms of above-ground biomass production, and to characterize the degree of niche overlap with respect to the use of above-ground resources, such as light and space. Four months after sowing, 15 species reached an above-ground biomass of more than 400 g m⁻². Highly productive monocultures adopted extremely different strategies of space filling in terms of canopy height, biomass density and centre of gravity of vertical biomass distribution. Regression tree analysis identified (1) the number of seedlings and (2) a trade-off between the development of a large number of small-sized shoots of species with intensive clonal growth in contrast to the establishment of fewer large-sized shoots as the two most important traits for successful establishment. Further variables associated with high above-ground biomass production by individual species were traits known to be relevant to the relative growth rate of herbaceous species, such as specific leaf area, leaf nitrogen or allocation between shoots and roots. The principle finding of this study is that the success of the 15 most productive species was not based on a single pathway but on a variable combination of traits. There are clearly many possible combinations of morphological and physiological features that will result in a species becoming productive, and these combinations differ among species in a local species pool.     

Juvenile/total foliage ratios inEucalyptus nitens and the growth of stands and individual trees

Summary Individual trees and stands of two provenances of Eucalyptus nitens which have marked differences in retention of juvenile foliage were studied in four plantations at different elevations. The proportion of juvenile to total foliage and growth was measured at the end of the 2nd, 3rd and 4th year from establishment. Between the ages of 2 and 4 years annual stem volume increment increased in proportion to the amount of juvenile foliage retained. By age 4 years, stem volume of trees of the juvenile persistent form was significantly larger than that of the early adult form. Increasing differences in height growth with age between provenances, which were highly significant across sites by age 4, contributed to these differences in performance. There was some evidence that the largest trees of the juvenile-persistent form were those which combined mature foliage above juvenile foliage for the longest period during the transition from juvenile to mature foliage. In the early-adult form the largest trees were those which completed the transition to mature foliage rapidly. There was no difference in the ratio of foliage mass to basal area between the two forms. It is suggested that the faster growth of the juvenile-persistent form is related to higher leaf area index and not to foliage type. A provenance of E. globulus which had a higher retention of juvenile foliage at age 4 than a second provenance had a lower stem volume, thus indicating that in this species early growth rate is not determined by foliage type.     

Phenotyping approaches for physiological breeding and gene discovery in wheat

Conceptual models of drought‐adaptive traits have been used in breeding to accumulate complementary physiological traits (PT) in selected progeny, resulting in distribution of advanced lines to rain‐fed environments worldwide by the International Maize and Wheat Improvement Center (CIMMYT). Key steps in PT breeding at CIMMYT include characterisation of crossing block lines for stress adaptive mechanisms, strategic crossing among parents that encompass as many target traits as possible and early generation selection (EGS) of bulks for canopy temperature (CT). The approach has been successful using both elite × elite crosses as well as three way crosses involving stress adapted landraces. Other EGS techniques that are amenable to high throughput include measurement of spectral reflectance indices and stomatal aperture‐related traits. Their genetic‐ and cost‐effectiveness are supported by realisation of genetic yield gains in response to trait selection, and by economic analysis, respectively. Continual reselection within restricted gene pools is likely to lead to diminishing returns, however, exotic parents can be used to introduce new allelic diversity. Examples include landraces from the primary gene pool, and products of inter‐specific hybridisation with the secondary gene pool consisting of closely related wheat genomes. Both approaches have been successful in introducing stress‐adaptive traits. The main problem with knowing which genetic resource to use in wide‐crossing is the uncertainty with which phenotypic expression can be extrapolated from one genome/genepool to another because of their unimproved or undomesticated genetic backgrounds. Nonetheless, their PT expression can be measured and used as a basis for investing in crossing or wide crossing. Discovering the genetic basis of PT is highly complex because putative QTLs may interact with environment and genetic background, including genes of major effect. Detection of QTLs was improved in mapping populations where flowering time was controlled, while new mapping populations have been designed by screening potential parents that do not contrast in the Rht, Ppd and Vrn alleles. Association genetics mapping is another approach that can be employed for gene discovery using exclusively agronomically improved material, thereby minimising the probability of identifying yield QTLs whose alleles have been already improved by conventional breeding.     

Between-forest variation in vertical stratification of drosophilid populations

Abstract 1. The spatial distribution pattern of forest‐dwelling drosophilid flies was compared among species and among four forests with markedly different vertical foliage structures, including secondary and primary forests, with special reference to vertical stratification. 2. All 20 drosophilid species analysed showed vertically stratified distribution patterns, which were detected statistically in at least one forest site during the 2‐year study period. As a whole, the vertical distribution patterns were stable, with or without stratification, over the 2 years. 3. The ratio of species with stratified distribution to the total number of species changed in a similar trend with the vertical foliage complexity of forests, both showing a large gap between secondary and primary forests, even for species common to all four forests. 4. Many species, most of which were tree‐sap feeders, showed highly predictable patterns of vertical stratification across forests and years. This was associated strongly with regular preference for microhabitat, mostly for the canopy, suggesting that the unvarying nature of canopy environments in any forest is very important in producing, maintaining, and promoting the vertical stratification in organism abundance, and contributes to the ubiquity of the pattern. 5. Some species with a wider range of food resources than other species, most of which showed no clear preference for the canopy, were characterised by large between‐forest differences in vertical distribution patterns and tended to show non‐stratified patterns in secondary forests, contributing to the lower ratio of species with stratified distribution in the two secondary forests than in the primary forests. 6. The role of forest structure in organising the environment is discussed; it is suggested that vertical complexity of foliage structure affects the prevalence of vertical stratification in animal communities indirectly through the vertical heterogeneity of environmental conditions.     

Analysis of stand density effects on canopy structure: a conceptual approach

Summary A few assumptions were used to generate a series of specific, quantitative predictions for the relationships between stand density and various dimensional measures of canopy structure. The predictions, each indicating an increase in mean crown size as density decreased, appeared to be reasonable and intuitive. Predictions were compared to data for two conifer species with different crown forms, Pinus contorta var. latifolia and Abies lasiocarpa. Results of these comparisons were mixed — the linear, directly measured dimensions were consistent with predicted relationships, but dimensions calculated from the linear measures were not. Re-examination of the original assumptions indicated that the model should account for crown shyness (engagement/disengagement) to adequately reflect the influence of stand density on canopy structure. The results also indicated a strong association between stand height and measures of mean crown size. Mean crown size of lodgepole pine was altered much more by density than was mean crown size of subalpine fir, due primarily to the different relative shade tolerances of the two species. Some of the observed differences between species may also reflect the range of densities examined and uneven spacing in the unmanaged natural stands.     

Contrasting leaf chemical traits in tropical lianas and trees: implications for future forest composition

Lianas are an important growthform in tropical forests, and liana abundance and biomass may be increasing in some regions. Explanations for liana proliferation hinge upon physiological responses to changing resource conditions that would favour them over trees. Testing a chemical basis for such responses, we assessed 22 foliar traits in 778 lianas and 6496 trees at 48 tropical forest sites. Growthform differences in chemical allocation occurred on a leaf mass and area basis. Light capture-growth and maintenance-metabolism chemicals averaged 14.5 and 16.7% higher mass-based concentration in lianas than in trees globally, whereas structure and defence chemicals averaged 9.0% lower in lianas. Relative differences in chemical allocation by lianas and trees were mediated by climate with peak differences at about 2500 mm year(-1) and 25 °C. Differences in chemical traits suggest that liana expansion could be greatest in forests undergoing increased canopy-level irradiance via disturbance and climate change.     

Plant competition for light analyzed with a multispecies canopy model

Summary A multispecies canopy photosynthesis simulation model was used to examine the importance of canopy structure in influencing light interception and carbon gain in mixed and pure stands of wheat (Triticum aestivum L.) and wild oat (Avena fatua L.), a common weedy competitor of wheat. In the mixtures, the fraction of the simulated canopy photosynthesis contributed by wheat was found to decline during the growing season and this decline was closely related to reductions in the amount of leaf area in upper canopy layers. For both species in mixture and in monoculture, simulated photosynthesis was greatest in the middle or upper-middle canopy layers and sensitivity analyses revealed that canopy photosynthesis was most sensitive to changes in leaf area and leaf inclination in these layers. Changes in LAI and leaf inclination affected canopy carbon gain differently for mixtures and monocultures, but the responses were not the same for the two species. Results from simulations where the structural characteristics of the two species were substituted indicated that species differences in leaf inclination, sheath area and the fraction of leaf area alive were of minor consequence compared with the differences in total leaf area in influencing relative canopy carbon gain in mixtures. Competition for light in these species mixtures appears to be influenced most by differences in the positioning of leaf area in upper canopy layers which determines, to a great extent, the amount of light intercepted.     

阔叶红松林林冠斑块特征及其与林木更新的关系

调查了长白山北坡海拔740m处阔叶红松林主要树种林冠斑块的特征,及其对林分结构与更新的影响,结果表明,林冠斑块主要由红松,椴树,色木,蒙古栎4个树种组成,其林冠斑块覆盖面积占林地面积的57．1％,占样地内林冠斑块总面积的87％,其中以红松和紫椴林冠为主,小林冠斑块数量多,分布广,大斑块则很少,林冠斑块面积基本在100m^2以上,只有个别的斑块超过100m^2,林冠斑块形状变异较大,呈现不规则的多边形,林冠树种对幼龄个体的分布存在一定的影响,色木槭幼龄个体几乎分布在所有的紫椴和红松林冠斑块内,最大密度出现32m^2的紫椴林冠斑块和28m^2的红松林冠斑块内,紫椴的幼龄个体只集中在少数林冠斑块内。     

柞蚕林冠层结构,林内光照分布与叶生物量研究

柞蚕林是以养蚕为目的的萌生栎林,林冠经剪伐而形成不同的冠型结构。本文对3种冠型的叶面积系数,冠层内叶量分布及消光系数等冠层结构特点进行了测定。结果表明,不同冠形的冠层结构特点明显影响林内光照环境及叶生物量。现有的3种冠型中,“阶梯”型结构有较高的叶面积系数和较合理的叶量分布,因而具有较高的叶生物量。“中干”型叶生物量与无干型虽无明显差异,但“中干”型消光系数较低,林内空间较大,林内光照环境比“无干”型结构优越。     

Plant interspecies competition for sunlight: a mathematical model of canopy partitioning

We examine the influence of canopy partitioning on the outcome of competition between two plant species that interact only by mutually shading each other. This analysis is based on a Kolmogorov-type canopy partitioning model for plant species with clonal growth form and fixed vertical leaf profiles (Vance and Nevai in J. Theor. Biol., 2007, to appear). We show that canopy partitioning is necessary for the stable coexistence of the two competing plant species. We also use implicit methods to show that, under certain conditions, the species’ nullclines can intersect at most once. We use nullcline endpoint analysis to show that when the nullclines do intersect, and in such a way that they cross, then the resulting equilibrium point is always stable. We also construct surfaces that divide parameter space into regions within which the various outcomes of competition occur, and then study parameter dependence in the locations of these surfaces. The analysis presented here and in a companion paper (Nevai and Vance, The role of leaf height in plant competition for sunlight: analysis of a canopy partitioning model, in review) together shows that canopy partitioning is both necessary and, under appropriate parameter values, sufficient for the stable coexistence of two hypothetical plant species whose structure and growth are described by our model. A. L. Nevai was supported in part by the National Institutes of Health, National Research Service Award (T32-GM008185) from the National Institute of General Medical Sciences (NIGMS).     

杂交粳稻超高产群体的冠层结构特点研究

研究了杂交粳稻超高产群体的苗穗粒结构及干物重在冠层不同层次、不同器官的分布,叶面积配置及光强分布等冠层结构特点．结果表明,杂交粳稻超高产群体冠层总干物重及40cm以下、40～60cm、60～80cm和80cm以上4个层次的干物重分别比常规粳稻高32．29％及29．12％、13．95％、16．45％和100．17％．杂交粳稻叶片(同化器官)与穗(库器官)干重分别占总干物重的248％与12．8％．高于常规稻;而叶鞘与茎(贮藏器官)的干重分别占总干物重的33．6％和28．9％,低于常规粳稻．杂交粳稻超高产群体冠层叶面积配置比较合理,齐穗期40cm以上冠层LAI达5．44．冠层光强上下分布比较均匀,60cm以下冠层光强比常规稻高13．1％～37．0％,而60cm以上冠层光强比常规稻低5．9％～12．2％;20cm、20～40cm、40～60cm和60～80cm各层消光系数分别比常规粳稻低35．1％、13．5％、29．1％和17．2％．     

Germination and early establishment of lower salt-marsh species in grazed and mown salt marsh

Abstract. Lower salt-marsh species occur more in the grazed than in the mown sites of a salt marsh in Schiermonnikoog in the Netherlands. This was demonstrated by a sowing experiment which determined which characteristic of the stand structure, height of the canopy or percentage of bare soil, is responsible for this difference. The total number of seedlings which emerged was negatively related to the height of the canopy and positively to the percentage of bare soil. The survival of seedlings of Suaeda maritima and Plantago maritima could be explained by the height of the canopy and for the latter species also by the percentage of bare soil. The survival of Aster tripolium and Plantago maritima was higher in grazed than in mown sites. Since the amount of bare soil was higher than seemed necessary for germination and survival, it was concluded that the taller canopy was responsible for the absence of lower salt-marsh species in the mown sites.     

Coordination theory of leaf nitrogen distribution in a canopy

It has long been observed that leaf nitrogen concentrations decline with depth in closed canopies in a number of plant communities. This phenomenon is generally believed to be related to a changing radiation environment and it has been suggested by some researchers that plants allocate nitrogen in order to optimize total whole canopy photosynthesis. Although optimization theory has been successfully utilized to describe a variety of physiological and ecological phenomena, it has some shortcomings that are subject to criticism (e.g., time constraints, oversimplifications, lack of insights, etc.). In this paper we present an alternative to the optimization theory of plant canopy nitrogen distribution, which we term coordination theory. We hypothesize that plants allocate nitrogen to maintain a balance between two processes, each of which is dependent on leaf nitrogen content and each of which potentially limits photosynthesis. These two processes are defined as W_c, the Rubiscolimited rate of carboxylation, and W_j, the electron transport-limited rate of carboxylation. We suggest that plants allocate nitrogen differentially to, leaves in different canopy layers in such a way that W_c and W_j remain roughly balanced. In this scheme, the driving force for the allocation of nitrogen within a canopy is the difference between the leaf nitrogen content that is required to bring W_c and W_j into balance and the current nitrogen content. We show that the daily carbon assimilation of a canopy with a nitrogen distribution resulting from this internal coordination of W_c and W_j is very similar to that obtained using optimization theory.     

The relation between above- and belowground biomass allocation patterns and competitive ability

Summary In a 2-year experiment, the evergreen shrubsErica tetralix andCalluna vulgaris (dominant on nutrient-poor heathland soils) and the perennial deciduous grassMolinia caerulea (dominant on nutrient-rich heathland soils) were grown in replacement series in a factorial combination of four competition types (no competition, only aboveground competition, only belowground competition, full competition) and two levels of nutrient supply (no nutrients and 10 g N+2 g P+10 g K m⁻² yr⁻¹). Both in the unfertilized and in the fertilized treatmentsMolinia allocated about twice as much biomass to its root system than didErica andCalluna. In all three species the relative amount of biomass allocated to the roots was lower at high than at low nutrient supply. The relative decrease was larger forMolinia than forErica andCalluna. In the fertilized monocultures biomass of all three species exceeded that in the unfertilized series.Molinia showed the greatest biomass increase. In the unfertilized series no effects of interspecific competition on the biomass of each species were observed in either of the competition treatments. In the fertilized mixtures where only belowground competition was possibleMolinia increased its biomass at the expense of bothErica andCalluna. When only aboveground competition was possible no effects of interspecific competition on the biomass of the competing species were observed. However, in contrast with the evergreens,Molinia responded by positioning its leaf layers relatively higher in the canopy. The effects of full competition were similar to those of only belowground competition, so in the fertilized series belowground competition determined the outcome of competition. The high competitive ability ofMolinia at high nutrient supply can be attributed to the combination of (1) a high potential productivity, (2) a high percentage biomass allocation to the roots, (3) an extensive root system exploiting a large soil volume, and (4) plasticity in the spatial arrangement of leaf layers over its tall canopy. In the species under study the allocation patterns entailed no apparent trade-off between the abilities to compete for above- and belowground resources. This study suggests that this trade-off can be overcome by: (1) plasticity in the spatial arrangement of leaf layers and roots, and (2) compensatory phenotypic and species-specific differences in specific leaf area and specific root length.