不同光照处理下青藏高原克隆植物黄帚橐吾(Ligulariavirgaurea)种子大小对其幼苗生长的影响

以高寒草甸克隆植物黄帚橐吾为实验材料,通过遮荫网模拟植被遮荫,研究种子大小与萌发及幼苗生长能力的关系和幼苗对光照条件的反应。结果表明：（1）在自然光照下,黄帚橐吾种子大小对种子萌发的影响显著,大种子的萌发率高于小种子。遮荫生境下,大、小种子萌发率有所降低,但遮荫对小种子萌发的影响比大种子显著。小种子的萌发率下降了近1/8,而大种子的萌发率仅下降了1/11。（2）黄帚橐吾种子大小对幼苗生物量积累影响显著,大种子幼苗总生物量（TB）大于小种子幼苗的。但生物量的分配与播种时间相关,播种后60 d,在自然光照条件下,大种子幼苗对根生物量的分配大于小种子幼苗,而对叶生物量的分配则正好相反。在遮荫环境中,大、小种子幼苗普遍对根的生物量分配增加,大种子幼苗根冠比（R/S）大于小种子幼苗。（3）黄帚橐吾种子大小对幼苗的生长也有明显影响。在自然光照下,小种子幼苗的相对生长速率（RGR）较大于大种子幼苗,但叶面积比率（LAR）、叶面积干质量比（SLA）、叶干质量（LWR）差别不明显。在遮荫条件下,幼苗的LAR、SLA、LWR显著增加,但大、小种子幼苗间差异不显著,幼苗的RGR减小,小种子幼苗的减小趋势大于大种子幼苗。     

New seed morphological features in Moehringia L. (Caryophyllaceae) and their taxonomic and ecological significance

Abstract

Relative growth rate (RGR) is a fundamental trait for comparative plant ecology but cannot be measured in situ, leading to problems in interpreting vegetation function. However, the components of RGR (net assimilation rate (NAR), leaf area ratio (LAR), leaf weight ratio (LWR), and specific leaf area (SLA)) can be calculated for wild plants from morphological measurements (leaf area, leaf dry mass, whole plant dry mass), which potentially reflect RGR. Seeds of 19 species from Italian prealpine calcareous grasslands were collected and seedlings were cultivated under controlled conditions. RGR, NAR, LAR, LWR and SLA were analysed. The results demonstrated that RGR was positively correlated with SLA and LAR (p < 0.01). Furthermore, LAR was positively correlated with LWR and negatively with NAR (p < 0.05). Monocotyledons showed significantly higher LAR, LWR and NAR than dicotyledons, as the latter allocated a greater proportion of biomass to stems, but RGR and SLA showed no such phylogenetic constraint. Therefore SLA is the most reliable indicator of RGR in ecological and functional surveys of prealpine calcareous grasslands, and has the additional advantage that it can be measured from leaf material alone. Lower mean RGR and SLA for calcareous grassland species suggests that this vegetation is less likely to recover from the effects of disturbance than meadows and dry meadows.     

Seed germination and seedling performance of two Mediterranean tree species,holm oak (shape Quercus ilex L.) and Aleppo pine (shape Pinus halepensis Mill.): a multifactor experimental approach

A two-level multifactor experimental approach was used to compare seed germination and seedling performance of two Mediterranean tree species: the early successional Aleppo pine (Pinus halepensis Mill.) and the late successional holm oak (Quercus ilex L.). In a first experiment germination rate was evaluated under the combined effects of shade, nitrogen availability, and pine or holm oak leaf litter. In a second experiment we tested for the effects of shade, nutrient availability, and litter type on seedling survival, growth and biomass allocation. Holm oak showed higher germination rates under shaded than under unshaded conditions, while Aleppo pine showed no differences between shaded and unshaded conditions. Nitrogen availability and litter type had no significant effect on germination of either species. Both species showed increased RGR, but also higher mortality rates, when grown in an enriched nutrient environment. While Aleppo pine showed no differences in RGR and mortality rate under different shading levels, RGR decreased and mortality increased for holm oak in full light. Increased radiation decreased LAR, SLA and height:diameter ratio, and increased RWR and R/S in both species, although Aleppo pine showed more pronounced changes. Unlike Aleppo pine, holm oak responded to increased nutrient availability by decreasing R/S and increasing LAR. From these results, no seed-seedling conflicts were found in either species, but a trade-off does seem to exist for holm oak between biomass allocation traits deployed in response to increased nutrient availability and radiation. Aleppo pine outperformed holm oak under most environmental conditions tested and showed a wider regeneration niche.     

Close relationship between leaf life span and seedling relative growth rate in temperate hardwood species

The life span of resource-acquiring organs (leaves, shoots, fine roots) is closely associated with species successional position and environmental resource availability. We examined to what extent leaf life span is related to inter- and intraspecific variation in seedling relative growth rate (RGR). We examined relationships between relative growth rate in mass (RGR_M) or height (RGR_H) and leaf life span, together with classical RGR_M components [net assimilation rate (NAR), specific leaf area (SLA), leaf weight ratio (LWR), and leaf area ratio (LAR)] for seedlings of five hardwood species of different successional position across a wide range of environmental resource availability, including the presence or absence of leaf litter in shaded forest understory, small canopy gaps, and large canopy gaps. Both SLA and LAR were negatively correlated with RGR_M along the environmental gradient for all species. However, positive correlations were observed among species within microsites, indicating that these two components cannot consistently explain the variation in RGR_M. Both NAR and LWR affect interspecific, but not intraspecific, variation in RGR_M. Leaf life span was negatively correlated with either RGR_M or RGR_H in both inter- and intraspecific comparisons. Species with short-lived, physiologically active leaves have high growth rates, particularly in resource-rich environments. Consequently, leaf life span is a good predictor of seedling RGR. Leaf life span affects plant performance and has a strong and consistent effect on tree seedling growth, even among contrasting environments.     

Roles of succession,light, nutrients and disturbance on population vigor and maintenance of the rare plant Solidago shortii (Asteraceae)

Results of field and glasshouse experiments on Solidago shortii, and our observations on this species over many years, were used to construct a conceptual model of the roles of succession, light, soil nutrients and disturbance on population vigor and maintenance of this federal-endangered species. As cover of woody vegetation increased at a population site between 1986 and 1992, number of flowering ramets of S. shortii significantly decreased but number of vegetative ramets remained nearly constant. Adult plants transplanted into a redcedar thicket and those shaded in a glasshouse produced many fewer flowering ramets and capitula per flowering ramet and less biomass and had higher mortality than those in the open. Seedlings/juveniles shaded in a glasshouse had significantly less dry biomass; lower RGR, NAR, leaf area and root/shoot ratio and higher LAR, SLA and LWR than nonshaded ones. In a field site and glasshouse, fertilized plants (NPK) consistently had more flowering ramets and capitula per flowering ramet than nonfertilized ones. Hierarchy of dry weight of plants grown in a glasshouse in soils derived from five types of bedrock was phosphatic limestone > calcareous shale > sandstone > black shale = dolomite. Flowering and biomass production in the field-fertilizer and soil-type experiments were associated closely with levels of P. Number of flowering ramets significantly increased in plants transferred from shaded to nonshaded glasshouse conditions, but no such increase occurred after opening the canopy above plants in a thicket. Both high light and high nutrient levels apparently are necessary to maintain high vigor of S. shortii. In areas subject to invasion by woody plants, periodic high intensity disturbance may be required to prevent population extirpation.     

Plasticity in seedling morphology,biomass allocation and physiology among ten temperate tree species in response to shade is related to shade tolerance and not leaf habit

• Mechanisms of shade tolerance in tree seedlings, and thus growth in shade, may differ by leaf habit and vary with ontogeny following seed germination. To examine early responses of seedlings to shade in relation to morphological, physiological and biomass allocation traits, we compared seedlings of 10 temperate species, varying in their leaf habit (broadleaved versus needle‐leaved) and observed tolerance to shade, when growing in two contrasting light treatments – open (about 20% of full sunlight) and shade (about 5% of full sunlight).
• We analyzed biomass allocation and its response to shade using allometric relationships. We also measured leaf gas exchange rates and leaf N in the two light treatments.
• Compared to the open treatment, shading significantly increased traits typically associated with high relative growth rate (RGR) – leaf area ratio (LAR), specific leaf area (SLA), and allocation of biomass into leaves, and reduced seedling mass and allocation to roots, and net assimilation rate (NAR). Interestingly, RGR was not affected by light treatment, likely because of morphological and physiological adjustments in shaded plants that offset reductions of in situ net assimilation of carbon in shade. Leaf area‐based rates of light‐saturated leaf gas exchange differed among species groups, but not between light treatments, as leaf N concentration increased in concert with increased SLA in shade.
• We found little evidence to support the hypothesis of a increased plasticity of broadleaved species compared to needle‐leaved conifers in response to shade. However, an expectation of higher plasticity in shade‐intolerant species than in shade‐tolerant ones, and in leaf and plant morphology than in biomass allocation was supported across species of contrasting leaf habit.

     

Differences in developmental plasticity and growth rate among drought-resistant and susceptible cultivars of durum wheat (Triticum turgidum L. var. durum)

Understanding how growth and development of durum wheat cultivars respond to drought could provide a basis to develop crop improvement programmes in drought-affected tropical and subtropical countries. A greenhouse experiment was conducted to study the responses of five durum wheat cultivars to moisture stress at different developmental phases. Phenology, total dry matter (TDM), relative growth rate (RGR), leaf area ratio (LAR), net assimilation rate (NAR), leaf weight ratio (LWR), specific leaf area (SLA) and shoot:root ratio were compared. Pre-anthesis moisture stress delayed phenological development, whereas post-anthesis moisture stress accelerated it. TDM accumulation rate was different between drought-resistant and susceptible cultivars. RGR and its components changed with age and moisture availability. Drought-resistant cultivars had a high RGR in favourable periods of the growing season and a low RGR during moisture stress. In contrast, the drought-susceptible cultivar (Po) showed an opposite trend. LAR explained the differences in RGR (r=0.788) best, whereas the relationship between NAR and RGR was not significant. Even though both LWR and SLA were important factors determining the potential growth rate, LWR was of major importance to describe cultivar differences in LAR, and consequently in RGR. The drought-resistant cultivars Omrabi-5 and Boohai showed vigorous root development and/or a low shoot:root ratio. It is concluded that biomass allocation is the major factor explaining variation in RGR among the investigated durum wheat cultivars.     

Ecology of Aglaia mackiana (Meliaceae) Seedlings in a New Guinea Rain Forest1

The large seeds of Aglaia mackiana (Meliaceae) germinate and produce vigorous seedlings under closed canopies or in large gaps. To assess seedling ecology after germination, we measured growth, herbivore damage, and survivorship of seedlings over one year. The sample included shaded seedlings from dispersed seeds, undispersed seeds under parent trees, and seedlings transplanted to gaps. We quantified the light environment using hemispherical canopy photographs taken above seedlings at the beginning and end of the one–year study. Seedlings transplanted to gaps grew faster and had more leaves, larger total leaf surface area, longer secondary roots, and greater root mass than shaded seedlings. Seedlings in gaps did not differ from shaded seedlings in survivorship or amount of herbivore– and pathogen–caused leaf damage. The canopy photographs taken one year apart suggest there is a rough equilibrium in closed canopies with slight changes occurring around an average light level. Sites with < 0.06 ISF (a unitless, relative measure of canopy openness or reflected sunlight) tended to remain the same with minor fluctuations toward brighter or darker. Sites with canopy openness > 0.06 ISF tended to close; few gaps grew larger. Seedlings under parenr trees and seedlings away from parent trees had similar amounts of leaf damage and virtually identical survivorship after 18 months, but seedlings under parent trees had slower growth rates and smaller total leaf surface areas. Dispersal did not strongly benefit seeds via escaping high levels of mortality or comperition around the parent.     

Relative growth rate in phylogenetically related deciduous and evergreen woody species

Relative growth rate (RGR) and other growth parameters were studied in eight pairs of closely related deciduous and evergreen species (within the same genus or family). The main objective of this study was to test the association between leaf turnover rate and RGR, specific leaf area (SLA, leaf area/leaf dry weight) and other growth variables. Plants were grown for 6 months in a greenhouse under favourable water and nutrient conditions. Variation in RGR among the 16 woody species was due mainly to differences in morphological parameters such as leaf area ratio (LAR, whole plant area/whole plant dry weight) and SLA). However, temporal variation in RGR within species was due mainly to variation in net assimilation rate. When phylogeny was not taken into account, analyses showed that deciduous species grew faster than evergreens. In contrast, when phylogeny was taken into account, the data analysis showed that a faster RGR is not consistently associated with the deciduous habit (in five pairs it was, but in the other three it was not). The faster growth of the deciduous trees (in the five positive contrasts) could be explained by their higher LAR and higher SLA relative to evergreens. The lack of differences in RGR between deciduous and evergreens (in three pairs) was due to the higher leaf mass ratio (LMR, leaf dry biomass/total dry biomass) for the evergreens, which offset the higher SLA of the deciduous species, resulting in a similar LAR in both functional groups (LAR=LMR2SLA). Deciduous species had consistently higher SLA than evergreens. We suggest that SLA, more than RGR, could be an important parameter in determining adaptive advantages of deciduous and evergreen species.     

Relative growth rate in relation to physiological and morphological traits for northern hardwood tree seedlings: species,light environment and ontogenetic considerations

The influence of ontogeny, light environment and species on relationships of relative growth rate (RGR) to physiological and morphological traits were examined for first-year northern hardwood tree seedlings. Three Betulaceae species (Betula papyrifera, Betula alleghaniensis and Ostrya virginiana) were grown in high and low light and Quercus rubra and Acer saccharum were grown only in high light. Plant traits were determined at four ages: 41, 62, 83 and 104 days after germination. In high light (610 mol m^–2 s^–1 PPFD), across species and ages, RGR was positively related to the proportion of the plant in leaves (leaf weight ratio, LWR; leaf area ratio, LAR), in situ rates of average canopy net photosynthesis (A) per unit mass (A_mass) and per unit area (A_area), and rates of leaf, stem and root respiration. In low light (127 mol m^–2 s^–1 PPFD), RGR was not correlated with A_mass and A_area whereas RGR was positively correlated with LAR, LWR, and rates of root and stem respiration. RGR was negatively correlated with leaf mass per area in both high and low light. Across light levels, relationships of CO₂ exchange and morphological characteristics with RGR were generally weaker than within light environments. Moreover, relationships were weaker for plant parameters containing a leaf area component (leaf mass per area, LAR and A_area), than those that were solely mass-based (respiration rates, LWR and A_mass). Across light environments, parameters incorporating the proportion of the plant in leaves and rates of photosynthesis explained a greater amount of variation in RGR (e.g. LWR^*A_mass, R²=0.64) than did any single parameter related to whole-plant carbon gain. RGR generally declined with age and mass, which were used as scalars of ontogeny. LWR (and LAR) also declined for seven of the eight species-light treatments and A declined in four of the five species in high light. Decreasing LWR and A with ontogeny may have been partially responsible for decreasing RGR. Declines in RGR were not due to increased respiration resulting from an increase in the proportion of solely respiring tissue (roots and stems). In general, although LWR declined with ontogeny, specific rates of leaf, stem, and root respiration also decreased. The net result was that whole-plant respiration rates per unit leaf mass decreased for all eight treatments. Identifying the major determinants of variation in growth (e.g. LWR^*A_mass) across light environments, species and ontogeny contributes to the establishment of a framework for exploring limits to productivity and the nature of ecological success as measured by growth. The generality of these relationships both across the sources of variation we explored here and across other sources of variation in RGR needs further study.     

Seedling growth strategies in Bauhinia species: comparing lianas and trees

BACKGROUND AND AIMS: Lianas are expected to differ from trees in their growth strategies. As a result these two groups of woody species will have different spatial distributions: lianas are more common in high light environments. This study determines the differences in growth patterns, biomass allocation and leaf traits in five closely related liana and tree species of the genus Bauhinia. METHODS: Seedlings of two light-demanding lianas (Bauhinia tenuiflora and B. claviflora), one shade-tolerant liana (B. aurea), and two light-demanding trees (B. purpurea and B. monandra) were grown in a shadehouse at 25% of full sunlight. A range of physiological, morphological and biomass parameters at the leaf and whole plant level were compared among these five species. KEY RESULTS: The two light-demanding liana species had higher relative growth rate (RGR), allocated more biomass to leaf production [higher leaf mass fraction (LMF) and higher leaf area ratio (LAR)] and stem mass fraction (SMF), and less biomass to the roots [root mass fraction (RMF)] than the two tree species. The shade-tolerant liana had the lowest RGR of all five species, and had a higher RMF, lower SMF and similar LMF than the two light-demanding liana species. The two light-demanding lianas had lower photosynthetic rates per unit area (A(area)) and similar photosynthetic rates per unit mass (A(mass)) than the trees. Across species, RGR was positively related to SLA, but not to LAR and A(area). CONCLUSIONS: It is concluded that the faster growth of light-demanding lianas compared with light-demanding trees is based on morphological parameters (SLA, LMF and LAR), and cannot be attributed to higher photosynthetic rates at the leaf level. The shade-tolerant liana exhibited a slow-growth strategy, compared with the light-demanding species.     

Variation in relative growth rate and its components in the annual Polygonum aviculare in relation to habitat disturbance and seed size

Polygonum aviculare is an annual weedy species showing extensive genetic variation in seed and leaf size and colonizing various types of man-disturbed habitats. A growth analysis was conducted on 12 genotypes representative of three regimes of disturbance of natural habitat (trampling, weeding, and no disturbance in the course of the growing season), grown under productive conditions in order to test whether relative growth rate (RGR) varies at the intraspecific level and, if so, which growth parameters may explain its variation. RGR showed significant genotypic variation (0.355–0.452 g g^-1 day^-1), positively correlated with specific leaf area (SLA) and leaf mass ratio (LMR) and negatively correlated with unit leaf rate per unit leaf area (ULR_A). Thus, the paramount importance of leaf area ratio (LAR=SLA×LMR) in determining growth rate variation between different herbaceous species is confirmed at the intraspecific level in this species. Genotypes originating from trampled habitats had smaller seeds and smaller leaves than genotypes from habitats subject to other disturbance regimes. Additionally, they showed a lower LAR, not entirely compensated for by a higher ULR_A, which resulted in a positive allometric relationship between seed size and RGR. It is hypothesized that their lower SLA, correlated with a higher leaf dry matter content (possibly a consequence of a higher cell wall content per unit leaf area) and their lower LMR have been co-selected with small leaf size as adaptations promoting resistance to trampling stress. It is suggested that variation in cell size and/or gibberellin content might be the mediators of the correlation found between seed size, leaf size and growth parameters within this species.     

The effect of nitrogen nutrition on growth and biomass partitioning of annual plants originating from habitats of different nitrogen availability

Summary The hypothesis was tested that faster growth of nitrophilic plants at high nitrogen (N) nutrition is counterbalanced by faster growth of non-nitrophilic plants at low N-nutrition. Ten annual plant species were used which originated from habitats of different N-availability. The species' preference for N was quantified by the N-number of Ellenberg (1979), a relative measure of nitrophily. The plants were cultivated in a growth cabinet at five levels of ammonium-nitrate supply. At low N-supply, the relative growth rate (RGR) was independent of nitrophily. At high N-supply, RGR tended to be higher in nitrophilic than in non-nitrophilic species. However, the response of RGR to N-supply was strongly and positively correlated with the nitrophily of species. Increasing N-supply enhanced partitioning to leaf weight per total biomass (LWR) and increased plant leaf area per total biomass (LAR). Specific leaf weight (SLW) and LWR were both higher in non-nitrophilic than in nitrophilic species at all levels of N-nutrition. NAR (growth per leaf area or net assimilation rate) increased with nitrophily only under conditions of high N-supply. RGR correlated positively with LAR, irrespective of N-nutrition. Under conditions of high N-supply RGR correlated with SLW negatively and with NAR positively.     

南亚热带森林24种乔木的种子萌发和幼苗生长

以膨胀珍珠岩为基质,在光和暗的条件下,对２４种南亚热带森林乔木的种子萌发和幼苗生长进行了研究。种子的形态和重量与种的演替阶段有关,种子的重量也与不同的种有关。不同种的萌发率差异较大,种子较大的种,萌发率较高。肉质果实的种子,开始萌发的时间较长。光和暗条件对萌发率和萌发速度无明显影响。幼苗高度和种子重量呈正相关,这一相关在光条件下比暗条件下更明显。暗条件明显地引起群落演替早期的树种的茎徒长,限制根的生长,而对演替后期种无显著的影响。幼苗地上部分和根系生物量的分配与种的演替阶段有关,也与不同的科有关。幼苗地上部分和根系生物量的分配也受光暗条件的影响,而不受种子重量的影响。根和叶的生物量分配和种在其群落演替阶段有关。幼苗的生物量与种子重量有显著的正相关。幼苗的相对生长率相差较大,在没有营养供应的生长基质中,以演替中间阶段的种的相对生长率较大。幼苗从种子的物质利用效率与种子重量呈负相关,而与种的演替阶段无明显的相关。     

What determines interspecific variation in relative growth rate of <Emphasis Type="Italic">Eucalyptus</Emphasis> seedlings?

The present study examines relative growth rate (RGR) and its determinants in seedlings of nine Eucalyptus species. Species were selected from mesic (1,800 mm a⁻¹ rainfall) through to semi-arid habitats (300 mm a⁻¹), and thus, notionally vary in “stress” tolerance. Seedlings were grown in a glasshouse during early summer and received between 33 mol and 41 mol PAR m⁻² day⁻¹ . The mean RGR varied among species—from a minimum of 66 mg g⁻¹ day⁻¹ in E. hypochlamydea to a maximum of 106 mg g⁻¹ day⁻¹ in E. delegatensis. RGR was positively related to rainfall at the sites of seed collection. Neither specific leaf area (SLA) nor net assimilation rate was related to rainfall or RGR. While the absence of relationships with SLA and net assimilation rate contrasts with other studies and species, we cannot rule out the effects of sample size (n=9 species) and modest ranges in SLA and RGR. The ratio of leaf mass to total mass (LMR) varied from 0.49±0.07 g g⁻¹ in E. socialis to 0.74±0.04 g g⁻¹ in E. delegatensis and was strongly positively related with rainfall (r ²=0.77). Interspecific differences in RGR were strongly related to LMR (positive relationship, r ²=0.50) and the rate of dry matter production per mol of leaf nitrogen (positive relationship, r ²=0.64). Hence, the slow RGR of low-rainfall species was functionally related to a lower growth rate per mol of leaf nitrogen than high-rainfall species. Furthermore, slow RGR of low-rainfall species was related to greater allocation to roots at the expense of leaves. Increasing allocation to roots versus leaves is likely an adaptation to soil and atmospheric water deficits, but one that comes at the expense of a slow RGR.     

Survival and growth of Virola surinamensis yearlings: Water augmentation in gap and understory

Summary Factors affecting seedling Virola surinamensis (Myristicaceae) survival and growth were investigated on Barro Colorado Island, Panama. Seedlings planted 3 months after germination were monitored in treefall gaps and understory using 2.25 ha irrigated and control plots through the first dry season. During the dry season, irrigated plants in gaps increased total leaf area significantly more than did irrigated plants in the shaded understory. Over the same dry season, control plants in gaps and in the shaded understory lost similar amounts of leaf area. Seedlings in understory were suppressed in stem height and biomass in both irrigated and control plots; these measures were greater in gaps and greatest in irrigated gaps (height). Roots were similar in length in all treatments, but greater in biomass in gaps than understory due to greater proliferation of secondary roots in control and irrigated gaps than in control and irrigated understory. This experiment demonstrates both water and light limitation during the first dry season after germination. V. surinamensis seedlings are capable of survival and modest growth of leaf area in the deep shade of the understory in moist locations; they are severely disadvantaged in shaded understory subject to drought, where most seeds fall and most seedlings establish. The broken canopy of a gap allows shoot and consequently root growth that permits seedlings to survive seasonal drought.     

Intra-specific variation in relative growth rate: impact on competitive ability and performance of Lychnis flos-cuculi in habitats differing in soil fertility

Plant species from unproductive or adverse habitats are often characterized by a low potential relative growth rate (RGR). Although it is generally assumed that this is the result of selection for specific trait combinations that are associated with a low rate of net biomass accumulation, few studies have directly investigated the selective (dis-)advantage of specific growth parameters under a set of different environmental conditions. Aim of the present study was to quantify the impact of inherent differences in growth parameters among phenotypes of a single plant species, Lychnis flos-cuculi, on their performance under different soil nutrient conditions. Growth analysis revealed significant variation in RGR among progeny families from a diallel cross between eight genotypes originating from a single population. Differences in RGR were due to variation in both leaf area ratio (LAR) and in net assimilation rate (NAR). A genetic trade-off was observed between these two components of growth, i.e. progeny families with high investment in leaf area had a lower rate of net biomass accumulation per unit leaf area. The degree of plasticity in RGR to nutrient conditions did not differ among progeny families. Inherent differences in growth parameters among progeny families had a significant impact on their yield in competition with Anthoxanthum odoratum and Taraxacum hollandicum. In nutrient-rich conditions, progeny families with an inherently high leaf weight ratio (LWR) achieved higher yield in competition, but variation in this trait could not explain differences in competitive yield under nutrient-poor conditions. Inherent differences in growth parameters among progeny families were poorly correlated with differences in survival and average rosette biomass (a good predictor of fecundity) among these progeny families sown in four field sites along a natural gradient of soil fertility. In the more productive sites none of the growth parameters was significantly correlated with rosette biomass, but in the least productive site progeny families with an inherently high specific leaf area (SLA) tended to produce smaller rosettes than low-SLA families. These results are consistent with the view that a selective advantage may accrue from either high or low values of individual RGR components, depending on habitat conditions, and that the selective advantage of low trait values in nutrient-poor environments may results in indirect selection for low RGR in these habitats.     

Latitudinal variation in plant size and relative growth rate in Arabidopsis thaliana

Latitude is an important determinant of local environmental conditions that affect plant growth. Forty ecotypes of Arabidopsis thaliana were selected from a wide range of latitudes (from 16°N to 63°N) to investigate genetic variation in plant size and relative growth rate (RGR) along a latitudinal gradient. Plants were grown in a greenhouse for 31 days, during which period three consecutive harvests were performed. Plants from high latitudes tended to have smaller plant size in terms of seed size, cotyledon width, rosette size, number of rosette leaves, size (leaf area) of the largest leaves, total leaf area, and total dry weight per plant than those from low latitudes. The mean (±SE) RGR across ecotypes was 0.229 (±0.0013) day⁻¹. There was, however, significant ecotypic variation, with RGR being negatively correlated with latitude. The two main components of RGR, leaf area ratio (LAR) and unit leaf rate (ULR), were also correlated with latitude: LAR increased with increasing latitude while ULR decreased with increasing latitude. It was also found that RGR tended to be negatively correlated with LAR, specific leaf area (SLA) and specific root length (SRL) but to be positively correlated with mean area per leaf (MAL) and ULR. The variation in RGR among ecotypes was relatively small compared with that in the other traits. RGR may be a conservative trait, whose variation is constrained by the trade-off between its physiological (i.e. ULR) and morphological (i.e. LAR) components. Received: 2 November 1997 / Accepted: 28 February 1998     

Light requirements of Australian tropical vs. cool-temperate rainforest tree species show different relationships with seedling growth and functional traits

Background and Aims

A trade-off between shade tolerance and growth in high light is thought to underlie the temporal dynamics of humid forests. On the other hand, it has been suggested that tree species sorting on temperature gradients involves a trade-off between growth rate and cold resistance. Little is known about how these two major trade-offs interact.

Methods

Seedlings of Australian tropical and cool-temperate rainforest trees were grown in glasshouse environments to compare growth versus shade-tolerance trade-offs in these two assemblages. Biomass distribution, photosynthetic capacity and vessel diameters were measured in order to examine the functional correlates of species differences in light requirements and growth rate. Species light requirements were assessed by field estimation of the light compensation point for stem growth.

Results

Light-demanding and shade-tolerant tropical species differed markedly in relative growth rates (RGR), but this trend was less evident among temperate species. This pattern was paralleled by biomass distribution data: specific leaf area (SLA) and leaf area ratio (LAR) of tropical species were significantly positively correlated with compensation points, but not those of cool-temperate species. The relatively slow growth and small SLA and LAR of Tasmanian light-demanders were associated with narrow vessels and low potential sapwood conductivity.

Conclusions

The conservative xylem traits, small LAR and modest RGR of Tasmanian light-demanders are consistent with selection for resistance to freeze–thaw embolism, at the expense of growth rate. Whereas competition for light favours rapid growth in light-demanding trees native to environments with warm, frost-free growing seasons, frost resistance may be an equally important determinant of the fitness of light-demanders in cool-temperate rainforest, as seedlings establishing in large openings are exposed to sub-zero temperatures that can occur throughout most of the year.     

Relative growth rate and biomass allocation in ten woody species with different leaf longevity using phylogenetic independent contrasts (PICs)

In this study, we compare the relative growth rate (RGR) and biomass allocation of 10 woody species (5 deciduous and 5 evergreen) from the Mediterranean region using phylogenetic independent contrasts (PICs) to test if these two functional groups differ in these traits. In general, the results were similar when using PICs or without taking into account phylogenetic relations. Deciduous species had a higher RGR than evergreen species, due to the higher net assimilation rate (NAR). Deciduous species had a higher specific leaf area (SLA) but a lower leaf mass ratio (LMR), resulting in a similar LAR for deciduous and evergreen species (LAR = SLA x LMR). In some cases, the use of PICs revealed patterns that would not have appeared if phylogeny had been overlooked. For example, there was no significant correlation between RGR and final dry mass (after 4 months of growth) but PICs revealed that there was a positive relation between these two variables in all deciduous-evergreen pairs. In general, RGR decreased with time and this temporal variation was due primarily to NAR variations (r = 0.79, p < 0.01), and also to variations in LAR (r = 0.69, p < 0.05). Considering the phylogeny, the only variable constantly different for all deciduous-evergreen pairs was SLA. This result, and the fact that SLA was the best correlated variable with RGR (r = 0.81, p < 0.01), reinforce the value of SLA as a variable closely associated to growth and to the functional groups (deciduous vs. evergreen).