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.     

Common leaf life span of co-dominant species in a continuously grazed temperate pasture

Short leaf life span is thought to be associated with low construction costs and consequently short payback times. Frequent defoliation could decrease the available payback time and force individuals to decrease construction costs to maintain a positive carbon balance. Therefore, we hypothesize that dominant species of frequently grazed pastures show similar and short leaf life span. Leaf life span was measured on four co-dominant species, two grasses and two dicots, in a pasture that was intensively grazed by cattle for six years. Leaf life span was analysed in terms of its components: the phyllotherm and the number of live leaves. Phyllotherm and number of live leaves were determined in eight to 20 individuals of Lolium perenne, Poa pratensis, Taraxacum officinale and Trifolium repens, during autumn 2006 and autumn and spring 2007. Leaf life span was estimated as the product of phyllotherm and number of live leaves. After critically assessing five operational definitions of leaf death, a leaf was considered alive during the time elapsed between its appearance and the senescence of 25% of its area. In general, leaf life span did not differ between species nor was there clear evidence for seasonal effects. The leaf life span ranged between 400 and 520 growing degree days (base temperature: 4 °C). The similarity of leaf life span between species was based on different mechanisms: dicots had shorter phyllotherms and more live leaves than grasses. Observed leaf life spans were short when compared with data from plants growing in less disturbed habitats, but similar to the dominant species of grazed grasslands with a similar disturbance regime. The similarity of leaf life spans resulted in a common community-level leaf life span. It is suggested that this trait could control community-scale biogeochemical features, such as the residence time of carbon in the aboveground structural biomass of this leafy ecosystem.     

Geography of Pinus elliottii Engelm. and Pinus palustris Mill. leaf life-spans in the southeastern U.S.A.

Aim Past studies have investigated differences in leaf life‐spans between deciduous and evergreen species. Environmental controls such as light, temperature, and nutrient and moisture availability explain differences in leaf life‐spans between species. This study examined intraspecific leaf life‐spans across climate and nutrient gradients within the geographical range of Pinus palustris Engelm (longleaf pine) and Pinus elliottii Mill. (slash pine). Location Five study areas in the southeastern United States were selected along the north–south geographical range of Pinus elliottii and Pinus palustris. Methods Leaf life‐span was calculated based on stand inventories and annual litterfall totals for each site, and allometric relationships between d.b.h. and foliar biomass. Results Leaf life‐span of P. elliottii ranged from 1.28 to 1.95 years between sites. Leaf life‐span of P.palustris varied by nearly a factor of 5 between the study site with the lowest and highest value (0.58–2.49 years). anova indicated that leaf life‐spans of P. elliottii were not significantly different among sites. In contrast, anova indicated a significant difference for P. palustris leaf life‐spans among sites (P < 0.05). The Tukey multiple comparisons tests showed that 2 study areas were the only pair of P. palustris sites with a significant difference in leaf life‐spans. Main conclusions The geographical variation in leaf life‐spans between two species illustrates the different phenotypic responses to environmental controls. The variation in leaf life‐spans by individuals of P. palustris across a geographical range illustrated in this study suggests that P. palustris may exhibit a greater phenotypic plasticity than P. elliottii.     

Leaf traits and leaf life spans of two xeric-adapted palmettos

Plants of nutrient-poor, arid environments often have leaf traits that include small size, sclerophylly, long life span, low nutrient concentration, and low photosynthetic rate. Hence, the success of two large-leaved palmettos in peninsular Florida's seasonally xeric, nutrient-impoverished uplands seems anomalous, given that their leaves are orders of magnitude larger than the leaves of sympatric species. An examination of a 16-yr data set of leaf traits and leaf life spans across four vegetative associations differing in available light showed that Serenoa repens and Sabal etonia had low rates of leaf production coupled with long leaf life spans reaching 3.5 yr in heavily shaded plants. The adaptation of these palmettos to xeric, nutrient-poor habitats has generated dwarf statures, diminished leaf sizes and numbers, increased leaf life spans, and reduced rates of leaf production relative to other palms and congeners of more mesic sites. Leaf and petiole size, plant leaf canopy area, and leaf life span increased in both palmettos with decreasing available light, helping to compensate for reduced photosynthetic rates under shaded conditions and for the high leaf construction costs of the large, thick palmetto leaves. Large leaf size in these palmettos, likely due to phylogenetic conservatism, is compensated by other leaf traits (e.g., heavily cutinized epidermises, thick laminas) that increase survival in seasonally xeric, nutrient-impoverished environments.     

Performance Trade-offs Driven by Morphological Plasticity Contribute to Habitat Specialization of Bornean Tree Species

Growth-survival trade-offs play an important role in niche differentiation of tropical tree species in relation to light-gradient partitioning. However, the mechanisms that determine differential species performance in response to light and soil resource availability are poorly understood. To examine responses to light and soil nutrient availability, we grew seedlings of five tropical tree species for 12 mo at < 2 and 18 percent full sunlight and in two soil types representing natural contrasts in nutrient availability within a lowland dipterocarp forest in North Borneo. We chose two specialists of nutrient-rich and nutrient-poor soils, respectively, and one habitat generalist. Across all species, growth was higher in high than low light and on more nutrient rich soil. Although species differed in growth rates, the ranking of species, in terms of growth, was consistent across the four treatments. Nutrient-rich soils improved seedling survival and increased growth of three species even under low light. Slower-growing species increased root allocation and reduced specific leaf area (SLA) and leaf area ratio (LAR) in response to decreased nutrient supply. All species increased LAR in response to low light. Maximum growth rates were negatively correlated with survival in the most resource-limited environment. Nutrient-poor soil specialists had low maximum growth rates but high survival at low resource availability. Specialists of nutrient-rich soils, plus the habitat generalist, had the opposite suite of traits. Fitness component trade-offs may be driven by both light and belowground resource availability. These trade-offs contribute to differentiation of tropical tree species among habitats defined by edaphic variation.     

Interspecific Variation in RGR and the Underlying Traits among 24 Grass Species Grown in Full Daylight

Abstract: A growth analysis was conducted with 24 central European grass species in full daylight to test whether traits underlying interspecific variation in relative growth rate (RGR) are the same in full daylight as they are at lower light, and whether this depends on the ecological characteristics of the studied species, i.e., their requirements with respect to nutrient and light availability.
In contrast to studies with herbaceous species at lower light, net assimilation rate (NAR) contributed more than leaf area ratio (LAR) or specific leaf area (SLA) to interspecific variation in RGR. This was associated with a larger interspecific variation in NAR than found in experiments with lower light. Without the two most shade-tolerant species, however, the contribution of LAR and its components to interspecific variation in RGR was similar or even higher than that of NAR.
Leaf dry matter content correlated negatively with RGR and was the only component of LAR contributing in a similar manner to variation in LAR and RGR. There was a positive correlation between NAR and biomass allocation to roots, which may be a result of nutrient-limited growth. RGR correlated negatively with biomass allocation to leaves. Leaf thickness did not correlate with RGR, as the positive effect of thin leaves was counterbalanced by their lower NAR.
Low inherent RGR was associated with species from nutrient-poor or shady habitats. Different components constrained growth for these two groups of species, those from nutrient-poor habitats having high leaf dry matter content, while those from shady habitats had thin leaves with low NAR.     

Infidelity of leafcutting ants to host plants: resource heterogeneity or defense induction?

Summary Leafcutting ants have strong among- and within-plant preferences, and generally abandon plants long before they are completely defoliated. Two tropical deciduous forest tree species preferred by the leafcutting ant Atta colombica were studied to determine how variation in resource quality affects ant selectivity and partial defoliation of plants. Significant differences in palatability and leaf characteristics of Spondias mombin and Bursera simaruba were found among trees and among leaf types within trees, but not among branches within trees. No short-term responses to experimental defoliation of up to 50% of total canopy were found in either species. Leaf nutrient and poisture content were positively correlated, and phenolic content negatively correlated, with the palatability of Spondias mombin, a species containing hydrolyzable tannins. Leaf moisture and phenolic content were both positively correlated with the palatability of Bursera simaruba, which contains predominantly condensed tannins. The results suggest that variation in leaf quality among and within plants is at least a partial explanation for ant selectivity and partial defoliation of preferred species. There is no evidence that rapidly induced changes in plant chemistry affect ant decisions to abandon these plants. Instead, it appears likely that ants abandon plants once high-quality leaf patches are exhausted. Quantitative variation in leaf nutrients, moisture, and secondary chemicals all appear to contribute to ant preferences for individuals and tissues of highly palatable plants.     

Blade life span,structural investment,and nutrient allocation in giant kelp

The turnover of plant biomass largely determines the amount of energy flowing through an ecosystem and understanding the processes that regulate turnover has been of interest to ecologists for decades. Leaf life span theory has proven useful in explaining patterns of leaf turnover in relation to resource availability, but the predictions of this theory have not been tested for macroalgae. We measured blade life span, size, thickness, nitrogen content, pigment content, and maximum photosynthetic rate (P _max) in the giant kelp (Macrocystis pyrifera) along a strong resource (light) gradient to test whether the predictions of leaf life span theory applied to this alga. We found that shorter blade life spans and larger blade areas were associated with increased light availability. In addition, nitrogen and P _max decreased with blade age, and their decrease was greater in shorter lived blades. These observations are generally consistent with patterns observed for higher plants and the prevailing theory of leaf life span. By contrast, variation observed in pigments of giant kelp was inconsistent with that predicted by leaf life span theory, as blades growing in the most heavily shaded portion of the forest had the lowest chlorophyll content. This result may reflect the dual role of macroalgal blades in carbon fixation and nutrient absorption and the ability of giant kelp to modify blade physiology to optimize the acquisition of light and nutrients. Thus, the marine environment may place demands on resource acquisition and allocation that have not been previously considered with respect to leaf life span optimization.     

Leaf life spans in wild perennial herbaceous plants: a survey and attempts at a functional interpretation

Summary Leaf longevity in 29 herbaceous plant species of Central Europe was studied by inspecting tagged leaves at weekly intervals. About half of the species are elements of the lowland meadow flora, the other half comprises a representative sample of species from the highest sites where vascular plants grow in the Alps. Shaded and water-stressed sites were avoided. Overall mean leaf longevity did not differ significantly between sites and amounted to 71±5 days at low and 68±4 days at high altitude. Leaf life spans ranged (with no clear altitudinal trend) from 41 to 95 days. Low-altitude forbs and grasses produced several leaf cohorts during their growth period, while most alpine species produced only one. Correlations were found between leaf duration and percent nitrogen content and carbon-cost/carbon-gain ratios, but not with leaf dry mass per unit leaf area and photosynthetic capacity alone. As leaf life spans increase, more C tends to be invested per unit CO₂ uptake and less N is invested per unit invested C. Thus, mass relationships rather than area relationships seem to be linked to leaf life span in these species, suggesting that leaf duration is associated with properties other than the efficiency of light utilization (e.g. mechanical strength, herbivory or pathogen resistance). It seems that the explanations of leaf duration that have been developed for evergreen/deciduous plants and for plants along steep light gradients do not apply to the variable life spans in leaves of perennial herbaceous plants of open habitats.     

Life history variation in Arabidopsis lyrata across its range: effects of climate,population size and herbivory

For plants with wide distributional areas, covering a wide range of ecologically distinct habitats, evolutionary divergence can lead to substantial phenotypic variation across a species’ range. These intraspecific trait differences can be very informative about the nature of the selective environment as they potentially reflect different environmental selection pressures while controlling for other species characteristics. In this study, multiple regression and structural equation models were used to examine the relative importance of environmental, ecological, population size and population density effects for variation in growth, reproduction and leaf morphology among 36 populations of the perennial plant Arabidopsis lyrata ssp. petraea across its northwest European range. Substantial variation in temperature, soil nutrient levels and herbivory was observed across the species’ range. In addition, large differences in flowering percentage and individual seed production were found. Leaf morphology varied considerably, with a substantial amount of variation in specific leaf area and trichome density among populations. Structural equation modeling suggested that this species is sensitive to small population sizes, eutrophication and herbivory. Reproductive output was negatively related to herbivory. In addition population size was negatively associated with soil nutrient concentrations. Leaf morphology was shown to be mainly associated with temperature and herbivory. Lower specific leaf areas and lower trichome densities were related to colder areas and high trichomes densities were related to high levels of herbivory. These model results are consistent with the interpretation that, in addition to changing environmental effects across its range, ecological effects such as herbivory contribute to the large variation in life history and morphology of this species. The results reveal a strong negative effect of herbivory on the reproductive output of this species, not only via direct effects of herbivory on flowers and seeds, but also indirectly via a shift in life history strategy.     

Biomass allocation and leaf chemical defence in defoliated seedlings of Quercus serrata with respect to carbon-nitrogen balance

BACKGROUND AND AIMS: Both nutrient availability and defoliation affect the carbon-nutrient balance in plants, which in turn influences biomass allocation (e.g. shoot-to-root ratio) and leaf chemical composition (concentration of nitrogen and secondary compounds). In this study it is questioned whether defoliation alters biomass allocation and chemical defence in a similar fashion to the response to nutrient deficiency. METHODS: Current-year seedlings of Quercus serrata were grown with or without removal of all leaves at three levels of nutrient availability. KEY RESULTS: Plant nitrogen concentration (PNC), a measure of the carbon-nutrient balance in the plant, significantly decreased immediately after defoliation because leaves had higher nitrogen concentrations than stems and roots. However, PNC recovered to levels similar to or higher than that of control plants in 3 or 6 weeks after the defoliation. Nitrogen concentration of leaves produced after defoliation was significantly higher than leaf nitrogen concentration of control leaves. Leaf mass per plant mass (leaf mass ratio, LMR) was positively correlated with PNC but the relationship was significantly different between defoliated and control plants. When compared at the same PNC, defoliated plants had a lower LMR. However, the ratio of the leaf to root tissues that were newly produced after defoliation as a function of PNC did not differ between defoliated and control plants. Defoliated plants had a significantly lower concentration of total phenolics and condensed tannins. Across defoliated and control plants, the leaf tannin concentration was negatively correlated with the leaf nitrogen concentration, suggesting that the amount of carbon-based defensive compounds was controlled by the carbon-nutrient balance at the leaf level. CONCLUSIONS: Defoliation alters biomass allocation and chemical defence through the carbon-nutrient balance at the plant and at the leaf level, respectively.     

Comparison of leaf life span, photosynthesis and defensive traits across seven species of deciduous broad-leaf tree seedlings

BACKGROUND AND AIMS: Leaf life span, photosynthetic parameters and defensive traits were compared across seven species of deciduous broad-leaved tree seedlings native to northern Japan to test the "cost-benefit hypothesis" that more productive leaves are more susceptible to herbivore attack than less productive leaves. METHODS: Studies were made on three early successional species, Alnus hirsuta, Betula maximowicziana and Betula platyphylla "japonica"; one mid-successional species, Ostrya japonica, and three late-successional species, Carpinus cordata, Quercus mongolica 'grosseserrata' and Acer mono. Photosynthetic parameters and defensive traits (total phenolics, condensed tannin and toughness) of leaves were measured for each species, and a bioassay test with Eri silkmoth larvae (Samia cynthia ricini) was undertaken to evaluate differences between species in susceptibility to herbivore attack. KEY RESULTS: Early successional species have a shorter leaf life span (62-88 d) than late successional species (155-187 d). Leaf nitrogen content and light-saturated photosynthetic rate per unit leaf area (P(sat)-area) and per unit leaf mass (P(sat)-mass) were negatively correlated with leaf life span. The nitrogen content of early successional species was about 30 mg g(-1) and that of late successional species was about 16 mg g(-1). Leaf toughness and the C/N ratio were positively correlated with leaf life span, although condensed tannin was not correlated with leaf life span. The bioassay test showed that the number of days the larvae survived was negatively correlated with leaf life span. Average survival of larvae feeding on leaves of A. hirsuta, which has the shortest leaf life span, was 14.4 d and that of Q. mongolica, which has the longest leaf life span, was 6.6 d. The number of days of larval survival was positively correlated with leaf nitrogen content. There was no correlation between days of larval survival and defensive traits. CONCLUSIONS: These results indicate that species with a shorter leaf life span have higher photosynthetic productivity and are more susceptible to herbivore attack than species with a longer leaf life span. This supports the "cost-benefit hypothesis".     

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.     

Theoretical habitat templets, species traits, and species richness: ostracods (Crustacea) in the Upper Rhône River and its floodplain

• 1 Ostracods occurring at two sections of the Upper Rhône River, France, were examined to determine relationships among species traits, habitat utilization, the relationship between species traits and habitat utilization, and trends in species traits and species richness in the context of spatial and temporal variability of habitats. Twenty regularly sampled species were used in this study and fifteen species traits were considered.
• 2 Throe groups can be distinguished according to their species traits: group 1 has species of mixed sizes with high reproductive rates, short life span, spherical shape, long swimming bristles, low thigmotactism, and high resistance to desiccation; group 2 has medium-sized species with low reproductive rates, long life span, low or no tolerance to desiccation, geometric (trapezoidal, triangular) or streamlined carapace shape, no swimming bristles, and a strong thigmotactism; group 3 has the largest species with parthenogenetic reproduction, medium-sized swimming bristles, and flattened or cylindric carapace shape.
• 3 Ostracod habitat utilization segregates the superficial and interstitial habitats along a gradient from the main channel to the abandoned arms and to the temporary waters.
• 4 The co-structure (= relationship) between species traits and habitat utilization indicates that the species use particular habitats with a particular set of species trait modalities. Species with long life spans, late maturity, low fecundity, and low migratory ability are restricted to the interstitial habitats; the epigean species with long life spans, large size, and parental care are more abundant in permanent flowing and standing surface waters; the epigean species with short life spans, high migratory ability, and high tolerance to desiccation are more abundant in temporary ponds.
• 5 The analyses of the distribution of the species traits in a river habitat templet of spatial and temporal variability emphasized that the main disturbance structuring the Rhône River ostracod assemblage is desiccation.
• 6 Of the trends predicted for species traits in the framework of the river habitat templet, five (size, body form, attachment, reproductive technique, and mobility) are clearly opposite for ostracods (because the predictions were mainly established for flood-related disturbances) but four (life span, number of reproductive cycles per year, age at first reproduction, and desiccation tolerance) are in agreement.
• 7 No trends in ostracod species richness in the framework of spatial–temporal habitat variability were evident.

     

Resource availability and growth responses to defoliation in seedlings of three early-successional,tropical, woody species

A growth experiment was conducted using seedlings of three early-successional, tropical, woody species:Dillenia suffruticosa (Dilleniaceae),Macaranga heynei (Euphorbiaceae) andTrema tomentosa (Ulmaceae). These species are characteristic of different positions along a soil fertility gradient in Singapore, withD. suffruticosa being the least andT. tomentosa the most demanding of high nutrient availability. The seedlings were grown in vermiculite at either low or high nutrient availabilities supplied by watering with different concentrations of a commercial plant food. Half the seedlings were subjected to a 50% defoliation at the start of the experiment by cutting off the distal half of each leaf. After 9 weeks the plants were harvested.Macaranga heynei andT. tomentosa seedlings showed no significant difference in parameters of growth such as total dry weight and total leaf area between the defoliated and control seedlings. The seedlings compensated completely for the loss of leaf area.Dillenia suffruticosa did show significant reductions in growth in some parameters due to defoliation, and these were more pronounced under the high nutrient treatment. These findings support the hypothesis that plants characteristic of resource-rich sites can readily recover from herbivory through fast growth, probably associated with a rapid turnover of leaves, whereas species of resource-poor habitats cannot easily replace losses due to herbivory and are adversely effected by defoliation.     

Trait-based community assembly of understory palms along a soil nutrient gradient in a lower montane tropical forest

Two opposing niche processes have been shown to shape the relationship between ecological traits and species distribution patterns: habitat filtering and competitive exclusion. Habitat filtering is expected to select for similar traits among coexisting species that share similar habitat conditions, whereas competitive exclusion is expected to limit the ecological similarity of coexisting species leading to trait differentiation. Here, we explore how functional traits vary among 19 understory palm species that differ in their distribution across a gradient of soil resource availability in lower montane forest in western Panama. We found evidence that habitat filtering influences species distribution patterns and shifts community-wide and intraspecific trait values. Differences in trait values among sites were more strongly related to soil nutrient availability than to variation in light or rainfall. Soil nutrient availability explained a significant amount of variation in site mean trait values for 4 of 15 functional traits. Site mean values of leaf nitrogen and phosphorus increased 37 and 64%, respectively, leaf carbon:nitrogen decreased 38%, and specific leaf area increased 29% with increasing soil nutrient availability. For Geonoma cuneata, the only species occurring at all sites, leaf phosphorus increased 34% and nitrogen:phosphorus decreased 42% with increasing soil nutrients. In addition to among-site variation, most morphological and leaf nutrient traits differed among coexisting species within sites, suggesting these traits may be important for niche differentiation. Hence, a combination of habitat filtering due to turnover in species composition and intraspecific variation along a soil nutrient gradient and site-specific niche differentiation among co-occurring species influences understory palm community structure in this lower montane forest.     

Leaf lifespan is positively correlated with periods of leaf production and reproduction in 49 herb and shrub species

Leaf life span and plant phenology are central elements in strategies for plant carbon gain and nutrient conservation. Although few studies have found that leaf life span correlate with the patterns of leaf dynamics and reproductive output, but there have not been sufficient conclusive tests for relationships between leaf life span and plant phenological traits, the forms and strengths of such relationships are poorly understood. This study was conducted with 49 herb and shrub species collected from the eastern portion of the Tibetan Plateau and grown together in a common garden setting. We investigated leaf life span, the periods of leaf production and death, the time lag between leaf production and death, and the period of plant reproduction (i.e., flowering and fruiting). Interspecific relationships of leaf life span with leaf dynamics and reproduction period were determined. Leaf production period was far longer than leaf death period and largely reflected the interspecific variation of leaf life span. Moreover, leaf life span was positively correlated with the length of reproduction (i.e., flowering and fruiting) period. These relationships were generally consistent across different subgroups of species (herbs vs. shrubs) and indicate potentially widely applicable relationships between LLS and aboveground phenology. We concluded that leaf life span is associated not simply with the dynamics of the leaf itself but with reproduction period. The results demonstrate a plant trade‐off in resource allocation between production and reproduction and a coordinated arrangement of leaves, flowers, and fruits in their time investment. Our results provide insight into the relationship between leaf life span and plant phenology.     

Variability in leaf optical properties among 26 species from a broad range of habitats

Leaves from 26 species with growth forms from annual herbs to trees were collected from open, intermediate, and shaded understory habitats in Mississippi and Kansas, USA. Leaf optical properties including reflectance, transmittance, and absorptance in visible and near infrared (NIR) wavelengths were measured along with leaf thickness and specific leaf mass (SLM). These leaf properties and internal light scattering have been reported to vary with light availability in studies that have focused on a limited number of species. Our objective was to determine whether these patterns in leaf optics and light availability were consistent when a greater number of species were evaluated. Leaf thickness and SLM varied by tenfold among species sampled, but within-habitat variance was high. Although there was a strong trend toward thicker leaves in open habitats, only SLM was significantly greater in open vs. understory habitats. In contrast, leaf optical properties were strikingly similar among habitats. Reflectance and reflectance/transmittance in the NIR were used to estimate internal light scattering and there were strong relationships (r1 > 0.65) between these optical properties and leaf thickness. We concluded that leaf thickness, which did not vary consistently among habitats, was the best predictor of NIR reflectance and internal light scattering. However, because carbon allocation to leaves was lower in understory species (low SLM) yet gross optical properties were similar among all habitats, the energy investment by shade leaves required to achieve optical equivalence with sun leaves was lower. Differences in leaf longevity and growth form within a habitat may help explain the lack of consistent patterns in leaf optics as the number of species sampled increases.     

Size-dependent leaf area ratio in plant twigs: implication for leaf size optimization

Background and Aims

Although many hypotheses have been proposed to explain variation in leaf size, the mechanism underlying the variation remains not fully understood. To help understand leaf size variation, the cost/benefit of twig size was analysed, since, according to Corner''s rule, twig size is positively correlated with the size of appendages the twig bears.

Methods

An extensive survey of twig functional traits, including twig (current-year shoots including one stem and few leaves) and leaf size (individual leaf area and mass), was conducted for 234 species from four broadleaved forests. The scaling relationship between twig mass and leaf area was determined using standardized major axis regression and phylogenetic independent comparative analyses.

Key Results

Leaf area was found to scale positively and allometrically with both stem and twig mass (stem mass plus leaf mass) with slopes significantly smaller than 1·0, independent of life form and habitat type. Thus, the leaf area ratio (the ratio of total leaf area to stem or twig mass) decreases with increasing twig size. Moreover, the leaf area ratio correlated negatively with individual leaf mass. The results of phylogenetic independent comparativeanalyses were consistent with the correlations. Based on the above results, a simple model for twig size optimization was constructed, from which it is postulated that large leaf size–twig size may be favoured when leaf photosynthetic capacity is high and/or when leaf life span and/or stem longevity are long. The model''s predictions are consistent with leaf size variation among habitats, in which leaf size tends to be small in poor habitats with a low primary productivity. The model also explains large variations in leaf size within habitats for which leaf longevity and stem longevity serve as important determinants.

Conclusions

The diminishing returns in the scaling of total leaf area with twig size can be explained in terms of a very simple model on twig size optimization.Key words: Allometry, leaf size, twig size, leaf area ratio, scaling relationship, broadleaved species     

Combined effects of climate,habitat, and disturbance on seedling establishment of <Emphasis Type="Italic">Pinus pinaster</Emphasis> and <Emphasis Type="Italic">Eucalyptus globulus</Emphasis>

The natural expansion of forestry trees into habitats outside plantations is a concern for managers and conservationists. We studied seedling emergence and survival of the two main forestry species in Portugal: Eucalyptus globulus (exotic) and Pinus pinaster (native); using a seed addition experiment. Our main objective was to evaluate the combined effects of climate (mild-summer and warm-summer climate), habitat (oak forest and shrubland), and disturbance (vegetation removal and non-disturbance) on the seedling establishment of species in semi- and natural habitats. Furthermore, we tested the effect of the “sowing season” (autumn and spring) on seedling emergence and survival. Overall, seedling establishment of both species was enhanced by light and water. However, we found important interactions among climate, habitat, and disturbance on both species’ emergence and survival. The differences between habitats were more evident in the mild-summer climate than in the warm-summer climate. Our results also suggested that seedling survival may be enhanced by shrub cover in drier conditions (warm-summer climate). Eucalyptus globulus appears more sensitive to drought and disturbance changes than P. pinaster. In shrublands and mild-summer climate conditions, disturbance especially promoted E. globulus seedling establishment, while the forest canopy and the shade appeared to control it in both climatic conditions. After the first summer life, very low seedling survival was observed in both species, although the colonization of new areas appeared to be more limited for E. globulus. Our study suggests that climate conditions influence the effect (direction and intensity) of habitat and disturbance (plant–plant interactions) on seedling survival. Thus, the effect of light availability (forest canopy) and disturbance (vegetation removal) on these species establishment is climate context-dependent. This study presents very useful information to understand future shifts in these species distribution and has direct applications for the management of natural establishment outside the planted areas, and the management of the understorey to favor forest regeneration or limit forest colonization.