Relative importance of photosynthetic traits and allocation patterns as correlates of seedling shade tolerance of 13 tropical trees

Among 13 tropical tree species on Barro Colorado Island, species with high seedling mortality rates during the first year in shade had higher reltive growth rates (RGR) from germination to 2 months in both sun (23% full sun) and shade [2%, with and without lowered red: far red (R:FR) ratio] than shade tolerant species. Species with higher RGR in sun also had higher RGR in shade. These interspecific trends could be explained by differences in morphological traits and allocation paterns among species. Within each light regime, seedlings of shade-intolerant species had lower root: shoot ratios, higher leaf mass per unit area, and higher leaf area ratios (LAR) than shade tolerant species. In contrast, leaf gas exchange characteristics, or acclimation potential in these traits, had no relationship with seedling mortality rates in shade. In both shade tolerant and intolerant species, light saturated photosynthesis rates, dark respiration, and light compensation points were higher for sungrown seedlings than for shade-grown seedlings. Differences in R:FR ratio in shade did not affect gas exchange, allocation patterns, or growth rates of any species. Survival of young tree seedlings in shade did not depend on higher net photosynthesis or biomass accumulation rates in shade. Rather, species with higher RGR died faster in shade than species with lower RGR. This trend could be explained if survival depends on morphological characteristics likely to enhance defense against herbivores and pathogens, such as dense and tough leaves, a well-established root system, and high wood density. High construction costs for these traits, and low LAR as a consequence of these traits, should result in lower rates of whole-plant carbon gain and RGR for shade tolerant species than shade-intolerant species in shade as well as in sun.     

A global study of relationships between leaf traits, climate and soil measures of nutrient fertility

Aim This first global quantification of the relationship between leaf traits and soil nutrient fertility reflects the trade‐off between growth and nutrient conservation. The power of soils versus climate in predicting leaf trait values is assessed in bivariate and multivariate analyses and is compared with the distribution of growth forms (as a discrete classification of vegetation) across gradients of soil fertility and climate. Location All continents except for Antarctica. Methods Data on specific leaf area (SLA), leaf N concentration (LNC), leaf P concentration (LPC) and leaf N:P were collected for 474 species distributed across 99 sites (809 records), together with abiotic information from each study site. Individual and combined effects of soils and climate on leaf traits were quantified using maximum likelihood methods. Differences in occurrence of growth form across soil fertility and climate were determined by one‐way ANOVA. Results There was a consistent increase in SLA, LNC and LPC with increasing soil fertility. SLA was related to proxies of N supply, LNC to both soil total N and P and LPC was only related to proxies of P supply. Soil nutrient measures explained more variance in leaf traits among sites than climate in bivariate analysis. Multivariate analysis showed that climate interacted with soil nutrients for SLA and area‐based LNC. Mass‐based LNC and LPC were determined mostly by soil fertility, but soil P was highly correlated to precipitation. Relationships of leaf traits to soil nutrients were stronger than those of growth form versus soil nutrients. In contrast, climate determined distribution of growth form more strongly than it did leaf traits. Main conclusions We provide the first global quantification of the trade‐off between traits associated with growth and resource conservation ‘strategies’ in relation to soil fertility. Precipitation but not temperature affected this trade‐off. Continuous leaf traits might be better predictors of plant responses to nutrient supply than growth form, but growth forms reflect important aspects of plant species distribution with climate.     

Leaf dry matter content and lateral spread predict response to land use change for six subalpine grassland species

Question: Land‐use change has a major impact on terrestrial plant communities by affecting fertility and disturbance. We test how particular combinations of plant functional traits can predict species responses to these factors and their abundance in the field by examining whether trade‐offs at the trait level (fundamental trade‐offs) are linked to trade‐offs at the response level (secondary trade‐offs). Location: Central French Alps. Methods: We conducted a pot experiment in which we characterized plant trait syndromes by measuring whole plant and leaf traits for six dominant species, originating from contrasting subalpine grassland types. We characterized their response to nutrient availability, shading and clipping. We quantified factors linked with different land usage in the field to test the relevance of our experimental treatments. Results: We showed that land management affected nutrient concentration in soil, light availability and disturbance intensity. We identified particular suites of traits linked to plant stature and leaf structure which were associated with species responses to these environmental factors. Leaf dry matter content separates fast and slow growing species. Height and lateral spread separated tolerant and intolerant species to shade and clipping. Discussion and Conclusion: Two fundamental trade‐offs based on stature traits and leaf traits were linked to two secondary trade‐offs based on response to fertilization shade and mowing. Based on these trade‐offs, we discuss four different species strategies which could explain and predict species distributions and traits syndrome at community scale under different land‐uses in subalpine grasslands.     

小兴安岭不同耐荫性树种枝叶性状变异及权衡

随着植株生长,不同耐荫性树种枝叶性状对资源的响应策略存在差异。探究不同耐荫性树种在不同径级间枝叶性状变异及其相关关系,对理解植物功能性状种内和种间变异以及植物对资源的响应策略具有重要意义。以黑龙江凉水国家级自然保护区阔叶红松（Pinus koraiensis）林中不同径级（小树、中等树和大树）的喜光树种（白桦Betula platyphylla、枫桦Betula costata）和耐荫树种（春榆Ulmus japonica、紫椴Tilia amurensis、色木槭Acer mono）为研究对象,测定其出叶强度、枝横截面积、枝干重、单叶面积、总叶面积、总叶干重共6个枝叶性状。利用单因素方差（LSD）分析检验不同耐荫性树种在不同径级间其枝、叶性状是否存在显著差异;以标准化主轴估计（SMA）对不同耐荫性树种枝叶性状间相关关系进行分析。结果表明：除枝干重外,耐荫树种枝叶性状均大于喜光树种;不同耐荫性树种大树枝横截面积均最大,中等树叶面积均最小;喜光树种不同径级间枝干重、总叶干重和出叶强度无显著差异,而耐荫树种均存在显著差异。随径级增大,不同耐荫性树种枝横截面积与叶面积均呈正异速生长关系,且除耐荫树种枝横截面积与单叶面积外,均存在共同斜率;不同耐荫性树种出叶强度与单叶面积存在共同斜率为-0.51和0.47,小树和中等树枝横截面积与总叶面积和总叶干重的异速生长指数均与1存在显著差异,而大树与1无显著差异。结果表明：不同耐荫性树种在不同径级间对资源的获取策略存在差异,不同耐荫性树种小树阶段均表现出资源获取策略,大树阶段则表现出资源保守型策略。与耐荫树种相比,喜光树种表现为快速生长策略,而随着径级增大不同耐荫性树种这种差异逐渐消失,本研究结果为森林演替过程中种内和种间的相互作用提供了一个新的视角。     

Inter- and intraspecific relationships between shade tolerance and shade avoidance in temperate trees

Patterns of physiological and architectural adaptation and acclimation to decreasing light availability were investigated along a light gradient for saplings of 12 common species of temperate deciduous trees in southeastern Ontario, Canada. Physiological adaptation and acclimation (shade tolerance physiology) were quantified at the leaf level by measuring leaf mass per unit area (LMA), dark respiration per unit leaf nitrogen, chlorophyll per unit leaf nitrogen and the chlorophyll a:b ratio for the newest fully expanded leaf on the leader. Architectural adaptation and acclimation (shade avoidance) were quantified by measuring branching intensity and side shoot:main shoot length ratios for the most recent three years of growth on the leader and selected side branches. Within species, increases in LMA, chlorophyll a:b ratio and respiration per unit nitrogen and decreases in chlorophyll per unit nitrogen indicated that shade tolerance physiology generally increased with decreasing canopy openness. Increases in the branching intensity and side shoot:main shoot ratios of the leader and side branches indicated that shade avoidance also increased with decreasing canopy openness for the majority of species; however, in some species, stem bending under deep shade resulted in lateral growth. Interspecific variation in shade tolerance physiology was minimal when species were compared under equal amounts of canopy openness. In contrast, interspecific variation in shade avoidance variables was relatively high under equal canopy openness, with saplings of shade tolerant canopy species exhibiting higher shade avoidance than saplings of shade tolerant understory species.     

Intraspecific variability improves environmental matching,but does not increase ecological breadth along a wet‐to‐dry ecotone

It is widely assumed that higher levels of intraspecific variability in one or more traits should allow species to persist under a wider range of environmental conditions. However, few studies have examined whether species that exhibit high variability are found in a wider range of environmental conditions, and whether variability increases the ability of a species to adapt to prevailing ecological gradients. We used four plant functional traits, specific leaf area (SLA), leaf dry matter content (LDMC), leaf carbon to nitrogen ratio (C:N) and maximum plant height in 49 species across a strong environmental gradient to answer three questions: 1) is there evidence for ‘high‐variability’ species (that is, species which show high variability in multiple traits, simultaneously)? 2) are species with more variable traits present across a wider range of environmental conditions than less variable species? And 3) whether more variable species show better trait–environment matching to the prevailing abiotic (soil moisture) gradient at the site? We found little evidence for a ‘high‐variability’ species. Variability was correlated for two leaf traits, SLA and LDMC, while variability in leaf traits and plant height were not correlated. We found little evidence that more variable species were present in more diverse conditions: only variation in SLA was correlated with a wider ecological niche breadth. For plant traits along the soil‐moisture gradient, higher variability led to better trait–environment matching in half of measured traits. Overall, we found little support for the existence of ‘high‐variability’ species, but that variability in SLA is correlated with a wider ecological breadth. We also found evidence that variation in traits can improve trait–environment matching, a relationship which may facilitate our understanding ecological breadth along prevailing gradients, and community assembly on the basis of traits.     

Long‐term tree fern dynamics linked to disturbance and shade tolerance

Question: Do New Zealand tree ferns have recognizable shade tolerance niches? Location: Lowland temperate rain forest of New Zealand (41°20′S, 174°58′E). Methods: Growth, death and recruitment of five tree fern species were estimated from a 38‐year record of stem heights, collected within a 2.25‐ha block of forest, and electron transport rates (ETR) of photosystem II of fronds were measured. Results: Two species of Cyathea were comparatively common (603 and 351 stems in total) and two were comparatively rare (155 and 17 stems in total) on the site. The common species had lower rates of growth, recruitment and mortality than the rare species, had skewed age distributions typical of shade‐tolerant species and were probably recruited soon after a catastrophic earthquake in 1855. The two rare species were failing to recruit under closed forests; their age distributions indicated that all had regenerated long after the earthquake. ETR were higher for faster‐growing than for the shade‐tolerant species. A tree fern that regenerates vegetatively from aerial buds, Dicksonia squarrosa, was common on the site (361 stems in total). Its age distribution suggested it was relatively shade tolerant, but its mortality and recruitment rates were much higher than those of the two shade‐tolerating Cyathea species, suggesting that this multi‐stemmed species functions differently from the monopodial Cyathea species. Conclusions: New Zealand Cyathea tree ferns occupy distinct niches along a shade tolerance spectrum and their relative abundances are strongly influenced by disturbance history. The study provides evidence that tree fern species differ strongly in their responses to canopy disturbance and are not ecologically equivalent.     

Is leaf dry matter content a better predictor of soil fertility than specific leaf area?

Background and Aims

Specific leaf area (SLA), a key element of the ‘worldwide leaf economics spectrum’, is the preferred ‘soft’ plant trait for assessing soil fertility. SLA is a function of leaf dry matter content (LDMC) and leaf thickness (LT). The first, LDMC, defines leaf construction costs and can be used instead of SLA. However, LT identifies shade at its lowest extreme and succulence at its highest, and is not related to soil fertility. Why then is SLA more frequently used as a predictor of soil fertility than LDMC?

Methods

SLA, LDMC and LT were measured and leaf density (LD) estimated for almost 2000 species, and the capacity of LD to predict LDMC was examined, as was the relative contribution of LDMC and LT to the expression of SLA. Subsequently, the relationships between SLA, LDMC and LT with respect to soil fertility and shade were described.

Key Results

Although LD is strongly related to LDMC, and LDMC and LT each contribute equally to the expression of SLA, the exact relationships differ between ecological groupings. LDMC predicts leaf nitrogen content and soil fertility but, because LT primarily varies with light intensity, SLA increases in response to both increased shade and increased fertility.

Conclusions

Gradients of soil fertility are frequently also gradients of biomass accumulation with reduced irradiance lower in the canopy. Therefore, SLA, which includes both fertility and shade components, may often discriminate better between communities or treatments than LDMC. However, LDMC should always be the preferred trait for assessing gradients of soil fertility uncoupled from shade. Nevertheless, because leaves multitask, individual leaf traits do not necessarily exhibit exact functional equivalence between species. In consequence, rather than using a single stand-alone predictor, multivariate analyses using several leaf traits is recommended.     

Sapling biomass allocation and growth in the understory of a deciduous hardwood forest

Above- and belowground tissues of co-occurring saplings (0.1-1 m height) of Acer saccharum Marsh. (very shade tolerant), Acer rubrum L. (shade tolerant), Fraxinus americana L. (intermediate shade tolerant), and Prunus serotina Ehrh. (shade intolerant) were harvested from a forest understory to test the hypothesis that the pattern of biomass allocation varied predictably with shade-tolerance rank. The placement and length of branches along the main axis were consistent with the formation of a monolayer of foliage for the tolerant and intermediate species. Other morphological characteristics did not vary predictably with shade-tolerance rank. The maintenance of high specific leaf area (SLA; leaf area/leaf mass) and leaf area ratio (LAR; leaf area/sapling mass) is considered important for growth under extreme shade, yet these traits were not clearly related to the shade-tolerance rank of these species. Fraxinus americana, an intermediate species, had the highest LAR and growth rate in the understory, and with the exception of P. serotina, the very shade-tolerant A. saccharum had the lowest LAR. Prunus serotina maintained a large starch-rich tap root and shoot dieback was common, yielding the largest root/shoot ratio for these species. The observed allocation patterns were not similar to the long-standing expectation for the phenotypic response of juvenile trees to shade, but were consistent with three hypothetical "growth strategies" in the understory: (1) the low SLA and LAR of A. saccharum may provide a measure of defense against herbivores and pathogens and thus promote persistence in the understory, (2) the high SLA for F. americana and high LAR for F. americana and A. rubrum may enable these species to achieve high growth rates in shade, and (3) the large carbohydrate stores of P. serotina may poise this species for opportunistic growth following disturbance. The relative importance of resistance to herbivores and pathogens vs. the maintenance of high growth rates may be important in evaluating the patterns of biomass allocation in the understory.     

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.

     

Competition- and resource-mediated tradeoffs between growth and defensive chemistry in trembling aspen (Populus tremuloides)

Costs of defense are thought to maintain genetic variations in the expression of defense within plant populations. As with many plant species, aspen exhibits considerable variation in allocation to secondary metabolites. This study examined the independent and interactive effects of genotype, soil fertility and belowground competition on defensive chemistry and growth in trembling aspen (Populus tremuloides). Four aspen genotypes were grown with high and low soil fertility, and with and without root competition. Physiological, morphological and allocational determinants of growth were measured to identify growth-defense tradeoffs. Nutrient limitation and competition decreased growth, leaf mass ratio, leaf nitrogen concentration and photosynthesis, and increased root : shoot ratio and leaf condensed tannin concentrations. The competition treatment also resulted in increased leaf phenolic glycoside (PG) concentrations. Aspen growth was negatively correlated with PG concentrations under low fertility with competition. The relationship between growth and its major determinants was also negatively related to foliar condensed tannins expressed as a proportion of tree mass, indicating an additional indirect cost of allocation to secondary metabolites.     

Contrasting patterns of diameter and biomass increment across tree functional groups in Amazonian forests

Species' functional traits may help determine rates of carbon gain, with physiological and morphological trade-offs relating to shade tolerance affecting photosynthetic capacity and carbon allocation strategies. However, few studies have examined these trade-offs from the perspective of whole-plant biomass gain of adult trees. We compared tree-level annual diameter increments and annual above-ground biomass (AGB) increments in eight long-term plots in hyper-diverse northwest Amazonia to wood density (rho; a proxy for shade tolerance), whilst also controlling for resource supply (light and soil fertility). rho and annual diameter increment were negatively related, confirming expected differences in allocation associated with shade tolerance, such that light-demanding species allocate a greater proportion of carbon to diameter gain at the expense of woody tissue density. However, contrary to expectations, we found a positive relationship between rho and annual AGB increment in more fertile sites, although AGB gain did not differ significantly with rho class on low-fertility sites. Whole-plant carbon gain may be greater in shade-tolerant species due to higher total leaf area, despite lower leaf-level carbon assimilation rates. Alternatively, rates of carbon loss may be higher in more light-demanding species: higher rates of litterfall, respiration or allocation to roots, are all plausible mechanisms. However, the relationships between rho and AGB and diameter increments were weak; resource availability always exerted a stronger influence on tree growth rates.     

How cellulose-based leaf toughness and lamina density contribute to long leaf lifespans of shade-tolerant species

Cell wall fibre and lamina density may interactively affect leaf toughness and leaf lifespan. Here, we tested this with seedlings of 24 neotropical tree species differing in shade tolerance and leaf lifespan under standardized field conditions (140-867 d in gaps; longer in shade). We quantified toughness with a cutting test, explicitly seeking a mechanistic linkage to fibre. Lamina density, but not fracture toughness, exhibited a plastic response to gaps vs shade, while neither trait was affected by leaf age. Toughness corrected for lamina density, a recently recognized indicator of material strength per unit mass, was linearly correlated with cellulose content per unit dry mass. Leaf lifespan was positively correlated with cellulose and toughness in shade-tolerant species but only weakly in gap-dependent species. Leaf lifespan was uncorrelated with lamina thickness, phenolics and tannin concentrations. In path analysis including all species, leaf lifespan was directly enhanced by density and toughness, and indirectly by cellulose via its effect on toughness. Different suites of leaf traits were correlated with early seedling survival in gaps vs shade. In conclusion, cellulose and lamina density jointly enhance leaf fracture toughness, and these carbon-based physical traits, rather than phenolic-based defence, explain species differences in herbivory, leaf lifespan and shade survival.     

Site fertility and leaf nutrients of sympatric evergreen and deciduous species of Quercus in central coastal California

Leaf and soil nutrient levels interact with and may each influence the other. We hypothesize that to the extent soil fertility influences the nutritional state of trees, soil fertility should correlate with summer leaf nutrient levels, whereas to the extent that trees influence soil nutrient levels, the quality of leaf litterfall should correlate with soil fertility. We examined these correlations for five sympatric oak species (genus Quercus) in central coastal California. Soil fertility, including both nitrogen and especially phosphorus, correlated significantly with summer leaf nutrient levels. In contrast, phosphorus, but not nitrogen, in the leaf litterfall correlated positively with soil nutrients. These results suggest that soil nitrogen and phosphorus influence tree nutrient levels and that leaf phosphorus, but not leaf nitrogen, influence soil fertility under the trees. Feedback between the soil and the tree for phosphorus, but not nitrogen, is apparently significant and caused by species-specific differences in leaf quality and not by litterfall quality differences within a species. We also compared functional differences between the evergreen and deciduous oak species at our study site. There were no differences in soil nitrogen and only small differences for soil phosphorus between the phenological types. Differences in leaf nutrient concentration were much more pronounced, with the evergreen species having substantially lower levels of both nitrogen and phosphorus. Evergreen species conserved more phosphorus, but not more nitrogen, than the deciduous species, but there was no consistent relationship between retranslocation and either soil nitrogen or phosphorus. These results do not support the hypothesis that evergreenness is an adaptation to low soil fertility in this system.     

Variation in juvenile survival and related physiological traits among dipterocarp species co‐existing in a Bornean forest

Question: What environmental factors and physiological traits determine the juvenile survival rates of 11 dipterocarp species (Dipterocarpaceae) co-occurring in a Bornean tropical rain forest? Location: Tropical northwestern Borneo. Methods: In 248 quadrats, distributed over 12 ha, the juvenile (height< 2.0 m) survival of 11 shade-tolerant dipterocarp species was monitored for one year. The probability of survival for each species was expressed as a logistic function of plant height and three environmental factors: canopy openness, soil water potential and conspecific individual density. The correlation between survival under shaded conditions and several physiological traits of juveniles was tested by normal Pearson correlation and a phylogeny based manner using independent contrasts. Results: Seven species had increased survival under more open canopies, one species had increased and two species had decreased survival at drier sites. Six species had higher survival as the density of conspecifics increased, whereas two species had lower survival. The magnitudes of the effects for the three environmental factors were similar. However, the interspecific difference in survival was largely determined by a dependence of small seedlings on the light environment. Survival under low light was correlated positively with root dry mass and negatively with total leaf area of juveniles. Conclusions: Responses of seedlings to the fine-scale gradient of light availability were considerably different among the 11 species. Species that suffered increased mortality under low light had a set of structural traits that would potentially allow rapid growth under bright conditions. Differential performance along the light gradient may contribute to the stable coexistence of these species.     

Physiological and morphological correlates of whole-plant light compensation point in temperate deciduous tree seedlings

A range of traits, including metabolic costs, biomass allocation and seed reserves, may contribute to interspecific variation in the shade tolerance of tree seedlings. In addition, shade tolerance may be affected by differential responses of species to soil resource availability at low light. We used a custom-built whole-plant gas-exchange chamber to quantify instantaneous whole-plant light compensation point (WPLCP) and to parameterize whole-plant daily C gain models for seedlings of eight temperate deciduous tree species. We examined the relationship of WPLCP to growth, biomass allocation and gas-exchange under high and low light and nutrient availabilities and compared it to WPCLP of naturally recruited saplings. For species showing a response, both increased light and nutrient availability resulted in increased WPLCP. However, species’ responses to resource availability did not correspond closely with shade tolerance as has generally been predicted. Variation in WPLCP within species was best predicted by whole-plant dark respiration rates, leaf-level light compensation point and leaf mass per area. Among species, seed size was a strong negative correlate of WPLCP, explaining 66% of the variation. Species with the lowest WPLCP maintained lower growth rates across treatments but greater biomass in the low-light treatment compared with more light-demanding species. These data suggest that a number of traits, in particular metabolic costs and seed size, contribute to WPLCP. However, gas-exchange-based WPLCP was 1.5–3.5 times lower than corresponding growth-based field estimates of WPLCP, suggesting that other factors such as biotic interactions or ontogenetic shifts in whole-plant light requirements may substantially increase species’ WPLCP under natural conditions.     

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.     

Plant traits link hypothesis about resource-use and response to herbivory

Grazing by large herbivores, in interplay with environmental productivity, is a key driver of the composition of the vegetation with important consequences on the ecosystem and, consequently, for land management. We tested the predictions of the resource availability – resource–acquisition theory by assessing the extent to which community averages of plant traits, known to be related to plant growth, competitive ability and response to grazing were correlated with resource gradients within local (200 km²) geographical ranges. Second, we assessed the applicability of the same set of plant traits to make inferences on ecological effects of grazing by sheep in alpine ecosystems in Norway, using a data set consisting of 16 sites in central Norway. We estimated grazing intensity by free-ranging sheep based on GPS telemetry, soil properties, plant species composition and species traits i.e. specific leaf area (SLA), leaf dry matter content (LDMC), leaf size and plant height. Soil fertility and the interaction between soil fertility and grazing, but not grazing intensity alone, were significantly related to plant species and traits composition. Generally, average SLA showed lower correspondence with soil fertility and grazing than the other traits. Leaf size and plant height were lowest at sites with high grazing intensity and in sites with low fertility, and increased with soil fertility in little and moderately grazed sites, but declined at high fertility sites when grazing was intense. LDMC showed the opposite trend. Grazing intensity was more related to the variability in plant composition and average plant traits when environmental productivity was high. Our results therefore are indicative of a convergence of responses to grazing and nutrient limitation.     

LEAF OPTICAL PROPERTIES ALONG A VERTICAL GRADIENT IN A TROPICAL RAIN FOREST CANOPY IN COSTA RICA

Leaf optical properties (400–1,100 nm) were compared for four species of rain forest trees with crowns in understory, mid-canopy, and canopy positions to test whether optical properties change with light environment. The species tested represent a spectrum of regeneration patterns ranging from shade tolerant to light demanding. Overall, leaf optical properties of the four species were similar. Differences in absorptance were small, but statistically significant among the species and positions along the canopy gradient. Species absorptance differences corresponded somewhat to shade tolerance; two of the shade species showed higher absorptance in lower light environments, while the sun species showed the reverse pattern. Specific leaf mass (leaf weight per unit area) and chlorophyll content per unit leaf weight also changed along the canopy gradient. Specific leaf mass was positively correlated and chlorophyll per unit leaf weight was negatively correlated with increasing light environment. Consequently, the efficiency of absorption, as represented by the absorptance per unit leaf weight, increased as light level decreased, largely due to changes in specific leaf mass. In contrast, efficiency of absorption per unit leaf chlorophyll was relatively constant with light environment for the two species measured for chlorophyll.     

Intraspecific trait variation and the differential decline of meadow species during conifer encroachment

Conifer encroachment has reduced the extent and habitat quality of mountain meadows throughout western North America. Past studies in the Cascade Range of Oregon reveal surprising variation in the pace at which meadow species are lost to encroachment. We hypothesized that this variation relates to intraspecific variability in plant functional traits that are adaptive as light levels decline during the transition from open meadow to closed forest. For 13 meadow species with varying sensitivities to encroachment, we compared how the rate of decline in abundance relates to intraspecific variation in three morphological traits (specific leaf area, shoot/root ratio, and shoot height) and to clonal ability (no, limited, or strong potential for vegetative spread). For each species, we computed an index of sensitivity to encroachment, ISE, the coefficient of variation in cover across the light gradient. For each trait of each species, we computed an index of variation, the linear slope of the relationship between trait values and available light. For most traits, the correlation between ISE and trait variation (or clonality) was weak. Although specific leaf area increased in the shade for all species, the magnitude of increase did not correlate with ISE. Only variability in leaf area was positively correlated with ISE, increasing in the shade for less sensitive species and decreasing for more sensitive species. Responses to encroachment may reflect differences in species’ physiological rather than morphological variability, or species may be responding to changes in resources other than, or in addition to, light.