Longer Leaf Life Span Research Articles

Controls on use efficiency of plant nutrients along subtropical to alpine gradients on the Tibetan Plateau

AbstractQuestionKnowledge of how nutrient use strategies differ between forest trees and alpine shrubs/grasses is important to understand the mechanisms of vegetation changes from montane forests to alpine shrubs/grasslands along altitudinal gradients. We tested the hypothesis that, to maximize the nitrogen use efficiency (NUE) of canopy production, forest trees tend to have a higher mean residence time (MRT) of nitrogen in the plants through increased leaf life span, whereas alpine shrublands and grasslands tend to have higher nitrogen productivity through increased below‐ground biomass fraction. We further tested whether similar patterns are found in phosphorus use efficiency (PUE).LocationTwenty‐one sampling sites along Tibetan Alpine Vegetation Transects (TAVT) at altitudes from 1900 m to 4900 m.MethodsWe measured the maximum biomass of new canopy leaves and twigs and the concentrations of their nutrients N and P and associated ecosystem variables along the TAVT. NUE (PUE) was calculated as the product of nutrient productivity (dry matter production per unit N or P in new canopy leaves and twigs) and MRT (the ratio of foliage nutrient pool to annual nutrient uptake).ResultsWith increasing altitude, leaf life span increased in forest trees but decreased in shrublands and grasslands, while below‐ground fraction increased when vegetation changed from montane forests to alpine shrubs/grasslands. In forest trees, higher N‐MRT and P‐MRT and lower P productivity were associated with longer leaf life span and lower below‐ground fraction, while N productivity varied little. In alpine shrublands and grasslands, N‐MRT, P‐MRT and P productivity varied little with leaf life span and below‐ground fraction, while N productivity was positively correlated with below‐ground fraction.ConclusionsOur data supported the hypothesis, suggesting that NUE of canopy production would be a measure of changes in ecosystem functioning from montane forests to alpine shrublands and grasslands along altitudinal gradients. The findings provide an insight into the linkage between biogeochemistry and phytogeographic processes across ecosystems.

Impact of longevity and wettability of fern leaves on epiphyll colonization in a Mexican lowland rainforest

AbstractQuestionsLeaf traits play an important role in the epiphyll colonization of leaves. Although ferns are common in forest understories, they have been widely ignored as possible host plants. We asked: How does epiphyll colonization (cover, richness, and composition) differ among fern species depending on their leaf longevity and surface wettability?LocationTropical Biological Station of “Los Tuxtlas”, Veracruz (Mexico).MethodsEpiphyll cover was monitored on leaves of six individuals per species after 2, 5, 12, 16.5, and 26 months. Leaf longevity was calculated from leaf turnover rates over 24 months. Leaf wettability was determined through the contact angle technique.ResultsAll studied fern species had hydrophilic leaf surfaces. Fern species with shorter leaf longevity (14.4–17.8 months) were colonized faster by epiphylls than species with longer leaf life spans (33.9–38.8 months). The area of the oldest leaves was covered to 1.2%–26.1% by foliicolous lichens and to 0.3%–2.2% by green algae. Foliicolous lichens were more species‐rich on Danaea nodosa (27 spp.) than on the other fern species (1–9 lichen spp.). Foliose liverworts thrived on the only riparian fern (3.57%). Only 0.04%–1.1% of the area of plastic substrates was colonized by epiphylls.ConclusionsThe hydrophilic leaf surfaces of ferns facilitated the colonization of epiphylls, especially of foliicolous lichens. The speed of epiphyll colonization was faster in ferns with shorter leaf longevity but increased in all species with leaf age. However, the relatively low epiphyll cover on fern leaves and artificial substrates in our study might be related to unfavorable microclimatic parameters.

Ring- and diffuse-porous tree species from a cold temperate forest diverge in stem hydraulic traits, leaf photosynthetic traits, growth rate and altitudinal distribution.

In cold and humid temperate forests, low temperature, late frost and frequent freeze-thaw cycles are the main factors limiting tree growth and survival. Ring- and diffuse-porous tree species differing in xylem anatomy coexist in these forests, but their divergent adaptations to these factors have been poorly explored. To fill this knowledge gap, we compared four ring-porous and four diffuse-porous tree species from the same temperate forest in Northeast China by quantifying their leaf and stem functional traits, their stem growth rates using tree ring analysis and their resistance to cold represented by upper altitude species distribution borders from survey data. We found that the ring-porous trees were characterized by traits related to more rapid water transport, carbon gain and stem growth rates than those of the diffuse-porous species. Compared with the diffuse-porous species, the ring-porous species had a significantly higher shoot hydraulic conductance (Ks-shoot, 0.52 vs 1.03kgm-1s-1MPa-1), leaf photosynthetic rate (An, 11.28 vs 15.83μmolm-2s-1), relative basal area increment (BAIr, 2.28 vs 0.72cmyear-1) and stem biomass increment (M, 0.34 vs 0.09kgyear-1m-1). However, the observed upper elevational distribution limit of the diffuse-porous species was higher than that of the ring-porous species and was associated with higher values of conservative traits, such as longer leaf life span (R2=0.52). Correspondingly, BAIr and M showed significant positive correlations with acquisitive traits such as Ks-shoot (R2=0.77) and leaf photosynthetic rate (R2=0.73) across the eight species, with the ring-porous species occurring at the fast-acquisitive side of the spectrum and the diffuse-porous species located on the opposite side. The observed contrasts in functional traits between the two species groups improved our understanding of their differences in terms of growth strategies and adaptive capabilities in the cold, humid temperate forests.

The consequence of leaf life span to virus infection of herbivorous insects.

Many herbivorous insects die of pathogen infections, though the role of plant traits in promoting the persistence of these pathogens as an indirect interaction is poorly understood. We tested whether winter leaf retention of bush lupines (Lupinus arboreus) promotes the persistence of a nucleopolyhedroviruses, thereby increasing the infection risk of caterpillars (Arctia virginalis) feeding on the foliage during spring. We also investigated whether winter leaf retention reduces viral exposure of younger caterpillars that live on the ground, as leaf retention prevents contaminated leaves from reaching the ground. We surveyed winter leaf retention of 248 lupine bush canopies across twelve sites and examined how it related to caterpillar infection risk, herbivory, and inflorescence density. We also manipulated the amount of lupine litter available to young caterpillars in a feeding experiment to emulate litterfall exposure in the field. Greater retention of contaminated leaves from the previous season increased infection rates of caterpillars in early spring. Higher infection rates reduced herbivory and increased plant inflorescence density by summer. Young caterpillars exposed to less litterfall were more likely to starve to death but less likely to die from infection, further suggesting foliage mediated exposure to viruses. We speculate that longer leaf life span may be an unrecognized trait that indirectly mediates top-down control of herbivores by facilitating epizootics.

Differences in leaf phenological traits between trees and shrubs are closely related to functional traits in a temperate forest

Leaf phenological traits vary greatly with plant species, are thought to relate to leaf functional traits, and play an important role in plant growth. In this study, we investigated leaf phenological and functional traits of 52 woody species (324 individuals) from a natural forest in the Loess Plateau in China. Shrubs obtain a longer leaf life span (LLS) through early leaf emergence and later leaf senescence. Shrubs have a longer LLS compared with trees because they have a greater leaf thickness than trees (to prevent early spring frost), leaf phosphorus content ([P]leaf, related to cell division), and C:N ratio and stem density (to prevent mechanical damage). Among all woody species, species with a small SLA and leaf area and species with thick leaves have a longer leaf emergence duration and LLS, which may be related to material energy reserves. In addition, shrubs had a greater [P]leaf and C:N ratio and a longer leaf emergence duration. Our results reveal differences in leaf phenological traits among life forms and indicate that plant functional traits can provide insight into the mechanisms underlying such differences. In addition, the results of this study provide insight into species coexistence and community assembly in temperate deciduous forests.

Vegetative anatomy and photosynthetic performance of the only known winter-greenCypripediumspecies: implications for divergent and convergent evolution of slipper orchids

AbstractCypripedium subtropicum is the only known winter-green species in the genus Cypripedium, whereas the other nearly 50 species keep their leaves for less than half the year. Life form has an important effect on carbon acquisition and adaptation of plants. However, the physiological mechanism behind it remains unclear. In this study, we investigated vegetative anatomy and photosynthetic performance of C. subtropicum across with its leaf ages. These anatomical and photosynthetic traits were also compared with typical Cypripedium spp. and other members of subfamily Cypripedioideae. The obtained results confirmed that this species exhibited many characters of shade plants, such as thin leaves, extremely low photosynthetic rate and light saturation point and high chlorophyll content. Unlike the strategy adopted by typical Cypripedium spp. that quickly achieve annual carbon gain with a high assimilation rate in a short growing season, C. subtropicum obtains its carbon through a low assimilation rate but a much longer leaf lifespan. The local climate and favourable light condition guaranteed the comparable carbon income in winter to compensate for its low photosynthetic capacity. The long-lived, thin leaves of C. subtropicum, differing from the long-lived, thick leathery leaves in conduplicate-leaved genera, represent a distinct adaptive strategy in subfamily Cypripedioideae. Our findings shed light on the divergent and convergent evolution in slipper orchids, and we hope these findings will contribute to the conservation of such an endangered orchid.

Pores versus spores: competition between photosynthesis and reproduction is constrained by leaf mass per unit area (LMA) in ferns

AbstractDiversity in plant life histories is primarily that found in the rate and duration of photosynthetic (vegetative) and reproductive growth. However, direct evidence for an anticipated trade-off between photosynthesis and reproduction is lacking in any plant lineage. Ferns allocate leaf space and resources to both photosynthesis and reproduction, potentially leading to competition for leaf resources between stomatal pores and reproductive spores. We hypothesized that a trade-off between stomatal density (StD; a proxy for photosynthetic capacity) and sporangial density (SpD; a measure of fertility) has evolved in monomorphic ferns due to the common space, time and resource constraints imposed by a highly conserved and globally low leaf mass per unit area (LMA) in ferns, where any increase in LMA indicated greater construction cost and longer leaf lifespan. We measured LMA, StD and SpD in 40 fern species in India that represented both monomorphic and dimorphic conditions from both terrestrial and epiphytic habits. Both StD and SpD showed a 50-fold range in monomorphic species whereas LMA was more conserved (six-fold range). LMA of terrestrial ferns was significantly lower than that of epiphytic ferns. Linear regression between LMA and StD was significantly positive in dimorphic terrestrial ferns (showing the lowest LMA among all ferns) and significantly negative in monomorphic epiphytic ferns (showing the highest LMA among all ferns). Dimorphic terrestrial ferns were highly fecund on their fertile leaves and showed a significantly higher StD to LMA ratio on their sterile leaves compared to monomorphic terrestrial ferns. Dimorphic ferns seem to maximize both StD and SpD by physical separation of photosynthesis and reproduction, and their characteristically low LMA (shorter leaf lifespan = smaller time window) potentially selects for high StD and high fertility. The regression between StD and SpD in monomorphic ferns was significantly linear and positive, although comparisons among closely related species (within families) showed negative correlations when both StD and SpD were high, captured also by a significant quadratic regression between StD and SpD in monomorphic ferns. Monomorphic terrestrial species bearing more spores per stomata showed relatively low LMA whereas those producing fewer spores per stomata possessed leaves with relatively high LMA. Monomorphic epiphytes produced as many spores as terrestrial species but showed significantly low StD for their high LMA. We discuss the evolutionary reasons behind these trends and conclude that monomorphic terrestrial ferns with high LMA (long leaf lifespan) tend to prioritize photosynthesis over reproduction, while monomorphic epiphytes (always high LMA) are significantly more fertile for lower photosynthesis. The role of LMA in framing the rules of competition between stomata and sporangia in monomorphic ferns provides a template for how photosynthesis may directly or indirectly influence reproductive strategies (and vice versa) in all land plants.

Revisiting the Functional Basis of Sclerophylly Within the Leaf Economics Spectrum of Oaks: Different Roads to Rome

Purpose of Review: Defining the mechanisms behind and the leaf economic consequences of the development of sclerophylly in woody plants will allow us to understand its ecological implications, anticipate the potential for adaptation of different tree species to global change, and define new woody plant ideotypes for stress tolerance. Recent Findings: Sclerophylly has evolved independently in different woody plant genera and has been traditionally considered as a stress-tolerance trait. However, the underlying drivers for this functional trait are still a matter of debate; it has been proposed as an adaptive response to miscellaneous stress factors, such as nutrient scarcity, drought stress, herbivory, and cold tolerance, and due to the large investment costs of sclerophylly, it is generally associated with a longer leaf life span. Summary: The genus Quercus constitutes a unique living laboratory to understand global adaptive patterns along the leaf economic spectrum in forest trees. With more than 400 species, oaks are distributed along six zonobiomes and its versatility has resulted in a wide range of variations in leaf functional traits and contrasting adaptive strategies. However, although this wide variability cannot be explained alone by any of the ecological factors considered, such as drought, nutrient scarcity, low temperatures during vegetative period, and physical damage, neither any of them could be fully discarded. Noteworthy, our study also suggests that these constraints may have a synergistic effect, and from a functional point of view, we can conclude that in oaks leaf habit largely modulates the physiological implications of sclerophylly.

Atlantic forest and leaf traits: an overview

Leaf traits in Atlantic forest are modulated by environmental filters and biotic interactions. More information about leaf traits and nutrient are needed. Categorizing tree species according to their relationships with ecosystem processes in the Atlantic forest (AF) biome is fundamental to better understand their functionality in this high biodiversity hotspot. For categorizing vegetation, leaf traits are extremely useful and this is the first compilation of leaf traits focusing on photosynthesis, water relations, nutrients, and functional diversity across major AF vegetation types: (1) rainforest (ombrophilous dense and mixed ombrophilous Araucaria forest), (2) seasonal semi-deciduous forest, (3) deciduous forests, and (4) coastal plain forests (restingas). Species showed high plasticity of photosynthetic traits from early developmental stages, including adjustments in Rubisco carboxylation capacity, maximum photosynthetic capacity, photochemical and non-photochemical quenching, specific leaf area (SLA), and chlorophyll and spongy parenchyma thickness. Montane rainforest tree species showed lower SLA and longer leaf lifespan in comparison to lowland rainforest species. Tree communities are structured by environmental filtering leading to trait convergence in early successional stages or in stressful ecosystems such as restingas, and by biotic interactions leading to trait divergence in later stages. Altitudinal gradients in litter production and below/aboveground biomass stocks in AF show contrasting patterns in comparison to other tropical forests, with important consequences for responding to climate change. However, the identity of limiting elements to growth still represents a critical gap in leaf trait data for AF species. Other gaps in knowledge of AF leaf traits are highlighted for consideration in future studies.

<b>Morphogenesis and structure of Tifton 85 cultivated in subtropical climate and fertilized with nitrogen

The experiment evaluated the morphogenesis and structure of Tifton 85 (Cynodon spp.) cultivated in subtropical climate and fertilized with nitrogen (N). The experiment was a completely randomized design with four levels of N (Zero; 75; 150 or 225 kg ha-1) in nine replicates per area. The experimental animals were Suffolk female lambs. The grazing method was continuous to maintain the sward height at 15 cm ± 10%. The stem expansion increased by 0.000027 cm degree-days-1 to each kg N applied (linear model). According to nonlinear model, the highest stem expansion (0.0226 cm degree-days-1) was observed with the use 220.1 kg ha-1 N. The leaf lifespan fitted a linear regression model, with increase of 50 degree-days leaf -1, comparing the levels zero and 225 kg ha-1 of N. According to nonlinear regression model, the longest leaf lifespan (407.1 degree-days) was observed with the use 208.8 kg ha-1 N. With the maintenance of sward height at approximately 15 cm, we recommended to use 200 kg ha-1 N in Tifton 85 cultivated in subtropical climate.

To recycle or steal? Nutrient resorption in Australian and Brazilian mistletoes from three low‐phosphorus sites

Resorption is the process by which nutrients are withdrawn from leaves prior to leaf fall. Mistletoes are generally thought not to rely on nutrient resorption; being xylem‐tapping parasites, they instead derive the nutrients required for new growth from their host plant, at little or no cost. We measured nutrient (N, P, K, Ca, Mg) resorption in 18 parasitic mistletoe–host species pairs distributed across three sites with notably low‐P soil, also quantifying relationships with leaf lifespan (LL) and specific leaf area (SLA). There was little or no evidence of N, Ca or Mg resorption. By contrast, on average ∼30% of P and ∼20% of K were resorbed prior to leaf fall. Longer LL in mistletoes was associated with lower N and P concentrations in mistletoes and in host leaves. We provide evidence that, even though mistletoes are relatively inefficient in terms of nutrient resorption compared to non‐parasite species, on low‐P soils their ecological and evolutionary strategies for conserving phosphorous involve modulation of both leaf lifespan and P concentration in senesced leaves.

Age of maturity in alpine herbaceous perennials in the northwest Caucasus

Sod transplantation experiment was carried out for 24 years in alpine communities, Teberda Reserve, the North-West Caucasus, Russia. Age of maturity (AOM) was estimated as a period between first registra- tion of a species on.a permanent plot and flowering shoot appearance. Mean species AOM varied from 2.3 years (Antennaria dioica with clonal propagation) to 13.7 years for Taraxacum stevenii (non-clonal plant). General gradient of alpine plant traits and population strategies was determined. It represents a continuum ranging from ruderal--stress-tolerants (shorter AOM, clonality, shorter leaf life span, low rate of mycorrhizal infection, low regrowth ability, low generative shoot number, low specific leaf area and leaf nitrogen content) to competitor--stress-tolerants (longer AOM, longer leaf life span, higher rate of mycorrhizal infection, higher regrowth ability, high generative shoot number, high specific leaf area and leaf nitrogen content).

Leaf economics of evergreen and deciduous tree species along an elevational gradient in a subtropical mountain.

The ecophysiological mechanisms underlying the pattern of bimodal elevational distribution of evergreen tree species remain incompletely understood. Here we used leaf economics spectrum (LES) theory to explain such patterns. We measured leaf economic traits and constructed an LES for the co-existing 19 evergreen and 15 deciduous species growing in evergreen broad-leaved forest at low elevation, beech-mixed forest at middle elevation and hemlock-mixed forest at high elevation in Mao'er Mountain, Guangxi, Southern China (25°50'N, 110°49'E). Leaf economic traits presented low but significant phylogenetic signal, suggesting trait similarity between closely related species. After considering the effects of phylogenetic history, deciduous species in general showed a more acquisitive leaf strategy with a higher ratio of leaf water to dry mass, higher leaf nitrogen and phosphorous contents, higher photosynthetic and respiratory rates and greater photosynthetic nitrogen-use efficiency. In contrast, evergreen species exhibited a more conservative leaf strategy with higher leaf mass per area, greater construction costs and longer leaf life span. With the elevation-induced decreases of temperature and soil fertility, both evergreen and deciduous species showed greater resource conservation, suggesting the increasing importance of environmental filtering to community assembly with increasing elevation. We found close inter-specific correlations between leaf economic traits, suggesting that there are strong genetic constraints limiting the independent evolution of LES traits. Phylogenetic signal increased with decreasing evolutionary rate across leaf economic traits, suggesting that genetic constraints are important for the process of trait evolution. We found a significantly positive relationship between primary axis species score (PASS) distance and phylogenetic distance across species pairs and an increasing average PASS distance between evergreen and deciduous species with increasing elevation, implying that the frequency of distantly related evergreen and deciduous pairs with wide spreading of leaf economic values increases with increasing elevation. Our findings thus suggest that elevation acts as an environmental filter to both select the locally adapted evergreen and deciduous species with sufficient phylogenetic variation and regulate their distribution along the elevational gradient based on their coordinated spreading of phylogenetic divergence and leaf economic variation.

Ontogenetic transition in leaf traits: a new cost associated with the increase in leaf longevity

Aims Recent work has identified a worldwide ‘economics’ spectrum of correlated leaf traits that mainly reflects the compromises between maximizing leaf longevity and short-term productivity. However, during the early stages of tree growth different species tend to exhibit a common strategy, because competition for soil water and nutrients forces the maximization of short-term productivity owing to the need for rapid growth during the most vulnerable part of the tree’s life cycle. Accordingly, our aim here was to compare the variations that occur during ontogeny in the different leaf traits (morphology and leaf chemical composition) of several coexisting Mediterranean woody species differing in their leaf life spans and to test our hypothesis that tree species with a long leaf life span should exhibit larger shifts in leaf characteristics along ontogeny. Methods Six Mediterranean tree species differing in leaf life span, selected from three plots located in central-western Spain, were studied during three growth stages: seedlings, juveniles and mature trees. Leaf life span, leaf morphology (leaf area, dry weight, thickness and mass per unit area) and chemical composition (N and fibre concentrations) were measured in all six species. The magnitude of the ontogenetic changes in the different traits was estimated and related to the mean leaf longevity of the different species. Important Findings Along ontogeny, strong changes were observed in all variables analysed. The early growth stages showed lower leaf thickness, leaf thickness and mass per unit area and N, cellulose and hemicellulose concentrations than mature trees, but a higher lignin content. However, these changes were especially marked in species with a longer leaf life span at maturity. Interspecific differences in leaf life span, leaf morphology and chemical composition were stronger at the mature stage than at the seedling stage. We conclude that greater plasticity and more intense strategy shifts along ontogeny are necessarily associated with long leaf life span. Our results thus provide a new aspect that should be incorporated into the analysis of the costs and benefits associated with the different strategies related to leaf persistence displayed by the different species. Accordingly, the intensity of the alterations in leaf traits among different growth stages should be added to the suite of traits that change along the leaf economics spectrum.

The allocation of assimilated carbon to shoot growth: in situ assessment in natural grasslands reveals nitrogen effects and interspecific differences

In grasslands, sustained nitrogen loading would increase the proportion of assimilated carbon allocated to shoot growth (A shoot), because it would decrease allocation to roots and also encourage the contribution of species with inherently high A shoot. However, in situ measurements of carbon allocation are scarce. Therefore, it is unclear to what extent species that coexist in grasslands actually differ in their allocation strategy or in their response to nitrogen. We used a mobile facility to perform steady-state (13)C-labeling of field stands to quantify, in winter and autumn, the daily relative photosynthesis rate (RPR~tracer assimilated over one light-period) and A shoot (~tracer remaining in shoots after a 100 degree days chase period) in four individual species with contrasting morpho-physiological characteristics coexisting in a temperate grassland of Argentina, either fertilized or not with nitrogen, and either cut intermittently or grazed continuously. Plasticity in response to nitrogen was substantial in most species, as indicated by positive correlations between A shoot and shoot nitrogen concentration. There was a notable interspecific difference: productive species with higher RPR, enhanced by fertilization and characterized by faster leaf turnover rate, allocated ~20% less of the assimilated carbon to shoot growth than species of lower productivity (and quality) characterized by longer leaf life spans and phyllochrons. These results imply that, opposite to the expected response, sustained nitrogen loading would change little the A shoot of grassland communities if increases at the species-level are offset by decreases associated with replacement of 'low RPR-high A shoot' species by 'high RPR-low A shoot' species.

Functional relationships between leaf hydraulics and leaf economic traits in response to nutrient addition in subtropical tree species

Leaves can be both a hydraulic bottleneck and a safety valve against hydraulic catastrophic dysfunctions, and thus changes in traits related to water movement in leaves and associated costs may be critical for the success of plant growth. A 4-year fertilization experiment with nitrogen (N) and phosphorus (P) addition was done in a semideciduous Atlantic forest in northeastern Argentina. Saplings of five dominant canopy species were grown in similar gaps inside the forests (five control and five N + P addition plots). Leaf lifespan (LL), leaf mass per unit area (LMA), leaf and stem vulnerability to cavitation, leaf hydraulic conductance (K(leaf_area) and K(leaf_mass)) and leaf turgor loss point (TLP) were measured in the five species and in both treatments. Leaf lifespan tended to decrease with the addition of fertilizers, and LMA was significantly higher in plants with nutrient addition compared with individuals in control plots. The vulnerability to cavitation of leaves (P50(leaf)) either increased or decreased with the nutrient treatment depending on the species, but the average P50(leaf) did not change with nutrient addition. The P50(leaf) decreased linearly with increasing LMA and LL across species and treatments. These trade-offs have an important functional significance because more expensive (higher LMA) and less vulnerable leaves (lower P50(leaf)) are retained for a longer period of time. Osmotic potentials at TLP and at full turgor became more negative with decreasing P50(leaf) regardless of nutrient treatment. The K(leaf) on a mass basis was negatively correlated with LMA and LL, indicating that there is a carbon cost associated with increased water transport that is compensated by a longer LL. The vulnerability to cavitation of stems and leaves were similar, particularly in fertilized plants. Leaves in the species studied may not function as safety valves at low water potentials to protect the hydraulic pathway from water stress-induced cavitation. The lack of rainfall seasonality in the subtropical forest studied probably does not act as a selective pressure to enhance hydraulic segmentation between leaves and stems.

Resource‐use strategies of native and invasive plants in Eastern North American forests

Studies in disturbed, resource-rich environments often show that invasive plants are more productive than co-occurring natives, but with similar physiological tradeoffs. However, in resource-limited habitats, it is unclear whether native and invasive plants have similar metabolic constraints or if invasive plants are more productive per unit resource cost - that is, use resources more efficiently. Using a common garden to control for environment, we compared leaf physiological traits relating to resource investments, carbon returns, and resource-use efficiencies in 14 native and 18 nonnative invasive species of common genera found in Eastern North American (ENA) deciduous forest understories, where growth is constrained by light and nutrient limitation. Despite greater leaf construction and nitrogen costs, invaders exhibited greater instantaneous photosynthetic energy-use efficiency (PEUE) and marginally greater photosynthetic nitrogen-use efficiency (PNUE). When integrated over leaf lifespan (LL), these differences were magnified. Differences in efficiency were driven by greater productivity per unit leaf investment, as invaders exhibited both greater photosynthetic abilities and longer LL. Our results indicate that woody understory invaders in ENA forests are not constrained to the same degree by leaf-based metabolic tradeoffs as the native understory flora. These strategy differences could be attributable to pre-adaptation in the native range, although other explanations are possible.

Influence of Disturbance on Temperate Forest Productivity

Climate, tree species traits, and soil fertility are key controls on forest productivity. However, in most forest ecosystems, natural and human disturbances, such as wind throw, fire, and harvest, can also exert important and lasting direct and indirect influence over productivity. We used an ecosystem model, PnET-CN, to examine how disturbance type, intensity, and frequency influence net primary production (NPP) across a range of forest types from Minnesota and Wisconsin, USA. We assessed the importance of past disturbances on NPP, net N mineralization, foliar N, and leaf area index at 107 forest stands of differing types (aspen, jack pine, northern hardwood, black spruce) and disturbance history (fire, harvest) by comparing model simulations with observations. The model reasonably predicted differences among forest types in productivity, foliar N, leaf area index, and net N mineralization. Model simulations that included past disturbances minimally improved predictions compared to simulations without disturbance, suggesting the legacy of past disturbances played a minor role in influencing current forest productivity rates. Modeled NPP was more sensitive to the intensity of soil removal during a disturbance than the fraction of stand mortality or wood removal. Increasing crown fire frequency resulted in lower NPP, particularly for conifer forest types with longer leaf life spans and longer recovery times. These findings suggest that, over long time periods, moderate frequency disturbances are a relatively less important control on productivity than climate, soil, and species traits.

Effects of experimental warming on phenology, growth and gas exchange of treeline birch (Betula utilis) saplings, Eastern Tibetan Plateau, China

Mountain birch (Betula utilis) is the most important treeline species in alpine forests of southwestern China. In order to understand the effects of future warming on treeline birch, this study was conducted to examine the effects of experimental warming on leaf phenology, growth and gas exchange of B. utilis saplings using the open top chamber (OTC) method in a treeline ecotone of eastern Tibetan Plateau. The OTCs enhanced daily mean air temperature by 2.9 K throughout the growing season. Conversely, soil moisture within the OTCs on average declined by 3% over the experimental period. Experimental warming did not affect the timing of bud break, although treeline birch saplings growing in the OTCs manifested later leaf abscission, resulting in longer leaf life span. Artificial warming significantly accelerated the leaf and shoot growth rates of treeline birch saplings, resulting in larger leaf area and longer shoot elongation late in the growing season. Moreover, experimental warming significantly reduced the leaf fluctuating asymmetry (FA) and tended to increase specific leaf area (SLA). Moreover, elevated temperatures significantly enhanced the transpiration rate (E), stomatal conductance (g s), maximum net assimilation rate (A max), dark respiration rate (R d) and apparent quantum yield (AQY) but did not influence the light compensation point (LCP) and light saturation point (LSP) of treeline birch saplings. Taken together, our results indicated that short-term experimental warming markedly altered structural/functional leaf traits and enhanced photosynthetic capacity of treeline birch saplings; such positive responses in treeline birch would be favorable for the growth of this species under future warmer world.

Relative Growth Rate and Functional Traits of a Hybrid Species Reflect Adaptation to a Low-Fertility Habitat

Hybrid species provide a unique opportunity to examine trait evolution. We previously demonstrated that the hybrid species Helianthus anomalus is more fit than its ancestral parents, Helianthus annuus and Helianthus petiolaris, in its habitat and hypothesized that nutrient limitations have been a major selective agent in that adaptive evolution. In this glasshouse study, we asked whether H. anomalus has more conservative growth and resource use traits than its ancestral parents, as expected for low-nutrient-adapted species. Helianthus anomalus had a lower maximum relative growth rate, leaf area ratio, and specific leaf area as well as less of a decrease in relative growth rate with nitrogen limitation than its ancestral parents. Helianthus anomalus also had higher nitrogen use efficiency, largely because of a longer mean residence time, as well as greater leaf toughness and longer leaf life span. However, H. anomalus did not have a lower photosynthetic rate, higher ratio of fine root length to leaf area, ...