Leaf-level Traits Research Articles

Leaf- and shoot-level plasticity in response to different nutrient and water availabilities

Phenotypic plasticity in response to environmental variation occurs at all levels of organization and across temporal scales within plants. However, the magnitude and functional significance of plasticity is largely unexplored in perennial species. We measured the plasticity of leaf- and shoot-level physiological, morphological and developmental traits in nursery-grown Populus deltoides Bartr. ex Marsh. individuals subjected to different nutrient and water availabilities. We also examined the extent to which nutrient and water availability influenced the relationships between these traits and productivity. Populus deltoides responded to changes in resource availability with high plasticity in shoot-level traits and moderate plasticity in leaf-level traits. Although shoot-level traits generally correlated strongly with productivity across fertilization and irrigation treatments, few leaf-level traits correlated with productivity, and the relationships depended on the resource examined. In fertilized plants, leaf nitrogen concentration was negatively correlated with productivity, suggesting that growth, rather than enhanced leaf quality, is an important response to fertilization in this species. With the exception of photosynthetic nitrogen-use efficiency, traits associated with resource conservation (leaf senescence rate, water-use efficiency and leaf mass per area) were uncorrelated with short-term productivity in nutrient- and water-stressed plants. Our results suggest that plasticity in shoot-level growth traits has a greater impact on plant productivity than does plasticity in leaf-level traits and that the relationships between traits and productivity are highly resource dependent.

CONTRASTING PLANT PHYSIOLOGICAL ADAPTATION TO CLIMATE IN THE NATIVE AND INTRODUCED RANGE OFHYPERICUM PERFORATUM

How introduced plants, which may be locally adapted to specific climatic conditions in their native range, cope with the new abiotic conditions that they encounter as exotics is not well understood. In particular, it is unclear what role plasticity versus adaptive evolution plays in enabling exotics to persist under new environmental circumstances in the introduced range. We determined the extent to which native and introduced populations of St. John's Wort (Hypericum perforatum) are genetically differentiated with respect to leaf-level morphological and physiological traits that allow plants to tolerate different climatic conditions. In common gardens in Washington and Spain, and in a greenhouse, we examined clinal variation in percent leaf nitrogen and carbon, leaf delta(13)C values (as an integrative measure of water use efficiency), specific leaf area (SLA), root and shoot biomass, root/shoot ratio, total leaf area, and leaf area ratio (LAR). As well, we determined whether native European H. perforatum experienced directional selection on leaf-level traits in the introduced range and we compared, across gardens, levels of plasticity in these traits. In field gardens in both Washington and Spain, native populations formed latitudinal clines in percent leaf N. In the greenhouse, native populations formed latitudinal clines in root and shoot biomass and total leaf area, and in the Washington garden only, native populations also exhibited latitudinal clines in percent leaf C and leaf delta(13)C. Traits that failed to show consistent latitudinal clines instead exhibited significant phenotypic plasticity. Introduced St. John's Wort populations also formed significant or marginally significant latitudinal clines in percent leaf N in Washington and Spain, percent leaf C in Washington, and in root biomass and total leaf area in the greenhouse. In the Washington common garden, there was strong directional selection among European populations for higher percent leaf N and leaf delta(13)C, but no selection on any other measured trait. The presence of convergent, genetically based latitudinal clines between native and introduced H. perforatum, together with previously published molecular data, suggest that native and exotic genotypes have independently adapted to a broad-scale variation in climate that varies with latitude.

Interspecific competition in a pecan–cotton alleycropping system in the southern United States: Production physiology

A study was conducted on a Red Bay sandy loam soil (Rhodic Paleudult) in Jay, Florida, USA, to investigate how interspecific interactions between pecan ( Carya illinoensis K. Koch) and cotton ( Gossypium hirsutum L.) would affect cotton leaf morphology and gas exchange and thereby biomass and lint yield. We quantified specific leaf area (SLA), specific leaf nitrogen (SLN), net photosynthesis (A), transpiration, stomatal conductance, and net canopy photosynthetic index (CNPI) from cotton with and without aboveground and belowground interactions. To separate roots of cotton and pecan, polyethylene-lined trenches were installed (barrier treatment) parallel to tree rows in half the number of plots. Results showed that SLA for barrier and nonbarrier plants was 61% and 47% higher, respectively, compared with the monoculture cotton. Monoculture plants exhibited higher CNPI (70.7 μmol·m–2·s–1) compared with the barrier (52.7 μmol·m–2·s–1) and nonbarrier plants (18.3 μmol·m–2·s–1). SLN was similar for both the barrier and nonbarrier plants; however, it was lower than the monoculture. A positive curvilinear relationship between A and SLN was observed, with peak A (28 μmol·m–2·s–1) observed between 2.2 and 2.4 mg N·m–2. Significant curvilinear relationships between CNPI and aboveground biomass and lint yield were also observed for all treatments. These findings indicate that competitive interactions in alleycropping regulate leaf level traits such as SLA and SLN by altering water and light availability, which in turn exert a profound influence on aboveground biomass and lint yield for cotton plants.

Production physiology of three fast-growing hardwood species along a soil resource gradient

We determined how specific leaf area (SLA), specific leaf nitrogen (SLN), leaf area index (LAI), light-saturated photosynthesis (Amax) and aboveground net primary productivity (ANPP) of three commercially important hardwood species, eastern cottonwood (Populus deltoides Bartr.), American sycamore (Platanus occidentalis L.) and cherrybark oak (Quercus falcata var.pagodafolia Ell.), vary across a soil resource gradient. Five treatments were applied in a randomized block design (control, irrigation only (IRR), and irrigation plus fertilization with 56, 112 or 224 kg N ha-1 year-1 (N56, N112 and N224)) with four replications per species. When trees were 6 years old, Amax, SLA, SLN, LAI and ANPP were quantified during peak leaf production within a single growing season. In all species, Amax for sun leaves was significantly higher than for shade leaves (34, 32 and 29 micromol m2 s-1 versus 27, 23 and 23 micromol m2 s-1 for cottonwood, cherrybark oak and sycamore sun and shade leaves, respectively) and tended to plateau in the N112 treatment. The SLA was significantly lower in sun than in shade leaves and reached a plateau in IRR-treated cottonwood and sycamore, and in N56-treated oak. Values of SLN peaked in the N122 treatment for cottonwood sun leaves (1.73 g N m2) and in the N56 treatment for sycamore and oak (1.54 and 1.90 g N m2, respectively). In sun and shade leaves of all species, Amax increased with increasing SLN. Cherrybark oak LAI reached a plateau across the resource gradient in the N56 treatment, whereas cottonwood and sycamore LAI reached a plateau in the IRR treatment. All species exhibited significant curvilinear relationships between canopy Amax and ANPP. These findings indicate that nutrients and water regulate leaf-level traits such as SLA and SLN, which in turn influence LAI and canopy photosynthesis, thereby affecting ANPP at the tree and stand levels.

A plot-based method for rapid estimation of forest canopy chemistry

In this study we present a rapid method to scale the leaf-level chemistry of forest stands to the whole-canopy level. The method combines simple leaf-level measurements of mass and chemistry with a camera-based technique to estimate the fractional distribution of species' foliage area in a forest canopy. Results using this methodology for the estimation of whole-canopy N concentration (g/100 g) are presented and are shown to be comparable with those derived directly from litter fall collection. The ability to efficiently scale leaf-level traits to whole forest canopies enhances our ability to examine key relationships associated with these traits at various levels from the leaf to the forest stand and, with remote sensing technologies, to larger landscapes.

Ontogenetic patterns of leaf CO2 exchange, morphology and chemistry in Betula pendula trees

In order to explore ontogenetic variation in leaf-level physiological traits of Betula pendula trees, we measured changes in mass- (Amass) and area-based (Aarea) net photosynthesis under light-saturated conditions, mass- (RSmass) and area-based (RSarea) leaf respiration, relative growth rate, leaf mass per area (LMA), total nonstructural carbohydrates (TNC), and macro- and micronutrient concentrations. Expanding leaves maintained high rates of Aarea, but due to high growth respiration rates, net CO2 fixation occurred only at irradiances >200 µmol photons m–2 s–1. We found that full structural leaf development is not a necessary prerequisite for maintaining positive CO2 balance in young birch leaves. Maximum rates of Aarea were realized in late June and early July, whereas the highest values of Amass occurred in May and steadily declined thereafter. The maintenance respiration rate averaged ≈8 nmol CO2 g–1 s–1, whereas growth respiration varied between 0 and 65 nmol CO2 g–1 s–1. After reaching its lowest point in mid-June, leaf respiration increased gradually until the end of the growing season. Mass and area-based dark respiration were significantly positively correlated with LMA at stages of leaf maturity, and senescence. Concentrations of P and K decreased during leaf development and stabilized or increased during maturity, and concentrations of immobile elements such as Ca, Mn and B increased throughout the growing season. Identification of interrelations between leaf development, CO2 exchange, TNC and leaf nutrients allowed us to define factors related to ontogenetic variation in leaf-level physiological traits and can be helpful in establishing periods appropriate for sampling birch leaves for diagnostic purposes such as assessment of plant and site productivity or effects of biotic or abiotic factors.

On the Relationship between Shade Tolerance and Shade Avoidance Strategies in Woodland Plants

Decreasing light availability with time is one of the primary factors that drives autogenic succession in woodland communities. In many successions, the pattern of species replacement appears to be largely a consequence of a negative correlation across species between adaptation to high irradiance and adaptation to low irradiance (Bazzaz 1979, Oliver and Larson 1990, Kobe et al. 1995). The mechanistic basis for this negative correlation can be explained with respect to at least two distinct fitness components: 1) the maximization of light interception and 2) the maximization of net carbon fixation per unit leaf protein (Givnish 1988). A plant grown under high irradiance can maximize light interception through a highly branched, bushy growth form (Steingraeber et al. 1979, Shukla and Ramakrishnan 1986). Maximum net carbon fixation under high irradiance is achieved through sun-adapted leaf physiology (Bj6rkman 1981). In contrast, for a plant grown under low irradiance, light interception is maximized through strong vertical growth (King 1990), and net carbon fixation is maximized through shadeadapted leaf physiology (Bjdrkman 1981). The term has long been used to describe the suite of leaf-level traits that afford maximal net carbon fixation under low irradiance. Less commonly, the analogous term has been used to describe the suite of architectural traits that contribute to strong vertical growth in plants grown under low irradiance (Grime 1966, Smith 1986, 1994). Although many components of shade tolerance and shade avoidance have been studied along light gradients, the relationship between these strategies has received very little theoretical and empirical attention (Kuppers 1994). Do they represent alternative adaptations to light limitation with patterns of covariance that reflect tradeoffs through life history or phylogenetic constraints (Smith 1986)? Alternatively, are the distributions across taxa, of shade avoidance and shade tolerance, independent of each other? In this paper, we briefly review the mechanistic basis of shade tolerance and shade avoidance to illustrate that both of these strategies can be expected to impose costs (trade-offs) to a plant in terms of reduced relative growth rate under high irradiance (Fig. 1). By interpreting these costs in the context of different selection regimes within vegetation, we arrive at the prediction that shade tolerance and shade avoidance strategies should be generally negatively correlated along light gradients.