Greater Specific Leaf Area Research Articles

Temperature response of CO2 exchange and dissolved organic carbon release in a maritime Antarctic tundra ecosystem

We examined the temperature response of CO2 exchange and soil biogeochemical processes in an Antarctic tundra ecosystem using laboratory incubations of intact tundra cores. The cores were collected from tundra near Anvers Island along the west coast of the Antarctic Peninsula that was dominated by the vascular plants Colobanthus quitensis and Deschampsia antarctica. After the initial 8-week incubation at moderate growth temperatures (12/7°C, day/night), the tundra cores were incubated for another 8 weeks at either a higher (17/12°C) or lower (7/4°C) temperature regime. Temperature responses of CO2 exchange were measured at five temperatures (4, 7, 12, 17, and 27°C) following each incubation and soil leachates were collected biweekly over the second incubation. Daytime net ecosystem CO2 exchange (NEE) per unit core surface area was higher across the five measurement temperatures after the warmer incubation (17/12°C > 7/4°C). Responses of ecosystem respiration (ER) were similar at each measurement temperature irrespective of incubation temperature regimes. ER, expressed on a leaf-area basis, however, was significantly lower following the warmer incubation, suggesting a downregulation of ER. Warmer incubation resulted in a greater specific leaf area and N concentration, and a lower δ13C in live aboveground C. quitensis, but a higher δ13C in D. antarctica, implying species-specific responses to warming. Concentrations of dissolved organic C and N and inorganic N in soil leachates showed that short-term temperature changes had no noticeable effect on soil biogeochemical processes. The results suggest that downregulation of ER, together with plant species differences in leaf-area production and N use, can play a crucial role in constraining the C-cycle response of Antarctic tundra ecosystems to warming.

RELATEDNESS AND ENVIRONMENT AFFECT TRAITS ASSOCIATED WITH INVASIVE AND NONINVASIVE INTRODUCED COMMELINACEAE

Understanding the traits of invasive species may improve the ability to predict, prevent, and manage invasions. I compared morphological and performance traits of five congeneric pairs of invasive and noninvasive Commelinaceae across a factorial experiment using a range of water and nutrient availabilities. Invasive species had greater fecundity and vegetative reproduction than their noninvasive relatives. The invasive species also had higher relative growth rates, greater specific leaf area, and more plastic root-to-shoot ratios than noninvasive species. However, whether a trait was associated with invasiveness often depended on both environment and relatedness. Invasives had greater sexual and vegetative reproduction, higher specific leaf area, and greater relative growth rates than noninvasive congeners, but only in some environments. Differences between invasive and noninvasive taxa were greatest at high nutrient availabilities. These results suggest that studies of invasive species' traits must incorporate information on conditions under which the trait was measured. In addition, incorporating information on relatedness improved our ability to detect associations between species traits, such as specific leaf area and relative growth rate, and invasiveness, suggesting that such information may be required for a complete understanding of what makes a species invasive.

Native and introduced populations of Solidago gigantea differ in shoot production but not in leaf traits or litter decomposition

Summary Invasive alien plants tend to have a greater specific leaf area and more nutrient‐rich tissues than the invaded native vegetation. To test whether these traits differ between introduced and native populations of the same species, we compared 20 European (introduced) and 22 American (native) populations of Solidago gigantea Aiton (Asteraceae) in a common‐garden experiment. Five plants per population were grown for 2 years in pots and for one summer outdoors in nutrient‐rich soil. We recorded shoot number and biomass, leaf production and senescence, flowering, leaf morphology and nutrient concentrations of leaves and litter. In laboratory assays, we compared litter decomposition and nutrient mineralization. Shoot growth and leaf traits varied three‐ to 10‐fold among the 42 populations. European plants produced, on average, more shoots than American plants, but did not differ in shoot size, leaf traits or litter decomposition. The shoot number and total shoot biomass per plant in the experiment correlated positively with the number of new rhizomes produced by shoots of the same populations at their original field sites. We conclude that introduced S. gigantea populations tend to produce more shoots through clonal growth than native populations. This may increase their ability to compete against the established vegetation in dense stands or at nutrient‐poor sites.

Adaptations of strangler figs to life in the rainforest canopy.

Figs are rainforest keystone species. Non-strangler figs establish on the forest floor; strangler figs establish epiphytically, followed by a dramatic transition from epiphyte to free-standing tree that kills its hosts. Free-standing figs display vigorous growth and resource demand suggesting that epiphytic strangler figs require special adaptations to deal with resource limitations imposed by the epiphytic environment. We studied epiphytic and free-standing strangler figs, and non-strangler figs in tropical rainforest and in cultivation, as well as strangler figs in controlled conditions. We investigated whether the transition from epiphyte to free-standing tree is characterised by morphological and physiological plasticity. Epiphyte substrate had higher levels of plant-available ammonium and phosphate, and similar levels of nitrate compared with rainforest soil, suggesting that N and P are initially not limiting resources. A relationship was found between taxonomic groups and plant N physiology; strangler figs, all members of subgenus Urostigma, had mostly low foliar nitrate assimilation rates whereas non-strangler figs, in subgenera Pharmacocycea, Sycidium, Sycomorus or Synoecia, had moderate to high rates. Nitrate is an energetically expensive N source, and low nitrate use may be an adaptation of strangler figs for conserving energy during epiphytic growth. Interestingly, significant amounts of nitrate were stored in fleshy taproot tubers of epiphytic stranglers. Supporting the concept of plasticity, leaves of epiphytic Ficus benjamina L. had lower N and C content per unit leaf area, lower stomatal density and 80% greater specific leaf area than leaves of conspecific free-standing trees. Similarly, glasshouse-grown stranglers strongly increased biomass allocation to roots under water limitation. Epiphytic and free-standing F. benjamina had similar average foliar δ13C, but epiphytes had more extreme values; this indicates that both groups of plants use the C3 pathway of CO2 fixation but that water availability is highly variable for epiphytes. We hypothesise that epiphytic figs use fleshy stem tubers to avoid water stress, and that nitrate acts as an osmotic compound in tubers. We conclude that strangler figs are a unique experimental system for studying the transition from rainforest epiphyte to tree, and the genetic and environmental triggers involved.

Response of Soybean, Sugar Beet and Spring Wheat to the Combination of Reduced Tillage and Fertilization Practices

To evaluate the feasibility of conservation tillage in combination with reduced biocide and fertilization regimes, we conducted a field experiment using conventional and reduced tillage, with or without reduced biocide, and fertilization regimes for growing soybean, sugar beet, and spring wheat in this order for three years. Root biomass and sugar yield of sugar beet did not differ with any combinations of conservation practice. Although leaf biomass was significantly reduced under reduced chemical fertilization (replaced partially with manure compost), it was compensated by a greater specific leaf area. Early crop growth of soybean, and spring wheat was increased to some extent under reduced tillage, which indicated a better nutrient utilization, as well when combined with reduced biocide application. However, reduced fertilization could not supply as much nitrogen as conventional chemical fertilization especially in the combination with reduced tillage. Larger amount or long-term application of organic fertilization may be necessary under reduced tillage compared to the requirement under conventional tillage to compensate for lower rate of nitrogen release from organic matter.

Evaluating the Impact of a Trait for Increased Specific Leaf Area on Wheat Yields Using a Crop Simulation Model

Early vigorous growth (also called early vigor) has been suggested as a characteristic to increase yields of wheat (Triticum aestivum L.) in rainfed environments. Breeders have tried to incorporate early vigor by selecting for increased specific leaf area (SLA). A crop simulation model (APSIM‐Nwheat) was used to evaluate the role of SLA on potential yield. Increased early vigor was simulated by increasing the SLA trait in the model. Grain yields were simulated for the mediterranean climate of Western Australia with winter‐dominant rainfall, for New South Wales with evenly distributed rainfall, and for Queensland with a subtropical climate with summer‐dominant rainfall. Independent, multiseason simulations were performed with historical weather records. The largest average yield increase with greater SLA was 15% on the sandy soil in Western Australia but only with additional N inputs. In the low‐rainfall region of Western Australia, with low N input, a yield increase of up to 5% was achieved on the sandy soil while yield responses were negative on the clay soil. With increased N application, the SLA trait increased yields by up to 7% on the clay soils in this region. The increased SLA trait failed to increase yields and even decreased yields on average in New South Wales and in Queensland, except in above‐average rainfall seasons and only at high N application. Crop management, in particular crop nutrition, is an important factor determining yield expression of new physiological traits and needs to be considered when evaluating traits.

Ecophysiological characteristics of four maize hybrids and Abutilon theophrasti

Combining empirical research with simulation modelling may improve our understanding of the dynamics of crop:weed competition and for testing hypotheses on the importance of specific traits for enhancing crop performance in mixtures. Two field experiments were conducted to quantify and compare estimates of traits important for radiation interception and utilization in four maize hybrids and Abutilon theophrasti grown in monoculture. Early leaf area growth rate did not vary among maize hybrids within a year, but varied among years. The response of CO2 assimilation rate to absorbed radiation and leaf nitrogen content did not differ among hybrids. Abutilon theophrasti and two old maize hybrids partitioned more new biomass to stem relative to reproductive organs than newer hybrids. Old hybrids had greater specific leaf area during the period of most rapid growth, grew taller, and leaf area was distributed higher in their canopy. Extinction coefficients for diffuse radiation did not differ among hybrids or between years. Results suggest that these four maize hybrids may differ in their ability to intercept incident radiation, which may influence their ability to compete for light.

The interactive effect of irradiance and source of nitrogen on growth and root respiration ofCalamagrostis epigejos

SUMMARYGrowth and respiratory processes ofCalamagrostis epigejos(L.) Roth were studied as part of a comprehensive ecological project aimed at explaining the acclimation potential of some perennial grasses upon changes in climatic and edaphic factors after logging activity in forests. We investigated the relative growth rate (RGR), the respiration rate and the contribution of the different respiratory pathways in roots ofCalamagrostis epigejosgrown at two levels of irradiance and with nitrate or ammonium as N‐source. The respiration rate as well as the RGR decreased significantly but the leaf area ratio (LAR) increased upon transfer to shade. The LAR increase was caused by both a greater specific leaf area (SLA) and a greater leaf weight ratio (LWR). The relative contribution of the alternative (AP) and the cytochrome pathways to total respiration rate was the same in both radiation regimes. The ammonium form of N‐nutrition had a significant stimulative effect on AP activity in shaded plants. The AP capacity was significantly higher in ammonium‐fed plants grown at both irradiance levels. A possible role of the AP in plants with low energy input and with ammonium ions as N‐source is discussed.

Facilitation between Higher Plant Species in a Semiarid Environment

A major role of facilitation between higher plant species, particularly in stressful environments, has recently been reported in several papers that suggest that beneficial interactions could be more important in the dynamics of plant communities than has been recognized to date. In a semiarid region in southeastern Spain we determined the effect of a leguminous shrub, Retama sphaerocarpa, on its environment, testing the hypothesis that facilitation by the shrub of one species of its understory, Marrubium vulgare, is reciprocal and that the shrub benefits from sheltering herbs beneath its canopy. Marrubium plants under Retama had greater specific leaf area, leaf mass, shoot mass, leaf area, more flowers, a higher nitrogen (N) concentration in leaf tissue, and more N per plant than isolated plants, suggesting a facilitative effect of Retama on Marrubium including increased availability of resources. Biomass of 1—yr—old cladodes, total biomass, total nitrogen content of 3—yr—old branches, and shoot water potential at midday were higher for Retama shrubs with Marrubium beneath them than in shrubs without Marrubium. Retama strongly improved its own environment, facilitated the growth of Marrubium and other species underneath its canopy, and at the same time obtained benefits from sheltering herbs underneath. The interaction between these two species was indirect, associated with differences in soil properties and with improved nutrient availability under shrubs compared with plants growing on their own. We propose that the mutual benefit of the association between Retama and Marrubium is best termed a facultative mutualism. Each partner benefits from greater availability of resources in the “island of fertility” that results from their association. If beneficial plant—plant interactions as described here are widespread, positive relationships may have a major role in determining the pattern and structure of plant communities.

RESPONSE OF FIELD-GROWN NORWAY MAPLE AND GREEN ASH TREES TO WHITE AND BROWN TREESHELTERS

We investigated microclimate, gas exchange, and growth of field-grown Norway maple (Acer platanoides) and green ash (Fraxinus pennsylvanica) trees in brown, white, or no treeshelters. Microclimate, tree growth, and gas exchange measurements were taken summer and winter. Treeshelter microclimate was greenhouse-like compared to ambient conditions, as short-wave radiation (S↓) was lower, and midday air temperature and relative humidity were higher. In both species, this resulted in less trunk growth and greater specific leaf area, which are growth responses characteristic of shade acclimation. Treeshelter microclimate did, however, substantially increase shoot elongation and stomatal conductance, but did not increase photosynthesis when compared to trees grown without shelters. White shelters allowed 25% more penetration of S↓ than brown shelters, but tree growth and climatic variables did not differ with treeshelter color. Stomatal conductance, however, was higher for trees in white shelters. Treeshelters also appeared to have a negative effect on plant hardiness. New shoot growth in shelters was more winter-damaged, particularly in maple, than nonsheltered trees. This may be related to winter bark (Tb) and air temperature (Ta). Winter midday Tb on trees grown in shelters was up to 15C higher than Tb on trees outside shelters, while midday Ta inside treeshelters was up to 20C higher than Ta outside treeshelters.

Photosynthesis-nitrogen relations in Amazonian tree species

Among species, photosynthetic capacity (Amax) is usually related to leaf nitrogen content (N), but variation in the species-specific relationship is not well understood. To address this issue, we studied Amax-N relationships in 23 species in adjacent Amazonian communities differentially limited by nitrogen (N), phosphorus (P), and/or other mineral nutrients. Five species were studied in each of three late successional forest types (Tierra Firme, Caatinga and Bana) and eight species were studied on disturbed sites (cultivated and early secondary successional Tierra Firme plots). Amax expressed on a mass basis (Amass) was correlated (p<0.05) with Nmass in 17 of 23 species, and Amax on an area basis (Aarea) was correlated (p<0.05) with Narea in 21 of 23 species. The slopes of Amax-N relationships were greater and intercepts lower for disturbance adapted early successional species than for late successional species. On a mass basis, the Amax-N slope averaged ≈15 μmol CO2 [g N]-1 s-1 for 7 early secondary successional species and ≈4 μmol CO2 [g N]-1 s-1 for 15 late successional species, respectively. Species from disturbed sites had shorter leaf life-span and greater specific leaf area (SLA) than late successional species. Across all 23 species, the slope of the Amass-Nmass relationship was related (p<0.001) positively to SLA (r2=0.70) and negatively to leaf life-span (r2=0.78) and temporal niche during secondary succession (years since cutting-and-burning, r2=0.90). Thus, disturbance adapted early successional species display a set of traits (short leaf life-span, high SLA and Amax and a steep slope of Amax-N) conducive to resource acquisition and rapid growth in their high resource regeneration niches. The significance and form of the Amax-N relationship were associated with the relative nutrient limitations in the three late successional communities. At species and community levels, Amax was more highly dependent on N in the N-limited Caatinga than in the P-and N-limited Bana and least in the P-and Ca-limited Tierra Firme on oxisol-and differences among these three communities in their massbased Amax-N slope reflects this pattern (6.0, 2.4, and 0.7 μmol CO2 [g N]-1 s-1, respectively). Among all 23 species, the estimated leaf Nmass needed to reach compensation (net photosynthesis ≈ zero) was positively related to the Amass-Nmass slope and to dark respiration rates and negatively related to leaf life-span. Variation among species in the Amax-N slope was well correlated with potential photosynthetic N use efficiency, Amax per unit leaf N. The dependence of Amax on N and the form of the relationship vary among Amazonian species and communities, consistent with both relative availabilities of N, P, and other mineral nutrients, and with intrinsic ecophysiological characteristics of species adapted to habitats of varying resource availability.

Seedling Response of Four Birch Species to Simulated Nitrogen Deposition: Ammonium vs. Nitrate.

Chronic nitrogen deposition has the potential to alter seedling shade tolerance and growth in the temperate forests of northeastern United States, by affecting both the form and the quantity of available nitrogen. Simulated deposition treatments were applied to seedlings of four birch species that co-occur at Harvard Forest (Betula lenta, B. alleghaniensis, B. populifolia, and B. papyrifera). Seedlings were individually potted in forest soil, and grown under light treatments representative of forest understory and treefall gap light levels. In a split-plot design, N was applied at 25 and 50 kg°ha-1 °yr-1 , as either nitrate, ammonium nitrate, or ammonium, within each light environment. While B. populifolia and B. papyrifera, and B. lenta all showed increased biomass allocation to leaves with increased N, only B. lenta showed a significant growth response to the type of N added, and this response was conditional on rate of N application and light environment. At low light, nitrate-fed B. lenta grew best, and also at low rate of supply, nitrate treatments out-performed ammonium treatments. Greater growth under these conditions is probably the result of higher biomass allocation to leaves, and greater specific leaf area, which increased the leaf area ratio, and improved the capacity for carbon gain. Under N deposition regimes that increase soil nitrate availability, the differences in response of B. lenta and B. alleghaniensis to nitrate at low light may potentially lead to changes in the species composition of the seedling communities in the understory. When a treefall occurs, a different species mix of seedlings will be released, with potential consequences for sapling and tree species mix.

Plant-Animal Interactions in a Continuously Grazed Mixture. I. Differences in the Physiology of Leaf Expansion and the Fate of Leaves of Grass and Clover

(1) Several features of the physiology of leaf expansion and the turnover and fate of leaves of two contrasting forage species were studied under continuous grazing to identify the mechanisms that control the relative performance of these species in a mixture. For this, swards of perennial rye-grass and white clover were maintained at three intensities of continuous grazing by sheep (sward surface heights of 3, 6 and 9cm) and a heavily fertilized grass sward was maintained at 6cm. (2) Grass and clover leaves appeared on average at a similar rate, but grass leaves appeared more slowly in the tall swards than in the short ones. In clover, leaf appearance was unaffected by grazing intensity but in the tall swards the leaves took longer to complete their expansion. (3) Growing clover leaves escaped grazing by appearing and expanding close to the soil surface, but once expanded near the top of the canopy, they were soon grazed, often in their entirety. Grass leaves were grazed earlier, often while they were still growing, but defoliation was only partial. This, and a difference in the pattern of leaf unfolding, led to differences between grass and clover in the age structure of the leaf area in the canopy. (4) Grass and clover also differed in their degree of investment in lamina. Ungrazed youngest expanded units of clover had a smaller proportion of lamina to total weight than grass when the total size of the unit was large (e.g. under lenient grazing). But clover had a greater specific leaf area than grass in all cases. (5) These findings explain how clover had an advantage over grass during summer grazing and increased its proportion in the mixture.