Short-term impacts of nutrient manipulations on leaf gas exchange and biomass partitioning in contrasting 2-year-old Pinus taeda clones during seedling establishment

We conducted a 1-year greenhouse experiment to assess the impact of nutrient manipulations on seedling growth, biomass partitioning, and leaf gas exchange between two fast growing Pinus taeda clones that differed in growth efficiency. After 1 year we observed significant treatment and treatment by clone effects on growth, biomass partitioning, and gas exchange parameters. Fertilization increased total seedling biomass 18% primarily through an increase in foliage and coarse-roots. Clones did not differ in total seedling biomass, however, clone 85 produced more stem than clone 93 leading to 37% greater stem:leaf, while clone 93 maintained more branch biomass. The logging residue treatment increased stem:leaf by 30%, but had no effect on total biomass or partitioning. Differences in leaf morphology resulted in significantly greater canopy leaf area in clone 93 than clone 85. Increased foliar N concentration from fertilization had only minor effects on specific photosynthesis under saturating light (A_Sat), but lowered stomatal conductance (g_s), transpiration (E), and internal to external CO₂ concentration ratio (C_i/C_a) as well as improved water use efficiency (WUE) independently of genotype. When gas exchange data was scaled to the canopy level both genotypes achieved similar canopy level CO₂ assimilation rates, but our data suggests they did this by different means. Although we did see a small effect of nutrient limitations in total canopy photosynthesis under saturating light (A_Canopy), A_Sat, and total leaf area (TLA), our foliar N concentration ([N]) indicated that our level of logging residue incorporation did not cause [N] to decrease below sufficiency limits. From a practical standpoint, a better understanding of strategies for capturing and partition C may lead to better selection of clonal material, thereby, optimizing productivity.     

CONTROL TECHNIQUE OF BACTERIAL CANKER OF POPLAR

The poplar bacterial canker caused by Erwinia sp is a serious disease spreading inmost parts of Northeast China,where poplars are cultivalted.The disease seriously infects thosepoplars belonging to section Aigeiros or the intersection hybrids of section Tacamahaca and section Aigeiros and their clones.Five fine poplar cultivars resistant to the bacterial canker have been ob-tained by screening tests.They are Lz_2(Populus simonii×nigra var.italica),A_(15)(P.×sp.)613(P.×sp.),A_(102)and A_(98)(P.simonii×sp.)T following chemicals are obviously effective,such as:50％DT germicide ,40％ XF-136 and 10％ C.C.M.A.or 12.5％ ramphencol and 50,000 ppm/mlvalidamycin,with which the stems or roots can be watered.     

Modulation of leaf physiology by age and in response to abiotic constraints in young cuttings of two Populus deltoides × P. nigra genotypes

? It is of importance, when comparing physiological responses of leaves to environmental constraints among different genotypes, to take into account any effect related to leaf position and age within the canopy that might interfere with the response to the constraints.

? To document such effects, photosynthetic capacity and tolerance to heat and to oxidation were measured on leaves from the top to the bottom of three-month-old single-stem rooted cuttings of Populus deltoides × P. nigra genotypes, ‘Dorskamp’ and ‘Luisa_Avanzo’, thus taking into account a gradient of ages from youngest and still expanding (top) to oldest and fully expanded (bottom) leaves.

? All recorded traits were tightly linked to the age of the leaves. Photosynthetic capacity gradually increased during leaf expansion, in parallel with chlorophyll content and relative nitrogen allocation to RuBisCO and to bioenergetics. On the contrary, dark respiration gradually decreased during leaf expansion until a minimum value was reached at maturity. Compared to expanding leaves, young mature leaves were characterized by a lower sensitivity to heat and a higher one to oxidations generated by methyl-viologen.

? Leaf characteristics appeared to vary along the stem to a larger extent than between the two genotypes that display largely different productivities in plantations.

     

Interspecific variation in leaf water use associated with drought tolerance in four emergent dipterocarp species of a tropical rain forest in Borneo

By use of tree-tower and canopy-crane systems we studied variations in the water use, including transpiration, stomatal conductance, and leaf water potential, of the uppermost sun-exposed canopy leaves of four emergent dipterocarp species in an aseasonal tropical rain forest in Sarawak, Malaysia. Midday depression in stomatal conductance and leaf water potential was observed in all the species studied. Interspecific differences were clearly observed in the maxima of transpiration rates and stomatal conductance and the minima of leaf water potential among the four dipterocarp species. These interspecific variations were closely related to wood density and to factors affecting ecological patterns of distribution. Specifically, Shorea parvifolia and S. smithiana, both of which have a relatively low wood density for Dipterocarpaceae and are found on clay-rich soil, had a high transpiration rate in the daytime but had a large midday depression and a low leaf water potential. In contrast, Dryobalanops aromatica, which has a high wood density and is found in sandy soil areas, consumed less water even during the daytime. Dipterocarpus pachyphyllus, which has a high wood density and is found on clay-rich soil, stood intermediate between Shorea and D. aromatica in leaf water use. The two Shorea species had higher mortality than the others during the severe drought associated with El Ni?o in 1998, so daily pattern of leaf water use in each dipterocarp species might be correlated with its susceptibility to unusual drought events.     

Ecophysiological responses of black walnut (Juglans nigra) to plantation thinning along a vertical canopy gradient

Ecophysiology of black walnut (Juglans nigra L.) along a vertical canopy gradient was studied in a mixed species plantation for the first (2007) and second (2008) growing seasons after thinning to understand response mechanisms to increased resource availability. Parameters were affected by the thinning treatment in 2008 only. Thinned trees showed increased light-saturated maximum photosynthesis (A_max) from 2007 to 2008 compared to non-thinned trees. This response was likely due to increased light levels near thinned trees (vs. changes in water or nutrient status), because thinning did not affect midday leaf water potential (Ψ_md), average daily soil water content (SWC), or leaf nitrogen content per unit area (N_a). Plantation thinning did not increase relative diameter growth during the experimental period. This may be due to low thinning intensity and mortality prior to thinning that reduced competition from first-tier neighbors. Certain leaf traits such as leaf mass per unit area (LMA) and N_a increased from the bottom to the upper canopy position, but did not influence thinning responses. Distribution patterns of photosynthetic parameters through the vertical canopy gradient were less defined than leaf structural traits such as LMA and N_a. Findings reflect black walnut's large variability in response to thinning.     

Improving predictions of forest growth using the 3-PGS model with observations made by remote sensing

Measurements made by remote sensing can characterize the leaf area density and nitrogen/chlorophyll content of forest canopies, as well as maximum photosynthetic capacity and above-ground structure and biomass. Combining these with climate data estimated from relationships based on temperature measurements and using an appropriate process-based model, it is possible to calculate, with useful accuracy, carbon sequestration and wood production by different forest types covering large land areas. To broaden its application and reduce the need for detailed information on stand characteristics, a satellite-driven version of the model 3-PG, was developed. The 3-PGS model incorporates the major first-order physiological processes that determine forest growth, and the biophysical factors that affect and govern those processes. It incorporates remotely sensed estimates of seasonal variation in canopy light interception (fPAR) and includes physiological variables (stomatal conductance and canopy quantum efficiency) that can be estimated by remote-sensing measurements of factors that influence those variables. 3-PGS therefore provides a useful framework within which to evaluate how data from the array of airborne and satellite-borne sensors now available might be used to initialize, drive, and test process-based growth models across regions with diverse soils and climates. We address the question: to what extent might additional remote-sensing techniques improve 3-PGS predictions?Sensitivity analyses indicate that model accuracy would be most improved through better estimates of seasonal changes in canopy photosynthetic capacity (α) and canopy conductance (G_c). Canopy photosynthetic capacity depends on the amount of light absorbed by the canopy, estimated as a fraction of photosynthetically active radiation (fPAR), and on foliage nitrogen or chlorophyll content, which can be estimated using multi-spectral imagery. G_c depends on canopy leaf area index (L) and stomatal conductance of the foliage (g_s), which is affected by the vapor pressure deficit of the air and soil water content. The onset and effects of drought can be determined from changes in canopy reflectance and fPAR identified from sequential measurements; the same measurements, coupled with calculations of evapotranspiration using climatic data and standard formulae, provide estimates of total available water in forest root zones. Periodic surveys with Light Detection and Ranging (LiDAR) and interferometric RADAR may serve to validate model predictions of above-ground growth (NPP_A), while progressive reduction in light-use efficiency (NPP_A/APAR) may identify forests with declining vigor that are likely to succumb to attack from insects and pathogens.     

Megasporogenesis,megagametogenesis, and induction of 2n eggs with colchicine in poplar section Aigeiros

Triploid breeding through 2n eggs induction has become one of the most powerful approaches for the improvement of the genus Populus. To enhance the efficiency of triploid production in section Aigeiros, female buds (catkins) of P. × euramericana (Dode) Guinier at different developmental stages were exposed to colchicine for 2n eggs induction. The relationship between flower bud morphological characteristics (or time after pollination) and female meiotic stage (or embryo sac development) was established to guide colchicine treatments. In the resulting progeny, three triploid seedlings were obtained via embryo rescue and identified by flow cytometric analyses. Cytological observation revealed that the leptotene to pachytene stage of meiosis and 12 h after pollination might be the suitable period for 2n eggs induction with colchicine in P. × euramericana. Results and protocols related to 2n eggs induction, effective methodologies for embryo rescue, and ploidy evaluation in this study might be applicable in polyploidy breeding in section Aigeiros.     

Morphological plasticity and regeneration strategies of velvet leaf blueberry (Vaccinium myrtilloides Michx.) following canopy disturbance in boreal mixedwood forests

The effects of canopy disturbance on the abundance, growth, morphological plasticity, biomass allocation and fruit production of velvet leaf blueberry (Vaccinium myrtilloides Michx.) were examined in 1996 in a second-growth boreal mixedwood forest near Nipigon, northwestern Ontario that had been logged by either shelterwood cutting or clearcutting in 1993. We found that V. myrtilloides was able to persist in both open and closed canopy boreal mixedwood forests managed for commercial timber extraction. Persistence under heavy shade conditions was accompanied by significant morphological and biomass allocation plasticity. Specific leaf area, leaf area, individual leaf weight, and the proportion of total biomass in stems and foliage changed along an understory light gradient from 0% to 67% percent photosynthetic photon flux density (% PPFD). The degree of above-ground morphological plasticity may explain blueberry's ability to survive under low light conditions. Reproductive performance of V. myrtilloides was greatest under the partial shade conditions associated with shelterwood cutting. Blueberry bushes growing in clearcuts overgrown with 3-year old aspen (Populous tremuloides Michx.) saplings remained mostly vegetative whereas the number, fresh weight and dry weight of berries in shelterwood cuts was 94% grater than that produced after clearcutting. We attributed the lower fruit yields in the clearcuts to heavy shading from regenerating hardwoods, and mechanical damage to above-ground biomass. The paucity of seedling regeneration as well as extensive mechanical damage to above-ground stems by logging equipment delayed vegetative regeneration of V. myrtilloides in large canopy openings of the clearcut blocks. Unlike other more aggressive ericaceous species (e.g. Kalmia angustifolia var. angustifolia L., Gaultheria shallon Pursh.), V. myrtilloides was unable to resist invasion from faster growing hardwood species (e.g. P. tremuloides) and was rapidly overtopped. V. myrtilloides plants in the uncut control blocks received 3.9% of full sunlight, whereas those growing in the partial cut and clearcut blocks received an average of 25.3% and 32.5% PPFD, respectively. Cover of vegetation over-topping blueberry plants was highest in the uncut forest (90.3%), but was not significantly different between the partial cut (45.5%) and clearcut (50.1%) treatment blocks.     

麻竹枝叶生长对钩梢的响应

为了解冠层生物量积累、分配及枝叶大小对麻竹立竹受营林措施干扰(钩梢)后的响应,对不同竹龄全梢、钩梢麻竹地上构件生物量、生物量比、单叶特征、大小枝生物量分配比例及商品竹叶数量进行了调查.结果表明:麻竹立竹地上现存生物量分配格局为秆＞枝＞叶.叶生物量、叶/枝和叶/秆生物量比为2年生＞3年生＞1年生.随着竹龄的增加,枝、秆生物量、地上生物量和枝/秆生物量比总体上呈增加的趋势.2年生和3年生立竹枝、叶生物量分配比例显著高于1年生立竹,秆生物量分配比例显著小于1年生立竹.此外,随着竹龄的增加,立竹减少了对0 ～8 mm枝生物量的分配,增加了对8～16 mm、16 mm以上枝生物量的投入,以提高空间拓展能力,截获更多光资源.钩梢强烈影响了生物量分配格局,显著减小了麻竹立竹枝、叶、秆、地上生物量.钩梢后麻竹立竹增加了枝、叶生物量分配比例,减少了秆生物量的分配比例,同时提高了单叶叶面积和单叶干质量,增加了8～16 mm、16 mm以上枝生物量分配比例,减小0～8 mm枝生物量分配比例,以权衡枝叶的生长,提高立竹对环境的适合度.钩梢后立竹叶/枝、叶/秆、枝/秆生物量比升高,表明生物量分配更多地向叶和枝倾斜.钩梢麻竹商品竹叶数量较全梢麻竹增加29.68％,且发生部位明显降低,钩梢后冠层下部商品竹叶数量增加79.73％,中部商品竹叶数量增加25.81％,降低了采摘高度.在钩梢后的一个生长季内,钩梢影响了麻竹立竹资源利用策略,表现为枝与叶之间关系的变化,但随钩梢年限的增加其变化规律如何尚需进一步研究.     

The successional status of tropical rainforest tree species is associated with differences in leaf carbon isotope discrimination and functional traits

We characterised the among species variability in leaf gas exchange and morphological traits under controlled conditions of seedlings of 22 tropical rainforest canopy species to understand the origin of the variability in leaf carbon isotope discrimination (Δ) among species with different growth and dynamic characteristics (successional gradient). Our results first suggest that these species pursue a consistent strategy in terms of Δ throughout their ontogeny (juveniles grown here versus canopy adult trees from the natural forest). Second, leaf Δ was negatively correlated with WUE and N, and positively correlated with g_s, but among species differences in Δ were mainly explained by differences in WUE. Finally, species belonging to different successional groups display distinct leaf functional and morphological traits. We confirmed that fast growing early successional species maximise carbon assimilation with high stomatal conductance. In contrast, fast and slow growing late successional species are both characterised by low carbon assimilation values, but by distinct stomatal conductance and leaf morphological features. Along the successional gradient, these differences result in much lower Δ for the intermediate species (i.e. fast growing late successional) as compared to the two other groups.     

Modeling forage growth in a Midwest USA silvopastoral system

Tree effects on understory pasture growth in a silvopastoral system were modeled by explicit simulation of tree canopy light and rainfall interception, evapotranspiration, and nutrient uptake. The algorithms to model these effects were incorporated into a multispecies grazing simulation model, GRASIM, to form the Silvopasture GRASIM model (SGRASIM). The new model was evaluated using forage biomass data and soil moisture data collected from a silvopasture field experiment with black walnut (Juglans nigra L.). The SGRASIM model performed well in simulating the growth of three competing dominant forage species (orchardgrass [Dactylis glomerata L.], Kentucky bluegrass [Poa pratensis L.], and tall fescue [Festuca arundinacea (Schreb.)] in the pasture both under tree canopy and in open pasture (linear regression of observed on simulated biomass for the species gave r ² values above 0.97). Model growth parameters for forage under tree canopy, compared with those for an open pasture, bear testament to the shading effects from the forest canopy in terms of reduced photosynthetic efficiency, increased leaf area ratio, and photosynthate partitioned to aboveground biomass. The new model reasonably followed the soil moisture time series in the upper soil layer (0–30 cm), where the bulk of the forage roots reside.     

Biomass production by two-year-old poplar clones on floodplain sites in the Lower Midwest, USA

Four Populus clones were grown for two years at 1×1 m spacing for study of total biomass production and carbon sequestration capacity on floodplain sites previously in forage grasses under climatic conditions of the lower Midwest, U.S.A. Total biomass (above-and below-ground) in the first year ranged from 3.9 Mg ha^–1 in a Populus deltoides x P. nigra clone (I45/51) to 1.9 Mg ha^–1 for a local-source Populus deltoides clone (2059). Second year total biomass production was substantially higher, ranging from 13.9 Mg ha^–1 in I45/51 to 7.4 Mg ha^–1 in P. deltoides clone 26C6R51. Second-year leaf area index (LAI) values for I45/51 plants reached 4 during mid-season, indicating essentially complete canopy closure in this clone by the second year after planting. In contrast, maximum mid-season, second-year LAI was significantly lower in P. deltoides clones ( 2.4). There was some evidence for differential allocation to roots and shoots among Populus clones, with 26C6R51 showing relatively more allocation to root biomass than other clones. Second-year growth in Populus deltoides clone 2059 accelerated substantially, and this genotype exhibited two-year biomass accumulation nearly equal to that of I45/51 despite having less leaf area. This result suggested a higher photosynthetic capacity or assimilation efficiency in the former. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of thinning on microbial biomass C, N and basal respiration in black pine forest soils in Mudurnu, Turkey

Reducing the canopy cover (e.g., forest thinning) is one of the most commonly employed forest silvicultural treatments. Trees are partially removed from a forest in order to manage tree competition, thus favoring the remaining and often the most valuable trees. The properties of the soil are affected by forest thinning as a result of changes in key microclimatic conditions, microbial communities and biomass, root density, nutrient budgets and organic matter turnover. The aim of this study was to determine the soil microbial biomass C, N and respiration (basal respiration) in a black pine (Pinus nigra Arn. subsp. pallasiana) forest in the Mudurnu district of Bolu Province (Western Black Sea Region, Turkey). Whereas forest thinning was found to cause increases in the soil temperature, microbial biomass C and N and organic C, it was found to decrease the soil moisture, basal respiration and metabolic quotient (qCO₂). As expected, soil organic C exhibited a strong impact on soil microbial biomass C, N and basal respiration. It was concluded that the influence of forest thinning on the microbial biomass and soil respiration was the combined result of changing microclimatic conditions and soil properties, such as forest litter, soil temperature, soil moisture, soil pH and soil organic matter.     

DNA methylation and histone acetylation: genotypic variations in hybrid poplars, impact of water deficit and relationships with productivity

? Several reports on annual plants have already shown the involvement of epigenetic modifiers such as DNA methylation in their adaptation to abiotic stresses.

? Nevertheless, the genotypic variations of epigenetic modifiers, their possible correlations with morphological traits and the impact of water deficit have not been described for perennial plants.

? Six genotypes of Populus deltoides × P. nigra were subjected or not to a moderate water deficit treatment. Various morphological traits such as the height of the plants, their biomass and the total leaf area were measured to characterize the productivity in both conditions. Levels of DNA methylation, histone acetylation and the activities and isoform accumulation of the corresponding enzymes were measured at the shoot apex, the site of morphogenesis. Genotypic variation was observed for the morphological traits and the epigenetic variables and correlations were established among them. Genotypic variation for DNA methylation was detected in hybrid poplars. A positive correlation was demonstrated between DNA methylation percentage and productivity under well watered conditions.

? While there was a general decrease of growth for all genotypes in response to a moderate water deficit, genotypic dependant variations of DNA methylation were found suggesting different strategies among hybrids.

     

Genetic variation in productivity, leaf traits and carbon isotope discrimination in hybrid poplars cultivated on contrasting sites

? We examined the relationships between productivity, leaf traits and carbon isotope discrimination in bulk leaf matter (Δ₁) and in phloem sap (Δ_s) from more than 5-year-old trees belonging to Populus deltoides × P. nigra and Populus trichocarpa × P. deltoides; trees were grown in alluvial and non alluvial sites in a commercial poplar plantation.

? On both sites, a large genetic variability was evidenced for all variables. The genotypic ranking remained stable between years for all variables, while it differed between sites. Δ₁ scaled positively with Δ_s and neither Δ₁ nor Δ_s were correlated with productivity. A significant genotype by site interaction was evident for all variables. The non alluvial site resulted in lower productivity, and in thicker/denser leaves with lower nitrogen and carbon contents. Noteworthy, the genotypic ranking for Δ₁ measured at the alluvial site was similar to that previously established in a glasshouse.

? As observed in previous studies from younger trees, there is a potential to select genotypes, combining high productivity and high water-use efficiency, for growth in moderately drought-prone areas.

     

Ramet population structure of Fargesia nitida in different canopy conditions of the subalpine dark coniferous forest in the Wolong Nature Reserve, China

The bamboo Fargesia nitida, one of the giant panda’s main food sources and the dominant shrub species of the forest understory, is mainly distributed in the dark coniferous belt in western Sichuan and southern Gansu in China. To study the impact of different forest canopy conditions on subalpine dwarf bamboo populations, ramet population structures of clonal Fargesia nitida were surveyed in: forest understory (FU), moderate gap (MG), large gap (LG) and marginal open space (MOS). In order to determine how the ramet structures could be affected and its effects on these four canopy conditions, a field survey of the age structure of Fargesia nitida population, its morphological traits and biomass was conducted in the Abies faxoniana forest situated in the Wolong Nature Reserve, western Sichuan, China. The main results were as follows. First, at the ramet level, the structures of the ramet populations in four canopy conditions were significantly different, and as the canopy density decreased, the mean height, basal diameter and biomass of the populations increased following the order: LG<MG<FU. Second, the biomass proportions of ramets modularly varied with different canopy conditions and leaf biomass proportion was positively related to the canopy density except for the MOS where the biomass proportions of rhizome and roots were both higher than those in the three other canopy conditions. Third, ramet specific leaf weight increased in parallel with the decrease in canopy density. In the MG, the values of the individual leaf biomass and leaf area were the largest, followed by those in the MOS. Both the individual leaf biomass and leaf area were significantly different from those in the FU and LG. Leaf number per ramet was significantly different among the four different canopy conditions and the biggest in the LG. Fourth, the ramet population mortality was the lowest in the FU (Chi-square test, p < 0.01), while there was no significant difference in the average population age (Mann-Whitney test, p > 0.05). All the results indicated that it was not ramet age, but the morphological changes and biomass distribution that exhibited the response of the ramet population of Fargesia nitida to the changed canopy conditions. Translated from Acta Ecologica Sinica, 2006, 40(1): 730–736 [译自: 生态学报]     

Na2SO4胁迫对沙枣幼苗生长和光合生理的影响

采用盆栽控制试验,研究了不同浓度(0、60、120和180 mmol·L-1)Na2SO4胁迫对沙枣幼苗生长和光合特性的影响。结果表明:(1)盐胁迫对沙枣幼苗生长具有显著的抑制效应。不同浓度Na2SO4胁迫沙枣的株高、侧枝数、总叶面积、单株叶片数、比叶面积以及各组织(除根)生物量均显著低于对照,且均随盐胁迫浓度的升高呈下降趋势,而根冠比值则由对照的0.153 1显著增加到180 mmol·L-1Na2SO4胁迫幼苗的0.348 7。(2)盐胁迫显著降低了沙枣幼苗的光合能力。随着Na2SO4胁迫的加剧,净光合速率(Pn)、气孔导度(Gs)、胞间CO2浓度(Ci)和蒸腾速率(Tr)均呈下降的趋势,而气孔限制值(Ls)和水分利用效率(WUE)则依次增加,且Pn下降主要受气孔限制;180 mmol·L-1Na2SO4胁迫沙枣幼苗的Pn、Gs、Ci和Tr分别为对照的71.57%、30.85%、67.15%和51.65%,而Ls和WUE则分别为对照的1.91、1.38倍。(3)盐胁迫强度与幼苗株高、总叶面积、单株叶片数、比叶面积、茎生物量、叶生物量、总生物量等生长指标以及Pn、Gs、Ci、Tr等光合参数呈极显著负相关,叶片的光合参数与总叶面积、单株叶片数呈显著或极显著正相关,而叶片的生长指标、光合参数与幼苗的株高生长和生物量累积也呈显著或极显著正相关。     

Biomass production physiology and soil carbon dynamics in short-rotation-grown Populus deltoides and P. deltoides × P. nigra hybrids

Fast-growing woody species grown in dense, short-rotation plantations on land previously in agriculture offer potential economic benefits in products such as engineered construction material, boiler fuel, non-food-based biofuel feed stocks and other carbon (C)-based products and credits. However, information on the effects on major C pools of short-rotation culture is relatively sparse. In this study, Populus deltoides and P. deltoides × P. nigra hybrid clones were grown for 5 years at 1 m × 1 m spacing in plantations on a former pasture of high native fertility in the Missouri River floodplain in the lower Midwest U.S.A. Above- and below-ground biomass production, leaf area-based production efficiency, photosynthetic attributes and soil C dynamics were studied.     

Leaf photosynthetic properties in a willow (Salix viminalis and Salix dasyclados) plantation in response to fertilization

Specific leaf area (SLA), nitrogen and chlorophyll concentrations and photosynthetic characteristics were studied in upper and lower canopy leaves of Salix viminalis and S. dasyclados grown at two nutrition levels. Fertilization increased SLA and leaf mass-based nitrogen concentration in most cases. Positive effects of fertilization on leaf light-saturated photosynthetic rate (A _max ^A ) and maximum carboxylation rate (V _cmax) were not detected. Significant differences between the leaves from upper and lower canopy layers in area-based nitrogen, A _max ^A , SLA, mass-based chlorophyll, V _cmax and stomatal conductance were found for most plots. We attempted to estimate the fraction of non-photosynthetic nitrogen and found that it tended to be higher due to fertilization. Thus, the insensitivity of leaf photosynthesis to fertilization could be caused by higher proportion of non-photosynthetic nitrogen in the leaves of fertilized plots. Though leaf-level photosynthesis was not increased by fertilization, considerably higher leaf area index of fertilized plots still resulted in increased canopy carbon gain.     

Evaluating changes in switchgrass physiology,biomass, and light-use efficiency under artificial shade to estimate yields if intercropped with Pinus taeda L.

There is growing interest in using switchgrass (Panicum virgatum L.) as a biofuel intercrop in forestry systems. However, there are limited data on the longevity of intercropped bioenergy crops, particularly with respect to light availability as the overstory tree canopy matures. Therefore, we conducted a greenhouse study to determine the effects of shading on switchgrass growth. Four treatments, each with different photosynthetically active radiation (PAR) levels, were investigated inside the greenhouse: control (no shade cloth, 49 % of full sunlight), low (under 36 % shade cloth), medium (under 52 % shade cloth), and heavy shade (under 78 % shade cloth). We determined the effect of shading from March to October 2011 on individually potted, multi-tillered switchgrass transplants cut to a stubble height of 10 cm. In the greenhouse, there was a reduction in tiller number, tiller height, gas exchange rates (photosynthesis and stomatal conductance), leaf area, above- and belowground biomass and light-use efficiency with increasing shade. Total (above- and belowground) biomass in the control measured 374 ± 22 compared to 9 ± 2 g pot^?1 under heavy shade (11 % of full sunlight). Corresponding light-use efficiencies were 3.7 ± 0.2 and 1.4 ± 0.2 g MJ^?1, respectively. We also compared PAR levels and associated aboveground switchgrass biomass from inside the greenhouse to PAR levels in the inter-row regions of a range of loblolly pine (Pinus taeda L.) stands from across the southeastern United States (U.S.) to estimate when light may limit the growth of intercropped species under field conditions. Results from the light environment of loblolly pine plantations in the field suggest that switchgrass biomass will be significantly reduced at a loblolly pine leaf area index between 1.95 and 2.25, which occurs on average between ages 6 and 8 years across the U.S. Southeast in intensively managed pine plantations. These leaf area indices correspond to a 60–65 % reduction in PAR from open sky.