Effects of host shading on consumption and growth of the geometrid Epirrita autumnata: interactive roles of water, primary and secondary compounds

Shading is assumed to reduce allocation to plant phenolics and to defense in general. We here report the results of experimental shading of individual branches or whole canopies in mountain birch on foliar chemistry and on the growth and consumption of a geometrid, Epirritaautumnata. Branch‐wide shading tended to have at least as strong effects on both leaf chemistry and herbivore performance as canopy‐wide shading, indicating local responses of the host to shading. Responses to shading varied among the key leaf traits. Leaf water content was higher and toughness lower in shaded than in non‐shaded leaves. Leaf sugars were lower and protein‐bound and free amino acids higher in shaded than in control leaves. Sucrose and galactose were at high levels in unshaded branches adjacent to shaded ones, suggesting that partial shading enhanced translocation of sugars within canopies. Total phenolics and soluble proanthocyanidins were low in both shading treatments. Of the other phenolic groups, concentrations of gallotannins and cell‐wall‐bound proanthocyanidins did not differ between shaded and non‐shaded leaves. Epirrita larvae grew better in both types of shading treatments compared to either unshaded control trees or to unshaded branches in the branch‐shading trees. By far the most important correlate of larval growth was the amount of water consumed with leaf mass (r=0.94). When variance in water intake was standardized (also largely eliminating parallel variation in proteins), fructose and glucose still had significant positive correlations and proanthocyanidins negative with larval growth on control but not on shade leaves. Concentrations of several phenolic compounds correlated negatively with intake of dry matter and especially water, and different phenolics were important in shaded (gallotannins) and in control (flavonoids) leaves. Our findings strongly suggest that the effects of putatively defensive leaf traits on insect consumption and growth interact with nutritive leaf traits, particularly with water.     

Effect of shade stress on growth,morphology, and carbon dynamics of loblolly pine branches

Summary Shade treatments were applied to the terminal portions of branches in loblolly pine trees to test whether portions of branches were autonomous with respect to carbohydrates. The shade treatments reduced light by 50% and 72% compared with full sun conditions resulting in significant reductions in net photosynthesis. Branch growth (branch diameter and length, branch and needle biomass) decreased significantly within the shaded portion of the branch. Branch and needle morphology were also affected by shading. Lateral branches subtending terminal shoots were labelled with ¹⁴C 2 times during the growing season. No movement of ¹⁴C into the terminal portions of the branch was evident during the first growth flush. However, during the second flush of growth small but statistically significant amounts of ¹⁴C were imported into the terminal portion of the shaded branches from subtending laterals. It was concluded that loblolly pine shoots were usually autonomous with respect to carbohydrate supply, but that carbohydrate movement into the terminal shoot from subtending foliage could occur under conditions of very high stress.     

Testing the branch autonomy theory: a 13C/14C double-labelling experiment on differentially shaded branches

The impact of a heterogeneous within‐crown light environment on carbon allocation was investigated on young walnut trees trained on two branches: one left in full sunlight, the other shaded until leaf fall resulting in 67% reduction in photosynthetically active radiation. In September, the two branches were separately labelled with ¹⁴CO₂ and ¹³CO₂, respectively, so that the photosynthates from each branch could be traced independently at the same time. Although some carbon movements could be detected within 5 d in both directions (including from the shaded branch to the sun branch), between‐branch carbon movements were very limited: approximately 1% of the diurnal net assimilation of a branch. At this time of the year branch autonomy was nearly total, leading to increased relative respiratory losses and a moderate growth deficit in the shaded branch. The ratio of growth to reserve storage rate was only slightly affected, indicating that reserves acted not as a mere buffer for excess C but as an active sink for assimilates. In winter, branch autonomy was more questionable, as significant amounts of carbon were imported into both branches, possibly representing up to 10% of total branch reserves. Further within‐plant carbon transfers occurred in spring, which totally abolished plant autonomy, as new shoots sprouted on each branch received significantly more C mobilized from tree‐wide reserves than from local, mother‐branch located reserves. This allowed great flexibility of tree response to environment changes at the yearly time scale. As phloem is considered not functional in winter, it is suggested that xylem is involved as the pathway for carbohydrate movements at this time of the year. This is in agreement with other results regarding sugar exchanges between the xylem vessels and the neighbouring reserve parenchyma tissues.     

Does foliage on the same branch compete for the same water? Experiments on Douglas-fir trees

Do branchlets within a branch have autonomous water supplies, or do they share a common water supply system? We hypothesized that if branchlets shared a common water supply, then stomatal conductance (g_s) on sunlit foliage would increase with reduced transpiration of competing foliage on the branch. We reduced transpiration of other foliage on the branch through bagging and shading, and we monitored the gas-exchange responses of the remaining sunlit foliage on the branch relative to control branches for several age classes of Douglas-fir trees (aged ~10 years, 20 years, and 450 years old). Contrary to our hypothesis, we found no increases in g_s in either young or old trees following transient reductions in the amount of transpiring leaf area. The diurnal change in water potential, mid-day stomatal closure and associated photosynthetic decline occurred at the same time and were of the same magnitude on both treated and untreated branches, with the exception of photosynthesis in one 450-year-old tree. Hydraulic conductance measurements of branch junctions indicate that xylem within branches is only partially interconnected which would reduce the effectiveness of shading as a means of increasing water supply to the remaining sunlit foliage. The lack of a response implies that when a branch is in partial shade, the remaining sunlit foliage has no advantage with respect to water status over foliage on a branch completely in the sun.     

An experimental study of colony-founding in pine saplings by queens of the arboreal ant, Crematogaster ashmeadi

Summary: Newly mated queens of the arboreal ant Crematogaster ashmeadi initiate colonies in old beetle galleries in the dead branches of longleaf pine trees. In a study by Hahn (1996), a number of tree characteristics were correlated with the number of newly-mated queens in those trees, with branch length the best indicator of queen presence. Three of these characteristics, tree height, dead branch length, and the number of dead branches were tested in an experiment to see which, if any, the queens were using to choose a tree. Both tree height and the number of dead branches significantly influenced queen choice: shorter trees (4-5 m) had more queens than tall ones (7-9 m), and trees with 8 branches had significantly more queens than trees with 2 branches. Branch length had no effect on the number of queens. These findings suggest that newly mated Crematogaster ashmeadi queens search for founding trees on the basis of the height of a sapling and its number of dead branches. Modes of searching are discussed.     

苗期遮荫对棉花产量与品质形成的影响

为揭示棉麦两熟共生期遮荫对棉花产量与品质形成的影响。在棉花苗期利用模拟棉麦两熟共生期遮荫的方法进行了研究。结果表明,遮荫对棉铃形成的影响因果枝,果节部位而异,遮荫有利于棉株下（1-3果枝）,中（4-6果枝）,上（7-9果枝）部果枝内围（1-2果节）铃的形成,对外围（≥3果节）尤其顶部果枝（≥10果枝）外围铃形成不利,从而决定铃重也随果枝,果节部位相应地变化,但遮荫对单株平均铃重的影响畔 小,变遮荫棉花籽棉产量而论,下,中部果枝的内围铃籽棉产量高于常规棉,在上,顶部果枝则相反,各部位果枝外围铃的籽棉产量均低于常规棉,遮荫棉花内,外围铃分布为1：0.36(常规棉为1：0.58),产量分布为1：0.42(常规棉为1：0.72)。苗期遮荫对棉纤维,棉籽品质性状的影响也主要在顶部果枝和上部果枝外围铃,综合分析遮荫棉花产量与品质的形成,棉花苗期耐遮荫性品种间存在差异。在本研究中以中9418耐遮荫性最强,中棉所19和春矮早次之。     

Sex ratios and clonal growth in dioecious Populus euphratica Oliv., Xinjiang Prov., Western China

Using a microsatellite assay, we investigated sex ratios at three levels (apparent, intrinsic, genet) for Populus euphratica stands in Xinjiang, China and possible consequences of sex-specific costs of reproduction in terms of clonal growth and individual growth or mortality. Sex ratios at all levels tended to be male biased (60 % of 3,295 flowering trees were male), although male excess was least pronounced at the genet level (52 % of 850 genets were male). Male clones comprised significantly more (708 vs. 572) trees than female clones. Reproductive investment was measured in terms of carbon (C) and nitrogen (N) contents of male and female reproductive organs: single flowers or fruit capsules, whole inflorescences or infructescences, and whole branches of ca. 2 cm diameter. Male flowers and catkins require less N than female fruits and catkins, but on average only 16 % of female catkins develop into fruits. This changes the measured investment for reproduction at branch level: now male branches spent 3.3 times more N than their female counterparts. This coincides with the annual increment of branches, measured as a possible trade-off for costs of reproduction: female branches needed 2 years less to reach a diameter of 2 cm. We conclude that full fruit set of females would give males a heavy comparative advantage, but frequent abortion of whole infructescences by females seems to be a powerful mechanism to compensate a higher reproductive effort, thus avoiding a pronounced runaway effect by more vigorous clonal growth of male trees over a long time.     

Effect of root competition and shading on resprouting dynamics of Erica multiflora L.

Abstract. Resprouting from underground structures is one of the main regeneration strategies of Mediterranean shrubs after aerial biomass disturbance such as fire or clear-cutting to reduce fire risk. In order to study the effect of root competition and shading (simulated shoot competition) on Erica multiflora, growth, morphology, flowering performance and sprout size variability during resprouting, a factorial field experiment was conducted in which neighbours around target plants were eliminated and plants were shaded with mesh for two years. Root competition reduced sprout recruitment and sprout density (number of sprouts per unit stump area) more strongly than did shading. The negative effect of root competition on sprout biomass was constant with time, while the reduction due to shading increased with time. There was an interaction between root competition and shading on the biomass of sprouts 22 months after treatment: genets without root competition and shading were four times larger than in any other treatment. Both shading and root competition also decreased percentage branching but did not modify maximum sprout height. Only shading decreased the leaf/shoot biomass ratio and the percentage of flowering genets. One year after resprouting, root competition counteracted the effect of shading on inducing sprout biomass variability within the genet because it decreased sprout density. 22 months after treatment, sprout biomass variability was not affected by any main effect. The results suggest that competition among sprouts within the genet is asymmetric. However, shading by genet neighbours may not always increase sprout biomass variability if root competition is also severe.     

Influence of a dominant macrophyte, <Emphasis Type="Italic">Juncus effusus</Emphasis>, on wetland plant species richness,diversity, and community composition

Few studies have experimentally investigated the influence of competition for light on structure and composition of wetland vascular plant communities, despite the well-documented high productivity in such systems. Influences of the dominant emergent wetland plant Juncus effusus on the surrounding macrophyte community were evaluated in a shallow freshwater wetland through two consecutive growing seasons. Permanent transects were constructed along diagonals of randomly oriented 1 m² plots centered around isolated, colonizing J. effusus tussocks. Percent areal cover was measured for each species or identifiable taxon group in 400 cm² sub-plots centered 10, 20, 30, and 40 cm from the tussock edge, to evaluate species richness, diversity, and dominance indices. Observational studies during the first growing season indicated that plots having larger, more heavily shading tussocks yielded significantly lower cover and species richness in the surrounding plant community than less shaded plots. Shading by J. effusus was reduced experimentally during the second growing season by holding culms in a vertical position in half of the J. effusus-occupied study plots in order to assess directly the influence of shading by J. effusus. Manipulated plots became more diverse and species-rich and developed higher total percent areal cover. Within shaded reference plots, reductions in cover, richness, and diversity were correlated with intensity of shading; each of these measures was lowest in sub-plots centered 10 cm from the tussock, where measured light reduction was highest. Neighborhood analyses of biomass, species composition, and light reduction indicated that individual species biomass varied with distance from J. effusus tussocks and shading intensity, an indication of the potential for shifts in community composition as populations of this dominant macrophyte expand to fill a wetland area. A mathematical model is presented to illustrate shading effects of J. effusus as a population grows from isolated, colonizing tussocks to an interacting system of tussocks in an established population of this dominant macrophyte.     

Acclimation responses of mature Abies amabilis sun foliage to shading

This paper addresses two main questions. First, can evergreen foliage that has been structurally determined as sun foliage acclimate physiologically when it is shaded? Second, is this acclimation independent of the foliage ageing process and source-sink relations? To investigate these questions, a shading and debudding experiment was established using paired branches on opengrown Abies amabilis trees. For each tree, one branch was either shaded, debudded, or both, from before budbreak until the end of summer, while the other branch functioned as a control. Foliage samples were measured both prior to and during treatment for photosynthesis at light saturation (A _max), dark respiration, nitrogen content, chlorophyll content, chlorophyll-to-nitrogen ratio and chlorophyll a:b ratio. All age classes of foliage responded similarly during the treatment, although pre-treatment values differed between age classes. Within 1 month after the treatment began, A _max was lower in shaded foliage and remained lower throughout the treatment period. For debudded branches, A _max was lower than the controls only during active shoot elongation. At the end of the treatments in September, A _max in shade-treated sun foliage matched the rates in the true shade-formed foliage, but nitrogen remained significantly higher. By 1.5 months after treatment, chlorophyll content in shaded foliage was higher than in controls, and the chlorophyll a:b ratio was lower for the shaded foliage. On debudded branches, chlorophyll content and chlorophyll a:b ratio were similar to the values in control samples. Shading lowered the rate of nitrogen accumulation within a branch, while removing debudding decreased the amount of sequestered N that was exported from the older foliage to supply new growth. By September, chlorophyll content in shade-treated foliage was higher than that in the control sun foliage or in true shade foliage. The chlorophyll increase as a result of shading was unexpected. However, the chlorophyll-to-nitrogen ratio was identical for the shade-treated sun foliage and the true shade foliage while being significantly lower than the control sun foliage. It appears that acclimation to shading in mature foliage involves a reallocation of nitrogen within the leaf into thylakoid proteins. A redistribution of resources (nitrogen) among leaves is secondary and appears to function on a slower time scale than reallocation within the leaf. Thus, A. amabilis foliage that is structurally determined as sun foliage can acclimate to shade within a few months; this process is most likely independent of ageing and is only slightly affected by source-sink relations within a branch.     

Environmental and stomatal control of transpiration, canopy conductance and decoupling coefficient in young lemon trees under shading net

Sap flow, as a measure of transpiration, was monitored in 2-year-old lemon trees growing in pots. Eight trees were used in the experiment, four of which were placed under a rectangular shading net, while the other four were maintained in the open air. Daily averages of canopy conductance and photosynthesis were not affected by shading; however, the daily transpiration was reduced in shaded plants, which displayed an increase in water use efficiency compared with exposed trees. The decoupling coefficient was higher in the shaded trees, indicating that the transpiration of lemon trees was efficiently controlled by stomata in exposed plants, while the transpiration rate was mainly influenced by radiation in the plants growing under the net. This influence was more pronounced in the afternoon, when the whole tree transpiration was largely dominated by equilibrium transpiration in the plants under netting, and the relationship between transpiration and radiation showed a steeper slope in shaded trees. The reduction in transpiration and the maintenance of photosynthesis in shaded plants with respect to exposed trees indicated that screen structures in semi-arid and arid environments could be considered as an intermediate solution for reducing plant water stress and increasing water use efficiency.     

Physiological basis of spacing effects on tree growth and form in Eucalyptus globulus

We examine the effects of spacing and layout on the growth and form of 3- to 4-year-old Eucalyptus globulus in a farm forestry context. Four planting layouts were chosen. These represented the range commonly in use in farm forestry: block plantings (2Ǹ m), triple rows (2Ǹ m) at 10-m intervals, single rows (2᎒ m) and isolated trees (10᎒ m). The physiological significance of key results is interpreted in terms of changes in the parameters of a simple plantation growth model. Under conditions where levels of direct light are high, for example during summer, block-planted trees intercepted only 38% of the light intercepted by isolated trees. On a stand basis, however, the combination of incident radiation and ground coverage declined with lower stand densities. While stand leaf area index declined from around 6 to 1 with increased spacing, individual tree leaf areas rose from around 50 m² in block plantings to 150 m² in isolated trees. The proportion of above-ground biomass found in stems declined with increasing spacing as the mass in foliage and branches increased. Stems accounted for 65% of above-ground biomass in block-planted trees but only 35% in isolated trees. The contributions of leaves and branches correspondingly rose from 19% to 35% and from 16% to 29%, respectively. Changes in biomass distribution were accompanied by increasing branch number, branch thickness, flatter branch angles and the longer retention of lower branches with greater spacing. These changes have implications for the merchantability of the timber. The efficiency of above-ground radiation conversion was constant at 0.67 g MJ^-1 irrespective of spacing. We estimated that foliar maintenance respiration (R_m) accounted for about 90% of above-ground R_m. On a stand basis R_m costs block plantings 23.90 t DM ha^-1 year^-1 (50% annual above-ground photosynthetic production) compared with 6.22 t DM ha^-1 year^-1 (40% annual above-ground photosynthetic production) in stands of isolated trees.     

光照条件、植株冠层结构和枝条寿命的关系——以桂花和水杉为例

根据衰老理论的代谢率假说,生物寿命与其代谢率有关,个体大小相同的生物体,在质量较好的微生境中通常比较差生境中具有更高的代谢速率。因此,生物体在资源供给较差的生境中通常比资源供给较充足的生境中具有更长的寿命。枝条是木本植物植冠构建的基本单元之一,如果枝条遵循代谢率假说,则可推测在光照较好环境下的植物枝条或小枝将比其在遮荫环境下具有更短的寿命,即枝条寿命与光照条件成反比。以常绿物种桂花(Osmanthus fragrans)和落叶物种水杉(Metasequoiaglyptostroboides)为研究对象,通过测量不同光照环境下,植株大小(株高和胸径)、冠层深度、冠层轮廓(冠层深度/冠层宽度)、相对冠层宽度(冠层宽度/植株高度)以及植株凋落枝条寿命等性状,探讨了光照条件对成年植株冠层形态结构和植株枝条寿命的影响。调查发现:1)枝条的寿命在遮荫条件下显著高于全光照条件下,与理论预测吻合;2)随遮荫程度增加,植株冠层深度和冠层轮廓增加,相对冠层宽度减小;3)枝条的平均寿命与植株冠层深度和冠层轮廓成正比,与植株相对冠层宽度成反比。这表明光照条件可能通过改变植株冠层结构来影响枝条寿命。未来需要进一步研究枝条生物量分配、叶片光合能力和呼吸速率在不同生活型物种之间的差异,以便更全面的理解枝条寿命与生境质量之间的关系。     

Impact of eastern dwarf mistletoe (Arceuthobium pusillum) on host white spruce (Picea glauca) development,growth and performance across multiple scales

Infection by eastern dwarf mistletoe (Arceuthobium pusillum) modifies needle and branch morphology and hastens white spruce (Picea glauca) mortality. We examined potential causal mechanisms and assessed the impacts of infection‐induced alterations to host development and performance across scales ranging from needle hormone contents to bole expansion. Needles on infected branches (IBs) possessed higher total cytokinin (CK) and lower abscisic acid contents than needles on uninfected branches (UBs). IBs exhibited greater xylem growth than same‐aged UBs, which is consistent with the promotive effect of CKs on vascular differentiation and organ sink strength. Elevated CK content may also explain the dense secondary and tertiary branching observed at the site of infection, i.e. the formation of ‘witches’ brooms' with significantly lower light capture efficiencies. Observed hormone perturbations were consistent with higher rates of transpiration, lower water use efficiencies (WUEs) and more negative needle carbon isotope ratios observed for IBs. Observed reductions in needle size allowed IBs to compensate for reduced hydraulic conductivity. Severe infections resulted in dramatically decreased diameter growth of the bole. It seems likely that the modifications to host hormone contents by eastern dwarf mistletoe infection led white spruce trees to dedicate a disproportionate fraction of their photoassimilate and other resources to self‐shaded branches with low WUE. This would have decreased the potential for fixed carbon accumulation, generating a decline in the whole‐tree resource pool. As mistletoe infections grew in size and the number of IBs increased, this burden was manifested as increasingly greater reductions in bole growth.     

Water-use efficiency as a means of modelling net assimilation in boreal forests

Although the processes governing photosynthesis are well understood, scaling from shoot to canopy in coniferous forests is complex. Development of different sap-flow techniques has made it possible to measure transpiration of whole trees and thereby also of whole canopies. There is a strong link between photosynthesis and transpiration, for which reason it would be interesting to test whether measurements of canopy transpiration could also be used to estimate canopy photosynthesis. As a first step towards this, water-use efficiency (WUE) was studied at branch and canopy scales on the basis of branch gas-exchange measurements, with half-hourly and daily temporal resolution. Half-hourly and daily WUE at both branch and canopy scales showed a strong dependency on vapour-pressure deficit ('e). Branch photosynthesis modelled from branch transpiration and 'e mimicked well measured branch photosynthesis. Also, modelled photosynthesis, scaled to canopy and compared to net forest CO₂ exchange measured by the eddy-covariance technique, occasionally showed good agreement. In spite of these seemingly promising results, there was a difference in the response to 'e between branches and between years, which needs to be better understood.     

Positive and negative interactions between environmental conditions affecting Cercocarpus ledifolius seedling survival

We evaluated the balance between positive and negative effects of environmental conditions on first-year seedling survival of the tree Cercocarpus ledifolius during two summers, 1996 and 1997. The experimental design was fully crossed with two levels of water, with and without supplementation, two levels of herbivory, with and without protection, and three major microhabitats, open interspaces, under the canopy of Artemisia tridentata shrubs, and under the canopy of mature C. ledifolius trees. Effects of drought and herbivory on seedling survival depended on the year. Water supplementation and herbivory protection during the dry summer of 1996 (27.7 mm) generally increased seedling survival. Additionally, survival tended to be greatest beneath C. ledifolius canopies. More important ecologically were the significant interactions. In 1996, water supplementation increased survival more with than without herbivory protection. The three-way interaction, treatment-microhabitat combination, was most important; by far the greatest survival was in the water supplementation and herbivory protection in the tree microhabitat. During the wet summer of 1997 (158.5 mm), neither water supplementation, herbivory protection, nor microhabitat were significant as main effects. The water-supplemented and herbivory-protected treatment again combined to yield highest survival, but this time in open interspaces rather than beneath trees. Our study shows how the importance of individual limiting factors and the relative favorableness of particular microhabitats appear to change across years depending on environmental conditions.     

Local weak light induces the improvement of photosynthesis in adjacent illuminated leaves in maize seedlings

To copy with highly heterogeneous light environment, plants can regulate photosynthesis locally and systemically, thus, maximizing the photosynthesis of individual plants. Therefore, we speculated that local weak light may induce the improvement of photosynthesis in adjacent illuminated leaves in plants. In order to test this hypothesis, maize seedlings were partially shaded, and gas exchange, chlorophyll a fluorescence and biochemical analysis were carefully assessed. It was shown that local shading exacerbated the declines in the photosynthetic rates, chlorophyll contents, electron transport and carbon assimilation‐related enzyme activities in shaded leaves as plants growth progressed. While, the decreases of these parameters in adjacent illuminated leaves of shaded plants were considerably alleviated compared to the corresponding leaves of control plants. Obviously, the photosynthesis in adjacent illuminated leaves in shaded plants was improved by local shading, and the improvement in adjacent lower leaves was larger than that in adjacent upper ones. As growth progressed, local shading induced higher abscisic acid contents in shaded leaves, but it alleviated the increase in the abscisic acid contents in adjacent leaves in shaded plants. Moreover, the difference in sugar content between shaded leaves and adjacent illuminated ones was gradually increased. Consequently, local weak light suppressed the photosynthesis in shaded leaves, while it markedly improved the photosynthesis of adjacent illuminated ones. Sugar gradient between shaded leaves and adjacent illuminated ones might play a key role in photosynthetic regulation of adjacent illuminated leaves.     

Clonal growth and its effects on male and female reproductive success in <Emphasis Type="Italic">Prunus ssiori</Emphasis> (Rosaceae)

Clonal growth can increase not only floral display but also geitonogamy and may affect sexual reproduction both positively and negatively. A clonal woody species, Prunus ssiori, was partially self-incompatible according to a pollination experiment. Its main pollinators, bumble bees, were often observed to consecutively visit inflorescences within a tree. Clone identification revealed that its genets formed mutually exclusive patches. These features suggest frequent geitonogamous pollination. In a 6.24-ha plot, 212 trees belonged to 59 genets, and 42 genets consisted of a single tree, whereas the rest contained two or more clonal trees. The largest genet had 65 trees and occupied 0.4 of a hectare. Fruit set was measured in 127 inflorescences sampled from nine maternal trees at the center of the plot. Paternal genets of 107 of their 300 seeds were assigned in the plot using microsatellites. There were no selfed seeds. Male reproductive success (the probability that individual trees of each genet sired a seed) increased as tree size increased, as the distance between the trees and maternal trees decreased, when the genet did not contain the maternal trees, and when the genet consisted of a single tree. Female reproductive success (fruit set in individual inflorescences of each maternal tree) increased as the within-tree geitonogamy index, which reflected the frequency of pollination within the maternal tree, decreased. These results suggest that clonal growth reduces male reproductive success, at least, in P. ssiori, because of pollen discounting.     

Uniform versus Asymmetric Shading Mediates Crown Recession in Conifers

In this study we explore the impact of asymmetrical vs. uniform crown shading on the mortality and growth of upper and lower branches within tree crowns, for two conifer species: shade intolerant lodgepole pine (Pinus contorta) and shade tolerant white spruce (Picea glauca). We also explore xylem hydraulics, foliar nutrition, and carbohydrate status as drivers for growth and expansion of the lower and upper branches in various types of shading. This study was conducted over a two-year period across 10 regenerating forest sites dominated by lodgepole pine and white spruce, in the lower foothills of Alberta, Canada. Trees were assigned to one of four shading treatments: (1), complete uniform shading of the entire tree, (2) light asymmetric shading where the lower 1/4–1/3 of the tree crown was shaded, (3) heavy asymmetric shading as in (2) except with greater light reduction and (4) control in which no artificial shading occurred and most of the entire crown was exposed to full light. Asymmetrical shading of only the lower crown had a larger negative impact on the bud expansion and growth than did uniform shading, and the effect was stronger in pine relative to spruce. In addition, lower branches in pine also had lower carbon reserves, and reduced xylem-area specific conductivity compared to spruce. For both species, but particularly the pine, the needles of lower branches tended to store less C than upper branches in the asymmetric shade, which could suggest a movement of reserves away from the lower branches. The implications of these findings correspond with the inherent shade tolerance and self-pruning behavior of these conifers and supports a carbon based mechanism for branch mortality – mediated by an asymmetry in light exposure of the crown.     

Benefits of clonal integration between interconnected ramets of Vallisneria spiralis in heterogeneous light environments

Interconnected ramets of the submersed macrophyte Vallisneria spiralis were subjected to two homogeneous treatments (shading or not shading whole plants) and two heterogeneous treatments (only shading basal or apical ramets of plants). The benefits and costs of clonal integration between connected ramets grown in heterogeneous treatments were examined. Results showed that shading apical ramets induced significant benefits to the performance of whole plant in terms of dry weight per plant (P < 0.01) and number of ramets per plant (P < 0.05). Especially for the unshaded basal ramets, their dry weight, number of ramets, number of branches and total stolon length were 89%, 30%, 29% and 58% higher than the corresponding ramets in homogeneous treatment, respectively. Compared to their controls in homogeneous treatments, unshaded basal ramets produced more leaf mass (0.15 g versus 0.11 g) whereas shaded apical ramets produced more root mass (0.012 g versus 0.008 g). However, there was a different pattern of integration when basal ramets were shaded. Shading basal ramets led to a significant decrease in stolon growth, but the individual performance of shaded ramets improved. Cost-benefit analyses revealed that dry weight per ramet of basal shaded ramets was 31% greater than that of basal shaded ramets in the homogeneous treatment. We can conclude that V. spiralis can benefit from clonal integration in heterogeneous light environments, but that the scale of these benefits is related to the quality of light environments where the clone become established.