The space-use strategy of plants with different growth forms,in a field experiment with manipulated nutrients and light

The biomass allocation pattern in plants is known to depend on the below and above-ground resource availabilities. In a herbaceous multi-species stand, it can be expected that the effects of nutrient and light availability on plants’ general space-use strategy are fundamentally different. We hypothesized that nutrient status alters the amount of biomass produced per unit canopy volume (biomass density), but not so much the biomass vertical distribution pattern. Changes in light availability, in contrast, should affect the vertical distribution pattern of biomass but not biomass density. We were also interested in whether the effect of resource manipulation on a plant’s space-use strategy depends on its basic morphological characteristics (growth form). The results from a four-year permanent plot experiment in a species-rich grassland, with fertilization and additional illumination from mirrors applied to 40 × 40 cm plots, showed that our main hypothesis was correct. Fertilization significantly affected biomass density above as well as below-ground, while additional illumination generally did not. Light addition altered the vertical distribution pattern of above-ground biomass, which remained unaffected by the fertilizer treatment.     

Structure and diversity of a species-rich grassland community, treated with additional illumination, fertilization and mowing

We examined the changes of species‐rich herbaceous community canopy structure and species diversity in a five‐year field experiment. Above‐ and below‐ground resource addition (fertilization and additional illumination with the help of mirrors) and cessation of annual mowing were applied as experimental treatments in 24 permanent plots of 40×40 cm. Canopy structure was studied by point quadrat sampling using laser beam to randomly position point quadrats at two different angles of observation – 60° and 120° from horizontal, north‐south direction. We hypothezised that the studied plant community is not light‐limited under the normal conditions (non‐fertilized and annually mown) but would become light‐limited after nutrient addition and/or cessation of mowing. In this case the effect of fertilization and cessation of mowing could be mitigated by introducing additional light resource (placing south‐facing mirrors).
Species richness in plots was not significantly altered by experimental treatments, most probably because of a strong input of species from surrounding areas by the means of lateral vegetative spread, compensating for the expected decrease of diversity with fertilization and cessation of mowing. Surprisingly, plant total coverage in a plot was the only variable significantly explaining plot richness, independent of experimental treatments and the year of observation. We therefore studied also species richness per unit coverage (standardized richness) which appeared to be significantly reduced by fertilization. Annual change of plant total coverage of a plot was good predictor of annual change of plot richness, except for the exceptionally dry period (1994–1995). There was no indication of light being limiting for the plants in control plots, but light obviously became limiting after nutrient addition – in fertilized plots additional illumination lead to a notably denser canopy. Generally, the average exposition of plant leaves surface was towards the north, independent of experimental treatments. However, in the exceptionally cloudy and cool year 1996 the situation was radically different – leaves were exposed predominantly towards the south, indicating that multi‐species canopy can plastically react to long‐term changes in light availability.     

Effects of additional illumination and fertilization on seasonal changes in fine-scale grassland community structure

Abstract. Fine-scale structure of a species-rich grassland was examined for seasonal changes caused by manipulated changes in the availability of above and below-ground resources (additional illumination with the help of mirrors and fertilization) in a field experiment. If changes induced by fertilization — which are expected to lead to a reduction in small-scale diversity —are due to intensified light competition, they should be compensated for by additional light input. Permanent plots of 40 cm × 40 cm were sampled by the point quadrat method at three angles (60°, 90° and 120° from the horizontal North-South direction), using a laser beam to position the quadrats, in early July and early September. The applied treatments did not cause apparent changes in plant leaf orientation. The degree of spatial aggregation of biomass increased seasonally in fertilized, non-illuminated plots: greater productivity at a constant light supply led to a faster growth rate of potentially dominant species, as compared to the subordinate ones. Additional illumination mitigated this effect of fertilization, indicating that the observed changes in biomass aggregation were due to increased light competition. There was a considerable seasonal decrease of variance ratio (ratio of observed variance of richness at a point and variance expected at random) in fertilized only and in illuminated only plots. In fertilized plots this was due to the positive relationship between biomass aggregation and expected variance of richness. Biomass constancy occurs to be inversely related to deficit in variance of richness. In illuminated plots, in contrast, only the observed variance of richness decreased seasonally, indicating a more uniform use of space by different species. Evidently, a deficit in variance of richness can be caused by drastically different processes, showing that the variance ratio statistic may not have a significant explanatory value in fine-scale community studies.     

Soil fertility alters the nature of plant–resource interactions in invaded grassland communities

Resource competition theory suggests that the nature of diversity–resource–invasibility interactions will vary along fertility gradients, concurrent with changes in the relative availability of limiting above- versus below-ground resources. Experimental support for this contingency is lacking. Here, we manipulated resident diversity, baseline fertility, and the availabilities of light and soil nitrogen in grassland communities invaded by two functionally distinct non-native plant species (Lolium arundinaceum and Melilotus alba). We tested the hypotheses that increased resident diversity reduces community invasibility and dampens the effects of light and soil nitrogen pulses, and that the relative effects of light versus soil nitrogen additions on diversity–invasibility relationships depend on the baseline fertility of the study system. Our results reveal an overall weak negative effect of resident diversity on Lolium performance, but in contrast to our expectations, this diversity effect did not vary with light or soil nitrogen additions or with baseline fertility. However, the relative effects of above- versus below-ground resource additions on invader performance varied with baseline fertility as expected: Lolium responded most strongly to soil nitrogen additions in low-fertility mesocosms and most strongly to increased light availability in high-fertility mesocosms. In contrast to Lolium, nitrogen-fixing Melilotus was overall less responsive to diversity and resource manipulations. Together, these patterns do not lend support for the dependence of diversity–resource–invasibility relationships on either baseline fertility or invasive species identity, but they do highlight the dominant role of resources over diversity in determining invader performance, as well as the manner in which fertility alters the relative importance of above- versus below-ground resource pulses in promoting invasions.     

Individual variability in tree allometry determines light resource allocation in forest ecosystems: a hierarchical Bayesian approach

Tree species differences in crown size and shape are often highlighted as key characteristics determining light interception strategies and successional dynamics. The phenotypic plasticity of species in response to light and space availability suggests that intraspecific variability can have potential consequences on light interception and community dynamics. Species crown size varies depending on site characteristics and other factors at the individual level which differ from competition for light and space. These factors, such as individual genetic characteristics, past disturbances or environmental micro-site effects, combine with competition-related phenotypic plasticity to determine the individual variability in crown size. Site and individual variability are typically ignored when considering crown size and light interception by trees, and residual variability is relegated to a residual error term, which is then ignored when studying ecological processes. In the present study, we structured and quantified variability at the species, site, and individual levels for three frequently used tree allometric relations using fixed and random effects in a hierarchical Bayesian framework. We focused on two species: Abies alba (silver fir) and Picea abies (Norway spruce) in nine forest stands of the western Alps. We demonstrated that species had different allometric relations from site to site and that individual variability accounted for a large part of the variation in allometric relations. Using a spatially explicit radiation transmission model on real stands, we showed that individual variability in tree allometry had a substantial impact on light resource allocation in the forest. Individual variability in tree allometry modulates species’ light-intercepting ability. It generates heterogeneous light conditions under the canopy, with high light micro-habitats that may promote the regeneration of light-demanding species and slow down successional dynamics.     

Plasticity and ontogenetic drift of biomass allocation in response to above- and below-ground resource availabilities in perennial herbs: a case study of <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis>

Changes in plant biomass allocation in response to varying resource availabilities may result from ontogenetic drift caused by allometric growth (i.e., apparent plasticity), a true adjustment of ontogenetic trajectories (true plasticity) or both (complex plasticity). Given that the root allocation of annual species usually decreases during the growth, the developmentally explicit model predicts that annual herbs will exhibit true plasticity in root allocation under above-ground resource limitation and apparent plasticity for moderate stress of below-ground resource. For perennial species, the root allocation of which increases during growth, the reverse patterns would be expected. In this study, we tested the developmentally explicit model with a perennial weed, Alternanthera philoxeroides (Mart.) Griseb. We report its adaptive changes and ontogenetic drift of root allocation in response to different resource levels (i.e., light, water and nutrient availability) by comparing root allocation on both an age and a size basis. The root allocation of A. philoxeroides increased with the size (i.e., ontogenetic drift) during the growth, and exhibited significant changes in response to different resource availabilities. Furthermore, the root allocation in response to water or nutrient availability exhibited typical complex plasticity, while the light stress only slowed down the growth, with the ontogenetic trajectory unchanged (apparent plasticity). The contrasting responses to above-ground and below-ground stresses were consistent with the prediction of the developmentally explicit model.     

Trade-off between light availability and soil fertility determine refugial conditions for the relict light-demanding species in lowland forests

Identifying potential refugial habitats in the face of rapid environmental change is a challenge faced by scientists and nature conservation managers. Relict populations and refugial habitats are the model objects in those studies. Based on the example of Actaea europaea from Central Poland, we analyse the habitat factors influencing relict populations of continental, light-demanding species in lowland forests and examine which habitats of studied species corresponding most closely to ancient vegetation. Our results indicate that the current refugial habitats of Actaea europaea include not only communities which are very similar to ancient open forest but also forests with a closed canopy. Although the populations are influenced by nitrogen and light availability, the co-occurrence of these two factors in forest communities is limited by dense canopy formation by hornbeam and beech trees on fertile soils and in more humid conditions. Our findings indicate that the future survival of relict, light-demanding communities in lowland forests requires low-intensity disturbances to be performed in tree-stands, according to techniques, which imitate traditional forests management.     

Conditional allelopathic potential of temperate lianas

Allelopathy is often treated as an innate characteristic of a species rather than a phenotypically plastic trait that can vary with environmental conditions. Lianas are a highly competitive, phenotypically plastic life form that typically occur in both shaded and unshaded environments. As such, we hypothesized that temperate lianas may conditionally change allocation to allelopathic chemicals in response to light availability though the expected direction of change is unclear. Shading may reduce resource availability and therefore reduce allocation to allelochemicals, induce allelopathy as a competitive mechanism, or may not be related to allelopathy. To test the conditionality of allelopathy, sun and shade leaves of five common liana species (Toxicodendron radicans, Parthenocissus quinquefolia, Celastrus orbiculatus, Lonicera japonica, and Vitis vulpina) were collected from a young deciduous forest in New Jersey, USA, and tested with laboratory bioassays to detect allelopathic potential. All liana species showed allelopathic potential, and three species exhibited induction of increased allelopathic potential in shaded environments. The two species that were not shade induced are late successional lianas that persist for long periods in forest canopies. In contrast, the inducible lianas were early successional species that typically decline with canopy closure. This research indicates that lianas have the potential to be allelopathic and allelopathic potential conditionally responds to shading only for species that would normally be excluded from the forest canopy. As early successional lianas are present throughout forest regeneration in a range of light environments, allelopathic plasticity may increase their success by differentially allocating resources based on environmental conditions.     

Effects of above- and below-ground competition from shrubs on photosynthesis, transpiration and growth in Quercus robur L. seedlings

For a tree seedling to successfully establish in dense shrubbery, it must maintain function under heterogeneous resource availability. We evaluated leaf-level acclimation in photosynthetic capacity, seedling-level transpiration, and seedling morphology and growth to gain an understanding of the effects of above- and below-ground competition on Quercus robur seedlings. Experimental seedlings were established in a typical southern Swedish shrub community where they received 1 of 4 competition levels (above-ground, below-ground, above- and below-ground, or no competition), and leaf-level responses were examined between two growth flushes. Two years after establishment, first-flush leaves from seedlings receiving above-ground competition showed a maximum rate of photosynthesis (A_max) 40% lower than those of control seedlings. With the development of a second flush above the shrub canopy, A_max of these seedlings increased to levels equivalent to those of seedlings free of light competition. Shrubby competition reduced oak seedling transpiration such that seedlings exposed to above- and below-ground competition showed rates 43% lower than seedlings that were not exposed to competition. The impaired physiological function of oak seedlings growing amid competition ultimately led to a 60-74% reduction in leaf area, 29-36% reduction in basal diameter, and a 38-78% reduction in total biomass accumulation, but root to shoot ratio was not affected. Our findings also indicate that above-ground competition reduced A_max, transpiration and biomass accumulation more so than below-ground competition. Nevertheless, oak seedlings exhibited the ability to develop subsequent growth flushes with leaves that had an A_max acclimated to utilize increased light availability. Our findings highlight the importance of flush-level acclimation under conditions of heterogeneous resource availability, and the capacity of oak seedlings to initiate a positive response to moderate competition in a shrub community.     

Canopy cover type,and not fine-scale resource availability,explains native and exotic species richness in a landscape affected by anthropogenic fires and posterior land-use change

Anthropogenic fires and land-use change, including the conversion from native to exotic species canopies, are two major types of disturbances that strongly affect the functioning of forest ecosystems around the world. These disturbances alter the resource availability for plants, which may lead to changes in species richness. Here we examined the relative effects of canopy cover type, light availability and soil nutrient (N and P) availability on species richness, including invasive species, at different post-fire plant systems. Additionally, we tested the resource heterogeneity hypothesis (RHH) for plant diversity, which proposes that diversity is higher in habitats with spatially heterogeneous resources. We evaluated four different canopy cover types, including mature and second-growth Nothofagus pumilio forests, treeless prairie, Pinus sylvestris afforestations, all of which were converted from mature N. pumilio forests. Using generalized mixed-effects model correlations, we determined (1) the relative influence of canopy cover type, light and soil nutrient availability on understory species richness and (2) the relationship between species richness and resource heterogeneity. We found that canopy cover type was the factor that best explained species richness, much more than fine-scale light and soil nutrient availability. Additionally, we found that the more homogeneous the light environment the higher the number of exotic species (mainly found in the prairie where the highest light intensity occurred), which is contrary to what the RHH states. In conclusion, canopy cover type, a stand-scale driver, and not fine-scale resource (light, N and P) availability, was most important for explaining native and exotic (including invasive) species understory richness in a landscape affected by anthropogenic fires and posterior land-use change.     

Effects of light and nutrient availability on dry matter and N allocation in six successional grassland species

Summary Recent discussions on determinants of competitive success during succession require the study of the combined effect of light and nutrient availability on growth and allocation. These effects can be used to predict the outcome of competition at changing resource availabilities. This work is part of a study on the successional sequence in permanent grassland starting after fertilizer application is stopped, but with continued mowing, in order to restore former species-rich communities. This yields a successional sequence which proceeds from grasslands with a high nutrient availability and a closed canopy, to grasslands with a low nutrient availability and an open canopy. If allocation is related to competitive ability, species from the productive stages would be expected to allocate more biomass and nitrogen to leaves, which could make them better competitors for light, while species from the unproductive stages would allocate more biomass to roots, which could make them better nutrient competitors. This study reports on growth, specific leaf area (SLA), vertical display of leaves, and allocation of biomass and nitrogen of six grassland species from this successional sequence at 16 combinations of light and nutrient supply. Species from the poorer successional stages reached a lower final dry weight than species from the richer stages, over all treatment combinations. The experimental design made it possible to test for unique effects of the resource ratio effect of light and nutrients on allocation characteristics. This resource-ratio effect was defined as the ratio light intensity/(light intensity + nutrient supply rate), using standardized levels for the treatments. The within-species variation (plasticity) in both allocation of dry matter and nitrogen was linearly related to this resource-ratio effect. Some interspecific differences in this relationship were found which could be related to the position of the species along the successional gradient. However, the range of plasticity in allocation pattern expressed within each species was much larger than the differences between species. It was concluded that allocation differences between these grassland species are relatively unimportant, given the large amount of plasticity in these traits. Interspecific differences in SLA and vertical stature seemed to be more important in explaining the position of species along the successional gradient.     

Above- and below-ground resource acquisition strategies determine plant species responses to nitrogen enrichment

Background and AimsKnowledge of plant resource acquisition strategies is crucial for understanding the mechanisms mediating the responses of ecosystems to external nitrogen (N) input. However, few studies have considered the joint effects of above-ground (light) and below-ground (nutrient) resource acquisition strategies in regulating plant species responses to N enrichment. Here, we quantified the effects of light and non-N nutrient acquisition capacities on species relative abundance in the case of extra N input.MethodsBased on an N-manipulation experiment in a Tibetan alpine steppe, we determined the responses of species relative abundances and light and nutrient acquisition capacities to N enrichment for two species with different resource acquisition strategies (the taller Stipa purpurea, which is colonized by arbuscular mycorrhizal fungi, and the shorter Carex stenophylloides, which has cluster roots). Structural equation models were developed to explore the relative effects of light and nutrient acquisition on species relative abundance along the N addition gradient.Key ResultsWe found that the relative abundance of taller S. purpurea increased with the improved light acquisition along the N addition gradient. In contrast, the shorter C. stenophylloides, with cluster roots, excelled in acquiring phosphorus (P) so as to elevate its leaf P concentration under N enrichment by producing large amounts of carboxylate exudates that mobilized moderately labile and recalcitrant soil P forms. The increased leaf P concentration of C. stenophylloides enhanced its light use efficiency and promoted its relative abundance even in the shade of taller competitors.ConclusionsOur findings highlight that the combined effects of above-ground (light) and below-ground (nutrient) resources rather than light alone (the prevailing perspective) determine the responses of grassland community structure to N enrichment.     

Within-canopy variation in photosynthetic capacity,SLA and foliar N in temperate broad-leaved trees with contrasting shade tolerance

Key message

The relative shade tolerance of T. cordata , F. sylvatica , and C. betulus in mature stands is based on different species-specific carbon and nitrogen allocation patterns.

Abstract

The leaf morphology and photosynthetic capacity of trees are remarkably plastic in response to intra-canopy light gradients. While most studies examined seedlings, it is not well understood how plasticity differs in mature trees among species with contrasting shade tolerance. We studied light-saturated net photosynthesis (A _max), maximum carboxylation rate (V _cmax), electron transport capacity (J _max) and leaf dark respiration (R _d) along natural light gradients in the canopies of 26 adult trees of five broad-leaved tree species in a mixed temperate old-growth forest (Fraxinus excelsior, Acer pseudoplatanus, Carpinus betulus, Tilia cordata and Fagus sylvatica), representing a sequence from moderately light-demanding to highly shade-tolerant species. We searched for species differences in the dependence of photosynthetic capacity on relative irradiance (RI), specific leaf area (SLA) and nitrogen per leaf area (N _a ). The three shade-tolerant species (C. betulus, T. cordata, F. sylvatica) differed from the two more light-demanding species by the formation of shade leaves with particularly high SLA but relatively low N _a and consequently lower area-based A _max, and a generally higher leaf morphological and functional plasticity across the canopy. Sun leaf morphology and physiology were more similar among the two groups. The three shade-tolerant species differed in their shade acclimation strategies which are primarily determined by the species’ plasticity in SLA. Under low light, T. cordata and F. sylvatica increased SLA, mass-based foliar N and leaf size, while C. betulus increased solely SLA exhibiting only low intra-crown plasticity in leaf morphology and N allocation patterns. This study with mature trees adds to our understanding of tree species differences in shade acclimation strategies under the natural conditions of a mixed old-growth forest.     

Determinants of community organization of a South African mesic grassland

Abstract. Question: What is the long‐term influence of nutrient availability, productivity and soil pH on grassland community organization? Location: Ukulinga research farm, KwaZulu‐Natal, South Africa. Methods: The influence of fertilization on soil pH, nitrogen (N) and phosphorus (P) on variation in plant traits, community composition and species richness were examined in a 50‐year grassland fertilization experiment. Results: Averaged over 30 years, above‐ground net primary production (ANPP) was 337, 428 and 518 g.m^‐2 in sites not fertilized, fertilized with N, and fertilized with N plus P respectively. ANPP depended directly on N‐fertilization but not on P‐fertilization or liming, and responded positively to the interaction of N (first limiting nutrient) and P (second limiting nutrient). Short narrow‐leaved grass species —Themeda triandra, Tristachya leucothrix and Setaria nigrirostris— dominated sites of lowest ANPP where N was limiting (unfertilized, P‐fertilized or limed sites). A tall narrow‐leaved species, Eragrostis curvula, dominated sites of intermediate ANPP where P was limiting (N‐fertilized sites). By contrast, a tall broad‐leaved species, Panicum maximum, dominated the most productive sites where neither N nor P were limiting (N‐ and P‐fertilized sites). Certain species responded to liming and type of N‐fertilizer apparently because of their effects on soil pH. N‐fertilization reduced the density of herbaceous dicots (forbs) from 14 (unfertilized) to two (high N, no P, no lime) and five species per m² (high N, no P, limed). This effect was attributed to increased ANPP and a decrease in soil pH from 4.6 (KCl) in unfertilized sites to 3.49 (high N, no lime) and 4.65 (high N and lime). Soil acidification had no effect on grass species richness but influenced the abundance of certain species. Conclusions: Grassland community organization is determined not only by the influence of N availability, but also by the hierarchical interaction of N and P availability, in part through their compounded effect on ANPP, and by individualistic species responses to soil pH.     

The role of below-ground competition during early stages of secondary succession: the case of 3-year-old Scots pine (Pinus sylvestris L.) seedlings in an abandoned grassland

In abandoned or extensively managed grasslands, the mechanisms involved in pioneer tree species success are not fully explained. Resource competition among plants and microclimate modifications have been emphasised as possible mechanisms to explain variation of survivorship and growth. In this study, we evaluated a number of mechanisms that may lead to successful survival and growth of seedlings of a pioneer tree species (Pinus sylvestris) in a grass-dominated grassland. Three-year-old Scots pines were planted in an extensively managed grassland of the French Massif Central and for 2 years were either maintained in bare soil or subjected to aerial and below-ground interactions induced by grass vegetation. Soil temperatures were slightly higher in bare soil than under the grass vegetation, but not to an extent explaining pine growth differences. The tall grass canopy reduced light transmission by 77% at ground level and by 20% in the upper part of Scots pine seedlings. Grass vegetation presence also significantly decreased soil volumetric water content (Hv) and soil nitrate in spring and in summer. In these conditions, the average tree height was reduced by 5% compared to trees grown in bare soil, and plant biomass was reduced by 85%. Scots pine intrinsic water-use efficiency (A/g), measured by leaf gas-exchange, increased when Hv decreased owing to a rapid decline of stomatal conductance (g). This result was also confirmed by δ ¹³C analyses of needles. A summer ¹⁵N labelling of seedlings and grass vegetation confirmed the higher NO₃ capture capacity of grass vegetation in comparison with Scots pine seedlings. Our results provide evidence that the seedlings' success was linked to tolerance of below-ground resource depletion (particularly water) induced by grass vegetation based on morphological and physiological plasticity as well as to resource conservation.     

A review of light interception in plant stands from leaf to canopy in different plant functional types and in species with varying shade tolerance

Changes in the efficiency of light interception and in the costs for light harvesting along the light gradients from the top of the plant canopy to the bottom are the major means by which efficient light harvesting is achieved in ecosystems. In the current review analysis, leaf, shoot and canopy level determinants of plant light harvesting, the light-driven plasticity in key traits altering light harvesting, and variations among different plant functional types and between species of different shade tolerance are analyzed. In addition, plant age- and size-dependent alterations in light harvesting efficiency are also examined. At the leaf level, the variations in light harvesting are driven by alterations in leaf chlorophyll content modifies the fraction of incident light harvested by given leaf area, and in leaf dry mass per unit area (M _A) that determines the amount of leaf area formed with certain fraction of plant biomass in the leaves. In needle-leaved species with complex foliage cross-section, the degree of foliage surface exposure also depends on the leaf total-to-projected surface area ratio. At the shoot scale, foliage inclination angle distribution and foliage spatial aggregation are the major determinants of light harvesting, while at the canopy scale, branching frequency, foliage distribution and biomass allocation to leaves (F _L) modify light harvesting significantly. F _L decreases with increasing plant size from herbs to shrubs to trees due to progressively larger support costs in plant functional types with greater stature. Among trees, F _L and stand leaf area index scale positively with foliage longevity. Plant traits altering light harvesting have a large potential to adjust to light availability. Chlorophyll per mass increases, while M _A, foliage inclination from the horizontal and degree of spatial aggregation decrease with decreasing light availability. In addition, branching frequency decreases and canopies become flatter in lower light. All these plastic modifications greatly enhance light harvesting in low light. Species with greater shade tolerance typically form a more extensive canopy by having lower M _A in deciduous species and enhanced leaf longevity in evergreens. In addition, young plants of shade tolerators commonly have less strongly aggregated foliage and flatter canopies, while in adult plants partly exposed to high light, higher shade tolerance of foliage allows the shade tolerators to maintain more leaf layers, resulting in extended crowns. Within a given plant functional type, increases in plant age and size result in increases in M _A, reductions in F _L and increases in foliage aggregation, thereby reducing plant leaf area index and the efficiency of light harvesting. Such dynamic modifications in plant light harvesting play a key role in stand development and productivity. Overall, the current review analysis demonstrates that a suite of chemical and architectural traits at various scales and their plasticity drive plant light harvesting efficiency. Enhanced light harvesting can be achieved by various combinations of traits, and these suites of traits vary during plant ontogeny.     

High shoot plasticity favours plant coexistence in herbaceous vegetation

Several theoretical considerations imply that high shoot morphological plasticity could increase competition symmetry and favour plant coexistence. We tested whether mean plasticity across co-occurring species is a key trait for explaining ramet density and species richness in herbaceous vegetation. We used three data sets to test the hypotheses: (a) experimentally achieved estimates of plasticity to light availability for 35 herbaceous species; (b) richness, ramet density and canopy architecture data from 17 herbaceous communities; (c) species richness data from a 5-year permanent-plot study in a calcareous grassland. In herbaceous communities containing species with relatively higher shoot plasticity, ramet density was significantly higher. Consequently, relatively more species were growing per unit area—a greater proportion of the community species pool was represented on 1 m². In the permanent plot study species-richness was higher in those 40×40 cm quadrats where species with high shoot plasticity prevailed—there was a positive regression of richness on the mean plasticity of species. This relationship was highly significant in five consecutive years. Our results suggest that shoot plasticity to light availability is evidently one of the key traits in processes that alter the density of co-existing plants and, therefore, species diversity in herbaceous communities. Electronic Supplementary Material Supplementary material is available for this article at     

Phenotypic plasticity and biomass allocation pattern in three <Emphasis Type="Italic">Dryopteris</Emphasis> (Dryopteridaceae) species on an experimental light-availability gradient

We were interested in whether the contrasting regional distribution patterns of three congeneric, frequently co-occurring fern species (Dryopteris carthusiana, D. dilatata and D. expansa) could be explained by differential biomass allocation strategies and different phenotypic plasticities to light availability. The morphology and habitat preference of these ferns are known to be very similar, but in Estonia, their frequencies of occurrence differ sharply––Dryopteris carthusiana is common, D. expansa grows in scattered localities, and D. dilatata is rare. We grew the species under different levels of illumination (100, 50, 25 and 10% of full daylight) in an experimental garden to compare their autecological responses to shading. After one growing season there were clear interspecific differences in total plant biomass accumulation––D. carthusiana > D. expansa > D. dilatata––indicating the possible competitive inferiority of the latter at the young sporophyte stage. D. expansa was the least shade-tolerant, with biomass decreasing sharply under less than 50% illumination; D. dilatata was the most shade-tolerant, with similar growth at all illumination levels. In relative biomass allocation patterns, the most notable differences among species were in the relative shares of biomass stored in rhizomes. In D. carthusiana and D. expansa this share was nearly constant and independent of the illumination conditions. D. dilatata allocated very little biomass into rhizome in deep shade, but was able to increase this share more than twofold in 50% light. Dryopteris dilatata was clearly shown to be morphologically the most plastic of the three. In four traits––rhizome mass, frond:below-ground biomass ratio, stipe length and specific leaf area––its degree of ontogenetic plasticity to light was significantly higher than that of D. expansa and D. carthusiana. While the general performance (biomass production) of species in the experiment coincided with that observed in nature, the results of plasticity estimation were somewhat surprising––it is difficult to explain the inferior performance of a species (D. dilatata) through high morphological plasticity. Probably, the species is rare either because of certain climatic restrictions, or because it is presently expanding its distribution and is in the phase of invading Estonian understory communities.     

Leaf age dependent changes in within-canopy variation in leaf functional traits: a meta-analysis

Within-canopy variation in leaf structural and photosynthetic characteristics is a major means by which whole canopy photosynthesis is maximized at given total canopy nitrogen. As key acclimatory modifications, leaf nitrogen content (N _A) and photosynthetic capacity (A _A) per unit area increase with increasing light availability in the canopy and these increases are associated with increases in leaf dry mass per unit area (M _A) and/or nitrogen content per dry mass and/or allocation. However, leaf functional characteristics change with increasing leaf age during leaf development and aging, but the importance of these alterations for within-canopy trait gradients is unknown. I conducted a meta-analysis based on 71 canopies that were sampled at different time periods or, in evergreens, included measurements for different-aged leaves to understand how within-canopy variations in leaf traits (trait plasticity) depend on leaf age. The analysis demonstrated that in evergreen woody species, M _A and N _A plasticity decreased with increasing leaf age, but the change in A _A plasticity was less suggesting a certain re-acclimation of A _A to altered light. In deciduous woody species, M _A and N _A gradients in flush-type species increased during leaf development and were almost invariable through the rest of the season, while in continuously leaf-forming species, the trait gradients increased constantly with increasing leaf age. In forbs, N _A plasticity increased, while in grasses, N _A plasticity decreased with increasing leaf age, reflecting life form differences in age-dependent changes in light availability and in nitrogen resorption for growth of generative organs. Although more work is needed to improve the coverage of age-dependent plasticity changes in some plant life forms, I argue that the age-dependent variation in trait plasticity uncovered in this study is large enough to warrant incorporation in simulations of canopy photosynthesis through the growing period.     

Scales of aboveground and below-ground competition in a semi-arid woodland detected from spatial pattern

Abstract. Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy patches. We hypothesized that both aboveground competition (within canopy patches), and below-ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis-Juniperus monosperma woodland in New Mexico using Ripley's K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density-dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below-ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- and below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above- and below-ground competition.