Photosynthetic acclimation of the liana Stigmaphyllon lindenianum to light changes in a tropical dry forest canopy

Tropical plant canopies show abrupt changes in light conditions across small differences in spatial and temporal scales. Given the canopy light heterogeneity, plants in this stratum should express a high degree of plasticity, both in space (allocation to plant modules as a function of opportunity for resource access) and time (photosynthetic adjustment to temporal changes in the local environment). Using a construction crane for canopy access, we studied light acclimation of the liana Stigmaphyllon lindenianum to sun and shade environments in a tropical dry forest in Panama during the wet season. Measured branches were randomly distributed in one of four light sequences: high- to low-light branches started the experiment under sun and were transferred to shade during the second part of the experiment; low- to high-light branches (LH) were exposed to the opposite sequence of light treatments; and high-light and low-light controls , which were exposed only to sun and shade environments, respectively, throughout the experiment. Shade branches were set inside enclosures wrapped in 63% greenhouse shade cloth. After 2 months, we transferred experimental branches to opposite light conditions by relocating the enclosures. Leaf mortality was considerably higher under shade, both before and after the transfer. LH branches reversed the pattern of mortality by increasing new leaf production after the transfer. Rates of photosynthesis at light saturation, light compensation points, and dark respiration rates of transferred branches matched those of controls for the new light treatment, indicating rapid photochemical acclimation. The post-expansion acclimation of sun and shade foliage occurred with little modification of leaf structure. High photosynthetic plasticity was reflected in an almost immediate ability to respond to significant changes in light. This response did not depend on the initial light environment, but was determined by exposure to new light conditions. Stigmaphyllon responded rapidly to light changes through the functional adjustment of already expanded foliage and an increase in leaf production in places with high opportunity for carbon gain. Received: 24 April 1998 / Accepted: 11 May 1999     

Photosynthetic responses of Miconia species to canopy openings in a lowland tropical rainforest

We examined the photosynthetic acclimation of three tropical species of Miconia to canopy openings in a Costa Rican rainforest. The response of photosynthesis to canopy opening was very similar in Miconia affinis, M. gracilis, and M. nervosa, despite differences in growth form (trees and shrubs) and local distributions of plants (understory and gap). Four months after the canopy was opened by a treefall, photosynthetic capacity in all three species had approximately doubled from closed canopy levels. There were no obvious signs of high light damage after treefall but acclimation to the gap environment was not immediate. Two weeks after treefall, A_max, stomatal conductance, apprarent quantum efficiency, and dark respiration rates had not changed significantly from understory values. The production of new leaves appears to be an important component of light acclimation in these species. The only variables to differ significantly among species were stomatal conductance at A_max and the light level at which assimilation was saturated. M. affinis had a higher stomatal conductance which may reduce its water use efficiency in gap environments. Photosynthesis in the more shade-tolerant M. gracilis saturated at lower light levels than in the other two species. Individual plant light environments were assessed after treefall with canopy photography but they explained only a small fraction of plant variation in most measures of photosynthesis and growth. In conclusion, we speculate that species differences in local distribution and in light requirements for reproduction may be more strongly related to species differences in carbon allocation than in carbon assimilation.     

Photosynthetic acclimation to light in juveniles of two cloud forest tree species

The photosynthetic response of juveniles of Decussocarpus rospigliosii, an emergent primary forest species and shade tolerant in its juvenile stages and Alchornea triplinervia, a gap-colonizing species of tropical cloud forest in Venezuela was studied. Daily courses of microenvironmental variables and gas exchange under contrasting light conditions (gap and understory) were carried out in their natural environment and transplanted to different light regimes (shade and sun) in the field. The photosynthetic response and some anatomical characteristics of plants from different treatments were analyzed in the laboratory. Photosynthetic rates were low for both species, and were negative during some diurnal periods, related to the low photosynthetically active radiation levels obtained at both gap (6% of total radiation) and understory (2%). A. triplinervia shows higher rates (1.5–3.0 molm^{-2 -1}) than D. rospigliosii (0.7–1.1 molm^-2s^-1). Both species showed increased photosynthetic rates when grown in gaps. A. triplinervia did not adjust its maximum photosynthetic rates to the prevailing light conditions. In contrast, D. rospigliosii responded to increased light levels. Both species showed low light compensation points when grown under total shade. There was a partial stomatal closure generally during midday in D. rospigliosii. A. triplinervia presented lower leaf conductances, transpiration rates and lesser stomatal control. Some leaf anatomical characteristics, in both species, were affected by variations in the light regime (i.e. increased leaf thickness, leaf specific weight and stomatal density). These results suggest that both species have the ability to respond to variations in their natural light environments, therefore maintaining a favorable carbon balance during the day.     

Bioassays of nutrient limitation in a tropical rain forest soil

Summary Six speices of shrubs and one large herb with contrasting life history patterns were used as bioassays of nutrient availability in a Costa Rican lowland rain forest soil. Growth responses of the herb (Phytolacca rivinoides, Phytolaccaceae) confirmed soil measurements indicating high availability of N and potentially limiting levels of P, K, Mg and Ca. Growth responses of the shrub species (Miconia spp., Melastomataceae and Piper spp., Piperaceae) to a complete nutrient fertilizer were generally less than that of Phytolacca. Lack of a strong shrub response to +P fertilization is probably due to mycorrhizal associations and slower growth rates of woody species. In general, increased growth did not occur at the expense of phenolic production in the leaves. The results emphasize that assessment of specific nutrient limitations to plant growth vary depending on species selected for the bioassay, even among species from the same community.     

Physiological response to complex environments in annual Polygonum species of contrasting ecological breadth

Individual physiological response to complex environments is a major factor in the ecological breadth of species. This study compared individual patterns of both long-term and short-term response to controlled, multifactorial environments in four annual Polygonum species that differ in field distribution (P. cespitosum, P. hydropiper, P. lapathifolium, and P. persicaria). To test long-term response, instantaneous net photosynthetic rate and stomatal conductance were measured in situ on one full-sib replicate from five inbred lineages from each of five field populations per species, raised in all possible combinations of low or high light; dry, moist, or flooded soil; and poor or rich nutrient status. Short-term plastic adjustment to changes in light level was examined by switching individual plants of the four species from one of six multifactorial growth environments to the contrasting light environment, and measuring assimilation rates 1 h after transfer. The Polygonum species differed significantly in their patterns of long-term photosynthetic response to particular resources and resource combinations. The species known to have relatively broad ecological distributions (P. persicaria and P. lapathifolium) maintained high photosynthetic performance in a variety of moisture and nutrient environments when grown in high light, while the more narrowly distributed P. hydropiper maintained such functional levels only if given both high light and ample macronutrients. P. cespitosum, a species limited to shaded habitats, maintained low photosynthetic rates across the environmental range. Complex differences among the species in instantaneous water use efficiency (WUE) reflected their highly specific and to some extent independent patterns of photosynthetic and stomatal response to the multifactorial environments. The species also differed significantly in short-term physiological adjustment to changes in light level. Plants of P. persicaria and P. cespitosum reached 78% and 98%, respectively, of their maximum photosynthetic rates 1 h after transfer from low to high light, but P. hydropiper and P. lapathifolium plants reached only c. 60% of their maximum rates. When switched from high to low light, P. persicaria and P. cespitosum plants maintained 64–76% of their maximum rates, while P. hydropiper and P. lapathifolium plants decreased photosynthetic rates sharply to less than 50% of their maximum rates. These results indicate that the latter two species will be less able to maintain effective functional levels in variable light environments, a result consistent with their distributions in the field. Received: 23 May 1997 / Accepted: 3 March 1998     

When sex is not enough: ecological correlates of resprouting capacity in congeneric tropical forest shrubs

In moist tropical forests resprouting may be an important component of life history, contributing to asexual reproduction through the clonal spread of individuals derived from shoot fragments. However, in contrast to other ecosystems where resprouting is common, the ecological correlates of resprouting capacity in tropical forests remain largely unexplored. In this study we characterized shade tolerance, resprouting capacity and sexual reproductive success of eight co-occurring Piper species from lowland forests of Panama. In field experiments we found that shade-tolerant Piper species had a higher capacity to regenerate from excised or pinned stem fragments than light-demanding species in both gap and understory light conditions. In contrast, shade-tolerant species had lower recruitment probabilities from seeds, as a consequence of lower initial seed viability, and lower seedling emergence rates. All Piper species needed gap conditions for successful seedling establishment. Of 8,000 seeds sown in the understory only 0.2% emerged. In gaps, seed germination of light-demanding species was between 10 and 50%, whereas for shade-tolerant species it was 0.5–9.8%. We propose that the capacity to reproduce asexually from resprouts could be adaptive for shade-tolerant species that are constantly exposed to damage from falling litter in the understory. Resprouting may allow Piper populations to persist and spread despite the high rate of pre-dispersal seed predation and low seed emergence rates. Across Piper species, we detected a trade-off between resprouting capacity and the annual viable seed production per plant but not with annual seed mass produced per plant. This suggests that species differences in sexual reproductive success may not necessarily result from differential resource allocation. Instead we suggest that low sexual reproductive success in the understory may in part reflect reduced genetic diversity in populations undergoing clonal growth, resulting in self-fertilization and in-breeding depression.     

Phenotypic plasticity in Phlox

Summary Three species of Phlox (Polemoniaceae) were grown in 6 greenhouse treatments. A variety of traits were recorded and the correlations among them were computed for each treatment. The phenotypic correlations between characters are significantly altered when plants are grown under different environmental conditions. These changes in correlation structure result from the differential phenotypic plasticity of traits. Partial correlations between flower production and other traits are also environment-dependent. Such changes can alter the intensity of, and possibly the response to, selection on traits correlated with fitness in natural plant populations.     

Morphological variation in a larval salamander: dietary induction of plasticity in head shape

We examined diet-dependent plasticity in head shape in larvae of the eastern long-toed salamander, Ambystoma macrodactylum columbianum. Larvae in some populations of this species exhibit trophic polymorphism, with some individuals possessing exaggerated trophic features characteristic of a cannibalistic morphology in larval Ambystoma; e.g. a disproportionately broad head and hypertrophied vomerine teeth. We hypothesized that 1) head shape variation results from feeding upon different types of prey and that 2) cannibal morphs are induced by consumption of conspecifics. To induce variation, we fed three groups of larvae different diets: 1) brine shrimp nauplii only; 2) nauplii plus anuran tadpoles; 3) nauplii, tadpoles and conspecific larval salamanders. Comparisons of size (mass)-adjusted means revealed that this manipulation of diet induced significant variation in six measures of head shape, but not in the area of the vomerine tooth patch. For five of the six head traits, larvae that ate tadpoles and brine shrimp nauplii developed significantly broader, longer and deeper heads than did larvae that only ate brine shrimp nauplii. The ingestion of conspecifics, in addition to nauplii and tadpoles, significantly altered two head traits (interocular-width and head depth), compared to larvae only fed nauplii and tadpoles. Canonical discriminant function analysis detected two statistically reliable canonical variables: head depth was most highly associated with the first canonical variable, whereas three measures of head width (at the jaws, gills and eyes) and interocular width were most highly associated with the second canonical variable. Despite this diet-enhanced morphological variation, there was no indication that any of the three types of diet (including conspecific prey) induced the exaggerated trophic features of the cannibal morph in this species. These results illustrate that ingestion of different types of prey contributes to plasticity in head shape, but that some other proximate cue(s), either alone or in combination with diet variation, is essential to induce the extremes of trophic polymorphism in this species.     

Crown light environments of saplings of two species of rain forest emergent trees

Summary The crown light environments of saplings of two Costa Rican rain forest tree species were simultaneously compared. The species, Dipteryx panamensis (Pitt.) Record & Mell., a relatively shade-intolerant species, and Lecythis ampla Miers, a shade-tolerant species, have contrasting growth and branching patterns. Quantum sensors were placed throughout the crowns of saplings up to 2.5 m tall and quantum fluxes were recorded with microloggers for seven-day periods. The shade-intolerant species had total quantum flux densities 35% larger than those of the shade-tolerant species, but totals for both species were less than 2% of full sun. More than 90% of the quantum flux densities measured within the crowns of both species were less than 25 mol m^-2s^-1. Lateral light was an important component of daily quantum flux totals; for saplings of both species, the half-hour with the maximum average irradiance for the day frequently occurred in mid-morning or midafternoon. Despite dissimilar crown and leaf display, there was no difference in the overall variability of irradiance within the crowns of the two species. However, quantum fluxes received within the crowns differed substantially in both species. Within-crown locations differed significantly from day to day because of variation in weather conditions. Daily total quantum flux densities and totals expressed as a percent of full sun were significantly correlated with height growth over the previous 12 months.     

Effects of behavioral and morphological plasticity on risk of predation in a Neotropical tadpole

Predator-induced phenotypic plasticity is widespread among aquatic animals, however the relative contributions of behavioral and morphological shifts to reducing risk of predation remain uncertain. We tested the phenotypic plasticity of a Neotropical tadpole (Rana palmipes) in response to chemical cues from predatory Belostoma water bugs, and how phenotype affects risk of predation. Behavior, morphology, and pigmentation all were plastic, resulting in a predator-induced phenotype with lower activity, deeper tail fin and muscle, and darker pigmentation. Tadpoles in the predator cue treatment also grew more rapidly, possibly as a result of the nutrient subsidy from feeding the caged predator. For comparison to phenotypes induced in the experiment, we quantified the phenotype of tadpoles from a natural pool. Wild-caught tadpoles did not match either experimentally induced phenotype; their morphology was more similar to that produced in the control treatment, but their low swimming activity was similar to that induced by predator cues. Exposure of tadpoles from both experimental treatments and the natural pool to a free-ranging predator confirmed that predator-induced phenotypic plasticity reduces risk of predation. Risk of predation was comparable among wild-caught and predator-induced tadpoles, indicating that behavioral shifts can substantially alleviate risk in tadpoles that lack the typical suite of predator-induced morphological traits. The morphology observed in wild-caught tadpoles is associated with rapid growth and high competition in other tadpole species, suggesting that tadpoles may profitably combine a morphology suited to competition for food with behaviors that minimize risk of predation.     

Loss of oak dominance in dry-mesic deciduous forests predicted by gap capture methods

Gap capture methods predict future forest canopy species composition from the tallest trees growing in canopy gaps rather than from random samples of shaded understory trees. We used gap capture methods and a simulation approach to forecast canopy composition in three old oak forests (Quercus spp.) on dry-mesic sites in southern Wisconsin, USA. In the simulation, a gap sapling is considered successful if it exceeds a threshold height of 13–17 m (height of maximum crown width of canopy trees) before its crown center can be overtopped by lateral crown growth of mature trees. The composition of both the tallest gap trees and simulated gap captures suggests that 68–90% of the next generation of canopy trees in the stands will consist of non-Quercus species, particularly Ulmus rubra, Carya ovata and Prunus serotina. Quercus species will probably remain as a lesser stand component, with Quercus alba and Quercus rubra predicted to comprise about 19% of successful gap trees across the three stands. Several methods of predicting future canopy composition gave similar results, probably because no gap opportunist species were present in these stands and there was an even distribution of species among height strata in gaps. Gap trees of competing species already average 11–13 m tall, and mean expected time for these trees to reach full canopy height is only 19 years. For these reasons, we suggest that dominance will shift from oaks to other species, even though late successional species (e.g., Acer and Tilia) are not presently common in the understories of these stands.     

Phenotypic plasticity inCrepis tectorum (Asteraceae)

Two experiments were carried out using two different approaches to compare populations ofCrepis tectorum (Asteraceae). One was based on a comparison of means of various vegetative and reproductive characters and another was based on a comparison of response patterns of the same characters in a series of environments. Population divergence within two earlier recognized form series, one from weed habitats and one from alvar habitats on Baltic islands, resulted in a partially overlapping pattern in cluster analyses based on character means. However, the pattern revealed by a comparison of the direction and amount of plastic response suggested that populations within the two form series had more similar response patterns than other combinations of populations. It was concluded that patterns of plasticity may provide useful additional information on the overall similarity among taxa. An hypothesis that plants in weed populations should exhibit a greater phenotypic response to the environments than plants in alvar populations was rejected.     

Microfungi from decaying leaves of two rain forest trees in Puerto Rico

Fungal species richness and abundance were compared in leaf litter of two tree species,Guarea guidonia andManilkara bidentata, in the Luquillo Mountains of Puerto Rico. Four litter samples yielded a total of 3337 isolates, ranging from 591 to 1259 isolates/sample. The number of species/sample ranged from 134 to 228. Many uncommon litter hyphomycetes were recovered as well as coelomycetes, sterile strains, endophytes, and phytopathogens. Species-abundance distributions revealed a typical pattern of a few abundant species and a high proportion of rare species. Similarities in fungal species composition were not correlated with host species or with the site. Replicate samples examined by the moist chamber technique yielded a total of 24 species among the four litter samples. The particle filtration method indicated that leaves ofG. guidonia were more species-rich, while moist chambers indicated leaves ofM. bidentata were more species-rich. The moist chamber technique underestimated the number and species of viable fungi.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

Leaf movement and photosynthetic plasticity of black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis>) alleviate stress under different light and water conditions

Leaf morphological, physiological and biochemical characteristics of Robinia pseudoacacia L. seedlings were studied under different stress conditions. The plants were subjected to drought and shade stress for one month. Leaf inclination, chlorophyll fluorescence and chlorophyll content were measured at the first day (short-term stress) and at the end of the stress period (long-term stress) and in the recovery period. Leaf inclination was affected mainly by light; a low level of irradiance caused leaves to be arranged horizontally. Diurnal rhythmicity was lost after the long-term stress, but resumed, in part, in the recovery period. Drought stress caused leaves to tilt more obviously and decreased damage to the photosystem. Sun avoiding movement in a single leaf and sun tracking movement in the whole plant coexisted. Significant physiological changes occurred under different conditions of light. Increased energy dissipation and light capture were the main responses to high and low level of irradiance, respectively, and these were reflected by changes of chlorophyll fluorescence and chlorophyll content. Phenotypic plasticity in the leaflet enhanced the protective response to stress. These adaptive mechanisms may explain better survival of R. pseudoacacia seedlings in the understory, especially during the drought periods, and made it to be the preponderant reforestation species in Shandong Province of China.     

Phenotypic plasticity and character differentiation in a subdivided population ofIris pumila (Iridaceae)

Variation patterns in phenotypic plasticity and broad sense heritability of 26 characters were examined within and among closely adjacent habitats of the bearded iris,Iris pumila. It was found thatI. pumila has considerable differentiation for phenotypic plasticity and genetic variation over short distances. An analysis of relationships between character differentiation and phenotypic plasticity suggests that they could have evolved independently. Possible mechanisms for maintaining local differentiation of the observed plastic and genetic variation are also discussed.     

Comparison of proline accumulation in two mediterranean shrubs subjected to natural and experimental water deficit

The effect of water stress on proline accumulation was tested in two contrasted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and Pistacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf water potential dropped below a threshold value of –3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag between decrease of leaf water potential and proline accumulation could be observed. In Halimium halimifolium, proline accumulation appeared to be associated with severe stress conditions as most plants with high proline contents suffered irreversible wilting, especially in the greenhouse. P. lentiscus showed a different pattern, accumulating proline at two different times of the year, as a response to cold or to drought. The results of our study indicated that the role of proline in this species, rather than an osmotic agent, seems to be more related to a protective action in cases of severe stress conditions.     

Leaf anatomy plasticity of Alchornea triplinervia (Euphorbiaceae) under distinct light regimes in a Brazilian montane Atlantic rain forest

 Alchornea triplinervia trees occur in a montane Atlantic rainforest at the Macaé de Cima State Ecological Reserve (Nova Friburgo, RJ, Brazil). They are found in two adjacent secondary forests at distinct successional stages: a late-secondary (shaded and humid) and an early-secondary forest (drier, higher light intensities reaching the understory). Leaf samples collected in these environments were compared in regard to various anatomic parameters. Histochemical tests, anatomic measurements and counting indicated no significant variation for the basic leaf anatomy. Nevertheless, as compared to the late-secondary forest, the leaves of the early-secondary forest individuals showed an increased bulk of sclerenchyma at the main nervation and petiole, gelatinous fibers with hygroscopic walls all along the central vascular system, thicker cuticle, a higher abaxial stomatal density, abundance of tannin in the mesophyll, a significantly thicker palisade and spongy parenchyma, and compactation of the spongy parenchyma. This anatomical variation indicates an increased xeromorphism of leaves under the drier conditions of the early-secondary forest. Leaf anatomy plasticity may contribute to the high ecological plasticity of this widespread neotropical species. Received: 11 November 1996 / Accepted: 5 February 1997     

山豆根叶片光合特性对光强和干旱的响应

山豆根(Euchresta japonica)为我国II级重点保护野生植物。该研究通过设置不同遮阴网的层数和采用不同浓度的聚乙二醇溶液浇灌,探讨了山豆根光合特性对光强和干旱的响应。结果表明, 山豆根叶片的饱和光强为683.06~907.07mmol/(m²·s); 单层遮阴处理叶片的最大电子传递速率和最大净光合速率整体高于双层遮阴处理,其中单层遮阴且未进行干旱处理的叶片最大电子传递速率和最大净光合速率最高,分别为55.36和6.73mmol/(m²·s);相同遮阴条件下,叶片的最大净光合速率及其对应的饱和光强均随干旱程度增加整体呈下降趋势;单层遮阴条件下的蒸腾速率和水分利用效率均整体高于双层遮阴条件下的。不同处理下叶片光系统II的实际光化学效率、光化学猝灭系数以及非光化学猝灭系数等整体上均无显著差异(P>0.05)。山豆根属半阳生植物,其叶片利用弱光能力较强,植株具有较强的耐干旱能力。因此,建议在开展野外回归、迁地保护、人工栽培等工作时,进行适当遮阴处理并保持充足的土壤含水量。