Leaf structure and patterns of photoinhibition in two neotropical palms in clearings and forest understory during the dry season

We studied the leaf structural, water status, and fast fluorescence responses of two palms, Socratea exorrhiza and Scheelea zonensis, under natural dry season conditions in a clearing (high light [HL] palms) and the forest understory (low light [LL] palms) on Barro Colorado Island, Panama. HL-Socratea leaves were more shade-adapted, less xeromorphic, and more strongly affected by drought than HL-Scheelea. F_v/F_m (the ratio of variable to maximum chlorophyll fluorescence) and t_½ (the half-rise time of F_m) was lower in HL-leaves of both species, indicating photoinhibition. In HL-Scheelea, the light-induced reduction of F_v/F_m was much less than in HL-Socratea, and F_v/F_m recovered completely overnight. Patterns of relative water content, specific leaf dry weight, stable carbon isotope composition, and leaf conductance suggest that increased drought resistance in Scheelea reduces susceptibility to photoinhibition. An increase in F_o indicated the inactivation of PSII reaction centers in HL-Socratea. The very low chlorophyll a/b ratio and alterations in chloroplast ultrastructure in HL-Socratea are consistent with photoinhibition. Under LL, the species showed no appreciable interspecific differences in chlorophyll fluorescence. Excess light leads to low values of F_v/F_m in HL-plants relative to LL-plants on both leaf surfaces, particularly on the lower surface, due to a decrease of F_m in both surfaces and an increase in F., of lower surface. For both species, F_o for the lower surfaces of HL-plants was higher and t_½ was markedly lower than for the upper surface, as is typical for shade-adapted leaves. Xeromorphic leaf structure may reduce susceptibility to photoinhibition during the dry season. Drought-enhanced photoinhibition could limit the ability of some species to exploit treefall gaps.     

Comparative life history and physiology of two understory Neotropical herbs

Summary Demography and physiology of two broad-leaved understory tropical herbs (Marantaceae) were studied in gaps and shaded understory in large-scale irrigated and control treatments during the dry season at Barro Colorado Island (BCI), Panama. Because photosynthetic acclimation potential may not predict light environments where tropical species are found, we studied a suite of physiological features to determine if they uniquely reflect the distribution of each species. Calathea inocephala and Pleiostachya pruinosa grow and reproduce in gaps, persist in shade, and have equivalent rates of leaf production. Calathea leaves survived 2 to 3 times as long as leaves of Pleiostachya and plants of Pleiostachya were 6 to 8 times more likely to die as plants of Calathea during 3.5 years of study. Pleiostachya had lowest survival in shade and when not irrigated during the dry season, while Calathea survived well in both habitats and both treatments. Pleiostachya had higher photosynthetic capacity and stomatal conductance than Calathea and acclimated to gaps by producing leaves with higher photosynthetic capacity. Calathea had lower mesophyll CO₂ concentrations than Pleiostachya. Both species had similar dark respiration rates and light compensation points, and water-use and nitrogen-use efficiencies were inversely related between species. Species showed no differences in leaf osmotic potentials at full turgor. Calathea roots were deeper and had tuberous swellings.Leaf-level assimilation and potential water loss are consistent with where these species are found, but photosynthetic acclimation to high light does not reflect both species' abilities to grow and reproduce in gaps. Pleiostachya's gap-dependent, rapid growth and reproduction require high rates of carbon gain in short-lived leaves, which can amortize their cost quickly. High rates of water loss are associated with reduced longevity during drought. Calathea's roots may confer greater capacitance, while its leaves are durable, long-lived and have lower water loss, permitting persistence long after gap closure.     

Seasonal changes in photosynthesis of four understory herbs in deciduous forests

Seasonal patterns of photosynthesis and respiration of single leaves of four understory perennial herbs in deciduous forests were investigated in relation to their leaf growth and light conditions on the forest floor.Anemone flaccida shows rapid growth of leaf area and high rates of gross photosynthesis at light saturation (Psat) in its early stage of development. Its photosynthetic activity is restricted to a brief period of high light intensity before the closure of overstory canopies.Disporum smilacinum possesses light-photosynthesis curves of the shade-leaf type throughout its whole growing period. A shading experiment has shown that this plant is low-light adapted and can utilize weak light efficiently. The light-photosynthesis curve ofSyneilesis palmata shifts from the sun-leaf type to the shade-leaf type in response to the seasonal change of light regime on the forest floor. Evergreen leaves ofPyrola japonica have three year longevity, and light-photosynthesis curves of the shade-leaf type. They maintain some photosynthetic activity even in late autumn and winter.     

Photosynthetic strategies of summergreen and evergreen understory herbs of the boreal mixedwood forest

Seasonal differences in photosynthesis and stomatal conductance of four herbaceous perennials from beneath a deciduous canopy was assessed at two light levels (60 and 400 μmol m⁻² s⁻¹ photosynthetic photon flux density, PPFD) and two leaf temperatures (7 and 20°C). Leaves of an evergreen, Pyrola asarifolia Michx., a wintergreen, Cornus canadensis L., and two summergreen species, Rubus pubescens Raf. and Aralia nudicaulis L., were collected at four times during the growing season. In addition, midsummer light response curves were obtained for one summergreen (A. nudicaulis) and one evergreen species (P. asarifolia) at both 7 and 20°C. Gas exchange measurements were made in the laboratory under controlled environmental conditions. For leaves collected in April, when insolation was high due to the leafless overstory, only P. asarifolia had green leaves, and there was no effect of temperature or light on this species' photosynthesis. P. asarifolia's net assimilation rate (NA) in April was about 30% of it's maximum in late summer. In early summer (June), A. nudicaulis and R. pubescens had higher NA at the higher temperature; at this time, these summergreen species also reached their maximum NA. Midsummer photosynthetic light response curves showed that the light-saturation point was higher and more responsive to leaf temperature in the summergreen A. nudicaulis than in the evergreen P. asarifolia. The summergreen species appear to have a photosystem which performs at high rates during early- and mid-summer, as well as a taller stature which allows them to intercept more light. The photosynthetic system of the ever/wintergreen species is adapted to the low ground-level light conditions in the summer and there does not appear to be an adjustment to take further advantage of the higher light in the spring and fall period. The adaptation of the evergreen and wintergreen understory species is tolerance to low temperatures, enabling them to photosynthesize into the fall till the first continuous frosts occur in the understory and also permitting the evergreen species to begin photosynthesis early in the spring. Received: 17 October 1996 / Accepted: 2 May 1997     

Trait coordination and environmental filters shape functional trait distributions of forest understory herbs

1. Understanding the drivers of trait selection is critical for resolving community assembly processes. Here, we test the importance of environmental filtering and trait covariance for structuring the functional traits of understory herbaceous communities distributed along a natural environmental resource gradient that varied in soil moisture, temperature, and nitrogen availability, produced by different topographic positions in the southern Appalachian Mountains.
2. To uncover potential differences in community‐level trait responses to the resource gradient, we quantified the averages and variances of both abundance‐weighted and unweighted values for six functional traits (vegetative height, leaf area, specific leaf area, leaf dry matter content, leaf nitrogen, and leaf δ¹³C) using 15 individuals of each of the 108 species of understory herbs found at two sites in the southern Appalachians of western North Carolina, USA.
3. Environmental variables were better predictors of weighted than unweighted community‐level average trait values for all but height and leaf N, indicating strong environmental filtering of plant abundance. Community‐level variance patterns also showed increased convergence of abundance‐weighted traits as resource limitation became more severe.
4. Functional trait covariance patterns based on weighted averages were uniform across the gradient, whereas coordination based on unweighted averages was inconsistent and varied with environmental context. In line with these results, structural equation modeling revealed that unweighted community‐average traits responded directly to local environmental variation, whereas weighted community‐average traits responded indirectly to local environmental variation through trait coordination.
5. Our finding that trait coordination is more important for explaining the distribution of weighted than unweighted average trait values along the gradient indicates that environmental filtering acts on multiple traits simultaneously, with abundant species possessing more favorable combinations of traits for maximizing fitness in a given environment.

     

Spatial distribution of understory fruit-eating birds and fruiting plants in a neotropical lowland wet forest

Spatial distribution of fruit-eating birds and fruiting shrubs of the Melastomataceae and Rubiaceae were examined on a 10 ha plot in tropical lowland wet forest of Costa Rica. Many plant species and most birds exhibited considerable spatial variation in their occurrence on the plot, as indicated by the distribution patterns of shrubs with ripe fruits and captures in mist nets, respectively. In many cases, captures of fruit-eating birds were correlated with abundance of fruiting plants, particularly for species that rely heavily on fruits. In general, fruit-eating birds concentrated their use of the plot to areas rich in fruiting shrubs. This differential use of certain areas likely results in differential visitation to fruiting plants located in these areas and in a heterogeneous dissemination of seeds into the habitat.     

间伐对落叶松人工林内草本植物多样性及其组成的影响

通过对落叶松人工林间伐林分和未间伐林分林下草本植物的调查以及林下光照条件的测定,分析间伐对林下草本植物多样性、组成的影响及其与光照条件的关系。结果表明,除均匀度指数在春、秋季表现不同外,间伐林分的丰富度指数、多样性指数均高于未间伐林分,说明间伐后草本植物多样性明显增加。依据草本物种在间伐前后出现与否和密度变化情况将其划分为正反应种(间伐后密度明显增加和间伐后新出现的种)、中性反应种(间伐后密度变化不明显或没有变化的种)和负反应种(间伐后密度明显减小和间伐后消失的种),间伐后出现大量新的草本种类,同时在间伐和未间伐林分的共有种中有许多表现为负反应种。在间伐林分中,间伐行和未间伐行的共有种春季有11个、秋季有10个,分别占间伐林分草本总种数的17.74%和15.15%,表明各行草本植物组成差异较大;各指数均呈现了从间伐行到未间伐行逐渐减小的趋势。主要的正反应种、中性反应种和负反应种均表现出与光照具有一定的相关性,且这种相关性与间伐和非间伐林分之间各草本物种与光照的相关性基本一致。     

Seeds and seedlings of two neotropical montane understory shrubs respond differently to anthropogenic edges and treefall gaps

Edges resulting from forest clear-cutting and treefall gaps can affect plant populations and consequently the distribution of species across landscapes. These two types of disturbance might interact to exacerbate or ameliorate “edge effects”, a rarely tested possibility. We focused on the effects of distance from forest edge (0–10, 30–40, 60–70, and 190–200 m) and habitat within forest fragments (treefall gaps and intact forest) on the early stages of development of Palicourea gibbosa and Faramea affinis, two common shrubs of montane forests in southwest Colombia. Seed germination and seedling growth did not change with distance from forest edge. Within forest fragments, however, seed germination and seedling growth were higher in treefall gaps than in intact forest understory for both species. In contrast, seed predation was influenced by distance from forest edge and in P gibbosa it depended on habitat. Seed predation was highest in the forest interior (190–200 m from forest edge) and in P. gibbosa this was true only in treefall gap habitats. These results suggest that animal mediated processes such as post-dispersal seed predation are more likely than physiological processes to be affected by anthropogenic edges. Our results provide some evidence that treefall gaps may interact with “edge effects”, however, they are inconclusive as to whether they exacerbate or ameliorate them. Received: 31 August 1998 / Accepted: 18 February 1999     

Influences of invasive herb Coreopsis lanceolata on riparian endemic herbs in relation to the understory light availability

In Japan, invasions of alien herb Coreopsis lanceolata are often observed in riparian endemic vegetation, but the invasion influences on light environments and endemic species are insufficiently evaluated. In this study, we investigated the influences of C. lanceolata and native dominant turf Zoysia japonica on the densities of riparian plants (two endemics Artemisia capillaris and Potentilla chinensis and one alien Lespedeza inschanica) in relation to the understory light availability using 594 0.2 m × 0.2 m plots. Available understory light was influenced negatively by C. lanceolata, not Z. japonica, due probably to the tall stature and dense foliar canopy of C. lanceolata. Understory light availability positively affected densities of riparian endemics, although the relation with L. inschanica was unclear. A. capillaris and P. chinensis densities were negatively related with C. lanceolata, not Z. japonica, implying that C. lanceolata reduces endemic populations. Shading by C. lanceolata would be one of prime factors reducing the endemics. L. inschanica density was not correlated with C. lanceolata cover. C. lanceolata could not shade L. inschanica effectively due to the taller stature of L. inschanica. We considered that C. lanceolata reduces endemic riparian species but coexists with L. inschanica. Preventing invasion and dominance of C. lanceolata is desirable to conserve endemic riparian vegetation.     

Leaf trait integration mediates species richness variation in a species-rich neotropical forest domain

Environmental gradients are known to drive changes in mean trait values, but changes in the trait integration strength across local communities are less well understood, particularly with regard to possible links with species richness variation. Here, we tested if climate, soil, and topography gradients drive species richness indirectly via constraints on trait integration in the Atlantic Forest of South America. We evaluated seven traits (from leaf, wood, seed, and plant size) of 1456 species occurring across 84 local communities. Generalized least square models and a path model were applied to test direct and indirect relationships. Correlations were higher between leaf traits (average r?=?0.28) and lower when other traits were included (average r?=?0.16). In line with this result, species richness was related to a multivariate index of interspecific trait integration (ITI) computed for leaf traits, but not to the ITI for all the seven traits. Abiotic gradients influenced species richness both directly and indirectly through the leaf trait integration. A total of 33% and 26% of the variation in species richness and ITI, respectively, were explained by the models, with climatic conditions showing higher contribution than topographic and edaphic factors. These results support a significant but reduced environmental selection role behind the trait-based community assembly and may suggest that other processes are involved in the constrain of trait integration at larger spatial scales. In addition, different directional trends in trait–trait relationships across local communities suggest that global trait relationships may not necessarily hold at local contexts.

     

Effects of nitrogen addition on root respiration of trees and understory herbs at different temperatures in Pinus tabulaeformis forest

Plant and Soil - Nitrogen (N) addition had differential effects on root respiration. However, the reasons and mechanism have not yet been analyzed. We speculated that the differential effects of N...     

Light environment and carbon gain of understory herbs associated with sunflecks in a warm temperate deciduous forest in Japan

Seasonal variation in the light environment on the forest floor of a deciduous forest was investigated with special reference to sunflecks. Diurnal variations and seasonal changes in frequency and irradiation period of the sunflecks (sunfleck duration) were measured. The hourly total sunfleck duration varied seasonally; that is, 30–40 min in spring and autumn and about 15–20 min in summer. There was no large variation in the hourly sunfleck duration during daytime hours (from 9.00 to 15.00 h). The emergence frequency of sunflecks was 1.3–4.8 per h with two peaks, one in the morning and one in the afternoon. The mean duration of a sunfleck, however, showed a characteristic daily pattern with a peak around noon. Sunfleck duration was long around noon, ranging from 12 to 18 min, and short around 10.00 and 14.00 h, ranging from 6 to 10 min. Using the light photosynthesis curves ofPyrola japonica andSyneilesis palmata (Koizumi & Oshima 1985), the contribution of sunflecks to the dry matter production of these understory species was evaluated. It was shown that the sunflecks contributed 7–10% of the carbon gain inS. palmata, but only 2–3% of that inP. japonica.     

Leaf and soil nitrogen and phosphorus availability in a neotropical rain forest of nutrient-rich soil

The nitrogen and phosphorus supply in a lowland rain forest with a nutrient-rich soil was investigated by means of the leaf N/P quotient. It was hypothesised a high N and P supply to the forest ecosystem with a N and P rich soil. Total N and extractable P were determined in the surface (10 cm) soil of three plots of the forest. Total N was analysed by the Kjeldahl method, and P was extracted with HCI and NH4F. The leaf N/P quotient was evaluated from the senesced leaves of 11 dominant tree species from the mature forest. Samples of 5 g of freshly fallen leaves were collected from three trees of each species. Nitrogen was analysed by microkjeldahl digestion with sulphuric acid and distilled with boric acid, and phosphorus was analysed by digestion with nitric acid and perchloric acid, and determined by photometry. Concentrations of total N (0.50%, n = 30) and extractable P (4.11 microg g(-1), n = 30) in the soil were high. As expected, P supply was sufficient, but contrary to expected, N supply was low (N/P = 11.8, n = 11).     

Leaf litter and understory canopy shade limit the establishment, growth and reproduction of Microstegium vimineum

The invasive grass Microstegium vimineum is widely distributed in closed-canopy forests, but often is patchily distributed under uniform canopy conditions. We hypothesized that the occurrence of patches of invasion may be related to two interacting factors, the presence of dense understory shrub layers and the presence of thick litter layers on the forest floor. Seeds of M. vimineum were sown in plots located under or distant from Lindera benzoin (spicebush) shrubs, and with or without litter manipulations (none, half of natural amount, naturally occurring amount, double the natural amount) in a mature forest in central New Jersey, USA. Populations were monitored for germination and survivorship, as well as growth and fecundity of surviving plants. Neither shrub-associated shade nor litter depth affected seed germination, but both factors affected survivorship, growth and reproduction. The presence of shade from the shrubs reduced survivorship and seed set. Seeds germinating on top of the litter layer also experience higher mortality than seeds germinating under litter and in contact with soil. These results suggest that the interacting effects of shade from understory strata and deep litter layers may limit the spread of M. vimineum. The loss of shrub layers due to intense deer browse and other factors may thus accelerate the spread of this highly invasive species.     

Leaf nonstructural carbohydrate concentrations of understory woody species regulated by soil phosphorus availability in a tropical forest

Leaf soluble sugars and starch are important components of nonstructural carbohydrates (NSCs), which are crucial for plant growth, development, and reproduction. Although there is a large body of research focusing on the regulation of plant NSC (soluble sugars and starch) concentrations, the response of foliar NSC concentrations to continuous nitrogen (N) and phosphorus (P) addition is still unclear, especially in tropical forests. Here, we used a long‐term manipulative field experiment to investigate the response of leaf NSC concentrations to continuous N and P addition (3‐, 5‐, and 8‐year fertilization) in a tropical forest in southern China. We found significant species‐specific variation in leaf NSC concentrations in this tropical forest. Phosphorus addition dramatically decreased both leaf soluble sugar and starch concentrations, while N addition had no significant effects on leaf soluble sugar and starch concentrations. These results suggest that, in plants growing in P‐limiting tropical soil, leaf NSC concentrations are regulated by soil P availability rather than N availability. Moreover, the negative relationships between NSC concentrations and leaf mass per area (LMA) revealed that NSCs could supply excess carbon (C) for leaf expansion under P addition. This was further supported by the increased structural P fraction after P fertilization in our previous study at the same site. We conclude that soil P availability strongly regulates leaf starch and soluble sugar concentrations in the tropical tree species included in this study. The response of leaf NSC concentrations to long‐term N and P addition can reflect the close relationships between plant C dynamics and soil nutrient availability in tropical forests. Maintaining relatively higher leaf NSC concentrations in tropical plants can be a potential mechanism for adapting to P‐deficient conditions.     

Photochemical efficiency of adult and young leaves of the neotropical understory shrub Psychotria limonensis (Rubiaceae) in response to changes in the light environment

We explored the short-term adjustment in photochemical efficiency (Fv/Fm) in adult and young leaves of the understory neotropical shrub Psychotria limonensis Krause (Rubiaceae) in response to rapid changes in the light environment. Leaves were collected from 20 individual plants growing under sun and shade conditions on Gigante Peninsula, Barro Colorado Natural Monument (Republic of Panama), during the wet season of 1996. Leaves were distributed in four sequences of light treatments (AB leaves were expanded under sun and were transferred to shade, BA leaves experienced the opposite transfer, and the controls AA and BB leaves that were expanded and maintained under sun or shade conditions). Adult and young leaves did not differ in overall photochemical efficiency. Instead, differences were found among light environments, for which leaves transferred from shade to sun showed the lowest Fv/Fm ratios. There was no relationship between photochemical efficiency and leaf temperature. In P. limonensis, understory plants are susceptible of photoinhibition independently of the leaf ontogenetic stage. The approach utilized in this experiment allowed the rapid exploration of this capacity, and could be applied to poorly studied understory species.     

Leaf litter age regulates the effect of native and exotic tree species on understory herbaceous vegetation of riparian forests

Decomposing litter is regarded as the most important source of allelochemicals released into soil. In this study, a greenhouse experiment was designed to assess the net effect of differently aged leaf litter from exotic (Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila) and native riverine trees (Populus alba, Populus nigra and Ulmus minor) on the germination and growth of three herb species (Trifolium repens, Dactylis glomerata and Chenopodium album). We also characterized the chemical composition of litter samples at different litter ages (0, 1, 2 and 3 months) based on phenolic compounds, fibers and ergosterol (as a measure of fungal biomass) contents. Overall, litter from both native and exotic species had a negative effect on shoot and root growth of target species, indicating that phytotoxic effects of litter predominate over positive effects. The inhibition effect of the exotic species was similar or even lower than that of the natives, which does not support the Novel Weapons Hypothesis. Among exotic trees, U. pumila showed the highest inhibition effect on the growth of the target species. T. repens was the most sensitive target species. The importance of litter age varied with both target and donor species. In general, D. glomerata was more inhibited by fresh litter, C. album by half-decomposed litter of U. pumila and R. pseudoacacia and by fresh litter of A. altissima and T. repens was more inhibited by fresh litter of A. altissima and P. alba and by highly decomposed litter of U. minor. The concentration of total phenolics and flavonoids decreased while acid detergent fiber, lignin and ergosterol increased with increasing litter age. Hydroxybenzoic and protocatechuic acids and the flavonoid quercetin were detected in all litter species and at most of the litter ages, while gallic, chlorogenic, vanillic, coumaric and rosmarinic acids were species-specific and they were only detected in fresh litter. Ergosterol concentration appeared as the strongest constrictor of inhibitory effects of litter on understory species. The results of this study contribute to the understanding of the net effect of fresh and decomposed litter from exotic and native trees on the growth of understory species.     

Determinants of fruit removal in <Emphasis Type="Italic">Geonoma pauciflora</Emphasis>, an understory palm of neotropical forests

Competition for seed dispersers is supposedly a selective force that drives the evolution of plant reproductive traits. In the understory of tropical forests, such competition should be especially severe among bird-dispersed plant species because (i) the production of copious fruit crops is limited by low light availability; (ii) there usually is a high density of fleshy fruited plants, and (iii) understory frugivorous birds are not abundant. In this paper, we took advantage of a high-density population of Geonoma pauciflora, a bird-dispersed palm species growing in the understory of the Brazilian Atlantic forest, to investigate the influence of plant traits and its immediate neighborhood on fruit removal. Intrinsic (crop and fruit sizes) and extrinsic traits (related to light availability) affected the fruit removal of G. pauciflora. Crop size had a greater influence than fruit size on the absolute number of fruits removed, whereas none of the investigated traits influenced decisively fruit removal efficiency (i.e., the proportion of an individual’s crop removed). The influence of light availability was mostly indirect, through its positive influence on fruit production. A significant positive spatial autocorrelation in removal efficiency occurred among neighboring plants within a 7-m radius, which is indicative of facilitation among neighboring individuals. The consequence of such positive spatial autocorrelation in removal efficiency for clonal plants such as G. pauciflora is that by attracting a frugivore a given ramet may promote the removal of fruits of other ramets, thus enhancing the reproductive output of the genet as a whole.     

Leaf production,reproductive patterns,field germination and seedling survival in Chamaedorea bartlingiana,a dioecious understory palm

Summary Chamaedorea bartlingiana is a dioecious palm that grows in the cloud forest understories of the Venezuelan Andes. Age and sexual differences in phenology and reproductive patterns were studied in labelled individuals of all age categories. This species has long-lived leaves and low leaf production, both characteristic of understory plants. Growth rates are lower in juveniles than in adults and in females than in males, as in other palms. Male and female individuals show different reproductive patterns. Male inflorescences are always produced at the same rate and the probability of surviving until anthesis is constant. Females produce reproductive buds at the same rate as males, but these buds have a 35% probability of becoming a ripe infrutescence if the plant has infrutescences already growing, and 70% if it does not. This pattern and the slow growth of inflorescences (1 year for males from bud to flowers, 2 years for females from bud to ripe fruits) cause a pluriannual reproductive pattern at the population level. Field germination does not follow this pattern, but shows one annual peak probably related to environmental conditions.