西双版纳热带雨林中两种生态型蕨类植物的光合特性比较研究

通过比较分布于西双版纳热带雨林林下生境中的附生鸟巢蕨（Neottopteris nidus）和地生网脉铁角蕨（Asplenium finlaysonianum）的光合特征和光合诱导特性,来研究不同生态型蕨类植物的光斑利用策略。研究结果表明,2种蕨类植物的最大净光合速率、暗呼吸速率、表观量子效率、光饱和点和光补偿点没有显著差异,但网脉铁角蕨的最大气孔导度远远高于鸟巢蕨,表明后者具有更强的光合水分利用效率。在暗处理3／J＇,时接着光照（光强为20I~mol-m-2,s。‘）30分钟后,网脉铁角蕨的初始气孔导度显著高于鸟巢蕨。连续照射饱和强光后,网脉铁角蕨达到最大净光合速率50％（T50％）和90％的时间（T90%）比鸟巢蕨短：网脉铁角蕨和鸟巢蕨的T50%分别为0．57和5．31分钟,T90％分别为5．85和26．33分钟。诱导过程中,气孔导度对强光的响应明显滞后于净光合速率。鸟巢蕨达到最大气孔导度的时间明显比网脉铁角蕨慢,但在光合诱导消失过程中2种蕨类植物的光合诱导维持能力却没有显著差异。上述结果表明,与大多数地生林下植物（如网脉铁角蕨）相比,附生鸟巢蕨的水分保护比碳获得更重要,但却限制了附生蕨对光斑的利用。     

西双版纳热带雨林中两种生态型蕨类植物的光合特性比较研究

通过比较分布于西双版纳热带雨林林下生境中的附生鸟巢蕨(Neottopteris nidus)和地生网脉铁角蕨(Asplenium finlaysonianum)的光合特征和光合诱导特性, 来研究不同生态型蕨类植物的光斑利用策略。研究结果表明, 2种蕨类植物的最大净光合速率、暗呼吸速率、表观量子效率、光饱和点和光补偿点没有显著差异, 但网脉铁角蕨的最大气孔导度远远高于鸟巢蕨, 表明后者具有更强的光合水分利用效率。在暗处理3小时接着光照(光强为20 mmol .m-2.s-1)30分钟后, 网脉铁角蕨的初始气孔导度显著高于鸟巢蕨。连续照射饱和强光后, 网脉铁角蕨达到最大净光合速率50%(T50%)和90%的时间(T90%)比鸟巢蕨短: 网脉铁角蕨和鸟巢蕨的T50%分别为0.57和5.31分钟, T90%分别为5.85和26.33分钟。诱导过程中, 气孔导度对强光的响应明显滞后于净光合速率。鸟巢蕨达到最大气孔导度的时间明显比网脉铁角蕨慢, 但在光合诱导消失过程中2种蕨类植物的 光合诱导维持能力却没有显著差异。上述结果表明, 与大多数地生林下植物(如网脉铁角蕨)相比, 附生鸟巢蕨的水分保护比碳获得更重要, 但却限制了附生蕨对光斑的利用。     

Crassulacean acid metabolism in the ZZ plant, Zamioculcas zamiifolia (Araceae)

Zamioculcas zamiifolia (Araceae), a terrestrial East African aroid, with two defining attributes of crassulacean acid metabolism (CAM) (net CO(2) uptake in the dark and diel fluctuations of titratable acidity) is the only CAM plant described within the Araceae, a mainly tropical taxon that contains the second largest number of epiphytes of any vascular plant family. Within the Alismatales, the order to which the Araceae belong, Z. zamiifolia is the only documented nonaquatic CAM species. Zamioculcas zamiifolia has weak CAM that is upregulated in response to water stress. In well-watered plants, day-night fluctuations in titratable acidity were 2.5 μmol H(+)·(g fresh mass)(-1), and net CO(2) uptake in the dark contributed less than 1% to daily carbon gain. Following 10 d of water stress, net CO(2) uptake in the light fell 94% and net CO(2) uptake in the dark increased 7.5-fold, such that its contribution increased to 19% of daily carbon gain. Following rewatering, dark CO(2) uptake returned to within 5% of prestressed levels. We postulate that CAM assists survival of Z. zamiifolia by reducing water loss and maintaining carbon gain during seasonal droughts characteristic of its natural habitat.     

Physiological and ecological significance of sunflecks for dipterocarp seedlings

Irradiance is highly dynamic in many plant canopies. Photosynthesis during sunflecks provides 10-90% of daily carbon gain. The survivorship of tree seedlings in the deeply shaded understorey of tropical rain forests is limited by their ability to maintain a positive carbon balance. Dipterocarp seedlings from the SE Asian rain forest were used as a model system to test novel aspects of the physiological and ecological significance of sunflecks. First, understorey seedlings experienced leaf temperatures up to 38 degrees C in association with sunflecks. Under controlled environment conditions, the inhibition of carbon gain at 38 degrees C, compared with 28 degrees C, was significantly greater during a sequence of sunflecks (-59%), than under uniform irradiance (-40%), providing the same total photosynthetic photon flux density (PPFD). Second, the relative enhancement effects of elevated [CO2] were greater under sunflecks (growth +60%, carbon gain +89%), compared with uniform irradiance (growth +25%, carbon gain +59%), supplying the same daily PPFD. Third, seedling growth rates in the forest understorey were 4-fold greater under a dynamic irradiance treatment characterized by long flecks, compared with a regime of short flecks. Therefore, stresses associated with dynamic irradiance may constrain photosynthetic carbon gain. Additionally, seedling photosynthesis and growth may be more responsive to interactions with abiotic factors, including future changes in climate, than previously estimated. The sensitivity of seedling growth to varying patterns of dynamic irradiance, and the increased likelihood of species-specific responses through interactions with environmental factors, indicates the potential for sunflecks to influence regeneration processes, and hence forest structure and composition.     

Patterns of dynamic irradiance affect the photosynthetic capacity and growth of dipterocarp tree seedlings

In the deeply shaded understorey of S.E. Asian rain forests the growth and survival of dipterocarp seedlings is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain in understorey plants. To test the sensitivity of photosynthesis and growth to variation in the pattern of dynamic irradiance, dipterocarp tree seedlings (Shorea leprosula and Hopea nervosa) were grown for 370 days under shaded forest light treatments of equal total daily photosynthetic photon flux density (approximately 3.3 mol m(-2) day(-1)), but characterised by either long flecks (LF) or short flecks (SF). Seedling growth was more than 4-fold greater under LF, compared with SF, in both species. Variation in the relative growth rates (RGR) and light saturated rates of photosynthesis (A(max)) were strongly positively correlated with the mean duration of sunflecks. Variation in RGR was strongly correlated with greater unit leaf rate growth, indicating that photosynthetic carbon gain per unit leaf area was greater under LF. The accumulation of starch in leaves over the diurnal period was 117% greater in both species under LF, compared with SF. Greater carbon gain in seedlings under LF is likely to have resulted from the combination of (1) greater A(max) (S. leprosula 35%, H. nervosa 40%), (2) more efficient dynamic photosynthesis, and (3) greater incident photosynthetic quantum yield, compared with seedlings receiving the SF irradiance treatment. The pattern of dynamic irradiance received by seedlings may significantly impact their growth and survival to a previously unrecognised extent, with important consequences for regeneration processes and hence forest structure and composition.     

A preliminary study on the photosynthetic induction response ofQuercus serrata seedlings

To understand the sunfleck utilization of leaves in heliophilic trees within grass canopies, we studied the photosynthetic induction response ofQuercus serrata seedlings grown for 5 months in different microsites in aMiscanthus sinensis canopy. Two phases, a rapid increase in CO₂ uptake and a following slow increase, were recognizabie in the time course of CO₂ uptake in response to an increase of photon flux densities (PFD). When the preceding period of low light became shorter, the period of the two phases became shorter. The capacity of response to a sudden light increase was evaluated by the relative photosynthetic induction efficiency (RPIE) defined as the ratio of integrated carbon gain measured to that calculated by assuming that a steady-state assimilation would be achieved instantaneously after the light increase. RPIEs estimated were negatively related to potential sunfleck PFD of microsite. The leaves of the seedlings grown in the microsite with a lower sunfleck PFD and a shorter sunfleck duration showed a more rapid response to a sudden increase of light. These findings suggest that the leaves ofQ. serrata seedlings growing under a lower sunfleck PFD are able to increase the photosynthetic capacity more rapidly to an increase of PFD.     

Leaf gas exchange of beech (Fagus sylvatica L.) seedlings in lightflecks: effects of fleck length and leaf temperature in leaves grown in deep and partial shade

Summary Responses of leaf gas exchange in shade and half-shade grown seedlings of the European beech, Fagus sylvatica L., to constant light conditions indicate different phases of photosynthetic induction: an immediate, a fast and a subsequent slow phase. The slow phase has both biochemical and stomatal components. The higher the induction, the higher the lightfleck utilization efficiency (LUE) attributable to a lightfleck. LUE can be higher than 100% compared to a theoretical instantaneous response. Lightfleck quantum yield (total carbon gain attributable to a lightfleck per incident quantum density in the fleck) is highest in short pulses of light. Post-illumination carbon gain initially increases with fleck length, levelling off above 20 s. The lower the induction, the longer carbon is fixed post-illuminatively (up to 84 s) but the less carbon is gained. Shade leaves are induced much faster than partial shade leaves. They utilize series of lightflecks to become fully induced, while half-shade (and sun) leaves depend on continuous high light. Half-shade leaves lose induction faster in low light between lightflecks. High as well as low temperatures strongly delay induction in half-shade but not in shade leaves. In general, shade leaves are much better adapted to the dynamic light environment of the forest understorey; however, their water-use efficiency during induction is lower.Dedicated to Prof. O. L. Lange on the occasion of his 65th birthday     

Physiological responses of the CAM epiphyte Tillandsia usneoides L. (Bromeliaceae) to variations in light and water supply

In an effort to understand the mechanisms that sustain rootless atmospheric plants, the modulation of Crassulacean acid metabolism (CAM) in response to variations in irradiance and water supply was investigated in the epiphyte Tillandsia usneoides. Plants were acclimated to three light regimes, i.e. high, intermediate and low, with integrated photon flux densities (PFD) of 14.40, 8.64 and 4.32 mol m-2 d-1 equivalent to an instantaneous PFD of 200, 100, and 50 mumol m-2 s-1, respectively. Daily watering was then withdrawn from half of the plants at each PFD for 7 d prior to sampling. In response to the three PFD treatments, chlorophyll content increased in plants acclimated to lower irradiances. Light response curves using non-invasive measurements of chlorophyll fluorescence demonstrated that photosystem II efficiency (phi PSII) was maintained in high PFD acclimated plants, as they exhibited a larger capacity for non-photochemical dissipation (NPQ) of excess light energy than low PFD acclimated plants. Net CO2 uptake increased in response to higher PFD, reflecting enhanced carboxylation capacity in terms of phosphoenolpyruvate carboxylase (PEPc) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities. After water was withdrawn, nocturnal net CO2 uptake and accumulated levels of acidity declined in all PFD treatments, concomitant with increased respiratory recycling of malate. Examining the strategies employed by epiphytes such as T. usneodies to tolerate extreme light and water regimes has demonstrated the importance of physiological mechanisms that allow flexible carboxylation capacity and continued carbon cycling to maintain photosynthetic integrity.     

Daily carbon gain by Adenocaulon bicolor (Asteraceae), a redwood forest understory herb,in relation to its light environment

Summary The relationships between carbon gain and availability of sunfleck- and diffuse-light were determined for Adenocaulon bicolor by following the daily courses of assimilation and incident PFD on different days and locations in a redwood forest understory. Total PFD for the days sampled ranged from 1 to 4% of full sun values. Sunflecks accounted for 50 to 90% of the total PFD and were responsible for the majority of variation among days and locations. Each day had several clusters of sunfleck activity separated by relatively long intervals of diffuse light. Most sunflecks had maximum PFDs below the photosynthetic light-saturation point, and they had a median length and diffuse light interval separating them of 2 s. Daily carbon gain varied from 14 to 40 mmol m^-2d^-1 and was more strongly correlated with differences among days in total sunfleck PFD (r ²=0.81) than with variation in diffuse PFD (r ²=0.54). The assimilation that was attributable to sunflecks ranged from essentially zero on one day to 30 to 65% of the total on the other days. Carbon gain on most days was 70 to 80% of that predicted by a model based on the measured light dependences of assimilation. This model assumed an instantaneous response to changes in PFD, whereas incomplete photosynthetic induction probably limited the capacity to respond to sunflecks and therefore limited carbon gain on most days.     

Vascular Epiphyte Communities in the Inner-Crown of Hyeronima alchorneoides and Lecythis ampla at La Selva Biological Station, Costa Rica

The community composition and percent cover of vascular epiphytes were examined in relation to photosynthetic photon flux density (PFD), temperature, vapor pressure, and tree characteristics in the inner-crown of two emergent tree species, Hyeronima alchorneoides and Lecythis ampla, at La Selva Biological Station, Costa Rica. A total of 53 species were found in a total sampled branch area of 32 m² in eight trees—four trees per species. Community composition varied both among individuals of the same tree species and between tree species. However, percent cover patterns of vascular epiphytes were significantly different between the two tree species; Hyeronima had a significantly greater percent cover of epiphytes than Lecythis . The higher percent light transmittance as well as lower humidity in Lecythis are likely causes of its lower percent cover of epiphytes.     

Climbing plants in a temperate rainforest understorey: searching for high light or coping with deep shade?

Background and Aims

While the climbing habit allows vines to reach well-lit canopy areas with a minimum investment in support biomass, many of them have to survive under the dim understorey light during certain stages of their life cycle. But, if the growth/survival trade-off widely reported for trees hold for climbing plants, they cannot maximize both light-interception efficiency and shade avoidance (i.e. escaping from the understorey). The seven most important woody climbers occurring in a Chilean temperate evergreen rainforest were studied with the hypothesis that light-capture efficiency of climbers would be positively associated with their abundance in the understorey.

Methods

Species abundance in the understorey was quantified from their relative frequency and density in field plots, the light environment was quantified by hemispherical photography, the photosynthetic response to light was measured with portable gas-exchange analyser, and the whole shoot light-interception efficiency and carbon gain was estimated with the 3-D computer model Y-plant.

Key Results

Species differed in specific leaf area, leaf mass fraction, above ground leaf area ratio, light-interception efficiency and potential carbon gain. Abundance of species in the understorey was related to whole shoot features but not to leaf level features such as specific leaf area. Potential carbon gain was inversely related to light-interception efficiency. Mutual shading among leaves within a shoot was very low (<20 %).

Conclusions

The abundance of climbing plants in this southern rainforest understorey was directly related to their capacity to intercept light efficiently but not to their potential carbon gain. The most abundant climbers in this ecosystem match well with a shade-tolerance syndrome in contrast to the pioneer-like nature of climbers observed in tropical studies. The climbers studied seem to sacrifice high-light searching for coping with the dim understorey light.     

Association between rainfall seasonality and the flowering of epiphytic plants in a Neotropical montane forest

The association between the reproductive phenology of epiphytic communities with environmental and ecological factors remains largely unexplored. Because epiphytes depend on environmental moisture, seasonal changes in moisture conditions likely act as the primary determinants of their reproductive timing. We examined whether water limitation or pollinator competition structures the flowering phenologies of an epiphytic community in a seasonal mountain forest in Costa Rica. Additionally, we addressed the environmental factors that might trigger floral induction. Using a 24‐month dataset of bimonthly flowering records from 104 species, we found high seasonality of flowering at the species level but somewhat lower seasonality at the community level. The flowering mid‐dates of most epiphytes, particularly from monocotyledonous species, occurred during the wettest months, as predicted if water limitation structures flowering. The increased moisture and nutrient availability during the rainy season give epiphytes the resources needed to complete floral development and anthesis, and later fruit and seed maturation. The observed flowering pattern of epiphytes coincides with reproductive patterns of terrestrial herbs and shrubs from seasonal tropical ecosystems, and suggests shared constraints to sexual reproduction in both ecological guilds under similar climatic conditions. In contrast, flowering patterns of congeneric epiphytes in the same pollination guild mostly did not follow the expectations of a pollinator competition scenario. Finally, we discuss the possible combined effect of precipitation, temperature, and daily insolation on floral induction of epiphytic plants.     

Elevated CO2 differentially affects photosynthetic induction response in two Populus species with different stomatal behavior

To understand dynamic photosynthetic characteristics in response to fluctuating light under a high CO(2) environment, we examined photosynthetic induction in two poplar genotypes from two species, Populus koreana 9 trichocarpa cv. Peace and Populus euramericana cv. I-55, respectively. Stomata of cv. Peace barely respond to changes in photosynthetic photon flux density (PFD), whereas those of cv. I-55 show a normal response to variations in PFD at ambient CO(2). The plants were grown under three CO2 regimes (380, 700, and 1,020 μmol CO(2) mol(-1) in air) for approximately 2 months. CO2 gas exchange was measured in situ in the three CO2 regimes under a sudden PFD increase from 20 to 800 μmol m(-2) s(-1). In both genotypes, plants grown under higher CO(2) conditions had a higher photosynthetic induction state, shorter induction time, and reduced induction limitation to photosynthetic carbon gain. Plants of cv. I-55 showed a much larger increase in induction state and decrease in induction time under high CO(2) regimes than did plants of cv. Peace. These showed that, throughout the whole induction process, genotype cv. I-55 had a much smaller reduction of leaf carbon gain under the two high CO(2) regimes than under the ambient CO(2) regime, while the high CO(2) effect was smaller in genotype cv. Peace. The results suggest that a high CO(2) environment can reduce both biochemical and stomatal limitations of leaf carbon gain during the photosynthetic induction process, and that a rapid stomatal response can further enhance the high CO(2) effect.     

Interactions between water stress, sun-shade acclimation, heat tolerance and photoinhibition in the sclerophyll Heteromeles arbutifolia

Gas exchange and chlorophyll fluorescence techniques were used to evaluate the acclimation capacity of the schlerophyll shrub Heteromeles arbutifolia M. Roem. to the multiple co-occurring summer stresses of the California chaparral. We examined the influence of water, heat and high light stresses on the carbon gain and survival of sun and shade seedlings via a factorial experiment involving a slow drying cycle applied to plants grown outdoors during the summer. The photochemical efficiency of PSII exhibited a diurnal, transient decrease (δF/F_m′) and a chronic decrease or photoinhibition (F_v/F_m) in plants exposed to full sunlight. Water stress enhanced both transient decreases of δF/F_m’and photoinhibition. Effects of decreased δF/F_m’and F_v/F_m on carbon gain were observed only in well-watered plants since in water-stressed plants they were overidden by stomatal closure. Reductions in photochemical efficiency and stomatal conductance were observed in all plants exposed to full sunlight, even in those that were well-watered. This suggested that H. arbutifolia sacrificed carbon gain for water conservation and photoprotection (both structurally via shoot architecture and physiologically via down-regulation) and that this response was triggered by a hot and dry atmosphere together with high PFD, before severe water, heat or high PFD stresses occur. We found fast adaptive adjustments of the thermal stability of PSII (diurnal changes) and a superimposed long-term acclimation (days to weeks) to high leaf temperatures. Water stress enhanced resistance of PSII to high temperatures both in the dark and over a wide range of PFD. Low PFD protected photochemical activity against inactivation by heat while high PFD exacerbated damage of PSII by heat. The greater interception of radiation by horizontally restrained leaves relative to the steep leaves of sun-acclimated plants caused photoinhibition and increased leaf temperature. When transpirational cooling was decreased by water stress, leaf temperature surpassed the limits of chloroplast thermostability. The remarkable acclimation of water-stressed plants to high leaf temperatures proved insufficient for the semi-natural environmental conditions of the experiment. Summer stresses characteristic of Mediterranean-type climates (high leaf temperatures in particular) are a potential limiting factor for seedling survival in H. arbutifolia, especially for shade seedlings lacking the crucial structural photoprotection provided by steep leaf angles.     

Induction of CO<Subscript>2</Subscript>-gas exchange and electron transport: comparison of dynamic and steady-state responses in <Emphasis Type="Italic">Fagus sylvatica </Emphasis>leaves

Photosynthetic induction was followed in a juvenile understorey beech (Fagus sylvatica L.) in a shaded habitat which was temporarily exposed to direct sunlight passing through a gap in the canopy. Simultaneous in situ measurements of leaf gas exchange and chlorophyll fluorescence were carried out and a steady-state light response curve of photosynthesis was recorded. The measured dynamic carbon gain was compared to the predicted carbon gain, calculated from the steady-state light response curve and the ambient photon flux density (PPFD) incident during the sunfleck event. Integration over the first 20 min, during which induction took place, resulted in a deviation of 27% if any induction effects are disregarded. The carbon gain was overestimated by 11% when the carbon gain was integrated over the whole measuring period of about 1 h including a 40-min period of full induction. In situ gas exchange measurements under constant saturating light, following dark adaptation, revealed an induction error of 21%. About 20 min was required to reach the final steady-state level of photosynthesis. The data show that the prediction of the carbon gain by steady-state models leads to a distinct overestimation of the CO₂ uptake, irrespective of whether the induction state rises concurrently with the incident radiation or saturating light induces photosynthesis. From chlorophyll fluorescence and absorptance data, rates of linear electron transport (ETR) were calculated. Uninduced leaves exposed to saturating light of constant PPFD show an initial fast down-regulation of ETR, due to excessive light, and a subsequent increase in electron transport attributable to the increasing energy demand during induction of carbon fixation. No difference in the ETR to PPFD relationship between data sets sampled during induction and in the fully induced state was found when both the incident light and the induction state of carbon fixation increased concomitantly. The proportion of electrons fed into alternative pathways besides carbon fixation was higher during induction as compared to the fully induced state. Thus, electrons were used for carbon fixation with a higher efficiency when full induction was reached.     

A hemiparasite in the forest understorey: photosynthetic performance and carbon balance of Melampyrum pratense

• Melampyrum pratense is an annual root‐hemiparasitic plant growing mostly in forest understorey, an environment with unstable light conditions. While photosynthetic responses of autotrophic plants to variable light conditions are in general well understood, light responses of root hemiparasites have not been investigated.
• We carried out gas exchange measurements (light response and photosynthetic induction curves) to assess the photosynthetic performance of M. pratense in spring and summer. These data and recorded light dynamics data were subsequently used to model carbon balance of the hemiparasite throughout the entire growth season.
• Summer leaves had significantly lower rates of saturated photosynthesis and dark respiration than spring leaves, a pattern expected to reflect the difference between sun‐ and shade‐adapted leaves. However, even the summer leaves of the hemiparasite exhibited a higher rate of light‐saturated photosynthesis than reported in non‐parasitic understorey herbs. This is likely related to its annual life history, rare among other understorey herbs. The carbon balance model considering photosynthetic induction still indicated insufficient autotrophic carbon gain for seed production in the summer months due to limited light availability and substantial carbon loss through dark respiration.
• The results point to potentially high importance of heterotrophic carbon acquisition in M. pratense, which could be of at least comparable importance as in other mixotrophic plants growing in forests – mistletoes and partial mycoheterotrophs. It is remarkable that despite apparent evolutionary pressure towards improved carbon acquisition from the host, M. pratense retains efficient photosynthesis and high transpiration rate, the ecophysiological traits typical of related root hemiparasites in the Orobanchaceae.

     

Heterogeneity of Light Availability and its Effects on Simulated Carbon Gain of Tree Leaves in a Small Gap and the Understory in a Tropical Rain Forest1

To clarify the small-scale heterogeneity of light regimes in a rain forest, photosynthetic photon flux density (PFD) was measured at 1-min intervals during six days at 12 microsites in each of two plots, a small gap and an understory in Pasoh Forest Reserve, Peninsular Malaysia. Frequency distribution of microsite PFD was unimodal with the peak value between 16 and 32 μmol/m²/sec in the small gap, but between 8 and 16 μmol/m²/sec in the understory. In the small gap, PFD was more variable among microsites; total daily PFD and daily sunfleck PFD exceeding 10 μmol/ m²/sec tended to be higher (P <0.05; t-test) compared to those in the understory. Sunfleck PFD exceeding 50 μmol/ m²/sec, however, showed no difference between the two plots. Diffuse PFD transmittance, defined as the ratio of PFD in the forest to that measured at 43 m above ground during the periods 0800-0810 and 1750-1800 h, was significantly higher in the small gap than in the understory plot. Diffuse PFD transmittance was also positively correlated with microsite total daily PFD. To examine the effects of the subtle heterogeneity of light regimes on leaf carbon gain, we simulated carbon gain by sun and shade leaves in a typical shade-tolerant species, Brosimum aticastrum Sw. (Moraceae). Despite the similarity in total daily PFD, total daily carbon gain was considerably higher in the gap than in the understory for both sun and shade leaves. This study suggests that frequency distribution of PFD is critical in describing microsite PFD regimes and determining leaf carbon gain in the tropical forest floor.     

Relative enhancement of photosynthesis and growth at elevated CO2 is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling

The survivorship of dipterocarp seedlings in the deeply shaded understorey of South‐east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO₂ upon photosynthesis and growth under sunflecks, seedlings of Shorealeprosula were grown in controlled environment conditions at ambient or elevated CO₂. Equal total daily photon flux density (PFD) (～7·7 mol m^?2 d^?1) was supplied as either uniform irradiance (～170 µmol m^?2 s^?1) or shade/fleck sequences (～30 µmol m^?2 s^?1/～525 µmol m^?2 s^?1). Photosynthesis and growth were enhanced by elevated CO₂ treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO₂ but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO₂ were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post‐irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO₂ under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.     

Seedling establishment of vascular epiphytes on isolated and enclosed forest trees in an Andean landscape, Ecuador

The impact of human disturbance on colonisation dynamics of vascular epiphytes is poorly known. We studied abundance, diversity and floristic composition of epiphyte seedling establishing on isolated and adjacent forest trees in a tropical montane landscape. All vascular epiphytes were removed from plots on the trunk bases of Piptocoma discolor. Newly established epiphyte seedlings were recorded after 2 years, and their survival after another year. Seedling density, total richness at family and genus level, and the number of families and genera per plot were significantly reduced on isolated trees relative to forest trees. Seedling assemblages on trunks of forest trees were dominated by hygrophytic understorey ferns, those on isolated trees by xerotolerant canopy taxa. Colonisation probability on isolated trees was significantly higher for plots closer to forest but not for plots with greater canopy or bryophyte cover. Seedling mortality on isolated trees was significantly higher for mesophytic than for xerotolerant taxa. Our results show that altered recruitment can explain the long-term impoverishment of post-juvenile epiphyte assemblages on isolated remnant trees. We attribute these changes to a combination of dispersal constraints and the harsher microclimate documented by measurements of temperature and humidity. Although isolated trees in anthropogenic landscapes are considered key structures for the maintenance of forest biodiversity in many aspects, our results show that their value for the conservation of epiphytes can be limited. We suggest that abiotic seedling requirements will increasingly constitute a bottleneck for the persistence of vascular epiphytes in the face of ongoing habitat alteration and atmospheric warming.     

Scaling sun and shade photosynthetic acclimation of Alocasia macrorrhiza to whole-plant performance – I. Carbon balance and allocation at different daily photon flux densities

We determined the carbon allocation patterns and construction costs of Alocasia macrorrhiza plants grown at different photon flux densities (PFD) as well as the whole-plant carbon gain of these plants at different daily PFDs. Growth at high PFD resulted in thicker leaves with a higher leaf mass per unit area, and increased biomass allocation to petioles and roots, as compared to growth at low PFD. Increased allocation to petioles may have been necessary to support the heavier leaves, whereas increased allocation to roots may have been necessary to supply sufficient water for the higher transpiration rates in high PFD. Root biomass was highly correlated with the daily, whole-plant transpiration rate. Tissue construction costs per unit dry mass were unchanged by acclimation, but, since the mass per unit areas of leaves, roots and petioles all increased, construction costs per unit leaf area were much higher for plants grown at high PFD. On a per unit leaf area basis, daily whole-plant carbon gain measured at high daily PFD was higher in high- than in low-PFD-grown plants. However, on a per unit leaf mass basis, low-PFD-grown plants had a daily carbon gain at least as high as that of high-PFD-grown plants at high daily PFD. At low daily PFD, low-PFD-grown plants maintained an advantage over high-PFD-grown plants in terms of carbon gain because of their larger leaf area ratios. Thus, in terms of carbon gain, low-PFD-grown plants performed better than sun plants at low PFD and as well as high-PFD-grown plants at high PFD, despite their lower photosynthetic capacities per unit area. For high-PFD-grown plants, the higher construction costs per unit leaf area resulted in lower leaf area ratios, which counteracted the advantage of higher photosynthetic rates per unit leaf area.