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.     

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.     

High-temperature inhibition of photosynthesis is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling

The survival of dipterocarp seedlings in the 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. Field measurements demonstrated that Shorea leprosula seedlings in a rain forest understorey received a high proportion of daily photon flux density at temperatures supra‐optimal for photosynthesis (72% at ≥30 °C, 14% at ≥35 °C). To investigate the effect of high temperatures on photosynthesis during sunflecks, gas exchange and chlorophyll fluorescence measurements were made on seedlings grown in controlled environment conditions either, under uniform, saturating irradiance (approximately 539 µmol m^?2 s^?1) or, shade/fleck sequences (approximately 30 µmol m^?2 s^?1/approximately 525 µmol m^?2 s^?1) at two temperatures, 28 or 38 °C. The rate of light‐saturated photosynthesis, under uniform irradiance, was inhibited by 40% at 38 °C compared with 28 °C. However, during the shade/fleck sequence, photosynthesis was inhibited by 59% at 38 °C compared with 28 °C. The greater inhibition of photosynthesis during the shade/fleck sequence, when compared with uniform irradiance, was driven by the lower efficiency of dynamic photosynthesis combined with lower steady‐state rates of photosynthesis. These results suggest that, contrary to current dogma, sunfleck activity may not always result in significant carbon gain. This has important consequences for seedling regeneration processes in tropical forests as well as for leaves in other canopy positions where sunflecks make an important contribution to total photon flux density.     

Leaf display and photosynthesis of tree seedlings in a cool-temperate deciduous broadleaf forest understorey

To examine a possible convergence in leaf photosynthetic characteristics and leaf display responses to light environment in seedlings of three canopy and two shrub tree species in understorey of cool-temperate deciduous broadleaf forest, relationships between light environment, leaf orientation and leaf light-photosynthetic response were measured. Light capture of the seedlings (17-24 individuals with 2-12 leaves for each species) was assessed with a three dimensional geometric modeling program Y-plant. Leaf photosynthetic characteristics of the five species were found to have acclimated to the understorey light environment, i.e., low light compensation point and high apparent quantum yield. In addition, light-saturated photosynthetic rates were higher in seedlings inhabiting microsites with higher light availability. Efficiencies of light capture and carbon gain of the leaf display were evaluated by simulating the directionalities of light capture and daily photosynthesis for each seedling using hemispherical canopy photography. The results showed that most of the seedlings orientated their leaves in a way to increase the daily photosynthesis during the direct light periods (sunflecks) rather than maximize daily photosynthesis by diffuse light. Simulations also showed that daily photosynthesis would increase only 10% of that on actual leaf display when the leaves orientated to maximize the diffuse light interception. Simulations in which leaf orientations were varied showed that when the leaf display fully maximized direct light interception, the time that leaves were exposed to excessive photon flux density of >800 mumol photons m(-2) s(-1) were doubled. The understorey seedlings studied responded to the given light environments in a way to maximize the efficiency of acquisition and use of light during their short (approximately 3 month) seasonal growth period.     

光照对鄂东南2种落叶阔叶树种幼苗生长、光合特性和生物量分配的影响

通过设置4个光照梯度(25%、12%、6%和3%自然光)模拟鄂东南低山丘陵地区落叶阔叶林林下的光环境,研究了2种耐荫性不同的树种幼苗--麻栎(Quercus acutissima)和化香(Platycarya strobilacea)不同光强下的存活率、光合特性、生长和生物量分配,探讨了低光环境中耐荫性不同的树种幼苗维持自身碳平衡的机制和权衡"存活-生长"选择的生活史策略。结果表明:(1)低光下的2个树种幼苗的生长、光合特性和生物量分配具有显著性差异。(2)各个光照梯度下麻栎幼苗都生长良好,存活率保持在35%以上,而化香幼苗遭遇高的死亡率,80d后3%和6%自然光下的幼苗全部死亡;低光环境中麻栎幼苗比化香幼苗具有更大的表观光量子(AQY)和最大净光合效率(Pmax),更低的光补偿点(LCP)和暗呼吸效率(Rd),即耐荫性较强的麻栎幼苗比耐荫性较弱的化香幼苗具有更高的低光碳同化率和碳捕获能力。(3)2个树种幼苗的成活率与RGR呈负相关关系,各个光照梯度下耐荫性较弱的化香幼苗的相对生长率(RGR)显著高于耐荫性较强的麻栎幼苗,而两个树种幼苗的净同化率(NAR)无明显差异。相对于麻栎幼苗较高的根生物量比(RMR),化香幼苗将更多的生物量分配给叶部,因而具有较高的叶生物量比(LMR)、叶面积比(LAR)和比叶面积(SLA)。不同耐荫性的幼苗生长及生物量分配方式的差异是植物"存活-生长"权衡后的结果,耐荫性弱的化香幼苗具有较高的生长潜力和较弱的自我保护能力,而耐荫性强的麻栎幼苗具有更高的低光碳储量,能够维持更好的低光碳平衡,具有竞争优势。     

Local texture convergence within three communities in Fiordland, New Zealand

Light environments in two lowland New Zealand podocarp rainforests are described using data from quantum sensors. Mean daily total photosynthetic photon flux density (PPFD) in the forest understorey varies from 2.6-5.2% incident PPFD in summer and 1.0-2.5% in autumn, and in gaps from 5.0-16.6% in summer and 6.3-8.3% in autumn. Pronounced differences in understorey PPFD occur between clear and overcast days. Overcast days tend to have a lower proportion of 2-minute periods with very low mean PPFD than clear days. In summer, 37.7-91.7% of PPFD occurs as sunflecks, but these only occur for 4.0-27.9% of the time. Most sunflecks are short duration (42.2-72.7% less than 4 minutes in summer) but some are very long (>32 minutes). Overall, light environments are similar to those found in other forests, both in New Zealand and elsewhere. However, canopy structure has a pronounced influence on spatial distribution of light environments within a forest, and differences in the size and frequency of canopy disturbances are a contributing factor to the floristic differences between the two forests studied.     

Fluctuation of photosynthetic photon flux density within aMiscanthus sinensis canopy

Detailed measurements of diurnal variations in photosynthetic photon flux density (PPFD) were made at seven locations within the canopy of aMiscanthus sinensis grassland to evaluate the light conditions of microsites for heliophilic tree seedlings. Multiple regression analysis revealed that the short-term light fluctuation on a clear day was highly dependent on the wind speed and solar elevation angle, whereas on a cloudy day it was mainly determined by the PPFD incident from above the canopy. The relative PPFD at 40 cm aboveground varied from 0.065 to 0.252, depending on sky conditions and the sensor's position in relation to clumped patches ofM. sinensis. On a clear day, the proportion of PPFD readings above 100 μmol·m⁻²·s⁻¹ contributed by sunflecks ranged between 25.4% and 82.0%. Computer simulation showed that the contribution of sunflecks to the daily carbon gain ofQuercus serrata seedlings may range from 11% to 65%. The mean relative PPFD measured under diffuse light conditions was linearly related to the daily total PPFD and the daily carbon gain by single leaves ofQ. serrata seedlings. This suggests that the relative PPFD under diffuse light conditions provides an appropriate measure of site-specific light availability within a grass canopy.     

The influence of nitrogen on rain forest dipterocarp seedlings exposed to a large increase in irradiance

Dipterocarps dominate the canopy of lowland tropical rain forest in South‐east Asia. Seedlings of these species form diverse assemblages on the forest floor where low irradiance severely limits their growth. Further growth depends largely upon the increased irradiance that can occur with the creation of canopy gaps. However, the response of dipterocarp seedlings to increased irradiance and their subsequent establishment in the canopy may be influenced by the availability of other resources, such as nutrient availability. We investigated the influence of nitrogen supply on aspects of the photosynthetic physiology and growth of seedlings of four dipterocarp species (Shorea leprosula, Shorea johorensis, Shorea oleosa and Dryobalanops lanceolata) growing under low irradiance, during transfer from low to high irradiance, and during subsequent growth at high irradiance. All four species increased growth and photosynthetic capacity in response to N‐supply at high irradiances but not at low irradiance approximating that which can be expected to occur in the forest understorey. When seedlings grown at low irradiances and varying N‐supply were exposed to a large increase in irradiance, all species showed some degree of initial photodamage (measured through chlorophyll fluorescence), the extent of which was similar between species but differed markedly depending on the pre‐exposure growth irradiance and N‐supply. Greater photodamage occurred in seedlings grown at lower compared with higher N‐supply and irradiance. Despite these initial difference in the extent of this photodamage, all seedlings demonstrated a similar capacity to recover from damage. However, the alterations in the photosynthetic physiology of leaves during this recovery differed between species and depended on N‐supply. Under high N‐supply all species apart from S. oleosa increased photosynthetic capacity per unit chlorophyll following exposure to high irradiance by increasing photosynthetic capacity per unit leaf area while, under low N‐supply, an increase in photosynthetic capacity per unit leaf only occurred in D. lanceolata. Our results suggest that variations in N‐availability may have a much greater impact on the relative competitiveness of dipterocarp seedlings during the regenerative phase following canopy gap formation than physiological differences between seedlings. Our results demonstrate a potentially significant role for N‐availability in the regeneration dynamics and distribution of canopy‐dominating dipterocarp species.     

Light environments within montane tropical rainforest,Mt Bellenden Ker,North Queensland

Irradiance and photosynthetic photon flux density (PPFD) at five sites within a montane tropical rainforest were compared to test for differences among characteristic environments within the same forest type. The five habitats investigated were a large clearing (800 m²). a small gap (150 m²), the lower canopy, and two shaded understorey sites. Daily average irradiance ranged from 32.3 W m^-2 in the heavily shaded understorey to over 400 W m^-2 in the large clearing. Similarly, daily average PPFD ranged from 10.2 to over 800 μmol m^-2 s^-1 at corresponding sites, respectively. Daily total irradiation in the understorey, small gap, and lower canopy were 7.9–8.8%, 11.8% and 11.5%, respectively, of irradiation in the large clearing. For daily total PPFD, relative values for the same sites were 1.3–3.8%, 6.0% and 10.3%, respectively. Comparisons of frequency distributions for irradiance showed that the only significant difference was between the large clearing and the four forest sites. Comparisons of frequency distributions for PPFD showed significant differences among all five sites. Median PPFDs were always above the range of known light compensation points for rainforest plants even in the heavily shaded understorey, and usually above the known light saturation levels of canopy members. These results indicate that the reduction in PPFD by cloud cover does not limit the rates of photosynthesis.     

Low temperature enhances photosynthetic down-regulation in French bean (Phaseolus vulgaris L.) plants

The mechanisms of photosynthetic adaptation to different combinations of temperature and irradiance during growth, and especially the consequences of exposure to high light (2000 micro mol m(-2) s(-1) PPFD) for 5 min, simulating natural sunflecks, was studied in bean plants (Phaseolus vulgaris L.). A protocol using only short (3 min) dark pre-treatment was introduced to maximize the amount of replication possible in studies of chlorophyll fluorescence. High light at low temperature (10 degrees C) significantly down-regulated photosynthetic electron transport capacity [as measured by the efficiency of photosystem II (PSII)], with the protective acclimation allowing the simulated sunflecks to be used more effectively for photosynthesis by plants grown in low light. The greater energy dissipation by thermal processes (lower F(v)'/F(m)' ratio) at low temperature was related to increased xanthophyll de-epoxidation and to the fact that photosynthetic carbon fixation was more limiting at low than at high temperatures. A key objective was to investigate the role of photorespiration in acclimation to irradiance and temperature by comparing the effect of normal (21 kPa) and low (1.5 kPa) O(2) concentrations. Low [O(2)] decreased F(v)/F(m) and the efficiency of PSII (Phi(PSII)), related to greater PSII down-regulation in cold pre-treated plants, but minimized further inhibition by the mild 'sunfleck' treatment used. Results support the hypothesis that photorespiration provides a 'safety-valve' for excess energy.     

Costs and benefits of photosynthetic light acclimation by tree seedlings in response to gap formation

Some shade leaves increase their photosynthetic capacity (P _max) when exposed to a higher irradiance. The increase in P _max is associated with an increase in chloroplast size or number. To accommodate those chloroplasts, plants need to make thick leaves in advance. We studied the cost and benefit of photosynthetic acclimation in mature leaves of a tree species, Kalopanax pictus Nakai, in a cool-temperate deciduous forest. Costs were evaluated as the additional investment in biomass required to make thick leaves, while the benefit was evaluated as an increase in photosynthetic carbon gain. We created gaps by felling canopy trees and examined the photosynthetic responses of mature leaves of the understorey seedlings. In the shade, leaves of K. pictus had vacant spaces that were not filled by chloroplasts in the mesophyll cells facing the intercellular space. When those leaves were exposed to higher irradiance after gap formation, the area of the mesophyll surface covered by chloroplasts increased by 17% and P _max by 27%. This increase in P _max led to an 11% increase in daily carbon gain, which was greater than the amount of biomass additionally invested to construct thicker leaves. We conclude that the capacity of a plant to acclimate to light (photosynthetic acclimation) would contribute to rapid growth in response to gap formation.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Water relations pattern of understorey species influenced by sunflecks

Water relations of plant cenopopulations of understorey species (7 herbaceous perennials, 1 shrub, and 1 tree seedling) growing in an oak-hornbeam forest in SW. Slovakia is markedly influenced by natural sunflecks moving on the forest floor, where stomatal conductance and transpiration rate are significantly higher than in shade areas. In the course of a day, the leaf stomatal conductance fluctuates, following irradiance of the leaf, from minimum in full shade to maximum in sunflecks, including intermediate values in transient conditions. Therefore, there is also a large variability in water saturation deficit and transpiration rate within the cenopopulations of the understorey species. These variations should be considered in modelling the physiology of woodland understorey species.     

Ecophysiological responses of Fagus sylvatica seedlings to changing light conditions. I. Interactions between photosynthetic acclimation and photoinhibition during simulated canopy gap formation

Natural regeneration of European beech (Fagus sylvatica L.) establishes under shade, but sudden exposure to high irradiance may occur due to openings in the canopy. To elucidate ecophysiological mechanisms associated with survival of European beech seedlings, the gas exchange, chlorophyll concentrations, and chlorophyll a fluorescence parameters of two different beech populations were studied under changing light conditions. Plants were grown both in a growth chamber and at a natural site (one population) where the seedlings were raised in containers placed in understory and in simulated canopy gaps. Upon exposure to high light in the growth chamber, photosynthetic rates of shade-acclimated leaves of seedlings from both populations increased severalfold and then decreased over several days to the rates of the low-light control seedlings. High-light seedlings always had the highest photosynthetic rates. Initial fluorescence displayed a trend opposite that of photosynthesis; it increased over time, and relative fluorescence and half-time rise declined continuously until the end of experiment to very low values. Exposure to high light of shade-acclimated seedlings resulted in a shift in chlorophyll concentrations to levels intermediate between high-light and low-light seedlings. The light treatment effects were statistically greater than population effects; however, seedlings from the Abetone population were found to be more susceptible to changing light conditions than seedlings from Sicily. Reciprocal light treatments on plants growing at the natural site confirmed the results obtained in the growth chamber experiment. Overall, beech seedlings grown in the field appeared to have a fairly large acclimation potential achieved by plasticity in the photosynthetic apparatus. The lack of pronounced acclimation to high light in seedlings grown in the growth chamber was ascribed to a threshold-type relationship between the acclimation capacity and the level of damage. These observations on the limited potential for acclimation to high light in leaves of European beech seedlings which show a clear capability to exploit sunflecks, are discussed in relation to regeneration following canopy gap formation and reinforce the view of the central role of gap formation in forest dynamics. We conclude that small forest gaps (in which sunflecks play a major role) may present a favorable environment for survival and growth of beech because of their limited ability to acclimate to a sudden increase in irradiance and because of the moderate levels of light stress found in small gaps.     

Leaves of Japanese oak (Quercus mongolica var. crispula) mitigate photoinhibition by adjusting electron transport capacities and thermal energy dissipation along the intra-canopy light gradient

We investigated the morphological and physiological acclimation of leaves grown within a canopy of Japanese oak tree (Quercus mongolica var. crispula) in terms of the susceptibility to photoinhibition under various growth light conditions. The maximum rates of photosynthesis (P(max) ) and electron transport (ETR(max) ) were higher in mature leaves grown under stronger light with higher area-based leaf nitrogen (N) content closely associated with higher leaf mass per area. The net photosynthetic (P(n) ) and electron transport (ETR) rates corresponding to the daily peak photosynthetic photon flux density (PPFD(max) ) during leaf maturation were almost comparable to P(max) and ETR(max) , respectively. Conversely, P(n) and ETR at the daily average PPFD (PPFD(avg) ) were substantially low in shade-grown leaves when compared with P(max) and ETR(max) . The susceptibility to photoinhibition at PPFD(max) , i.e. at sunflecks for the shade-grown leaves, was assessed by the rate of excess energy production. Although sun leaves showed higher rates of electron transport and thermal energy dissipation than shade leaves under PPFD(max) conditions, the rate of excess energy production was almost constant across shade to sun leaves. The shade leaves of the Japanese oak grown within a crown were suggested to adjust their N investment to maintain higher photosynthetic capacities compared with those required to maximize the net carbon gain, which may facilitate the dissipation of the excessive light energy of sunflecks to circumvent photoinhibition in cooperation with thermal energy dissipation.     

Light environment under Rhododendron maximum thickets and estimated carbon gain of regenerating forest tree seedlings

Canopy tree recruitment is inhibited by evergreen shrubs in many forests. In the southern Appalachian mountains of the USA, thickets of Rhododendron maximum L. restrict dominant canopy tree seedling survival and persistence. Using R. maximum as a model system, we examined available light under the thickets and the photosynthetic responses of seedlings of canopy tree species. We tested the hypothesis that the additional shading from under R. maximum drives carbon gain in seedlings below the threshold for growth and survival. A reduction in light under the thicket was found where canopy openness (derived from canopy photographs) under R. maximum was half the amount measured in forest without R. maximum. R.␣maximum also reduced direct radiation by 50% and diffuse radiation by 12–29% compared to forest without the shrub layer. Mean mid-day PPFD (photosynthetically active photon flux density between 1000 and 1400 h) under R. maximum (obtained from quantum sensors) was below 10 mol m⁻² s⁻¹ on both clear and overcast days and the amount of sunflecks greater than 10 mol m⁻² s⁻¹ PPFD was only 0–20 min per day. In contrast, forest without R. maximum received a mean PPFD of 18–25 mol m⁻² s⁻¹ on clear days and a cumulative sunfleck duration of 100–220 min per day in all sky conditions. Consistent with light availability between the sites, daily carbon gain in Quercus rubra L. seedlings was lower in forest with R. maximum compared to forest where the shrub was absent. The presence of the shrub layer also significantly suppressed average mid-day photosynthesis of both Q. rubra and Prunus serotina Ehrt. seedlings on 8 out of 11 measurement dates. However, parameters derived from light response curves between seedlings growing in forest sites with or without a thicket of R. maximum was significantly different only in A _max (maximum photosynthetic rate), indicating a lack of further acclimation to the deeper shade under R. maximum. While the additional shade cast by R. maximum is sufficient to prevent the regeneration of tree seedlings under this shrub, there was sufficient heterogeneity in light under the thicket to imply that deep shade only partially explains the complete inhibition of regenerating canopy trees under R. maximum.     

Analysis of the ��shelter tree-effect�� of natural and exotic forest canopies on the growth of young Podocarpus falcatus trees in southern Ethiopia

Plantations of exotic trees on areas where tropical forest has been clear-felled have been reported to facilitate regrowth of indigenous tree species. This so-called nurse- or shelter tree effect was investigated in a montane semihumid site in southern Ethiopia where plantations of Pinus patula and Eucalyptus saligna grow in close vicinity to the natural Podocarpus falcatus mixed forest. Physiological performance of P. falcatus saplings growing in the exotic and the natural forests was investigated over the year. Compared with the natural forest, photosynthetic carbon gain and growth of the saplings were significantly enhanced under Pinus patula while likewise retarded under Eucalyptus saligna. Diverging effects of the differently dense shelter canopies on the saplings could be traced to differences in the sub-canopy microclimates and particularly to the intensities and temporal distribution of photosynthetic active radiation (PAR). Moisture also played an important role for photosynthetic carbon gain: while the morning patterns of CO₂ uptake were significantly correlated with the patterns of sunflecks, stomatal conductance was the determinant factor in the afternoon. Analysis of the photosynthetic efficiency of sunflecks revealed a lower quantum yield than the basic diffuse PAR intensity. Compared with a theoretically even distribution of the daily PAR, CO₂ uptake under the real light climate was 70% under Pinus and in the natural forest, and 59% under Eucalyptus. Relating growth rates of Podocarpus saplings to photosynthesis the microclimate under Pinus was 2.5 times as effective as that in the natural forest and five times more effective than under Eucalyptus.     

光照对两种热带雨林树种幼苗光合能力、热耗散和抗氧化系统的影响

 研究了生长于不同光照条件下（100%、25%和8%光强）热带雨林冠层树种绒毛番龙眼（Pometia tomentosa）和中层树种滇南风吹楠（Horsfieldia tetratepala）幼苗的光合能力、热耗散、活性氧和保护性酶的活性。结果表明,绒毛番龙眼的最大光合速率随着生长光强的增加而提高,而滇南风吹楠在全光条件下的最大光合速率反比25%光照条件下的低。全光条件下两个树种光系统II的最大光化学效率（Fv/Fm）都显著降低,表明发生了长期光抑制。当把生长于遮荫条件下的幼苗移到全光下,从凌晨到中午随着光强的增加光抑制加剧,日落时生长于8%光照条件下的绒毛番龙眼及生长于8%和25%光照条件下的滇南风吹楠的光抑制不能完全恢复。非光化学猝灭对光强的响应曲线表明,随着生长光强的增加滇南风吹楠的热耗散能力增强,而生长在全光和25%光照条件下的绒毛番龙眼的热耗散能力都比滇南风吹楠的弱。两个树种叶片中O-[]·2、H2O2含量、SOD和CAT活性均随着生长光强的增加而提高;在同一光照条件下,绒毛番龙眼叶片中O-[]·2、H2O2含量、SOD和CAT活性显著高于滇南风吹楠。上述结果表明,在光抑制条件下,冠层树种绒毛番龙眼较大程度通过提高保护性酶的活性来保护光合机构免受损伤,而中层树种滇南风吹楠却较大程度通过增强非光化学猝灭来耗散过量光能;滇南风吹楠对强光的适应性差。     

Climate change reverses the competitive balance of ash and beech seedlings under simulated forest conditions

This study identifies the important role of climate change and photosynthetic photon flux density (PPFD) in the regenerative competence of ash and beech seedlings in 12 inter- and intra-specific competition designs in simulated mixed ash-beech forest gaps under conditions of non-limiting soil volume, water and nutrient supply. The growth conditions simulated natural forest conditions as closely as possible. Simulations were performed by growing interacting seedling canopies for one season in temperature-regulated closed-top chambers (CTCs). Eight CTCs were used in a factorial design with replicate treatments of [CO2] x temperature x PPFD x competition design. [CO2] tracked ambient levels or was 360 micromol mol-1 higher. Temperature tracked ambient levels or was 2.8 degrees C higher. PPFD on two plant tables inside each CTC was 16% and 5% of open-field levels, respectively, representative of typical light flux levels in a natural forest gap. In several of the competition designs, climate change made the ash seedlings grow taller than the beech seedlings and, at the same time, attain a larger leaf area and a larger total biomass. Advantages of this type for ash were found particularly at lower PPFD. There was a positive synergistic interaction of elevated temperature x [CO2] for both species, but more so for ash. There are many uncertainties when a study of chambered seedlings is to be projected to real changes in natural forests. Nevertheless, this study supports a possible future shift towards ash in north European, unmanaged, mixed ash-beech forests in response to the predicted climate change.     

Morning vs afternoon sun patches in experimental forest gaps: consequences of temporal incongruency of resources to birch regeneration

We investigated whether the timing of high light availability as sun patches within forest gaps, independent of total or peak photosynthetic photon flux (PPF), influences the physiology and growth of four coexisting birch species (Betula alleghaniensis, B. lenta, B. papyrifera, and B. populifolia). Birch seedlings were grown for two years along either the east or west sides of experimental gap structures and at two moisture levels. Seedlings positioned in the west received sun patches earlier in the day than those in the east, and environmental conditions for carbon gain were generally more favorable during the earlier sunpatches in the west; air and leaf temperatures were lower, and relative humidity higher, relative to conditions during sun patches in the cats, simulating patterns observed in natural forest gaps. Seedlings positioned along the west edges of gaps fixed more carbon earlier in the day than those in the east, and in many cases, peak net photosynthetic rates were greater for west positioned seedlings. In year two, leaf-level integrated daily carbon gain was greater for west- than eastpositioned plants, and for the most pioneer species, B. populifolia, differences between west and east seedlings were greatest at lower soil moisture levels. Despite some small effects on leaf gas exchange, the timing of high light availability, and its temporal congruence with other factors critical to carbon gain, had no significant effects on first or second year seedling biomass. The responses of birch seedlings to controlled variations in the timing of high light availability were generally much smaller than birch seedling responses to variations in other components of daily light regimes such as total integrated and peak PPF.