Herbivory on Temperate Rainforest Seedlings in Sun and Shade: Resistance,Tolerance and Habitat Distribution

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.     

光对热带雨林冠层树种绒毛番龙眼种子萌发及其幼苗早期建立的影响

绒毛番龙眼(Pometia tomentosa)为西双版纳热带季节雨林标志树种,被列为中国珍稀濒危植物。对其开展种子及幼苗生态学研究可为珍稀濒危树种的保护及当地森林恢复提供科学依据。试验采用遮荫试验和野外试验相结合的方法,研究光对绒毛番龙眼种子萌发及幼苗早期建立的影响。结果表明:在遮荫试验30%(林窗中心光)、10%(林窗边缘光)和3.5%(林下光)3个光处理及森林3种生境(林窗中心、林窗边缘和林下)条件下,种子萌发率差异无显著性差异,均达到95%以上,且平均萌发周期小于6 d。3个光处理下的幼苗生长、生物量分配模式及气体交换参数差异显著。30%光下绒毛番龙眼幼苗的根重比(RMR)和茎重比(SMR)最高,10%和3.5%光下幼苗的叶重比(LMR)最高,3.5%光下的叶面积比(LAR)显著高于30%光下。30%光下绒毛番龙眼幼苗的最大净光合速率(P_nmax)、暗呼吸速率(R_d)和光饱和点(I_sat)在3个光处理中均最大,光补偿点(I_comp)则无显著性差异。绒毛番龙眼幼苗具耐荫性,能够在低光环境下长期存活且能缓慢生长;同时具有适应林窗光环境生长的能力,如高的质量相对生长速率(RGR_M)和高度相对生长速率(RGR_H)。林窗的出现是绒毛番龙眼进行成功更新的必要条件,水分可能是限制其幼苗生境选择的另一环境因子。     

中国木本植物幼苗生长对光照强度的响应

光照是影响植物幼苗生长的重要的环境因子,定量化研究光照对木本植物不同生活型幼苗生长的影响具有重要意义。系统收集了有关光照对我国木本植物幼苗生长影响的国内外文献,采用Meta分析的方法对幼苗的生长效应进行评估。研究结果表明:(1)与全光照下幼苗生长相比,由于遮阴处理对幼苗基径增长量的抑制作用强于对株高增长量的作用,幼苗出现株高基面积比增高的形态特征变化;(2)与灌木相比,乔木树种幼苗对不同光照强度的响应更加敏感;(3)在小于20%和20%—40%全光照条件下,落叶阔叶和常绿阔叶树种幼苗基径均受到显著抑制,而常绿针叶树种的基径并没有受到遮阴的明显抑制;(4)落叶阔叶和常绿阔叶树种的株高基面积比均显著高于全光照,而常绿针叶树种的株高基面积比与全光照相比差异较小。我们的这一研究将有助于更好的理解木本植物幼苗生长对光照的响应机理。     

Growth and survivorship of dipterocarp seedlings: differences in shade persistence create a special case of dispersal limitation

A series of growth experiments and observations on natural populations have been carried out on dipterocarp species of contrasting ecology growing in artificial gaps and the forest understorey. These studies have demonstrated that although differences exist between species in photosynthetic and growth responses to the high-light environment, competition for light in canopy gaps is highly asymmetrical and tends to reinforce any pre-existing dominance hierarchy. We propose that differences in seedling persistence in forest canopy shade are highly influenced by species-specific biotic and abiotic interactions. Our experiments suggest that as seedlings, dipterocarp species trade off traits which enhance persistence and growth in shade against those that enhance their ability to exploit gaps. Less competitive species survive for progressively longer periods of time after a gregarious fruiting event. This leads to significant shifts with time in the number of species present in the seedling bank and hence in the importance of interspecific competition in determining which species dominates regrowth in gaps. We propose that this special case of dispersal limitation is more likely to account for coexistence of dipterocarp species than differences in growth responses to gaps of different size, with stochastic and environmental variables interacting to determine species distribution and abundance.     

Different responses to shade of evergreen and deciduous oak seedlings and the effect of acorn size

An evergreen oak species, Cyclobalanopsis multinervis, and a deciduous oak species, Quercus aliena var. acuteserrata were grown from acorns under two light levels (full sunlight and shade at about 18 % of full sunlight, simulating the light intensities in forest clearings and gaps, respectively) for one growing season. Three hypotheses were tested: (i) the deciduous species grows faster than the evergreen species in forest gaps and clearings; (ii) the deciduous species responds more strongly in terms of growth and morphology to variation in light climate than the evergreen species; and (iii) seedling size is positively correlated to acorn size. The results showed: (i) at both light levels, the deciduous seedlings gained significantly more growth in biomass and height than the evergreen seedlings; (ii) both species produced significantly more biomass in full sunlight than in shade, without showing any significant difference in height between treatments. Increase in light intensity improved the growth of the deciduous seedlings more strongly; (iii) at a similar age, the deciduous seedlings showed a greater response in leaf morphology and biomass allocation to variation in light levels, but when compared at a similar size, biomass allocation patterns did not differ significantly between species; (iv) bigger acorns tended to produce larger seedlings, larger leaf sizes and more leaf area, between and within species. These differences demonstrate that the deciduous species is gap-dependent and has the advantage over the evergreen species in forest gaps and clearings.     

山地落叶阔叶林优势树种米心水青冈幼苗的定居

在郁闭的林冠下、模拟林窗和林间空旷地3种光照环境中,研究了施肥和未施肥的米心水青冈幼苗生存和生长发育过程．结果表明,幼苗在林下的发育受到光照不足的严重抑制,生长在林下的幼苗比生长在林窗和空旷地幼苗死亡快,死亡率高,幼苗在郁闭林下能够生存的时间不超过12周．施肥处理的幼苗比对照的幼苗死亡快,死亡率高．幼苗死亡很可能与土壤中的病原生物有关．虽然由林下光照强度增加到模拟林窗的光照强度后,幼苗发育有明显改善,但幼苗在模拟林窗环境和空旷地的生长没有显著差异．结果说明,米心水青冈新出土的幼苗在郁闭林冠下易受伤害,更新立苗阶段需要有比较好的光照条件,肥沃土壤可能也无助于改善郁闭林冠下幼苗的定居．     

Photoinhibition and recovery in tropical plant species: response to disturbance

Disturbance or rainforest is often followed by mass mortality of understorey seedlings. Transitions of shade grown plants to full sunlight can cause reductions in the efficiency with which light is used in photosynthesis, called photoinhibition. In order to assess the influence of photoinhibition on mortality and growth after rainforest disturbance this study examined photoinhibition in both simulated and real forest disturbances in northern Papua New Guinea. In an experiment simulating rainforest disturbance, exposure of shade-grown plants to full sunlight resulted in abrupt decreases in the chlorophyll fluorescence parameter F _v/F _m that is characteristic of photoinhibition. However, in the well-watered plants used in these experiments there were no fatalities during 3 weeks after exposure to full sunlight. Thus, it is unlikely that photoinhibition, alone, is responsible for seedling fatalities after rainforest disturbances, but more likely that fatalities are due to photoinhibition in conjunction with other environmental stress. There were differences between the response of species to the simulated disturbance that concurred with their preferred habitats. For example, species form the genus Barringtonia, which is commonly found in shaded understorey environments, underwent greater reductions in F _v/F _m and were slower to recover than species that usually inhabit high solar radiation environments. The extent of photoinhibition and the rate of recovery appeared to be dependent on avoidance of direct solar radiation by altering leaf angles and on increasing maximum photosynthetic rates. A field survey of photoinhibition in man-made rainforest gaps corroborated the findings of the simulated disturbance experiment showing that plant species commonly found in shaded environments showed a greater degree of photoinhibition in forest gaps at midday than those species which are classified as species that benefit from gaps or specialist gap inhabitors.     

Global patterns of insect herbivory in gap and understorey environments,and their implications for woody plant carbon storage

Insect herbivory is thought to favour carbon allocation to storage in juveniles of shade‐tolerant trees. This argument assumes that insect herbivory in the understorey is sufficiently intense as to select for storage; however, understoreys might be less attractive to insect herbivores than canopy gaps, because of low resource availability and – at temperate latitudes – low temperatures. Although empirical studies show that shade‐tolerant species in tropical forests do allocate more photosynthate to storage than their light‐demanding associates, the same pattern has not been consistently observed in temperate forests. Does this reflect a latitudinal trend in the relative activity of insect herbivory in gap versus understorey environments? To date there has been no global review of the effect of light environment on insect herbivory in forests. We postulated that if temperature is the primary factor limiting insect herbivory, the effect of gaps on rates of insect herbivory should be more evident in temperate than in tropical forests; due to low growing season temperatures in the oceanic temperate forests of the Southern Hemisphere, the effect of gaps on insect herbivory rates should in turn be stronger there than in the more continental temperate climates of the Northern Hemisphere. We examined global patterns of insect herbivory in gaps versus understories through meta‐analysis of 87 conspecific comparisons of leaf damage in contrasting light environments. Overall, insect herbivory in gaps was significantly higher than in the understorey; insect herbivory was 50% higher in gaps than in understoreys of tropical forests but did not differ significantly between gaps and understories in temperate forests of either hemisphere. Results are consistent with the idea that low resource availability – and not temperature – limits insect herbivore activity in forest understoreys, especially in the tropics, and suggest the selective influence of insect herbivory on late‐successional tree species may have been over‐estimated.     

Light energy partitioning in photosystems I and II during development of Nothofagus nitida growing under different light environments in the Chilean evergreen temperate rain forest

Nothofagus nitida (Phil.) Krasser (Nothofagaceae) regenerates under the shade. Nonetheless, older seedlings are commonly found at full sun. We tested the hypothesis that light capture and photochemical and non-photochemical energy dissipation of both photosystems PSI and PSII adjust with ontogeny and brighter environment. Light energy partitioning in both photosystems was studied in seedlings of different developmental stages (small 9.7 cm, tall 36 cm) under contrasting light environments (8–200 and 1,800–2,043 μmol photons m⁻² s⁻¹) in the Chilean evergreen temperate forest. Higher A _max, dark respiration, and light compensation and saturation points in sun seedlings of both developmental stages were accompanied by higher rates of electron transport. These seedlings also showed a high fraction of open PSII reaction centres and similar non-photochemical quenching at high-light in both photosystems, showing no effect of developmental stage in these parameters. Conversely, light capture, total thermal dissipation after photoinhibition, active down-regulation of antenna efficiency, and state transitions were higher in smaller seedlings than in taller ones. These changes maintain photostasis, preventing photodamage, while favouring a more oxidized quinone pool. There is an independent effect of seedling development and light acclimation on this transition from shade to sun during early ontogeny. This transition reflects short-term responses of the photosynthetic apparatus to light and longer term responses that depend on seedling developmental stage.     

Responses of temperate woody seedlings to shade and drought: do trade-offs limit potential niche differentiation?

Diversity in seedling responses to combined shade and drought can drive species niche differences, and thus natural forest and scrub establishment dynamics and diversity. However, inherent trade-offs between shade tolerance and drought tolerance, as hypothesized in the literature, would constrain potential niche differentiation. For thirteen species of European trees and shrubs the impacts were determined of moderate and extreme drought on the growth and survival of first-year seedlings in a given soil volume, in irradiances in the range typical for temperate forest and scrub understories (3% daylight) and large gaps (30% daylight). Pots were watered differentially each second day to equalize soil water content across species and irradiances. Comparisons within and across species supported independent tolerances of shade and drought rather than trade-offs. For all species, drought reduced relative growth rate in dry mass by the same proportion in 3% daylight and in 30% daylight. Consequently, drought generally reduced final dry mass significantly more strongly in 30% daylight than in 3% daylight. Extreme drought led to earlier mortality in 30% daylight than in 3% daylight for nine of the eleven species tested, to earlier mortality in 3% daylight for one species, and to equal mortality rate in both irradiances for one species. For each species, growth-based shade tolerance was quantified as the ratio of absolute growth in low irradiance to that in high irradiance, and growth-based drought tolerance as the ratio of absolute growth in low water supply to that in high water supply. Across species, growth-based shade tolerance varied ≈9-fold, and growth-based drought tolerance ≈2-fold; species’ tolerances correlated respectively with indices of field establishment in shade and drought. Growth-based drought tolerance correlated with survival time in extreme drought. Experimentally quantified shade and drought tolerances varied independently for the 13 species tested, indicating the potential for extensive species niche differentiation in combinations of irradiance and water supply.     

Root/Shoot Allocation and Root Architecture in Seedlings: Variation among Forest Sites, Microhabitats, and Ecological Groups1

I analyzed patterns of variation in root mass allocation and root morphology among seedlings of woody species in relation to environmental factors in four Neotropical forests. Among forests, I explored the response of root traits to sites varying in water or nutrient availability. Within each forest, I explored the plastic response of species to different microhabitats: gaps and understory. Additionally, I explored evidence for life history correlation of root and shoot traits by comparing species differing in their successional group (light‐demanding [22 spp.] or shade tolerant [27 spp.]) and germination type (species with photosynthetic cotyledons or species with reserve cotyledons). At each forest site, young seedlings from 10 to 20 species were excavated. A total of 55 species was collected in understory conditions and 31 of them were also collected in gaps. From each seedling, six morphological ratios were determined. Allocation to roots was higher in forest sites with the lowest soil resources. Roots were finer and longer in the most infertile site, while roots were deeper in the site with the longest dry season. Seedling traits did not differ between germination types. Shade tolerant species allocated more to roots and developed thicker roots than light‐demanding species. Light‐demanding species showed stronger plastic responses to habitat than shade tolerant species, and species with photo‐synthetic cotyledons showed lower plasticity than species with reserve cotyledons. Overall, these results suggest that among Neotropical species, root allocation and root morphology of seedlings reflect plant adjustments to water or nutrient availability at geographic and microhabitat scales. In addition, life history specialization to light environments is suggested by differences among groups of species in their allocation to roots and in their root morphology.     

Pathogen mortality of tropical tree seedlings: experimental studies of the effects of dispersal distance,seedling density,and light conditions

Summary We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.     

Plasticity in seedling morphology,biomass allocation and physiology among ten temperate tree species in response to shade is related to shade tolerance and not leaf habit

• Mechanisms of shade tolerance in tree seedlings, and thus growth in shade, may differ by leaf habit and vary with ontogeny following seed germination. To examine early responses of seedlings to shade in relation to morphological, physiological and biomass allocation traits, we compared seedlings of 10 temperate species, varying in their leaf habit (broadleaved versus needle‐leaved) and observed tolerance to shade, when growing in two contrasting light treatments – open (about 20% of full sunlight) and shade (about 5% of full sunlight).
• We analyzed biomass allocation and its response to shade using allometric relationships. We also measured leaf gas exchange rates and leaf N in the two light treatments.
• Compared to the open treatment, shading significantly increased traits typically associated with high relative growth rate (RGR) – leaf area ratio (LAR), specific leaf area (SLA), and allocation of biomass into leaves, and reduced seedling mass and allocation to roots, and net assimilation rate (NAR). Interestingly, RGR was not affected by light treatment, likely because of morphological and physiological adjustments in shaded plants that offset reductions of in situ net assimilation of carbon in shade. Leaf area‐based rates of light‐saturated leaf gas exchange differed among species groups, but not between light treatments, as leaf N concentration increased in concert with increased SLA in shade.
• We found little evidence to support the hypothesis of a increased plasticity of broadleaved species compared to needle‐leaved conifers in response to shade. However, an expectation of higher plasticity in shade‐intolerant species than in shade‐tolerant ones, and in leaf and plant morphology than in biomass allocation was supported across species of contrasting leaf habit.

     

Physiological and morphological correlates of whole-plant light compensation point in temperate deciduous tree seedlings

A range of traits, including metabolic costs, biomass allocation and seed reserves, may contribute to interspecific variation in the shade tolerance of tree seedlings. In addition, shade tolerance may be affected by differential responses of species to soil resource availability at low light. We used a custom-built whole-plant gas-exchange chamber to quantify instantaneous whole-plant light compensation point (WPLCP) and to parameterize whole-plant daily C gain models for seedlings of eight temperate deciduous tree species. We examined the relationship of WPLCP to growth, biomass allocation and gas-exchange under high and low light and nutrient availabilities and compared it to WPCLP of naturally recruited saplings. For species showing a response, both increased light and nutrient availability resulted in increased WPLCP. However, species’ responses to resource availability did not correspond closely with shade tolerance as has generally been predicted. Variation in WPLCP within species was best predicted by whole-plant dark respiration rates, leaf-level light compensation point and leaf mass per area. Among species, seed size was a strong negative correlate of WPLCP, explaining 66% of the variation. Species with the lowest WPLCP maintained lower growth rates across treatments but greater biomass in the low-light treatment compared with more light-demanding species. These data suggest that a number of traits, in particular metabolic costs and seed size, contribute to WPLCP. However, gas-exchange-based WPLCP was 1.5–3.5 times lower than corresponding growth-based field estimates of WPLCP, suggesting that other factors such as biotic interactions or ontogenetic shifts in whole-plant light requirements may substantially increase species’ WPLCP under natural conditions.     

Interactive effects of nitrogen and irradiance on sustained xanthophyll cycle engagement in Eucalyptus nitens leaves during winter

Eucalyptus nitens is a species that is adapted to low temperature. This study examines xanthophyll-cycle engagement in E. nitens seedlings exposed to cold-induced photoinhibitory conditions under different levels of irradiance and nutrient status. Xanthophyll-cycle pool size indicated an increased requirement for light energy dissipation under high irradiance and low nutrient status. Greater sensitivity to photoinhibition of non-shaded seedlings indicated that sustained xanthophyll-cycle engagement may occur in response to damaged chlorophyll. Within irradiance treatments, fertilised seedlings had higher photochemical efficiency and faster recovery from photoinhibition than unfertilised seedlings. These results demonstrate that fertilised compared to unfertilised seedlings can utilise a greater proportion of incident light under cold temperature conditions     

A season- and gap-detection mechanism regulates seed germination of two temperate forest pioneers

The survival of seedlings in temperate climate habitats depends on both temporal and spatial factors. The interaction between an internal seed dormancy mechanism and the ruling environmental conditions allows accurate cueing of germination. We analysed how environmental signals interact in seeds of temperate forest pioneer species, increasing the seed's chances of germinating in the right place at the right time. Digitalis purpurea and Scrophularia nodosa are two small-seeded herbaceous species that typically grow in vegetation gaps in European temperate forests. Seeds of both species are partially dormant at the time of dispersal in summer. This primary dormancy is released in autumn and early winter, resulting in a minimal level of physiological dormancy by late winter and early spring. We observed that physiological dormancy was induced again in seeds exhumed in late spring and in summer. Experiments in laboratory conditions revealed that primary dormancy in seeds of S nodosa was broken by cold stratification, whereas primary dormancy in D. purpurea seeds was broken by both a cold and a warm stratification. The two species differed in their response to the tested gap-detection signals, as light was the most important factor stimulating germination of D. purpurea, and seeds of S. nodosa germinated best when subjected to daily fluctuating temperatures. This study clearly indicates that the ability to germinate in response to gap-detection signals changes seasonally in temperate forest pioneers. Additionally, seeds of both species responded differently to these environmental signals, probably reflecting differences in the regeneration niche.     

On the adaptive value of large seeds for tropical moist forest trees: A review and synthesis

In moist temperate and tropical environments species that typically become established in closed, shaded habitats tend to have larger seeds than those that regenerate in open, secondary habitats. Despite this common pattern and the frequency with which benefits of small seed size for early successional species (large number, enhanced dispersal potential) have been discussed, little attention has focused on the advantages of large seeds for species that regenerate in closed, late successional associations. It is generally considered that large seeds enhance seedling survivorship at low light intensities. However, light intensity is only one of several factors that differ between shaded and sunlit habitats. This review examines microclimatic and biotic differences between shaded subcanopy habitats in mature tropical forests and those in sunlit, light gap habitats in which the early stages of tropical forest succession occur. Each factor is examined as a possible selective agent responsible for maintaining seed size differences between two guilds of tropical rainforest trees; the pioneer species that have small seeds and typically become established in large, sunlit gaps in the forest canopy and the persistent, relatively shade-tolerant species that have larger seeds and produce seedlings that survive for variable periods of time in the shade beneath the forest canopy. Three microclimatic factors that differ in subcanopy and gap habitats are examined; temperature, moisture, and light intensity. It is unlikely that temperature has been an important selective agent in maintaining the differences in seed sizes observed between the pioneer and persistent tree guilds. However, greater desiccation stress in light gaps might prevent successful regeneration of larger seeds in this habitat and thus might impose the smaller mean seed sizes of pioneer species. Reduced light intensities in subcanopy habitats also could favor larger seeds in the persistent guild because large seed reserves might 1) enhance the abilities of seeds to persist until suitable light (or moisture) conditions arise by providing for metabolic requirements of seeds during quiescent periods, 2) provide secondary compounds for defense of persistent seedlings against pathogens and predators during periods of low energy availability, 3) provide energy for construction of large amounts of photosynthetic tissue needed to maintain a positive net energy balance when light conditions are just above the leaf light compensation point of the plant, 4) provide energy for growth into higher light intensity strata, and 5) provide nutrients for replacement of lost or damaged tissues in persistent seedlings. Differences between soils in light gaps and subcanopy habitats are considered briefly. It is concluded that too little is known for predictions to be made regarding the probable effects of soil differences on the sizes of seeds able to survive in each habitat. Finally, differences between the two habitats in four biotic factors (competition, predation, pathogens, and mycorrhizal availability) are considered. Of these, greater competition for nutrients in the subcanopy habitat, and competition among co-germinating seedlings for light could have been important in favoring large seeds in the guild of persistent species. Pathogens are known to be more effective in shaded habitats, but data on seedling resistance to pathogens do not provide support for a role of seed size in enhancing resistance. Although differences in predation intensity and in mycorrhizal abundance in the two habitats have not been evaluated in the field, potential roles of these two factors in maintaining the seed size differential between these two guilds of forest trees are discussed. Despite the existence of numerous potential benefits of large seed reserves, seed sizes often must reflect compromises between conflicting selective pressures. Environmental conditions (e.g., moisture availability) can impose upper limits on seed size. Enhanced dispersal potential and greater total propagule numbers from maternal energy reserves are benefits of small seed size that can counterbalance selection for large seed reserves. The interactions between selective forces in molding seed sizes are discussed in a final section.     

Light requirements of Australian tropical vs. cool-temperate rainforest tree species show different relationships with seedling growth and functional traits

Background and Aims

A trade-off between shade tolerance and growth in high light is thought to underlie the temporal dynamics of humid forests. On the other hand, it has been suggested that tree species sorting on temperature gradients involves a trade-off between growth rate and cold resistance. Little is known about how these two major trade-offs interact.

Methods

Seedlings of Australian tropical and cool-temperate rainforest trees were grown in glasshouse environments to compare growth versus shade-tolerance trade-offs in these two assemblages. Biomass distribution, photosynthetic capacity and vessel diameters were measured in order to examine the functional correlates of species differences in light requirements and growth rate. Species light requirements were assessed by field estimation of the light compensation point for stem growth.

Results

Light-demanding and shade-tolerant tropical species differed markedly in relative growth rates (RGR), but this trend was less evident among temperate species. This pattern was paralleled by biomass distribution data: specific leaf area (SLA) and leaf area ratio (LAR) of tropical species were significantly positively correlated with compensation points, but not those of cool-temperate species. The relatively slow growth and small SLA and LAR of Tasmanian light-demanders were associated with narrow vessels and low potential sapwood conductivity.

Conclusions

The conservative xylem traits, small LAR and modest RGR of Tasmanian light-demanders are consistent with selection for resistance to freeze–thaw embolism, at the expense of growth rate. Whereas competition for light favours rapid growth in light-demanding trees native to environments with warm, frost-free growing seasons, frost resistance may be an equally important determinant of the fitness of light-demanders in cool-temperate rainforest, as seedlings establishing in large openings are exposed to sub-zero temperatures that can occur throughout most of the year.     

Growth Ecology of Carapa nicaraguensis Aublet. (Meliaceae): Implications for Natural Forest Management1

The probability of achieving a sustainable tropical logging operation is increased if the growth of surviving trees is maximized after logging. This research examined the growth ecology of seedlings and trees of the Neotropical timber species, Carapa nicaraguensis (Meliaceae). Shadehouse experiments, field plantings, and growth rate analyses tested the hypothesis that Carapa seedlings and trees experience higher growth rates in high light levels (recently formed logging gaps or logged forest plots) than in low light levels (old logging gaps or undisturbed forest plots). Consistently poor seedling growth rates under low light conditions suggest that seedlings establishing in newly formed gaps will be more successful than those establishing in building-phase gaps. Thus, the first year after logging is a critical time for gap recolonization by Carapa, and seed casting into gaps after logging is recommended. Growth of Carapa trees was significantly faster in logged forest than in undisturbed forest, and was positively associated with light availability. The results provide evidence that harvest models can be both diameter- and illumination-based. Crown illumination was not significantly higher in logged forest than in undisturbed forest, indicating that the illumination scale for this study was not sensitive enough to detect increases in light availability brought about by selective logging. Potential silvicul-tural methods (particularly poison-girdling) that maintain adequate forest light levels in Carapa swamps without significantly altering long-term tree diversity or allowing invasion of secondary species should be explored.     

Gap size effects on photoinhibition in understorey saplings in tropical rainforest

The sudden increase in irradiance after canopy disturbance in primary forest together with the accompanying increase in leaf temperatures is known to cause photoinhibition in shade acclimated foliage of understorey plants. We hypothesized that there is species specific variation among understorey saplings in the magnitude of photoinhibition in response to gap creation, which is related to their requirement for overstorey disturbance. Eleven more or less circular gaps were created varying in size from 60 up to 1459 m². Photoinhibition was assessed by determining predawn and midday F_v/F_m using chlorophyll fluorescence at two occasions during the first 3 weeks after creation of the gaps. The light environment was assessed using hemispherical photography. Five species that occurred in sufficient numbers in the understorey after gap creation were measured. They all showed an increase of photoinhibition with increasing gap size. Variation in exposure to direct sunlight within gaps contributed also to variation in photoinhibition. Dynamic photoinhibition, the overnight increase in F_v/F_m, was about 20% of total photoinhibition as measured at midday. The species responded quantitatively different. Oxandra asbeckii was most sensitive as evident from a decrease of predawn F_v/F_m from 0.79 in the understorey of undisturbed forest to 0.70 in the smallest and further to 0.41 in the largest gaps. Catostemma fragrans, the least sensitive species showed hardly any photoinhibition in the smallest gaps and less in the largest ones, whereas Lecythis concertiflora, Licania heteromorpha, and Chlorocardium rodiei had intermediate responses. Species rank order in sensitivity to photoinhibition was maintained across the whole range of gap sizes. The relationship between sensitivity to photoinhibition and species-specific gap size preference for regeneration is discussed.