Characteristics of Light Availability Under Forest Canopies and Its Influences on Photosynthesis of Understory Plants

Available light under forest canopies includes two components, diffuse light and direct light (sunflecks), and is characterized as low and highly dynamic. Understory habitats under different forest types experience different light conditions. Sunflecks as a critical resource for understory plants have great importance on carbon gain of understory plants. Under the light-limiting habitat, understory plants exhibit a high light utilization efficiency attributed by a post-illumination CO2 uptake. Although different species have different photosynthetic responses, shade plants appear to be acclimated to respond more quickly and efficiently to sunflecks. This acclimation includes a faster induction, relatively lower rate of induction loss, lower photosynthetic compensation point, and higher water use efficiency. The process that shade plants harvest light energy is not well known. Studies of photosynthetic responses to sunflecks in natural conditions are rare. Little is known about constraints on sunfleck utilization, which may change seasonally. Extensive field studies in conjunction with laboratory investigations will be needed to further understand potential and actual constraints on sunfleck utilization. Most studies on photosynthetic responses to fluctuating light condition were done in the level of leaves. Fluctuating light utilization on the basis of whole plants and populations presents future challenges to ecologists.

Increased light availability enhances tolerance against ocean acidification-related stress in the calcifying macroalga Halimedaopuntia

Although the adverse impacts of ocean acidification(OA)on marine calcifiers have been investigated extensively,the anti-stress capabilities regulated by increased light availability are unclear.Herein,the interactive effects of three light levels(30μmol photons/(m^(2)·s),150μmol photons/(m^(2)·s),and 240μmol photons/(m^(2)·s)combined with two pCO_(2)concentrations(400 ppmv and 1400 ppmv)on the physiological acclimation of the calcifying macroalga Halimeda opuntia were investigated using a pCO_(2)-light coupling experiment.The OA negatively influenced algal growth,calcification,photosynthesis,and other physiological performances in H.opuntia.The relative growth rate under elevated pCO_(2)conditions significantly declined by 13.14%−41.29%,whereas net calcification rates decreased by nearly three-fold under OA conditions.Notably,increased light availability enhanced stress resistance through the accumulation of soluble organic molecules,especially soluble carbohydrate,soluble protein,and free amino acids,and in combination with metabolic enzyme-driven activities,OA stress was alleviated.The carotenoid content under low light conditions increased markedly,and the rapid light curve of the relative electron transport rate was enhanced significantly by increasing light intensities,indicating that this new organization of the photosynthetic machinery in H.opuntia accommodated light variations and elevated pCO_(2)conditions.Thus,the enhanced metabolic performance of the calcifying macroalga H.opuntia mitigated OA-related stress.

Effects of elevated carbon dioxide concentration and soil temperature on the growth and biomass responses of mountain maple (Acer spicatum) seedlings to light availability

Aims Some shade-tolerant understory tree species such as mountain maple(Acer spicatum L.)exhibit light-foraging growth habits.Changes in environmental conditions,such as the rise of carbon dioxide concentration([CO_(2)])in the atmosphere and soil warming,may affect the performance of these species under different light environments.We investigated how elevated[CO_(2)]and soil warm-ing influence the growth and biomass responses of mountain maple seedlings to light availability.Methods The treatments were two levels of light(100%and 30%of the ambient light in the greenhouse),two[CO_(2)](392μmol mol^(−1)(ambient)and 784μmol mol^(−1)(elevated))and two soil tempera-tures(Tsoil)(17 and 22℃).After one growing season,we measured seedling height,root collar diameter,leaf biomass,stem biomass and root biomass.Important findings We found that under the ambient[CO_(2)],the high-light level increased seedlings height by 70%and 56%at the low Tsoil and high Tsoil,respectively.Under the elevated[CO_(2)],however,the high-light level increased seedling height by 52%and 13%at the low Tsoil and high Tsoil,respectively.The responses of biomasses to light generally followed the response patterns of height growth under both[CO_(2)]and Tsoil and the magnitude of biomass response to light was the lowest under the elevated[CO_(2)]and warmer Tsoil.The results suggest that the elevated[CO_(2)]and warmer Tsoil under the projected future climate may have negative impact on the colonization of open sites and forest canopy gaps by mountain maple.

Importance of overstorey attributes for understorey litter production and nutrient cycling in European forests

Background:In contrast with the negligible contribution of the forest understorey to the total aboveground phytobiomass of a forest,its share in annual litter production and nutrient cycling may be more important.Whether and how this functional role of the understorey differs across forest types and depends upon overstorey characteristics remains to be investigated.Methods:We sampled 209 plots of the FunDivEUROPE Exploratory Platform,a network of study plots covering local gradients of tree diversity spread over six contrasting forest types in Europe.To estimate the relative contribution of the understorey to carbon and nutrient cycling,we sampled non-lignified aboveground understorey biomass and overstorey leaf litterfall in all plots.Understorey samples were analysed for C,N and P concentrations,overstorey leaf litterfall for C and N concentrations.We additionally quantified a set of overstorey attributes,including species richness,proportion of evergreen species,light availability(representing crown density)and litter quality,and investigated whether they drive the understorey’s contribution to carbon and nutrient cycling.Results and conclusions:Overstorey litter production and nutrient stocks in litterfall clearly exceeded the contribution of the understorey for all forest types,and the share of the understorey was higher in forests at the extremes of the climatic gradient.In most of the investigated forest types,it was mainly light availability that determined the contribution of the understorey to yearly carbon and nutrient cycling.Overstorey species richness did not affect the contribution of the understorey to carbon and nutrient cycling in any of the investigated forest types.

The Potential for Genotype-by-Environment Interactions to Maintain Genetic Variation in a Model Legume–Rhizobia Mutualism

The maintenance of genetic variation in mutualism-related traits is key for understanding mutualism evolution,yet the mechanisms maintaining variation remain unclear.We asked whether genotype-byenvironment(G3E)interaction is a potential mechanism maintaining variation in the model legume–rhizobia system,Medicago truncatula–Ensifer meliloti.We planted 50 legume genotypes in a greenhouse under ambient light and shade to reflect reduced carbon availability for plants.We found an expected reduction under shaded conditions for plant performance traits,such as leaf number,aboveground and belowground biomass,and a mutualism-related trait,nodule number.We also found G3E for nodule number,with83%of this interaction due to shifts in genotype fitness rank order across light environments,coupled with strong positive directional selection on nodule number regardless of light environment.Our results suggest that G3E can maintain genetic variation in a mutualism-related trait that is under consistent positive directional selection across light environments.

Effects of logging on the trade-off between seed and sprout regeneration of dominant woody species in secondary forests of the Natural Forest Protection Project of China

Background:Promoting natural regeneration(including seed and sprout regeneration)of dominant woody species is essential for restoring secondary forests.However,such restoration processes have been decelerated by the enclosure under Natural Forest Protection Project of China(NFPP).It remains unclear how to implement appropriate management measures(e.g.,whether to apply logging and the suitable intensity)to facilitate natural regeneration according to the responses of two regeneration modes to management measures.We monitored the early stages of seed regeneration(seed rain,soil seed bank,and 1–3-year-old seedlings)and sprout regeneration(stump sprout rate,stump survival rate,probability of sprouting,and number of sprouts per stump)over the first 3 years(2017–2019)after logging under three intensity regimes(control[0%],25%,and 50%logging intensity)in secondary forests.Results:The seed rain density decreased markedly,seedling density increased insignificantly after logging,and logging promoted seedling survival at an increasing conversion rate of 3-year-old seedlings(37.5%under 0%,100%under 25%,and 80.95%under 50%logging)compared to those of the control.The proportion of 3-year-old seedlings increased with logging intensity and was the highest(16.2%)at 50%logging intensity.Sprout density was not affected by logging intensity,however,under 25%and 50%logging,it decreased by 27%and 6%in 2018,and by 37%and 33%in 2019,respectively.Seedling density was 41.65-and 15.31-fold higher than that of sprouts in the 50%and 25%logging treatments,respectively.Based on the relative contributions of the two regeneration modes after logging,three groups of natural regeneration patterns were classified for dominant woody species in temperate secondary forests,i.e.,seed regeneration preference(Betula dahurica,Carpinus cordata and Fraxinus mandshurica),sprout regeneration preference(Acer mono and Acer pseudosieboldianum)and no preference(Quercus mongolica,Fraxinus rhynchophylla,and Juglans mandshurica).Conclusion:In addition to enclosure,appropriate logging can be applied according to the responses of various natural regeneration patterns of dominant woody species to logging in temperate secondary forests under the NFPP.