Variation In Light Availability Research Articles

Water clarity monitoring in complex coastal environments: Leveraging seagrass light requirement toward more functional satellite ocean color algorithms

Florida's Big Bend region is home to the second largest contiguous seagrass bed in the continental United States, but the extent and integrity of this seagrass bed is declining as a result of degraded water clarity. Seagrass beds provide important ecological services and support high levels of biodiversity, thus monitoring the spatial and temporal variability of water clarity in the Big Bend region, particularly during the seagrass growth season (April to October), is of vital importance. Satellite remote sensing of coastal water clarity is often challenging due to the optical complexity and heterogeneity of coastal regions as well as frequent clouds and sun glint in the satellite imagery. As such, we developed machine learning (ML) approaches to derive the diffuse attenuation coefficient at 490 nm [Kd(490)] from satellite data with partial atmospheric correction. Traditionally, such algorithms would be developed to capture the full range and variation of the parameter of interest. Here, we demonstrate deficiencies in this method, and instead advocate for an alternate approach which leverages seagrass light requirements to improve the algorithm design. Specifically, we developed and implemented a 2-stage ML approach, which first classifies the coastal water into two types, then retrieves Kd(490) for the water type within which measurable changes in water clarity may impact seagrass health. We developed and compared the traditional ML approach and the proposed 2-stage ML approach based on data from MODIS/Aqua, OLCI/Sentinel-3A, and VIIRS/SNPP. Given the outperformance of the 2-stage ML approach, we applied it to the MODIS/Aqua data archive (2003−2020) to generate time-series Kd(490) maps and analyzed how often different areas across the Northern Big Bend region are suitable for seagrass growth each year. The derived information is important toward understanding how the spatial and temporal variation in light availability impacts seagrass growth and distribution in the study region, while the 2-stage ML approach appears applicable to other coastal regions with similar optical complexity.

Subsurface phytoplankton vertical structure from lidar observation during SCS summer monsoon onset.

Subsurface phytoplankton vertical structure was observed for the first time by lidar during the onset of the SCS summer monsoon. Based on the lidar data that were obtained by continuous day-and-night measurements over a two-week period, a hybrid retrieval method to determine the vertical structure of the seawater chlorophyll-a concentrations using lidar data was proposed. We compared the data obtained from the lidar retrievals with the ocean color data and studied the spatial variations and hourly diurnal variations in the subsurface chlorophyll-a maximum layer (SCML). The significant changes in the depth of the SCML in the SCS may be due to the variations in light availability and nutrient supply during the onset of the SCS summer monsoon. The preliminary results indicated that lidar measurements allow the submesoscale oceanic dynamics mechanisms to be understood from a new perspective.

Seedling performance, allocation patterns and phenotypic plasticity of two sympatric hawthorn species and their natural hybrid

Hybridization followed by introgression is common between European Crataegus species, while hybrids of numerous sympatric taxa are abundant and widespread in several natural and seminatural environments. No study to the present day has compared the development of Crataegus species and their hybrids in their seedling stage. Our aim was to compare seedling performance, biomass allocation and phenotypic plasticity of two sympatric Crataegus species (C. monogya and C. rhipidophylla) with contrasting shade tolerance and their natural hybrid along light and water availability gradients in a controlled pot experiment. By this means we tested which of the existing models can successfully explain the field distribution and habitat preference of the hybrid. The performance of both species and their hybrid decreased in deep shade as well as under high water availability. Variation in light availability had a more pronounced effect on seedling development than that in water availability. These results evidence that habitat differentiation between the two species under natural conditions is probably mainly driven by competitive interactions. The shade tolerant forest species C. rhipidophylla had shorter, more dense stems compared to the shade intolerant C. monogyna, with the hybrid being intermediate in these characters. This suggests a steadier growth and higher resource allocation into stems in the case of the shade tolerant species. Furthermore, the hybrid was phenotypically more plastic in biomass allocation relative to at least one of its parental species under various water availabilities, suggesting an advantage on behalf of the hybrid in establishment under a wider range of environmental conditions. Our results support the “mosaic-model” that describes most accurately the hybrid zone.

Domestication reduces phenotypic plasticity in chaya (<em>Cnidoscolus aconitifolius</em> (Mill.) I.M. Johnst)

Background: Natural selection optimizes phenotypic plasticity in plants found in environmentally variable habitats. However, it is unclear how artificial selection has affected the phenotypic plasticity of crops. Reduced plasticity in crop yield is often considered a desirable feature in cultivated plants; however, limited phenotypic plasticity in this and other traits may also affect the ability of crops to cope with environmental variation. Study species: Wild and domesticated chaya (Cnidoscolus aconitifolius (Mill.) I.M. Johnst). Question: How domestication has affected the phenotypic plasticity of vegetative traits in response to the light environment? Methods: Leaf area, leaf perimeter, leaf specific area, leaf production, trichome density, stem elongation, growth in stem diameter and slenderness were measured in clones of wild and domesticated plants. These clones were allocated to two contrasting light treatments: fully exposed to sun vs. placement beneath a shade cloth. The phenotypic traits and reaction norms were compared between the genotypes of wild and domesticated plants. Results: Lower plasticity in leaf production and slenderness was observed in the domesticated compared to the wild plants. Leaf production and slenderness are associated with the shade avoidance syndrome, which was evident in wild plants but not manifested in domesticated plants. Reduced plasticity in leaf production also suggests yield stability. Conclusion: Artificial selection reduces phenotypic plasticity in the yield of chaya and in its response to variation in light availability.

Can coppicing planted saplings improve the growing position of mid-tolerant northern hardwood tree species in harvest gaps?

Group harvesting is often used to regenerate mid-tolerant tree species. However, in managed northern hardwood forests of the Great Lakes region, regeneration failures are common among mid-tolerant species regardless of gap size. Coppicing advance regeneration has the potential to alter recruitment patterns within harvest gaps, as some mid-tolerant species have large dormant season carbohydrate reserves in the root system. However, the comparative sprouting abilities of northern hardwood saplings are not well quantified. Furthermore, variation in light availability and chronic browsing from white-tailed deer may modify comparative sprouting abilities. In this study, we tracked the effects of dormant season coppicing and deer exclusion on the initial sprouting response and five-year survival, height growth, and final height of seven-year-old volunteer white ash saplings and planted yellow birch, paper birch, northern red oak, sugar maple, red maple, and American beech saplings across a gradient of harvest gap sizes (10–35 m diameter, <0.5–1.25 tree height) in a mature even-aged northern hardwood stand in northern Lower Michigan, USA. In addition, saplings were given a growing position ranking, which integrated sapling final height and survival status relative to competitors to evaluate the effects of coppicing and deer exclusion on sapling development. Overall, coppicing produced a strong initial sprouting response with nearly all red maple and northern red oak stems initially sprouting. In the absence of deer, appreciable differences in sprout survival and height growth occurred among mid-tolerant and intolerant species and contributed to changes in growing position. Northern red oak sprouts significantly gained growing position over all species through a combination of high survival and modest height growth across all gap sizes. In contrast, coppiced yellow birch and paper birch sprout survival was significantly lower than uncoppiced stems, which contributed to declines in growing position for both species across all gap sizes. Coppicing in the absence of deer had less effect on the growing position of mid-tolerant red maple and white ash and shade tolerant sugar maple and American beech. Without deer exclusion, browsing limited the effectiveness of coppicing by reducing height growth and final height of all mid-tolerant species. Collectively, our results indicate that coppicing does not benefit mid-tolerant species equally and is unlikely to benefit browsing-preferred species in areas with high deer density. Nevertheless, in areas of lower deer density or in situations where sprouts can be protected, coppicing may improve the competitiveness of northern red oak saplings within harvest gaps.

Effects of light availability on leaf attributes and seedling growth of four tree species in tropical dry forest

BackgroundIn tropical dry forests, variation in understory light availability due to season and canopy tree density could be a governing factor in establishment and growth of tree seedlings. Species with varying life history traits are expected to respond differentially to such heterogeneity. We investigated the response of seedlings of four tree species in a tropical day forest in relation to spatiotemporal variability of light. We attempt to explore the role of leaf attributes in explaining intra- and inter-specific variations in relative growth rate. Four study sites, each with three contrasting canopy conditions, were selected along a soil moisture gradient. Seedlings of four tree species (viz., Acacia catechu, Bridelia retusa, Lagerstroemia parviflora, and Shorea robusta), varying in life history traits, were monitored for seasonal variations in growth traits across canopy condition and sites for 2 years.ResultsWe observed a larger variation in leaf attributes for pioneer species. A. catechu showed highest mean values for leaf dry matter content, leaf nitrogen concentration, leaf phosphorus concentration, net stomatal conductance, net photosynthetic rate, and relative growth rate in high light conditions. S. robusta and B. retusa demonstrated highest mean values for all the leaf attributes (except leaf dry matter content) in low light conditions. However, intermediate values for leaf attributes were observed in L. parviflora which preferred moderate light conditions.ConclusionsSeasonal variations in light availability at the forest floor appear to play an important role in the establishment and growth of tree seedlings in seasonal dry forests. Leaf attributes can be used to explain intra- and inter-specific variation in response to light availability. Leaf attributes in combinations can be used to predict relative growth rate of tree species in tropical dry environment, which apart from soil moisture is also determined by light availability due to seasonal changes and canopy tree density.

Diurnal variation in the convection-driven vertical distribution of phytoplankton under ice and after ice-off in large Lake Onego (Russia)

ABSTRACTWhen sunlight penetrates the ice layer covering lakes in winter, it warms the top water layer and sets up convection, with several potentially contrasting effects on phytoplankton. While convective mixing keeps cells in suspension and prevents sedimentation losses, it may also transport phytoplankton well below the euphotic zone. We investigated diurnal variations in the vertical distribution of phytoplankton under ice and just after ice-off in Lake Onego (Russia), a lake with moderate to high colored dissolved organic carbon (CDOM) levels. We showed that diurnal variation in convection under ice restricts phytoplankton access to light in the morning hours to a narrow euphotic zone, whereas cells are mixed through a deep aphotic layer in the afternoon. After ice-off, low chlorophyll a was found on the open-water side of the thermal bar as convection distributed cells throughout the water column. By contrast, the inshore side had significantly higher concentrations of chlorophyll a (p < 0.001) because the mixing depth brought about by diurnal microstratification was reduced, resulting in greater access to light in the afternoon. Overnight, convective cooling broke down microstratification, which redeveloped the next day. Our work highlights the importance of studying diurnal variation in light availability for photoautotrophic growth, both under ice and after ice-off in lakes characterized by high CDOM.

Foraging behaviour of Weddell seals (Leptonychotes weddellii) in connection to oceanographic conditions in the southern Weddell Sea

The region of the Filchner Outflow System (FOS) in the southeastern Weddell Sea is characterized by intensive and complex interactions of different water masses. Dense Ice Shelf Water (ISW) emerging from beneath the ice shelf cavities on the continental shelf, meets Modified Warm Deep Water (MWDW) originating from the Antarctic Circumpolar Current at the sill of the Filchner Trough. These hydrographic features convert the FOS into an oceanographic key region, which may also show enhanced biological productivity and corresponding aggregations of marine top predators. In this context, six adult Weddell seals (Leptonychotes weddellii) were instrumented with CTD-combined satellite relay data loggers in austral summer 2014. By means of these long-term data loggers we aimed at investigating the influence of environmental conditions on the seals’ foraging behaviour throughout seasons, focussing on the local oceanographic features. Weddell seals performed pelagic and demersal dives, mainly on the continental shelf, where they presumably exploited the abundant bentho-pelagic fish fauna. Diurnal and seasonal variations in light availability affected foraging activities. MWDW was associated with increased foraging effort. However, we observed differences in movements and habitat use between two different groups of Weddell seals. Seals tagged in the pack ice of the FOS focussed their foraging activities to the western and, partly, eastern flank of the Filchner Trough, which coincides with inflow pathways of MWDW. In contrast, Weddell seals tagged on the coastal fast ice exhibited typical central-place foraging and utilized resources close to their colony. High foraging effort in MWDW and high utilization of areas associated with an inflow of MWDW raise questions on the underlying biological features. This emphasizes the importance of further interdisciplinary ecological investigations in the near future, as the FOS may soon be impacted by predicted climatic changes.

Seasonal variations overwhelm temperature effects on microbial processes in headwater streams: insights from a temperate thermal spring

Carbon cycling in headwater streams is mostly driven by the decomposition of allochthonous organic matter, and to a lesser extent by primary production. Quantifying the influence of temperature on these processes is therefore essential to better anticipate the consequences of global warming for stream ecological functioning. In this study, we measured alder litter microbial decomposition and associated fungal biomass and diversity, using leaf discs enclosed in fine-mesh bags along a natural geothermal temperature gradient, in both spring and winter. We monitored the chlorophyll-a accrual in biofilms growing on ceramic tiles. The temperature gradient, from upstream to downstream, ranged from 15.3 to 14.2 °C in spring and 18.2 to 13.2 °C in winter. Autotrophs and heterotrophs exhibited contrasting responses to temperature. The expected positive effect of temperature was actually observed for chlorophyll-a accrual only, while an apparent temperature-independence of litter decomposition rate was found. Moreover, temperature effects on heterotrophic and autotrophic organisms depended on the season, with higher litter decomposition rates, sporulation rates, fungal biomass and chlorophyll-a in spring, despite a lower mean water temperature than in winter. Together, these results suggest that the influence of temperature remained largely overrode by seasonal effects. This result is likely due to annual variations in light availability, and may involve indirect positive interactions between microbial primary producers and decomposers.

Resolving coral photoacclimation dynamics through coupled photophysiological and metabolomic profiling.

Corals continuously adjust to short-term variation in light availability on shallow reefs. Long-term light alterations can also occur as a result of natural and anthropogenic stressors, as well as management interventions such as coral transplantation. Although short-term photophysiological responses are relatively well understood in corals, little information is available regarding photoacclimation dynamics over weeks of altered light availability. We coupled photophysiology and metabolomic profiling to explore changes that accompany longer-term photoacclimation in a key Great Barrier Reef coral species, Acropora muricata High light (HL)- and low light (LL)-acclimated corals were collected from the reef and reciprocally exposed to high and low light ex situ Rapid light curves using pulse-amplitude modulation (PAM) fluorometry revealed photophysiological acclimation of LL corals to HL and HL corals to LL within 21 days. A subset of colonies sampled at 7 and 21 days for untargeted LC-MS and GC-MS metabolomic profiling revealed metabolic reorganization before acclimation was detected using PAM fluorometry. Metabolomic shifts were more pronounced for LL to HL corals than for their HL to LL counterparts. Compounds driving metabolomic separation between HL-exposed and LL control colonies included amino acids, organic acids, fatty acids and sterols. Reduced glycerol and campesterol suggest decreased translocation of photosynthetic products from symbiont to host in LL to HL corals, with concurrent increases in fatty acid abundance indicating reliance on stored lipids for energy. We discuss how these data provide novel insight into environmental regulation of metabolism and implications for management strategies that drive rapid changes in light availability.

Persistent nitrogen limitation of stream biofilm communities along climate gradients in the Arctic.

Climate change is rapidly reshaping Arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in Arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll-a accumulation. For unamended biofilms, activity and biomass accrual were not closely related to any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition was: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, Arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.

Winter mixing impacts gene expression in marine microbial populations in the Gulf of Aqaba

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 80:223-242 (2017) - DOI: https://doi.org/10.3354/ame01854 Winter mixing impacts gene expression in marine microbial populations in the Gulf of Aqaba D. Miller1, U. Pfreundt2,3, S. Hou2, S. C. Lott2, W. R. Hess2, I. Berman-Frank1,* 1Mina and Everard Goodman Faculty of Life Sciences, Bar Ilan University, Ramat Gan, Israel 2Genetics and Experimental Bioinformatics, Faculty of Biology, University of Freiburg, Schänzlestr. 1, 79104 Freiburg, Germany 3Present address: ETH Zürich, Department of Civil, Environmental and Geomatic Engineering, Institute of Environmental Engineering, Stefano-Franscini-Platz 5, 8093 Zürich, Switzerland *Corresponding author: ilana.berman-frank@biu.ac.il ABSTRACT: In aquatic systems, changes in temperature and irradiance fundamentally characterize the water column and regulate microbial population structure and function. In systems with stable thermal stratification, the warm surface mixed layer is typically nutrient impoverished, limiting biological production. In periods of destratification, convective mixing of the water column exposes the microorganisms inhabiting these mixed systems to rapid variations in light availability and spectra. We explored the impact of winter deep-mixing (500 m deep mixed layer) on microbial communities from the surface (2.5 m) and the aphotic waters (440 m) in the Gulf of Aqaba by examining changes in both population composition and function via DNA and RNA sequencing. The greatest fraction of 16S sequences was assigned to Euryarchaeota, while metatranscriptomes were dominated by Synechococcus transcripts. Community composition was highly similar at both depths, yet transcription profiles differed. Phototrophic organisms found at the photic surface overexpressed genes related to catabolism and energy metabolism, while genes affiliated with biosynthesis were overexpressed at the aphotic depth. Similar transcriptional trends were observed in the non-photoautotrophs SAR11, Euryarchaeota, and Thaumarchaeota, with niche partitioning based on differential utilization of nitrogen and phosphorus occurring between the 2 archaeal groups. We did not detect upregulated expression of cyanobacterial genes indicative of mixotrophy or glycogen metabolism in the aphotic zone, suggesting they survive the aphotic period by utilizing photosynthates produced in the photic zone. Indications for a mixotrophic lifestyle were observed for prasinophytes, with genes related to phagocytosis overexpressed at the aphotic depth compared with the surface. KEY WORDS: Deep mixing · Red Sea · Metatranscriptomics · Cyanobacteria · SAR11 · Archaea · Prasinophytes Full text in pdf format Supplement 1Supplement 2 PreviousNextCite this article as: Miller D, Pfreundt U, Hou S, Lott SC, Hess WR, Berman-Frank I (2017) Winter mixing impacts gene expression in marine microbial populations in the Gulf of Aqaba. Aquat Microb Ecol 80:223-242. https://doi.org/10.3354/ame01854 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 80, No. 3. Online publication date: November 24, 2017 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2017 Inter-Research.

How tree species identity and diversity affect light transmittance to the understory in mature temperate forests.

Light is a key resource for plant growth and is of particular importance in forest ecosystems, because of the strong vertical structure leading to successive light interception from canopy to forest floor. Tree species differ in the quantity and heterogeneity of light they transmit. We expect decreases in both the quantity and spatial heterogeneity of light transmittance in mixed stands relative to monocultures, due to complementarity effects and niche filling. We tested the degree to which tree species identity and diversity affected, via differences in tree and shrub cover, the spatiotemporal variation in light availability before, during, and after leaf expansion. Plots with different combinations of three tree species with contrasting light transmittance were selected to obtain a diversity gradient from monocultures to three species mixtures. Light transmittance to the forest floor was measured with hemispherical photography. Increased tree diversity led to increased canopy packing and decreased spatial light heterogeneity at the forest floor in all of the time periods. During leaf expansion, light transmittance did differ between the different tree species and timing of leaf expansion might thus be an important source of variation in light regimes for understory plant species. Although light transmittance at the canopy level after leaf expansion was not measured directly, it most likely differed between tree species and decreased in mixtures due to canopy packing. A complementary shrub layer led, however, to similar light levels at the forest floor in all species combinations in our plots. Synthesis. We find that a complementary shrub layer exploits the higher light availability in particular tree species combinations. Resources at the forest floor are thus ultimately determined by the combined effect of the tree and shrub layer. Mixing species led to less heterogeneity in the amount of light, reducing abiotic niche variability.

Contrasting hydrography and phytoplankton distribution in the upper layers of cyclonic eddies in the Mozambique Basin and Mozambique Channel

Hydrographic data collected in cyclonic eddies in the Mozambique Channel and Basin revealed notable differences in temperature and salinity at a depth of 100 m, the upper mixed layer, the nitracline depths, and vertical distribution of chlorophyll-a (Chl-a). Differences in temperature and salinity did not show any consistent patterns. In contrast, the differences in the upper mixed layer, nitracline depths and the vertical Chl-a profile appeared to be driven by combined effects of eddy dynamics (i.e. shoaling of isopleths) and the seasonal variation in light availability and mixing conditions in the upper layers. Cyclonic eddies studied during austral spring and summer in the Mozambique Channel exhibited shallower upper mixed layers and nitracline depths, and deeper euphotic zones. Distinct subsurface Chl-a maxima (SCM) were associated with the stratified conditions in the upper layers of these eddies. In contrast, a cyclonic eddy studied during mid-austral winter in the Mozambique Basin had a shallower euphotic zone, deeper upper mixed layer and uniform Chl-a profiles. Another eddy sampled in the Mozambique Basin toward the end of winter showed a less pronounced SCM and roughly equal euphotic zone and upper mixed layer depths, suggestive of a transition from a well-mixed upper layer during winter to stratified conditions in summer.

Photosynthetic refixation varies along the stem and reduces CO2 efflux in mature boreal Pinus sylvestris trees.

Trees are able to reduce their carbon (C) losses by refixing some of the CO2 diffusing out of their stems through corticular photosynthesis. Previous studies have shown that under ideal conditions the outflowing CO2 can be completely assimilated in metabolically active, young stem and branch tissues. Fewer studies have, however, been carried out on the older stem sections of large trees and, accordingly, the importance of refixation is still unclear under natural environmental conditions. We investigated the spatial and temporal variation in refixation in ~90-year-old boreal Scots pine (Pinus sylvestris L.) trees by utilizing month-long continuous measurements of stem CO2 efflux (Ec) made at four heights along the bole. Refixation rates were found to vary considerably along the bole, leading to a 28% reduction in long-term Ec in the upper stem compared with a negligible reduction at breast height. This vertical pattern correlated with variation in light availability, bark chlorophyll content and bark type. Analysis of the vertical and diurnal patterns in Ec further suggested that the influence of sap flow on the observed daytime reduction in Ec was small. The areal rates of corticular photosynthesis were much lower than previous estimates of photosynthetic rates per unit leaf area from the same trees, implying that the impact of refixation on tree-scale C uptake was small. However, upscaling of refixation indicated that 23-27% of the potential Ec was refixed by the bole and the branches, thereby significantly reducing the woody tissue C losses. Thus, our results suggest that refixation needs to be considered when evaluating the aboveground C cycling of mature P. sylvestris stands and that breast-height estimates should not be extrapolated to the whole tree.

The effects of local variation in light availability on pollinator visitation, pollen and resource limitation of female reproduction in Hosta ventricosa

BackgroundLight availability may have direct effects on reproduction through resource availability, and indirect effects on female reproduction by influencing plant-pollinator interactions. Floral display size, pollinator visitation per flower, resource and pollen limitation of fruit and seed production were quantified in a forested patch and an adjacent open patch of two populations of the perennial herb Hosta ventricosa.ResultsPlants in the open patch produced significantly larger floral displays than those in the forested patch in both populations. Floral display size had a positive effect on pollinator visitation rate per flower in one population, but no effect in the other. Plants in the open patch received approximately 8–11 times more visitation rates and produced significantly more fruit and seeds per flower than those in the forested patch. However, supplemental pollination resulted in significantly more fruit and seed production per flower compared to natural pollination in the forested patch but not in the open patch in one population, and did not enhance fruit and seed production in either the forested or the open patch in the other. In both populations, supplementally pollinated plants in the open patch produced significantly more fruit and seeds per flower than supplementally pollinated plants in the forested patch.ConclusionsIn H. ventricosa, local variation in light conditions could affect pollinator activity and influence female reproduction through resource availability; however differences in the degree of pollen limitation between local habitats were found to be population-specific.

Canopy gradients in leaf functional traits for species that differ in growth strategies and shade tolerance

In temperate deciduous forests, vertical gradients in leaf mass per area (LMA) and area-based leaf nitrogen (Narea) are strongly controlled by gradients in light availability. While there is evidence that hydrostatic constraints on leaf development may diminish LMA and Narea responses to light, inherent differences among tree species may also influence leaf developmental and morphological response to light. We investigated vertical gradients in LMA, Narea and leaf carbon isotope composition (δ13C) for three temperate deciduous species (Carpinus caroliniana Walter, Fagus grandifolia Ehrh., Liriodendron tulipifera L.) that differed in growth strategy (e.g., indeterminate and determinate growth), shade tolerance and leaf area to sapwood ratio (Al:As). Leaves were sampled across a broad range of light conditions within three vertical layers of tree crowns to maximize variation in light availability at each height and to minimize collinearity between light and height. All species displayed similar responses to light with respect to Narea and δ13C, but not for LMA. Light was more important for gradients in LMA for the shade-tolerant (C. caroliniana) and -intolerant (L. tulipifera) species with indeterminate growth, and height (e.g., hydrostatic gradients) and light were equally important for the shade-tolerant (F. grandifolia) species with determinate growth. Fagus grandifolia had a higher morphological plasticity in response to light, which may offer a competitive advantage in occupying a broader range of light conditions throughout the canopy. Differences in responses to light and height for the taller tree species, L. tulipifera and F. grandifolia, may be attributed to differences in growth strategy or Al:As, which may alter morphological and functional responses to light availability. While height was important in F. grandifolia, height was no more robust in predicting LMA than light in any of the species, confirming the strong role of light availability in determining LMA for temperate deciduous species.

Traits and functional strategies as predictors of demographic variations over a chronosequence

Traits and functional strategies when related to demographic rates provide important information about the distribution of species and their performance in different environments. Therefore, predictions were made that: (1) plant communities in tropical forests, distributed over a chronosequence, have different trade-offs between demographic rates and functional traits due to variations in light availability and (2) acquisitive, intermediate and conservative species have different demographic rates throughout the chronosequence. To test these predictions, data from eight functional traits (maximum plant height; specific leaf area; leaf dry matter content; leaf nitrogen, phosphorus and chlorophyll concentrations; wood density; and seed shape) were collected from 64 tree species, distributed in six secondary (three 17 and three 25 years old) and three mature (>40 years old) Ombrophilous Dense Forest fragments. Demographic rates (mortality, recruitment and growth) of the species were determined for periods of three and five years, in each forest. Mortality was higher and growth rate was lower in the 17-year-old than in the mature forest, and the acquisitive group had the highest recruiting rate, but interactions were not significant. Thus, the majority of relationships between demographic rates and functional traits did not correspond to the predictions, as some traits were not better predictors of the demography of species in a determined forest than others and that these relationships did not vary across the chronosequence. Therefore, the trade-offs between functional and demographic characteristics are related to the species and not to environmental differences across the age gradient, and when the species are separated into groups, the predicted changes in demography are valid across the gradient.

Sward structural characteristics and herbage accumulation of Piatã palisade grass (Brachiaria brizantha) in a crop–livestock–forest integration area

The aim of this study was to evaluate the effects of shading on structural characteristics and herbage accumulation of palisade grass (Brachiaria brizantha cv. Piatã), in a crop–livestock–forest integration area. Three shade regimes were evaluated: no shade (NS), moderate shade (MS, 338 trees ha–1), and intense shade (IS, 714 trees ha–1). The effects of shade regime on soil-water content and nitrogen nutrition index were also evaluated. Grazing management was established based on sward light interception targets (95% LI pre-gazing). The MS and IS regimes were divided into two shade strips (central and lateral) in order to describe the variation in light availability within the paddocks. Two methods for taking the reference readings of LI were tested in IS paddocks: LI reading inside (under the tree canopy) and outside (under full sunlight) the experimental area. Increased tree density reduced photosynthetically active radiation on lateral and central shade strips by 37% and 12% for MS, and 52% and 50% for IS, respectively. For NS, MS, IS (outside) and IS (inside), grazing intervals were 23, 26, 28 and 35 days, pre-grazing heights corresponded to 33, 35, 43 and 52 cm, and pre-grazing herbage mass to 5.37, 4.44, 2.96 and 3.23 t ha–1, respectively. Leaf percentage remained relatively stable across shade regimes, but stem percentage was greater and dead material percentage was smaller for IS than for NS and MS. Soil water content was lower near the trees, especially during the dry season. Relative to NS, herbage accumulation decreased by 39% and 68% for MS and IS, respectively. Under natural shade, small variations in light availability affect sward structure and herbage production. The use of the 95% LI target developed under full sunlight conditions is not suitable for use under trees, emphasising the need to generate specific grazing-management targets for pastures cultivated under trees.

Changes in species composition and richness in an alluvial hardwood forest over 52 yrs

AbstractQuestions(1) How has the species richness of an alluvial forest changed over the past 52 yrs, and what are the main drivers of the observed temporal changes; (2) has the species composition changed over this period in response to changes in environmental variables; and (3) what are the main drivers of species richness (change) in this forest?LocationHaseder Holz, Innerste floodplain in lowland northern Germany.MethodsIn 2012 we resurveyed the plant species composition of 19 permanent plots analysed for the first time in 1960 and for a second time in 2002. At the most recent survey, several environmental variables (photosynthetically active radiation, soil water content, groundwater table, pH and content of soil P, Ca, Mg, K, C and N) were measured in five locations within each plot. GLM were used to examine the relationship between explanatory variables and species richness, while variation in species composition, in time and space, was assessed by DCA.ResultsThe number of species generally increased with decreasing soil nutrient (except Mg) content, with increasing variation in light availability and with increasing variation in K content. The significant increase in species richness from 1960 to 2012 was mainly attributed to the increase in true forest species, such as Paris quadrifolia and Mercurialis perennis, and woody species. The most pronounced increase in species richness was found in plots with lower soil phosphate content. Species typical for open habitats showed the most pronounced decrease from 1960 to 2012. A significant homogenization of the plots over time was observed.ConclusionsAgainst the common trend in European forests, we found a significant increase in mean species richness, especially in plots with relatively low nutrient content. While the total pool of species has not consistently increased, we found a strong increase in plot‐scale species richness of woody and herbaceous species. This overall increase and the slight decrease in the proportion of species typical of open habitats were probably driven by a decrease in light availability caused by less intensive management. Our results demonstrate that fine‐scale spatial environmental heterogeneity positively affects species richness. In contrast to recently reported findings, we found a decrease in the number of species with increasing nutrient content.