Lorentzian model of roots for understory yellow birch and sugar maple saplings

Total 66 small (<50m(2)), 24 medium (101-200m(2)) and 36 large (201-500m(2)) canopy gaps at the three sites of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh) forests were established in southern Québec, Canada. Half of the gaps were covered by 8x8m(2) shading cloths to mimic a closed canopy. From these gaps, 46 understory yellow birch and 46 sugar maple saplings with different tree ages and sizes were sampled. Single- and multi-variable linear and nonlinear models of root biomass and traits (root surface area, volume, length and endings) were developed and examined. Lorentzian model as a multi-variable nonlinear model was firstly applied to the simulations using both base diameter and height, and performed the best fit to total root biomass in both species with the highest correlation coefficients (R(2)=0.96 and 0.98) and smallest root mean squared deviations (RMSD=7.85 and 7.02) among all the examined models. The model also accurately simulated small fine root (2.0mm in diameter), coarse fine root (>2.0-5.0mm) and coarse root (>5.0mm) biomass (R(2)=0.87-0.99; RMSD=2.24-6.41), and the root traits (R(2)=0.71-0.99; RMSD=0.19-19.38). The study showed yellow birch roots were longer, larger, had more endings (tips) and grew faster than sugar maple roots. The root traits were largely distributed to small fine roots, sharply decreased from small fine roots to coarse fine roots, the fewest in coarse roots except for root volume. When trees were large, coarse root biomass increased more rapidly than fine root biomass, but vise versa when the trees were small.     

Light and tree size influence belowground development in yellow birch and sugar maple

The effects of light and tree size on the root architecture and mycorrhiza of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh) growing in the understory of deciduous forests in southern Québec, Canada were studied. At the study site, small (<50 m²), medium (101–200 m²) and large (201–500 m²) canopy gaps were investigated. From within these gaps, 17 yellow birch and 23 sugar maple saplings from 40 to 600 cm in height were sampled. In both species, root biomass and morphological traits were strongly correlated with tree size, but only weakly with light availability. Increased root biomass was primarily allocated to coarse roots and secondarily to fine roots. Yellow birch roots were longer, had a larger area, more endings and branches and grew more rapidly than sugar maple roots. Mycorrhizal colonization increased with available light and declined with tree age in sugar maple and was positively associated with tree size in yellow birch. The study demonstrates that tree size is a very important determinant of how belowground systems acclimate to understory conditions.     

Leaf litter quality and decomposition rates of yellow birch and sugar maple seedlings grown in mono-culture and mixed-culture pots at three soil fertility levels

Seedlings of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh.) were grown for 2 years in mono-culture and mixed-culture and at three fertility levels. Following the second growing season, senescent leaves were analysed for N concentration, acid hydrolysable substances (AHS), and nonhydrolysable remains (NHR). A litter sub-sample was then inoculated with indigenous soil microflora, incubated 14 weeks, and mass loss was measured. Litter-N was significantly higher at medium than at poor fertility, as well as in yellow birch than in sugar maple litter. The species effect on litter-N increased with increasing fertility. At medium fertility, litter-N of sugar maple litter was lower in mixed-culture than in mono-culture. AHS, NHR as well the NHR/N ratio were significantly higher in yellow birch than in sugar maple litter. At medium fertility, the NHR/N ratio of sugar maple litter was significantly lower in mono-culture than in mixed-culture. Mass loss was significantly greater at medium and rich fertility than at poor fertility, and in yellow birch than in sugar maple litter. At poor fertility, mixed-litter decomposed at a rate comparable to yellow birch, whereas at medium and rich fertility, mixed-litter decomposed at a rate comparable to sugar maple. There was a significant positive relationship between litter-N and mass loss. A similar positive relationship between NHR and mass loss was presumed to be a species effect on decomposition. Results support the hypothesis that species × fertility and species × mixture interactions can be important determinants of litter quality and, by implication, of site nutrient cycling.     

Patterns of rhizosphere carbon flux in sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) saplings

Despite its importance in the terrestrial C cycle rhizosphere carbon flux (RCF) has rarely been measured for intact root–soil systems. We measured RCF for 8‐year‐old saplings of sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) collected from the Hubbard Brook Experimental Forest (HBEF), NH and transplanted into pots with native soil horizons intact. Five saplings of each species were pulse labeled with ¹³CO₂ at ambient CO₂ concentrations for 4–6 h, and the ¹³C label was chased through rhizosphere and bulk soil pools in organic and mineral horizons for 7 days. We hypothesized yellow birch roots would supply more labile C to the rhizosphere than sugar maple roots based on the presumed greater C requirements of ectomycorrhizal roots. We observed appearance of the label in rhizosphere soil of both species within the first 24 h, and a striking difference between species in the timing of ¹³C release to soil. In sugar maple, peak concentration of the label appeared 1 day after labeling and declined over time whereas in birch the label increased in concentration over the 7‐day chase period. The sum of root and rhizomicrobial respiration in the pots was 19% and 26% of total soil respiration in sugar maple and yellow birch, respectively. Our estimate of the total amount of RCF released by roots was 6.9–7.1% of assimilated C in sugar maple and 11.2–13.0% of assimilated C in yellow birch. These fluxes extrapolate to 55–57 and 90–104 g C m^?2 yr^?1 from sugar maple and yellow birch roots, respectively. These results suggest RCF from both arbuscular mycorrhizal and ectomycorrhizal roots represents a substantial flux of C to soil in northern hardwood forests with important implications for soil microbial activity, nutrient availability and C storage.     

Fig trees at the northern limit of their range: the distributions of cryptic pollinators indicate multiple glacial refugia

Climatic oscillations during the last few million years had well‐documented effects on the distributions and genomes of temperate plants and animals, but much less is known of their impacts on tropical and subtropical species. In contrast to Europe and North America, ice‐sheets did not cover most of China during glacial periods, and the effects of glacial cycles were less dramatic. Fig trees are a predominantly tropical group pollinated by host‐specific fig wasps. We employed partial mitochondrial COI (918 bp) and nuclear ITS2 (462 bp) gene sequences to investigate the genetic structure and demographic histories of the wasps that pollinate the subtropical Ficus pumila var. pumila in Southeastern China. Deep genetic divergence in both mitochondrial (7.2–11.6%) and nuclear genes (1.6–2.9%) indicates that three pollinator species are present and that they diverged about 4.72 and 6.00 Myr bp . This predates the Quaternary ice ages, but corresponds with the formation of the Taiwan Strait and uplifting of the Wuyi–Xianxia Mountains. The three pollinators have largely allopatric distribution patterns in China and display different postglacial demographic histories. Wiebesia spp. 1 and 2 occupy, respectively, the northern and southern regions of the mainland host range. Their populations both underwent significant postglacial spatial expansions, but at different times and at different rates. Wiebesia sp. 3 is largely restricted to northern islands and shows less evidence of recent population expansion. Their mainly allopatric distributions and different demographic histories are consistent with host plant postglacial expansion from three distinct refugia and suggest one mechanism whereby fig trees gain multiple pollinators.     

Frontier mutualism: coevolutionary patterns at the northern range limit of the leaf-cutter ant-fungus symbiosis

Tropical leaf-cutter ants cultivate the fungus Attamyces bromatificus in a many-to-one, diffuse coevolutionary relationship where ant and fungal partners re-associate frequently over time. To evaluate whether ant-Attamyces coevolution is more specific (tighter) in peripheral populations, we characterized the host-specificities of Attamyces genotypes at their northern, subtropical range limits (southern USA, Mexico and Cuba). Population-genetic patterns of northern Attamyces reveal features that have so far not been observed in the diffusely coevolving, tropical ant-Attamyces associations. These unique features include (i) cases of one-to-one ant-Attamyces specialization that tighten coevolution at the northern frontier; (ii) distributions of genetically identical Attamyces clones over large areas (up to 81 000 km(2), approx. the area of Ireland, Austria or Panama); (iii) admixture rates between Attamyces lineages that appear lower in northern than in tropical populations; and (iv) long-distance gene flow of Attamyces across a dispersal barrier for leaf-cutter ants (ocean between mainland North America and Cuba). The latter suggests that Attamyces fungi may occasionally disperse independently of the ants, contrary to the traditional assumption that Attamyces fungi depend entirely on leaf-cutter queens for dispersal. Peripheral populations in Argentina or at mid-elevation sites in the Andes may reveal additional regional variants in ant-Attamyces coevolution. Studies of such populations are most likely to inform models of coextinctions of obligate mutualistic partners that are doubly stressed by habitat marginality and by environmental change.     

Insect-related differences in growth of birch and pine at northern treeline in Swedish Lapland

The tree-ring patterns of birch and pine in the Torneträsk area of Swedish Lapland were compared. In order to dendrochronologically identify years of major insect attacks on birch, pine being unaffected by these outbreaks, served as control. The sharp growth increase in birch following the historically confirmed, heavy outbreak in 1954–55 is discussed and evidence pointing to possible earlier outbreaks is presented.     

Pollination partners of Mucuna macrocarpa (Fabaceae) at the northern limit of its range

Mucuna macrocarpa is a plant found in tropical and subtropical regions that requires an “explosive opening.” Explosive opening is the process that exposes the stamen and pistil from the opening of the carina. This process is needed for cross pollination; however, the plant cannot open itself and opening by an animal is needed. The most common opener of Mucuna flowers is several nectar‐eating bats (e.g., Syconycteris), but the flying fox, Pteropus dasymallus, is the only opener of M. macrocarpa on the subtropical island of Okinawajima. Here, we present the explosive openers and possible pollinators in the northernmost and temperate Kamae region, Kyushu, Japan, where nectar‐eating bats are absent. The Japanese macaque, Macaca fuscata, and the Japanese marten, Martes melampus, were the explosive openers observed during our survey in Kamae. Martens opened flowers using their snout in a manner similar to that of the flying fox, whereas macaques opened flowers using their hands. This is the first time that an animal has been observed opening these flowers with its hands rather than snout. In total, 97% (n = 283) of explosively opened flowers were opened by macaques, and the macaque largely contributed to the overall flower opening. Because many pollen grains become attached to the explosive openers, they are considered to be primary pollinators. Furthermore, two bee species, Apis cerana japonica and Bombus ardens ardens, also visited opened flowers and collected pollen, and they were possibly secondary pollinators.     

Living on the edge: Fig tree phenology at the northern range limit of monoecious Ficus in China

Fig trees (Ficus) are a species-rich group of mainly tropical and subtropical plants that are of ecological importance because of the large numbers of vertebrates that utilise their figs for food. Factors limiting their distributions to warmer regions are still poorly understood, but are likely to include factors linked to their specialised pollination biology, because each Ficus species is dependent on one or a small number of host-specific fig wasps (Agaonidae) for pollination. Adult fig wasps are short-lived, but some species are capable of dispersing extremely long distances to pollinate their hosts. Close to its northern range limit we investigated the phenology of Ficus virens, the monoecious fig tree that reaches furthest north in China. Relatively few trees produced any figs, and very few retained figs throughout the winter. Despite this, new crops produced in spring were pollinated, with seasonally migrant pollinators from plants growing further south the most likely pollen vectors. An inability to initiate new crops at low temperatures may limit the distribution of monoecious fig trees to warmer areas.     

Population at the edge: increased divergence but not inbreeding towards northern range limit in Acer campestre

Ecological conditions shape natural distribution of plants. Populations are denser in optimal habitats but become more fragmented in the areas of suboptimal environmental conditions. Usually, fragmentation increases towards the limits of species distribution. Fragmented populations are often characterised by decreased genetic variation, and this effect is frequent in peripheral populations, mostly due to the reduced effective population size. Interestingly, the genetic consequences of fragmentation seem to be relatively weak in forest trees. Using microsatellite markers, we assessed the impact of population fragmentation on the genetic structure of a European tree species Acer campestre. Within the study area, this medium-size wind-dispersed and insect-pollinated tree reveals a gradual decrease in population density towards the northern range limit. Over the distance of 150 km, we detected the significant decrease in allelic richness, heterozygosity as well as an increase in the rate of population divergence along with latitude. On the other hand, we failed to show that the observed patterns of genetic structure result from the variation in population densities. Moreover, inbreeding levels revealed no association with both density and geographic location, suggesting that pollen limitation does not occur, even at the range margin. As we showed that there is no difference in a dispersal scale between low- and high-density populations in the study species, we argue that the genetic structure is a result of postglacial recolonization. However, unlike many other forest trees, A. campestre showed the sharp latitudinal genetic pattern at a very restricted spatial scale. Limited dispersal and high fragmentation are likely the reasons.     

Comparison of climatic effects on radial growth of evergreen broad-leaved trees at their northern distribution limit and co-dominating deciduous broad-leaved trees and evergreen conifers

Using dendrochronological techniques, this study examined whether tree-ring width of two evergreen broad-leaved species (Cleyera japonica, Eurya japonica) at their inland northern distribution limit in central Japan is more limited by low temperature compared with two co-dominating deciduous broad-leaved species (Fagus japonica, Magnolia hypoleuca) and two evergreen conifer species (Chamaecyparis obtusa, Abies firma), whose distribution limits are further north. The two deciduous broad-leaved species and the two evergreen conifers are tall tree species. Evergreen broad-leaved Cleyera japonica is a sub-canopy species and Eurya japonica is a small tree species. The tree-ring widths of four of the six species (except for Eurya japonica and Magnolia hypoleuca) correlated positively with the March temperature just before the start of the growth period. For deciduous broad-leaved Magnolia hypoleuca, the tree-ring width was correlated positively and negatively with July temperature and precipitation, respectively. However, the other deciduous broad-leaved Fagus japonica showed no such relationships. For the evergreen broad-leaved Cleyera japonica and evergreen conifers Chamaecyparis obtusa and Abies firma, tree-ring widths correlated positively with winter temperatures, probably because evergreen species can assimilate during warm winters. The tree-ring width of Cleyera japonica also correlated positively with temperatures of many months of the growth period. By contrast, the tree-ring width of the other evergreen broad-leaved Eurya japonica showed no positive correlation with the temperature in any month. Most Eurya japonica trees were suppressed by tall trees, which might disguise any climate effect. Thus, there were species differences in response to climate for each life form, and the tree-ring width of Cleyera japonica at the northern distribution limit was more limited by low temperatures compared with co-dominating species. It is suggested that growth of Cleyera japonica is increased by global warming at the latitudinal ecotone.     

Allopatric speciation in the desert: diversification of cichlids at their geographical and ecological range limit in Iran

Collated electrofishing data from wadeable riffles of boreal rivers in Finland revealed a substantial shift in the fish assemblage composition, accompanied by a decline in total fish density and a reduction in species richness from early summer to late autumn. As the major changes in fish assemblages, the density of cyprinids decreased sharply from June towards autumn, whereas salmonids peaked in September. These shifts were considered to originate mainly from spawning migrations, fish movements to winter refuge habitat and other temperature-related responses of rheophilic versus eurytopic fish species. Temporal change in fish assemblages induced a prominent variation in the monthly fish-based index values used for bioassessment. The proportion of sampled sites classified as high or good in ecological status within the Water Framework Directive (WFD) based on fish increased from 25.9% in July to 68.3% in October. These results, combined with the observed timing of young of the year (0+) fish recruitment to electrofishing catch, suggest that sampling of fish in boreal rivers for WFD monitoring should be restricted to a considerably shorter period than the prevailing practice in order to avoid temporal bias.     

Photosynthetic energy storage of Photosystems I and II in the spectral range of photosynthetically active radiation in intact sugar maple leaves

The relative activity of Photosystems (PS) I and II in the spectral range between 400 and 720 nm was studied by measuring photosynthetic energy storage (ES) of an intact sugar maple leaf using photoacoustic spectroscopy. ES, determined with a modulated (80 Hz) monochromatic light beam in the presence of saturating intensity of background non-modulated white light, indicated the total energy stored by both photosystems (ES_T). Using background far-red light, ES of PS I (ES_{PS I}) was quantified. ES_{PS II} was derived from ES_T-ES_{PS I}. ES_T dependence on intensity and wavelength of modulated light was studied at 470, 560, 640 and 680 nm. ES_T was maximum in red light and minimum in blue light. It decreased with an increase in modulated light intensity. The ratio ES_{PS II}/ES_{PS I}, measured at 640 nm, remained nearly constant with an increase in modulated light intensity. The relative quantum yield of ES_T spectrum showed two peaks around 610 and 660 nm, and declined sharply after 680 nm, revealing a clear red drop. ES_{PS I} spectrum presented peaks around 610 and 670 nm, and a minimum between 440 and 470 nm. ES_{PS I} was observed beyond 700 nm up to 720 nm, indicating the energy stored by cyclic electron transport. ES_{PS II} spectrum showed broad peaks, around 460, 490, 600 and 660 nm, and a shoulder between 530 and 560 nm. ES_{PS II} was always higher than ES_{PS I} between 400 and 690 nm and reached zero around 700 nm.Abbreviations ES energy storage - ES_{PS I} energy storage of PS I - ES_{PS II} energy storage of PS II - ES_T energy storage of PS I and PS II - PA photoacoustic - PS I Photosystem I - PS II Photosystem II - Q_m PA signal in the absence of any background light - Q_ma PA signal in the presence of background white light - Q_mfrl PA signal in the presence of background far-red light - S/N signal to noise     

Localization of flavonoids in the yellow lupin seedlings and their UV-B-absorbing potential

Quantification of the flavonoids in yellow lupin (Lupinus luteus; Leguminosae) seedlings revealed that a flavone glucoside, 7-O-beta-(2-O-beta-rhamnosyl)glucosyl-4',5,7-trihydroxyflavone (apigenine 7-O-beta-neohesperidoside), is rich in the epicotyl and cotyledon. In hypocotyls and roots, 8-C-beta-glucosyl-4',5,7-trihydroxyisoflavone (genistein 8-C-beta-glucoside) was a predominant flavonoid constituent. The roles of the localized flavonoids are briefly discussed relating to defense against biotic and abiotic external stresses.     

Wrens on the edge: feeders predict Carolina wren Thryothorus ludovicianus abundance at the northern edge of their range

The marked range shifts seen with global warming suggest that ranges are limited by temperature. Other findings, however, suggest winter survival is directly tied to food availability. We studied Carolina wrens Thryothorus ludovicianus at the northern edge of their range to understand the roles of temperature and food in limiting this species. We established 21 transects over 3 habitats (residential, city park, and rural) with varying degrees of human influence on temperature and food supply. The three habitat types showed variations in wren density, temperature, and feeder presence. While wren densities showed similar seasonal patterns in all habitats, significantly higher densities of birds were observed in the city park and residential habitats. Post‐winter densities of Carolina wrens were predicted by the presence of bird feeders, and not by January mean minimum temperatures. Our findings suggest the winter range limits for endotherms is more directly related to food supply, and only indirectly related to temperature. Therefore supplemental feeding and other changes in food supply may modify the range shifts predicted from temperature changes alone.