Light and VPD gradients drive foliar nitrogen partitioning and photosynthesis in the canopy of European beech and silver fir

While foliar photosynthetic relationships with light, nitrogen, and water availability have been well described, environmental factors driving vertical gradients of foliar traits within forest canopies are still not well understood. We, therefore, examined how light availability and vapour pressure deficit (VPD) co-determine vertical gradients (between 12 and 42 m and in the understorey) of foliar photosynthetic capacity (Amax), 13C fractionation (∆), specific leaf area (SLA), chlorophyll (Chl), and nitrogen (N) concentrations in canopies of Fagus sylvatica and Abies alba growing in a mixed forest in Switzerland in spring and summer 2017. Both species showed lower Chl/N and lower SLA with higher light availability and VPD at the top canopy. Despite these biochemical and morphological acclimations, Amax during summer remained relatively constant and the photosynthetic N-use efficiency (PNUE) decreased with higher light availability for both species, suggesting suboptimal N allocation within the canopy. ∆ of both species were lower at the canopy top compared to the bottom, indicating high water-use efficiency (WUE). VPD gradients strongly co-determined the vertical distribution of Chl, N, and PNUE in F. sylvatica, suggesting stomatal limitation of photosynthesis in the top canopy, whereas these traits were only related to light availability in A. alba. Lower PNUE in F. sylvatica with higher WUE clearly indicated a trade-off in water vs. N use, limiting foliar acclimation to high light and VPD at the top canopy. Species-specific trade-offs in foliar acclimation to environmental canopy gradients may thus be considered for scaling photosynthesis from leaf to canopy to landscape levels.     

Acclimation of photosynthetic characteristics of the moss Pleurozium schreberi to among‐habitat and within‐canopy light gradients

Light availability varies strongly among moss habitats and within the moss canopy, and vertical variation in light within the canopy further interacts with the age gradient. The interacting controls by habitat and canopy light gradient and senescence have not been studied extensively. We measured light profiles, chlorophyll (Chl), carotenoid (Car) and nitrogen (N) concentrations, and photosynthetic electron transport capacity (J_max) along habitat and canopy light gradients in the widespread, temperate moss Pleurozium schreberi to separate sources of variation in moss chemical and physiological traits. We hypothesised that this species, like typical feather mosses with both apical and lateral growth, exhibits greater plasticity in the canopy than between habitats due to deeper within‐canopy light gradients. For the among‐habitat light gradient, Chl, Chl/N and Chl/Car ratio increased with decreasing light availability, indicating enhanced light harvesting in lower light and higher capacity for photoprotection in higher light. N and J_max were independent of habitat light availability. Within the upper canopy, until 50–60% above‐canopy light, changes in moss chemistry and photosynthetic characteristics were analogous to patterns observed for the between‐habitat light gradient. In contrast, deeper canopy layers reflected senescence of moss shoots, with pigment and nitrogen concentrations and photosynthetic capacity decreasing with light availability. Thus, variation in chemical and physiological traits within the moss canopy is a balance between acclimation and senescence. This study demonstrates extensive light‐dependent variation in moss photosynthetic traits, but also that between‐habitat and within‐canopy light gradient affects moss physiology and chemistry differently.     

Vertical canopy gradients in δ13C correspond with leaf nitrogen content in a mixed-species conifer forest

Stable carbon isotope composition varies markedly between sun and shade leaves, with sun leaves being invariably more enriched (i.e., they contain more¹³C). Several hypotheses have emerged to explain this pattern, but controversy remains as to which mechanism is most general. We measured vertical gradients in stable carbon isotope composition (δ¹³C) in more than 200 trees of nine conifer species growing in mixed-species forests in the Northern Rocky Mountains, USA. For all species except western larch, δ¹³C decreased from top to bottom of the canopy. We found that δ¹³C was strongly correlated with nitrogen per unit leaf area (N _area), which is a measure of photosynthetic capacity. Usually weaker correlations were found between δ¹³C and leaf mass per area, nitrogen per unit leaf mass, height from the ground, or depth in the canopy, and these correlations were more variable between trees than for N _area. Gradients of δ¹³C (per meter canopy depth) were steeper in small trees than in tall trees, indicating that a recent explanation of δ¹³C gradients in terms of drought stress of upper canopy leaves is unlikely to apply in our study area. The strong relationship between N _area and δ¹³C here reported is consistent with the general finding that leaves or species with higher photosynthetic capacity tend to maintain lower CO₂ concentrations inside leaves. We conclude that photosynthetic capacity is a strong determinant of δ¹³C in vertical canopy profiles, and must be accounted for when interpreting δ¹³C values in conifer forests.     

Variation in crown light utilization characteristics among tropical canopy trees

BACKGROUND AND AIMS: Light extinction through crowns of canopy trees determines light availability at lower levels within forests. The goal of this paper is the exploration of foliage distribution and light extinction in crowns of five canopy tree species in relation to their shoot architecture, leaf traits (mean leaf angle, life span, photosynthetic characteristics) and successional status (from pioneers to persistent). METHODS: Light extinction was examined at three hierarchical levels of foliage organization, the whole crown, the outermost canopy and the individual shoots, in a tropical moist forest with direct canopy access with a tower crane. Photon flux density and cumulative leaf area index (LAI) were measured at intervals of 0.25-1 m along multiple vertical transects through three to five mature tree crowns of each species to estimate light extinction coefficients (K). RESULTS: Cecropia longipes, a pioneer species with the shortest leaf life span, had crown LAI <0.5. Among the remaining four species, crown LAI ranged from 2 to 8, and species with orthotropic terminal shoots exhibited lower light extinction coefficients (0.35) than those with plagiotropic shoots (0.53-0.80). Within each type, later successional species exhibited greater maximum LAI and total light extinction. A dense layer of leaves at the outermost crown of a late successional species resulted in an average light extinction of 61% within 0.5 m from the surface. In late successional species, leaf position within individual shoots does not predict the light availability at the individual leaf surface, which may explain their slow decline of photosynthetic capacity with leaf age and weak differentiation of sun and shade leaves. CONCLUSION: Later-successional tree crowns, especially those with orthotropic branches, exhibit lower light extinction coefficients, but greater total LAI and total light extinction, which contribute to their efficient use of light and competitive dominance.     

A bounded null model explains juvenile tree community structure along light availability gradients in a temperate rain forest

Ecologists have proposed that tree species may coexist by specialising on light environments associated with gaps of different sizes. Remarkably few community‐level studies, however, have actually examined juvenile tree distributions along light availability gradients. Here we describe distributions of juvenile trees in relation to canopy openness in a temperate rainforest, and test the hypothesis that competitive sorting causes coexisting species to overlap less in light environment occupancy than would be expected by chance. Average overlap of species’ interquartile ranges on the canopy openness gradient was tested against a bounded domain null model of community structure which used range‐size criteria to constrain random placement of species optima. Microsite availability was strongly skewed towards low light, with 43% of microsites occurring at <5% canopy openness. We therefore transformed canopy openness values to ranks, so that equal intervals on the transformed gradient represented equal areas of microsite availability. We then calculated the interquartile range (25–75%) of sample ranks occupied by juveniles of each species. About half the assemblage was non‐randomly distributed in relation to canopy openness, providing evidence of niche expression. Average overlap of species’ interquartile ranges did not depart significantly from that predicted by the bounded null model, indicating that community structure in relation to canopy openness was mainly explained by a mid‐domain effect. As predicted by the null model, species’ interquartile range mid‐points were concentrated in the centre of the rank‐transformed gradient, and species richness (overlap of interquartile ranges) peaked close to the median light environment. Most species therefore had intermediate light requirements. The apparent lack of constraints on pairwise overlap suggest that differences in light use are not a prerequisite for tree species coexistence. As far as we are aware, this is the first study to identify a mid‐domain effect on a resource availability axis.     

Impact of tree leaf phenology on growth rates and reproduction in the spring flowering species Trillium erectum (Liliaceae)

We investigated the impact of overstory tree leaf phenology on growth rates, carbon allocation pattern, and fruit characteristics in the spring flowering species, Trillium erectum (Liliaceae). Air temperature, overstory canopy closure, and T. erectum phenology were monitored at three locations following a latitudinal gradient in Québec, Canada. Northern sugar maple trees leaf out at cooler temperatures than more southern populations, while Trillium development was initiated at the same soil temperature irrespective of the latitude. Therefore, in northern areas, the time between initiation of T. erectum leaf expansion and canopy closure was shorter than in southern areas, which left less time for northern plants to accumulate reserves before canopy closure. Differences in growth patterns were noted between T. erectum populations. From a south-north gradient, investment to reproduction, total plant biomass, and annual growth rate decreased, while specific leaf area and stem height increased, indicating shade acclimation. The length of the high light period in early spring seems to be a determinant for spring flowering plants' growth and reproduction and may explain the northern distribution limit of some of these species.     

Tree species growth response to climate warming varies by forest canopy position in boreal and temperate forests

Reports of forest sensitivity to climate change are based largely on the study of overstory trees, which contribute significantly to forest growth and wood supply. However, juveniles in the understory are also critical to predict future forest dynamics and demographics, but their sensitivity to climate remains less known. In this study, we applied boosted regression tree analysis to compare the sensitivity of understory and overstory trees for the 10 most common tree species in eastern North America using growth information from an unprecedented network of nearly 1.5 million tree records from 20,174 widely distributed, permanent sample plots across Canada and the United States. Fitted models were then used to project the near-term (2041–2070) growth for each canopy and tree species. We observed an overall positive effect of warming on tree growth for both canopies and most species, leading to an average of 7.8%–12.2% projected growth gains with climate change under RCP 4.5 and 8.5. The magnitude of these gains peaked in colder, northern areas for both canopies, while growth declines are projected for overstory trees in warmer, southern regions. Relative to overstory trees, understory tree growth was less positively affected by warming in northern regions, while displaying more positive responses in southern areas, likely driven by the buffering effect of the canopy from warming and climate extremes. Observed differences in climatic sensitivity between canopy positions underscore the importance of accounting for differential growth responses to climate between forest strata in future studies to improve ecological forecasts. Furthermore, latitudinal variation in the differential sensitivity of forest strata to climate reported here may help refine our comprehension of species range shift and changes in suitable habitat under climate change.     

Recruitment patterns and northward tree migration through gap dynamics in an old-growth white pine forest in northern Ontario

Decreases in abundances and declines in growth of eastern white pine over the past century due mainly to human activities have resulted in few large intact old-growth white pine forests in Ontario. These stands may be vulnerable to replacement by deciduous species from temperate forests further south, where recruitment in canopy gap disturbances can greatly define the regeneration process. We investigated recruitment dynamics in canopy gaps of an old-growth white pine forest of Temagami, northern Ontario, Canada, the northern limit of the temperate?Cboreal ecotone. White pine, red pine, black spruce and eastern white cedar represented 85?% of the mature canopy abundance, where trees and saplings established equally in gaps and the closed canopy. Balsam fir and paper birch were more abundant in gaps, showing increases of abundance and basal area with increases in gap size representing canopy self-replacement (balsam fir) and autogenic succession (paper birch). Red maple, at its northernmost range limit, was the only species to show linear increases of abundance and basal area with increases in gap size and gap age. This result, along with adult red maples present in gaps but absent from the closed canopy, identifies the establishment of a northward migrating species in gaps as hypothesized for pine forests at the northern limit of this broad ecotone. We discuss how migration pressures, coupled with pine recruitment limitation through reduced fire frequency by regional fire suppression and predicted future increased warming of 2?C4?°C over the next century, threatens replacement of old-growth white pine forests at this latitude with northward migrating tree species found further south.     

Intraspecific variation in epiphyte functional traits reveals limited effects of microclimate on community assembly in temperate deciduous oak canopies

Forest canopies are heterogeneous environments where changes in microclimate over short distances create an opportunity for niche‐based filtering of canopy‐dwelling species assemblages. This environmental filtering may not occur if species' physiological capacities are flexible or if rapid dispersal alleviates compositional differences. I assess the role of humidity, light and temperature gradients in structuring epiphyte communities in temperate deciduous oak (Quercus) canopies and determine whether gradients filter species with fixed traits or whether environmental constraints act primarily to alter individual phenotypes. I measured environmental conditions and seven functional traits related to water and light acquisition on individual macrolichens at 60 sample locations in northern red oaks Quercus rubra in two Piedmont forests in North Carolina, USA. The effects of environmental variables on individual‐level traits and community composition were evaluated using linear mixed models and constrained ordination (RDA). In general, traits and community composition responded weakly to environmental variables and trait variation within taxa was high. Cortex thickness exhibited the strongest response, such that individuals with thicker cortices were found in samples experiencing lower humidity and higher light levels. Overall, gradients of humidity, light and temperature were not strong environmental filters that caused large changes in community composition. This was probably due to phenotypic variability within taxa that enabled species to persist across the full range of environmental conditions measured. Thus, humidity affected the phenotype of individuals, but did not limit species distributions or alter community composition at the scale of branches within trees. Community and trait responses were primarily associated with site‐level differences in humidity, suggesting that in these forests landscape‐scale climatic gradients may be stronger drivers of epiphyte community assembly than intra‐canopy environmental gradients.     

Comparison of light harvesting and resource allocation strategies between two rhizomatous herbaceous species inhabiting deciduous forests

Light conditions on the floor of deciduous forests are determined by the leaf dynamics of canopy trees and gap formation. Such spatiotemporal variations of light availability should affect the resource partitioning strategies of understory herbs. Although rhizomatous species are common in understory, relationships between rhizome structure, vegetative growth, and sexual reproduction are unclear in terms of carbon allocation. We compared the photosynthetic characteristics and carbon translocation patterns in the under-canopy and light-gap sites between two summer-green perennial species: Cardamine leucantha with an annual long rhizome, and Smilacina japonica with a perennial short rhizome system. Flowering of both species occurs in early summer under decreasing light availability. In the light-gap, C. leucantha maintained high photosynthetic activity due to continuous leaf production, resulting in higher seed production than in the under-canopy. In contrast, the photosynthetic rate of S. japonica, producing leaves simultaneously, decreased with time irrespective of light conditions, resulting in stable seed production in both sites. Although seasonally decreasing light availability commonly restricts carbon assimilation of understory herbs, the responses of resource partitioning to variations in light availability depend greatly on the belowground structure of individual species.     

Ecophysiology of exotic and native shrubs in Southern Wisconsin

Summary We compared seasonal trends in photosynthesis of two naturalized exotic shrubs (Rhamnus cathartica and Lonicera X bella) and two native shrubs (Cornus racemosa and Prunus serotina) in open and understory habitats in southern Wisconsin. We examined the relationships between resource availability and leaf photosynthetic performance in these four species. All four species had similar relationships between leaf nitrogen (N) content and photosynthetic rate, but the species differed in absolute leaf N content and therefore in photosynthetic rates. Maximum daily photosynthetic rates of all species were significantly correlated with leaf N content in the open habitat, but not in the understory, where low light availability was the major limitation to photosynthesis. Extended leaf longevity was important in the forest understory because it allowed shrubs to take advantage of high light availability at times when the overstory canopy was leafless. Early leaf emergence was more important than late senescence: from 27% to 35% of the annual carbon gain of P. serotina, R. cathartica, and L. X bella occurred prior to leaf emergence of C. racemosa, the species with the shortest leaf life span. Extended leaf longevity of exotic shrubs may help explain their persistence in the understory habitat, but it contributed relatively less to their annual carbon gain in the open habitat.     

Can light-saturated photosynthesis in lowland tropical forests be estimated by one light level?

Leaf-level net photosynthesis (A_n) estimates and associated photosynthetic parameters are crucial for accurately parameterizing photosynthesis models. For tropical forests, such data are poorly available and collected at variable light conditions. To avoid over- or underestimation of modeled photosynthesis, it is critical to know at which photosynthetic photon flux density (PPFD) photosynthesis becomes light-saturated. We studied the dependence of A_n on PPFD in two tropical forests in French Guiana. We estimated the light saturation range, including the lowest PPFD level at which A_sat (A_n at light saturation) is reached, as well as the PPFD range at which A_sat remained unaltered. The light saturation range was derived from photosynthetic light-response curves, and within-canopy and interspecific differences were studied. We observed wide light saturation ranges of A_n. Light saturation ranges differed among canopy heights, but a PPFD level of 1,000 µmol m⁻² s⁻¹ was common across all heights, except for pioneer trees species that did not reach light saturation below 2,000 µmol m⁻² s⁻¹. A light intensity of 1,000 µmol m⁻² s⁻¹ sufficed for measuring A_sat of climax species at our study sites, independent of the species or the canopy height. Because of the wide light saturation ranges, results from studies measuring A_sat at higher PPFD levels (for upper canopy leaves up to 1,600 µmol m⁻² s⁻¹) are comparable with studies measuring at 1,000 µmol m⁻² s⁻¹.     

Mild experimental climate warming induces metabolic impairment and massive mortalities in southern African quartz field succulents

Current thermal regimes for many southern African succulent species of the subfamily Ruschioideae, which rapidly diversified during the cooler Pleistocene period, may be close to their tolerable extremes, and likely exceeded with anticipated future climate warming. This hypothesis was tested by exposing succulent species of different size and architecture to differently elevated temperatures approximating future African climate scenarios (2.5–3.8 °C increases in mean annual daily temperature maxima) using transparent hexagonal open-top chambers of different heights. Air temperatures, soil water potentials and amounts of fog and dew precipitation were monitored hourly in the differently heated open-top chambers and ambient environment, and changes in species leaf densities and canopy covers precisely determined in these chambers and ambient environment from high resolution digital images taken at 3-monthly intervals spanning a 12-month monitoring period. Photochemical efficiencies and activities of the photosynthetic enzyme Rubisco were also measured in one widespread dwarf succulent species following 2-h exposures of its populations in a forced draft oven to eight different heat intensities (range: 40–54 °C), the highest closely matching the temperature extreme of 54.8 °C recorded in the most intensively heated open-top chambers. After 12-months warming, all succulent species displayed massively (up to 90.2%) reduced leaf densities and canopy covers in the differently heated open-top chambers, with small sparsely branched species comprising single leaf pairs per axis exhibiting much greater reductions than large, shrubby or creeping species with multiple leaves. Noteworthy, was that fog and dew precipitation levels and soil water potentials at the centres of the least intensively heated chambers did not differ significantly from those in the ambient environment, even during the critical dry summer and early autumn seasons. However, leaf density and canopy reductions in these chambers were of similar magnitude to those in the most intensively heated chambers where fog and dew precipitation levels and soil water potentials were significantly reduced. These findings identified elevated temperatures as the principal cause of the observed massive reductions in leaf density and canopy cover and supported the hypothesis that mild anthropogenic warming could exceed the thermal thresholds of many southern African quartz field succulents leading to metabolic impairment. This impairment explained by an observed loss in the catalytic efficiency of Rubisco at daytime temperature extremes exceeding 54 °C, preceded by a decrease in PSII electron transport commencing at temperatures much lower than the threshold for Rubisco de-activation.     

Stem and leaf gas exchange and their responses to fire in a north Australian tropical savanna

We measured stem CO2 efflux and leaf gas exchange in a tropical savanna ecosystem in northern Australia, and assessed the impact of fire on these processes. Gas exchange of mature leaves that flushed after a fire showed only slight differences from that of mature leaves on unburned trees. Expanding leaves typically showed net losses of CO2 to the atmosphere in both burned and unburned trees, even under saturating irradiance. Fire caused stem CO2 efflux to decline in overstory trees, when measured 8 weeks post-fire. This decline was thought to have resulted from reduced availability of C substrate for respiration, due to reduced canopy photosynthesis caused by leaf scorching, and to priority allocation of fixed C towards reconstruction of a new canopy. At the ecosystem scale, we estimated the annual above-ground woody-tissue CO2 efflux to be 275 g C m(-2) ground area year(-1) in a non-fire year, or approximately 13% of the annual gross primary production. We contrasted the canopy physiology of two co-dominant overstory tree species, one of which has a smooth bark on its branches capable of photosynthetic re-fixation (Eucalyptus miniata), and the other of which has a thick, rough bark incapable of re-fixation (Eucalyptus tetrodonta). Eucalyptus miniata supported a larger branch sapwood cross-sectional area in the crown per unit subtending leaf area, and had higher leaf stomatal conductance and photosynthesis than E. tetrodonta. Re-fixation by photosynthetic bark reduces the C cost of delivering water to evaporative sites in leaves, because it reduces the net C cost of constructing and maintaining sapwood. We suggest that re-fixation allowed leaves of E. miniata to photosynthesize at higher rates than those of E. tetrodonta, while the two invested similar amounts of C in the maintenance of branch sapwood.     

Response of floodplain understorey species to environmental gradients and tree invasion: a functional trait perspective

Plants are connected to habitats by functional traits which are filtered by environmental gradients. Since tree species composition in the forest canopy can influence ecosystem processes by changing resource availability, litter accumulation, and soil nutrient content, we hypothesised that non-native invasive trees can establish new environmental filters on the understorey communities. In the hardwood floodplain forests in Northern Italy, the invasive trees Robinia pseudoacacia L. and Prunus serotina Ehrh. are the dominant canopy species. We used trait data assembled from databases and iterative RLQ analysis to identify a parsimonious set of functional traits responding to environmental variables (soil, light availability, disturbance, and stand structure) and the dominant native and invasive canopy species. Then, RLQ and fourth-corner analysis was conducted to investigate the joint structure between macro-environmental variables and species traits and functional groups were identified. The trait composition of the herb-layer was significantly related to the main environmental gradients and the presence of the invaders in the canopy showed significant relationships with several traits. In particular, the presence of P. serotina may mitigate or even erase the effect of disturbances, maintaining a stable forest microclimate and thus favouring ‘true’ forest species, while R. pseudoacacia may slow down forest succession and regeneration by establishing new stable associations with a graminoid-dominated understorey. The impact of the two invasive trees on herb layer composition appears to differ, indicating that different management and control strategies may be needed.     

Leaf carbon isotope ratios of plants from a subtropical monsoon forest

Summary Carbon isotope ratios were used to survey the distribution of photosynthetic pathways among taxa, the relationship between photosynthetic pathway and habitat light levels, and the relationship between intercellular CO₂ levels of C₃ plants and habitat light levels within a subtropical monsoon forest in southern China. Of 128 species, most (94) possessed the C₃ photosynthetic pathway; 33 species possessed the C₄ pathway and all of these were restricted to high light locations. There was one epiphytic CAM species. The C₃ species were classified as occurring in open, intermediate, and closed canopy sites. Among C₃ species, carbon isotope ratios tended to become more negative with decreasing light availability in the habitat.C.I.W.D.P.B. Pub no 931     

Differences in nitrogen use efficiency between leaves from canopy and subcanopy trees

We examined the effects of increasing light availability along a vertical gradient within a forest community on the efficiency of leaf nitrogen (N) use in individual trees. The N contents of green and senescent leaves in canopy and subcanopy trees of an evergreen coniferous species, Podocarpus nagi, and an evergreen hardwood species, Neolitsea aciculata, were analyzed in a mixed forest community at Mt Mikasa, Nara City, Japan. The inverse of N concentration (N_C) in senescent leaves was used as an index of N use efficiency (NUE) at the leaf-level. The leaf-level NUE was higher in canopy trees than in subcanopy trees in both P.nagi and N.aciculata, although soil N mineralization rates around canopy and subcanopy trees did not differ significantly. The N_C in green leaves was lower in canopy trees than in subcanopy trees. The ratio of resorbed N in senescent leaves to the N content in green leaves was higher in canopy trees than in subcanopy trees. The higher leaf-level NUE of canopy trees was partly a result of lower N_C in living tissues and partly because of greater N resorption during senescence. The present study suggested that the leaf-level NUE could be increased in response to an imbalance between soil N and light availability caused by spatial community structure.     

Linking light attenuation,sunflecks, and canopy architecture in mesic shrub thickets

Expansion of shrubs into grasslands is often accompanied by a reduction in understory light and an associated reduction of shade-intolerant species. However, effects of specific canopy architectural characteristics on the light environment under shrub thickets are unknown. Our objective was to determine what characteristics of canopy architecture most influence understory light in monospecific shrub thickets. We quantified understory light and canopy architecture for five shrub species in the eastern United States that have a history of expansion, and we used multiple regression to determine which canopy characteristics best predicted light attenuation and relative contribution of sunflecks. Measurements included leaf angle, leaf azimuth, branch bifurcation ratio, leaf area index (LAI), canopy depth (the vertical distance from the bottommost leaf to the top of the canopy), and leaf area density (LAD) as well as understory photosynthetic photon flux density (PPFD). The best predictor of light attenuation and the occurrence of sunflecks for all species was canopy depth. Multiple leaf and plant-level traits were correlated with canopy depth but not with LAI or LAD. The invasive shrub Elaeagnus umbellata had the lowest understory light levels of the species examined although LAI values for Morella cerifera and Rhododendron maximum were higher. Branch bifurcation ratios for E. umbellata were significantly higher than for other species and this likely contributed to the differences in light attenuation and suppression of sunflecks. The potential of shrubs to intercept light is primarily dependent on vertical distribution of leaves in the canopy which is itself correlated with fine-scale, species-specific variations in leaf display.     

Historical and recent processes shaping the geographic range of a rocky intertidal gastropod: phylogeography,ecology, and habitat availability

Factors shaping the geographic range of a species can be identified when phylogeographic patterns are combined with data on contemporary and historical geographic distribution, range‐wide abundance, habitat/food availability, and through comparisons with codistributed taxa. Here, we evaluate range dynamism and phylogeography of the rocky intertidal gastropod Mexacanthina lugubris lugubris across its geographic range – the Pacific coast of the Baja peninsula and southern California. We sequenced mitochondrial DNA (CO1) from ten populations and compliment these data with museum records, habitat availability and range‐wide field surveys of the distribution and abundance of M. l. lugubris and its primary prey (the barnacle Chthamalus fissus). The geographic range of M. l. lugubris can be characterized by three different events in its history: an old sundering in the mid‐peninsular region of Baja (~ 417,000 years ago) and more recent northern range expansion and southern range contraction. The mid‐peninsular break is shared with many terrestrial and marine species, although M. l. lugubris represents the first mollusc to show it. This common break is often attributed to a hypothesized ancient seaway bisecting the peninsula, but for M. l. lugubris it may result from large habitat gaps in the southern clade. Northern clade populations, particularly near the historical northern limit (prior to the 1970s), have high local abundances and reside in a region with plentiful food and habitat – which makes its northern range conducive to expansion. The observed southern range contraction may result from the opposite scenario, with little food or habitat nearby. Our study highlights the importance of taking an integrative approach to understanding the processes that shape the geographic range of a species via combining range‐wide phylogeography data with temporal geographic distributions and spatial patterns of habitat/food availability.     

The demography of range boundaries versus range cores in eastern US tree species

Regional species–climate correlations are well documented, but little is known about the ecological processes responsible for generating these patterns. Using the data from over 690 000 individual trees I estimated five demographic rates—canopy growth, understorey growth, canopy lifespan, understorey lifespan and per capita reproduction—for 19 common eastern US tree species, within the core and the northern and southern boundaries, of the species range. Most species showed statistically significant boundary versus core differences in most rates at both boundary types. Differences in canopy and understorey growth were relatively small in magnitude but consistent among species, being lower at the northern (average −17%) and higher at the southern (average +12%) boundaries. Differences in lifespan were larger in magnitude but highly variable among species, except for a marked trend for reduced canopy lifespan at the northern boundary (average −49%). Differences in per capita reproduction were large and statistically significant for some species, but highly variable among species. The rate estimates were combined to calculate two performance indices: R₀ (a measure of lifetime fitness in the absence of competition) was consistently lower at the northern boundary (average −86%) whereas Z^* (a measure of competitive ability in closed forest) showed no sign of a consistent boundary–core difference at either boundary.