Co-occurring Tree Species Research Articles

Physiological homeostasis and morphological plasticity of two tree species subjected to precipitation seasonal distribution changes

General circulation models and empirical observations show that rainfall patterns will become increasingly erratic in the future as the paradigm “dry gets drier, wet gets wetter” becomes globally prevalent. Currently, few field studies have evaluated the long-term effects of extreme precipitation on forest ecosystems based on tree physiology. To provide a better mechanistic understanding of these effects, a two-year “Precipitation Seasonal Distribution Changes” experiment (PSDC) was conducted in lower subtropical China. The treatments included “Drier dry season and wetter wet season (DD)”, ‘Extended dry season and wetter wet season (ED)” and “Ambient control (AC)’. The throughfall of the DD treatment was excluded during the dry season (October-March of the following year) to simulate drought, whereas the throughfall of the ED treatment was excluded in the spring (April-May) to simulate spring drought without significant altering the total annual precipitation input. The sap flow, water use efficiency, leaf and wood nutrient content and morphological parameters of two co-occurring tree species, Michelia macclurei Dandy and Schima superba Gardner & Champ., were quantified to elucidate the physiological mechanisms that occur in response to rainfall pattern changes. Our results indicated that the whole-tree transpiration per unit sapwood area (Js) and intrinsic water use efficiency (WUEi) showed homeostatic responses. Tree transpiration and response parameters to environmental variables (vapor pressure deficit [VPD] and photosynthetically active radiation [PAR]) did not show significant treatment effects regardless of drought, spring drought or irrigation periods. However, the leaf nitrogen (Narea) and phosphorus (Parea) content showed fluctuations, particularly the leaf N:P ratio, which showed that rainfall pattern changes had adverse effects on nutrient acquisition in this dual nitrogen- and phosphorus- limited forest ecosystem. The wood stoichiometry was more conserved than the leaf stoichiometry. For a given leaf 13C discrimination (≈ci/ca), the wood 13C discrimination did not show treatment or species effects, suggesting that the carbon transfer of trees was not affected by rainfall patterns. The branch As:Al was significantly negatively correlated with the parameters representing the sensitivity of tree transpiration to VPD, indicating that the trees maintained homeostatic responses by combining stomatal control and morphological adjustments. In mesic environments, the two studied tree species showed high resilience to seasonal rainfall pattern changes and homeostatic responses to nutrient acquisition, light use and water uptake, which is consistent with the ‘ecohydrological equilibrium theory’. In conclusion, the shallow-rooted M. macclurei presented a growth advantage in mesic environments or under mild water stress, whereas the deeply rooted S. superba is more competitive under predicted prolonged drought scenarios, such as DD and ED rain patterns, because of its greater morphological plasticity and increased water uptake from groundwater. Both species are common in south and eastern China but present inherently different responses to seasonal precipitation changes, which implies that changes in the timing and magnitude of precipitation may have consequences for plant nutrient acquisition and water balance and the potential to alter community composition.

Distinct genecological patterns in seedlings ofNorway spruce and silver fir from a mountainous landscape

Understanding the genecology of forest trees is critical for gene conservation, for predicting the effects of climate change and climate change adaptation, and for successful reforestation. Although common genecological patterns have emerged, species-specific details are also important. Which species are most vulnerable to climate change? Which are the most important adaptive traits and environmental drivers of natural selection? Even though species have been classified as adaptive specialists vs. adaptive generalists, large-scale studies comparing different species in the same experiment are rare. We studied the genecology of Norway spruce (Picea abies) and silver fir (Abies alba), two co-occurring but ecologically distinct European conifers in Central Europe. For each species, we collected seed from more than 90 populations across Switzerland, established a seedling common-garden test, and developed genecological models that associate population variation in seedling growth and phenology to climate, soil properties, and site water balance. Population differentiation and associations between seedling traits and environmental variables were much stronger for Norway spruce than for silver fir, and stronger for seedling height growth than for bud phenology. In Norway spruce, height growth and second flushing were strongly associated with temperature and elevation, with seedlings from the lowlands being taller and more prone to second flush than seedlings from the Alps. In silver fir, height growth was more weakly associated with temperature and elevation, but also associated with water availability. Soil characteristics explained little population variation in both species. We conclude that Norway spruce has become an adaptive specialist because trade-offs between rapid juvenile growth and frost avoidance have subjected it to strong diversifying natural selection based on temperature. In contrast, because silver fir has a more conservative growth habit, it has evolved to become an adaptive generalist. This study demonstrates that co-occurring tree species can develop very different adaptive strategies under identical environmental conditions, and suggests that Norway spruce might be more vulnerable to future maladaptation due to rapid climate change than silver fir.

Acclimation of leaf water status and stem hydraulics to drought and tree neighbourhood: alternative strategies among the saplings of five temperate deciduous tree species

Adjustment in leaf water status parameters and modification in xylem structure and functioning can be important elements of a tree's response to continued water limitation. In a growth trial with saplings of five co-occurring temperate broad-leaved tree species (genera Fraxinus, Acer, Carpinus, Tilia and Fagus) conducted in moist or dry soil, we compared the drought acclimation in several leaf water status and stem hydraulic parameters. Considering the extremes in the species responses, Fraxinus excelsior L. improved its leaf tissue hydration in the dry treatment through osmotic, elastic and apoplastic adjustment while Fagus sylvatica L. solely modified its xylem anatomy, which resulted in increased embolism resistance at the cost of hydraulic efficiency. Our results demonstrate the contrasting response strategies of coexisting tree species and how variable trait plasticity among species can be. The comparison of plants grown either in monoculture or in five-species mixture showed that the neighbouring species diversity can significantly influence a tree's hydraulic architecture and leaf water status regulation. Droughted Carpinus betulus L. (and to a lesser extent, Acer pseudoplatanus L.) plants developed a more efficient stem hydraulic system in heterospecific neighbourhoods, while that of F. sylvatica was generally more efficient in conspecific than heterospecific neighbourhoods. We conclude that co-occurring tree species may develop a high diversity of drought-response strategies, and exploring the full diversity of trait characteristics requires synchronous study of acclimation at the leaf and stem (and possibly also the root) levels, and consideration of physiological as well as morphological and anatomical modifications.

Foliar Nutrient Distribution Patterns in Sympatric Maple Species Reflect Contrasting Sensitivity to Excess Manganese.

Sugar maple and red maple are closely-related co-occurring tree species significant to the North American forest biome. Plant abiotic stress effects including nutritional imbalance and manganese (Mn) toxicity are well documented within this system, and are implicated in enhanced susceptibility to biotic stresses such as insect attack. Both tree species are known to overaccumulate foliar manganese (Mn) when growing on unbuffered acidified soils, however, sugar maple is Mn-sensitive, while red maple is not. Currently there is no knowledge about the cellular sequestration of Mn and other nutrients in these two species. Here, electron-probe x-ray microanalysis was employed to examine cellular and sub-cellular deposition of excessively accumulated foliar Mn and other mineral nutrients in vivo. For both species, excess foliar Mn was deposited in symplastic cellular compartments. There were striking between-species differences in Mn, magnesium (Mg), sulphur (S) and calcium (Ca) distribution patterns. Unusually, Mn was highly co-localised with Mg in mesophyll cells of red maple only. The known sensitivity of sugar maple to excess Mn is likely linked to Mg deficiency in the leaf mesophyll. There was strong evidence that Mn toxicity in sugar maple is primarily a symplastic process. For each species, leaf-surface damage due to biotic stress including insect herbivory was compared between sites with acidified and non-acidified soils. Although it was greatest overall in red maple, there was no difference in biotic stress damage to red maple leaves between acidified and non-acidified soils. Sugar maple trees on buffered non-acidified soil were less damaged by biotic stress compared to those on unbuffered acidified soil, where they are also affected by Mn toxicity abiotic stress. This study concluded that foliar nutrient distribution in symplastic compartments is a determinant of Mn sensitivity, and that Mn stress hinders plant resistance to biotic stress.

Growth and Physiological Responses to an Elevation Gradient by Co-Occurring Tree Species in a Shola forest of Kerala, India

Shola forests of Kerala in peninsular India belong to the broad category of tropical montane forests represented in the continents of Asia, Africa and America. In Kerala, typical shola forests are distributed along the crest of the Western Ghats, where the altitude goes beyond 1800 m (ref. 1). In these forests, like in other tropical forest belts, altitudinal gradient is one of the ecological factors which influence structure, composition and diversity of plant community.

Seasonal variation in N uptake strategies in the understorey of a beech-dominated N-limited forest ecosystem depends on N source and species.

In forest ecosystems, species use different strategies to increase their competitive ability for nitrogen (N) acquisition. The acquisition of N by trees is regulated by tree internal and environmental factors including mycorrhizae. In this study, we investigated the N uptake strategies of three co-occurring tree species [European beech (Fagus sylvatica L.), sycamore maple (Acer pseudoplatanus L.) and Norway maple (Acer platanoides L.)] in the understorey of a beech-dominated, N-limited forest on calcareous soil over two consecutive seasons. For this purpose, we studied (15)N uptake capacity as well as the allocation to N pools in the fine roots. Our results show that European beech had a higher capacity for both inorganic and organic N acquisition throughout the whole growing season compared with sycamore maple and Norway maple. The higher capacity of N acquisition in beech indicates a better adaption of beech to the understorey conditions of beech forests compared with the seedlings of other tree competitors under N-limited conditions. Despite these differences, all three species preferred organic over inorganic N sources throughout the growing season and showed similar seasonal patterns of N acquisition with an increased N uptake capacity in summer. However, this pattern varied with N source and year indicating that other environmental factors not assessed in this study further influenced N acquisition by the seedlings of the three tree species.

Differential effect of landscape structure on the large-scale dispersal of co-occurring bird-dispersed trees

Seed dispersal enables plants to reach favorable sites for population renewal or expansion far from conspecifics. However, the ability of plants to respond to habitat heterogeneity at large spatial scale is strongly mediated by seed dispersal vectors, e.g. animals, which usually restrict seed deposition to sites with specific environmental conditions, and at short distances from source plants. This spatial constraint, together with the technical difficulties of following the movement of seeds in the wild, makes the estimation of plant response to large-scale heterogeneity a challenge. Here, we applied an isotope-based technique to track bird-mediated seed dispersal of two co-occurring tree species in eight replicated landscapes in the Cantabrian Range (N Spain): the hawthorn Crataegus monogyna and the holly Ilex aquifolium. These species bear very similar fruits, partially overlap in the timing of fruit production, and their seeds are dispersed by the same species of frugivorous birds. Thus we expected that landscape structure, as represented by the amount of forest cover in the landscape, would affect the large-scale seed dispersal of neighboring individuals of C. monogyna and I. aquifolium in a similar fashion. Contrary to our expectation though, the effects of forest cover on the dispersal patterns of co-occurring hawthorn and holly were opposite: high forest cover in the landscape decreased large-scale dispersal for hawthorn, but enhanced it for holly. Our results suggest that small differences in the traits of plant and frugivore species, such as phenology patterns, can interact with the distribution of adult plants to generate strong differences in the response to landscape structure through seed dispersal, even for neighboring trees belonging to different species.

Latitudinal distribution, co-occurring tree species and structural diversity of the threatened palm Borassus aethiopum (Arecaceae) in Benin, West Africa

Background and aims – Borassus aethiopum Mart. is a wild palm species with high subsistence importance in West Africa. Extensive agriculture and overharvesting of its stem and fruits for multiple uses have caused a decrease of its natural populations in its native range. For conservation purposes, the distribution, abundance and structural diversity of the species were investigated across ten phytodistricts in three biogeographical zones in Benin. Two hypotheses were tested (i) tree floristic composition of B. aethiopum natural habitat changes with phytodistricts and (ii) structural diversity of B. aethiopum changes with phytodistricts, both as potential adaptation strategies to changing ecological conditions. Methods – Geographical coordinates of the species occurrence were recorded. Abundance was assessed in 852 one-ha plots. Structural diversity was studied using structural indices on data from ecological inventories and neighbourhood survey in 70 one-ha plots. Key results – The two hypotheses proved true. B. aethiopum was found in all phytodistricts but with strong variations in abundance. Overall, floristic composition of its natural habitats showed dissimilarities among phytodistricts. Three main vegetation types sheltered B. aethiopum : mixed grass and shrub savannas, savanna woodlands and woodlands, all of which were found in gallery forest landscapes. The density of B. aethiopum was lower in grass savannas but larger, shorter and distant individuals were found there than in savanna woodlands and woodlands. In the latter vegetation types, its density was high with thin, tall and closely spaced individuals. B. aethiopum tolerates mingling with several other tree species but increased mingling tends to lead to positive differentiation in diameter and height. Conclusions – Borassus aethiopum is a sun-demanding species and establishes successful populations in various ecological conditions. It could be mixed with other tree species in tree plantations and modern agroforestry systems as long as water requirements are met. However, it would be preferable that the species is associated with shade tolerant or medium sun-demanding species.

Seed trait-mediated selection by rodents affects mutualistic interactions and seedling recruitment of co-occurring tree species

As mutualists, seed dispersers may significantly affect mutualistic interactions and seedling recruitment of sympatric plants that share similar seed dispersers, but studies are rare. Here, we compared seed dispersal fitness in two co-occurring plant species (Armeniaca sibirica and Amygdalus davidiana) that inhabit warm temperate deciduous forest in northern China. We tested the hypothesis that seed trait-mediated selection by rodents may influence mutualistic interactions with rodents and then seedling establishment of co-occurring plant species. A.davidiana seeds are larger and harder (thick endocarps) than A.sibirica seeds, but they have similar levels of nutrients (crude fat, crude protein), caloric value and tannin. More A.sibirica seedlings are found in the field. Semi-natural enclosure tests indicated that the two seed species were both harvested by the same six rodent species, but that A.sibirica had mutualistic interactions (scatter hoarding) with four rodent species (Apodemus peninsulae, A. agrarius, Sciurotamias davidianus, Tamias sibiricus), and A.davidiana with only one (S. davidianus). Tagged seed dispersal experiments in the field indicated that more A.sibirica seeds were scatter-hoarded by rodents, and more A.sibirica seeds survived to the next spring and became seedlings. A.sibirica seeds derive more benefit from seed dispersal by rodents than A.davidiana seeds, particularly in years with limited seed dispersers, which well explained the higher seedling recruitment of A.sibirica compared with that of A.davidiana under natural conditions. Our results suggest that seed dispersers may play a significant role in seedling recruitment and indirect competition between co-occurring plant species.

Potential Shift in Plant Communities with Climate Change: Outcome on Litter Decomposition and Nutrient Release in a Mediterranean Oak Forest

The ongoing decline in biodiversity has fuelled concerns about its impact on ecosystem functioning. Mediterranean oak forests may prove very sensitive to global change, which could strongly influence the species composition of plant communities and thereby affect ecosystem processes. To determine the potential outcome of shifts in species composition on litter decomposition dynamics, we conducted a full-factorial decomposition experiment over a gradient of litter species diversity in a Mediterranean Downy oak (Quercus pubescens Willd.) forest. We used litter from the three dominant tree species naturally present in the Downy oak forest and litter from Aleppo pine (Pinus halepensis Mill.) in anticipation of its possible spread in the future with global change. Litter water holding capacity and N/P ratio were the most important and positive drivers of decomposition process. In contrast to other ecosystems where synergistic non-additive effects are prevalent, we observed 54% of additive and 46% of non-additive effects on litter mass loss in our Mediterranean ecosystem. These results could indicate less complementarity among decomposers for decomposition of diverse plant material in such a stressful climate. Moreover, dominant tree species are of key importance for nutrient availability, and the arrival of Aleppo pine would strongly reduce the N release during the decomposition process. Based on calculations of an annual partial nutrient budget at the ecosystem level, we showed that a shift in plant communities could affect nutrient release, ranging from 5 to 36% for N and 63 to 83% for P depending on the co-occurring tree species in the Downy oak forest.

Effects of precipitation regime and soil nitrogen on leaf traits in seasonally dry tropical forests of the Yucatan Peninsula, Mexico.

Leaf traits are closely associated with nutrient use by plants and can be utilized as a proxy for nutrient cycling processes. However, open questions remain, in particular regarding the variability of leaf traits within and across seasonally dry tropical forests. To address this, we considered six leaf traits (specific area, thickness, dry matter content, N content, P content and natural abundance (15)N) of four co-occurring tree species (two that are not associated with N2-fixing bacteria and two that are associated with N2-fixing bacteria) and net N mineralization rates and inorganic N concentrations along a precipitation gradient (537-1036 mm per year) in the Yucatan Peninsula, Mexico. Specifically we sought to test the hypothesis that leaf traits of dominant plant species shift along a precipitation gradient, but are affected by soil N cycling. Although variation among different species within each site explains some leaf trait variation, there is also a high level of variability across sites, suggesting that factors other than precipitation regime more strongly influence leaf traits. Principal component analyses indicated that across sites and tree species, covariation in leaf traits is an indicator of soil N availability. Patterns of natural abundance (15)N in foliage and foliage minus soil suggest that variation in precipitation regime drives a shift in plant N acquisition and the openness of the N cycle. Overall, our study shows that both plant species and site are important determinants of leaf traits, and that the leaf trait spectrum is correlated with soil N cycling.

High foliar nutrient concentrations and resorption efficiency in Embothrium coccineum (Proteaceae) in southern Chile.

Southern South American (SA) Proteaceae species growing in volcanic soils have been proposed as potential ecosystem engineers by tapping phosphorus (P) from soil through their cluster roots and shedding nutrient-rich litter to the soil, making it available for other species. We tested whether Embothrium coccineum (Proteaceae) has effectively lower P nutrient resorption efficiency and higher litter P concentrations than co-occurring, non-Proteaceae species. In southern Chile, we assessed the P and nitrogen (N) resorption efficiency of senescent leaves and fresh litter of E. coccineum and co-occurring tree species in a soil fertility and moisture gradient (600-3000 mm of annual precipitation) in Patagonia, Chile. We determined P and N concentrations, leaf mass per area (LMA), and ratios of N/P and C/N in mature and senescent leaf cohorts and fresh litter. Embothrium coccineum showed significantly higher P and N resorption efficiency than co-occurring species; in fact, E. coccineum fresh litter had the lowest P-content. While E. coccineum showed significantly lower fresh litter P concentrations than the rest of the species, it showed significantly higher N concentrations. Embothrium coccineum also had lower LMA and similar N/P and C/N ratios when compared with co-occurring tree species. We found that E. coccineum efficiently mobilized P and, to a lesser extent, N before leaf shedding. We did not find support for the ecosystem engineering hypothesis via shedding P-rich litter. We suggest that southern South American Proteaceae may be taking up other nutrients besides P, probably N, from the young, volcanic soils of this region.

Geographic range predicts photosynthetic and growth response to warming in co-occurring tree species

Populations may exhibit different sensitivity to climate change depending on where they occur in their climatic range. Experimental results now show that tree species growing nearest to their warm range limit exhibit reduced net photosynthesis and growth in response to warming, whereas those near their cold range limit show improved functioning.

Tree size and fecundity influence ballistic seed dispersal of two dominant mast-fruiting species in a tropical rain forest

Seed production, seed dispersal, and seedling recruitment are integral to forest dynamics, especially in masting species. Often these are studied separately, yet scarcely ever for species with ballistic dispersal even though this mode of dispersal is common in legume trees of tropical African rain forests. Here, we studied two dominant main-canopy tree species, Microberlinia bisulcata and Tetraberlinia bifoliolata (Caesalpinioideae), in 25ha of primary rain forest at Korup, Cameroon, during two successive masting events (2007/2010). In the vicinity of c. 100 and 130 trees of each species, 476/580 traps caught dispersed seeds and beneath their crowns c. 57,000 pod valves per species were inspected to estimate tree-level fecundity.Seed production of trees increased non-linearly and asymptotically with increasing stem diameters. It was unequal within the two species’ populations, and differed strongly between years to foster both spatial and temporal patchiness in seed rain. The M. bisulcata trees could begin seeding at 42–44cm diameter: at a much larger size than could T. bifoliolata (25cm). Nevertheless, per capita life-time reproductive capacity was c. five times greater in M. bisulcata than T. bifoliolata owing to former’s larger adult stature, lower mortality rate (despite a shorter life-time) and smaller seed mass.The two species displayed strong differences in their dispersal capabilities. Inverse modelling (IM) revealed that dispersal of M. bisulcata was best described by a lognormal kernel. Most seeds landed at 10–15m from stems, with 1% of them going beyond 80m (<100m). The direct estimates of fecundity significantly improved the models fitted. The lognormal also described well the seedling recruitment distribution of this species in 121 ground plots. By contrast, the lower intensity of masting and more limited dispersal of the heavier-seeded T. bifoliolata prevented reliable IM. For this species, seed density as function of distance to traps suggested a maximum dispersal distance of 40–50m, and a correspondingly more aggregated seedling recruitment pattern ensued than for M. bisulcata.From this integrated field study, we conclude that the reproductive traits of M. bisulcata give it a considerable advantage over T. bifoliolata by better dispersing more seeds per capita to reach more suitable establishment sites, and combined with other key traits they explain its local dominance in the forest. Understanding the linkages between size at onset of maturity, individual fecundity, and dispersal capability can better inform the life-history strategies, and hence management, of co-occurring tree species in tropical forests.

Radial growth of two dominant montane conifer tree species in response to climate change in North-Central China.

North-Central China is a region in which the air temperature has clearly increased for several decades. Picea meyeri and Larix principis-rupprechtii are the most dominant co-occurring tree species within the cold coniferous forest belt ranging vertically from 1800 m to 2800 m a.s.l. in this region. Based on a tree-ring analysis of 292 increment cores sampled from 146 trees at different elevations, this study aimed to examine if the radial growth of the two species in response to climate is similar, whether the responses are consistent along altitudinal gradients and which species might be favored in the future driven by the changing climate. The results indicated the following: (1) The two species grew in different rhythms at low and high elevation respectively; (2) Both species displayed inconsistent relationships between radial growth and climate data along altitudinal gradients. The correlation between radial growth and the monthly mean temperature in the spring or summer changed from negative at low elevation into positive at high elevation, whereas those between the radial growth and the total monthly precipitation displayed a change from positive into negative along the elevation gradient. These indicate the different influences of the horizontal climate and vertical mountainous climate on the radial growth of the two species; (3) The species-dependent different response to climate in radial growth appeared mainly in autumn of the previous year. The radial growth of L. principis-rupprechtii displayed negative responses both to temperature and to precipitation in the previous September, October or November, which was not observed in the radial growth of P. meyeri. (4) The radial growth of both species will tend to be increased at high elevation and limited at low elevation, and L. principis-rupprechtii might be more favored in the future, if the temperature keeps rising.

Patterns of Forest Phylogenetic Community Structure across the United States and Their Possible Forest Health Implications

The analysis of phylogenetic relationships among co-occurring tree species offers insights into the ecological organization of forest communities from an evolutionary perspective and, when employed regionally across thousands of plots, can assist in forest health assessment. Phylogenetic clustering of species, when species are more closely related than expected by chance, suggests a process of evolutionary niche conservatism. Because such communities share much evolutionary history and an affinity for similar environmental conditions, they may be particularly susceptible to threats such as insects and diseases and shifting climate conditions. Meanwhile, a pattern of phylogenetic evenness, in which the species are less closely related than by chance, may indicate competitive exclusion or interspecies facilitation. The ecological integrity of such communities may be less at risk because they may encompass a wider variety of evolutionary adaptations. Using a network of more than 100,000 forest inventory plots across the conterminous United States, we tested whether community phylogenetic structure was significantly clustered or even at multiple scales. Clustering predominated across most of the study area, indicating the widespread significance of evolutionary niche conservatism, except in areas of the west. Phylogenetic structure varied along environmental gradients, suggesting that clustering predominates in more favorable locations and evenness predominates in areas with harsher environments. These results have implications for broad-scale forest health monitoring.

Water-use advantage for lianas over trees in tropical seasonal forests.

Lianas exhibit peak abundance in tropical forests with strong seasonal droughts, the eco-physiological mechanisms associated with lianas coping with water deficits are poorly understood. We examined soil water partitioning, sap flow, and canopy eco-physiological properties for 99 individuals of 15 liana and 34 co-occurring tree species in three tropical forests that differed in soil water availability. In the dry season, lianas used a higher proportion of deep soil water in the karst forest (KF; an area with severe seasonal soil water deficit (SSWD)) and in the tropical seasonal forest (TSF, moderate SSWD), permitting them to maintain a comparable leaf water status than trees in the TSF or a better status than trees in the KF. Lianas exhibited strong stomatal control to maximize carbon fixation while minimizing dry season water loss. During the dry period, lianas significantly decreased water consumption in the TSF and the KF. Additionally, lianas had a much higher maximum photosynthetic rates and sap flux density in the wet season and a lower proportional decline in photosynthesis in the dry season compared with those of trees. Our results indicated that access to deep soil water and strong physiological adjustments in the dry season together with active wet-season photosynthesis may explain the high abundance of lianas in seasonally dry forests.

Floodplain ecohydrology: Climatic, anthropogenic, and local physical controls on partitioning of water sources to riparian trees.

Seasonal and annual partitioning of water within river floodplains has important implications for ecohydrologic links between the water cycle and tree growth. Climatic and hydrologic shifts alter water distribution between floodplain storage reservoirs (e.g., vadose, phreatic), affecting water availability to tree roots. Water partitioning is also dependent on the physical conditions that control tree rooting depth (e.g., gravel layers that impede root growth), the sources of contributing water, the rate of water drainage, and water residence times within particular storage reservoirs. We employ instrumental climate records alongside oxygen isotopes within tree rings and regional source waters, as well as topographic data and soil depth measurements, to infer the water sources used over several decades by two co-occurring tree species within a riparian floodplain along the Rhône River in France. We find that water partitioning to riparian trees is influenced by annual (wet versus dry years) and seasonal (spring snowmelt versus spring rainfall) fluctuations in climate. This influence depends strongly on local (tree level) conditions including floodplain surface elevation and subsurface gravel layer elevation. The latter represents the upper limit of the phreatic zone and therefore controls access to shallow groundwater. The difference between them, the thickness of the vadose zone, controls total soil moisture retention capacity. These factors thus modulate the climatic influence on tree ring isotopes. Additionally, we identified growth signatures and tree ring isotope changes associated with recent restoration of minimum streamflows in the Rhône, which made new phreatic water sources available to some trees in otherwise dry years.Key PointsWater shifts due to climatic fluctuations between floodplain storage reservoirsAnthropogenic changes to hydrology directly impact water available to treesEcohydrologic approaches to integration of hydrology afford new possibilities

A comparative study of physiological and morphological seedling traits associated with shade tolerance in introduced red oak (Quercus rubra) and native hardwood tree species in southwestern Germany

Northern red oak (Quercus rubra L.), a moderately shade-tolerant tree species, is failing to regenerate throughout its native North American range, while successful recruitment in Central Europe has been observed since its introduction. To examine whether comparative photosynthetic performance could explain the regeneration success of this non-native species in Central Europe, we compared the physiological and morphological seedling traits of red oak with three co-occurring tree species under three canopy types in southwestern Germany. Native species included a moderately shade-tolerant native oak (Quercus robur L.) and two shade-tolerant species (Acer pseudoplatanus L. and Carpinus betulus L.). The photosynthetic traits of non-native red oak seedlings were similar to those reported for this species in the native range, where shade-tolerant competitors readily outperform red oak under low light conditions. However, compared with native shade-tolerant species in Europe, red oak seedlings photosynthesized efficiently, especially under closed canopies and in small canopy gaps, exhibiting high photosynthetic capacity, low leaf dark respiration and leaf-level light compensation points that were similar to the more shade-tolerant species. The superior net carbon gain of red oak seedlings at low and moderate light levels was likely facilitated by high leaf areas and reflected by seedling dry masses that were greater than the observed native European species. A competitive advantage for red oak was not evident because relative height growth was inferior to seedlings of co-occurring species. In North America, the inability of seedlings to compete with shade-tolerant tree species in deeply shaded understories is central to the problem of poor oak recruitment. Our study suggests that the ability of non-native red oak to perform equally well to native shade-tolerant species under a variety of light conditions could contribute to the consistent success of red oak regeneration in Europe.

A taxonomic comparison of local habitat niches of tropical trees

The integration of ecology and evolutionary biology requires an understanding of the evolutionary lability in species' ecological niches. For tropical trees, specialization for particular soil resource and topographic conditions is an important part of the habitat niche, influencing the distributions of individual species and overall tree community structure at the local scale. However, little is known about how these habitat niches are related to the evolutionary history of species. We assessed the relationship between taxonomic rank and tree species' soil resource and topographic niches in eight large (24-50 ha) tropical forest dynamics plots. Niche overlap values, indicating the similarity of two species' distributions along soil or topographic axes, were calculated for all pairwise combinations of co-occurring tree species at each study site. Congeneric species pairs often showed greater niche overlap (i.e., more similar niches) than non-congeneric pairs along both soil and topographic axes, though significant effects were found for only five sites based on Mantel tests. No evidence for taxonomic effects was found at the family level. Our results indicate that local habitat niches of trees exhibit varying degrees of phylogenetic signal at different sites, which may have important ramifications for the phylogenetic structure of these communities.