Coexistence Of Tree Species Research Articles

Heterogeneity in soil water and light environments and dispersal limitation: what facilitates tree species coexistence in a temperate forest?

In the present study, we analysed the habitat association of tree species in an old-growth temperate forest across all life stages to test theories on the coexistence of tree species in forest communities. An inventory for trees was implemented at a 6-ha plot in Ogawa Forest Reserve for adults, juveniles, saplings and seedlings. Volumetric soil water content (SMC) and light levels were measured in 10-m grids. Relationships between the actual number of stems and environmental variables were determined for 35 major tree species, and the spatial correlations within and among species were analysed. The light level had no statistically significant effect on distribution of saplings and seedlings of any species. In contrast, most species had specific optimal values along the SMC gradient. The optimal values were almost identical in earlier life stages, but were more variable in later life stages among species. However, no effective niche partitioning among the species was apparent even at the adult stage. Furthermore, results of spatial analyses suggest that dispersal limitation was not sufficient to mitigate competition between species. This might result from well-scattered seed distribution via wind and bird dispersal, as well as conspecific density-dependent mortality of seeds and seedlings. Thus, both niche partitioning and dispersal limitation appeared less important for facilitating coexistence of species within this forest than expected in tropical forests. The tree species assembly in this temperate forest might be controlled through a neutral process at the spatial scale tested in this study.

Seed shadow of Swietenia macrophylla remnant trees in a Mexican rainforest: implications for forest management

The understanding of processes affecting the regeneration and coexistence of tree species is of high concern in tropical landscapes disturbed by anthropogenic activities. In this study, we evaluated the seed shadow of eight small remnant Swietenia macrophylla trees to determine the possible consequences of selective logging on the first stages of natural regeneration. We expected to find a restricted dispersal ability and a marked loss of seeds due to biotic interactions. To test this, seed shadow per tree was quanti?ed along four radial transects (30 × 1 m) that provided 960 - 1m2 squares for sampling. The wind direction and some tree characteristics as diameter at breast height and crown projection area were also considered as factors in?uencing seed shadow. We also scored the number of seeds predated or infested by fungi. In total, 540 seeds were found along transects. Around 86.1% of the 960 squares presented 0 or 1 seed, whereas 80% of the seeds were found within the first 20 m from the parent tree. The number of seeds lost through biotic interactions regressed on the distance to the parent tree ?tted a quadratic model that explained 52.4% of the total variance. These results suggest that small remnant S. macrophylla trees display low dispersal ability caused by a restricted seed shadow and a high rate of seed damaged produced by biotic interactions. Therefore, removal of largest trees for trading could be negatively impacting the first stages of the natural regeneration of this high value species. Consequently, harvesting could threat the local permanence of S. macrophylla in these managed forests of the Mayan zone.

Impact of interspecific interactions on the soil water uptake depth in a young temperate mixed species plantation

• We studied whether differences in water uptake exists among tree species in a mixed plantation. • We tested the influence of species interactions on the water extraction depth. • Complementarity for water extraction was found between functional groups. • Species interactions did not clearly influence soil water extraction depth. • Complementarity could facilitate the coexistence of tree species in mature forests. Interactions between tree species in forests can be beneficial to ecosystem functions and services related to the carbon and water cycles by improving for example transpiration and productivity. However, little is known on below- and above-ground processes leading to these positive effects. We tested whether stratification in soil water uptake depth occurred between four tree species in a 10-year-old temperate mixed species plantation during a dry summer. We selected dominant and co-dominant trees of European beech, Sessile oak, Douglas fir and Norway spruce in areas with varying species diversity, competition intensity, and where different plant functional types (broadleaf vs. conifer) were present. We applied a deuterium labelling approach that consisted of spraying labelled water to the soil surface to create a strong vertical gradient of the deuterium isotope composition in the soil water. The deuterium isotope composition of both the xylem sap and the soil water was measured before labelling, and then again three days after labelling, to estimate the soil water uptake depth using a simple modelling approach. We also sampled leaves and needles from selected trees to measure their carbon isotope composition (a proxy for water use efficiency) and total nitrogen content. At the end of the summer, we found differences in the soil water uptake depth between plant functional types but not within types: on average, coniferous species extracted water from deeper layers than did broadleaved species. Neither species diversity nor competition intensity had a detectable influence on soil water uptake depth, foliar water use efficiency or foliar nitrogen concentration in the species studied. However, when coexisting with an increasing proportion of conifers, beech extracted water from progressively deeper soil layers. We conclude that complementarity for water uptake could occur in this 10-year-old plantation because of inherent differences among functional groups (conifers and broadleaves). Furthermore, water uptake depth of beech was already influenced at this young development stage by interspecific interactions whereas no clear niche differentiation occurred for the other species. This finding does not preclude that plasticity-mediated responses to species interactions could increase as the plantation ages, leading to the coexistence of these species in adult forest stands.

Variability in potential to exploit different soil organic phosphorus compounds among tropical montane tree species

Summary We hypothesized that tropical plant species with different mycorrhizal associations reduce competition for soil phosphorus (P) by specializing to exploit different soil organic P compounds. We assayed the activity of root/mycorrhizal phosphatase enzymes of four tree species with contrasting root symbiotic relationships – arbuscular mycorrhizal (AM) (angiosperm and conifer), ectomycorrhizal (EM) and non‐mycorrhizal – collected from one of three soil sites within a montane tropical forest. We also measured growth and foliar P of these seedlings in an experiment with P provided exclusively as inorganic orthophosphate, a simple phosphomonoester (glucose phosphate), a phosphodiester (RNA), phytate (the sodium salt of myo‐inositol hexakisphosphate) or a no‐P control. The EM tree species expressed twice the phosphomonoesterase activity as the AM tree species, but had similar phosphodiesterase activity. The non‐mycorrhizal Proteaceae tree had markedly greater activity of both enzymes than the mycorrhizal tree species, with root clusters expressing greater phosphomonoesterase activity than fine roots. Both the mycorrhizal and non‐mycorrhizal tree species contained significantly greater foliar P than in no‐P controls when limited to inorganic phosphate, glucose phosphate and RNA. The EM species did not perform better than the AM tree species when limited to organic P in any form. In contrast, the non‐mycorrhizal Proteaceae tree was the only species capable of exploiting phytate, with nearly three times the leaf area and more than twice the foliar P of the no‐P control. Our results suggest that AM and EM tree species exploit similar forms of P, despite differences in phosphomonoesterase activity. In contrast, the mycorrhizal tree species and non‐mycorrhizal Proteaceae appear to differ in their ability to exploit phytate. We conclude that resource partitioning of soil P plays a coarse but potentially ecologically important role in fostering the coexistence of tree species in tropical montane forests.

Regeneration of Abies pinsapo within gaps created by Heterobasidion annosum-induced tree mortality in southern Spain

We explored the effects of tree-fall gaps caused by Heterobasidion annosum-induced tree death on the regeneration of the relict Abies pinsapo in southern Spain. We analyzed the structure and regeneration of A. pinsapo in 44 circular closed-canopy plots and 11 gaps (mean size 75 m2) located in H. annosum infection foci. We measured adult density, basal area, and height of all tree species with a diameter at breast height greater than 7.5 cm and quantified regeneration abundance. Diameter structures were analyzed using Weibull functions. A. pinsapo comprised 49-81% of total adult tree density (358-1418 trees ha-1) and total basal area (15.70-39.22 m2 ha-1). Density of A. pinsapo juveniles ranged from 129 to 416 individuals ha-1, while that of juveniles of other tree species such as Quercus ilex was much lower. A. pinsapo saplings and seedlings were more abundant within gaps than in closed-canopy sites. A. pinsapo regeneration was related to the tree species composition of the stands adjacent to gaps. The main potential species replacing A. pinsapo in H. annosum-induced gaps were, in decreasing order of importance, Quercus ilex, Q. faginea, and Juniperus oxycedrus. The current stand structure and gap-infilling processes suggest that A. pinsapo regenerates adequately within H. annosum-induced gaps. However, the relative proportion of other coexisting tree species within H. annosum-related gaps will increase substantially over time as compared with closed-canopy areas.

Complementarity in the use of nitrogen forms in a temperate broad-leaved mixed forest

Background: The complementary use of different forms of soil nitrogen (N) might lead to a higher productivity of mixed forests than monocultures, but convincing evidence for temperate mixed forests is scarce.Aims: We searched for species differences in N uptake rates and the preference for NH4+, NO3− or glycine among five temperate broad−leaved tree species (Acer pseudoplatanus, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Tilia cordata) in a mature mixed stand.Methods: 15N tracer was added to the soil and its accumulation in fine root biomass was analysed after 10 min, 1 h and 1 d.Results: The estimated root uptake rates of the species were in the range of 5–46 µg N g−1 root h−1 for NH4+, 6–86 µg N g−1 h−1 for NO3− and 4–29 µg N g−1 h−1 for glycine during the first hour after tracer application. Carpinus, Tilia and Acer tended to prefer NH4+ over NO3−, while Fraxinus showed equal preference for both N forms and Fagus seemed to prefer NO3−.Conclusions: The five co-existing tree species differed in uptake rates and partly in their N form preference, but complementarity in the use of different N forms seems to be of minor importance in this forest because tree species appear to be rather flexible in their N form use.

Daily and seasonal courses of gas exchange and niche partitioning among coexisting tree species in a tropical montane forest

We investigated the seasonal gas exchange patterns of three different functional types of tropical afromontane trees, namely Podocarpus falcatus (Thunb.) Mirb. (evergreen gymnosperm), Prunus africana (Hook. f.) Kalkm. (broad-leaf evergreen), and Croton macrostachyus Hochst. ex Del. (broad-leaf deciduous) which grow side-by-side in the Munessa forest, southern Central Ethiopia. The hypothesis is that the trees can make different use of the environmental conditions which change seasonally and also from year to year. These changes can be understood as fluctuating niches, the utilization of which allows coexistence through balanced carbon gain. In this study, light and moisture were considered the two main fluctuating niches. Porometry was used to measure the daily and seasonal responses of the leaves to environmental variables under dry and wet season conditions in the course of the climatically differing years 2009 and 2010. Correlations of the patterns of these responses were analyzed by regression analysis. From daily integrated rates of irradiation, photosynthesis and transpiration, light and water use efficiencies of photosynthetic carbon gain and their mutual interdependences were determined. Except an extended dry season in 2009 when C. macrostachyus had shed its foliage, all three species continued photosynthetic net CO2 uptake concomitant with transpiration at all seasons, although at varying rates. Ecophysiological performance of P. falcatus leaves is mainly light-driven and responds relatively little to a change in moisture conditions. Its carbon and water relations are striving for stability rather than flexibility. As of a typically subdominant species of the forest, P. africana leaves can efficiently use low light intensities but suffer from photoinhibition at full light. Their performance, showing more dynamic response to the environment than P. falcatus, P. africana appears driven more by moisture than by light. Compared to the two evergreens C. macrostachyus exhibited the highest flexibility in its leaves’ physiological responses to environmental conditions, in particular to the light climate which is additionally potentiated by the fast turnover of its foliage. This species optimizes its carbon gain during the wet season and during the early dry period when cloud cover is minimal. Our findings thus reveal that elasticity of the response to fluctuating environmental conditions is an additional aspect in the assessment of the utilization of temporally fluctuating niches by adult tree individuals.

Response of tree growth and species coexistence to density and species evenness in a young forest plantation with two competing species

There is considerable evidence for the presence of positive species diversity-productivity relationships in plant populations, but the population parameters determining the type and strength of the relationship are poorly defined. Relationships between species evenness and tree survival or species coexistence are not well established. The objective of this study was to quantify the joint effects of density and species evenness on tree productivity and species coexistence. A 12-year-old experimental tree plantation mixing two species according to a double gradient of density and species proportion was used. A neighbourhood approach was employed and descriptors of local competition were used to model individual tree growth. Fagus sylvatica and Acer pseudoplatanus were used as model species, as they can be considered as ecologically equivalent in their young stages. Density and tree size were primary factors determining individual growth and stand productivity. Species identity had a significant, but less pronounced, role. Stand productivity was highest when species evenness was close to 1 and slightly lower in uneven mixtures. The reduction in stand productivity when species evenness decreased was of similar magnitude irrespective of which species became dominant, indicating symmetric effects for the two species. When examining individual tree growth in response to species proportion for each species separately, it was observed for both species that individual trees exhibited greater growth in uneven mixtures in which the other species was more frequent. The results suggest that mixtures of these two functionally similar species have the highest production at maximum evenness, indicating a complementary effect between them. The presence of a mixture combines both stabilizing mechanisms (individuals from both species show higher growth when surrounded by individuals from the other species) and equalizing mechanisms (the two species have very similar growth curves) that, in turn, determine the species' relative dominance. These processes should act to ensure the long-term coexistence of species.

Climatic turning point for beech and oak under climate change in Central Europe

The growth behavior of coexisting tree species under climate change is important from an ecological, silvicultural and economic perspective. While many previous studies are concerned with climatic limits for species occurrence, we focus on climate related shifts in interspecific competition. A landmark for these changes in competition is the ‘climatic turning point' (CTP): those climate conditions under which a rank reversal between key tree species occurs. Here, we used a common type of temperate mixed forest in Central Europe with European beech (Fagus sylvaticaL.) and sessile oak (Quercus petraea(Matt.) Liebl.) to explore the CTP under a future climate projection of increasing temperature and aridity. We selected a dry region where the prerequisite of differential climate sensitivity in mixed beech‐oak forests was fulfilled: In‐situ dendrochronological analyses demonstrated that the currently more competitive beech was more drought sensitive than sessile oak. We then used two complementary forest growth models, namely SILVA and LandClim, to investigate the climate induced rank‐reversal in species dominance and to quantify it as the CTP from beech to oak by simulating future forest development from the WETTREG 2010 A1B climate projection. Utilizing two models allowed us to draw conclusions robust against the assumptions of a particular model. Both models projected a CTP at a mean annual temperature of 11–12°C (July temperature >18°C) and a precipitation sum of 500–530 mm. However, the change in tree species composition can exhibit a time‐lag of several decades depending on past stand development and current stand structure. We conclude that the climatic turning point is a simple yet effective reference measure to study climate related changes in interspecific competition, and confirm the importance of competition sensitivity in climate change modeling.

Trade‐off between light interception efficiency and light use efficiency: implications for species coexistence in one‐sided light competition

Summary Taller plant species can pre‐empt solar energy and suppress growth of subordinate species in vegetation stands, which is described through one‐sided competition. Yet, in much of the world's vegetation species of different statures coexist. This study aims to clarify the mechanisms underlying this apparent paradox. We quantified how co‐occurring species and individuals intercepted and used light for growth in a mature, warm‐temperate evergreen forest. This was performed by determining the 3D distribution of foliage and light with a ground‐based lidar system in combination with nondestructive measurements of plant growth. Taller trees intercepted light more efficiently per unit of above‐ground biomass than shorter trees did (=higher light interception efficiency, LIE). However, taller trees tended to have lower biomass production per unit light interception (=lower light use efficiency, LUE). Reduced LUE in taller trees was associated with their higher biomass allocation to nonphotosynthetic organs and probably with over‐saturated light intensity for photosynthesis at high canopy positions. Due to the increased LIE and decreased LUE with tree heights, a trade‐off between LIE and LUE was found, and this trade‐off resulted in trees of different statures having similar relative growth rates. Synthesis. Light competition drives trees to grow taller, and the light interception efficiency is higher in taller trees; however, this benefit comes at a cost of decreased efficiency of light use for growth. This trade‐off allows trees of different statures to grow at proportionally comparable rates and may promote coexistence of tree species in one‐sided light competition.

Differential growth responses to water balance of coexisting deciduous tree species are linked to wood density in a Bolivian tropical dry forest.

A seasonal period of water deficit characterizes tropical dry forests (TDFs). There, sympatric tree species exhibit a diversity of growth rates, functional traits, and responses to drought, suggesting that each species may possess different strategies to grow under different conditions of water availability. The evaluation of the long-term growth responses to changes in the soil water balance should provide an understanding of how and when coexisting tree species respond to water deficit in TDFs. Furthermore, such differential growth responses may be linked to functional traits related to water storage and conductance. We used dendrochronology and climate data to retrospectively assess how the radial growth of seven coexisting deciduous tree species responded to the seasonal soil water balance in a Bolivian TDF. Linear mixed-effects models were used to quantify the relationships between basal area increment and seasonal water balance. We related these relationships with wood density and sapwood production to assess if they affect the growth responses to climate. The growth of all species responded positively to water balance during the wet season, but such responses differed among species as a function of their wood density. For instance, species with a strong growth response to water availability averaged a low wood density which may facilitate the storage of water in the stem. By contrast, species with very dense wood were those whose growth was less sensitive to water availability. Coexisting tree species thus show differential growth responses to changes in soil water balance during the wet season. Our findings also provide a link between wood density, a trait related to the ability of trees to store water in the stem, and wood formation in response to water availability.

Herbivores differentially limit the seedling growth and sapling recruitment of two dominant rain forest trees

Resource heterogeneity may influence how plants are attacked and respond to consumers in multiple ways. Perhaps a better understanding of how this interaction might limit sapling recruitment in tree populations may be achieved by examining species' functional responses to herbivores on a continuum of resource availability. Here, we experimentally reduced herbivore pressure on newly established seedlings of two dominant masting trees in 40 canopy gaps, across c. 80 ha of tropical rain forest in central Africa (Korup, Cameroon). Mesh cages were built to protect individual seedlings, and their leaf production and changes in height were followed for 22 months. With more light, herbivores increasingly prevented the less shade-tolerant Microberlinia bisulcata from growing as tall as it could and producing more leaves, indicating an undercompensation. The more shade-tolerant Tetraberlinia bifoliolata was much less affected by herbivores, showing instead near to full compensation for leaf numbers, and a negligible to weak impact of herbivores on its height growth. A stage-matrix model that compared control and caged populations lent evidence for a stronger impact of herbivores on the long-term population dynamics of M. bisulcata than T. bifoliolata. Our results suggest that insect herbivores can contribute to the local coexistence of two abundant tree species at Korup by disproportionately suppressing sapling recruitment of the faster-growing dominant via undercompensation across the light gradient created by canopy disturbances. The functional patterns we have documented here are consistent with current theory, and, because gap formations are integral to forest regeneration, they may be more widely applicable in other tropical forest communities. If so, the interaction between life-history and herbivore impact across light gradients may play a substantial role in tree species coexistence.

Light and competition gradients fail to explain the coexistence of shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings.

The coexistence of forest tree species has often been linked to differences among species in terms of their response to light availability during the regeneration stage. From this perspective, species coexistence results from growth-growth or mortality-growth trade-offs along spatial light gradients. Experimental evidence of growth-growth trade-offs in natural conditions is sparse due to various confounding factors that potentially hinder the relationship. This study examined growth hierarchies along light gradients between two tree species with contrasting shade tolerance by controlling potential confounding factors such as seedling size, seedling status, seedling density and species composition. Natural regenerated shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings were used, and growth rankings over a 4-year period were compared in 8- to 10-year-old tree seedlings. No rank reversal occurs between the two species along the light gradient, or along the density, mixture or seedling size gradients. The shade-tolerant species was always the more competitive of the two. Pronounced effects of initial size on seedling growth were observed, whereas the effects of light and competition by neighbours were of secondary importance. The paramount effect of size, which results from the asymmetric nature of interseedling competition, gives a strong advantage to tall seedlings over the long term. This study extends previous efforts to identify potential drivers of rank reversals in young tree mixtures. It does not support the classical assumption that spatial heterogeneity in canopy opening explains the coexistence of the two species studied. It suggests that spatial variation in local size hierarchies among seedlings that may be caused by seedling emergence time or seedling initial performance is the main driver of the dynamics of these mixed stands.

Growth and carbon isotopes of Mediterranean trees reveal contrasting responses to increased carbon dioxide and drought

Forest dynamics will depend upon the physiological performance of individual tree species under more stressful conditions caused by climate change. In order to compare the idiosyncratic responses of Mediterranean tree species (Quercus faginea, Pinus nigra, Juniperus thurifera) coexisting in forests of central Spain, we evaluated the temporal changes in secondary growth (basal area increment; BAI) and intrinsic water-use efficiency (iWUE) during the last four decades, determined how coexisting species are responding to increases in atmospheric CO2 concentrations (C(a)) and drought stress, and assessed the relationship among iWUE and growth during climatically contrasting years. All species increased their iWUE (ca. +15 to +21%) between the 1970s and the 2000s. This increase was positively related to C(a) for J. thurifera and to higher C(a) and drought for Q. faginea and P. nigra. During climatically favourable years the study species either increased or maintained their growth at rising iWUE, suggesting a higher CO2 uptake. However, during unfavourable climatic years Q. faginea and especially P. nigra showed sharp declines in growth at enhanced iWUE, likely caused by a reduced stomatal conductance to save water under stressful dry conditions. In contrast, J. thurifera showed enhanced growth also during unfavourable years at increased iWUE, denoting a beneficial effect of C(a) even under climatically harsh conditions. Our results reveal significant inter-specific differences in growth driven by alternative physiological responses to increasing drought stress. Thus, forest composition in the Mediterranean region might be altered due to contrasting capacities of coexisting tree species to withstand increasingly stressful conditions.

Different light acclimation strategies of two coexisting tree species seedlings in a temperate secondary forest along five natural light levels

To understand how temperate coexisting tree species acclimate to natural light levels, seedlings of two coexisting species, Pinus koraiensis Sieb. et Zucc. and Quercus mongolica Fisch., were transplanted to five light levels (100%, 20%, 10%, 5%, and 1%) under natural forest stands. The survival/growth rates, nonstructural carbohydrate (NSC) concentrations and pools in each plant organ of the two species seedlings were monitored in two consecutive years. The survival rate and relative growth rate (RGR) of both species peaked at the 100% light level. No seedlings of both species survived at the 1% light level. The NSC concentration varied insignificantly with light levels in the roots of Q. mongolica and the stems of both species in 2010, while the NSC concentration and pool decreased with light reduction for both species in 2011. With a light level decrease from 20% to 5%, Q. mongolica showed insignificant differences, but P. koraiensis exhibited a significant decrease in survival rate and NSC levels. The survival rates of P. koraiensis and Q. mongolica seedlings were positively and significantly related to the NSC concentration in leaves (r1=0.9898) and NSC concentration in roots (r2=0.9976), respectively. These results suggested that: (1) full sunlight was the optimum light environment for the initial survival and growth of both tree species; (2) low light induced carbohydrate deficiency, which might be the key factor resulting in seedling mortality in forest understory; (3) different NSC allocation patterns in response to light alteration might promote the coexistence of P. koraiensis and Q. mongolica in the early stage of regeneration in the forest stands.

Impact de la structure spatiale de Strombosia scheffleri Engl. et Xymalos monospora (Harv.) Baill. sur la régénération naturelle et la coexistence des espèces arborescentes dans la réserve naturelle forestière de Bururi, Burundi

Dans le but d'analyser l'influence de la structure spatiale sur le fonctionnement écologique des écosystèmes forestiers, une étude sur la distribution spatiale, la régénération naturelle et la coexistence des principales espèces arborescentes a été menée dans la réserve naturelle forestière de Bururi au Burundi. Cette aire protégée constitue la partie la plus méridionale du système de forêts de la crête Congo-Nil. La présente étude cherche à tester l'hypothèse selon laquelle la structure spatiale agrégée d'une espèce arborescente aurait un impact sur la régénération naturelle et la coexistence avec des espèces arborescentes du même habitat. Deux méthodes, l'une basée sur la distance au plus proche voisin et l'autre sur la variation du nombre d'individus de l'espèce dans les placettes échantillonnées, ont été appliquées pour déterminer le type de distribution spatiale des espèces arborescentes. Leur régénération naturelle a été évaluée sur la base de la distribution de leurs diamètres. L'analyse de la distribution spatiale a montré que Strombosia scheffleri et Xymalos monospora sont caractérisées par une distribution spatiale agrégée. La présente étude a ensuite mis en évidence l'impact négatif de la distribution agrégée de S. scheffleri et X. monospora sur la régénération naturelle de Entandrophragma excelsum et Prunus africana, deux espèces menacées de disparition au Burundi. Ainsi, la dominance locale d'une espèce arborescente peut réduire la diversité comme le stipule l'hypothèse d'échappement.

Revisiting the fate of buds: size and position drive bud mortality and bursting in two coexisting Mediterranean Quercus species with contrasting leaf habit

Understanding the relationships between bud size and position and bud fate through time is crucial for identifying and subsequently modeling the mechanisms underlying tree architecture. However, there is a lack of information on how bud size drives crown architectural patterns in coexisting tree species. We studied bud demography in two coexisting Mediterranean oak species with contrasting leaf habit (Quercus ilex, evergreen; Q. faginea, deciduous). The main objective was to analyse the effect of bud size on the fate of buds with different positions along the shoot (apical, leaf axillary and scale-cataphyll axillary buds). The number, length and position of all buds and stems were recorded in marked branches during 4 years. Study species presented different strategies in bud production and lifespan. The evergreen species showed greater mortality rate than the deciduous one, which produced larger buds. Bud size and position were highly related since apical buds where longer than axillary ones and bud length declined basipetally along the stem. Apical buds had also higher chances of bursting than axillary ones. Within positions, longer buds presented a higher probability of bursting than shorter ones, although no absolute size threshold was found below which bud bursting was impaired. In Q. ilex, four-year-old buds were still viable and able to burst, whereas in Q. faginea practically all buds burst in their first year or died soon after. Such different bud longevities may indicate contrasting strategies in primary growth between both species. Q. ilex is able to accumulate viable buds for several ages, whereas Q. faginea seems to rely on the production of large current-year buds with high bursting probability under favourable environmental conditions.

Variation of first‐order root traits across climatic gradients and evolutionary trends in geological time

AbstractAimPlant roots are crucial for water and nutrient absorption, but large‐scale patterns and underlying mechanisms of root trait variation and evolution are poorly understood. Here we quantify the degree of variation in functional traits for the first‐order roots across large geographical scales and examine the potential mechanisms underlying these patterns.LocationChina.MethodsWe collected first‐order roots (stream‐based ordering system) and leaf samples of 65 tree species in six forests from subtropical to temperate zones and determined the key morphological, architectural and chemical traits.ResultsWe found wider variation in root morphology in the subtropical zone, where species with very thick and very thin first‐order roots coexisted, in contrast with narrower variation among species in the temperate zone. Inconsistent with the predictions of trait economics spectrum, root nitrogen concentration was uncorrelated with root morphology. Furthermore, average root diameter at the plant family level decreased markedly with their divergence time in both piecewise and phylogenetic independent contrast regression analyses.Main conclusionsHigher variation in root morphology in the subtropical zone appears to result from the coexistence of tree species with thick and thin roots, probably because of a more favourable water supply. Patterns of root evolution towards thinner roots were driven by both phylogeny and possible adaptation of newly diverged species to drier habitats from mid to late Cretaceous. Our findings reveal contrasting selection pressures at the root and leaf level in different climatic zones during plant evolution, and suggest that a single vector of variation linking morphology to resource acquisition or life span (‘trait economics’) remains elusive in roots.

The need for a canopy perspective to understand the importance of phenotypic plasticity for promoting species coexistence and light‐use complementarity in forest ecosystems

AbstractBecause of their overwhelming size over other organisms, trees define the structural and energetic properties of forest ecosystems. From grasslands to forests, leaf area index, which determines the amount of light energy intercepted for photosynthesis, increases with increasing canopy height across the various terrestrial ecosystems of the world. In vertically well‐developed forests, niche differentiation along the vertical gradient of light availability may promote species coexistence. In addition, spatial and temporal differentiation of photosynthetic traits among the coexisting tree species (functional diversity) may promote complementary use of light energy, resulting in higher biomass and productivity in multi‐species forests. Trees have evolved retaining high phenotypic plasticity because the spatial/temporal distribution of resources in forest ecosystems is highly heterogeneous and trees modify their own environment as they increase nearly 1,000 times in size through ontogeny. High phenotypic plasticity may enable coexistence of tree species through divergence in resource‐rich environments, as well as through convergence in resource‐limited environments. We propose that the breadth of individual‐level phenotypic plasticity, expressed at the metamer level (leaves and shoots), is an important factor that promotes species coexistence and resource‐use complementarity in forest ecosystems. A cross‐biome comparison of the link between plasticity of photosynthesis‐related traits and stand productivity will provide a functional explanation for the relationship between species assemblages and productivity of forest ecosystems.

Janzen‐Connell effects in a broadcast‐spawning Caribbean coral: distance‐dependent survival of larvae and settlers

The Janzen-Connell hypothesis states that host-specific biotic enemies (pathogens and predators) promote the coexistence of tree species in tropical forests by causing distance- or density-dependent mortality of seeds and seedlings. Although coral reefs are the aquatic analogues of tropical forests, the Janzen-Connell model has never been proposed as an explanation for high diversity in these ecosystems. We tested the central predictions of the Janzen-Connell model in a coral reef, using swimming larvae and settled polyps of the common Caribbean coral Montastraea faveolata. In a field experiment to test for distance- or density-dependent mortality, coral settler mortality was higher and more strongly density dependent in locations down-current from adult corals. Survival did not increase monotoilically with distance, however, revealing the influence of fluid dynamics around adult corals in structuring spatial patterns of mortality. Complementary microbial profiles around adult coral heads revealed that one potential cause of settler mortality, marine microbial communities, are structured at the same spatial scale. In a field experiment to test whether factors causing juvenile mortality are host specific, settler mortality was 2.3-3.0 times higher near conspecific adults vs. near adult corals of other genera or in open reef areas. In four laboratory experiments to test for distance-dependent, host-specific mortality, swimming coral larvae were exposed to water collected near conspecific adult corals, near other coral genera, and in open areas of the reef. Microbial abundance in these water samples was manipulated with filters and antibiotics to test whether the cause of mortality was biotic (i.e., microbial). Juvenile survivorship was lowest in unfiltered water collected near conspecifics, and survivorship increased when this water was filter sterilized, collected farther away, or collected near other adult coral genera. Together these results demonstrate for the first time that the diversity-promoting mechanisms embodied in the Janzen-Connell model can operate in a marine ecosystem and in an animal. The distribution of adult corals across a reef will thus influence the spatial pattern of juvenile survival. When rare coral species have a survival advantage, coral species diversity per se becomes increasingly important for the persistence and recovery of coral cover on tropical reefs.