Successional Status Research Articles

The role of exotic and native hybrids during ecological succession in salt marshes

Knowledge of factors that influence species colonization and abundance during ecological succession is key for conservation and restoration efforts. The tolerance of species to environmental stresses and interspecific interactions influence stages of ecological succession. Species with high tolerance to stress or high competitive ability, such as invasive species or transgressive hybrids, may acquire a relevant role in the succession, inhibiting its development. We studied the role of native Sarcocornia and exotic Spartina hybrids on vegetation succession in salt marshes. In a time span of ca. 12 years, halophyte community structure and key sedimentary characteristics were recorded in marshes that differ in geomorphology, physiography, and successional status. We evaluated vegetation change in a young marsh undergoing rapid primary succession, and in a mature marsh undergoing slow changes in composition within established vegetation. Native hybrids increased their abundance in both types of marshes over time. Their presence and persistence was concurrent with colonization by other native halophytes. While they did not dominate the community and impede native succession, it is interesting to note their colonization coincided with a decrease in cover of invasive Spartina densiflora. In contrast, exotic hybrids formed from S. densiflora changed successional trajectories through competitive displacement of native halophytes in the mature marsh. However, these hybrids were not yet present in the young marsh where S. densiflora had just started to invade. This study reveals contrasting roles of invasive and native hybrids during ecological succession, providing valuable new knowledge for the management and conservation of salt marshes.

Variation and relationships between twig and leaf traits of species across successional status in temperate forests

ABSTRACTRelationships among twig size, leaf size and leafing intensity are essential for predicting structure and function of twigs and leaves and ability of plants to acquire different environmental resources. We aimed to test whether trade-offs between twig and leaf traits for species would be influenced by forest successional status and sampling orientation within canopy. We measured four twig and leaf traits for seven coexisting broadleaf species across successional statuses in mixed broadleaved-Korean pine (Pinus koraiensis) forests in Northeast China. The results showed that both twig and leaf traits varied with successional status and twig level explained the largest variations in traits. Slopes of regression of leaf traits (i.e. individual leaf area, total leaf area and volume-based leafing intensity) against twig cross-sectional area were late-successional species > middle-successional species > early-successional species. Relationships between twig cross-sectional area and total leaf area in upper south and lower north canopy orientations did not vary with successional status of species. However, slopes of regression of twig cross-sectional area against total leaf area varied with successional status in other four sampling orientations. Our results highlight the importance of successional status and sampling orientation within canopy in controlling variations and trade-offs between twig and leaf traits.

Sudden Aspen Decline: A Review of Pattern and Process in a Changing Climate

The American quaking aspen (Populus tremuloides Michx.) and its close relative, the Eurasian quaking aspen (Populus tremula L.), cover a realm that is perhaps the most expansive of all tree species in the world. In North America, sudden aspen decline (SAD) is a growing concern that marks the rapid decline of quaking aspen trees leading to mortality at the stand and landscape scale. Research suggests that drought and water stress are the primary causes of SAD. Predisposing factors (age, structure, and landscape position), as well as associated stressors (i.e., pests and pathogens), have been linked to mortality in affected stands. The conflation of multiple interacting factors across the aspen’s broad geographic range in North America has produced significant debate over the classification of SAD as a disease and the proper management of affected stands. Interestingly, no such effects have been reported for the Eurasian aspen. We here review and synthesize the growing body of literature for North America and suggest that SAD is a novel decline disease resulting from multiple inciting and interacting factors related to climate, land-use history, and successional dynamics. We suggest that the range of aspen observed at the onset of the 21st Century was bolstered by a wet period in western North America that coincided with widespread regional cutting and clearing of late-successional forests for timber and grazing. No comparable land-use history, successional status, or age-class structure is apparent or linked for Eurasian forests. Eurasian aspen is either absent or young in managed forests, or old and decadent in parks in Fenno-Scandinavia, or it grows more intimately with a more diverse mixture of tree species that have arisen from a longer period of frequent timber cutting in Russia. Based on these insights we provide recommendations for practical management techniques that can promote stand resilience and recovery across a range of stand conditions in North America. Managers should attempt to identify SAD-prone stands using the presence of predisposing conditions and focus treatments such as coppice or prescribed fire on stands with suitable topographies, elevations, and climates. We conclude that SAD will persist throughout the coming decades, given the enormity of past cutting history, fire exclusion, and current changes in climate until a more active restoration agenda is implemented.

Classification of successional stages in native forests of the Argentine Spinal through neural networks

AbstractNative forests are subject to variations in the structure of the different strata. These modifications can have a natural origin due to phenomena such as climate variation, fires, or floods; or they can be anthropic, due to the unsustainable use of forest resources. In general, according to the dominance of the different strata, the forests are classified as virgin, successional, or renoval. In the realization of inventories, there are situations in which the operator can offer a doubtful classification with a high degree of subjectivity. The purpose of this work has been to provide to technicians an objective tool for the classification of successional status in native forests. In this work, it has been found that there is a relationship between the presence of combinations of different plant species, both arboreal and shrub, related to the classification of forests. This relationship has been analyzed through the application of neural networks in two steps: First, a perceptron was applied, then a probabilistic neural network. The analysis through an artificial neuronal network of these two stages has allowed us to develop equations that through the presence/absence of 16 species allows to classify objectively the successional state. This analysis demonstrated an agreement of 83% with the subjective classification of trained field assessors.

Physiological parameters and plasticity as key factors to understand pioneer and late successional species in the Atlantic Rainforest

Although there has been a long tradition in plant ecology of grouping species into functional groups, there is a lack of consensus regarding both the successional status of species and the dynamics of forest recovery. Therefore, plant species are commonly classified into two groups: early successional pioneers and late successional or even climax species. Here, we aimed to answer the following question: if pioneer and late successional species have different photosynthetic characteristics, will these differences be translated to improved performance when plants are grown under similar conditions, particularly high irradiance and different water regimes? To this end, we investigated gas exchange, photoprotection, plasticity, and nutritional traits in ten native species that were subject to natural variations in photosynthetically active radiation, rainfall, and air temperature throughout the year. Our results provided evidence that photosynthesis is directly dependent on nitrogen and phosphorus. Remarkably, this characteristic increased in importance when the species were grouped into pioneer and late successional species compared with each species separately. Furthermore, principal component analysis demonstrated that physiological traits are excellent parameters for characterizing the pioneer and late successional plants growing in situ under the same irradiance. Our findings indicate that the responses of trees to seasonal variations depend on their ability to absorb nutrients to meet the mineral requirements to sustain long-term growth. We further analyzed the mechanisms that allow woody species to cope with drought stress.

Composition of natural phytoplankton community has minor effects on autochthonous dissolved organic matter characteristics

ABSTRACTDissolved organic matter (DOM) is an important component of nutrient cycling, but the role of different organisms controlling the processing of autochthonous DOM remains poorly understood. Aiming to characterize phytoplankton-derived DOM and the effects of complex pelagic communities on its dynamics, we incubated natural plankton communities from a temperate mesohaline estuary under controlled conditions for 18 days. The incubations were carried out in contrasting seasons (spring and autumn) and changes in the planktonic community (phytoplankton, bacteria and microzooplankton), nutrients and DOM were assessed. Our results highlight the complexity of DOM production and fate in natural planktonic communities. Small changes in DOM composition were observed in the experiments relative to the orders-of-magnitude variations experienced in the phytoplankton assembly. We argue that the tight coupling between microbial processing and DOM production by phytoplankton and grazers stabilizes variations in quantity and characteristics of autochthonous DOM, resulting in apparently homogeneous semi-labile DOM pool throughout the experiments. However, seasonal differences in the production and processing of DOM were observed, reflecting differences in the nutrient regimes and initial DOM characteristics in each experiment, but also likely influenced by changes in the successional status of the pelagic community. Acknowledging that characteristics of the DOM derived from phytoplankton growth can vary broadly, heterotrophic processing and successional status of the community are synergistically important factors for shaping those characteristics, and thus affecting the seasonal signature of the semi-labile autochthonous DOM pool.

Hydraulic and Photosynthetic Traits Vary with Successional Status of Woody Plants on the Loess Plateau

Research highlights: Water transport and CO2 diffusion are two important processes that determine the CO2 assimilation efficiency in leaves. The integration of leaf economic and hydraulic traits will help to present a more comprehensive view of the succession of woody plants in arid regions. However, studies on hydraulic traits of plants from different successional stages are still rare compared to that on economic traits in arid regions. Materials and methods: We selected 31 species from shrub stage, pioneer tree stage and late successional stage on the Loess Plateau, and measured five economic traits and five hydraulic traits of these species. Results: We found species from the pioneer tree stage exhibited "fast-growing" characteristics with high maximum net photosynthesis rate (Pmax) and vein density (VD). Species from the late successional stage exhibited "slow-growing" characteristics with low Pmax and VD. Economic traits showed no significant differences among the three stages except for Pmax. Hydraulic traits, such as VD, leaf area to sapwood area ratio and vessel frequency, exhibited significant differences among different stages. Conclusions: Hydraulics may play an important role in the succession of woody plants in arid regions. Hydraulic traits and Pmax, should be combined to investigate succession of woody plants in future studies. The "fast-growing" characteristics of pioneer trees and "slow-growing" characteristics of late successional trees may induce the succession of woody plants.

A height-wood-seed axis which is preserved across climatic regions explains tree dominance in European forest communities

Environmental constraints on both functional traits and trait–trait correlation patterns are extremely relevant research questions to understand assembling mechanisms and scaling processes in ecology. In fact, community weighted mean (CWM) and functional diversity (FD) metrics assimilate meaningful complementary information from species to community level. Relating species abundance data from national forest inventories (France, Italy and Spain) with species trait values from published sources, we explored CWM and FD variations along climatic gradients for traits related to leaf, hydraulic and live history strategies, and how the CWM or the FD of single traits co-vary. Results show how the combined effects of water availability and temperature modulate CWM, and to a lower extent FD, of Western European forests. PCA on CWM traits data showed that main axis accounts for 46.6% of the variability and is represented by wood density, seed dry weight (SDW) and maximum height. This axis is similarly preserved in humid and dry areas and potentially related to competition ability, growth rates and successional status of tree species dominants. Furthermore, the second axis is associated with leaf wilting and cavitation resistance (P50), but not consistent in dry areas. Correlation patterns of FD data, which were partially preserved across climatic regions, showed one main axis involving the joint variability of all traits. However, between humid and dry areas determinants of SDW and P50 differ from all other traits. Results showed that at least a part of the co-variation patterns among traits change with climate conditions, suggesting scale-dependent effects which should be taken into account when scaling assembly mechanisms.

Functional Crown Architecture of Five Temperate Broadleaf Tree Species: Vertical Gradients in Leaf Morphology, Leaf Angle, and Leaf Area Density

The morphology, inclination, and spatial distribution of leaves in different parts of tree crowns are important determinants of the radiation, momentum, and gas exchange between the canopy and the atmosphere. However, it is not well known how these foliage-related traits vary among species differing in successional status. We measured leaf size, leaf mass area (LMA), leaf inclination (angle to the horizontal), leaf area density (LAD), total leaf area (leaf area index, LAI), and leaf area distribution across the crown in adult trees of five common, early to late-successional tree species (Betula pendula Roth, Quercus petraea (Matt.) Liebl., Carpinus betulus L., Tilia cordata Mill., and Fagus sylvatica L.) using different canopy access techniques and the harvest of foliated trees (29 trees in total). Leaf size increased continuously with crown depth in B. pendula and T. cordata but peaked at mid-crown in Q. petraea, C. betulus, and F. sylvatica to decrease toward the shade crown. By contrast, LMA and leaf angle decreased continuously with crown depth in all species, but the pattern of vertical change varied. The mid/late- and late-successional species had higher LAI, lower shade-leaf LMA, lower leaf angles (shade and sun crown), and higher LAD in the uppermost sun crown in comparison to early successional B. pendula. We assume that the most peripheral sun leaf layer is partly acting as a shield against excess radiation, with foliage properties depending on the structure of the shade crown. We conclude that the vertical change in leaf morphology, inclination, and spatial distribution in tree crowns is highly species specific, with partial dependence on the species’ position in succession.

Variation of leaf angle distribution quantified by terrestrial LiDAR in natural European beech forest

Leaf inclination angle and leaf angle distribution (LAD) are important plant structural traits, influencing the flux of radiation, carbon and water. Although leaf angle distribution may vary spatially and temporally, its variation is often neglected in ecological models, due to difficulty in quantification.In this study, terrestrial LiDAR (TLS) was used to quantify the LAD variation in natural European beech (Fagus Sylvatica) forests. After extracting leaf points and reconstructing leaf surface, leaf inclination angle was calculated automatically. The mapping accuracy when discriminating between leaves and woody material was very high across all beech stands (overall accuracy = 87.59%). The calculation accuracy of leaf angles was evaluated using simulated point cloud and proved accurate generally (R2 = 0.88, p < 0.001; RMSE = 8.37°; nRMSE = 0.16). Then the mean (θmean), mode (θmode), and skewness of LAD were calculated to quantify LAD variation.Moderate variation of LAD was found in different successional status stands (θmean∈ [36.91°, 46.14°], θmode∈ [17°, 43°], skewness ∈ [0.07, 0.48]). Rather than the previously assumed spherical distribution or reported planophile distribution, here we find that LAD tended towards a uniform distribution in young and medium stands, and a planophile distribution in mature stands. A strong negative correlation was also found between plot θmean and plot median canopy height, making it possible to estimate plot specific LAD from canopy height data.Larger variation of LAD was found on different canopy layers (θmean∈ [33.64°, 52.97°], θmode∈ [14°, 64°], skewness ∈ [−0.30, 0.71]). Beech leaves grow more vertically in the top layer, while more obliquely or horizontally in the middle and bottom layer.LAD variation quantified by TLS can be used to improve leaf area index mapping and canopy photosynthesis modelling.

Variación de la diversidad taxonómica y funcional de la avifauna en un bosque seco tropical (bs-T) en diferentes estados de sucesión en el sur del Valle del Magdalena, Huila, Colombia

El Bosque Seco Tropical es uno de los ecosistemas más amenazados en Colombia y aún se desconocen diferentes aspectos sobre su avifauna y ecología, especialmente en el sur del Valle del Magdalena en Colombia. En este estudio se evaluaron las variaciones en la diversidad taxonómica y funcional en un ensamblaje de aves en un área con diferentes estados sucesionales en un bosque seco en el sur del valle del río Magdalena. Para la caracterización de la avifauna se llevaron a cabo puntos de conteos de radio fijo y para la complejidad de hábitat se cuantificaron variables estructurales de la vegetación en cada estado sucesional, se realizó un análisis de componentes principales, donde se utilizó el primer componente principal como una medida de la complejidad de hábitat. Los resultados muestran que la diversidad taxonómica medida como riqueza basada en el número efectivo de especies, y la diversidad funcional evaluada mediante el índice de riqueza funcional del ensamblaje se correlacionan significativamente con la complejidad del hábitat, aumentando en estados sucesionales más complejos. Esta tendencia fue positiva para frugívoros e insectívoros, y negativa para los granívoros, por lo que el estado sucesional y la complejidad de hábitat pueden estar jugando un papel clave en la estructuración de las comunidades de avifauna. Estos resultados contribuyen al conocimiento de la avifauna y su relación ecológica con el bosque seco en el sur del valle geográfico del río Magdalena.

Towards Conservation of Genetic Variation of Tropical Tree Species With Differing Successional Status: The Case of Mansonia altissima A. Chev and Triplochiton scleroxylon K. Schum

Two important West African timber tree species with differing successional status, Mansonia altissima A. Chev and Triplochiton scleroxylon K. Schum were investigated in this study. Triplochiton scleroxylon is a pioneer species found in open forests, whereas Mansonia altissima is a nonpioneer light-demanding tree species occurring in closed forests. Amplified fragment length polymorphism markers were used to compare the genetic diversities of these two timber species in stands with different degrees of human impact (isolated forest patch, logged forest, farmland, plantation, and primary forest). Contrasting effects of human impact on genetic diversity were detected for these two timber species. The results suggested severe effects of human impact on the genetic diversity of Mansonia altissima, a nonpioneer species. However, no adverse effect was recorded in Triplochiton scleroxylon, a pioneer species. These findings indicate that nonpioneer tree species could be more prone to genetic erosion than pioneer tree species as a result of adverse human impacts. Therefore, conservation of genetic diversity in both pioneer and nonpioneer tree species populations would likely necessitate different measures.

Differential herbivory and successional status in five tropical tree species

Using 4 restoration plots, we performed a common garden experiment to test the hypothesis that inter-specific variation in leaf herbivory depends on the successional status of tree species of the seasonally dry tropical forest. In July of 2011, we calculated the standing levels of herbivory in 5 species at the beginning of the rainy season: Ipomoea pauciflora (early-successional), Swietenia humilis, and Pseudobombax ellipticum (intermediate-successional) and Jacaratia mexicana and Bursera linanoe (late-successional). From each individual tree, we selected 14 leaves to measure herbivory (N = 84 plants, 821 leaves). The mean leaf area lost by herbivory across the 5 tree species was 5.25%. The results evidencing differences among the categories evaluated supported our hypothesis, as herbivory increased from the late to the early-successional species. We discussed the proximate (i.e, differential leaf traits) and ultimate causes (i.e., differential selective pressures) operating on trees situated at the extreme of the successional stages that could explain the inter-specific differences in herbivory we observed. In accordance with our results, successional status should be recognized as a factor affecting herbivory in tree species in tropical dry forests.

Edaphic factors, successional status and functional traits drive habitat associations of trees in naturally regenerating tropical dry forests

Abstract Many studies have examined individual environmental drivers of tropical tree species distributions, but edaphic and successional gradients have not been considered simultaneously. Furthermore, determining how functional traits influence species distributions along these gradients may help to elucidate mechanisms behind community assembly. To assess the influence of environmental filtering on tropical dry forest (TDF) tree species distributions, we used forest inventory data from sites with large edaphic and successional gradients in NW Costa Rica. Our goals were to determine (1) whether edaphic or successional factors are more important determinants of the abundance of individual tree species in regenerating TDF, (2) how species‐level functional traits are related to edaphic and/or successional niche associations of tree species and (3) correlations between species‐level edaphic and successional niche associations. The distributions of 82 focal tree species were strongly driven by both edaphic and successional gradients. Overall, 94% of species responded to soil chemistry, 89% to soil texture and 94% to stand age gradients. Some functional traits were correlated with the edaphic and successional niche associations of TDF tree species. Species that specialized on soils with high total nutrient concentrations had higher foliar nutrient concentrations (nitrogen and phosphorus) and lower leaf dry matter content (LDMC). Species with wider stand age niches had lower LDMC and wood density. There were no correlations between edaphic and successional niche optima of TDF tree species. Our results indicate that successional and edaphic gradients concurrently drive community assembly in regenerating TDF. Moreover, our work underscores the importance of considering how the functional characteristics of TDF trees dictate species distributions across environmental gradients. A plain language summary is available for this article.

Photosynthesis of subtropical forest species from different successional status in relation to foliar nutrients and phosphorus fractions

The ecophysiological linkages of leaf nutrients to photosynthesis in subtropical forests along succession remain elusive. We measured photosynthetic parameters (Amax, Vcmax, Jmax, PPUE), leaf phosphorus (P) and nitrogen (N), foliar P fractions and LMA from 24 species (pioneer, generalist, and climax). Amax was significantly related to N and P for the pooled data, while significant relationship between Amax and P was only found in climax species. The mixed-effect model including variables (N, P, and SLA or LMA) for predicting Vcmax and Jmax best fitted but varied remarkably across succession. Climax species had higher N: P ratios, indicating an increasing P limitation at later succession stage; photosynthesis, however, did not show stronger P than N limitations across all species. Nevertheless, climax species appeared to increase nucleic acid P allocation and residual P utilization for growth, thereby reducing the overall demand for P. Our results indicate that the scaling of photosynthesis with other functional traits could not be uniform across succession, growth variables (e.g. photosynthesis) and species trait identity (e.g. successional strategy) should be considered in combination with N: P ratio when we investigate P limitation in subtropical forests, and variations in P allocation state further influencing photosynthetic rates and P-use efficiency.

Size-related shifts in carbon gain and growth responses to light differ among rainforest evergreens of contrasting shade tolerance.

Recent work suggests that plant size affects light requirements and carbon balance of juvenile trees, and such shifts may be greater in light-demanding species than in their more shade-tolerant associates. To explore the physiological basis of such shifts, we measured juvenile light interception, carbon gain and growth of four subtropical Australian rainforest trees differing in shade tolerance, comparing individuals ranging from 13 to 238cm in height, across a wide range of understory environments. We hypothesized that even in a standardized light environment, increasing sapling size would lead to declines in net daily carbon gain of foliage and relative growth rates (RGR) of all species, with declines more pronounced in light-demanding species. Crown architecture of individuals was recorded using a 3-dimensional digitizer, and the YPLANT program was used to estimate the self-shaded fraction of each crown and model net carbon gain. Increased sapling size caused a significant increase in self-shading, and significant declines in net daily carbon gain and RGR of light-demanding species, while such ontogenetic variations were minimal or absent in shade-tolerant species. Additionally, differences in the slope of the relationship between light and RGR led to crossovers in RGR among shade-tolerant and light-demanding species at low light. Our results show that the magnitude of ontogenetic variation in net daily carbon gain and RGR can be substantial and may depend on successional status, making it unsafe to assume that young seedling performance can be used to predict or model responses of larger juvenile trees.

Are species photosynthetic characteristics good predictors of seedling post-hurricane demographic patterns and species spatiotemporal distribution in a hurricane impacted wet montane forest?

In situ measurements of leaf level photosynthetic response to light were collected from seedlings of ten tree species from a tropical montane wet forest, the John Crow Mountains, Jamaica. A model-based recursive partitioning ('mob') algorithm was then used to identify species associations based on their fitted photosynthetic response curves. Leaf area dark respiration (RD) and light saturated maximum photosynthetic (Amax) rates were also used as 'mob' partitioning variables, to identify species associations based on seedling demographic patterns (from June 2007 to May 2010) following a hurricane (Aug. 2007) and the spatiotemporal distribution patterns of stems in 2006 and 2012. RD and Amax rates ranged from 1.14 to 2.02 μmol (CO2) m−2s−1 and 2.97–5.87 μmol (CO2) m−2s−1, respectively, placing the ten species in the range of intermediate shade tolerance. Several parsimonious species 'mob' groups were formed based on 1) interspecific differences among species response curves, 2) variations in post-hurricane seedling demographic trends and 3) RD rates and species spatiotemporal distribution patterns at aspects that are more or less exposed to hurricanes. The composition of parsimonious groupings based on photosynthetic curves was not concordant with the groups based on demographic trends but was partially concordant with the RD - species spatiotemporal distribution groups. Our results indicated that the influence of photosynthetic characteristics on demographic traits and species distributions was not straightforward. Rather, there was a complex pattern of interaction between ecophysiological and demographic traits, which determined species successional status, post-hurricane response and ultimately, species distribution at our study site.

Physiological responses of saplings of rainforest timber species differing in successional group to light regimes and nitrogen

ABSTRACTSaplings of 19 valuable rain forest timber species representative of three successional status groups (early secondary, late secondary and climax) were grown in a polyhouse to examine their responses to three light intensity/quality treatments and nitrogen supply. Solar radiation was modified using painted polyethylene sheet to mimic natural light environments across a rain forest vertical column as follows: 1. Transparent plastic, 80% of full sunlight, R:FR = 0.95, 2. Blue shade, 14% of full sunlight, R:FR = 0.69; 3. Green shade, 7% of full sunlight, R:FR = 0.50. Transparent plastic conditions promoted an increase in stem height and diameter (i.e., growth), leaf thickness and gas exchange per unit leaf area. Additional nitrogen availability enhanced growth and specific leaf area (i.e., leaves were thinner), particularly in the full sun environment and on early secondary and late secondary successional species, but did not influence photosynthetic rate. Successional status of the species did not affect photosynthetic rate although early secondary successional species grew faster and had fewer branches than species of the other successional groups. We recommend that for a successful mixed stand the high-light requiring species should be planted first, with increased nitrogen supply, and the shade tolerant species should be introduced later with no extra nitrogen supply required.

Associations among arbuscular mycorrhizal fungi and seedlings are predicted to change with tree successional status.

Arbuscular mycorrhizal (AM) fungi in the soil may influence tropical tree dynamics and forest succession. The mechanisms are poorly understood, because the functional characteristics and abundances of tree species and AM fungi are likely to be codependent. We used generalized joint attribute modeling to evaluate if AM fungi are associated with three forest community metrics for a sub-tropical montane forest in Puerto Rico. The metrics chosen to reflect changes during forest succession are the abundance of seedlings of different successional status, the amount of foliar damage on seedlings of different successional status, and community-weighted mean functional trait values (adult specific leaf area [SLA], adult wood density, and seed mass). We used high-throughput DNA sequencing to identify fungal operational taxonomic units (OTUs) in the soil. Model predictions showed that seedlings of mid- and late-successional species had less leaf damage when the 12 most common AM fungi were abundant compared to when these fungi were absent. We also found that seedlings of mid-successional species were predicted to be more abundant when the 12 most common AM fungi were abundant compared to when these fungi were absent. In contrast, early-successional tree seedlings were predicted to be less abundant when the 12 most common AM fungi were abundant compared to when these fungi were absent. Finally, we showed that, among the 12 most common AM fungi, different AM fungi were correlated with functional trait characteristics of early- or late-successional species. Together, these results suggest that early-successional species might not rely as much as mid- and late-successional species on AM fungi, and AM fungi might accelerate forest succession.

Phosphorus limitation of aboveground production in northern hardwood forests.

Forest productivity on glacially derived soils with weatherable phosphorus (P) is expected to be limited by nitrogen (N), according to theories of long-term ecosystem development. However, recent studies and model simulations based on resource optimization theory indicate that productivity can be co-limited by N and P. We conducted a full factorial N × P fertilization experiment in 13 northern hardwood forest stands of three age classes in central New Hampshire, USA, to test the hypothesis that forest productivity is co-limited by N and P. We also asked whether the response of productivity to N and P addition differs among species and whether differential species responses contribute to community-level co-limitation. Plots in each stand were fertilized with 30kg N·ha-1 ·yr-1 , 10kg P·ha-1 ·yr-1 , N + P, or neither nutrient (control) for four growing seasons. The productivity response to treatments was assessed using per-tree annual relative basal area increment (RBAI) as an index of growth. RBAI responded significantly to P (P=0.02) but not to N (P=0.73). However, evidence for P limitation was not uniform among stands. RBAI responded to P fertilization in mid-age (P=0.02) and mature (P=0.07) stands, each taken as a group, but was greatest in N-fertilized plots of two stands in these age classes, and there was no significant effect of P in the young stands. Both white birch (Betula papyrifera Marsh.) and beech (Fagus grandifolia Ehrh.) responded significantly to P; no species responded significantly to N. We did not find evidence for N and P co-limitation of tree growth. The response to N + P did not differ from that to P alone, and there was no significant N × P interaction (P=0.68). Our P limitation results support neither the N limitation prediction of ecosystem theory nor the N and P co-limitation prediction of resource optimization theory, but could be a consequence of long-term anthropogenic N deposition in these forests. Inconsistencies in response to P suggest that successional status and variation in site conditions influence patterns of nutrient limitation and recycling across the northern hardwood forest landscape.