Allometric growth and carbon storage in the mangrove Sonneratia apetala

Allometric growth reflects different allocation patterns and relationships of different components or traits of a plant and is closely related to ecosystem carbon storage. As an introduced species, the growth and carbon storage of Sonneratia apetala are still unclear. To derive allometric relationships of the mangrove S. apetala and to estimate carbon storage in mangrove ecosystems, we harvested 12 individual Sonneratia apetala trees from four different diameter classes in the Futian National Nature Reserve, Guangdong, China. Allometric growth models were fitted. The results showed that diameter at breast height (DBH) and wood density were better variables for predicting plant biomass (including above- and below-ground biomass) than plant height. There were significant power function relationships between biomass and DBH, with a mean allometric exponent of 2.22, and stem biomass accounted for 97% of the variation in S. apetala total biomass. Nearly isometric scaling relationships were developed between stem biomass and other biomass components. To better understand the carbon stocks of the S. apetala ecosystem, we categorized all trees into five age classes and quantified vegetation carbon storage. The S. apetala vegetation carbon storage ranged from 96.48 to 215.35 Mg C ha^?1, and the carbon storage significantly increased with stand age. The allometric equations developed in this study are useful to estimate biomass and carbon storage of S. apetala ecosystems.

     

Variation in wood density determines spatial patterns inAmazonian forest biomass

Uncertainty in biomass estimates is one of the greatest limitations to models of carbon flux in tropical forests. Previous comparisons of field‐based estimates of the aboveground biomass (AGB) of trees greater than 10 cm diameter within Amazonia have been limited by the paucity of data for western Amazon forests, and the use of site‐specific methods to estimate biomass from inventory data. In addition, the role of regional variation in stand‐level wood specific gravity has not previously been considered. Using data from 56 mature forest plots across Amazonia, we consider the relative roles of species composition (wood specific gravity) and forest structure (basal area) in determining variation in AGB. Mean stand‐level wood specific gravity, on a per stem basis, is 15.8% higher in forests in central and eastern, compared with northwestern Amazonia. This pattern is due to the higher diversity and abundance of taxa with high specific gravity values in central and eastern Amazonia, and the greater diversity and abundance of taxa with low specific gravity values in western Amazonia. For two estimates of AGB derived using different allometric equations, basal area explains 51.7% and 63.4%, and stand‐level specific gravity 45.4% and 29.7%, of the total variation in AGB. The variation in specific gravity is important because it determines the regional scale, spatial pattern of AGB. When weighting by specific gravity is included, central and eastern Amazon forests have significantly higher AGB than stands in northwest or southwest Amazonia. The regional‐scale pattern of species composition therefore defines a broad gradient of AGB across Amazonia.     

Effect of initial fertilisation on biomass and nutrient contentof Norway spruce and Douglas-fir plantations at the same site

Norway spruce (NS) and Douglas-fir (DF) are among the main species used for production forestry in France. In low-elevation mountains and under-acidic conditions, they often occupy the same ecological situations. It is therefore of paramount interest to have a good understanding of how the two species behave under similar conditions and how they react to site improvement by fertilisation. The study stands are part of an experimental stand located in the estate forest of Breuil-Chenue in the Morvan (east central part of France). Its aim is to compare the impact of change in species on ecosystem functions. Destructive sampling of 10 trees per stand, distributed over the whole spectrum of inventoried classes of circumference at breast height (c _1.30), was carried out within four stands, e.g., fertilised and control (non-fertilised) NS; fertilised and control (non-fertilised) DF. Allometric relationships between c _1.30 and biomass or nutrient content per tree compartment were calculated. These equations were applied to the stand inventory for quantifying stand biomass and nutrient content on a hectare basis. The standard deviations of results were estimated using Monte-Carlo simulations. Specific emphasis was given to explain the origin of differences observed between species and treatments, i.e., changes in carbon allocation leading to specific allometric relationships, changes in stand structure (tree size distributions) and changes in stand density due to mortality.DF was more productive than NS (+28% for total tree biomass, +50% for ligneous biomass and +53% for stem wood). Both NS and DF were affected by fertilisation but in the case of NS, effects on the crown_c _1.30 relationship and on average tree growth were predominant while in the case of DF, the stem_c _1.30 relationship and stand density were affected by changes in soil fertility. The general fertilisation effect was an increment of 40% of ligneous dry matter for DF and only 22% for NS. In both cases, the amount of wood biomass produced per unit of leaf biomass (on a tree basis and, to a lesser extent, on a per hectare basis) was greater in fertilised plots. However, in the case of NS, the same amount of wood biomass was produced from a smaller quantity of leaves while in the case of DF, the same amount of leaves produced more wood biomass.The amount of nutrients in total ligneous biomass was higher for N, P and K, but lower for Ca and Mg, in DF than in NS. A high variability was observed between nutrient content of the different compartments, e.g., DF < NS for needles (except Mg), DF < NS for K, Ca and Mg for stem wood and DF > NS for N and P of stem wood. Fertilisation did not considerably change the hierarchy. On the basis of this study, all the indexes concerning stand production, wood density, nutrient use efficiency and response to fertilisation gave a net advantage to DF. This information is highly relevant for both ecological and practical purposes.     

Improved allometric models to estimate the aboveground biomass of tropical trees

Terrestrial carbon stock mapping is important for the successful implementation of climate change mitigation policies. Its accuracy depends on the availability of reliable allometric models to infer oven‐dry aboveground biomass of trees from census data. The degree of uncertainty associated with previously published pantropical aboveground biomass allometries is large. We analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types (4004 trees ≥ 5 cm trunk diameter). When trunk diameter, total tree height, and wood specific gravity were included in the aboveground biomass model as covariates, a single model was found to hold across tropical vegetation types, with no detectable effect of region or environmental factors. The mean percent bias and variance of this model was only slightly higher than that of locally fitted models. Wood specific gravity was an important predictor of aboveground biomass, especially when including a much broader range of vegetation types than previous studies. The generic tree diameter–height relationship depended linearly on a bioclimatic stress variable E, which compounds indices of temperature variability, precipitation variability, and drought intensity. For cases in which total tree height is unavailable for aboveground biomass estimation, a pantropical model incorporating wood density, trunk diameter, and the variable E outperformed previously published models without height. However, to minimize bias, the development of locally derived diameter–height relationships is advised whenever possible. Both new allometric models should contribute to improve the accuracy of biomass assessment protocols in tropical vegetation types, and to advancing our understanding of architectural and evolutionary constraints on woody plant development.     

Functional traits determine tree growth and ecosystem productivity of a tropical montane forest: Insights from a long‐term nutrient manipulation experiment

Trait‐response effects are critical to forecast community structure and biomass production in highly diverse tropical forests. Ecological theory and few observation studies indicate that trees with acquisitive functional traits would respond more strongly to higher resource availability than those with conservative traits. We assessed how long‐term tree growth in experimental nutrient addition plots (N, P, and N + P) varied as a function of morphological traits, tree size, and species identity. We also evaluated how trait‐based responses affected stand scale biomass production considering the community structure. We found that tree growth depended on interactions between functional traits and the type or combination of nutrients added. Common species with acquisitive functional traits responded more strongly to nutrient addition, mainly to N + P. Phosphorous enhanced the growth rates of species with acquisitive and conservative traits, had mostly positive effects on common species and neutral or negative effects in rare species. Moreover, trees receiving N + P grew faster irrespective of their initial size relative to trees in control or to trees in other treatment plots. Finally, species responses were highly idiosyncratic suggesting that community processes including competition and niche dimensionality may be altered under increased resource availability. We found no statistically significant effects of nutrient additions on aboveground biomass productivity because acquisitive species had a limited potential to increase their biomass, possibly due to their generally lower wood density. In contrast, P addition increased the growth rates of species characterized by more conservative resource strategies (with higher wood density) that were poorly represented in the plant community. We provide the first long‐term experimental evidence that trait‐based responses, community structure, and community processes modulate the effects of increased nutrient availability on biomass productivity in a tropical forest.     

Fungi inhabiting knotwood of Pinus sylvestris infected by Porodaedalea pini

Abundance and diversity of fungi in naturally formed knots of Pinus sylvestris affected by Porodaedalea pini were investigated. Samples were taken from trees that were (i) affected, with internal heartwood decay and no conks, (ii) affected, with internal heartwood decay and conks and (iii) controls. The Illumina sequencing technology was used for amplification of DNA, sequencing and analysis. In total, 566,279 raw sequences were obtained from six samples. Sequences included 74% of culturable and 8.4% of non‐culturable fungi and 17.6% of organisms with no reference sequences in NCBI. Abundance of organisms in knotwood, measured as number of OTUs, ranged from 36,272 (29,506 for fungi) to 178,535 (177,484 for fungi) and differed significantly between two trees in a stand and between stands. The highest and lowest average number of fungal OTUs occurred in infected trees with no conks and in trees with conks, respectively. Number of taxa ranged from 171 to 415 and often differed significantly between two trees in one stand and between stands. Greatest diversity occurred in control trees. The number of fungal taxa shared by two trees in one stand was 67–152 and that shared by two stands was 51–141. The majority of fungi were Ascomycota. Those most common in pines affected by P. pini were Coniochaeta hoffmannii and C. fodinicola (19.65%–59.92%). Infundichalara microchona, Leotiomycetes spp. and Rhinocladiella atrovirens were also present. Another common species, Lecanora conizaeoides, occurred most often in control trees (0.30%–8.82%). Porodaedalea pini was detected only sporadically. Non‐culturable fungi were most frequent in the control trees. The greater average abundance and smaller average diversity of fungi in knots of trees infected by P. pini suggest that the pathogen successfully competes with some fungal species and does not inhibit the growth of survivors. Some fungi detected may contribute to production of natural biocides.     

林龄和竞争对日本落叶松各组分生物量异速关系的影响

基于7-、17-、30-和40年生日本落叶松生物量测定数据,应用方差分析和多重比较分析了林龄和林分内树木竞争类型(优势木、平均木和被压木)对各组分生物量分配比例和异速关系的影响,构建了含林龄和树木竞争类型作为哑变量的生物量异速方程,为准确估算日本落叶松人工林生物量和碳储量提供依据。结果表明:(1)林龄显著影响生物量分配比例的异速关系。随林龄增加干生物量比例增大,枝叶生物量比例减小,根生物量逐渐稳定。加入林龄的干、枝和叶生物量方程显著改善。年龄效应在幼龄林阶段作用最显著,需单独构建生物量模型。(2)树木竞争类型对生物量分配的影响小于林龄。立地条件一致下,虽然相同胸径的优势木比劣势木积累更多的枝叶生物量和少的干生物量,但它们分配生物量到不同器官的比例和方式是基本相同的,林内竞争不会导致生物量分配规律由"异速关系理论"向"环境优先理论"转化。因此,常规采用平均木法估算各组分生物量是可行的。(3)在近成熟林分中不同竞争类型树木的根生物量分配比例均较为稳定,采用根茎比比值来估算根生物量是可行的。     

Native tree regeneration in native tree plantations: understanding the contribution of Araucaria angustifolia to biodiversity conservation in the threatened Atlantic Forest in Argentina

Deforestation is a global process that has strongly affected the Atlantic Forest in South America, which has been recognised as a threatened biodiversity hotspot. An important proportion of deforested areas were converted to forest plantations. Araucaria angustifolia is a native tree to the Atlantic Forest, which has been largely exploited for wood production and is currently cultivated in commercial plantations. An important question is to what extent such native tree plantations can be managed to reduce biodiversity loss in a highly diverse and vulnerable forest region . We evaluated the effect of stand age, stand basal area, as a measure of stand density, and time since last logging on the density and richness of native tree regeneration in planted araucaria stands that were successively logged over 60 years, as well as the differences between successional groups in the response of plant density to stand variables. We also compared native tree species richness in planted araucaria stands to neighbouring native forest. Species richness was 71 in the planted stands (27 ha sampled) and 82 in native forest (18 ha sampled) which approximate the range of variation in species richness found in the native forests of the study area. The total abundance and species richness of native trees increased with stand age and time since last logging, but ecological groups differed in their response to such variables. Early secondary trees increased in abundance with stand age 3–8 times faster than climax or late secondary trees. Thus, the change in species composition is expected to continue for a long term. The difference in species richness between native forest and planted stands might be mainly explained by the difference in plant density. Therefore, species richness in plantations can contribute to local native tree diversity if practices that increase native tree density are implemented.     

Estimating carbon stock in lowland Papua New Guinean forest: Low density of large trees results in lower than global average carbon stock

Papua New Guinean forests (PNG), sequestering up to 3% of global forest carbon, are a focus of climate change mitigation initiatives, yet few field‐based studies have quantified forest biomass and carbon for lowland PNG forest. We provide an estimate for the 10 770 ha Wanang Conservation Area (WCA) to investigate the effect of calculation methodology and choice of allometric equation on estimates of above‐ground live biomass (AGLB) and carbon. We estimated AGLB and carbon from 43 nested plots at the WCA. Our biomass estimate of 292.2 Mg AGLB ha^?1 (95% CI 233.4–350.6) and carbon at 137.3 Mg C ha^?1 (95% CI 109.8–164.8) is higher than most estimates for PNG but lower than mean global estimates for tropical forest. Calculation method and choice of allometric model do not significantly influence mean biomass estimates; however, the most recently calibrated allometric equation generates estimates 13% higher for lower 95% confidence intervals of mean biomass than previous allometric models – a value often used as a conservative estimate of biomass. Although large trees at WCA (>70 cm diameter at breast height) accounted for 1/5 total biomass, their density was lower than that seen in SE Asian and Australia forests. Lower density of large trees accounts for lower AGLB than in neighbouring forests – as large trees contribute disproportionately to forest biomass. Reduced frequency of larger trees at WCA is explained by the lack of diversity of large dipterocarp species common to neighbouring SE Asian forests and, potentially, higher rates of local disturbance dynamics. PNG is susceptible to the El Niño Southern Oscillation (ENSO) extreme drought events to which large trees are particularly sensitive and, with still over 20% carbon in large trees, differential mortality under increasing ENSO drought stress raises the risk of PNG forest switching from carbon sink to source with reduced long‐term carbon storage capacity.     

Effects of European colonization on indigenous ecosystems: post-settlement changes in tree stand structures in Eucalyptus–Callitris woodlands in central New South Wales, Australia

Aim There has been considerable debate about pre‐settlement stand structures in temperate woodlands in south‐eastern Australia. Traditional histories assumed massive tree losses across the region, whereas a number of recent histories propose that woodlands were originally open and trees regenerated densely after settlement. To reconcile these conflicting models, we gathered quantitative data on pre‐settlement stand structures in Eucalyptus–Callitris woodlands in central New South Wales Australia, including: (1) tree density, composition, basal area and canopy cover at the time of European settlement; and (2) post‐settlement changes in these attributes. Location Woodlands dominated by Eucalyptus species and Callitris glaucophylla, which originally occupied approximately 100,000 km² in central New South Wales, Australia. Methods We recorded all evidence of pre‐settlement trees, including stumps, stags and veteran trees, from 39 relatively undisturbed 1‐ha stands within 16 State Forests evenly distributed across the region. Current trees were recorded in a nested 900 m² quadrat at each site. Allometric relationships were used to estimate girth over bark at breast height, tree basal area, and crown diameter from the girth of cut stumps. A post‐settlement disturbance index was developed to assess correlations between post‐settlement disturbance and attributes of pre‐settlement stands. Results The densities of all large trees (> 60 cm girth over bark at breast height) were significantly greater in current stands than at the time of European settlement (198 vs. 39 trees ha^?1). Pre‐settlement and current stands did not differ in basal area. However, the proportional representation of Eucalyptus and Callitris changed completely. At the time of settlement, stands were dominated by Eucalyptus (78% of basal area), whereas current stands are dominated by Callitris (74%). On average, Eucalyptus afforded 83% of crown cover at the time of settlement. Moreover, the estimated density, basal area and crown cover of Eucalyptus at the time of settlement were significantly negatively correlated with post‐settlement disturbance, which suggests that these results underestimate pre‐settlement Eucalyptus representation in the most disturbed stands. Main conclusions These results incorporate elements of traditional and recent vegetation histories. Since European settlement, State Forests have been transformed from Eucalyptus to Callitris dominance as a result of the widespread clearance of pre‐settlement Eucalyptus and dense post‐settlement recruitment of Callitris. Tree densities did increase greatly after European settlement, but most stands were much denser at the time of settlement than recent histories suggest. The original degree of dominance by Eucalyptus was unexpected, and has been consistently underestimated in the past. This study has greatly refined our understanding of post‐settlement changes in woodland stand structures, and will strengthen the foundation for management policies that incorporate historical benchmarks of landscape vegetation changes.     

Restoration of Old Forest Features in Coast Redwood Forests Using Early‐stage Variable‐density Thinning

To accelerate development of old forest features in coast redwood, two thinning treatments and an unthinned control were compared in three treatment areas in north coastal California. One thinning treatment was designed to restore old forest densities of 125 trees/ha and the other 250 trees/ha representing a one‐step and partial treatments to the desired stand density. Four years after treatment, numbers of trees had increased in the thinning treatments due to recruitment of new trees, but had decreased in the control due to self‐thinning. Residual trees increased in stem volume following thinning by 128% in low‐density thinning compared to 70% in the controls indicating thinning accelerated stand development. The thinning treatments also moved the species composition of these stands to a greater proportion of redwood. Considerable slash was produced by the thinning treatments but was decomposing rapidly. Black bears damaged approximately 15% of all trees and more than 38% of residual trees in the thinned treatments compared to less than 2% of all trees in the control. This damage included killing some trees and damaging other trees that survived. Decisions over restoration densities in these stands are complicated by prolonged stand development, and balancing risks and costs. In this case, the bears represent a stochastic factor that dramatically increases risk. Thinning appears to be an effective means of enhancing old forest development by accelerating tree growth, modifying species composition, and increasing stand‐level variability. Continued monitoring will be necessary to evaluate long‐term trends in density relative to effects of bear damage.     

Effects of forest management on the carbon dioxide emissions of wood energy in integrated production of timber and energy biomass

The aim of this work was to study the sensitivity of carbon dioxide (CO₂) emissions from wood energy to different forest management regimes when aiming at an integrated production of timber and energy biomass. For this purpose, the production of timber and energy biomass in Norway spruce [Picea abies (L.) Karst] and Scots pine (Pinus sylvestris L.) stands was simulated using an ecosystem model (SIMA) on sites of varying fertility under different management regimes, including various thinning and fertilization treatments over a fixed simulation period of 80 years. The simulations included timber (sawlogs, pulp), energy biomass (small‐sized stem wood) and/or logging residues (top part of stem, branches and needles) from first thinning, and logging residues and stumps from final felling for energy production. In this context, a life cycle analysis/emission calculation tool was used to assess the CO₂ emissions per unit of energy (kg CO₂ MWh^?1) which was produced based on the use of wood energy. The energy balance (GJ ha^?1) of the supply chain was also calculated. The evaluation of CO₂ emissions and energy balance of the supply chain considered the whole forest bioenergy production chain, representing all operations needed to grow and harvest biomass and transport it to a power plant for energy production. Fertilization and high precommercial stand density clearly increased stem wood production (i.e. sawlogs, pulp and small‐sized stem wood), but also the amount of logging residues, stump wood and roots for energy use. Similarly, the lowest CO₂ emissions per unit of energy were obtained, regardless of tree species and site fertility, when applying extremely or very dense precommercial stand density, as well as fertilization three times during the rotation. For Norway spruce such management also provided a high energy balance (GJ ha^?1). On the other hand, the highest energy balance for Scots pine was obtained concurrently with extremely dense precommercial stands without fertilization on the medium‐fertility site, while on the low‐fertility site fertilization three times during the rotation was needed to attain this balance. Thus, clear differences existed between species and sites. In general, the forest bioenergy supply chain seemed to be effective; i.e. the fossil fuel energy consumption varied between 2.2% and 2.8% of the energy produced based on the forest biomass. To conclude, the primary energy use and CO₂ emissions related to the forest operations, including the production and application of fertilizer, were small in relation to the increased potential of energy biomass.     

Relationship between size hierarchy and density of trees in a tropical dry deciduous forest of western India

Questions: Density dependence is thought to restrict exponential growth as well as give rise to size structure in populations. Size hierarchy in trees from tropical dry deciduous forests is studied to ask (1) whether nature of competition is symmetric or asymmetric and (2) what is the self thinning trajectory under a natural gradient of tree density. Location: Western India. Methods: Density was measured as the number of trees in 10‐m radius circular plots (n= 96) and size was measured at DBH. Size variation was evaluated by the Gini coefficient (n= 1239 trees). Results: Size inequality between neighbours decreased with density but in a non‐linear manner. In the backdrop of existing theory this indirectly suggests that competitive interactions may be symmetric over a ‘depletive’ resource such as below‐ground water (rather than a ‘pre‐emptive’ resource such as light), which is very plausible in a semi‐arid environment. The self thinning coefficient derived from the relationship between stem diameter and density (γ～?1/4), is higher than expected from existing models of allometric plant growth (γ=?1/3) which are based on above‐ground interactions alone. Seen in conjunction, these results suggest that above‐ground structures, such as stem size, do not adequately represent the outcome of competitive interactions when below‐ground resources, such as water, may be more important under semi‐arid conditions. Conclusions: The non‐linear relationship between size inequality and density indicates that there exists a density threshold beyond which investment in above‐ground biomass becomes sluggish in semi‐arid, deciduous forests. Since current allometric models do not incorporate below‐ground biomass for trees, these data suggest that a more comprehensive allometric growth model may have higher predictive power and wider applicability.     

A Unified Law of Spatial Allometry for Woody and Herbaceous Plants

Abstract: The objective of the present paper is to provide both proof and theoretical deduction of an overlapping, valid law of allometry for woody and herbaceous plants used in agriculture and forestry. In his attempt to find an adequate expression for stand density, independent of site quality and age, Reineke (1933^[281]) developed the following equation for even‐aged and fully stocked forest stands in the northwest of the USA: ln(N) = a ‐ 1.605 . ln(dg), based on the relationship between the average diameter dg and the number N of trees per unit area. With no knowledge of these results, Kira et al. (1953^[281]) and Yoda et al. (1957^[281] and 1963^[281]) found the boundary line ln(m) = b ‐ 3/2 . ln(N) in their study of herbaceous plants. This self‐thinning rule ‐ also called the ‐ 3/2‐power rule ‐ describes the relationship between the average weight m of a plant and the density N in even‐aged herbaceous plant populations growing under natural development conditions. It is possible to make a transition from Yoda's rule to Reineke's stand density rule if mass m in the former rule is substituted by the diameter dg. From biomass analyses for the tree species spruce (Picea abies [L.] Karst.) and beech (Fagus sylvatica L.), allometric relationships between biomass m and diameter d are derived. Using the latter in the equation ln(m) = b ‐ 3/2 . ln(N) leads to allometric coefficients for spruce (Picea abies [L.] Karst.) and beech (Fagus sylvatica L.), that come very close to the Reineke coefficient. Thus Reineke's rule (1933^[281]) proves to be a special case of Yoda's rule. Both rules are based on the simple allometric law governing the volume of a sphere v and its surface of projection s: v = c₁ . s^3/2. If the surface of projection s, is substituted by the reciprocal value of the number of stems s = 1/N and the isometric relationship between volume v and biomass m is considered v = c₂ . m^1.0 we come to Yoda's rule m = c₃ . N^‐3/2 or, in logarithmic terms, ln(m) = ln c₃ ‐ 3/2 . ln(N).     

Predicting tree diameter and height from above-ground biomass for four eucalypt species

Summary Previous work has suggested that tree stems are structured dimensionally to resist the forces to which they are subjected by the weight of the crown and the action of wind, snow and other loads on the crown. This proposition has been used to develop allometric relationships relating diameter at breast height or height of individual trees growing in even-aged monoculture to their above-ground fresh biomass. These models have practical application as estimators of tree diameters or heights from tree biomass as extensions of mechanistically based models of forest tree growth which predict tree biomasses. The present work applied these models to Eucalyptus regnans F. Muell, E. delegatensis R. Baker, E. nitens (Deane: Maiden) Maiden and E. grandis Hill ex Maiden trees, growing in plantation or regrowth stands, aged between 1.5 and 20 years, at eight geographically diverse sites extending from temperate to sub-tropical regions of Australia. While the models held for the various species at the various sites, their parameter values differed significantly between sites and/or species. This suggested there may be some inadequacy in the models. However, the differences were small and it was found reasonable to fit single models across all species and sites for practical use in estimating diameter or height. The errors about predicted values of height and diameter from these models were quantified. The models were also found to estimate diameter or height with little loss of precision when dry biomass was used in place of fresh biomass.     

Factors affecting species richness of tree regeneration in mixed‐wood stands of central Maine

Question: Two questions about within‐stand spatial variability are addressed in this paper. How does species richness of tree regeneration respond to small‐scale ecological gradients, and what effect does natural Abies balsamea abundance have on the species richness of other tree regeneration? Location: A long‐term, gap‐silviculture experiment, Acadian mixed‐wood forest, Maine, USA. Methods: Eight stands treated with and without gap harvesting were sampled to capture sub‐stand heterogeneity of understorey tree regeneration concurrently with patterning of local stand conditions. Spatial and non‐spatial models were developed to test the relationships between two response variables [species richness of small (height ≥0.1 m, but <0.75 m) and large (height ≥0.75 m, but <1.4 m) regeneration] and five explanatory variables (depth to water table, percentage canopy transmittance, A. balsamea regeneration density, and overstorey basal area and species richness). Results: Despite high unexplained variance for all models, consistent associations among variables were found. Negative associations were found between: (1) the species richness of small regeneration and A. balsamea regeneration density and (2) the species richness of large regeneration and overstorey basal area. Positive associations were found between: (1) the species richness of small regeneration and both overstorey basal area and species richness and (2) the species richness of small and large regeneration and canopy transmittance. Conclusions: Promoting tree species diversity in Acadian mixed‐wood stands may not be achievable through the use of gap‐harvesting alone if the density of understorey Abies balsamea is not reduced either naturally or through silvicultural intervention.     

Do understorey or overstorey traits drive tree encroachment on a drained raised bog?

• One of the most important threats to peatland ecosystems is drainage, resulting in encroachment of woody species. Our main aim was to check which features – overstorey or understorey vegetation – are more important for shaping the seedling bank of pioneer trees colonising peatlands (Pinus sylvestris and Betula pubescens). We hypothesised that tree stand parameters will be more important predictors of natural regeneration density than understorey vegetation parameters, and the former will be negatively correlated with species diversity and richness and also with functional richness and functional dispersion, which indicate a high level of habitat filtering.
• The study was conducted in the ‘Zielone Bagna’ nature reserve (NW Poland). We assessed the structure of tree stands and natural regeneration (of B. pubescens and P. sylvestris) and vegetation species composition. Random forest and DCA were applied to assess relationships between variables studied.
• Understorey vegetation traits affected tree seedling density (up to 0.5‐m height) more than tree stand traits. Density of older seedlings depended more on tree stand traits. We did not find statistically significant relationships between natural regeneration densities and functional diversity components, except for functional richness, which was positively correlated with density of the youngest tree seedlings.
• Seedling densities were higher in plots with lower functional dispersion and functional divergence, which indicated that habitat filtering is more important than competition. Presence of an abundant seedling bank is crucial for the process of woody species encroachment on drained peatlands, thus its dynamics should be monitored in protected areas.

     

The Allometry of Coarse Root Biomass: Log-Transformed Linear Regression or Nonlinear Regression?

Precise estimation of root biomass is important for understanding carbon stocks and dynamics in forests. Traditionally, biomass estimates are based on allometric scaling relationships between stem diameter and coarse root biomass calculated using linear regression (LR) on log-transformed data. Recently, it has been suggested that nonlinear regression (NLR) is a preferable fitting method for scaling relationships. But while this claim has been contested on both theoretical and empirical grounds, and statistical methods have been developed to aid in choosing between the two methods in particular cases, few studies have examined the ramifications of erroneously applying NLR. Here, we use direct measurements of 159 trees belonging to three locally dominant species in east China to compare the LR and NLR models of diameter-root biomass allometry. We then contrast model predictions by estimating stand coarse root biomass based on census data from the nearby 24-ha Gutianshan forest plot and by testing the ability of the models to predict known root biomass values measured on multiple tropical species at the Pasoh Forest Reserve in Malaysia. Based on likelihood estimates for model error distributions, as well as the accuracy of extrapolative predictions, we find that LR on log-transformed data is superior to NLR for fitting diameter-root biomass scaling models. More importantly, inappropriately using NLR leads to grossly inaccurate stand biomass estimates, especially for stands dominated by smaller trees.     

Soil-surface carbon dioxide efflux and microbial biomass in relation to tree density 13 years after a stand replacing fire in a lodgepole pine ecosystem

The effects of fire on soil‐surface carbon dioxide (CO₂) efflux, F_S, and microbial biomass carbon, C_mic, were studied in a wildland setting by examining 13‐year‐old postfire stands of lodgepole pine differing in tree density (< 500 to > 500 000 trees ha^?1) in Yellowstone National Park (YNP). In addition, young stands were compared to mature lodgepole pine stands (～110‐year‐old) in order to estimate ecosystem recovery 13 years after a stand replacing fire. Growing season F_S increased with tree density in young stands (1.0 µmol CO₂ m^?2 s^?1 in low‐density stands, 1.8 µmol CO₂ m^?2 s^?1 in moderate‐density stands and 2.1 µmol CO₂ m^?2 s^?1 in high‐density stands) and with stand age (2.7 µmol CO₂ m^?2 s^?1 in mature stands). Microbial biomass carbon in young stands did not differ with tree density and ranged from 0.2 to 0.5 mg C g^?1 dry soil over the growing season; C_mic was significantly greater in mature stands (0.5–0.8 mg C g^?1 dry soil). Soil‐surface CO₂ efflux in young stands was correlated with biotic variables (above‐ground, below‐ground and microbial biomass), but not with abiotic variables (litter and mineral soil C and N content, bulk density and soil texture). Microbial biomass carbon was correlated with below‐ground plant biomass and not with soil carbon and nitrogen, indicating that plant activity controls not only root respiration, but C_mic pools and overall F_S rates as well. These findings support recent studies that have demonstrated the prevailing importance of plants in controlling rates of F_S and suggest that decomposition of older, recalcitrant soil C pools in this ecosystem is relatively unimportant 13 years after a stand replacing fire. Our results also indicate that realistic predictions and modeling of terrestrial C cycling must account for the variability in tree density and stand age that exists across the landscape as a result of natural disturbances.