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Context Allometric equations describing the relationships between tree height (H) and breast height diameter (D) should be both statistically efficient and biologically relevant. Aims To determine whether selected allometric equations can meet established criteria for both efficiency and relevance. Methods Nine equations were compared to define the H–D relationships of 1122 individuals and 18 species from an Australian subtropical rainforest. Key results Three-parameter asymptotic equations described initial slope (a), curvature (b), and asymptotic height (Ha). Each equation was evaluated for precision (root mean square error, RMSE) and bias in H estimates, and ease of interpretation of function parameters. For both individual species and all stems, a non-rectangular hyperbola (NRH) provided almost equally high precision and low bias as did the statistically most parsimonious generalised Michaelis–Menten function, plus linear parameter values easily relatable to tree structural and functional attributes. The value of NRH a increased linearly with wood density for canopy species, but not for understorey and subdominant species, whereas the value of NRH b decreased as Ha increased from understorey to canopy species. Conclusions Species within understorey, subdominant, and canopy structural groups shared similar ranges of parameter values within groups that reflect both intrinsic architectural and developmental patterns, and environmental limitations to Ha. Implications The NRH can be used to visualise both early and later tree development stages and differences among the growth patterns of species occupying different positions within a forest.

The relationships between climate and forest dynamics can help us to interpret patterns of ecosystem carbon and to predict how forests react to climatic changes. We report mass dynamics of deadwood (necromass) from tropical forest ecosystems subject to some of the highest frequency of tropical cyclones in the world and to regular, persistent seasonal monsoon winds. Plots that are influenced by typhoons but exposed to different degrees of monsoon winds were monitored. We expected that stocks and inputs of necromass would reflect the seasonal intensity of wind events and be higher in the high wind exposure forest than in the low wind exposure forest, especially for fallen woody debris. The results showed that necromass input was indeed influenced by the magnitude of typhoons and aggravated by monsoon winds. However, while there was no significant difference in stock of necromass between plots, inputs of standing necromass were significantly higher in the high wind exposure plot; these were mostly derived from dead resprouts. Both our forests had very low values of total necromass stocks (3.47–4.32 Mg C ha−1) and inputs (2.1–2.5 Mg C ha−1 y−1) compared with tropical forests worldwide. Our results show that both monsoon and typhoon winds shape these tropical forests, favouring low stature individuals and trees with ability to resprout and that these strategies provide these forests with remarkable resistance and resilience to wind disturbances. Our findings from some of the most wind-affected forests in the world indicate how woody carbon dynamics and forest structure in other regions may respond to future changes in the frequency and intensity of winds.

Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity., The relationship of mycorrhizal associations with latitudinal gradients in tree beta-diversity is unexplored. Using a global dataset approach, this study examines how trees with arbuscular mycorrhizal and ectomycorrhizal associations contribute to latitudinal beta-diversity patterns and the environmental controls of these patterns.

Background Abandoned human-modified forests are refuges for remnant biodiversity. However, there are very few studies on the biodiversity and regeneration of native species in human-modified forests which are rich in exotic trees. Our research aim is to evaluate the regeneration status and biodiversity of two adjacent human-modified forests. The two forests have distinct overstorey exotic species richness prior to abandonment: one is an exotic tree plantation low in species richness, and the other is an exotic arboretum high in species richness. The original management practices of the two forests have been neglected for more than 20 years. A primary forest was selected as a reference forest to compare their diversity and regeneration status. We asked: (1) Is there a structural difference among the three forests? (2) What are the proportions of native saplings in the human-modified forests? (3) Are the introduced exotic species able to naturalize? Results We recorded 1316 individuals from 88 species, belonging to 69 genera and 34 families in the three forests [each sampled 16 quadrats (10 m × 5 m)]. Both human-modified forests were similar in their height structure, diameter structure, and sapling density, but differed in species diversity (characterized by rarefaction curves) and floristic composition (indicated by a quantitative similarity index). In the arboretum, only 50% of the sapling individuals were native. Surprisingly, when sampling efforts were standardized, the arboretum had higher native sapling species richness than the exotic species-poor plantation. Moreover, both human-modified forests had conserved a few rare and endemic species. Nevertheless, some exotic species in the arboretum had escaped to the nearby plantation. Conclusions After 20 years of abandonment, the two human-modified forests had converged in structure, but not in diversity patterns of native saplings. This could be due to that the diversity of exotic overstorey composition can influence the natural regeneration of understorey plants. Our study also raised concerns about conserving native species and managing naturalized exotic species in these human-modified forests. Electronic supplementary material The online version of this article (doi:10.1186/s40529-017-0190-x) contains supplementary material, which is available to authorized users.

Carbon stored in woody debris is a key carbon pool in forest ecosystems. The most widely-used method to convert woody debris volume to carbon is by first multiplying field-measured volume with wood density to obtain necromass, and then assuming that a fixed proportion (often 50%) of the necromass is carbon. However, this crucial assumption is rarely tested directly, especially in the tropics. The aim of this study is to verify the field carbon concentration values of living trees and woody debris in two distinct tropical forests in Taiwan. Wood from living trees and woody debris across five decay classes was sampled to measure density and carbon concentrations. We found that both wood density and carbon concentration (carbon mass/total mass) declined significantly with the decay class of the wood. Mean (±SE) carbon concentration values for living trees were 44.6 ± 0.1%, while for decay classes one to five they were respectively 41.1 ± 1.4%, 41.4 ± 1.0%, 37.7 ± 1.3%, 30.5 ± 2.0%, and 19.6 ± 2.2%. Total necromass carbon stock was low, only 3.33 ± 0.55 Mg C ha −1 in the windward forest (Lanjenchi) and 4.65 ± 1.63 Mg C ha −1 in the lowland forest (Nanjenshan). Applying the conventional 50% necromass carbon fraction value would cause a substantial overestimate of the carbon stocks in woody debris of between 17% and 36%, or about 1 Mg of carbon per hectare. The decline in carbon concentration and the increase of variances in the heavily decayed class suggest that in high-diversity tropical forests there are diverse decomposition trajectories and that assuming a fixed carbon fraction across woody pieces is not justified. Our work reveals the need to consider site-specific and decay class-specific carbon concentrations in order to accurately estimate carbon stocks and fluxes in forest ecosystems. If the marked decline in carbon content with necromass decay is typical of tropical forests, the dead wood carbon pool in the biome needs revision and is likely to be overestimated.

Background Hemispherical photography (HP) is a popular method to estimate canopy structure and understorey light environment, which analyses photographs acquired with wide view-angle lens (i.e. fisheye lens). To increase HP accuracy, the approaches of most previous studies were to increase the preciseness of exposure and thresholding of photographs, while ours quantified effects of canopy properties (gap fraction and length of sky-canopy border (SCB)) and errors of exposure and thresholding on the accuracy of HP. Results Through analysing photographs of real and model canopies, it was showed that HP inaccuracy resulted from the mismatch between exposure and thresholding rather than exposure or thresholding errors alone. HP inaccuracy was a function of the SCB length and the extent of exposure and thresholding errors, but independent of gap fraction. Discussion In photographs, SCBs are recorded as grey pixels which greyness is in between that of sky and canopy pixels. When there are exposure and thresholding errors, grey pixels are those prone to be misclassified in image analysis. Longer (vegetation with taller canopies) and wider (lower image sharpness) SCBs in photographs can both result in a higher amount of grey pixels and ultimately higher HP inaccuracy for a given extent of exposure and threshold errors. Conclusions Using lenses with view angle narrower rather than that of fisheye lens can shorten the SCB length in photographs and in turn reduce HP estimation inaccuracy for canopy structure and understorey light environment. Since short SCBs and low levels of exposure and thresholding errors can both result in low HP inaccuracy, to identify the true performance of new exposure and thresholding methods for HP, photographs recording canopies with long SCBs and acquired with fisheye lenses should be used. Because HP inaccuracy in a function of the amount of grey pixels resulting from SCBs, the amount of these pixels in photographs can be used as a universal parameter to quantify canopy properties influential to HP estimation and in turn make cross-study comparisons feasible. Electronic supplementary material The online version of this article (10.1186/s40529-018-0235-9) contains supplementary material, which is available to authorized users.

Wood density and carbon concentration are the two major parameters needed for carbon pool estimation. However, the two parameters are rarely measured in tropical forests. In this dataset, physical properties (wood density) and chemical properties (i.e. carbon, nitrogen, and hydrogen concentration) of woody debris across decay classes in tropical Taiwan were presented. The decay classes of woody debris were subjectively classified into five classes. Based on the dataset, a more precise estimation of carbon stocks in woody debris across decay classes can be obtained. For further description, interpretation and discussion about the dataset please refer to Chao et al. (submitted).

High image discrimination threshold values tend to be given to canopy hemispherical photographs (CHPs) with high exposure (resulting in bright images), but the effects of exposure on image threshold have been overlooked. A model canopy system was developed to precisely manipulate exposure (in relation to reference exposure measured from an unobscured overcast model sky), canopy openness, gap fragmentation and sky illumination of CHPs. We showed that there was a numerical trade-off relationship between exposure and image threshold of CHPs, whereas the last three factors had negligible effects on image threshold. A new thresholding method based on this relationship was shown to be applicable in the field and to allow users of hemispherical photography (HP) to compensate for the effects of higher exposure on CHPs with higher empirical threshold values. The advantages of this method are as follows: (i) it is an objective method; (ii) its overall performance is better than that of automatic thresholding methods throughout a wide range of canopy openness and exposure; and (iii) its efficiency of image analysis is higher than that of conventional manual and automatic thresholding methods, because all CHPs can be analysed with a constant empirical threshold value. Our results highlight the necessity of integrating both exposure and thresholding for optimal CHP results and the contribution that model canopies can make to methodological studies of HP.

Questions: Although forest ecosystems harbour many animal species and animal non-trophic effects are as ubiquitous as trophic effects, few studies have examined animal non-trophic effects on plant-plant interactions. Can animal non-trophic behaviour influence plant-plant interactions and thus, change the net effect of interactions, especially those between understorey vegetation and tree seedlings? How do the species traits of understorey vegetation and animals contribute to their interactions with seedlings? Location: The Main Range National Park, southeast Queensland, Australia. Methods: Seedling survival related to the cover of understorey vegetation (mainly the fern Lastreopsis decomposita) was monitored in a 0.5-m wide and 200-m long transect for 2 yrs. Ten per cent of the transect was caged to estimate the effects of non-trophic disturbances from two pheasant-size, ground-dwelling birds (Menura alberti and Alectura lathami) for 1 yr. Two hundred plastic tags (5 × 5 cm) were placed in the transect to quantify bird disturbance and litter input. Results: The negative effects of the fern understorey on seedlings were increased light deficits, greater risk of herbivory and wilting or rotting; the positive effects were reduced uprooting and litter smothering due to abiotic and bird disturbances. Notably, the exclusion of bird activity changed the net effect of the fern understorey from positive to neutral, and seedling survival was positively correlated with fern cover. Conclusions: The net effect of plant-plant interactions is subject to change when additional species are involved. In addition to trophic effects, non-trophic effects are such that they can change interactions between plants. A conceptual model of species traits contributing to interactions is presented. An understorey fern facilitates tree seedling survival through reducing nontrophic disturbances of birds and litter stress. Our results show that the net effect of plant-plant interactions is subject to change if additional species are involved. In addition to trophic effects, nontrophic effects of animals can be significant forces influencing interactions between plants.

There is a growing need to understand, and ultimately manage, carbon storage by forest ecosystems. Broad-leaved evergreen forests of Taiwan provide an outstanding opportunity to examine factors that regulate ecosystem carbon storage. We utilized data from three Taiwan Forest Dynamics Plots (FS, LHC, and PTY) in which every tree is identified, measured, tagged and mapped, to examine factors regulating carbon storage as estimated from aboveground biomass. Allometric equations were used to estimate the aboveground biomass of each tree, and a model building procedure was used to examine relationships between plot-level aboveground biomass (AGB; Mg/ha) and a suite of topographic and biotic factors. We found that our study sites have AGB values comparable to some of the most carbon dense forests in the world. Across all three sites, maximum biomass was contained in the taxonomic families Fagaceae, Lauraceae and Theaceae. In the FS site, we identified slope convexity (P = 0.03) and elevation (P < 0.001) as topographic predictors of AGB and found that maximum AGB was found in topographically flat areas. In FS, stem density (P < 0.001) was a significant biotic predictor of AGB and the maxima occurred at intermediate densities. In LHC, we found that convexity (P < 0.001) and slope (P < 0.001) were significantly related to AGB which was maximized along a topographic ridge in the plot. Species richness (P < 0.001) was a significant biotic predictor of AGB in LHC, and the relationship indicated slightly higher AGB at higher levels of species richness. The only significant factor related to AGB in PTY was species richness (P = 0.02). Further work is needed to seek a mechanistic understanding of topographic factors and species richness as drivers of carbon storage in forests.

In order to reveal the characteristics of the vegetation affected by monsoons at the northern border of Paleotropics, a tree-by-tree census was conducted in the lowland forests in the southernmost Taiwan (Nanjenshan) and an adjacent islet (Lanyu). The census recorded a total of 109,060 individuals (≥1-cm diameter at breast height) belonging to 255 vascular tree species in 1330 quadrats (10 × 10 m). Two-way Indicator Species Analysis first classified forest types into two groups, Lanyu and Nanjenshan, reflecting biogeographical differences. Five subgroups were further classified, showing correlations with topographic position indices. Forests located on wind-exposed slopes, regardless of elevations, were characterised by low canopy height, high stem density, high proportion of small stems, and high proportion of warm-temperate-related species, compared with the wind-sheltered communities. However, there were no significant differences in basal area and species diversity. In comparison with other tropical forests, our forests are characterised by high stem density, low diversity and a lack of the pan-Paleotropical dominant Dipterocarpaceae. In conclusion, vegetation in the studied regions not only showed a transition characteristic between Paleotropics and Holarctic Kingdoms in terms of composition, but also showed differentiations caused by their biogeographical history and the interaction between topographic positions and wind stress from monsoons.