A Novel Method for Evaluating Diversity of Stand Structure Based on Relationship of Adjacent Trees

Background Improving the diversity and complexity of stand structure is the basis for maintaining and increasing forest ecosystem biodiversity. Measures of stand structural diversity is important for predicting stand growth and evaluating forest management activities. Based on the relationship of adjacent trees, we present a new method for the quantitative analysis of stand structure diversity that allows comparison of stand structural heterogeneity between different stands and forest types and to quantify the impact of forest management on structural diversity. Method: The diversity of structural unit types was defined and then we derive a new index of forest structural diversity () according to the additivity principle of Shannon-Weiner index. The effectiveness and sensitivity to management were verified by sixteen field survey samples in different locations and six different simulated management datasets based on Pinus koraiensis broad-leaved forest survey sample. Results (1) The mountain rainforest in Hainan had the highest \({{S}^{\text{'}}}_{D}\) value at 5.287, followed by broad-leaved Korean pine forest in Jiaohe (2), Jiaohe (1) and oak broadleaved mixed natural forest in Xiaolongshan (2), with values of 5.144, 5.014 and 5.006, respectively. The \({{S}^{\text{'}}}_{D}\) values of plantations and natural pure forest were lower. (2) Different thinning methods and intensities reduced \({{S}^{\text{'}}}_{D}\) compared with no treatment and magnitude of the with the differences were greater as thinning intensity increased. The \({{S}^{\text{'}}}_{D}\) value of thinning from above decreased more than thinning from below at the same thinning intensity. Conclusion The\({{S}^{\text{'}}}_{D}\) well describes differences in stand structural diversity of different forest types and allows comparison of stand structural heterogeneity. It is also sensitive to forest management activities and to quantify the impact of forest management on structural diversity. The application of this new index \({{S}^{\text{'}}}_{D}\) could greatly facilitate forest management and monitoring.

Stand Structural Diversity and Species with Leaf Nitrogen Conservation Drive Aboveground Carbon Storage in Tropical Old-Growth Forests

Tropical old-growth forests are essential for global carbon regulation. Although there is increasing evidence that species and functional diversity, stand structural diversity, functional compositions, and elevation play roles in ecosystem functioning, the relative strengths of these drivers and the underlying mechanisms (mass-ratio hypothesis or niche complementarity hypothesis) are not clear. Aboveground carbon storage, species diversity, stand structural diversity, community-weighted mean (CWM), and functional diversity (FDvar) of 12 leaf traits were analyzed using data from 56 old-growth forest communities in the Dawei Mountain area of Southwest China. Multiple regression models were used to test the relative importance of the predictor variables and the structural equation model was used to explore the direct and indirect influences on aboveground carbon storage. High structural diversity moderately enhanced aboveground carbon storage. CWM leaf nitrogen concentration in young leaves weakly affected aboveground carbon storage. Our final multiple regression model showed that aboveground carbon storage is mostly affected by diameter at breast height (DBH) diversity, followed by FDvar of dry matter concentration in mature leaves and CWM nitrogen concentration in young leaves. The structural equation model indicated that elevation negatively affects aboveground carbon storage via diameter at breast height (DBH) diversity. Our results suggest that niche complementarity effects moderately drive aboveground carbon storage in tropical old-growth forests, but do not fully support the importance of the mass-ratio hypothesis.

Stand structural diversity and species with leaf nitrogen conservative strategy drive aboveground carbon storage in tropical old-growth forests

Background: Tropical old-growth forest ecosystems are essential for global carbon regulation. Even there are mounting evidences for the significance of species and functional composition, stand structure and elevation gradients on aboveground carbon storage, the relative strengths of these drivers and whether elevation effects via biotic factors are not clear. Furthermore, the mechanisms (the mass-ratio hypothesis or niche complementarity hypothesis) are still poorly understood.Methods: We analyzed aboveground carbon storage, species diversity, stand structural diversity, community-weighted mean (CWM) of functional traits and functional diversity (FDvar) using date from 56 old-growth forest communities with different elevation gradients in Dawei mountain of southwestern China. Multiple regression models were used to test the relative importance of the predictor variables and structural equation model was used to explore the direct and indirect influences on the aboveground carbon storage.Results: Our optimal multiple regression model show aboveground carbon storage is mostly affected by diameter at breast height (DBH) diversity, followed by FDvar of dry matter concentration in mature leaves and CWM nitrogen concentration in young leaves. The final structural equation model indicates elevation indirectly affected aboveground carbon storage via DBH diversity. The stand structural diversity, but not species diversity or functional diversity, enhanced aboveground carbon storage.Conclusions: Our results indicate mass-ratio and niche complementarity effect promote aboveground carbon storage simultaneously. The complex stand structure and species with leaf nitrogen conservative strategy were the crucial drivers of aboveground carbon storage in tropical old-growth forests.

Stand structural diversity and species with leaf nitrogen conservative strategy drive aboveground carbon storage in tropical old-growth forests

Background: Tropical old-growth forest ecosystems are essential for global carbon regulation. Even there are mounting evidences for the significance of species and functional composition, stand structure and elevation gradients on aboveground carbon storage, the relative strengths of these drivers and whether elevation effects via biotic factors are not clear. Furthermore, the mechanisms (the mass-ratio hypothesis or niche complementarity hypothesis) are still poorly understood. Methods: We analyzed aboveground carbon storage, species diversity, stand structural diversity, community-weighted mean (CWM) of functional traits and functional diversity (FDvar) using date from 56 old-growth forest communities with different elevation gradients in Dawei mountain of southwestern China. Multiple regression models were used to test the relative importance of the predictor variables and structural equation model was used to explore the direct and indirect influences on the aboveground carbon storage. Results: Our optimal multiple regression model show aboveground carbon storage is mostly affected by diameter at breast height (DBH) diversity, followed by FDvar of dry matter concentration in mature leaves and CWM nitrogen concentration in young leaves. The final structural equation model indicates elevation indirectly affected aboveground carbon storage via DBH diversity. The stand structural diversity, but not species diversity or functional diversity, enhanced aboveground carbon storage. Conclusions: Our results indicate mass-ratio and niche complementarity effect promote aboveground carbon storage simultaneously. The complex stand structure and species with leaf nitrogen conservative strategy were the crucial drivers of aboveground carbon storage in tropical old-growth forests.

Stand structural diversity rather than species diversity enhances aboveground carbon storage in secondary subtropical forests in Eastern China

Stand structural diversity, typically characterized by variances in tree diameter at breast height (DBH) and total height, plays a critical role in influencing aboveground carbon (C) storage. However, few studies have considered the multivariate relationships of aboveground C storage with stand age, stand structural diversity, and species diversity in natural forests. In this study, aboveground C storage, stand age, tree species, DBH and height diversity indices, were determined across 80 subtropical forest plots in Eastern China. We employed structural equation modelling (SEM) to test for the direct and indirect effects of stand structural diversity, species diversity, and stand age on aboveground C storage. The three final SEMs with different directions for the path between species diversity and stand structural diversity had a similar goodness of fit to the data. They accounted for 82 % of the variation in aboveground C storage, 55–59 % of the variation in stand structural diversity, and 0.1 to 9 % of the variation in species diversity. Stand age demonstrated strong positive total effects, including a positive direct effect (β = 0.41), and a positive indirect effect via stand structural diversity (β = 0.41) on aboveground C storage. Stand structural diversity had a positive direct effect on aboveground C storage (β = 0.56), whereas there was little total effect of species diversity as it had a negative direct association with, but had a positive indirect effect, via stand structural diversity, on aboveground C storage. The negligible total effect of species diversity on aboveground C storage in the forests under study may have been attributable to competitive exclusion with high aboveground biomass, or a historical logging preference for productive species. Our analyses suggested that stand structural diversity was a major determinant for variations in aboveground C storage in the secondary subtropical forests in Eastern China. Hence, maintaining tree DBH and height diversity through silvicultural operations might constitute an effective approach for enhancing aboveground C storage in these forests.

Relative contribution of stand characteristics on carbon stocks in subtropical secondary forests in Eastern China

Stand structural diversity, which is characterized by species diversity, variances in tree diameter at breast height (DBH) and height, plays an important role in influencing forest carbon (C) stocks. However, the relative contribution of stand structural diversity in contrast to other stand characteristics on the variation in C stocks in subtropical forests have not been fully explored. In this study, aboveground C stock, soil organic C stock, tree species, DBH and height diversities, stand age, and stand density, and site productivity were determined across 80 subtropical forest plots in Eastern China. Using simple regression analysis, we found that DBH and height diversities, site productivity, and stand age explained 49 %, 13 %, 41 %, and 50 % of the variation in aboveground C stock, respectively, whereas species diversity and stand density did not explained any variation (i.e., < 1 %). Multiple regression analysis indicated that variation in aboveground C stock was explained to a higher degree (83 %) by the joint effects of DBH diversity, stand age, site productivity, species diversity and height diversity than by stand structural diversity (54 %), and the other three stand characteristics (79 %) alone. The structural equation modelling (SEM) showed that the effect of stand age on aboveground C stock was stronger directly (beta = 0.59) than indirectly (beta = 0.11). Stand age has also significant and strong effect on DBH (beta = 0.63) and height (beta = 0.55) diversities. Six stand characteristics did not explain any variation in soil organic C stock (i.e., < 2 %), based on both simple and multiple regressions analyses, as well as SEM analysis. Our analyses suggest that, rather than species and height diversities, DBH diversity, stand age and site productivity cumulatively contributed to variation in aboveground C stock during stand development in subtropical secondary forests in Eastern China. Therefore, improving tree DBH diversity and stand condition could be an effective approach for enhancing C storage in subtropical forests.

Relationships between stand growth and structural diversity in spruce-dominated forests in New Brunswick, Canada

Relationships between stand growth and structural diversity were examined in spruce-dominated forests in New Brunswick, Canada. Net growth, survivor growth, mortality, and recruitment represented stand growth, and tree species, size, and height diversity indices were used to describe structural diversity. Mixed-effects second-order polynomial regressions were employed for statistical analysis. Results showed stand structural diversity had a significant positive effect on net growth and survivor growth by volume but not on mortality and recruitment. Among the tested diversity indices, the integrated diversity of tree species and height contributed most to stand net growth and survivor growth. Structural diversity showed increasing trends throughout the developmental stages from young, immature, mature, and overmature forest stands. This relationship between stand growth and structural diversity may be due to stands featuring high structural diversity that enhances niche complementarities of resource use because trees exist within different horizontal and vertical layers, and strong competition resulted from size differences among trees. It is recommended to include effects of species and structural diversity in forest growth modeling initiatives. Moreover, uneven-aged stand management in conjunction with selective or partial cutting to maintain high structural diversity is also recommended to maintain biodiversity and rapid growth in spruce-dominated forests.

Does Variable-Density Thinning Increase Wind Damage in Conifer Stands on the Olympic Peninsula?

Silvicultural treatments designed to enhance stand structural diversity may result in increased wind damage. The ability to avoid conditions that might lead to excessive wind damage would benefit forest managers. We analyzed wind damage following implementation of a variable-density thinning at four sites on the Olympic National Forest in northwest Washington. The prescription created small canopy gaps and retained unthinned patches within a uniformly thinned matrix, thus creating substantial amounts of internal edge. Our objective was to determine whether variable-density thinning resulted in elevated wind damage and whether the damage was spatially related to elements of the treatment, i.e., canopy gaps and uncut patches. Wind damage on the thinned plots averaged slightly more than 8.0 trees/ha. Although precise determinations of residual stem densities were not available, we estimate that total wind damage amounted to 1.3% of total stems remaining following treatment. Approximately 80% of the wind damage was blowdown, the remaining damage being stem breakage, leaning, or bowing. Nearly 54% of the damaged stems were less than 20 cm dbh. The maximum amount of damage observed was 51 trees/ha, but only 3 of 13 thinned plots had wind damage exceeding 7 trees/ha. The overall level of wind damage across all thinned plots after two growing seasons was not statistically greater than on unthinned control plots. Internal edges created by gaps, skid trails, and unthinned patches did not inherently increase wind damage risk; however, where gaps were located in topographically vulnerable positions, greater wind damage did occur. Overall wind damage was not excessive on any of the plots, and after 2 years, all residual stands remained intact and in a manageable condition. Our preliminary results suggest that variable-density thinning that includes creation of small canopy gaps does not necessarily predispose stands to greater risk of wind damage than uniform thinning. However, care must be taken in locating gaps and skid trails away from topographically vulnerable positions.