Stand Structure Factors Research Articles

Importance of forest stand structures for gross rainfall partitioning on China’s Loess Plateau

Large-scale ecological restoration in the arid and semi-arid Loess Plateau region of northern China is challenged by the intensifying water stress due to the changes in precipitation regimes. Although the importance of Gross Precipitation Partitioning (GRP) has been well recognized in forest hydrology, especially in dry areas, the processes are not understood because site- or region-specific factors complicate the accurate quantifications. This study integrates meta-analysis and field observation to compare patterns of GRP in forest stands that had different origins (plantations vs. native forests) and tree species (deciduous vs. evergreen) compositions. We aimed at quantifying the dominant biotic and abiotic factors controlling each of the GRP components to form empirical relationships between the GRP components and the influencing factors via the Boosted Regression Trees (BRT) model. We found a convergence in the GRP components among the examined stands that have diverse tree species and origins. Our results indicated that, unlike stem flow (SF) and canopy interception (Ic), the throughfall (TF) did not differ between natural and plantation stands. Meanwhile, from the perspective of species composition, the broadleaf forest stands had significantly higher SF rates than the coniferous stands. We found that stand structures exerted limited influence on TF, but significantly affected SF and Ic. We hypothesized that the residence time of the GRP components was important in explaining the matrix of influencing factors. The cumulative influences of stand structure factors played a more important role than species in most conditions. Based on the most influential factors (cumulative explanatory power > 70 %) selected from the BRT model, we successfully built a general formula to represent the GRP processes across the diverse forest stands in the Loess Plateau. Our study provides insights into the underlying mechanism of GRP for developing more realistic forest hydrological models. An improved understanding of GRP and better models are important for guiding large scale reforestation efforts that account for hydrological processes across stand species and origins.

Effects of Stand Structure of Artificial Shelter Forest on Understory Herb Diversity in Desert-Oasis Ecotone

The relationship between the spatial structure of shelter forests and the diversity of understory herbaceous plants in desert–oasis ecotones is important for maintaining biodiversity indices and protecting the oasis ecosystem. In this paper, we explore the coupling relationship between tree layer structure (competition index, angle scale, neighborhood comparison, DBH, etc.) and understory herb diversity in the transition zone of shelter forest plots near oases and near deserts; in addition, we also aim to elucidate the dominant stand structure factors affecting herb biodiversity. The results indicated the following: A total of 13 herbaceous plant species were discovered in the transitional zone, with 11 species found near the oasis area and 4 species near the desert region. The Shannon, Simpson, and Pielou indices of understory herbaceous plants were significantly higher near the oasis area compared to the desert region. The Margalef index mean was higher in the oasis area compared to the desert region. Pearson and canonical correlation analyses revealed significant associations between specific stand structure indicators and diversity in the herbaceous layer. The results of the multiple linear regression analysis revealed that the competition index had a significant impact on the Shannon, Simpson, and Pielou diversity indices of the herbaceous layer in the understory of the shelterbelt forest near the oasis, with corresponding impact coefficients of 0.911, 0.936, and 0.831, respectively. The mingling degree was found to be the primary influencing factor for the Margalef index, with an impact coefficient of 0.825. However, in the understory of the shelterbelt forest near the desert, the neighborhood comparison ratio negatively affected the Shannon and Margalef indices, with impact coefficients of −0.634 and −0.736, respectively. Additionally, tree height negatively impacted the Simpson and Pielou indices, with impact coefficients of −0.645 and −0.677, respectively. In order to enhance the diversity of understory herbaceous species in the transitional zone and preserve the ecological system of the oasis, specific modifications to the forest structure and arrangement are essential. Pruning and thinning are necessary for shelterbelt forests located near desert regions, while shelterbelt forests near oases should use a suitable mix of tree species. These measures can help preserve or enhance the diversity of understory herbaceous plants.

Estimating the self-thinning boundary line for oak mixed forests in central China by using stochastic frontier analysis and a proposed variable density model.

A suitable self‐thinning model is fundamental to effective density control and management. Using data from 265 plot measurements in oak mixed forests in central China, we demonstrated how to estimate a suitable self‐thinning line for oak mixed forests from three aspects, i.e., self‐thinning models (Reineke's model and the variable density model), statistical methods (quantile regression and stochastic frontier analysis), and the variables affecting stands (topography and stand structure factors). The proposed variable density model, which is based on the quadratic mean diameter and dominant height, exhibited a better goodness of fit and biological relevance than Reineke's model for modeling the self‐thinning line for mixed oak forests. In addition, the normal‐truncated normal stochastic frontier model was superior to quantile regression for modeling the self‐thinning line. The altitude, Simpson index, and dominant height–diameter ratio (Hd/D) also had significant effects on the density of mixed forests. Overall, a variable density self‐thinning model may be constructed using stochastic frontier analysis for oak mixed forests while considering the effects of site quality and stand structure on density. The findings may contribute to a more accurate density management map for mixed forests.

Optimization of stand structure in Robinia pseudoacacia Linn. based on soil and water conservation improvement function

The analysis of complex relationship between stand structure and soil and water conservation function (SWCF) of forests and the precise adjustment of stand structure based on an improvement in function are the key technical problems of forestry ecological engineering construction in the Loess Plateau. This study was conducted in the Caijiachuan watershed of the Loess Plateau in western Shanxi, China. Sixty standard plots (20 × 20 m2), including 20 stand densities (900 ∼ 2,800 plants·hm−2), of Robinia pseudoacacia L. were established in the watershed. The 10 factors of stand structure and four functional indices of the SWCF, including 29 sub-functional indices, were investigated and monitored each year from June to September in 2016 to 2019. The results indicated that there are differences in the SWCF under different stand structures, and the differences are concentrated in the water conservation function (FWC) and soil conservation function (FSC) of functional indices of the SWCF. In particular, they are reflected in the soil moisture and hydrological characteristics of the litter of sub-functional indices. Stand structure factors could effectively explain the differences of functional indices of the SWCF of a R. pseudoacacia forest, and a rate of 81.4% of functional indices could be explained by stand structure factors. Based on the configuration of the optimal stand structure (included a stand density of 1,800 plants·hm−2, mean diameter at breast height of 15.19 cm, mean tree height of 10.49 m, leaf area index of 2.86, canopy density of 0.85, uniform angle index of 0.50, story differentiation of 0.59, diameter dominance of 0.45, and biomass dominance of 0.46), the density of existing stands was concentrated in the range of 1,500 ∼ 1,900 plants·hm−2 and was more effective at optimizing the SWCF. The optimization strategy of the stand structure of the R. pseudoacacia forest in the study area should focus on increasing the differentiation of forest layer, enrich the vertical structure of the stand and maintain a random distribution pattern of trees. The overall growth of the stand remained uniform, avoiding the excessive dominance of individual trees and unreasonable stand density. These results will provide a more scientific and complete reference to optimize the stand structure of the R. pseudoacacia forest in the Loess Plateau.

Structure of black walnut (Juglans nigra L.) stands on sandy soils in Hungary

Understanding the various processes and relationships that take place in forest ecosystems is generally possible only through long-term observations. This is especially true of the biological production of forests, through the in-depth exploration of their structure. In Hungary the black walnut (Juglans nigra L.) is one of the most valuable exotic tree species, mainly because of its very valuable wood, with fine tissues and unique colour, for furniture industry. Generally, the species is established by manual seeding, and can also be regenerated well by coppice shoots, but not by natural seeding. Black walnut regularly produces seeds from the age of 25–30 years and its rotation age is 70–80 years. It is also used outside forests due to the very decorative stem and crown shapes. In this paper, out of the stand structure factors, the relationships between age and height (r² = 0.7205), age and diameter (r² = 0.7719), age and number of stems per ha (r² = 0.3485) as well as between diameter and number of stems per ha (r² = 0.4595), all considered important for cultivation technology, were examined (based on the data collected in 34 black walnut stands, age of 7 to 67 years). We also analysed the diameter distributions in two black walnut stands with tending operations, reporting them as a case study. Its future role may be increased mainly on sandy soils of better quality, therefore the more accurate exploration of the structure of black walnut stands growing under such conditions can be considered as gap-filling.

A simplified method for application of natural regeneration in black locust (Robinia pseudoacacia L.) stands in Hungary

Black locust (Robinia pseudoacacia L.) is one of the most important forest tree species in Hungary, covering approximately 24% of the forest land and providing 25% of the annual timber supply. One third of these black locust stands are high forests (planted with seedlings) and the remaining stands are coppices. An auxiliary table was developed for planning the black locust natural regeneration in order to determine the sprouting criteria based on stand volume at the final cutting age. Twenty forest subcompartments were selected for investigating the possibility of black locust regeneration by root suckers. The basic yield and stand structure factors were determined using the numerical yield table for black locust stands. The results show that the regeneration of black locust stands from root suckers can be recommended on good and medium quality sites without a decrease in yield or stem quality. Black locust stands of good and medium quality (yield classes I, II, III and IV) may be regenerated from suckers in general until their growing stock attains and if their health is adequate. The simplicity of the practice-oriented auxiliary table based on the black locust yield table may further the development of management and wood utilization of the species.

Effects of forest structure on natural regeneration of Excentrodendron hsienmu population in Southwest Guangxi, China.

The effects of forest vertical structural variables (crown index, tree height, and large and small tree proportions) and density variables (basal area, adult tree density, shrub density, shrub coverage, and Excentrodendron hsienmu proportion in the forest canopy) on the natural rege-neration of the typical E. hsienmu populations in the karst mountains of Southwest Guangxi, China were studied. The results showed that the average regeneration density of E. hsienmu populations was 1742-3861 trees·hm-2 and relatively low. Forest vertical structure and density variables did not significantly affect the number of E. hsienmu seedlings, but significantly affected seedling ground diameter and height to some extent. Among the vertical structure variables, a significant nega tive correlation was observed between crown index and seedling height, but the association between crown index and seedling ground diameter was not significant. The large tree proportion and tree height were significantly negatively correlated with both seedling ground diameter and height. However, the proportion of small trees was significantly positively related to both seedling ground diameter and height. Among the density variables, adult tree density showed a significant positive correlation with both seedling ground diameter and height. A significant negative association was observed between the proportion of E. hsienmu and seedling ground diameter. Multiple regression analysis indicated that the model based on stand structure factors showed a poor fitness with regard to the number of E. hsienmu seedlings (P>0.05), and the quantitative distribution of E. hsienmu seedlings was affected to some extent by vertical structural variables. The stand structure factor model fitted well for the diameter and height of the seedlings (P<0.01). Seedling ground diameter was mainly affected by adult tree density, and seedling height was mainly determined by the combined effects of adult tree density and basal area.

Canopy precipitation interception in urban forests in relation to stand structure

Urban forests provide important ecosystem services. In terms of hydrological benefits, forest ecosystems in urban environments represent qualitative and quantitative filter for rainwater. We quantified the canopy interception in relation to urban forest stand structure and rainfall intensity in an urban transect of the mixed (upland) forest in the city centre, towards a riparian pine forest and a floodplain hardwood forest in the City of Ljubljana, Slovenia. Bulk precipitation in open areas and throughfall were measured with fixed rainfall collectors in each forest. Stemflow was estimated from a review of relevant literature. We found that canopy interception in selected urban forests was mainly affected by tree species composition and other stand structure variables, such as canopy cover and tree dimensions. Average annual canopy interception was highest in the mixed forest (18.0% of bulk precipitation), while the riparian pine forest had the lowest level (3.9% of bulk precipitation) and the floodplain hardwood forest had the intermediate level for interception (7.1% of bulk precipitation). The mixed forest exhibited the stand structure factors that contributed to the highest canopy interception among the studied forests: high assemblage of dominant coniferous trees, denser canopy cover and the highest growing stock. Furthermore, rainfall intensity has proven to be an important factor for the seasonal partitioning (comparing the leafed and leafless period) of canopy interception. A better understanding of precipitation interception processes in urban forests is needed to assist urban forest managing and planning, aiming at maximizing canopy interception for the mitigation of stormwater runoff and flooding in urbanized watershed.

Estimating aboveground biomass of broadleaf, needleleaf, and mixed forests in Northeastern China through analysis of 25-m ALOS/PALSAR mosaic data

Aboveground biomass (AGB) of temperate forest plays an important role in global carbon cycles and needs to be estimated accurately. ALOS/PALSAR (Advanced Land Observing Satellite/Phased Array L-band Synthetic Aperture Radar) data has recently been used to estimate forest AGB. However, the relationships between AGB and PALSAR backscatter coefficients of different forest types in Northeastern China remain unknown. In this study, we analyzed PALSAR data in 2010 and observed AGB data from 104 forest plots in 2011 of needleleaf forest, mixed forest, and broadleaf forest in Heilongjiang province of Northeastern China. “Poisson” regression in generalized linear models (GLMs) and BRT (boosted regression tree) analysis in generalized boosted models (GBMs) were used to test whether the constructed PALSAR/AGB models based on individual forest types have better performance. We also investigated whether adding topographical and stand structure factors into the regression models can enhance the model performance. Results showed that GBM model had a better performance in fitting the relationships between AGB and PALSAR backscatter coefficients than did GLM model for needleleaf forest (RMSE=3.81Mgha−1, R2=0.98), mixed forest (RMSE=17.72Mgha−1, R2=0.96), and broadleaf forest (RMSE=19.94Mgha−1, R2=0.96), and performance of nonlinear regression models constructed on individual forest types were higher than that on all forest plots. Moreover, fitting results of GLM and GBM models were both enhanced when topographical and stand structure factors were incorporated into the predictor variables. Regression models constructed based on individual forest types outperform than that based on all forest plots, and the model performance will be enhanced when incorporating topographical and stand structure factors. With information of forest types, topography, and stand features, PALSAR data can express its full ability in accurate estimation of forest AGB.

Estimating Stand Structure Using Digitalized Aerial Photography

This study estimates the structure of a forest stand using high-resolution data from digitized aerial photography. The study area was conducted on the fifteen plots with the size of 20 m × 20 m or 15 m × 15 m taken as samples of sugi (Cryptomeria japonica D. Don) stands. The stand factor was investigated based on its relation with the digitalized number (DN) on the black and white aerial ortho photos (scale 1:5,000, taken in 1995). We analyzed various stand structure factors and the distribution of DN from the image data using the Weibull function, which comforms well to every shape of distributions. Using the Weibull distribution, a significant negative correlation was observed between the quantity of stand factors (Mean DBH and the stand volume) and the distribution parameterc of DN. A significant positive correlation was also observed between the distribution parameterc of the stand factor (DBH, height and volume) and the distribution parameterc of DN.

The natural regeneration pattern and process of Korean pine population under natural Poplar-birch forest in Xiaoxing’an Mountains

According to a great deal of field investigation and detailed indoor analysis, the natural regeneration pattern and process of Korean pine population under natural Poplar-birch forest has been uncovered. The results show that the regeneration quantity and quality of Korean pine population under mountainous Poplar-birch forest and mountainous white birch forest are better than those under valley moss-grass White Birch forest and virgin broad-leaved Korean pine forest. Korean pine population shows aggregated distribution, the mass-occurrence period of Korean pine population is behind the mass-occurrence period of Poplar-birch population. Various aged Korean pines growth is affected by different stand structure factors. Man-made lighting tending can accelerate the regeneration process and increase the regeneration quality of Korean pine population. It is feasible to set up regeneration recombination of upper layer White Birch and lower layer Korean pines.