Croatian Journal of Forest Engineering (Apr 2011)
Impact of Stand, Site and Structural Characteristics on Stand Regeneration Planning in Pedunculate Oak Forests
Abstract
Starting from the fact that stand structure is significantly disturbed and from the assumption that all stands of pedunculate oak at forest level older than 100 years are potentially considered for stand regeneration, it is obvious that forest planning and management have to define their priorities on the basis of which individual, potentially mature and mature stands, will be regenerated. The general objective of this paper is to research the structure and mutual impacts of stand, site and economic factors on the elements of stand structure in a greater area of pedunculate oak forests consisting of old or potentially mature pedunculate oak stands with a disturbed structure. Based on obtained structural relations, the goal was to determine by modeling the easily obtainable elements of the variable (model), which will represent the objective criterion for stand ranking according to the regeneration priority and structural and economic requirements.The subject of the research are pedunculate oak forests in the Central Posavina region, where 37 stands, with stocking of less than 0.8, were randomly selected and divided into 3 age groups (101–120 years, 121–135 years, >135 years) in 16 management units.On a total of 146 established plots of 25 m radius, estimates and measurements were carried out of variables at the level of individual model trees (6 trees of the main species – upper storey layer – nearest to the plot centre) and of stand and site variables (2 breast-height diameters, crown diameter, tree height, base height of the crown and height of the widest part of the crown, varietal trunk structure, crown defoliation, increment core, breast-height diameter of all trees higher than 1.3 m, seedlings, potential intermediate cutting, died trees, diameters of stump of cut trees, shrub layer, microrelief, canopy, health status, litter, understorey vegetation). The estimated and measured data were processed and appropriate variables for data analysis derived. The complex potential revenue (rent) difference was defined for stand regeneration (Equation 3) as a dependent variable and objective criterion for stand ranking according to regeneration priority. By means of partial linear analyses, multivariate analysis and modeling (generalized linear modeling), mutual and complex impacts of stand, site and economic factors on elements of stand structure and potential revenue (rent) difference were investigated. Statistical analyses were performed by application of the SAS software package, whereas descriptive statistics, correlation analyses, integration of functions and development of graphical presentations were carried out by means of the STATISTICA 8.2 software package.The research included different management, stand and structural characteristics (Table 1) and determined a statistically significant mutual impact of individual variables of stand, site and economic factors on certain elements of stand structure and potential revenue (rent) difference (Table 2). Multivariate analysis revealed models of complex impacts of stand, site and economic factors on standing volume of pedunculate oak and revenue (rent) difference (Table 3). According to the obtained data, all three models are statistically significant for the estimated standing volume of pedunculate oak and also for the estimated revenue (rent) difference. The stand factors in total account for 35% variability of standing volume of pedunculate oak and for 22% variability of revenue (rent) difference. The site factors account for 39% variability of standing volume of pedunculate oak and for 34% variability of revenue (rent) difference, whereas economic factors account for even 45% variability of standing volume of pedunculate oak and for 43% variability of revenue (rent) difference. The results of the Tukey post hoc (HSD) test for statistically significant variables of multivariate analysis of the revenue (rent) difference and the standing volume of pedunculate oak (Table 4) indicate that there exists a relation between the standing volume of pedunculate oak as a complex structure element and the potential revenue (rent) difference. The results proved the existence of a direct impact of individual elements of main tree species as well as a complex impact of structure elements on the variable of potential revenue (rent) difference (Fig. 1 and Fig. 2).The standing volume of pedunculate oak as a complex structure element is a suitable dependent variable, along with the revenue (rent) difference, for comparative analysis of impacts of statistically significant stand, site and economic factors on stand regeneration priority. Due to a significantly smaller standing volume of pedunculate oak as a consequence of its disturbed structure, there is a higher regeneration priority for the youngest stand group (100 to 119 years) in comparison with the older stands. Accordingly, in forests with disturbed structure age cannot be an objective criterion for planning stand regeneration. Stands with disturbed canopy of understorey layer with the smallest and lowest value of standing volume expectedly have the highest regeneration priority, i.e. their maintenance would lead to both economic losses and disturbed site conditions. In view of site preservation, the problem is particularly marked in case of pure stands, where pedunculate oak trees do not cover the soil in a sufficient measure, and have a worsening tendency due to weakening and dying of trees. According to the obtained results, which indicate that there is a relation between site and standing volume of pedunculate oak and revenue (rent) difference, stands with better site have lower regeneration priority. This is related to the fact that stand structure on a higher quality site is characterized by a lower number of pedunculate oak trees of larger dimensions and potentially large value increase, which is also characteristic of forest communities of pedunculate oak and common hornbeam. On average, a smaller standing volume of pedunculate oak in a floodplain pedunculate oak forest and a higher regeneration priority indicate disturbed structure conditions. Due to hydro-ameliorative works, this community has suffered even greater site changes and is more in danger of dieback, so regeneration will be more difficult and expectably less successful in case of stands with disturbed site and structure conditions, both at early and delayed regeneration. Regarding the proportional relation between the quantity of prescribed cut and the standing volume of pedunculate oak, stands with better structure and higher prescribed intermediate cut are expected to have a lower regeneration priority. A prescribed cut below 15 m3/ha (50% sampled plots) indicates a significant share of preserved stands left to natural development, with necessary felling of only weak and dying trees by the beginning of regeneration or recovery. The stand structure characteristics according to their belonging to individual management units are more a consequence of spatial differences in site and structure conditions than a management characteristic. Thus we can separate 3 management units in the centre of the Lonjsko polje floodplain (Žutica, Kutina floodplain forests and Brezovica), where dieback of pedunculate oak was the highest and which have a more marked stand regeneration priority in comparison to the management units with a more preserved structure of old stands (Trstika, Josip Kozarac). The impact of individual structure elements of pedunculate oak, unlike the rent, has a high correlation, with the exception of numbers of pedunculate oak trees. This can be explained by the fact that the same number of pedunculate oak trees can represent very different structural relations in terms of average dimensions of pedunculate oak trees, basal area and standing volume of pedunculate oak as well as different rate of other tree species in the structure. A particular significance with regards to the application of the results can be seen in the fact that easily obtainable structural elements, such as pedunculate oak basal area, number of pedunculate oak trees and diameter increment of pedunculate oak, proved to be very good indicators, which can explain nearly 70% variability of the complex impact on the revenue (rent) difference.The obtained research results prove the existence of a direct relation between stand structure and regeneration priority. There is, therefore, a practical possibility to apply the established complex variable of revenue (rent) difference for ranking of stands or stand parts (stand dividing) according to regeneration priority, which can be determined by means of easily obtainable elements of structure (tree numbers, basal area and diameter increment of pedunculate oak). An extension of the research into site-structure relations to include a greater, or even the entire area of pedunculate oak forests in Croatia as well as spatial and multiobjective forest management criteria (spatial distribution and area of stands for regeneration, habitat requirements) for stand regeneration planning would be a welcome continuation of this work.
