Šumarski list, Vol. 133 No. 7-8, 2009.
Izvorni znanstveni članak
Development Structure Elements in Mixed Oak Stands in Aria of ForestAdministration Bjelovar with Retrospect on Modelling Growth andYield of Mixed Stands
Krunoslav Godina
; Hrvatske šume Šumarija Pakrac
Sažetak
Most yield tables are constructed for pure stands. From them can be expor ted yield tables for mixed stands. In them, due to a simple calculated construction, refers among tree species according to the number of trees, height and diameter of the mean tree, often do not represent realistic relationships in mixed stands. For modelling growth and yield, it is important to understand biodiversity, dynamics mixed stands and growth mo dels. The growth model of a stand is an abstraction of a natural dynamics in forest stands, and it can be simple or complex, according to its construction. Among complex models for modelling dynamics of mixed stands the most appropriate are gap models.
In unmanaged mixed stands of three subassociations within three age groups (12–15; 19–22; 27–30 yrs.), the sample plots (table 1) were used for measurement and evaluating the vitality of each tree as a consequence of competition for light among tree crowns. Based on consideration of the vertical structure, diameter and crown luminance, the su bassociation of pendunculate oak and common hornbeam has the greatest share of vital trees of the main species (picture 2). With age increasing in all subassociations, growing the share of too suppressed (-+) and dry trees (-) of pendunculate oak, but expressed in basal area, their share is less (picture 3). In the subassociation of pendunculate oak and narrow ash within the third age group, a great number of dominant and some predomi nant trees, influences the reduce in number of trees in lower layer, which is recognised by the omission of semidry and dry trees of secondary species (picture 4). The subassociation of pendunculate oak, narrow ash and black alder distinguishes itself by even more in tense extraction of the main tree species, so there are a great number of suppressed yet perspective (+-) and dry (-) penduculate oak trees (picture 5). After simulated harvest, the growth of mean tree was projected according to the tree species. Simultaneously with the vitality evaluation, the competition among neighbouring dominant trees was being deter mined, and the influence on lower perspective trees, primarily of the main species, was evaluated. The simulated harvest gave the advantage to pendunculate oak and the predo minant and dominant trees of the secondary species were mainly removed. The structure of the stand did not involve dry (-) and too suppressed trees (-+), which, with the previou sly mentioned, gives the total simulated harvest (picture 6 and 7). This was the way the main stand was gained. A growth projections of the mean tree was performed for the main stand according to the tree species, and the structural elements were determined in this way after five years. The growth projection for a single subassociation was gained from samples of circumference and tree heights after total simulated harvest that could more represented the regularly managed stands.
The height growth models during a period of time, for each of the three researched su bassociations, are gained by equalizing of tree heights samples according to the modified Mihajlov function (formula 1) in aplication programme Statistica 6.0. The same way were obtained circumference growth models. A portion of the explained variability (R2) is the largest in height growth models after total simulated harvest, and it is considerably lower in circumference growth models before the simulated harvest. Variability (S.D.) in creases with the age increase, and there is a greater circumference variability in relation to the heights.
The current diameter and height increment of the pendunculate oak is the biggest in subassociation with common hornbeam, while it is considerably lower and roughly equal in correlation within the remaining subassociations. The competition of the main stand trees is mostly manifested through the height increment in purpose of conquering the local area for undisturbed crown development. For all tree species in research stands, the height increment is mainly bigger, while the diameter increment is lower in relation to the yield table (picture 13–15).
The basal areas and volume projections of researched mixed stands for the three age groups after the simulated harvest are considerably different in relation to the mensura tion stand (picture 10–12). The differences in relation to the mensuration stand appear due to equalization of data by Mihajlov’s growth function, because the biggest amount of the diameter and height growth are realised within the first age group. Then, the volume of died and unqualitative trees, since they were not adequately and in time cut, increases mainly successively in relation to the first age group (table 2).
Different volume projections reflect on the current volume increment, which is conside red as accumulated iVt(a) and productive iVt(p). The accumulated current volume incre ment is the volume difference of the main stand at the end and at the beginning of the period (picture 16). The accumulated increment is under a strong influence of harvest, and it can be positive, negative or equal to zero. For pendunculate oak as the main species, a positive amount accumulates, while secondary species can also have negative amounts of the accumulated increment. The production current volume increment is an volume in crease of the main stand until the next harvest. The productive increment depends mainly on structural elements of the mean tree and regards to prominent vertical structure of the mixed stand, on the number of trees and on giving the advantage to the main tree species for the undisturbed development. The productive current volume increment for all mixedstands is considerably bigger in relation to the yield tables (Špiranec, 1975), and in the second age group, the subassociations of the pendunculate oak and narrow ash are even bigger by 182 % (picture 17). This points to a very high development potential of young unmanaged mixed stands, which decreases with an increase in age as a consequence of mortality, falling off increments and harvest. There is a question of how to preserve the de velopmental potential in mixed stands. Silviculture interventions should be done in time in order to enable trees supstitutions from lower to higher layers and, on the basis of a spatial structure, an optimal diameter and height increment for the trees of the main stand.
Ključne riječi
Hrčak ID:
40467
URI
Datum izdavanja:
31.8.2009.
Posjeta: 1.341 *