Stand structure of an uneven-aged fir–beech forest with an irregular diameter structure: modeling the development of the Belevine forest, Croatia

Abstract

This study focuses on the problem of irregular diameter structure in a silver fir-beech selection (plenter) forest with a ''surplus'' of large diameter trees and a lack of natural regeneration and small diameter trees. We sampled 274 plots (900 m 2 each) in the Belevine research site (266.24 ha) in the mountain region Gorski Kotar (Croatia), where diameter (dbh) distribution, diameter increment, and natural regeneration were analyzed in detail. A low density of natural regeneration, weak annual recruitment of small (10 cm dbh) diameter trees (only five trees per hectare), delayed diameter growth of trees, and a low annual rate of trees reaching the next dbh class were attributed to the current irregular dbh structure. The stand development prediction for the next 50 years is based on a simulation model, which considers the current diameter structure, increment, recruitment, and future cutting regime. Intensive cutting in the first of five 10-year cutting cycles (intensity higher of 25%) is needed to initiate natural regeneration and to accelerate growth of young silver fir trees. In the next 50 years, the irregular diameter structure will be gradually improved.

Stand Regeneration Characteristics of Beech and Fir Forests in Gorski Kotar Region

“…This is specially emphasized in the case of conifers where regenerated area in private forests is four times greater than in the state forests. High growing stock with great share of conifers and almost even-aged structure is obviously not favourable for fir regeneration, as already reported in previous studies [19,20,23]. It is also interesting that thin broadleaved trees are significantly more abundant in state forests but also in private non-managed forests (without management plans), than in private forests managed according to management plans.…”

“…It can be tracked in diameter structure from stand-wise inventories, sporadic research on permanent plots in managed forests [19,20] and old growth forests [21], and also from the results of the first national forest inventory [14].…”

“…An insufficient level of natural regeneration leads inevitably to the collection and production of seeds and seedlings of silver fir, and also requires planning the planting procedures. At the same time adequate management activities should raise the level and success of natural regeneration [10,19,20] wherever and as much as it is possible.…”

“…It can generally be concluded that saplings are of good quality and survive when they appear, thus indicating favourable habitat conditions. On the other hand, management activities do not ensure enough level of sapling occurrence on most areas [20,23]. Future management influence is, however, supposed to change due to significant change in selection management approach recorded in the past decade [24].…”

Generating Virtual Even-Aged Silver Fir Stand Structure Based on the Measured Sample Plots

“…According to Čavlović et al [49], and Čavlović and Božić [50], that is the minimal area on which it is possible to establish a selective structure in the range of beech-fir forests of the Croatian Dinarides. On the other hand, the minimal stand area for sustainable even-aged forest management is one hectare.…”