engleski

Estimating forest stand structure based on re- measurement on concentric circular sample plots

Field plots serve as a basic data source on forest stand structure – number of trees (N), basal area (G) and tree volume (V). Concentric circular sample plots for measuring trees of different size are designed to balance work efficiency and precision of results. However, these type of plots present difficulties when used as permanent plots for repeated measurements, because of ingrowth trees and change of plot size with tree size. The aim of this research was to evaluate the outcomes of stand structure (N, G, and V) from re- measurement on concentric circular plots. Tree data from 29 field plots with 20 m radius measured in selection fir-beech stand in Gorski kotar, Croatia, were used as reference values. Concentric plots (7, 13 and 20 m radii with dbh thresholds 10, 30 and 50 cm) were generated at measurement time 1 (T1). Modelling increment and ingrowth resulted in stand structure at time 2 (T2) for both plot types. Differences in N, G, V between plot types were examined on stand, plot and tree level and tested on stand level using paired t- test with 0, 05 significance level. The concentric plots slightly underestimated fixed plot results on stand level at T1: N by -38, 0 stems/ha (7, 7%), G by -0, 9 m2/ha (-2, 5%) and V by -7, 4 m3/ha (1, 6%), and resulted with higher variability, especially for number of stems. The differences at plot level ranged from 18, 8 to 320, 8 stems/ha (N), 0, 5 to 9, 9 m2/ha (G), and 0, 0 to 131, 5 m3/ha (V). At T2 concentric plots statistically significantly overestimated N by 88, 0 stems/ha (15, 6%, p=0, 01) on stand level, and gave very close estimates of G (+0, 6 m2/ha) and V (+1, 2 m3/ha), with higher variability than fixed plots. Individual differences at plot level ranged from 1, 1 to 377, 4 stems/ha (N), 0, 0 to 14, 6 m2/ha (G), and 0, 5 to 194, 4 m3/ha (V). The re-measurement on concentric plots showed reliable estimates of G and V, likely resulting from two opposite effects that cancel each other at stand level. The inclusion of new trees that reach dbh thresholds 30 and 50 cm in second measurement increases, while larger plot area for trees that shift to higher dbh class decreases the results. Number of stems is apparently more influenced by ingrowth than reduction of tree factor and its estimate is not found reliable. These effects should be taken into account when using this plot type and investigated in different stand conditions.

forest inventory ; field plots ; repeated measurement ; sampling ; stand structure

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