Osnovni elementi strukture sastojine (broj stabala, temeljnica i volumen) procjenjuju se na temelju uzorka primjernih ploha. Cilj ovog istraži vanja je usporediti rezultate procjene strukturnih elemenata sastojine dobive ne na primjernim plohama različite veličine, te posredno ocijeniti učinkovitost iz mjere glede veličine ploha. Mjerenja su provedena u bukovo-jelovoj sastojini Nastavno-pokusnoga šumskog objekta “Zalesina” na području prebornih šuma Gorskoga kotara u Hrvatskoj. Na sistematskom uzorku 17 koncentričnih kruž nih primjernih ploha izmjereni su prsni promjeri stabala te njihov položaj (azi mut i udaljenost) u odnosu na središte plohe. Pri tome su stabla iznad 10 cm promjera mjerena na krugovima radijusa 13 m, stabla iznad 30 cm promjera na krugovima radijusa 19 m, te samo stabla deblja od 50 cm na krugovima radi jusa 26 m. Načinjen je računalni program za izračun strukturnih elemenata sastojine te njihovu simulaciju i obračun po plohama radijusa različitih od onih izmjerenih. Uspoređeno je osam veličina kružnih ploha – osim mjerenih ploha (radijusi 13, 19 i 26 m) uzete su plohe radijusa korištenih u uređajnoj inventuri (7,98 m; 12,62 m; 5 i 12 m), nacionalnoj inventuri (7, 13 i 20 m)i nekoliko po kus nih veličina krugova (9,77 m; 11,28 m te 7 i 13 m). Na svim stajalištima su za svaku veličinu plohe izračunate prosječne vrijednosti broja stabala, temelj nice i volumena po hektaru. Razlike u procjeni strukturnih elemenata na ra zini sastojine između ploha različitih veličina nisu statistički značajne, uz ra zinu značajnosti 0,05. Za rezultate procjene izračunata je preciznost procje ne uz 95 % pouzdanosti koja izravno ovisi o odabranoj veličini ploha. Bolja preciznost procjene strukturnih elemenata dobivena je na većim plohama, gdje je zbog većeg broja uključenih stabala dobivena manja prostorna varija bilnost. Primjenom koncentričnih krugova posebice je kod procjene broja sta bala zbog manje mjerenih stabala povećana varijabilnost i pogreška uzorka. Preciznost procjene temeljnice i volumena na koncentričnim krugovima nije znatno pogoršana, što upućuje na opravdanost njihove primjene zbog moguće uštede pri izmjeri. Dobiveni rezultati omogućuju odabir željene veličine ploha uzorka na temelju omjera broja mjerenih stabala na plohama i kakvoće (pre ciz nosti) procjene strukturnih elemenata. Prije praktične primjene potrebno je istraživanje provesti na većem uzorku ploha u više sastojina različitog pro stornog rasporeda stabala te analizirati razdiobe broja stabala, temeljnice i volumena po debljinskim razredima. Mjerenje vremena potrebnog za izmjeru ploha omogućilo bi točniji izračun učinkovitost izmjere na plohama različite veličine.Stand structure estimate is based on data from sample plots. The aim of this research was to compare the stand structure estimates based on sample of circular plots with different radii. Through this influence of plot size on structure estimate and efficiency of stand measurement was also indi rectly assessed. Measurements were made in beech-fir selection stand in the Educational and experimental forest site “Zalesina” in Gorski kotar region, Croatia. Stand size is 20,63 ha, it is situated from 790 to 850 m above sea level, and belongs to site class II. Stand exposition is south to east, terrain slope is 5–10°. Tree breast height diameters (DBH) were measured on syste matic sample of 17 concentric circular sample plots. Tree location from plot centre was recorded by azimuth and distance. All trees of DBH 10 cm or more were measured on 13 meter radius plot, trees of DBH 30 cm and more were measured on 19 m radius plot and trees of DBH 50 cm and more on 26 m ra dius plot. Computer programme CirCon for calculation of stand parameters based on measured plots and simulated plots, with radii different from measu red ones, has been developed. Plots based on real measurements were simula ted according to ones used in forest management practice (singular and concentric circle plots). We simulated 8 methods: K7,98 (7.98 m radius plots), K9,77 (9.77 m radius plots), K11,28 (11.28 m radius plots), K12,62 (12.62 m radius plots); K5-12 (concentric circle plots with radii 5 and 12 m), K7-13 (concentric plots with radii 7 and 13 m), K7-13-20 (concentric plots with radii 7, 13 and 20 m) and K13-19-26 (three concentric circles of 13, 19 and 26 m radius). Calculated estimates for number of trees, basal area and volume on the same standing points differed between methods depending on spatial tree distribution and size of plots. Descriptive statistics (arithmetic mean, stan dard deviation, standard error) was made for each variable (number of trees, basal area and volume) on stand level. Sample error with 95 % confidence (SE/mean*t0.05,) was also calculated to express the precision of estimates. Different estimates by methods depending on plot size were compared by repeated measures ANOVA, due to lack of independence between methods (plot sizes) on the same standpoints.
Estimates of number of trees by methods (Figure 2) ranged between 275.4 and 303.5 per hectare, although differences were not statistically significant at 0.05 confidence level (Repeated measures ANOVA: F = 0.6027, df = 7, p = 0.7526). Precision expressed by relative sample error varied from 13.58 % (K13-19-26) to 28.34 % (K5-12). Better results (lesser sample error) were ob tained on bigger plots, though concentric circles (K5-12, K7-13 and K7-13-20) have considerably greater sample error due to fewer trees per plot.
Basal area estimates by methods ranged from 34.80 to 37.76 m2per hec tare (Figure 3), making no statistically significant differences (Repeated mea sures ANOVA: F = 0.2948, df = 7, p = 0.9547). Relative precision ranged from 10.13 % (K13-19-26) to 26.96 % on smallest plots (K7,98). Sample error of basal area estimate on concentric circles was just slightly bigger in spite of fewer trees per plot. Reason for that is stability of basal area on plots regardless to fewer trees: concentric circles include fewer trees but have great share of bigger ones that contribute to basal area more than smaller ones.
Estimate of stand volume by methods ranged from 457.93 to 496.47 m3per hectare (Figure 4). There was no statistical difference in volume estimates between analysed methods (Repeated measures ANOVA: F = 0.2650, df = 7, p = 0.9661). Relative precision ranged between 10.14 % (K7-13-20) and 30.36%(K7,98). Better precision was obtained with bigger plots, due to more trees per plot. Concentric circles produce just slight increase in sample error while lowering the cost of measurement by reducing the number of trees per plot.
Number of measured trees per plot was computed as an indicator of plot ef ficiency. Differences in number of trees per plot between plot sizes were statistically significant at 0.05 level (Repeated measures ANOVA: F = 187.621, df = 7, p = 0.0000), except for: K7,98 and K5-12; K7-13 and K9,77; K7-13-20 and K12,62 (Fisher LSD Post-hoc test).
Evident increasing trend of number of trees per plot by increasing of plot size is the main cause of better precision. Although concentric circles reduce number of trees per plot, loss of precision for basal area and volume are mini mal (Figure 5). Therefore plots K5-12 are acceptable for use in this kind of stands, with remark that they require well trained staff and modern instru ments. Plots K7-13 do not improve precision while increasing number of trees per plot (9), therefore are not recommended. Triple concentric circles K7-13-20 reduce sample error almost by 10 %, although by significant increase of mea sured trees per plot.
Plots K11,28 reduce number of trees per plot with minimal increase in sample error compared to K12,62 plots. That fact makes them acceptable choice for gain in efficiency. However, K11,28 sample should be adjusted with more plots to satisfy required sampling intensity (5 % of stand), which would increase costs. In order to simplify the sampling plan, legislation does not re quire precision rather sampling intensity (5 % of stand area), which restricts opportunity to optimize sample size.
The choice of plot size is based on inventory goals and should depend on cost of measurements and expected precision. This kind of research can pro vide useful base for determining plot size by costs and precision of data. Exact ratio of cost and precision could be computed by including time measurement per plots of different sizes
