10. The mean individual biomass of carabid beetles in forest ecosystems

In 2002, the mean individual biomass (MIB) of epigeic carabid beetles was assessed on 16 second-level POPs located in the Regional Directorates of the State Forests of Gdańsk, Katowice, Kraków, Lublin, ŁódŸ and Szczecin.

The catches conducted in 2002, close a four-year series of the MIB studies on the forest monitoring plots. The present work is the recapitulation of the whole study period.

The catches of carabid beetles were carried out on 148 second-level POPs during the four years of studies. The results obtained from 10 plots were left out in estimations for methodical reasons.

The MIB values assessed for stands with dominating pine ranged from 64 to 660 mg. The MIB value on the plots representing oak stands oscillated between 61 and 532 mg. The MIB evaluated for spruce stands ranged from 66 to 392 mg, and the MIB value for beech stands - from 180 to 581 mg.

The MIB values depended on the forest-site type - Fig. 21. The highest mean MIB value 303 mg was found for stands representing the fresh mixed broadleaved forest-site type (LMśw), while the lowest values 138 mg for stands representing the mixed upland broadleaved forest-site type (LMwyż). For the stands representing other forest-site types, the mean MIB value oscillated between 215 and 276 mg. Dry coniferous (Bs), moist coniferous (Bw) and riparian (Lł) forest-site types were represented on single plots, so the results obtained are not the mean values. The MIB value for the dry coniferous forest site type, the poorest among the analysed site types, was nearly 450 mg and was one of the highest obtained MIB values.

While analysing the MIB values on the monitoring plots, attention should be drawn to the environmental factors such as age of a stand, site fertility, type of humus, as well as the closing of the canopy and degree of defoliation. The two letter factors influence the amount of light penetrating the forest floor, and thus, they can modify processes going on in this layer of the forest. The statistical analysis confirmed that the interactions of the MIB with the environmental factors were not as strong as could be expected. The coefficients of the Spearman's rank correlation between the MIB and some of the forest-related and environmental characteristics attained the values close to zero. The strongest negative relationship occurred between the MIB and the type of humus although the correlation was weak. Other characteristics of stands on POPs including stand age did not prove to significantly affect the state of carabid assemblages.

In a number of studies, the emphasis is put on the relationship between the MIB value and the threat posed by the foliophagous insects to forest stands [7]. In this study the strength of the relationship between the state of the populations of the most dangerous pine foliophages and the MIB value has been assessed. The analyses were based on the data from the last four years on the state of foliophagous populations on POPs with pine as a dominant species. The increased threat posed by pine beauty moth to stands was not observed on the plots where the MIB value was particularly low (<180 mg). The correlation coefficient between the MIB and the abundance of larvae was very low and showed negative values (r=-0.040; p>0.05). The correlation between the MIB values and the abundance of pupae of pine looper were at the average level (r=0.316; p>0.05). The correlation coefficient between the MIB values and the number of larvae of pine webworm did not exceed the average value (r=0.304; p>0.05). A strong correlation (r=0.646; p=0.166) was found between the number of larvae of pine lappet moth and the MIB values.

The overall threat posed by all the above-mentioned pine foliophages was poorly correlated with the MIB value (r=0.162; p>0.05).

The obtained results do not confirm the hypothesis about the relationship between the carabid MIB and the threat posed by the folivorous insects to forest stands. The positive values of the correlation coefficient for pine lappet moth and pine looper reveal that the state of soil processes expressed by the MIB value does not always have to indicate the vulnerability of forests stands to pest attacks.