Features of Spatial Snag Distribution in a Beech-Fir Forest

With the participation of about 65% in the total growing stock of conifers, silver fir is the most important and the best represented conifer species in Croatia. The increasing share of sanitary felling in the annual felling plans demands that special attention be paid to the health status of fir forests and the quantity of unplanned felling. This is the reason why dieback of silver fir in beech-fir forests in the Dinaric Alps is an acute and topical management-ecological problem. In the past 10 years, the share of total unplanned felling in the overall annual felling for fir has amounted to 57%. Such a high share of unplanned yield (mainly snags) has had a negative effect on sustainable forest management. Determining the spatial distribution of damaged trees and snags, as well as their monitoring is one of the priorities of sustainable management. Hence, it is of utmost importance to detect less healthy stands and apply timely measures for the purpose of maintaining their vitality and productivity at an optimal level. Due to the seriously disturbed stand stability caused by forced canopy opening, which in turn affects the health condition and natural regeneration of the stand, it is necessary not only to detect snags but also determine the causes of dieback. According to the results of past research, increased tree dieback is associated with the impact of different site and stand characteristics (altitude, exposition, slope, soil, structure, etc.), as well as abiotic and biotic factors (fir needle moth, mistletoe); however, the real causes are yet to be discovered. It is difficult to make efficient snag inventories with standard field methods. For large areas, a remote sensing method (the application of CIR aerial photographs) is much more practical, more cost effective and more reliable. All research activities so far have proved that, in terms of accuracy, this method is equal to field working methods, while in terms of speed and objectivity it is far more efficient. Snag inventories by means of CIR aerial photography interpretation and GIS enable efficient enumeration (detection) and spatial distribution (mapping) of snags. The proposed method is highly important not only for the detection and monitoring of the occurrence, distribution and direction of dieback progress, but also for the detection of possible dieback causes, on the basis of which experts will prescribe adequate measures and treatments. The objective of CIR aerial photography interpretation is to determine the current situation and acquire an objective picture of forest condition (data