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ŠUMARSKI LIST 3-4/2017 str. 29     <-- 29 -->        PDF

Adamopoulos et al. (2012) have used soil depth in combination to several other parameters, for the site quality distinction in reforestations found in the study area of the present study.
Similarly, topography is the second more important variable (importance = 0.21) in the classification of the stands in site qualities using boosting method. Topography incorporates slope, and slope is related to soil depth. Usually higher slope leads to lower soil depth (see Dafis 1986). In the present study, low productivity sites (site quality III) exhibit greater lowest and highest slope, compared to productive and medium productivity sites (site qualities I and II).
The most important variable (importance = 0.79) in the classification of the stands in site qualities is canopy density. A higher value of canopy density implies higher amount of photosynthetic tissues for a given area, being an indication of greater production ability. Canopy photosynthetic capacity is related to site productivity (Coops et al. 1998). According to Dafis (1986), in productive sites plants can endure heavier shade. As a result, in productive areas a higher canopy density is expected, compared to that of medium and low productive locations, since under the crowns of overstory more trees can survive. Moreover, in poor sites trees’ crowns are small, as a result of slow or lack of differentiation (Oliver and Larson 1996). In the present study, low productivity sites (site quality III) exhibit lower lowest canopy density, compared to sites of high and medium productivity (site qualities I and II). However, canopy density is influenced by many factors, such as silvicultural treatments or other disturbances, stand development stage and species ecology. In particular, in the present study cuttings from the local residents and grazing strongly affected vegetation structure in the past, while silvicultural interventions in the frame of forest management are taking place (Consorzio Forestale Del Ticino 2005). Moreover, as it is referred in the description sheets of the stands, pine formations (mainly) of Pinus brutia, oak formations as well as mixed formations of pines and oaks occur in the study area (Consorzio Forestale Del Ticino 2005). The different shade tolerance of the species, with their different position in the stand stories (overstory, middlestorey or understory) undoubtedly determines canopy density.
Even though canopy density is estimated in the form of lowest and highest values, and the influence of species mixture and stand vertical structure are not weighted up in the model, the correct classification of stand in site qualities I and II was 4% and 77%, respectively, while in site quality III was 99%. If we consider that the initial site classification comprised over 70% of the Dadia-Lefkimi –Soufli forest area in site quality III (Consorzio Forestale Del Ticino 2005), then the usage of the boosting method for creating a collective classifier for site qualities in the studied forest can be characterized as fully successful.
The development of alternative methods of site classification, using topographic features instead of growth of trees having certain characteristics, will be notably useful in bare and intensively degraded areas with sparse distribution of plants (see Smith et al. 1997). In this case, the usage of three site productivity categories (site qualities) instead of five or six is more practical and feasible in restoration of former forest lands. Furthermore, in areas were microtopography is highly variable, resulting in chances of site condition in rather small spatial scales (see also Papalexandris and Milios 2010), topographic features in combination with canopy density estimations can be used for site classification. In this case, two or three site productivity categories are preferable, since the frequent changes of site conditions make difficult to apply a more complex system. On the other hand, the adoption of canopy density as a variable for site classification models is very efficient, in cases where the vegetation is disturbed and the growth of most trees has been affected by these disturbances. Site index models based on dominant and codominant trees assume that these trees where always in that status and their growth did not retarded from disturbance factors (Smith et al. 1997, Raulier et al. 2003). Highest and lowest canopy density seems to be affected in a lower degree than the growth of dominant and codominant trees by disturbances (including silvicultural treatments). Only in extreme cases where trees’ density in the whole area has been determined by intense disturbances, then the two extreme values of canopy density is affected. On the other hand, mean canopy density is a parameter that is more sensitive to disturbances that influence trees’ density in any area of the stand.
In this study, site classification is not based on soil analysis (Bravo and Montero 2001; McKenney and Pedlar 2003) or