In most of European countries, basic information about forests that is used for its monitoring and formulation of national policies comes from the National Forest Inventories (NFIs). Assessment of forest resources at the national level was initiated in Poland in 2005. In 2014, the second cycle of NFI was completed. Since 2010, results of NFI are the main source of information about the amount of forest resources in Poland, their structure and condition. In the paper, we analyse the indicators of forest structure. Classification of forest categories (forested, temporary non−forested, related to forestry), vertical stand structure, species composition and age structure determined directly from NFI's sample plots were compared with descriptions of stands in which plots are located. Data from more than 29,000 NFI sample plots measured in 2010−2014 were analysed. Our results demonstrate that the share of temporary non−forested areas (resulting from management practices) assessed from NFI plots is higher than this based upon the stands description (3.9% and 1.8% respectively). We also observed that results of NFI show the huge discrepancy in percentage of land related to forestry (forest roads generally) in forests of private property comparing to cadastral data (1,2% and 0,04% respectively). Most of forests in Poland (92% based on NFI) are single−layer stands. But it should be emphasised that NFI indicate more than twice higher share of two−layer stands (4.8%) than that resulting from description of stands in which samples are located (2.2%). However, only one third of NFI plots located in two−generation and uneven−aged stands was assessed in the same way. On 80% of sample plots dominant species was in accordance with stand description. On 85% plots there were compatibility of the age of dominant species with age recorded in forest management plans. The results of our research confirm that description of the entire diversity of forests by any of the indicators is not practically possible. Simplifying the forests description occurs during NFI measurements as well as within forest management planning. However, some issues need clarification and additional analysis when NFI is used as a source of forest structure. NFI plots with maximal size of a few hundred square meters seem to be too small to observe vertical structure of stands. On the other hand, it should be recognized whether division of the sample plots into smaller sections does not cause overestimation of the area of temporary non−forested land.

Sylwan 161 (4): 267-276