Accuracy of sample-based forest inventory obtained from different plot configurations

Abstract

Introduction. The current demands of society in precise and actual information on forest resources require improving methods of forest mensuration used in operational forest inventory in Ukraine. Prevailing ocular estimates during field examination of forest stands leads to biased estimation of growing stock volume. Thus, any improvement of current technology of forest survey requires reliable field data, usually collected in sample-based forest inventory relaying on predefined sampling design and accuracy. Nowadays, the technology of national forest inventory of Ukraine is under discussion therefore an analysis of different approaches for collecting data based on local case studies is of great importance.

Analysis of recent research and publication. The modern technology of forest inventory has faced a gradual shift from sample unit of big size (strip sampling or rectangular plots) towards small plots typically having a form of circle. One good example of continuous advancement of methodology and application of different sampling approaches is multi-source national forest inventory of Finland. The circular form of sample units facilitates overcoming at least three problems: the shape of circle has the smallest area compared to its perimeter, so circular plot minimizes the number of bordering trees; the circular plot could be positioned using merely its center; to find the plot perimeter minimal time is needed as it defined by radius.

By this time, there is no common plot size used in forest inventory. Many researchers think that optimal number of tally trees to be sampled per plot should not exceed 20–30 trees. Increasing plot size does not lead to improving the accuracy since some trees could be skipped during counting. From that point of view sampling with variable radius plots was developed. The studies had been conducted in Ukraine proved that optimal plot size includes in average 7 tally trees.

The sample size for fixed radius plots depends on the area, but for angle counting plots it relies on basal area factor (BAF). The countries of Western and North Europe use the BAF from 1.5 to 4. If BAF > 1 is applied the number of bordering trees which diameters and distance from plot center should be checked decreases. To optimize field work, methods that combine fixed- and variable-area sampling were developed. The simplest way to improve the efficiency of field work is nested plot design. It refers to two or three circular plots of different radius so that thin trees are measured in plots of smaller area. Nested plots are widely used in many national forest inventories throughout of the world. Rather cost-efficient is also combination of fixed-area sampling and angle counting approach, so called “truncated angle sampling”. This technique is actively discussed in literature to optimize the combination of plot radius and BAF used for tree selection.

Objective. The aim of the study is to investigate accuracy and efficiency of different plot configuration relied on two main methodological principles – tree selection with probabilities proportional to tree sizes and stand density.

Methods. The study is based on empirical data collected within study area established in Kyiv region (Ukraine). The test polygon is located between longitudes 30° 00' E and 30° 12' E and latitudes 50° 12' N та 50° 20' N. We used systematic sampling by rectangular grid spacing 1՛ for both longitude and latitude to locate sample units within forested area. The forests are characterized by pine (Pinus sylvestris L.) accompanied by several other tree species like oak (Querqus robur L., Querqus rubra L.), birch (Betula pendula Roth.), aspen (Populus tremula L.) and alder (Alnus glutinosa L.). We used Field-Map software for data collecting. Each tally tree within circular plot of 500 m² was mapped, afterwards a list of attributes has been measured including tree species and diameter at 1.3 m. For model trees (1/4 of total quantity) that were randomly selected among tally trees we also measured tree height. As an option, angle counting plots were established using BAF = 1. Mapping trees allowed us to calculate parameters of several additional plot configurations using diameters of trees and local coordinates defining their location within plot. We examined: 1) three nested plot configuration incorporating maximum radius of 12.62 m and three subplots radii – 3.98, 5.64, 7.98 m; 2) one nested plot design with maximum radius of 9.77 m and subplot radius of 5.64 m; 3) two truncated angle plot configurations with maximum radius of 12.62 m where BAF = 1 and BAF = 2 used; 4) two truncated angle plot configurations with BAF = 1 but having maximum radius 11.28 m and 9.77 m; 5) angle count plot with BAF = 1. In the study we focused on accuracy of estimation of basal areas and number of trees per hectare. As a reference, we used data obtained on fixed area plots of 500 m².

Results and discussion. Besides of common assumption that nested plot design is most suitable for natural forest with negatively J-shaped tree sizes distribution we outlined some advances of the configuration for the study area. The nested plots having maximum radius of 12.62 m were rather precise regardless of subplot radius. But, for practical use we would recommend establishing subplots having radius of 3.98 m since such design had least sample size and at the same time demonstrated good accuracy (standard deviation of basal area estimates did not exceed 0.5 m²ha^-1). At the same time, we obtained biased estimates of basal area and stands density using nested plots with maximum radius of 9.77 m.

The performance of truncated angle sampling was moderate if maximum radius was decreased or BAF = 2 was applied. The most accurate plot configuration was accepted which had maximum plot radius of 12.62 m and BAF = 1. The angle count approach was biased on average of 4 m²‧ha^-1 because of omission of some trees located a long distance from plot center.