Uncertainties of forest area estimates caused by the minimum crown cover criterion

Abstract

Defining “forest land” is a complex issue and has been discussed for decades. Today, a confusing multitude of definitions of forest land are in use making comparison of forest area figures difficult. But currently, comparability is receiving much attention when it comes to install market mechanisms for ecosystem services. Minimum crown cover is among the most frequently employed criteria of forest definitions. However, the size of the reference area on which the crown cover percent is to be measured is usually not defined. But how does a change of the size of the reference area affect the derived forest cover? In this study, we analyze the interactions of the crown cover threshold and the size of the reference area. We start with analyzing the interactions using a simple geometric model of the forest edge. Then, we extend the analysis by simulating artificial landscapes where we study how the interaction is affected by different degrees of forest fragmentation, crown cover proportion, and spatial resolution of the data source used. The simulation showed that large differences in forest cover (>50 %) may result for a fixed crown cover threshold value, just by changing the size of the reference area, where the magnitude of this effect is a function of the chosen threshold value and the spatial configuration of the crowns. As a consequence of the findings, we see an urgent need to complete forest definitions by defining a reference area in order to reduce uncertainties of forest cover estimates.

Wall-to-Wall Forest Mapping Based on Digital Surface Models from Image-Based Point Clouds and a NFI Forest Definition

“…In the Swiss NFI, the minimum crown coverage threshold is 20% for forest. For the calculation of crown coverage, a defined reference area is essential [45]. In the Swiss NFI, it is an interpretation area of 50 × 50 m around the sample plot center, which is used to estimate the percentage of crown coverage in the field.…”

“…Figure 9 illustrates that the greatest differences between the forest cover map and the reference plots occur at or near forest borders. Moreover, according to Magdon and Kleinn [45], the simplified use of a constant buffer distance (15 m when using a CC of 20% and an interpretation area size of 51 m, see Equation (1)) independent on the degree of forest fragmentation might be too generalized. Thus, the forest mapping approach might have introduced a bias which depends on the degree of fragmentation of the landscape, and in particular on the forest pattern and the resulting variability of the forest border (straight and irregular forest edges).…”

“…Thus, the forest mapping approach might have introduced a bias which depends on the degree of fragmentation of the landscape, and in particular on the forest pattern and the resulting variability of the forest border (straight and irregular forest edges). It is assumed to be higher in highly fragmented landscapes than in compact ones although the differences become smaller the larger the interpretation area is [45].…”

“…The effect of different forest pattern on the landscape level, especially regarding forest edges for the proposed method should be further investigated as described in [45]. -Further research is also needed to improve the accuracy of the forest cover map at the upper tree line in the Alps.…”

Operationalizing the Definition of Forest Degradation for REDD+, with Application to Mexico

“…In this, forests are defined on the basis of thresholds values for three quantitative criteria: crown cover, area and height, all of which can be related to biomass and carbon stocks. However, the reference area to measure such criteria, mainly canopy cover, is usually not defined which results in major uncertainty in forest cover estimates [48]. Another problem with thresholds is that some tree-dominated land uses are classified as forest whereas others are classified as non-forest, through the application of commodity-based definitions of "land use" by FAO and other organisations.…”