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The year 2020 will be the 25th global anniversary for research on use of airborne laser scanning (ALS) as a technology to improve operational forest management inventories.
The first ALS test data were acquired in 1995, the first test of the so-called area-based method was conducted in 1999 (1000 ha), the first commercial contract to undertake an operational forest inventory with ALS was signed in 2002 (49000 ha) and, finally, the first operational inventories in the second cycle with ALS was started in 2017. All these milestones were reached in Scandinavia, and basically in Norway.
Looking back, in particular Norway seemed to have ideal conditions for these early achievements. Four factors seemed to be essential, which perhaps also can inspire activities in other countries:
DIGITAL PLATFORM: there was already a fully digitized production pipeline of forest management plans in the inventory companies based on digital photogrammetry (1997).
INDUSTRY: Norway had at this time a leading surveying industry that experimented and adopted ALS as early as in 1995. A few other countries were in a similar position (e.g. Germany, The Netherlands and Sweden), but with Sweden as an exception, the inte-raction with the forest sector seemed weaker in many countries.
RESEARCH: a strong interaction between the operational forest sector and the acade-mic community, and with researchers who already worked actively with the most recent technologies to improve forest inventory.
ECONOMY/ORGANIZATION: public subsidies that encouraged small forest holdings to join forces and collaborate on data collection over vast areas – a necessary requirement to provide a sufficient land base needed to benefit from economies of scale. Also, there was a strong competition among the forest inventory companies which stimulated development and adoption of more efficient methods.
As the area-based method utilizing ALS took over a greater share of the market for wall-to-wall forest inventories in Norway as well as in Sweden in the period up to 2010,
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Finland did not fully enter into the new era until around 2010. However, Finland soon be-came the largest ALS-based forest inventory market with an annually inventoried area of around 3 million ha per year. By 2010, most of the inventories in Finland and Norway were conducted by ALS. The adopted methodology in Norway and Sweden was more or less the same, with dedicated systematic sample plots surveys in the target area and dedicated ALS campaigns. Statistical regression estimation was the technique adopted to develop relationships between the target variables and the ALS data. Typical target variables at stand level is mean height, dominant height, mean stem diameter, basal area, stem number and volume. Information on for example tree species and site productivity is typically taken from other sources than ALS; in Norway, often as a result of stereo aerial image interpre-tation. Most forest owners have only requested data at stand level. In Norway in particular, stands are small in size – on average around 1.5 ha. More advanced users of forest resource information have more frequently had a demand for more detailed data, for example data for every single spatial unit subject to prediction of the mentioned target variables. These smaller units are typically around 250 m2 in size, and many forest managers have apprecia-ted this high level of detail in their daily forest management. In Finland, the methodology for the area-based method has basically been the same as in the other countries, but while a typical project area in Norway has been around 20000–80000 ha, the areas in Finland tend to be larger. In Finland, the estimation has been based on non-parametric methods like kNN as opposed to regression. kNN usually requires a larger sample of field plots. The list of variables on stand level in Finland is typically more extensive with more dedicated tree species-wise information.
Over the years, there have been incremental improvements of the methods. In the early days, the ALS inventories focused on mature forest stands or at least forests that had reached a stage where thinning was a relevant treatment (>8–10 m in height). In recent years, also younger stands – from the time of regeneration and planting (typical tree heights of 0–8 m), have been included. This implies that other variables are estimated than those related to growing stock, like for example stem numbers and species distribution. Other types of infor-mation are nowadays also commonly derived from ALS, such as indices indicating need for tending, thinning and other silvicultural and harvest operations. Additional products found useful for harvest operations, such as terrain wetness maps, are produced on a routine basis.
The single-tree method has remained a rather marginal method in Scandinavia. Only a few projects have been conducted in Norway and Sweden.
Some of the same commercial inventory companies are present in all the mentioned countries, and some of the companies also operate throughout Europe, especially in the Baltic countries, although there were attempts to establish business activities in forest inventory even in the south (e.g. Spain) and outside Europe (e.g. Canada) a decade ago.
The Nordic market is today more segmented than some years ago, with companies that basically operate in only one country.
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At least five trends in operational forest management inventory should be noticed – some of them are technology-driven, some are purely research-driven, and some are driven by pure profit considerations.
Wykorzystanie danych z lotniczego skaningu laserowego do inwentaryzacji zasobów leśnych…
48 Erik Næsset
MORE DETAILED INFORMATION: Even the relatively low-density ALS campaigns that have been most common (1–2 ALS points/m2) probably provide data that contain more information about the structure of the forest than what is currently being exploited for esti-mation of the traditional forest variable (height, volume etc.). This includes inforesti-mation on distribution on size classes, e.g. diameter distributions. Such products have been promoted in the market for almost 15 years, and seem to become more important in the future. This also includes more detailed species-wise information, which may be improved with sensor improvements (multispectral ALS) and sensor fusion (ALS and aerial imagery).
NEW TYPES OF INFORMATION: Because the ALS data contains structural information about the terrain and the trees and forest stands, the data may most likely also be used to extract proxies for other ecosystem services in the forests that the forest sector often has an obligation by law to identify and preserve. Examples are cultural remains and various aspects of biodiversity, which in one form or the other is related to the certification process of a forest entity.
NATIONAL PRODUCTS: Many countries in Europe have conducted nation-wide ALS campaigns in recent years to established new and improved digital terrain models for their entire territory. Several national forest inventory institutions have taken the opportunity to combine these nation-wide ALS data with the nation-wide datasets of sample plots to pro-duce gridded products with all relevant forest variables at cell level (cells typically around 200–250 m2 in size). The methodology behind these products is the area-based method.
In Sweden, this product which can be accessed freely on the internet for download of data, has already been adopted by many forest owners as the primary source of information for management planning. The same trend may to some extent be expected in other countries.
However, the accuracy is expected to be smaller than in dedicated local ALS inventories, so there will clearly be an issue of trade-off between the very low costs of the data and the costs that may be expected in terms of poorer decisions and loss of money due to suboptimal management recommendations cause by erroneous data.
IMAGE MATCHING: Many countries in Europe have nation-wide programs for regular aerial image acquisition. Recent advances in digital camera technology, software for image matching and 3D point cloud extraction, and computer processing capacity have made 3D image matching a viable alternative to ALS for production 3D data for forest inventory, provided that there already is an existing and accurate terrain model from ALS in place.
Such data are cheaper to acquire than ALS data and in cases such data are part of an existing national data infrastructure, the forestry sector can often get access to these data at a low cost or even free of charge. Recent studies suggest that the area-based method applying 3D data from image matching can provide estimates of volume, mean height and other variab-les with an accuracy comparable to that of ALS. Image matching is now recommended as a source of data for forest management inventories in Norway and is to some extent also used as primary source of data for the gridded national products detailed above.
TEMPORAL INFORMATION: Repetition of forest inventories with ALS (or image matching) through time, as illustrated by the second inventory cycle with ALS, opens up for estimation of a new suite of variables that is of temporal nature. This pertains to information on changes in the state of the forest caused by harvests, other management actions and natural changes. Of particular relevance and importance, is estimation of site productivity.
Site productivity – in many countries expressed in the form of site index based on dominant height and age, can be accurately determined in bi-temporal 3D data from ALS (or image
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matching). Two alternatives exist, namely 1) to estimate site index by the area-based me-thod using site index values from the field plots and 3D metrics from both points in time reflecting height growth, or 2) to estimate site index from the length of the time period between the two acquisitions and the change in height due to growth over the same period.
Recent research in Norway suggests that the two methods are equally accurate. Site index is one of the least accurate forest stand variables in current management inventories, and in the second cycle of the ALS-based inventories ALS-supported estimates of site index from bi-temporal data will be provided for those parts of the inventory area that have not been subject to any treatment since the previous inventory.
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The area-based management inventories using ALS represented a change of paradigm in forest inventory. Since the early experiments almost 25 years ago, there has been a gradual improvement of the methodology to lower the costs, increase accuracy and introduce new information products that were previously not economically or technically feasible to pro-vide. Future technological developments are expected to nurse a continued improvement of inventory practices.
Wykorzystanie danych z lotniczego skaningu laserowego do inwentaryzacji zasobów leśnych…
Lars T. Waser
Federalny Instytut Badawczy Lasu, Śniegu i Krajobrazu, Szwajcaria waser@wsl.ch