Britta Allgöwer

Modeling airborne laser scanning data for the spatial generation of critical forest parameters in fire behavior modeling

Abstract Methods for using airborne laser scanning (also called airborne LIDAR) to retrieve forest parameters that are critical for fire behavior modeling are presented. A model for the automatic extraction of forest information is demonstrated to provide spatial coverage of the study area, making it possible to produce 3-D inputs to improve fire behavior models. The Toposys I airborne laser system recorded the last return of each footprint (0.30–0.38 m) over a 2000 m by 190 m flight line. Raw data were transformed into height above the surface, eliminating the effect of terrain on vegetation height and allowing separation of ground surface and crown heights. Data were defined as ground elevation if heights were less than 0.6 m. A cluster analysis was used to discriminate crown base height, allowing identification of both tree and understory canopy heights. Tree height was defined as the 99 percentile of the tree crown height group, while crown base height was the 1 percentile of the tree crown height group. Tree cover (TC) was estimated from the fraction of total tree laser hits relative to the total number of laser hits. Surface canopy (SC) height was computed as the 99 percentile of the surface canopy group. Surface canopy cover is equal to the fraction of total surface canopy hits relative to the total number of hits, once the canopy height profile (CHP) was corrected. Crown bulk density (CBD) was obtained from foliage biomass (FB) estimate and crown volume (CV), using an empirical equation for foliage biomass. Crown volume was estimated as the crown area times the crown height after a correction for mean canopy cover.

Forest Canopy Gap Fraction From Terrestrial Laser Scanning

A terrestrial laser scanner (TLS) was used to measure canopy directional gap fraction distribution in forest stands in the Swiss National Park, eastern Switzerland. A scanner model was derived to determine the expected number of laser shots in all directions, and these data were compared with the measured number of laser hits to determine directional gap fraction at eight sampling points. Directional gap fraction distributions were determined from digital hemispherical photographs recorded at the same sampling locations in the forest, and these data were compared with distributions computed from the laser scanner data. The results showed that the measured directional gap fraction distributions were similar for both hemispherical photography and TLS data with a high degree of precision in the area of overlap of orthogonal laser scans. Analysis of hemispherical photography to determine canopy gap fraction normally requires some manual data processing; laser scanners offer semiautomatic measurement of directional gap fraction distribution plus additional three-dimensional information about tree height, gap size, and foliage distributions

Inversion of a lidar waveform model for forest biophysical parameter estimation

Due to its measurement principle, light detection and ranging (lidar) is particularly suited to estimate the horizontal as well as vertical distribution of forest structure. Quantification and characterization of forest structure is important for the understanding of the forest ecosystem functioning and, moreover, will help to assess carbon sequestration within forests. The relationship between the signal recorded by a lidar system and the canopy structure of a forest can be accurately characterized by physically based radiative transfer models (RTMs). A three-dimensional RTM is capable of representing the complex forest canopy structure as well as the involved physical processes of the lidar pulse interactions with the vegetation. Consequently, the inversion of such an RTM presents a novel concept to retrieve biophysical forest parameters that exploits the full lidar signal and underlying physical processes. A synthetic dataset and data acquired in the Swiss National Park (SNP) successfully demonstrated the feasibility and the potential of RTM inversion to retrieve forest structure from large-footprint lidar waveform data. The SNP lidar data consist of waveforms generated from the aggregation of small-footprint lidar returns. Derived forest biophysical parameters, such as fractional cover, leaf area index, maximum tree height, and the vertical crown extension, were able to describe the horizontal and vertical forest canopy structure.

Do Factors Causing Wildfires Vary in Space? Evidence from Geographically Weighted Regression

This paper describes the results of a geo-statistical analysis carried out at the provincial level in Southern Europe to model wildfire occurrence from socio-economic and demographic indicators together with land cover and agricultural statistics. We applied a classical ordinary least squares (OLS) linear regression together with a geographically weighted regression (GWR) to explain long-term wild-fire occurrence patterns (mean annual density of >1 ha fires). The explanatory power of the OLS model increased from 52% to 78% as a result of the non-constant relationships between fire occurrence and the underlying explanatory variables throughout the Mediterranean Basin. The global model we developed (i.e., OLS regression) was not sufficient to fully describe the underlying causal factors in wildfire occurrence modeling. Indeed, local approaches (i.e., GWR) can complement the global model in overcoming the problem of non-stationarity or missing variables. Our results confirm the importance of agrarian activities, land abandonment, and development processes as underlying factors of fire occurrence. The identification of regions with spatially varying relationships can contribute to the better understanding of the fire problem, especially over large geographic areas, while at the same time recognizing its local character. This can be very important for fire management and policy.

Assessment of the influence of flying altitude and scan angle on biophysical vegetation products derived from airborne laser scanning

Airborne Laser Scanning (ALS) has been established as a valuable tool for the estimation of biophysical vegetation properties such as tree height, crown width, fractional cover and leaf area index (LAI). It is expected that the conditions of data acquisition, such as viewing geometry and sensor configuration influence the value of these parameters. In order to gain knowledge about these different conditions, we test for the sensitivity of vegetation products for viewing geometry, namely flying altitude and scanning (incidence) angle. Based on two methodologies for single tree extraction and derivation of fractional cover and LAI previously developed and published by our group, we evaluate how these variables change with either flying altitude or scanning angle. These are the two parameters which often need to be optimized towards the best compromise between point density and area covered with a single flight line, in order to reduce acquisition costs. Our test‐site in the Swiss National Park was sampled with two nominal flying altitudes, 500 and 900 m above ground. Incidence angle and local incidence angle were computed based on the digital terrain model using a simple backward geocoding procedure. We divided the raw laser returns into several different incident angle classes based on the flight path data; the TopoSys Falcon II system used in this study has a maximum scan angle of ±7.15°. We compared the derived biophysical properties from each of these classes with field measurements based on tachymeter measurements and hemispherical photographs, which were geolocated using differential GPS. It was found that with increasing flying height the well‐known underestimation of tree height increases. A similar behaviour can be observed for fractional cover; its respective values decrease with higher flying height. The minimum distance between first and last echo increases from 1.2 metres for 500 m AGL to more than 3 metres for 900 m AGL, which does alter return statistics. The behaviour for incidence angles is not so evident, probably due to the small scanning angle of the system used. fCover seems to be most affected by incidence angles, with significantly higher differences for locations further away from nadir. As expected, incidence angle appears to be of higher importance for vegetation density parameters than local incidence angle.

Wildfire history and fire ecology of the Swiss National Park (Central Alps): new evidence from charcoal, pollen and plant macrofossils

Microscopic (> 10 mm) and macroscopic (> 200 mm) charcoal particles were analysed in sediments from two mires in subalpine coniferous forests at c. 1800 m a.s.l. in southeastern Switzerland. Pollen and plant macrofossils suggest that since 6000 BC, Pinus mugo ssp. uncinata (DC) Domin (‘upright mountain pine’) has mostly been the dominant tree species at one of the study sites (Il Fuorn). In contrast, forests dominated by Picea abies (Norway spruce) have formed the vegetation since c. 4000 BC around the mire ‘Fuldera-Palü Lunga’. Mean fire-return intervals (MFI) varied from 250 to > 600 years, depending on forest type, climate and land use. In mountain-pine forests (Il Fuorn), local fires occurred approximately every 250 years, even before the region was agriculturally used (ie, before 3600 BC). About 2000 years ago, intensified human impact as documented by the pollen record resulted in increased fire activity at Fuldera. Post-fire vegetation dynamics suggest that the mountain-pine stands at Il Fuorn had a moderate fire regime with a mix of surface and crown fires. In alpine ecosystems, the impact of fire is generally overshadowed by other disturbance factors such as windthrow, landslides, fungal decay and by climate changes or human land use. Nevertheless, our results show for the first time that natural wildfires exerted a major control on the subalpine coniferous forest ecosystems of the Swiss National Park and its neighbouring areas, eg, by contributing to maintain Pinus mugo ssp. uncinata forests throughout the mid and late Holocene.

Assessment of the hazard potential of ice avalanches using remote sensing and GIS‐modelling

Ice avalanches typically occur when a large mass of ice breaks off from steep glaciers. Since the reach of ice avalanches is usually low, their hazard potential is generally restricted to high mountain areas that are densely populated or frequently visited by tourists. However, far‐reaching disasters are possible in combination with other processes such as rockfalls or snow avalanches. In addition, the hazard potential of ice avalanches is presently increasing as a consequence of climatic and socio‐economic changes in mountain areas. Dealing with ice‐avalanche hazards requires robust tools for systematic area‐wide detection of hazard potentials. Corresponding techniques have not been developed so far. To bridge this methodological gap, a three‐level downscaling approach was developed. This method chain is based on statistical parameters, geographic information system (GIS) modelling techniques and remote sensing. The procedure permits to perform a fast and systematic first‐order mapping of potentially dangerous steep glaciers and their runout paths for an entire region. To validate the approach, a case study was carried out in the Bernese Alps, Switzerland. The results correspond well with local studies using dynamic avalanche models. Improvements can be obtained by expanding the method chain by including basic data of higher spatial resolution as satellite data and digital terrain models (DTM).

Virtual Worlds—Real Decisions: Model- and Visualization-based Tools for Landscape Planning in Switzerland

Abstract Prominent construction projects in Switzerland, such as the Sawiris luxury resort in Andermatt planned by Orascom Hotels & Development, Cairo (Egypt), or the idea of a hotel and apartment tower at Schatzalp, Davos, demonstrate how rapidly Alpine landscapes may undergo major changes. Decisions on whether or not such changes are supported by policymakers should be based on the best information available and in agreement with the local population to ensure long-term sustainable development. The present article investigates the potential and limitations of computer-based tools to support such decisions in the area of landscape planning, with a particular focus on Alpine landscapes.

Multi-resolution imaging spectroscopy resolving the structure of heterogeneous canopies for forest fire fuel properties mapping

Coniferous forests represent canopies with a high heterogeneity in the horizontal and as well in the vertical dimension. Consequently the interaction of incident radiation is dominated by the complex 3-D canopy structure and architecture. Radiative transfer approaches based on coupled leaf and canopy radiative transfer models still allow the simulation of the canopy reflectance as a function of leaf optical properties, canopy structure and viewing geometry as well as the retrieval of biophysical and biochemical canopy variables. High resolution imaging spectrometry supported by LIDAR data and radiative transfer models of different levels of complexities (SAIL, GeoSAIL) are employed to assess the influence of canopy heterogeneity and structure at different spatial scales. We discuss the relevance of single scene components and canopy structure to the recorded canopy reflectance and present a strategy to support radiative transfer models for biophysical and biochemical parameter retrieval relevant for forest fires.

Real-time 4D visualization of migratory insect dynamics within an integrated spatiotemporal system

This paper presents a new approach of spatiotemporally visualizing the simulation output of migratory insect dynamics and resultant vegetation changes in real-time. The visualization is capable of displaying simulated ecological phenomena in an intuitive manner, which allows research results to be easily understood by a wide range of users. In order to design a fast and efficient visualization technique, a simplified mathematical model is applied to intelligibly represent migrating groups of insects. In addition, impostors are used to accelerate rendering processes. The presented visualization method is implemented in an integrated spatiotemporal analysis system, which models, simulates and analyzes ecological phenomena such as insect migration through time at a variety of spatial resolutions.