Carina Kübert

Synthetic RapidEye data used for the detection of area-based spruce tree mortality induced by bark beetles

Tree mortality caused by outbreaks of the bark beetle Ips typographus (L.) plays an important role in the natural dynamics of Norway spruce (Picea abies L.) stands, which could cause far-reaching changes in the occurrence and duration of vegetation phenology. Field-based early detection of tree disturbances is hampered by logistic, terrain, and technical shortcomings, and by the inability to continuously monitor disturbances over large areas. Despite achievements in remote mapping of bark-beetle-induced tree mortalities, early warning has been mostly unsuccessful mainly because of the lack of spectral sensitivity and discrepancies in definitions of field- and image-based disturbance classes. Here we applied a method based on inter-annual phenology of Norway spruce stands derived from synthetic multispectral data to part of the Bavarian Forest National Park in Germany. We fused temporally continuous Moderate Resolution Imaging Spectroradiometer and discrete RapidEye data using a flexible spatiotemporal data fusion method to achieve validated 8-day RapidEye-like composites of normalized difference vegetation index for 2011. We assumed that the dead trees delineated on 2012 aerial photographs were those in which bark beetle infestations were initiated in 2011. Samples were drawn with variable-sized buffering to represent the areas prone to infestations and their surroundings. We applied a conditional inference random forest to select the best image date among the entire 46 synthetic datasets to best discriminate between the core infestation patches and their surroundings from the subsequent year. Of the discrete time points identified, day 281 of the year represented the highest discrepancy between aerial image-based dead trees and their surroundings. Classification results were significantly correlated with beetle count data obtained using pheromone traps. Our method provided valuable information for management purposes and enabled wall-to-wall mapping of stands prone to infestation and its uncertainty. The results offer potential implications for rapid and cost-effective monitoring of bark beetle outbreaks using satellite data, which would be of great benefit for both management and research tasks.

Land Surface Phenology from MODIS data in Germany

Changes in the timing of phenological events are a critical parameter for quantifying the impact of climate alterations on vegetation development that in reverse alters patterns of biosphere-atmosphere interactions. By the means of remote sensing, numerous approaches have been developed for deriving Land Surface Phenology. This study aims at identifying the best combination of data quality integration, filtering and threshold choice for the Start of Season (SOS) in Germany using MODIS data. The resulting remote sensing based SOS is related to phenological ground observations. For the year 2010, SOS data was produced by 5 different methods and the best method was identified by the comparison of the SOS dates to ground observations of the leaf unfolding of Broad-Leaved forest and beginning of turning green for Pastures, respectively. The results show that the impact of time series data quality screening, filter choice and threshold setting differs among the selected land cover classes.