Biology subjects

Sanders, T. G.

Publications and source records attributed to Sanders, T. G..

3 recordsLinked to original sources

bioRxiv2022

European Beech Spring Phenological Phase Prediction with UAV-derived Multispectral Indices and Machine Learning Regression

The acquisition of phenological events play an integral part in investigating the effects of climate change on forest dynamics and assessing the potential risk involved with the early onset of young leaves. Large scale mapping of forest phenological timing using earth observation data, could facilitate a better understanding of phenological processes due to an added spatial component. The translation of traditional phenological ground observation data into reliable ground truthing for the purpose of the training and validation of Earth Observation (EO) mapping applications is a challenge. In this study, we explored the possibility of predicting high resolution phenological phase data for European beech (Fagus sylvatica) with the use of Unmanned Aerial Vehicle (UAV)-based multispectral indices and machine learning. Using a comprehensive feature selection process, we were able to identify the most effective sensors, vegetations indices, training data partitions, and machine learning models for phenological phase prediction. The best performing model that generalised well over various sites was the model utilising the Green Chromatic Coordinate (GCC) and Generalized Addictive Model (GAM) boosting. The GCC training data was derived from the radiometrically calibrated visual bands from a multispectral sensor and predicted using uncalibrated RGB sensor data. The final GCC/GAM boosting model was capable in predicting phenological phases on unseen datasets within a RMSE threshold of 0.5. This research shows the potential of the interoperability among common UAV-mounted sensors in particular the utility of readily available low cost RGB sensors. Considerable limitations were however discovered with indices implementing the near-infrared (NIR) band due to oversaturation. Future work involves adapting models to facilitate the ICP Forests phenological flushing stages.

ecology↗

bioRxiv2021

Long-term forest monitoring unravels constant mortality rise in European forests

European forests are an important source for timber production, human welfare, income, protection and biodiversity. During the last two decades, Europe has experienced a number of droughts which were exceptionally within the last 500 years both in terms of duration and intensity and these droughts seem to left remarkable imprints in the mortality dynamics of European forests. However, systematic observations on tree decline with emphasis on single species together with high-resolution drought data has been scarce so far so that deeper insights into mortality dynamics and drought occurrence is still limiting our understanding at continental scale. Here we make use of the ICP Forest crown defoliation dataset, permitting us to retrospectively monitor tree mortality for four major conifers, two major broadleaves as well as a pooled dataset of nearly all minor tree species in Europe. In total, we analysed more than 3 million observations gathered during the last 25 years and employed a high-resolution drought index which is able to assess soil moisture anomaly based on a hydrological water-balance and runoff model every ten days globally. We found significant overall and species-specific increasing trends in mortality rates accompanied by decreasing soil moisture. A generalized linear model identified previous-year soil moisture anomaly as the most important driver of mortality patterns in European forests. Significant interactions appeared between previous-year soil moisture and stand water regime in conifers, strongly suggesting that conifers growing at productive sites are more vulnerable under drought. We conclude that mortality patterns in European forests are currently reaching a concerning upward trend which could be further accelerated by global change-type droughts. Key messageForest mortality has significantly increased over the last 25 years and remained above the long-term mean since 2012.

ecology↗

bioRxiv2021

European-wide forest monitoring substantiate the neccessity for a joint conservation strategy to rescue European ash species (Fraxinus spp.)

European ash (Fraxinus excelsior) and narrow-leafed ash (F. angustifolia) are keystone forest tree species in Europe with a broad ecological amplitude and significant economic importance. Besides global warming both species are currently under significant thread by an invasive fungal pathogen that has been progressively spreading throughout the continent for almost three decades. Ash dieback caused by the invasive ascomycete Hymenoscyphus fraxineus is capable of damaging ash trees of all age classes and often leads to the ultimate death of a tree after years of progressively developing crown defoliation. While studies at national and regional level already suggested rapid decline of ash populations as a result of ash dieback, a comprehensive survey at European level with harmonized crown assessment data across countries could shed more light into the population decline from a pan-European perspective and could also pave the way for a new conservation strategy beyond national boarders. Here, we present data from the ICP Forests Level I crown condition monitoring including 27 countries, covering the timespan from 1987-2020. In total, 407 survey plots randomly distributed across these countries were analyzed resulting in >36,000 individual observations. We found a substantial increase in defoliation and mortality over time indicating that crown defoliation has almost doubled during the last three decades. Hotspots of mortality are currently situated in southern Scandinavia and north-eastern Europe, well corresponding to the fact that the disease spread fast from north-east to north-west. Overall survival probability after nearly 30 years of infection has already reached a critical value of 0.51, but with large differences among regions (0.00-0.907). Both a Cox proportional hazard model as well as an Aalen additive regression model strongly suggest that survival of ash is significantly lower in locations with excessive water regime and which experienced more extreme precipitation events during the last two decades. Our results underpin the neccessity for fast governmental acting and joint rescue efforts beyond national boarders since overall mean defoliation will likely reach 50% as early as 2030 as suggested by time series forecasting. We strongly recommend to develop a pan-European conservation strategy before the decline will reach its tipping point resulting into non-reversible loss of diversity in the European forest landscape.

ecology↗