Robinia pseudoacacia L. in Short Rotation Coppice: Seed and Stump Shoot Reproduction as well as UAS-based Spreading Analysis

Abstract

Varying reproduction strategies are an important trait that tree species need in order both to survive and to spread. Black locust is able to reproduce via seeds, stump shoots, and root suckers. However, little research has been conducted on the reproduction and spreading of black locust in short rotation coppices. This research study focused on seed germination, stump shoot resprout, and spreading by root suckering of black locust in ten short rotation coppices in Germany. Seed experiments and sample plots were analyzed for the study. Spreading was detected and measured with unmanned aerial system (UAS)-based images and classification technology-object-based image analysis (OBIA). Additionally, the classification of single UAS images was tested by applying a convolutional neural network (CNN), a deep learning model. The analyses showed that seed germination increases with increasing warm-cold variety and scarification. Moreover, it was found that the number of shoots per stump decreases as shoot age increases. Furthermore, spreading increases with greater light availability and decreasing tillage. The OBIA and CNN image analysis technologies achieved 97% and 99.5% accuracy for black locust classification in UAS images. All in all, the three reproduction strategies of black locust in short rotation coppices differ with regards to initialization, intensity, and growth performance, but all play a role in the survival and spreading of black locust.Forests 2019, 10, 235 2 of 23 energy policies aimed towards a reduction in the use of fossil fuels and greater utilization of renewable energy since 2004 [7] marked an increase in fast-growing tree production, including black locust in short rotation coppices (harvested every 2-20 years) [8,9]. Nevertheless, some studies have declared black locust to be an invasive non-native tree species in Europe [6,[10][11][12]. However, other studies consider black locust to be an alternative tree species, for example for European ash (Fraxinus excelsior) [13,14], whose high dieback is caused by the fungal pathogen Hymenoscyphus fraxineus [14].Black locust possesses many growth characteristics that makes it ideal for short-rotation biomass production such as: rapid growth, high drought tolerance, and nitrogen-fixation [15,16], as well as the ability to reproduce via stump shoots in response to harvest. Black locust starts flowering at the age of six years [17]. Many insects benefit from this characteristic, including honeybees [18]. Moreover, the production of black locust honey is very common and economically important, especially in Hungary [17]. A maximum of 15,559 flowers per tree were counted in an eight-year-old black locust plantation [18], and 12,000 seeds/m 2 were identified in a monodominant stand [6,19]; black locust thus produces an abundance of seeds. As black locust belongs to the Fabaceae family, the seed coat is hard and impermeable [20,21]. Hence, seeds require scarification for successful germination [21][22][23] and priming of seeds is favorable [21,24,25]. Seeds pre...

Editorial for the Special Issue “Forestry Applications of Unmanned Aerial Vehicles (UAVs)”

“…One study evaluated the feasibility of adopting the low-cost, flexible, high-resolution, sensor-capable UAV platform for collecting reference data to use in thematic map accuracy assessments for complex environments [4]. Another research study focused on seed germination, stump shoot resprout, and spreading by root suckering of black locust in ten short rotation coppices [5]. The evaluation of density and canopy cover of western juniper in a treated (juniper removed) and an untreated watershed and an assesment of the effectiveness of using low altitude UAV-based imagery to measure juniper-sapling population density and canopy cover were also studied [6].…”

“…Various data processing methods were used in this Special Issue. Image analysis workflow, where a three-step, object-based process consisting of image segmentation, automated classification using a classification and regression tree (CART) machine- Seven papers in this special issue reported results from UAV platform using RGB cameras [1][2][3][4][5][6][7], three paper used multispectral cameras [6,8,9], while one paper used a hyperspectral camera [6].…”

“…Pixel-based and object-based classification reference data methods were used in [4]. Object-based image analysis (OBIA) and convolutional neural network (CNN) were used in [5]. Vegetation indices and support vector machine classification were used in [6].…”

The perspective of optical measurement methods in forestry

“…Additionally, UAS have quick turnaround times, are very cost-efficient and are useful supplements to data from satellites, airplanes, terrestrial manual, and other data analyses. In forestry drones were used to detect trees [7], as well as to monitor the infestation levels of pests [8], to find bark beetles as early as possible [9], to monitor wildlife animals, develop forest inventories [10], monitor wind damage [11], invasiveness analysis [12,13], to quantify the expected flower [6] and seeds [14] as well as for autonomous reforestation.…”

Growth Response of Sessile Oak and European Hornbeam to Traditional Coppice-with-Standards Management

“…It has been documented that the growth and wood production of a coppice is more intensive during the first 40 years after harvest when compared to seed origin trees growing on the same site due to assimilation reserves allocated in the coppice root system preserved from the previous generation [8]. Coppicing is being reintroduced for many reasons, but mainly as a source of renewable energy (biomass production) [9], increasing biodiversity [10], or urban forestry [11]. Coppice-with-standards is a management type where a certain density of quality trees of generative origin (so called standards) are not harvested in order to enhance the coppice regeneration with seeds [12].…”