New technologies for forest monitoring in Alpine region

The science of forest digitalization via technological innovation offers an opportunity to develop new methods for mass monitoring forest resources. A key constraint is the ability to collect data, store and analyze said retrieved data. The TreeTalker® (TT+) is a multisensory IoT-driven platform designed to detect and collect information on individual trees where its nested sensor approach captures several key eco-physiological parameters autonomously and in semi-real-time. Key parameters are: 1) tree radial growth, as an indicator of photosynthetic carbon allocation in biomass; 2) sap flux density, as an indicator of tree transpiration and functionality of xylem transport; 3) stem water content as an indicator of hydraulic functionality 4) light penetration in the canopy in terms of fractional absorbed radiation 5) light spectral components related to foliage dieback and phenology and 6) tree stability parameters to allow real time forecast of potential tree fall. The focus of this study is to design/calibrate and validate sensors for stem water content and sap flow measurement using the TreeTalker platform with the application of these platforms for monitoring ecophysiological parameters at a single tree scale providing time series data for forest monitoring. i. Stem water content To demonstrate the capability of the TreeTalker, a three-phase experimental process was performed including (1) sensor sensitivity analysis, (2) sensor calibration, and (3) long-term field data monitoring. A negative linear correlation was demonstrated under temperature sensitivity analysis, and for calibration, multiple linear regression was applied on harvested field samples, explaining the relationship between the sample volumetric water content and the sensor output signal. Furthermore, in a field scenario, TreeTalkers were mounted on adult Fagus sylvatica L. and Quercus petraea L. trees, from June 2020 to October 2021, in a beech-dominated forest near Marburg, Germany, where they continuously monitored sap flux density and stem volumetric water content (stem VWC). The results show that the range of stem VWC registered is highly influenced by the seasonal variability of climatic conditions. Depending on tree characteristics, edaphic and microclimatic conditions, variations in stem VWC and reactions to atmospheric events occurred. Low sapwood water storage occurs in response to drought, which illustrates the high dependency of trees on stem VWC under water stress. Consistent daily variations in stem VWC were also clearly detectable. Stem VWC constitutes a significant portion of daily transpiration (using TreeTalkers, up to 4% for the beech forest in our experimental site). The diurnal–nocturnal pattern of stem VWC and sap flow revealed an inverse relationship. ii. Sap flow: an empirical approach Here, a new IoT-based multisensing device, TreeTalker® with its tailored firmware is exploited to input different heating duration to capture high-frequency data of both heating and cooling phases. Using this advance in technology, its application, we aim to assess the applicability and thus merit of the TreeTalker toward sap flux density measurement and computation. Capability analysis of TT+ is verified both under a lab scenario using an artificial hydraulic column of sawdust and a stem segment of F. sylvatica L. in the field via mounted TT+ devices and with the comparison of commercial sap flow sensors on different species. Installing a TT+ on the artificial flow system, temperature evolution data from heating and reference probes are recorded both in heating and cooling phases to compute values of different flow indices under different flux densities. Applied continuous heating mode and a transient regime with four different combinations of heating and cooling times (in minutes) 10/10, 5/10, 15/45, and 10/50 are tested by TT+ and calibration of flux density vs flow indices conducted by applying optimal fitting curve on the source data up to 8 (L dm-2 h-1). Nonetheless, comparing TT+ set on the transient regime (10H/50C) performance across different species of Norway spruce, European beech, and oak in situ with well know thermal approaches (TDP: Continous Heating and HPV: Heat Pulse Velocity method) proved that the TT+ is capable to measure sap flow with reasonable accuracy (≈80%) for network-based mass monitoring in remote areas with low power consumption. iii. Semi-analytical solution for transient regime Measurement of xylem sap flow via thermal dissipation probes (TDP) and the transient regime (TTD), which is essentially derived from the TDP system, are two widely accepted and applied methods for estimating whole-tree transpiration. So far, thermal dissipation approaches use empirical equations to estimate sap flow and although robust, by nature, are limited by their accuracy. To overcome the limitations typically associated with the empirical approach, a novel method is introduced to solve the heat partial differential equation driven by the mechanisms of conduction/convection for the transient thermal dissipation method (TTD) with heating/cooling cycles. Also, a simple semi-analytical method was developed to exploit the convolution integral of the heat flow equation. The capability of the novel solution is approved by comparing its results with observations under the controlled condition as well as the output of the available well-known empirical equations under field circumstances. An essential feature of the TreeTalker platform, therefore, is to capture the full heat flow curve at the microprocessor level and integrate a semi-analytical approach to mathematically evaluate the amount of sap velocity and thermal diffusivity at a large scale and in real-time. iv. TT+ applications at forest monitoring In this investigation, two sites (Molveno and Val Canali) are established with a total of 84 TT+ in the Alpine zone, Northern Italy. The Italian Alps are important ecosystems supporting rich landscapes and biodiversity with their forests supporting several key ecosystem services. Thus, monitoring these ecosystems is of critical importance to track the variation of individuals’ ecological demands in different species. For this study, we focus on two of the most dominant tree species across the Central European Forest, Fagus Sylvatica L. and Picea Abies L., to evaluate the TT+ as a novel biosensing platform for mass monitoring. Furthermore, we explore the relationships between site characteristics and abiotic factors using collected TreeTalkers data. Although not a complete substitute for field data collection, platforms such as the Treetalker can enhance established methods for mass monitoring, offers big data solutions on individual trees, and further the pursuit of forest digitalization. Yet, as with any new technology challenges remain related to obstacles such as sensor green character, durability, flexible design, maintenance, precision, and accuracy

Toward a unified TreeTalker data curation process

The Internet of Things (IoT) development is revolutionizing environmental monitoring and research in macroecology. This technology allows for the deployment of sizeable diffuse sensing networks capable of continuous monitoring. Because of this property, the data collected from IoT networks can provide a testbed for scientific hypotheses across large spatial and temporal scales. Nevertheless, data curation is a necessary step to make large and heterogeneous datasets exploitable for synthesis analyses. This process includes data retrieval, quality assurance, standardized formatting, storage, and documentation. TreeTalkers are an excellent example of IoT applied to ecology. These are smart devices for synchronously measuring trees’ physiological and environmental parameters. A set of devices can be organized in a mesh and permit data collection from a single tree to plot or transect scale. The deployment of such devices over large-scale networks needs a standardized approach for data curation. For this reason, we developed a unified processing workflow according to the user manual. In this paper, we first introduce the concept of a unified TreeTalker data curation process. The idea was formalized into an R-package, and it is freely available as open software. Secondly, we present the different functions available in “ttalkR”, and, lastly, we illustrate the application with a demonstration dataset. With such a unified processing approach, we propose a necessary data curation step to establish a new environmental cyberinfrastructure and allow for synthesis activities across environmental monitoring networks. Our data curation concept is the first step for supporting the TreeTalker data life cycle by improving accessibility and thus creating unprecedented opportunities for TreeTalker-based macroecological analyse

Real-time Continuous IoT-TT Spectrum as a Predictor of Plant Health and Fruit Quality in 'Soreli' Kiwifruit

The kiwifruit, for which Italy is the second largest producer in the world, is derived from a fruit vine that requires a significant amount of water for proper growth and the production of high-quality fruit. This study aims to investigate the ideal irrigation strategy to ensure an adequate water supply for robust root development, vibrant foliage, and successful fruit production. To achieve this goal, three experimental plots of Soreli kiwifruit vineyards were subjected to different water regimes: full irrigation (100%) and two plots with 80% and 60% irrigation. Each irrigation group was equipped with an Internet of Things (IoT) TT Spectrum, which measured reflected spectrometer data in 12 bands. Additionally, a soil sensor was used to determine moisture content. Fruit quality analysis, including weight loss, soluble solid content, flesh color and firmness, titratable acidity, ascorbic acid, polyphenols and flavonoids, was conducted to correlate productivity with water availability. By analyzing the IoT-TT Spectrum data for each kiwifruit plant, crucial indicators for assessing physiological status, health, and growth were examined. Maintaining a balance between leaf nitrogen content and photosynthetic capacity, with low AI and higher NDVI and CVI values, was used to determine the optimal irrigation level for each of the three plots. Furthermore, qualitative data obtained from fruits revealed that overwatering or underwatering kiwifruit plants affects yield production and fruit quality. Fruits subjected to full irrigation exhibited lower sugar levels compared to those under deficit irrigation. This study demonstrates that excessive or limited irrigation regimes negatively impact plant health and on the fruit quality

Towards continuous stem water content and sap flux density monitoring: IoT-based solution for detecting changes in stem water dynamics

Taking advantage of novel IoT technologies, a new multifunctional device, the “TreeTalker”, was developed to monitor real-time ecophysical and biological parameters of individual trees, as well as climatic variables related to their surrounding environment, principally, air temperature and air relative humidity. Here, IoT applied to plant ecophysiology and hydrology aims to unravel the vulnerability of trees to climatic stress via a single tree assessment at costs that enable massive deployment. We present the performance of the TreeTalker to elucidate the functional relation between the stem water content in trees and respective internal/external (stem hydraulic activity/abiotic) drivers. Continuous stem water content records are provided by an in-house-designed capacitance sensor, hosted in the reference probe of the TreeTalker sap flow measuring system, based on the transient thermal dissipation (TTD) method. In order to demonstrate the capability of the TreeTalker, a three-phase experimental process was performed including (1) sensor sensitivity analysis, (2) sensor calibration, and (3) long-term field data monitoring. A negative linear correlation was demonstrated under temperature sensitivity analysis, and for calibration, multiple linear regression was applied on harvested field samples, explaining the relationship between the sample volumetric water content and the sensor output signal. Furthermore, in a field scenario, TreeTalkers were mounted on adult Fagus sylvatica L. and Quercus petraea L. trees, from June 2020 to October 2021, in a beech-dominated forest near Marburg, Germany, where they continuously monitored sap flux density and stem volumetric water content (stem VWC). The results show that the range of stem VWC registered is highly influenced by the seasonal variability of climatic conditions. Depending on tree characteristics, edaphic and microclimatic conditions, variations in stem VWC and reactions to atmospheric events occurred. Low sapwood water storage occurs in response to drought, which illustrates the high dependency of trees on stem VWC under water stress. Consistent daily variations in stem VWC were also clearly detectable. Stem VWC constitutes a significant portion of daily transpiration (using TreeTalkers, up to 4% for the beech forest in our experimental site). The diurnal–nocturnal pattern of stem VWC and sap flow revealed an inverse relationship. Such a finding, still under investigation, may be explained by the importance of water recharge during the night, likely due to sapwood volume changes and lateral water distribution rather than by a vertical flow rate. Overall, TreeTalker demonstrated the potential of autonomous devices for monitoring sap density and relative stem VWC in the field of plant ecophysiology and hydrolog

Effects of wildfire on growth, transpiration and hydraulic properties of Pinus pinaster Aiton forest

In the last decades, forests have been seriously threatened by the effects of climate change. In Mediterranean regions, increasing fire occurrence and drought have a substantial impact on forests, possibly pushing them towards tipping points. Indeed, although trees could survive to fire events of medium severity in the short term, severe defoliation and environmental stressors could affect the capacity to transport water and to fix and use carbon, amplifying the possibility of a delayed post fire mortality. Therefore, it is urgent to better understand trees eco-physiological processes in response to wildfire in order to plan suitable forest management strategies. In this research an innovative continuous monitoring system of sap flow and micro-stationary climatic conditions of trees was combined with dendro-anatomical analyses and xylem cavitation vulnerability studies to assess fire impact on a Pinus pinaster forest affected by a severe fire and compared to an unburned control site. Xylem analyses showed that the forest-fire did not influence the xylem hydraulic parameters and did not increased vulnerability to embolism. However, burned trees with high defoliation rate, presented reduced assimilation capacity and carbon availability for growth, as well as a limited development of thicker xylem conduit walls compared to unburned trees. Furthermore, continuous monitoring highlighted an increase in sap flow of defoliated trees compared to the control ones during the hottest and driest months of the year indicating a higher stomatal conductance. Overall, our results suggested that although fire did not affect the xylem hydraulics properties of the vascular system, the severe crown damage caused growth reduction and transpiration dysfunction in the monitored period. Those effect could negatively affect the survival of burned trees, especially in areas subjected to hot and dry periods