TreeWatch.net : a water and carbon monitoring and modeling network to assess instant tree hydraulics and carbon status

TreeWatch.net is an initiative that has been developed to watch trees grow and function in real-time. It is a water- and carbon-monitoring and modeling network, in which high quality measurements of sap flow and stem diameter variation are collected on individual trees. Automated data processing using a cloud service enables instant visualization of water movement and radial stem growth. This can be used to demonstrate the sensitivity of trees to changing weather conditions, such as drought, heat waves, or heavy rain showers. But TreeWatch.net's true innovation lies in its use of these high precision harmonized data to also parameterize process-based tree models in real-time, which makes displaying the much needed mechanisms underlying tree responses to climate change possible. Continuous simulation of turgor to describe growth processes and long-term time series of hydraulic resistance to assess drought-vulnerability in real-time are only a few of the opportunities our approach offers. TreeWatch.net has been developed with the view to be complementary to existing forest monitoring networks and with the aim to contribute to existing dynamic global vegetation models. It provides high-quality data and real-time simulations in order to advance research on the impact of climate change on the biological response of trees and forests. Besides its application in natural forests to answer climate-change related scientific and political questions, we also envision a broader societal application of TreeWatch.net by selecting trees in nature reserves, public areas, cities, university areas, schoolyards, and parks to teach youngsters and create public awareness on the effects of changing weather conditions on trees and forests in this era of climate change

Influence of drought on foliar water uptake capacity of temperate tree species

Foliar water uptake (FWU) has been investigated in an increasing number of species from a variety of areas but has remained largely understudied in deciduous, temperate tree species from non- foggy regions. As leaf wetting events frequently occur in temperate regions, FWU might be more important than previously thought and should be investigated. As climate change progresses, the number of drought events is expected to increase, basically resulting in a decreasing number of leaf wetting events, which might make FWU a seemingly less important mechanism. However, the impact of drought on FWU might not be that unidirectional because drought will also cause a more negative tree water potential, which is expected to result in more FWU. It yet remains unclear whether drought results in a general increase or decrease in the amount of water absorbed by leaves. The main objectives of this study are, therefore: (i) to assess FWU- capacity in nine widely distributed key tree species from temperate regions, and (ii) to investigate the e ff ect of drought on FWU in these species. Based on measurements of leaf and soil water potential and FWU- capacity, the e ff ect of drought on FWU in temperate tree species was assessed. Eight out of nine temperate tree species were able to absorb water via their leaves. The amount of water absorbed by leaves and the response of this plant trait to drought were species- dependent, with a general increase in the amount of water absorbed as leaf water potential decreased. This relationship was less pronounced when using soil water potential as an independent variable. We were able to classify species according to their response in FWU to drought at the leaf level, but this classification changed when using drought at the soil level, and was driven by iso- and anisohydric behavior. FWU hence occurred in several key tree species from temperate regions, be it with some variability, which potentially allows these species to partly reduce the e ff ects of drought stress. We recommend including this mechanism in future research regarding plant- water relations and to investigate the impact of di ff erent pathways used for FWU

The twittering tree

Bossen zijn het dominante terrestrische ecosysteem op aarde. Ze bedekken 31% van het landoppervlak, i.e. 4.03 miljard hectare (FAO 2010), zijn verantwoordelijk voor 75% van de terrestrische bruto primaire productie en vormen 80% van de totale plant biomassa (Beer et al. 2010). Bomen zijn hierbij fundamenteel voor het leven op aarde en zijn ware ecosysteem ingenieurs (Jones et al. 1996). Ze zetten zonne-energie om in chemische energie, spelen een belangrijke rol in de nutriënten-, koolstof- en watercyclus, en creëren een habitat voor diverse species (Taiz en Zeiger 2010). Dit gehele arsenaal aan functies dreigt echter verloren te gaan en wordt reeds direct en indirect beïnvloed door ‘global change’ (Foley et al. 2007, Mooney et al. 2009, Montoya en Raffaelli 2010, Seidl et al. 2011, IPCC 2013). Niettegenstaande reeds veel informatie en data beschikbaar is over hoe individuele variabelen (e.g. verhoogde atmosferische CO2 concentraties, verhoogde temperatuur en droogte) een effect hebben op planten, is de geïntegreerde kennis van de in situ effecten gering (Morison and Lawlor 1999). Wat is de globale status van onze bossen? Wat is de geïntegreerde respons van de versnelde klimaatsverandering? Hoe veerkrachtig zijn bomen bij snel veranderende milieucondities? Zullen bomen hun cruciale rol in terrestrische ecosystemen kunnen blijven vervullen? Meer informatie en data is hiervoor nodig en deze masterproef is er om de aanzet te geven. In het experimenteel proefbos Aelmoeseneie (UGent) worden beuk (Fagus sylvatica L.) en eik (Quercus robur L.) sinds mei 2014 gemonitord met plantsensoren (sapstroom, stam-diametervariaties). Deze continue plantmetingen worden aangevuld met metingen van het microklimaat en fenologie om de groei en het hydraulisch functioneren van beuk en eik beter te begrijpen, en hun verschil in respons op wijzigingen in het microklimaat te karakteriseren. Het doel is om in real-time de respons van beuk en eik te visualiseren en eventuele stress te detecteren, i.e. ‘The Twittering Tree’. Visualisatie van de metingen gebeurt via PhytoSense. Diverse hydraulische parameters die inherent zijn aan een bepaalde boomspecies, e.g. geleidbaarheid, weerstand en capaciteit worden berekend voor beuk en eik. Reeds uit de eerste resultaten blijkt een significant verschil aanwezig te zijn tussen beuk en eik inzake gevoeligheid voor grote neerslaghoeveelheden. Onderstaande figuur geeft dit duidelijk weer waarbij eik meer invloed ondervindt van neerslag en minder goed herstelt dan beuk. Voornamelijk de extreme regenbui die plaatsvond tussen 25 en 26 augustus 2014 zorgde voor verminderde groei bij eik na de regenbui. Beuk en eik in het experimentele proefbos Aelmoeseneie (UGent) zullen het startpunt vormen van een netwerk waarbij de real-time status van bomen digitaal zal kunnen waargenomen worden. Voorlopig resultaten leiden ertoe dat dit zeker haalbaar is en dat het potentieel van dit netwerk zeer groot is. De data zal tevens gebruikt worden in een mechanistisch plantmodel, om zo de radiale stamgroei te verklaren op basis van onderliggende mechanismen. Door de boomrespons als stressindicator te gebruiken, zal de Twittering Tree als mogelijke ‘klimaatvoorspeller’ kunnen optreden

Modelling tree hydraulic responses to drought in beech

TreeWatch.net is a water and carbon monitoring and modeling network developed to watch trees grow and function in real-time (Steppe et al., 2016). High-quality measurements of sap flow and stem diameter variation are collected on individual trees, which are processed using a cloud service that enables real-time visualization of the data. TreeWatch.net’s true innovation lies in its use of this high-precision data and combination with process-based tree models. This makes displaying the much-needed mechanisms underlying tree responses to climatic changes possible and allows a well-founded assessment of the tree’s vitality. Due to its widespread distribution, the primary aimed tree species in the network is beech (Fagus sylvatica L.). The focus of this work lies in optimization of the process-based tree model linking sap flow and stem diameter variations (Steppe et al., 2006). While the model performs well and generates identifiable parameters under well-watered conditions, achieving this level of model accuracy is challenging when the tree is subjected to drought. During summer of 2017, drought responses were monitored in a mature beech tree and compared with a control tree. Continuous and discrete measurements were used to improve and calibrate the process-based tree model by Steppe et al. (2006). By enabling certain parameters to vary with time (e.g., hydraulic resistance of the root-to-leaf segment (Salomón et al., 2017) and conductance (Steppe et al., 2012)), it was possible to correctly simulate the tree responses and derive parameters related to tree vitality. Because the model can be run in real-time using the PhytoSense cloud service, it is possible to display the tree’s vitality status with high temporal resolution. This approach has the potential to become a very precise vitality monitoring tool. This can alleviate some of the currently labor intensive manual vitality measurements, and assist forest managers and policy makers in their decision taking

Variation in nocturnal stomatal conductance and development of predawn disequilibrium between soil and leaf water potentials in nine temperate deciduous tree species

It is widely acknowledged that many plant species can keep stomata open during night. We examined how nocturnal stomatal conductance differs among potted saplings of nine temperate tree species from diverse native habitats in wet and dry soil conditions, and how it affects plant predawn water status. Nocturnal stomatal conductance in dry soil conditions was low in all the species (with a maximum value of 14.6 mmol m(-2) s(-1)); in wet conditions, it was the highest in Populus tremula L., a fast-growing and anisohydric pioneer species, and the lowest in Quercus robur L., a late-successional and isohydric species. Relatively high nocturnal stomatal conductance in wet conditions in P. tremula compared with the other species resulted in the highest difference in water potential values between the leaves and soil at predawn. As drought progressed, different species tended to keep stomata almost closed at night, and the observed differences between anisohydric and isohydric species disappeared. At an ample soil water supply, nocturnal stomatal behaviour was species dependent and varied according to both the water-use and the life strategies of the species. Keeping that in mind, one should therefore be careful when using predawn leaf water potential as a proxy for soil water potential, sampling different species

An improved single probe method for sap flow measurements using finite heating duration

Because of its low cost and simple fabrication, it is easy to advocate for the single probe method as a method of choice in sap flow studies. An improved single probe method with finite heating duration (F-SPHP) is verified both in cut stem segments and in the field using mature beech (Fagus sylvatica L.) trees. The F-SPHP method is based on an analytical solution of the partial differential equation for combined heat conduction and convection, which shows large relative sensitivity to thermal conductivity (K). The F-SPHP method is able to measure sap flux densities (SFD) between 2 and 36 cm(3) cm(-2) h(-1) (heat pulse velocity (V-h): 3-60 cm h(-1)) in the cut stem egment experiment. This is an improvement compared to the instantaneous single probe heat pulse (I-SPHP) method which cannot accurately measure low (V-h < 20 cm h(-1)) sap flow. Sapflow+ (a four-needle heat pulse method) is used for field validation, and when compared with gravimetric measurements, slopes of 0.853 and 0.730 are obtained for F-SPHP and Sapflow+, respectively. Patterns measured with F-SPHP and Sapflow+ in mature beech trees in the forest are similar at all measurement positions and temperature correction is needed for both methods when natural temperature gradients are steep. Compared with multi-probe methods, the single probe method with finite heating duration has the advantage of causing less damage to conductive tissue