Hydraulic redistribution under moderate drought among English oak, European beech and Norway spruce determined by deuterium isotope labeling in a split-root experiment

Laser ablation tomography (LATscan) as a new tool for anatomical studies of woody plants

Traditionally, botanists study plant anatomy by carefully sectioning samples, histological staining to highlight tissues of interests, then imaging slides under light microscopy. This approach generates significant details; however, this workflow is laborious, particularly in woody vines (lianas) with heterogeneous anatomies, and ultimately yields two-dimensional (2D) images. Laser ablation tomography (LATscan) is a high-throughput imaging system that yields hundreds of images per minute. This method has proven useful for studying the structure of delicate plant tissues; however, its utility in understanding the structure of woody tissues is underexplored. We report LATscan-derived anatomical data from several stems of lianas (c. 20 mm) of seven species and compare these results with those obtained through traditional anatomical techniques. LATscan successfully allows the description of tissue composition by differentiating cell type, size, and shape, but also permits the recognition of distinct cell wall composition (e.g. lignin, suberin, cellulose) based on differential fluorescent signals on unstained samples. LATscan generate high-quality 2D images and 3D reconstructions of woody plant samples; therefore, this new technology is useful for both qualitative and quantitative analyses. This high-throughput imaging technology has the potential to bolster phenotyping of vegetative and reproductive anatomy, wood anatomy, and other biological systems.

Magnitude and determinants of plant root hydraulic redistribution: A global synthesis analysis

Plant root hydraulic redistribution (HR) has been widely recognized as a phenomenon that helps alleviate vegetation drought stress. However, a systematic assessment of the magnitude of HR and its drivers at the global scale are lacking. We collected 37 peer-reviewed papers (comprising 47 research sites) published in 1900-2018 and comprehensively analyzed the magnitude of HR and its underlying factors. We used a weighting method to analyze HR magnitude and its effect on plant transpiration. Machine learning algorithms (boosted regression trees) and structural equation modeling were used to determine the influence of each factor on HR magnitude. We found that the magnitude of HR was 0.249 mm H2O d-1 (95% CI, 0.113-0.384) and its contribution to plant transpiration was 27.4% (3-79%). HR varied significantly among different terrestrial biomes and mainly occurred in forests with drier conditions, such as temperate forest ecosystems (HR = 0.502 mm H2O d-1), where HR was significantly higher than in other ecosystems (p < 0.01). The magnitude of HR in angiosperms was significantly higher than that in gymnosperms (p < 0.05). The mean magnitude of HR first increased and then decreased with an increase in humidity index; conversely, the mean magnitude of HR decreased with an increase in water table depth. HR was significantly positively correlated with root length and transpiration. Plant characteristics and environmental factors jointly accounted for 61.0% of the variation in HR, and plant transpiration was the major factor that directly influenced HR (43.1% relative importance; p < 0.001), and soil texture was an important indirect driver of HR. Our synthesis offers a comprehensive perspective of how plant characteristics and environmental factors influence HR magnitude.

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 hydrology.

Split down the middle: studying arbuscular mycorrhizal and ectomycorrhizal symbioses using split-root assays

Most land plants symbiotically interact with soil-borne fungi to ensure nutrient acquisition and tolerance to various environmental stressors. Among these symbioses, arbuscular mycorrhizal and ectomycorrhizal associations can be found in a large proportion of plants, including many crops. Split-root assays are widely used in plant research to study local and systemic signaling responses triggered by local treatments, including nutrient availability, interaction with soil microbes, or abiotic stresses. However, split-root approaches have only been occasionally used to tackle these questions with regard to mycorrhizal symbioses. This review compiles and discusses split-root assays developed to study arbuscular mycorrhizal and ectomycorrhizal symbioses, with a particular emphasis on colonization by multiple beneficial symbionts, systemic resistance induced by mycorrhizal fungi, water and nutrient transport from fungi to colonized plants, and host photosynthate allocation from the host to fungal symbionts. In addition, we highlight how the use of split-root assays could result in a better understanding of mycorrhizal symbioses, particularly for a broader range of essential nutrients, and for multipartite interactions.

Friendly neighbours: Hydraulic redistribution accounts for one quarter of water used by neighbouring drought stressed tree saplings

Hydraulic redistribution (HR) can buffer drought events of tree individuals, however, its relevance for neighbouring trees remains unclear. Here, we quantified HR to neighbouring trees in single- and mixed-species combinations. We hypothesized that uptake of HR water positively correlates with root length, number of root tips and root xylem hydraulic conductivity and that neighbours in single-species combinations receive more HR water than in phylogenetic distant mixed-species combinations. In a split-root experiment, a sapling with its roots split between two pots redistributed deuterium labelled water from a moist to a dry pot with an additional tree each. We quantified HR water received by the sapling in the dry pot for six temperate tree species. After 7 days, one quarter of the water in roots (2.1 ± 0.4 ml), stems (0.8 ± 0.2 ml) and transpiration (1.0 ± 0.3 ml) of the drought stressed sapling originated from HR. The amount of HR water transpired by the receiving plant stayed constant throughout the experiment. While the uptake of HR water increased with root length, species identity did not affect HR as saplings of Picea abies ((L.) Karst) and Fagus sylvatica (L.) in single- and mixed-species combinations received the same amount of HR water.

Drought Resistance of Norway Spruce (Picea abies [L.] Karst) and European Beech (Fagus sylvatica [L.]) in Mixed vs. Monospecific Stands and on Dry vs. Wet Sites. From Evidence at the Tree Level to Relevance at the Stand Level

Frequency of drought years is expected to increase through climate warming. Mixed stands have often shown to be more productive than monospecific stands in terms of yield and of resistance against windthrows and bark beetle attacks. Mixture of beech and spruce is of particular interest. However, little is known about its growth reaction to drought. Therefore, we investigated the drought reaction of beech and spruce in mixed vs. monospecific stands along an ecological gradient. In particular, we sought evidence for mixture-related resilience on the individual tree level. Therefore, we quantified the response of tree ring width to drought. Moreover, we attempted to explain the relevance of individual tree response on the stand level by quantifying the stand level loss of volume growth after drought. At the individual tree level, beech was found to be more resilient and resistant in pure vs. mixed stands. Spruce, in contrast, was favored by mixture, and this was especially evident on drier sites. Along the gradient, growth losses at stand level increased in both mixed and pure stands in 2015, with growth gains on the drier sites observed in the same drought year, in accordance with the Stress Gradient Hypothesis. However, the stand level difference of growth loss between mixed and pure stands was not statistically significant. Mitigating mixture effects on the level of the individual tree thus did not become evident on the level of the whole stand.

Hydraulic Water Redistribution by Silver Fir (Abies alba Mill.) Occurring under Severe Soil Drought

Hydraulic redistribution (HR) of water from wet- to dry-soil zones is suggested as an important process in the resilience of forest ecosystems to drought stress in semiarid and tropical climates. Scenarios of future climate change predict an increase of severe drought conditions in temperate climate regions. This implies the need for adaptations of locally managed forest systems, such as European beech (Fagus sylvatica L.) monocultures, for instance, through the admixing of deep-rooting silver fir (Abies alba Mill.). We designed a stable-isotope-based split-root experiment under controlled conditions to test whether silver fir seedlings could perform HR and therefore reduce drought stress in neighboring beech seedlings. Our results showed that HR by silver fir does occur, but with a delayed onset of three weeks after isotopic labelling with 2H2O (δ2H ≈ +6000‰), and at low rates. On average, 0.2% of added ²H excess could be recovered via HR. Fir roots released water under dry-soil conditions that caused some European beech seedlings to permanently wilt. On the basis of these results, we concluded that HR by silver fir does occur, but the potential for mitigating drought stress in beech is limited. Admixing silver fir into beech stands as a climate change adaptation strategy needs to be assessed in field studies with sufficient monitoring time.

Diversity and forest productivity in a changing climate

Contents Summary 50 I. Introduction 50 II. Drivers of the diversity-productivity relationship 51 III. Patterns of the diversity-productivity relationship 55 IV. Responses of mixed stands to climate change 57 V. Conclusions 60 Acknowledgements 61 References 61 SUMMARY: Although the relationship between species diversity and biomass productivity has been extensively studied in grasslands, the impact of tree species diversity on forest productivity, as well as the main drivers of this relationship, are still under discussion. It is widely accepted that the magnitude of the relationship between tree diversity and forest stand productivity is context specific and depends on environmental conditions, but the underlying mechanisms of this relationship are still not fully understood. Competition reduction and facilitation have been identified as key mechanisms driving the diversity-productivity relationship. However, contrasting results have been reported with respect to the extent to which competition reduction and facilitation determine the diversity-productivity relationship. They appear to depend on regional climate, soil fertility, functional diversity of the tree species involved, and developmental stage of the forest. The purpose of this review is to summarize current knowledge and to suggest a conceptual framework to explain the various processes leading to higher productivity of species-rich forests compared with average yields of their respective monocultures. This framework provides three pathways for possible development of the diversity-productivity relationship under a changing climate.