Alexia Stokes

Vegetation as a driver of temporal variations in slope stability: The impact of hydrological processes

Although vegetation is increasingly used to mitigate landslide risks, how vegetation affects the temporal variability of slope stability is poorly understood, especially in earthquake-prone regions. We combined 3-year long soil moisture monitoring, measurements of soil physical properties and plant functional traits, and numerical modeling to compare slope stability under paired land uses with and without trees in tropical, sub-tropical, and temperate landslide- and earthquake-prone regions. Trees improved stability for 5-12 months per year from drawdown of soil moisture and resulted in less interannual variability in the duration of high-stability periods compared to slopes without trees. Our meta-analysis of published data also showed that slopes with woody vegetation were more stable and less sensitive to climate and soil factors than slopes with herbaceous vegetation. However, estimates of earthquake magnitude necessary to destabilize slopes at our sites suggest that large additional stabilization from trees is necessary for meaningful protection against external triggers.Although vegetation is increasingly used to mitigate landslide risks, how vegetation affects the temporal variability of slope stability is poorly understood, especially in earthquake-prone regions. We combined 3-year long soil moisture monitoring, measurements of soil physical properties and plant functional traits, and numerical modeling to compare slope stability under paired land uses with and without trees in tropical, subtropical, and temperate landslide- and earthquake-prone regions. Trees improved stability for 5–12 months per year from drawdown of soil moisture and resulted in less interannual variability in the duration of high-stability periods compared to slopes without trees. Our meta-analysis of published data also showed that slopes with woody vegetation were more stable and less sensitive to climate and soil factors than slopes with herbaceous vegetation. However, estimates of earthquake magnitude necessary to destabilize slopes at our sites suggest that large additional stabilization from trees is necessary for meaningful protection against external triggers. (Resume dauteur)

Linkages between root traits, soil fungi and aggregate stability in tropical plant communities along a successional vegetation gradient

AimsDetermining which abiotic and biotic factors influence soil aggregate stability (MWD) in tropical climates is often confounded by soil type. We aimed to better understand the influence of soil physical and chemical components, vegetation and fungal abundance on MWD of a Ferralsol along a successional gradient of vegetation in New Caledonia.MethodsFive plant communities (sedge dominated, open sclerophyllous shrubland, Arillastrum forest, Nothofagus forest and mixed rainforest) were studied. For each community, MWD, soil texture, soil organic carbon (SOC), iron (Fe) and aluminium (Al) sesquioxides, root length density (RLD), specific root length (SRL), root mass density (RMD) and fungal abundance were measured. Generalized linear models were used to predict MWD from soil and plant trait data.ResultsThe best prediction of MWD combined abiotic and biotic factors. Along the gradient, Fe increased MWD, while root traits, fungal abundance and SOC modified MWD. From the sedge-dominated community to Arillastrum forest, RMD and SOC increased MWD, while between Nothofagus and mixed rainforest, it was likely that floristic composition and fungal communities influenced MWD.ConclusionsPlant community, the intrinsic nature of Ferralsol and fungal abundance all modified MWD. However, the specific effect of microbial communities should be addressed through a metagenomics approach to elucidate microbial interactions with plant communities.

An evaluation of inexpensive methods for root image acquisition when using rhizotrons

BackgroundBelowground processes play an essential role in ecosystem nutrient cycling and the global carbon budget cycle. Quantifying fine root growth is crucial to the understanding of ecosystem structure and function and in predicting how ecosystems respond to climate variability. A better understanding of root system growth is necessary, but choosing the best method of observation is complex, especially in the natural soil environment. Here, we compare five methods of root image acquisition using inexpensive technology that is currently available on the market: flatbed scanner, handheld scanner, manual tracing, a smartphone application scanner and a time-lapse camera. Using the five methods, root elongation rate (RER) was measured for three months, on roots of hybrid walnut (Juglans nigraxa0×xa0Juglans regia L.) in rhizotrons installed in agroforests.ResultsWhen all methods were compared together, there were no significant differences in relative cumulative root length. However, the time-lapse camera and the manual tracing method significantly overestimated the relative mean diameter of roots compared to the three scanning methods. The smartphone scanning application was found to perform best overall when considering image quality and ease of use in the field. The automatic time-lapse camera was useful for measuring RER over several months without any human intervention.ConclusionOur results show that inexpensive scanning and automated methods provide correct measurements of root elongation and length (but not diameter when using the time-lapse camera). These methods are capable of detecting fine roots to a diameter of 0.1xa0mm and can therefore be selected by the user depending on the data required.

Linking above- and belowground phenology of hybrid walnut growing along a climatic gradient in temperate agroforestry systems

Background and aimsPlant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France.MethodsEight rhizotrons were installed at each site for 21xa0months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1xa0mm, 1–2xa0mm and 2–5xa0mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year.ResultsRoot growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1xa0mm and 1–2xa0mm diameter classes lived for significantly shorter periods compared to those in the 2–5xa0mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1xa0mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter.ConclusionsWe conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined.

Mechanical traits of fine roots as a function of topology and anatomy

Background and AimsnRoot mechanical traits, including tensile strength (Tr), tensile strain (εr) and modulus of elasticity (Er), are key functional traits that help characterize plant anchorage and the physical contribution of vegetation to landslides and erosion. The variability in these traits is high among tree fine roots and is poorly understood. Here, we explore the variation in root mechanical traits as well as their underlying links with morphological (diameter), architectural (topological order) and anatomical (stele and cortex sizes) traits.nnnMethodsnWe investigated the four tropical tree species Pometia tomentosa, Barringtonia fusicarpa, Baccaurea ramiflora and Pittosporopsis kerrii in Xishuangbanna, Yunnan, China. For each species, we excavated intact, fresh, fine roots and measured mechanical and anatomical traits for each branching order.nnnKey ResultsnMechanical traits varied enormously among the four species within a narrow range of diameters (<2 mm): <0.1-65 MPa for Tr, 4-1135 MPa for Er and 0.4-37 % for εr. Across species, Tr and Er were strongly correlated with stele area ratio, which was also better correlated with topological order than with root diameter, especially at interspecific levels.nnnConclusionsnRoot topological order plays an important role in explaining variability in fine-root mechanical traits due to its reflection of root tissue development. Accounting for topological order when measuring fine-root traits therefore leads to greater empirical understanding of plant functions (e.g. anchorage) within and across species.