Is sensitivity to xylem cavitation a relevant physiological trait for fodder production in dry season?

In the Sahel parklands, trees can provide fodder during the dry season if they keep functional foliage. In such conditions, the xylem vessels sustain high tension which can induce cavitation, decrease the conduction of sap and lead to branch mortality. Therefore, xylem resistance to cavitation is a good index of tree tolerance to water stress. However, information about this parameter remains scarce in Sahel tree species. Moreover, its measurement is difficult and requires equipment limiting its feasibility in field conditions. To measure the vulnerability to xylem cavitation in 10 tree species with fodder potential in North Senegal and to assess if this ecophysiological trait can be correlated to morphological traits that are easier to measures: wood density, xylem anatomy, and vessel length. Branch samples were collected in parklands in Louga region, Northern Senegal. Wood density and xylem anatomy were determined. Branch sections were wrapped to keep wet and sent to France within 1 week. The Cavitron spinning technique was used to construct cavitation vulnerability curves and to compute the P50 (water potential inducing 50 % loss of conductivity). Vessel length was assessed by flushing air at low pressure through gradually shortened branch sections. The Cavitron spinning technique was not relevant on nine out of ten species. Most vulnerability curves had a sharp increase in loss of conductivity, starting at low water stress. The resulting P50 which were not consistent with water potential measured in the field. All this species but Tamarindus indica had xylem elements longer than the device rotor. They were severed during measurement, inducing artificial loss of conductivity. Only Boscia senegalensis showed correct curves, and revealed very resistant to cavitation (P50 = ?7.9 Mpa). Wood densities ranged between 0.41 - Adansonia digitata - to 0.71 - Acacia tortilis. P50 estimated from these densities were between - 2 and - 6 MPa. Vulnerability to xylem cavitation could not be assessed by the spinning technique 'Cavitron', due to long xylem vessels elements in most the species. The probable low vulnerabilities estimated from high wood densities could be corroborated by xylem anatomy. (Texte intégral

Defining functional groups of tree species according to rural stakeholder perceptions in Central-Mali

In the Tiby area (Segou region, Mali), the intensive exploitation of timber resources and the lack of tree regeneration lead to a degradation of agroforestry parklands for years. In 2010, a study was conducted concerning local knowledge and uses of tree species in a same territory, depending on different kind of stakeholders. Qualitative surveys have been carried out to identify farmer's perceptions of tree functions and corresponding functional traits. The results show that most of woody species have a significant multifunctionality. Only few species are unifunctional. The most multifunctional species cover all types of production and service functions but not all the uses, particularly for medicinal uses. Some specific uses are fulfilled by only one species. Correspondence analysis with tree functions data show the possibility of grouping species in relevant functional groups. Socio?economic analyses emphasize the fact that a same species does not fulfill the same functions depending on user groups (cultivators, livestock farmers, women, etc.) and depending on agri?environmental and socio?economical contexts (activities, knowledge, means and practices). The surveys reveal common perceptions between farmers of certain functional traits linked to strategic functions, and specific knowledge depending on farmer activities. But data on functional traits were tricky to analyze because of their sparse character and a number of interviews too low. Nevertheless, comparisons between traits quoted by farmers and some functional traits known in scientific literature will be helpful in the aim of trying to build functional groups of tree species. Finally, it seems that each user group seeks for satisfying a given need (maintain of soil fertility in cropped fields, fodder, fruits, fences, timber, firewood, medicines, etc.) by targeting a group of species among available woody species, on the base of functional traits as they discern them, and by mobilizing their knowledge and their means. It means that scientific databases composed of lists of functions and traits for a given species have low generic value and must be valued depending on the context. The notion of functional group gathering substitutable species for a same function takes thus all sense. These results aimed to emphasize the functions and specific uses of tree species that must be taken into account in perspective of improving the management of agroforestry systems. (Texte intégral

Modélisation de l'architecture racinaire

Pour décrire et modéliser l'architecture du système racinaire on analyse d'abord la position d'apparition des axes et leurs caractéristiques morphogénétiques. On établit ainsi, par observation dans des milieux divers et à différents stades, une typologie des axes racinaires. La construction d'un modèle permet de synthétiser ces informations et de produire des simulations. Nous présentons l'application de la démarche à l'hévéa, et montrons les intérêts de cet outil pour l'agronome. (Résumé d'auteur

Biofunctool®: a new set of indicators to assess the impact of land management onsoil functioning

The concept and methods to assess soil quality have been widely debated in the literature for the last twenty years. We developed a new framework to assess soil quality following an integrative approach based on the measurement of soil dynamic functions rather than stocks, namely Biofunctool®. Biofunctool® accounts for the interactions between soil physico-chemical properties and soil biological activity. It consists of twelve in-field, time- and cost- effective indicators to assess three main soil functions: carbon transformation, nutrient cycling and structure maintenance. Firstly, the capacity of Biofunctool® to assess the impact of land management on soil quality was validated through a reliability, redundancy and sensitivity analysis. The results over 250 sampling points in Thailand showed the relevance of each of the twelve indicators to assess soil functioning. Secondly, we applied Biofunctool® and aggregated the indicators in a Soil Quality Index that synthetize the impact of land management on soil quality. Biofunctool® was applied within various contexts (tree plantations, agroforestry, conservation agriculture etc.) and two cases study will be presented: i.) impacts of a disturbance gradient based on various land uses and rubber tree stands in Thailand ii.) impacts of conservation agriculture practices in Cambodia. The overall results proved that Biofunctool® index provides a synthetic soil functioning score that is sensitive to land management and is robust in various pedo-climatic contexts. Therefore, Biofunctool® is a reliable tool to assess the soil integrated functioning, i.e., soil quality, and could be included within larger environmental impact assessment frameworks

Effects of drought and tapping for latex production on water relations of Hevea brasiliensis trees

The impact of drought conditions on the water relations of rubber trees is important because the trees could shed leaves while they are being continuously tapped. The aim of this work was to study the effect of soil drought and the tapping of latex on the water relations of rubber trees. This study distinguished between untapped trees and trees affected by tapping activity. The experiment was conducted on 14-15 year-old rubber trees from the clone, RRIM 600, at the Chachoengsao Rubber Research Center (CRRC). Climatic measurement, soil water content, predawn and midday leaf water potential, stand transpiration and percentage loss of hydraulic conductivity (PLC) were measured from January 2009 to January 2010, especially in the dry season. Under drought conditions, during the rainy season, stand transpiration and predawn leaf water potential decreased in response to soil drought in August. The midday leaf water potential and PLC did not change following soil water stress. In the dry season, the plant water status was related to climatic variables and soil drought. The results indicated that drought induces a reduction in the plant water status of rubber trees. Tapping activity did not have a marked infl uence on the water balance of the rubber trees. However, tapping reduced the sap fl ux density and stomatal conductance; thus, attention must be paid to tapping in the dry season, particularly in dry areas. (Résumé d'auteur

Impact of tapping and soil water status on fine root dynamics in a rubber tree plantation in Thailand

Fine roots (FR) play a major role in the water and nutrient uptake of plants and contribute significantly to the carbon and nutrient cycles of ecosystems through their annual production and turnover. FR growth dynamics were studied to understand the endogenous and exogenous factors driving these processes in a 14-year-old plantation of rubber trees located in easternThailand. FR dynamicswere observed using field rhizotrons from October 2007 to October 2009.This period covered two complete dry seasons (November to March) and two complete rainy seasons (April to October), allowing us to study the effect of rainfall seasonality on FR dynamics. Rainfall and its distribution during the two successive years showed strong differences with 1500 and 950 mm in 2008 and 2009, respectively. FR production (FRP) completely stopped during the dry seasons and resumed quickly after the first rains. During the rainy seasons, FRP and the daily root elongation rate (RER)were highly variable and exhibited strong annual variations with a total FRP of 139.8 and 40.4mm?2 and an average RER of 0.16 and 0.12 cm day?1 in 2008 and 2009, respectively. The significant positive correlations found between FRP, RER, the appearance of new roots, and rainfall at monthly intervals revealed the impact of rainfall seasonality on FR dynamics. However, the rainfall patterns failed to explain the weekly variations of FR dynamics observed particularly during the rainy seasons. At this time step, FRP, RER, and the appearance of new FR were negatively correlated to the average soil matric potential measured at a depth of between 30 and 60 cm. In addition, our study revealed a significant negative correlation between FR dynamics and the monthly production of dry rubber. Consequently, latex harvesting might disturb carbon dynamics in the whole tree, far beyond the trunk where the tapping was performed. These results exhibit the impact of climatic conditions and tapping system in the carbon budget of rubber plantations. (Résumé d'auteur

Can the thermal stability of soil organic matter reflect disturbance and resilience in rubber tree-based agrosystems?

The capacity of soils to mitigate climate change through carbon sequestration depends on the quantitative but also the qualitative changes in soil organic carbon (SOC). In particular, the stability of SOC has to be considered to evaluate the long-term fate of the soil carbon stocks. The aim of this study was to assess the accurateness of thermal analyzes (Rock-Eval pyrolysis) to monitor the evolution of SOC quality and quantity in different land management situations. Using new I/R diagram (Sebag et al., 2016), dynamics of SOC was assessed in rubber tree-based agrosystems in Thailand. This case study allowed analyzing the changes in SOC properties after land use change (cassava to rubber) and over the ageing of the rubber plantation (up to 25 years). Soil from secondary forest plots were included in the study as a reference of an undisturbed system. While I (contribution of fresh OM) and R indices (global thermal stability) were well correlated in forest soils, our results show a higher I value in cassava field, and increasing R values with rubber plantation age. These results indicate that both SOC stocks and SOC stability increased with the age of plantations. However, R-index in the oldest rubber plantations was lower than in forest. Finally, this study highlights the potential of thermal analyses for monitoring SOC quantity and quality with promising application under the 4‰ initiative

Diurnal dynamics of stand transpiration and stomatal conductance in rubber (Hevea brasiliensis Muell. Arg.)

Transpiration at the leaf and stand level is mainly controlled by climatic conditions. The diurnal dynamics of stand transpiration and stomatal conductance were analyzed in rubber trees. The experiment was conducted using 13 yr-old rubber trees from clone RRIM 600 under natural conditions at the Chachoengsao Rubber Research Center during January to December 2007, with sampling in the dry season, the rainy season and early in the dry season. At the stand level, the stand transpiration was monitored using sap fl ow measurement and estimated evapotranspiration from meteorological data. At the leaf level, the stomatal conductance was measured using a portable photosynthesis system on the leaves exposed to the sun. The leaf water potential was measured using a pressure chamber and the soil water content was measured by the gravimetric method. Climatic measurements were recorded using a weather station above the canopy. The plant water status was correlated with the climatic conditions and soil water content. The diurnal dynamics of stand transpiration and stomatal conductance were mainly controlled by the vapor pressure defi cit and net radiation. The stomatal conductance was more sensitive to climatic variations than stand transpiration. There were different time lags between the stomatal conductance and stand transpiration throughout the year. The relationship between the climatic factors and transpiration varied throughout the year. (Résumé d'auteur

Relationships between carbon stock in the rubber tree and latex production

The relationship between tapping systems and the balance between latex production and carbohydrate availability would be very informative. This present work aims to study the effects of tapping systems on enzyme activities involved with Non-structural carbohydrates (NSCs) dynamic. The result indicated that enzyme activities from RR IM600 with upward tapping was higher from that of downward tapping. This could be partly due to the gradient sucrose along the t ruck as it is more concentrated in higher position than the lower one. Sucrose synthase was proportional to sucrose content in bark of upward tapping. Meanwhile, Sucrose Phosphate Synthase and Amylase were higher in wood than bark. These enzymes were also significantly higher in upward tapping than downward tapping. Using immunolocalization test, amylase was located in both vascular rays of bark and wood. It was noticed that enzyme activities strongly related to sucrose content while the link with starch content could not be clarified yet. To eventually explain the whole picture of NSC balance and latex production, information of latex production yield and enzyme involved in starch synthesis must be taken into account. (Résumé d'auteur

"Biofunctool": a framework to assess the impact of agricultural practices on soil quality based on soil functions

The assessment of soil quality is a concern that has been widely developed in the literature over the last twenty years. According to Karlen et al. (1997), the soil quality can be defined as the “the capacity of the soil to function”. However, most indicators of the soil quality are based on the evaluation of soil characteristics rather than soil functions. This leads to some discrepancies such as the high productivity of some tropical soils despite their status of “poor soil” conferred by these indicators. Kibblewhite et al. (2008) proposed a theoretical framework to study soil functions related to the major ecosystem services. This framework underlines the need to consider the interactions between soil physico-chemical properties and its biological activity. Based on this concept, we aimed at developing a set of functional soil indicators to assess three central soil functions: carbon transformation, nutrient cycling and soil structure maintenance. We intend to integrate all indicators within the “Biofunctool” package, i.e. a set of easy, cost, time-effective in situ measurements to assess the soil functions. The Biofunctool package is currently being developed in the context of rubber plantations in Thailand. It will be then tested in a plantation network along a gradient of pedo-climatic and agronomic conditions. In each situation, Biofunctool measurements will be completed with soil biota diagnosis in order to better link soil functions and biological assemblages. The approach and outputs developed will be used to evaluate the impact of agricultural practices on agro-ecosystem performances and sustainability. Biofunctool framework will thus potentially complete existing one, such as the regional agronomic diagnosis (Doré et al., 2008) or the Life Cycle Assessment. (Résumé d'auteur