Short term effects of latex tapping on micro-changes of trunk girth in [i]Hevea Brasiliensis[/i]

Latex tapping has a well-known negative effect on the long term radial growth of rubber trees (Hevea Brasiliensis). The additional carbon sink induced by latex yield is considered as the main cause. However the potential contribution of a tapping induced water stress has received little attention. In Northeast Thailand, we applied an exploring approach comparing the diel cycle of girth change between days of rest and days with tapping in conditions of relatively stable evaporative demand and soil water availability. Trees were tapped at dark in the early morning for two consecutive days and rested for one day. Five standard trees were equipped with high accuracy girth bands above the tapping panel. The sampling included one tree with additional measurements, one below the tapping cut and the other at the trunk bottom. Data were recorded at 30 min interval over 14 days at the onset of the dry season in November. Results demonstrated a significant short-term shrinkage within two hours after tapping. However, the nighttime expansion maximum diurnal shrinkage and midnight recovery were not significantly influenced by the tapping cycle. As a result the daily growth was not negatively impacted on tapping days. Finally, in conditions of low average growth, our results refute the hypothesis of a negative impact of tapping on radial growth at a daily scale through a simple dehydration. A substantial loss of turgor was confirmed but trees seem to quickly react and smooth the consequences on nighttime recovery and diurnal shrinkage

A simple framework to analyze water constraints on seasonal transpiration in rubber tree (Hevea brasiliensis) plantations

Climate change and fast extension in climatically suboptimal areas threaten the sustainability of rubber tree cultivation. A simple framework based on reduction factors of potential transpiration was tested to evaluate the water constraints on seasonal transpiration in tropical sub-humid climates, according pedoclimatic conditions. We selected a representative, mature stand in a drought-prone area. Tree transpiration, evaporative demand and soil water availability were measured every day over 15 months. The results showed that basic relationships with evaporative demand, leaf area index and soil water availability were globally supported. However, the implementation of a regulation of transpiration at high evaporative demand whatever soil water availability was necessary to avoid large overestimates of transpiration. The details of regulation were confirmed by the analysis of canopy conductance response to vapor pressure deficit. The final objective of providing hierarchy between the main regulation factors of seasonal and annual transpiration was achieved. In the tested environmental conditions, the impact of atmospheric drought appeared larger importance than soil drought contrary to expectations. Our results support the interest in simple models to provide a first diagnosis of water constraints on transpiration with limited data, and to help decision making toward more sustainable rubber plantations. (Résumé d'auteur

Change of whole-tree transpiration of mature Hevea brasiliensis under soil and atmospheric droughts: analyze in intermittent and seasonal droughts under the framework of the hydraulic limitation hypothesis

The changes of whole-tree transpiration under atmospheric and soil droughts were investigated for mature rubber trees (Hevea brasiliensis, clone RRIM 600) in a drought-prone area of North-East Thailand. The changes were analyzed and compared between intermittent drought in the rainy season and seasonal drought where occur periods of lower leaf area (leaf senescence, massive leaf shedding and leaf flushing) and subsoil root growth. The physiological responses were analyzed under the framework of a simple hydraulic limitation model called 'RER_ET0'. This model is mainly based on a critical minimum leaf water potential (ψcrit) and the responses of whole-tree hydraulic conductance (gL) and predawn leaf water potential (ψpredawn) to relative extractable water (REW). Continuous and accurate estimate of whole tree transpiration over a whole year was a key measurement of this study with concurring measurement of environmental conditions and leaf water potential (ψLeaf). We applied the transient thermal dissipation method (TTD) developed by Do and Rocheteau (2002b) which has several advantages but which was never tested in rubber wood The TTD method was calibrated in the laboratory with cut stems of Hevea brasiliensis and two other species (Mangifera indica and Citrus maxima). The results provided a simple linear calibration which was independent of the woody species tested (R² = 0.88, n = 276, P<0.0001). Field comparison with soil water depletion in the dry season validated to a reasonable extent the absolute estimates of transpiration. In rainy season, the first remarkable result was the assessment of saturating transpiration above reference evapotranspiration (ET0) c.a. 2.2 mm day-1 in period of well soil watered conditions. Intermittent soil drought provoked a dramatic decrease of transpiration below a threshold of 50% REW in the top soil which corresponded to a ψpredawn c.a. -0.45 MPa. Transpiration was reduced by 40% at 0.3 REW and 80% at 0.1 REW. The minimum leaf water potential (ψminimum) for sunny day did not decrease according to soil drought and was stable around -1.95 MPa which supported an isohydric behaviour. The decrease of transpiration was mainly due to the change of gL. Results in the seasonal drought showed that the principles of the hydraulic limitation model hold despite changing phenology and leaf area. Moreover, the deciduous pattern showed little influence upon the details of parameters and relationships. Finally, the framework of a simple hydraulic limitation model was successful to describe the main change of whole-tree transpiration under both atmospheric and soil drought. Both our experimental and modeling results demonstrate the isohydric behaviour of the studied mature rubber trees.Les variations de transpiration totale sous contraintes hydriques, à la fois atmosphérique et édaphique, sont étudiées pour des arbres matures d'Hevea brasiliensis (clone RRIM600) dans une zone limitée en eau du Nord-Est de la Thaïlande. Les variations sont analysées et comparées entre une sécheresse transitoire en saison des pluies et la sécheresse saisonnière caractérisée par des fluctuations de la surface foliaire (sénescence foliaire, chute des feuilles and re-feuillaison) et de la croissance racinaire en profondeur. Les réponses physiologiques sont analysées dans le cadre d'un modèle hydraulique simple appelé "RER_ET0". Ce modèle est principalement basé sur une valeur critique du potentiel hydrique foliaire (ψcrit) et sur les réponses, de la conductivité hydraulique totale de l'arbre (gL) et du potentiel hydrique de base (ψpredawn), à la disponibilité en eau du sol (REW). Une estimation précise et en continue de la transpiration totale des arbres durant une année complète était une mesure clé de cette étude avec des mesures concomitantes des conditions environnementales et du potentiel hydrique foliaire (ψLeaf). Nous avons appliqué la méthode à dissipation thermique transitoire (TTD) développée par Do et Rocheteau (2002b) qui a plusieurs avantages mais n'avait pas été testée sur bois d'hévéa. La méthode TTD a été calibrée en laboratoire sur des segments de branches d'Hevea brasiliensis et de deux autres espèces (Mangifera indica and Citrus maxima). Les résultats fournissent une calibration linéaire unique indépendante des espèces étudiées (R² = 0.88, n = 276, P<0.0001). Une comparaison en plantation via un bilan hydrique du sol en saison sèche valide les ordres de grandeurs de transpiration ainsi estimés par les flux de sève. Les résultats démontrent une remarquable saturation de la transpiration au dessus d'un certain seuil d'évapotranspiration de référence (ET0), environ 2.2 mm day-1, indépendamment de la disponibilité en eau du sol (REW). Ensuite, le dessèchement du sol en saison des pluies provoque une chute marquée de la transpiration en dessous d'un seuil de 50% du REW dans l'horizon de surface du sol, ce qui correspond à ψpredawn autour de -0.45 MPa. La transpiration est réduite de 40% à REW 0.3 et 80% à REW 0.1. La valeur minimale de potentiel hydrique foliaire (ψminimum) en journée ensoleillée ne décroit pas en fonction du dessèchement du sol mais est stable autour de -1.95 MPa ce qui suggère un comportement iso-hydrique. La décroissance de la transpiration est essentiellement liée à la variation de gL. Les résultats durant la sécheresse saisonnière montrent étonnamment que les principes du modèle à limitation hydraulique tiennent malgré les variations de phénologie et de surface foliaire. De plus, les variations phénologiques ont peu influencé sur les détails des paramètres et relations du modèle. Finalement, le cadre du simple modèle hydraulique a été suffisant pour décrire correctement les principales variations de transpiration de l'arbre sous contrainte hydrique édaphique comme climatique. Un comportement iso-hydrique des arbres matures étudiés est démontré à la fois par les résultats expérimentaux et la modélisation

Global transpiration data from sap flow measurements : the SAPFLUXNET database

Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land-atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/, last access: 8 June 2021). We harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants, mostly trees, of 174 species. SAPFLUXNET has a broad bioclimatic coverage, with woodland/shrubland and temperate forest biomes especially well represented (80 % of the datasets). The measurements cover a wide variety of stand structural characteristics and plant sizes. The datasets encompass the period between 1995 and 2018, with 50 % of the datasets being at least 3 years long. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings. SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes. SAPFLUXNET version 0.1.5 is freely available from the Zenodo repository (https://doi.org/10.5281/zenodo.3971689; Poyatos et al., 2020a). The "sapfluxnetr" R package - designed to access, visualize, and process SAPFLUXNET data - is available from CRAN.Peer reviewe

Rubber trees affected by necrotic Tapping Panel Dryness exhibit poor transpiration regulation under atmospheric drought

International audienceThe objective was to study the responses in water status and transpiration regulation of rubber trees affected by the necrotic Tapping Panel Dryness (N) by comparison with healthy trees (H). The experiment was done with 5 N trees and 5 H trees of clone RRIM600 during well soil watered periods differing in evaporative demand intensity, May and August 2007. The study compared predawn leaf water potential (Psi(pd)), midday leaf water potential (Psi(mid)), whole tree hydraulic conductance (K), midday sapflow density (J(s)) and tree transpiration (E-T) with the average girth size 51.54 cm of H tree and 52.66 cm of N tree. These variables, investigated in the high evaporative demand day (ETo = 3.71 mm day(-1)) on 23 May 2007 and low evaporative demand day (ETO = 1.75 mm day(-1)) on 22 Aug 2007, did not significantly differ between tree types. However, over a long period, in high evaporative demand, E-T tended to be higher in N trees. Expression of E-T versus ETO confirmed different relationships between the two tree types with a higher plateau of maximum transpiration for N trees. Our results suggested that individuals with relatively poor transpiration regulation could be more sensitive to necrotic Tapping Panel Dryness syndrome

Dendroclimatic Reconstruction of Mean Annual Temperatures over Treeline Regions of Northern Bhutan Himalayas

The Himalayan region is likely particularly exposed to climate change indicated by the high regional rate of change. The number of high-resolution, well-calibrated, and long-term paleoclimate reconstructions are however regrettably few, to set this change in a longer-term context. The dendroclimatic reconstructions over Himalaya that do exist have only reconstructed summer season temperatures, and rarely or never attempted to reconstruct mean annual temperatures. The paucity of long meteorological records is a matter of concern when developing chronologies of climate sensitive tree-ring data in Bhutan, but the chronologies would theoretically be of high potential for extending short meteorological records back in time using trees in high-elevation ecotones. The objectives of this study were to explore dendroclimatic signals in tree-ring width chronologies of Abies densa growing in these extreme ecotones and to reconstruct, if possible, annual temperatures over Northern Bhutan. A point-by-point regression analysis revealed that the regional composite chronology was significantly and positively correlated with temperatures of all months of the current year, i.e., January to December. The chronology was highly correlated with annual temperatures (calibration period R = 0.67 and validation period R = 0.50; p &lt; 0.001) allowing a reconstruction of temperature over Northern Bhutan (NB-TEMR). The NB-TEMR reveals some common variations with summer temperature reconstructions of the Northern Hemisphere as well as the Himalayan region, particularly w.r.t to the recent warming trend. The reconstruction covers the period of 1765 to 2017. This reconstruction reveals a warming trend since 1850 with higher rates of warming 1935 to 2017, but with a pause around 1940&ndash;1970. The warming is consistent with reduced volcanic activity and increase of greenhouse gases. We anticipate that our new reconstruction of annual mean temperature could be an important contribution for future climate change studies and assessments of climate models

Ecological Dynamics and Regeneration Expansion of Treeline Ecotones in Response to Climate Change in Northern Bhutan Himalayas

The alpine treeline ecotones are an early indicator of vegetation’s response to changes in climate, and the advancement of diffuse treeline ecotones has been associated with mean annual warming temperatures. However, the knowledge of how tree demographic size, age and population distribution, and regeneration decrease with increasing elevation and mean annual temperature remain fragmentary in Bhutan. There was no explanation of how treelines migrate in response to the climate. Therefore, the objectives of this study were to investigate tree demographic size and age and population distribution, as well as the regeneration expansion of treeline ecotones of Abies densa trees in response to climate change. Demographic data from thirty transect bands from treeline ecotones and reconstructed mean annual temperatures from tree-rings were used. Regression analysis was used to establish a relationship between elevation/temperature and demographic tree size and age, as well as to determine recruitment frequency distributions and whether these could be driven by climate change. The tree demography indicated that the treeline ecotone in our sampling site is temperature limited. Hence, cooler temperatures at higher elevations should drive decreases in basal diameter, age and recruitment frequencies. From the dendroecological analysis, the diffuse treeline ecotones appear to be climbing on average 1.00 m per year in Northern Bhutan. We also found that the recruitment frequency has increased over recent years (1850–2017), as temperatures continue to rise. The thermal treeline ecotones will be likely to serve as a line of bioclimatic reference against which other zones of bioclimate can be defined. With documented responses of treeline ecotones toward mean annual temperatures, the expectation is that additional warming will continue to influence regeneration expansion in the future. This dynamic response of treeline ecotones towards the climate acts as an indicator of climate change. Information about climbing treelines and altered ecotones should be a vital part of the material for decision makers to consider, to assess impacts and threats to Himalayan alpine biota

Multi-species test and calibration of an improved transient thermal dissipation system of sap flow measurement with a single probe

International audienceTransient thermal dissipation (TTD) systems provide a simple way to measure xylem sap flow with dual or single Granier-type probe, which gives lower energy consumption and higher accuracy due to its lessor sensitivity to thermal interferences. A new system, TTD heat within 5 min (TTD5), proposed on the reduction of the heating duration. This evolution captured interest through decreased energy consumption and increased temporal resolution. Within our study, the first objective was to test and calibrate this new system with a single probe for young rubber tree - Hevea brasiliensis. The second objective was to explore the sources of variability in calibration such as species, individual cut-stems and probe-wood contact. The complementary species consisted of two diffuse-porous species (mango tree - Mangifera indica, eucalyptus tree - Eucalyptus camaldulensis) and one ring-porous species (teak tree - Tectonia grandis). Twenty-eight response curves were assessed over a large range of flux densities from 0.5 to 10 l dm-2 h-1. The incremental rise of temperature from 30 to 300 s (T300-30) after commencement of heating was sensitive to flux density over the complete range. Compared with the full signal at 300 s, the incremental signal markedly reduced the variability between response curves within species and between species. Moreover, a new index K2, defined as (T0 - Tu)/T0, normalized the responses between 0 and 1. However, the responses had a non-linear trend above 5 l dm-2 h-1. Within diffuse-porous wood type, the species did not differ in calibration, whereas the ring-porous species was markedly different. A sigmoid function provided the best fit for the diffuse-porous species. Individual stems were identified as the main source of within-species variability in calibration. The normalizing K2 index removed the influence of probe-wood contacts, controlled through drilling difference; however, there was still an effect of individual stems interacting with flux density (P = 0.019). Replications of cut-stems and response curves are necessary to assess a reliable averaged calibration. In conclusion, the applicability of the TTD5 system with a single probe has been confirmed and several sources of variability in calibration have been evaluated

Effect of trunk locations on micro-change of trunk girth in mature rubber trees (Hevea brasiliensis)

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