Alain Rocheteau

Growth duration is a better predictor of stem increment than carbon supply in a Mediterranean oak forest: implications for assessing forest productivity under climate change

Understanding whether tree growth is limited by carbon gain (source limitation) or by the direct effect of environmental factors such as water deficit or temperature (sink limitation) is crucial for improving projections of the effects of climate change on forest productivity. We studied the relationships between tree basal area (BA) variations, eddy covariance carbon fluxes, predawn water potential (Ψpd ) and temperature at different timescales using an 8-yr dataset and a rainfall exclusion experiment in a Quercus ilex Mediterranean coppice. At the daily timescale, during periods of low temperature (< 5°C) and high water deficit (< -1.1 MPa), gross primary productivity and net ecosystem productivity remained positive whereas the stem increment was nil. Thus, stem increment appeared limited by drought and temperature rather than by carbon input. Annual growth was accurately predicted by the duration of BA increment during spring (Δtt0-t1 ). The onset of growth (t0 ) was related to winter temperatures and the summer interruption of growth (t1 ) to a threshold Ψpd value of -1.1 MPa. We suggest that using environmental drivers (i.e. drought and temperature) to predict stem growth phenology can contribute to an improvement in vegetation models and may change the current projections of Mediterranean forest productivity under climate change scenarios.

Why do mixotrophic plants stay green? A comparison between green and achlorophyllous orchid individuals in situ

Some forest plants adapt to shade by mixotrophy, i.e., they obtain carbon both from photosynthesis and from their root mycorrhizal fungi. Fully achlorophyllous species using exclusively fungal carbon (the so-called mycoheterotrophic plants) have repeatedly evolved from such mixotrophic ancestors. However, adaptations for this evolutionary transition, and the reasons why it has happened a limited number of times, remain unknown. We investigated this using achlorophyllous variants (i.e., albinos) spontaneously occurring in Cephalanthera damasonium, a mixotrophic orchid. In two populations, we compared albinos with co-occurring green individuals in situ. We investigated vegetative traits, namely, shoot phenology, dormancy, CO2 and H2O leaf exchange, mycorrhizal colonization, degree of mycoheterotrophy (using 13 C abundance as a proxy), and susceptibility to pathogens and herbivores. We monitored seed production (in natural or experimental crosses) and seed germination. Albinos displayed (1) more frequent shoot drying at fruiting, possibly due to stomatal dysfunctions, (2) lower basal metabolism, (3) increased sensitivity to pathogens and herbivores, (4) higher dormancy and maladapted sprouting, and, probably due to the previous differences, (5) fewer seeds, with lower germination capacity. Over the growing season, green shoots shifted from using fungal carbon to an increasingly efficient photosynthesis at time of fruiting, when fungal colonization reached its minimum. Conversely, the lack of photosynthesis in fruiting albinos may contribute to carbon limitation, and to the above- mentioned trends. With a 10 3 3 fitness reduction, albinos failed a successful transition to mycoheterotrophy because some traits inherited from their green ancestors are maladaptive. Conversely, mycoheterotrophy requires at least degeneration of leaves and stomata, optimization of the temporal pattern of fungal colonization and shoot sprouting, and new defenses against pathogens and herbivores. Transition to mycoheterotrophy likely requires progressive, joint evolution of these traits, while a sudden loss of photosynthesis leads to unfit plants. We provide explanations for the evolutionary stability of mixotrophic nutrition and for the rarity of emergence of carbon sinks in mycorrhizal networks. More broadly, this may explain what prevents the emergence of fully heterotrophic taxa in the numerous other mixotrophic plant or algal lineages recently described.

Transient thermal dissipation method of xylem sap flow measurement: multi-species calibration and field evaluation

The transient thermal dissipation (TTD) method developed by Do and Rocheteau (2002b) is a close evolution of the original constant thermal dissipation (CTD) method of Granier (1985). The TTD method has the advantage of limiting the influence of passive natural temperature gradients and of yielding more stable zero-flux references at night. By analogy with the CTD method, the transient method was first calibrated on synthetic porous material (sawdust) on the assumption that the relationship was independent of the woody species. Here, our concern was to test the latter hypothesis with a 10-min heating time in three tropical species: Hevea brasiliensis Müll. Arg., Mangifera indica L. and Citrus maxima Merr. A complementary objective was to compare the field estimates of daily transpiration for mature rubber trees with estimates based on a simplified soil water balance in the dry season. The calibration experiments were carried out in the laboratory on cut stems using an HPFM device and gravimetric control of water flow up to 5 L dm(-2) h(-1). Nineteen response curves were assessed on fully conductive xylem, combining 11 cut stems and two probes. The field evaluation comprised five periods from November 2007 to February 2008. Estimates of daily transpiration from the measurement of sap flow were based on the 41 sensors set up on 11 trees. Soil water depletion was monitored by neutron probe and 12 access tubes to a depth of 1.8 m. The calibrations confirmed that the response of the transient thermal index to flow density was independent of the woody species that were tested. The best fit was a simple linear response (R(2) = 0.88, n = 276 and P < 0.0001). The previous calibration performed by Do and Rocheteau (2002b) on sawdust fell within the variability of the multi-species calibration; however, there were substantial differences with the average curve at extreme flow rates. Field comparison with soil water depletion in the dry season validated to a reasonable extent the absolute estimates of transpiration acquired with the 10-min TTD method. In conclusion, evidence for the independence of calibration from woody species and the simple linear response of the thermal index strengthen the interest of the TTD method with 10-min heating.

Non-steady-state modelling of water transfer in a Mediterranean evergreen canopy

A model simulating the diurnal pattern of water transfer within a Holm oak (Quercus ilex) canopy in Mediterranean conditions has been designed. It combines a non-steady-state hydraulic model with a transpiration model. The hydraulic model includes a reservoir represented by a capacitance, a soil‐plant hydraulic resistance and a storage hydraulic resistance connected to the capacitance. It simulates the diurnal variation of water uptake and storage flow from the diurnal course of transpiration used as input. The transpiration model is based upon the Penman‐Monteith equation and a Jarvis-type representation of the stomatal resistance (i.e., a minimum stomatal resistance multiplied by the product of independent stress functions). Simultaneous measurements of canopy evaporation by an eddy covariance system and water uptake from the soil by sap flow measurements have allowed one to calibrate and validate the model. The capacitance has been found to be equal to 0.17 mm MPa 1 (with a storage hydraulic resistance of about 2 MPa h mm 1 ), generating a time lag of about 1 h between the transpiration rate and the water uptake from the soil. The hydraulic model correctly represents the experimental data. The transpiration model provides reasonable estimates, but with a significant scatter. The combined model simulates the diurnal variation of water uptake, storage flow and transpiration rate directly from environmental variables, but in this latter case, the storage flow is estimated with a rather poor accuracy.

Water loss regulation in mature Hevea brasiliensis: effects of intermittent drought in the rainy season and hydraulic regulation

Effects of soil and atmospheric drought on whole-tree transpiration (E(T)), leaf water potential (Ψ(L)) and whole-tree hydraulic conductance (K(T)) were investigated in mature rubber trees (Hevea brasiliensis, clone RRIM 600) during the full canopy stage in the rainy season in a drought-prone area of northeast Thailand. Under well-watered soil conditions, transpiration was tightly regulated in response to high evaporative demand, i.e., above reference evapotranspiration (ET(0)) ~2.2 mm day(-1) or maximum vapor pressure deficit ~1.8 kPa. When the trees experienced intermittent soil drought E(T) decreased sharply when relative extractable water in the top soil was < 0.4. The midday leaf water potential (Ψ(md)) on sunny days did not change as a function of soil drought and remained stable at approximately - 1.95 MPa, i.e., displaying isohydric behavior. The decrease in E(T) was mainly due to the change in K(T). K(T) remained constant over a wide range of environmental conditions and decreased sharply at low soil water availability. A simple hydraulic model incorporating critical minimum water potential and the response of whole-tree hydraulic conductance to relative extractable water correctly simulated patterns of transpiration over 6 months. We conclude that an explicit and simplified framework of hydraulic limitation hypothesis was sufficient to describe water use regulation of a mature rubber tree stand in water-limited conditions. Given the complexity of constraints in the soil-plant-atmosphere pathway, our results confirm the relevance of this approach to synthesize the overall behavior of trees under drought.

Transient thermal dissipation method for xylem sap flow measurement: implementation with a single probe

Comparisons of tree water relations between treatments, species and sites are facilitated by the use of simple and low-cost measurements of xylem sap flow rates. The transient thermal dissipation (TTD) method is a variant of the constant thermal dissipation (CTD) method of Granier. It has the advantages of limiting thermal interference and of saving electrical energy. Here, our concern was to test a new step towards simplicity and low cost: the applicability of the TTD method with a single probe, i.e., without a reference sensor, following a cycle of 10 min heating and 10 min cooling, and using the same thermal index and multi-species calibration previously assessed with a dual probe. First, the responses of the dual and single probes were compared in an artificial hydraulic column of sawdust in the laboratory over a complete range of flux densities, from 0.3 to 4.0 l dm⁻² h⁻¹. Second, diurnal kinetics were compared in a young tree with rapid changes in the sapwood reference temperature of up to 5 °C h⁻¹ for 5 consecutive days. With a relatively stable reference temperature, laboratory results showed that a single probe yielded the same temperature signal and thermal index as a dual probe for the full range of sap flux densities. Within the tree, the cooled temperature of the heated probe, linearly interpolated, proved to be an accurate indicator of the change in the reference temperature over time. Logically, the temperature signals and estimates of sap flux density with the single probe did not differ from the dual-sensor measurements when the cooled temperature was interpolated. Additionally, the responses of the thermal index, yielded in the hydraulic experiment with the sawdust column, fell within the variability of the multi-species calibration. This result supports the previous assessment of a non-species-specific calibration for the TTD method with diffuse porous media. In conclusion, our results showed that the TTD method can be directly applied with a single probe. Limitations and possible future progress are pointed out. This measurement system is probably the simplest technique currently available to measure xylem sap flow.