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Fertilization is commonly used to increase growth in forest plantations, but it may also affect tree water relations and responses to drought. Here, we measured changes in biomass, transpiration, sapwood-to-leaf area ratio (A s :A l ) and sap flow driving force (ΔΨ ) during the 6-year rotation of tropical plantations of Eucalyptus grandis under controlled conditions for throughfall and potassium (K) fertilization. K fertilization increased final tree height by 8 m. Throughfall exclusion scarcely affected tree functioning because of deep soil water uptake. Tree growth increased in Ksupplied plots and remained stable in K-depleted plots as tree height increased, while growth per unit leaf area increased in all plots. Stand transpiration and hydraulic conductance standardized per leaf area increased with height in K-depleted plots, but remained stable or decreased in K-supplied plots. Greater A l in K-supplied plots increased the hydraulic constraints on water use. This involved a direct mechanism through halved A s :A l in K-supplied plots relative to K-depleted plots, and an indirect mechanism through deteriorated water status in K-supplied plots, which prevented the increase in ΔΨ with tree height. K fertilization in tropical plantations reduces the hydraulic compensation to growth, which could increase the risk of drought-induced dieback under climate change.

<p>Potassium (K) is essential for a wide range of physiological functions in plants, and a limiting element for wood productivity in numerous forest ecosystems. However, the contribution of each of the K-sensitive physiological processes to the limitation of wood productivity is poorly known. In trees, K deficiency acts both on the source and the sinks of carbon making it difficult to disentangle its effects on wood productivity. The literature dealing with the influence of K-limitation on tree physiologywhile disparate, shows some converging results. Furthermore, K-limited tropical <em>Eucalyptus</em> plantations have been studied extensively over the last 2 decades. Large scale fertilization experiments, run over multiple rotations, allow us to gain insight into the ecosystem’s K-cycle as a whole and the physiological processes that are impacted the most by K deficiency. Mechanistic modeling of this system should allow us to quantify the relative contribution of each process when it comes to wood productivity limitation by K. We have thus adapted an eco-physiological model (CASTANEA-CNP), previously used in temperate forest settings, to use in tropical eucalypt plantations. This has led us to adapt existing nutrient (N and P) eco-physiological modeling frameworks specifically for K as well as focus on processes that are little impacted by N and P availability but greatly by K availability. The biological K-cycle model was calibrated using the comprehensive experimental data. Carbon and water fluxes were calibrated using data from a flux tower site (Eucflux) with the same environmental conditions as the experimental plots. The development of a new canopy generation model was mandated by both the continuous nature of leaf generation in <em>Eucalyptus grandis </em>and the major interaction between leaf ontogeny and the K-cycle. At first we focus mainly on carbon assimilation at the canopy level. Here we present the preliminary results obtained by this model.</p>

Climate change is imposing drier atmospheric and edaphic conditions on temperate forests. Here, we investigated how subsoil (down to 300 cm) water extraction contributed to the provision of water in the Fontainebleau-Barbeau temperate oak forest over two years, including the 2018 record drought. Deep water provision was key to sustain canopy transpiration during drought, with layers below 150 cm contributing up to 60% of the transpired water in August 2018, despite their very low density of fine roots. We further showed that soil databases used to parameterize ecosystem models clearly underestimated the amount of water extractable from the soil by trees. The consensus database established for France gave an estimate of 207 mm for the soil water holding capacity (SWHC) at our study site, when our estimate based on the analysis of soil water content measurements was 1.9 times higher, reaching 390+/-17 mm. Running the CASTANEA forest model with the database-derived SWHC yielded a 350 gC m-2 y-1 average underestimation of annual gross primary productivity under Global Change Biology For Review Only current climate, reaching up to 700 gC m-2 y-1 under climate change scenario RCP8.5. Correctly estimating SWHC is a challenge for accurate simulations of the carbon cycle in a changing climate and we showed that subsoil SWC measurements are needed for this.

Abstract. The extent of the potassium (K) limitation of forest productivity is probably more widespread than previously thought, and K limitation could influence the response of forests to future global changes. To understand the effects of K limitation on forest primary production, we have developed the first ecophysiological model simulating the K cycle and its interactions with the carbon (C) and water cycles. We focused on the limitation of the gross primary productivity (GPP) by K availability in tropical eucalypt plantations in Brazil. We used results from stand-scale fertilisation experiments as well as C flux measurements in two tropical eucalypt plantations to parameterise the model. The model was parameterised for fertilised conditions and then used to test for the effects of contrasting additions of K fertiliser. Simulations showed that K deficiency limits GPP by more than 50 % during a 6-year rotation, a value in agreement with estimations in K-limited eucalypt stands. Simulations showed a decrease of modelled canopy transpiration of around 50 % and a decrease in modelled water-use efficiency WUEGPP of 10 %. Through a sensitivity analysis, we used the model to identify the most critical processes to consider when studying K limitation of GPP. The inputs of K to the stands, such as the atmospheric deposition and weathering fluxes, and the regulation of the cycle of K within the ecosystem were critical for the response of the system to K deficiency. Litter leaching processes were of lower importance, since residence time of K in litter was low. The new forest K-cycle model developed in the present study includes multiple K processes interacting with the carbon and water cycles, and strong feedbacks on GPP were outlined. This is a first step in identifying the source or sink limitation of forest growth by K.

Atmospheric deposition K atmosphere→soil 0.5 gK.m −2 .yr −1 Measured in Laclau et al. (2010) Initial K contained in litter K ini litter 1.92 gK.m −2 Measured in Laclau et al. (2010)

Number of leaves produced by height increment κ 345 nb leaves .m −2 .m −1 tree Calibrated using leaf production on the +K Itatinga stand Leaf Lifespan LLS 400 days Calibrated using leaf production, biomass and fall measurements on the +K Itatinga stand Target leaf area Smax 2750 mm 2 Measured in scans from the +K stand