Afforestation is an important approach to mitigate global warming. Its complex interactions with the climate system, however, makes it controversial. Afforestation is expected to be effective in the tropics where biogeochemical and biogeophysical effects act in concert; however, its potential in the large semi-arid regions remains insufficiently explored. Here, we use a Global Climate Model to provide a process-based demonstration that implementing measured characteristics of a successful semi-arid afforestation system (2000 ha, ~300 mm mean annual precipitation) over large areas (~200 million ha) of similar precipitation levels in the Sahel and North Australia leads to the weakening and shifting of regional low-level jets, enhancing moisture penetration and precipitation (+0.8 ± 0.1 mm d −1 over the Sahel and +0.4 ± 0.1 mm d −1 over North Australia), influencing areas larger than the original afforestation. These effects are associated with increasing root depth and surface roughness and with decreasing albedo. This results in enhanced evapotranspiration, surface cooling and the modification of the latitudinal temperature gradient. It is estimated that the carbon sequestration potential of such large-scale semi-arid afforestation can be on the order of ~10% of the global carbon sink of the land biosphere and would overwhelm any biogeophysical warming effects within ~6 years.Afforestation is considered as climate change mitigation strategy 1-3 , but it is also associated with potential climate feedbacks 2,4 and rarely considers the importance of semi-arid regions 5 . Attempts to explain the records that indicate the "greening of the Sahara" 6 to 9 thousand years ago 6 , showed that the "top-down" effects of changes in the earth-system that ultimately result in land cover changes, such as changes in the sea surface temperature (SSTs) of the Atlantic, Indian and Pacific Ocean basins, can be associated with changes in land-ocean circulation and teleconnection that influence moisture transport and local precipitation over semi-arid monsoon regions such as the Sahel 7,8 . The resulting changes in land cover can consequently lead to changes in surface temperature gradients and, in turn, to changes in the characteristics of local atmospheric circulation, such as the African easterly jet (AEJ) and the intensity of the monsoon westerly (MW) winds in regions such as the Sahel 9-11 . Similar processes to those described for the Sahel were also indicated for the Asian monsoon in Northern Australia 12,13 , including the development of the Australian Easterly Jet (AUSEJ).In contrast to the "top-down" effects noted above, locally driven, "bottom-up" climatic effects triggered by first changing the land-cover in semi-arid regions, such as desertification in the Sahel, were demonstrated by the pioneering work of Charney 14 , Ornstein 11 and others 15,16 . Inverse feedbacks also operated on re-forestation in such regions (e.g. 11,[17][18][19] ). While these pioneering studies dealt with some aspects of land cover and climate in t...

-Nation wide estimates of the changes in forest biomass are needed for the greenhouse gas (GHG) reporting under the Climate Convention. The bases for national GHG reporting concerning forest sector are the national forest inventory (NFI) programmes. Since these programmes were mostly established for monitoring of timber resources, one of the current challenges for the NFIs is the development of methodology, such as biomass expansion factors (BEFs). The methodology for carbon stock change estimation should be transparent and verifiable, but this demand is not currently met due to the fact that the source data and uncertainty in the applied BEFs are not known. Here we developed BEFs with uncertainty estimation applicable to stand wise inventory of Norway spruce forests in the Czech Republic. BEFs were constructed, based on tree wise data from permanent research plots, by applying biomass and volume models to tree-level data. These BEFs were age-dependent and their uncertainty was sensitive to the dependencies among errors. Most of the uncertainty in the BEFs was due to uncertainty in the biomass and volume models applied.biomass expansion factor / Monte Carlo simulation / greenhouse gas inventory / national forest inventory Résumé -Incertitudes pour l'estimation des facteurs d'expansion de la biomasse chez l'épicéa en République tchèque. Les estimations de la biomasse des forêts servent à évaluer les effets des changements climatiques et à dresser des rapports internationaux. En foresterie, les rapports nationaux sur les gaz à effet de serre (GES) sont basés sur l'inventaire forestier national. L'objectif premier de cet inventaire étant la prévision des ressources de bois brut, l'inventaire des GES appelle des méthodes de prévision basée sur les facteurs d'expansion de la biomasse. Les méthodes d'estimation des GES devraient être transparentes et vérifiables -objectif souvent non atteint parce que l'origine et l'incertitude des facteurs d'expansion de la biomasse ne sont pas connues. Dans cette étude nous avons développé des facteurs d'expansion de la biomasse des forêts de sapin dans la République tchèque et calculé l'incertitude de ceux-ci. Les facteurs ont été estimés en fonction des mesures au niveau de l'arbre dans des zones d'échantillonnage permanentes et de l'application des modèles de volume et de biomasse. Ces facteurs d'expansion étaient dépendants de l'âge et leur incertitude sensible aux corrélations entre les erreurs. L'incertitude des facteurs d'expansion était principalement due aux modèles de biomasse et de volume appliqués. facteur d'expansion de la biomasse / simulation Monte Carlo / inventaire des gaz à effet de serre / inventaire forestier national

Drought-related tree mortality is increasing globally, but the sequence of events leading to it remains poorly understood. To identify this sequence, we used a 2016 tree mortality event in a semi-arid pine forest where dendrometry and sap flow measurements were carried out in 31 trees, of which seven died. A comparative analysis revealed three stages leading to mortality. First, a decrease in tree diameter in all dying trees, but not in the surviving trees, 8 months "prior to the visual signs of mortality" (PVSM; e.g., near complete canopy browning). Second, a decay to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting the loss of stem radial water flow in the dying trees, 6 months PVSM. Third, cessation of stem sap flow 3 months PVSM. Eventual mortality could therefore be detected long before visual signs were observed, and the three stages identified here demonstrated the differential effects of drought on stem growth, water storage capacity and soil water uptake. The results indicated that breakdown of stem radial water flow and phloem function is a critical element in defining the "point of no return" in the sequence of events leading to mortality of mature trees.

Global warming and drying trends, as well as the increase in frequency and intensity of droughts, may have unprecedented impacts on various forest ecosystems. We assessed the role of internal water storage (WS) in drought resistance of mature pine trees in the semi-arid Yatir forest. Transpiration (T), soil moisture, and sap flow (SF) were measured continuously, accompanied by periodical measurements of leaf and branch water potential (Ψleaf) and water content (WC). The data were used to parameterize a tree hydraulics model to examine the impact of WS capacitance on the tree water-relations. The results of the continuous measurements showed a 5-hour time lag between T and SF in the dry season, which peaked in the early morning and early afternoon, respectively. A good fit between model results and observations was only obtained when the empirically estimated WS capacitance was included in the model. Without WS during the dry season, Ψleaf would drop below a threshold known to cause hydraulic failure and cessation of gas exchange in the studied tree species. Our results indicate that tree WS capacitance is a key drought resistance trait that could enhance tree survival in a drying climate contributing up to 45% of the total daily transpiration during the dry season.

Drought-related tree mortality is increasing globally, but the sequence of events leading to it remains poorly understood. To identify such sequence, we used a 2016 tree mortality event in the semi-arid pine forest of Yatir were dendrometry and sap flow measurements were carried out in 31 trees, of which seven died. A comparative analysis revealed three stages leading to mortality. First, a decrease in tree diameter in all dying trees, but not in the living ones, eight months 'prior to the visual signs of mortality' (PVSM; e.g., brown needles). Second, a decay to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting the loss of stem radial water flow in the dying trees, six months PVSM. Third, cessation of stem sap flow three months PVSM. Eventual mortality could therefore be detected long before visual signs are observed, and the three stages identified here demonstrated the differential effects of drought on stem growth, water storage capabilities, and soil water uptake. The results indicated that breakdown of radial stem water flow and phloem functionality is a critical element in defining the 'point of no return' in the sequence of events leading to mortality of mature trees.