Assessment of N uptake by tree leaves via liquid and gas phase N-15 pulse labelling

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Water relations and drought tolerance of young African tamarind (**Tamarindus indica** L.) trees

AbstractTamarindus indica L. is an important multipurpose tree, indigenous to Africa, now introduced worldwide and known for its drought tolerance. The effects of drought on tamarinds, especially at seedling stage, are hardly investigated. However, this information is important for its conservation and domestication. In a growth chamber experiment we investigated the water relations of African tamarind seedlings under short-term soil drought stress. Initially tamarind seedlings can be considered as drought-tolerant at the expense of internal water storage reserves as they keep on transpiring (sap flow) and growing (diameter fluctuations). They finally spent 20% of their stem and root water storage reserves and experienced stem water potentials near −3MPa. Therefore, they can be classified as anisohydric. Their risk-taking behavior led to a high rate of seedling mortality (50%) because of whole plant hydraulic failure. They were not hydraulically efficient and they possessed low water storage capacity in stem and root (45%) due to high tissue density. When re-irrigated, remaining seedlings recovered slowly as a consequence of non-stomatal limitations and partial shoot dieback. Although tamarind seedlings show traits related to drought tolerance, we suggest that the species contains some water saving mechanisms. Contrasts with the co-occurring water-conserving tree species baobab (Adansonia digitata L.) are also discussed

Quantifying aboveground N uptake from wet deposition: effect of tree species, N load and leaf phenology

Semi-natural ecosystems such as forests are exposed to higher inputs of atmospheric pollutants originating from anthropogenic sources. This increased deposition has caused changes in biogeochemical processes, which may have adverse effects on forest structure and functioning. Therefore, an accurate quantification of total atmospheric deposition is required. One widely known technique is the throughfall method, in which the amount and quality of water above and beneath the canopy are compared. Total atmospheric deposition is then quantified by a so-called canopy budget model (Draaijers and Erisman, 1995), which estimates ion-exchange reactions between the canopy and throughfall water. Although this model is applied worldwide, it has rarely been validated (Staelens et al., 2008), and dry deposition and canopy exchange of nitrogen (N) generally cannot be quantified accurately from throughfall measurements. Therefore, this study focused on the assessment of aboveground N uptake from wet deposition, which can be quantified directly by the application of 15N labelled sources. A better understanding of the direct aboveground N uptake by different tree species under varying N pollution levels may lead to an improved canopy budget model