Land use change impacts on floods at the catchment scale: Challenges and opportunities for future research.

Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage, and terracing are identified. Potential strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long-term experiments on physical-chemical-biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology, and geomorphology.

Mistletoes and mutant albino shoots on woody plants as mineral nutrient traps.

BACKGROUND AND AIMS: Potassium, sulphur and zinc contents of mistletoe leaves are generally higher than in their hosts. This is attributed to the fact that chemical elements which are cycled between xylem and phloem in the process of phloem loading of sugars are trapped in the mistletoe, because these parasites do not feed their hosts. Here it is hypothesized that mutant albino shoots on otherwise green plants should behave similarly, because they lack photosynthesis and thus cannot recycle elements involved in sugar loading. METHODS: The mineral nutrition of the mistletoe Scurrula elata was compared with that of albino shoots on Citrus sinensis and Nerium oleander. The potential for selective nutrient uptake by the mistletoe was studied by comparing element contents of host leaves on infected and uninfected branches and by manipulation of the haustorium-shoot ratio in mistletoes. Phloem anatomy of albino leaves was compared with that of green leaves. KEY RESULTS: Both mistletoes and albino leaves had higher contents of potassium, sulphur and zinc than hosts or green leaves, respectively. Hypothetical discrimination of nutrient elements during the uptake by the haustorium is not supported by our data. Anatomical studies of albino leaves showed characteristics of release phloem. CONCLUSIONS: Both albino shoots and mistletoes are traps for elements normally recycled between xylem and phloem, because retranslocation of phloem mobile elements into the mother plant or the host is low or absent. It can be assumed that the lack of photosynthetic activity in albino shoots and thus of sugars needed in phloem loading is responsible for the accumulation of elements. The absence of phloem loading is reflected in phloem anatomy of these abnormal shoots. In mistletoes the evolution of a parasitic lifestyle has obviously eliminated substantial feeding of the host with photosynthates produced by the mistletoe.

Mineral nutrition and water relations of hemiparasitic mistletoes: a question of partitioning. Experiments with Loranthus europaeus on Quercus petraea and Quercus robur.

Hemiparasitic mistletoes thrive on xylem sap diverted from the host through direct xylem connections. There is no phloem link and no exchange of photosynthates. Mineral nutrition is therefore closely coupled with water consumption.Mineral nutrient partitioning between the mistletoe Loranthus europaeus and its host Quercus petraea was studied by monitoring nutrient incorporation into 3-year-old twigs of both species over 1 year. The mistletoes accumulated large amounts of potassium in comparison to other elements. It is hypothesized that this phenomenon is a consequence of the absence of a phloem connection between host and hemiparasite. In the host potassium is cycled between leaves and sites of photosynthate utilization in the wake of photosynthate transport in the phloem. In the hemiparasite it cannot cycle beyond the host-parasite interface and potassium imported with the xylem sap is thus trapped in the hemiparasite phytomass.The control of water partitioning in a host-hemiparasite system is another important aspect. As host and hemiparasite receive water from a common supply the otherwise independent stomatal control systems of both species become linked when transpiration demand exceeds supply. By measuring diurnal curves of transpiration and leaf water status in both species, by use of a special cuff-freezing technique to study stomatal response when water supply is cut off, and by measurements of leaf water capacitance with a pressure-volume curve technique, the fundamental data for a descriptive model of water partitioning in a L. europaeus - Q. robur system were collected. This model may be used to gain deeper insight into stomatal control of plant water status.