A document by Fabian Bernhard. Click on the document to view its contents.

Pine mistletoe is a hemiparasitic shrub that can produce its own photosynthates. There is a lack of knowledge about the interaction of mistletoe and host under varying environmental condition that might influence carbon gain and allocation. In a 13C-pulse labeling experiment with mature Pinus sylvestris (pine) infected by mistletoes grown in naturally dry or irrigated conditions, (1) mistletoe clusters were shielded from 13CO 2 added , and (2) mistletoes or host needles were removed to manipulate the local assimilate and water availability. No 13C signal was found in shielded mistletoes, indicating no carbon transfer from the host to the mistletoe. When the pine needles were removed from girdled branches, no 13C signal was found in the host tissues, implying no carbon transfer from mistletoe to the host. However, mistletoes on needle-removed pine trees accumulated more labelled assimilates and had higher non-structural carbohydrate (NSC) concentrations only under naturally dry conditions but not in irrigated plots. Our results suggest that mistletoes show full carbon autonomy, as they neither receive carbon from nor provide carbon resource to the host trees. Moreover, the high assimilation capacity of mistletoes seems to be constrained by the host water use under dry conditions, suggesting that drought stress is not only negatively impacting trees but also mistletoes. Therefore, we conclude that the hemiparasites live on their own in terms of carbon gain which, however, depends on the water provided by the host tree.

Current increases in not only the intensity and frequency but also the duration of drought events could affect the growth, physiology, and mortality of trees. We experimentally studied the effects of drought duration in combination with fertilization on leaf water potential, gas exchange, growth, tissue levels of non-structural carbohydrates (NSCs), tissue NSC consumption over winter, and recovery after drought release in oak (Quercus petraea) and beech (Fagus sylvatica) saplings. Long drought duration (> 1 month) decreased leaf water potential, photosynthesis, and NSC concentrations in both oak and beech saplings. Nitrogen fertilization did not mitigate the negative drought effects on both species. The photosynthesis and relative height increment recovered in the following rewetting year. Height growth in the rewetting year was significantly positively correlated with both pre- and post-winter root NSC levels. Root carbon reserve is critical for tree growth and survival under long-lasting drought. Our results indicate that beech is more sensitive to drought and fertilization than oak. The present study, in a physiological perspective, experimentally confirmed the view of Ellenberg (2009) that the European beech, compared to oak, may be more strongly affected by future environmental changes.

The oxygen and hydrogen isotopic composition (δ 18O, δ 2H) of plant tissues integrates hydrological, physiological, and metabolic functions differently, which may allow disentangling reasons of tree mortality. To test this, we performed a greenhouse study and determined predisposing fertilization and lethal drought effects on δ 18O and δ 2H values of plant water and organic matter (OM) in leaf and woody tissues of living and dead saplings of five European tree species. Additionally, we measured physiological and metabolic traits. Compared to controls, drought reduced leaf gas-exchange, predawn water potential, and stem starch concentrations and increased δ 18O and δ 2H values of leaf and twig water in all tested species. These drought-induced changes generally caused an 2H-enrichment in leaf and tree-ring OM, but a low and heterogenous δ 18O response. δ 2H values of tree-ring OM were correlated with those of leaf and twig water across treatments and species. In contrast, the predisposing fertilization had generally no significant effect on any isotopic, physiological, and metabolic traits. We propose that the 2H-enrichment in the dying trees is related to (i) the plant water isotopic composition, (ii) metabolic processes shaping leaf non-structural carbohydrates , (iii) the use of carbon reserves for growth, and (iv) species-specific physiological adjustments. This stress imprint on δ 2H but not on δ 18O suggests that the further could be used as a proxy to understand mechanisms of drought-induced tree mortality.