The photosynthetic capacity of pine mistletoes is suppressed by the host trees under drier conditions
Although we did not find a significant effect of the irrigation treatment on mistletoe 13C uptake and incorporation in the wrapping experiment (Fig. 3), mistletoe leaves and shoots accumulated more13C-labeled assimilates in the control trees than in the irrigated trees, but only when the needles were removed from the girdled branches (Table 2, Fig. 4). This is surprising, as photosynthetic carbon acquisition is normally greater in an environment with higher soil moisture (Joseph et al., 2020; Reich et al., 2018; Wang, Hawkins, & Letchford, 1998). Since we have shown in Exp. 1 that no direct carbon transport occurred from the host to the mistletoes, this phenomenon needs to be attributed to the mistletoe photosynthetic activity. We need to consider that mistletoe leaves were larger and had a higher water content under the dry control conditions (Fig. 2). This might indicate a higher photosynthetic potential of mistletoe leaves in drier sites, which is, however, only expressed when the host’s needles are removed. These results further signify that needle removal not only affected source–sink carbon relationships in the girdled branch but might also result in more available water for the mistletoe due to discontinued host transpiration. The reduced competition for water may allow the mistletoes to keep stomates more open, thus allowing for higher photosynthesis rates.
We also found that mistletoe leaf NSCT concentrations were significantly higher in the control trees compared with the irrigated trees when pine needles were not present anymore (Table 2, Fig. 5), which corroborates our assumption of higher assimilation by mistletoes when there were no needles to demand water under dry conditions. The high carbon accumulation capacity of mistletoes in a stressful environment also demonstrates the competitive ability of the hemiparasite. However, it seemed that mistletoes with bigger leaf size and higher assimilation potential benefit more from pine needle removal than the pine needles benefit from mistletoe removal (see discussion below). Still, mistletoes are known to exacerbate tree mortality in drought-exposed regions (M Dobbertin & Rigling, 2006; Durand-Gillmann, Cailleret, Boivin, Nageleisen, & Davi, 2014; Rigling et al., 2010; Sangüesa-Barreda et al., 2013; Zweifel et al., 2012). Our findings support our hypothesis H2b that expected an increased carbon level in mistletoes due to increased carbon assimilation by the hemiparasite itself as a result of decreased competition for water with the host after host needle removal. In contrast to our hypothesisH2a , an increase in carbon level and carbon assimilation of mistletoes after host needle removal was found, which is not related to a change in source activity of the host but is rather due to the released water restrictions of the mistletoe by removing the transpiring host needles under dry conditions.