Parasitic plants are known for high transpiration rates and low water use efficiency (WUE). Because water availability has a large bearing on community structure and function, understanding the drivers and impacts of their water use is of great interest. The N-parasitism hypothesis posits that N limitation drives high transpiration rates. Xylem-tapping parasites acquire dilute N from the host xylem stream. The low concentration then requires profligate transpiration. Research on this hypothesis has focused on stem parasites and yielded mixed results. Root hemi-parasites are common and also typically have high transpiration rates. I used N-fixing hosts to investigate the N-parasitism hypothesis in root hemi-parasites. I also looked at consequences to the host, surrounding plants and soil moisture, focusing on the following:
- How does parasitism alter N-status in a N-fixing host and a parasite?
- How does parasitism alter WUE in a N-fixing host and a parasite?
- How does the combination of a parasite and a N-fixing host affect soil moisture?
I established 120 1x1m plots within Sagehen Reserve, CA. 50% included C. applegatei (a root hemi-parasite). I surveyed each plot noting in particular the presence/absence of C. applegatei and C. prostratus (a N-fixing shrub), and measured volumetric water content in the soil. In a subset of plots I collected leaf samples of C. applegatei, C. prostratus, and two non N-fixing possible hosts. Leaf samples were processed for δ13C, δ15N and C/N.
Results/Conclusions
Availability of N-fixing hosts corresponded with a significant increase in leaf %N in parasites. Availability of N-fixing hosts also corresponded with a distinct δ15N signature in parasites, which was closer to the N-fixer’s δ15N signature, further suggesting that the increased in parasite leaf N is derived in part from the N-fixing host. Availability of a N-fixing host corresponded with an increase in WUE (signified by δ13C) in the parasite. In addition, a higher leaf N in individual parasites corresponded to higher WUE. The presence of the parasite was associated with a significant decrease in WUE in N-fixing hosts. Parasite presence, however, had no effect on the other two non N-fixing host species sampled. The presence of parasites and N-fixing hosts, either alone or in combination, had no effect on soil moisture within each plot. These results suggest that transpiration decreases with higher N availability, broadly supporting the N-parasitism hypothesis and indicating that host type affects parasite’s physiology. I did not find evidence for community-level effects of this host-parasite interaction.