Parasitic plants are common fixtures in ecosystems. Although traditionally studied primarily for their negative impacts on their hosts, the range of interactions that parasitic plants have and their role in shaping ecosystem structure and function is increasingly recognized. Parasitic plants are defined by a unique set of ecophysiological traits. Accordingly, here I take a primarily ecophysiological approach to understanding parasitic plants and their role in ecosystems. This dissertation is largely organized from narrow to broad in terms of focal species, and explores three major topics within parasitic plant ecophysiology: N-parasitism, nighttime transpiration, and leaf traits.

In the first chapter, I focus on two species of root hemiparasites, Castilleja applegatei and Castilleja wightii. The N-parasitism hypothesis posits that N limitation drives high transpiration rates in xylem-tapping parasites. Thus, availability of N-fixing hosts may affect parasite’s WUE and in turn impact the surrounding plant community. I investigate how the availability of an N-fixing host affects the root hemiparasite, Castilleja applegatei, and examine host-mediated effects on community structure and soil moisture. I contrast this work with a removal experiment testing the impact of Castilleja wightii on a N-fixing host species. In C. applegatei availability of N-fixing hosts corresponded to a significant increase in leaf %N, a distinct δ15N signature, and an increase in WUE (signified by δ13C). The presence of parasites was associated with a significant decrease in WUE in N-fixing neighbors, but had no effect on the non-N-fixing species. The presence of parasites significantly affected soil moisture but did not impact diversity or percent cover. In contrast to the observational work on C. applegatei, I did not find strong evidence for host-parasite interactions between C. wightii and available N-fixers in the experimental removal.

In the second chapter, I look at nighttime stomatal conductance in eight species or subspecies of Castilleja. Parasitic plants are theoretically released from two of the major drivers of nighttime stomatal closure. First, instead of relying solely on photosynthesis, xylem parasites also derive dilute carbon from their host xylem, a source unaffected by darkness. Second, their access to host xylem also reduces the need to conserve water. Here I measured nighttime stomatal conductance in eight species of Castilleja, a widespread genus of hemiparasites that access host xylem via the roots, and common neighboring plants at eight sites in California. All the plants measured displayed some nighttime stomatal conductance, but on average, nighttime stomatal conductance in Castilleja was 235% higher than in non-parasites. These data demonstrate that many Castilleja commonly transpire at night, adding these root hemiparasites to the growing group of plants understood to open their stomata at night.

In the third chapter, I use a wider lens to examine leaf traits in parasitic plants across the globe. Utilizing the TRY database, I characterize the state of knowledge on leaf traits in parasitic plants and explore how parasitic plants, with their unique ecophysiology, fit into or deviate from the global leaf economic spectrum (LES). I also compile a dataset of all the known parasitic genera, which is freely available. Heterotrophy in parasitic plants undermines some of the essential functions of leaves, namely C acquisition via photosynthesis, and in theory could lead to departures from the LES. However, despite their unique physiology, parasitic plants largely adhere to the LES although they do have some tendency towards the ‘fast’ end of the spectrum, that is, towards leaves with shorter lifespans but higher short-term photosynthetic yield. Further research on the physiology of parasitic plants will improve our understanding of patterns in resource acquisition and utilization.