Effect of honey mesquite size and distribution on desert mistletoe parasitism Angelique Leonard1, Monica Rivas2, Thomas Savoie3, Olivia Zanzonico4 1University of California, Merced; 2University of California, Los Angeles; 3University of California, Santa Cruz; 4University of California, Davis Parasitic plants must rely on their hosts to survive. However, the distribution and survival of parasitic plants often depends on other organisms in addition to the host. Desert mistletoe (Phoradendron californicum) has a unique interdependent relationship with its preferred host, honey mesquite (Prosopis glandulosa), and its primary disperser, Phainopepla (Phainopepla nitens). We sought to study this relationship by testing the effects of honey mesquite size and distribution on the likelihood of infection by mistletoe. At four different sites in and around Anza Borrego State Park, we measured the size of honey mesquite thickets and distance between thickets. We found that there was no effect of distance between host and neighbors on the presence of mistletoe. However, the host height, volume of neighboring mesquite thickets, and height of neighboring thickets had a positive relationship with mistletoe presence. In addition, there was a noticeable effect of sampling site on the presence of mistletoe. The two sites near a source of water had a higher percentage of infected hosts than the two without a water source. Implications of our findings may include conservation of Phainopepla in the future by protecting desert environments where honey mesquite is found by water sources, thereby increasing presence of mistletoe. Keywords: Phainopepla nitens, Phoradendron californicum, Prosopis glandulosa, plant parasite, spatial distributions INTRODUCTION 1992). Plant parasites have negative effects on their hosts, ranging from a decrease in Host-parasite interactions can give way to size, limited water, reduced nutrients, and an evolutionary arms race that establishes a even death but often depend on biotic or complex network of unique vectors, hosts, abiotic factors in order to spread (Yule and and parasite dispersal strategies. Bronstein 2018, Candia et al. 2014, Understanding the spatial patterns that Mathiasen et al. 2008). lead to the spread of a parasite can provide Mistletoes (Viscaceae) are a highly diverse insight into the dynamics which drive these group of aerial hemi-parasites that rely on interdependent relationships (Kareiva animal vectors to pollinate and disperse 1994). Patterns of the spread of a disease them (Aukema 2003). Hemi-parasites are are dependent on the spatial distribution of capable of photosynthesis but need a host both host and parasite, as well as vector for water and nutrients (Calder 1983). prevalence and preference (Real et al. Desert mistletoe (Phoradendron californicum) CEC Research | https://doi.org/10.21973/N39M1P Fall 2019 1/7 depends upon pollinators, mainly insects Phainopepla can more easily see their and birds, to exchange genetic material, territorial displays, therefore leading to and dispersers which determine the higher rates of mistletoe infection on taller location of the establishment of offspring mesquite (Aukema 2004). However, little is on hosts (Yule and Bronstein 2018, known about how other spatial factors of Mathiasen et al. 2008). In the desert, mesquite plants may contribute to the mistletoe maintains a mutualistic distribution of desert mistletoe. relationship with its vectors, as it is one of In this study, we investigated the effects the only available resources during its fall- of honey mesquite size and density on winter fruiting period (Aukema and desert mistletoe presence across the Martinez 2002). Perhaps its most landscape. We hypothesized that honey specialized relationship is with the mesquite thickets that were both larger and Phainopepla (Phainopepla nitens). growing closer together would be more Phainopeplas are drawn to the Sonoran likely to be infected by mistletoe, due to Desert in the winter to breed and gorge Phainopepla preference to exert less energy themselves on mistletoe berries (Aukema flying between sparsely growing hosts. We and Martinez 2002). A sticky viscin coats the also predicted that infection rates would be berries, allowing them to adhere to higher in larger thickets due to increased branches after being deposited by area available for mistletoe infection. defecation, regurgitation, or bill wiping (Reid et al. 1995). Phainopepla have a METHODS digestive system specialized for mistletoe berries, and in the desert will defend 2.1 Site Description territories of host plants infected with Sampling for this study was conducted at mistletoe (Aukema 2004). four sites in and around Anza Borrego State Desert mistletoe has several hosts in the Park in early November 2019. Anza Borrego Fabaceae family in the Sonoran Desert, but State Park is in the Sonoran Desert in is most successful on mesquite, receiving southeastern California, at the eastern edge more pollen, growing larger, and with more of the Colorado Desert. Field samples were berries than on trees such as acacia (Yule taken at four sites adjacent to San Felipe and Bronstein 2018). Yet mistletoe, in Road (San Felipe), Borrego Springs Water contrast, can have a number of detrimental Treatment Ponds (Water Treatment Ponds), effects on mesquite, killing branches, Yaqui Wells Road (Yaqui Wells), and the reducing growth and fecundity, and Great Southern Overland Stage Route sometimes killing the host (Hawksworth (Stage Route). Two of the sites, Yaqui Wells 1983). Honey mesquite (Prosopis and Water Treatment Plant, were located glandulosa) is a prime host for mistletoe near a water source, a small seep and water due to its sturdy branches and deep root treatment settling ponds, respectively. system which optimizes water uptake in the Vegetation varied slightly among sites but dry desert (Aukema and Martinez 2002). was primarily dominated by honey Previous study has shown that mesquite, catclaw acacia (Senegalia Phainopeplas prefer to defend territories on greggii), and creosote bush (Larrea taller mesquite trees because other CEC Research | https://doi.org/10.21973/N39M1P Fall 2019 2/7 tridentata). Desert willow (Chilopsis Longest side and height measurements linearis), alternatively, was only found at were also recorded for the two nearest Yaqui Wells. Sampling was performed neighbors. Neighbors were only recorded if approximately 1500 meters south of Hwy they were also honey mesquite. 78 on the Great Southern Overland Stage Route and along a 1500-meter stretch of 2.3 Bird Count San Felipe Road located 428 meters north of Hwy 78. Twelve host thickets and their Estimates for the population of two closest neighbors were sampled at Phainopepla nitens at each site were each site. However, some host thickets did performed using a point count. At each of not have a neighbor within 50 m and were the four sites, point counts were done two considered isolated. As a result, a total of hours after sunrise for a five-minute period. 48 host thickets and 83 neighbors were All Phainopepla that were seen or heard sampled overall. were included in the count. 2.2 Mesquite Selection and Measurement 2.4 Statistical Tests For each sample, a host patch of honey Statistical tests were performed using mesquite was randomly selected at a JMP 14 software. We utilized a multiple minimum distance of 50 m from other host logistic regression to test the effect of site, patches. Patch height, length of longest host height, average neighbor height, size, and mistletoe presence were average neighbor volume, and average measured. Host honey mesquite plants neighbor distance on the presence of were both individuals and patches of many mistletoe on the host plant. Average individuals. The honey mesquite sampled nearest neighbor data was log transformed was categorized as a single patch so long as to make it normally distributed. We the individuals were no further than one executed a multiple linear regression to test meter apart. Mistletoe infection was the effect of the same factors on mistletoe categorically recorded as either present or density on a host plant. The linear regression absent for each host patch. Mistletoe plants excluded samples with no mistletoe present. could easily be seen and identified on the RESULTS host thicket. Mistletoe was not recorded for neighboring plants. 3.1 Site Dependent Effects The longest side was measured using a rangefinder and used as a proxy for area We found a significant difference in covered by the patch, rather than mistletoe presence among our four sample measuring both sides. When the longest sites (Figure 1, Table 1). The Water Treatment side was not measurable using the Ponds had the highest proportion of host rangefinder due to dense vegetation, we patches infected by mistletoe (83%), used GPS markers to find distance on followed by Yaqui Wells (67%), San Felipe Google Earth. Height measurements of the (42%) and Stage Route (8%). Among host thicket were estimated using a two-meter mesquite hosts with mistletoe presence, site stick and rounded to the nearest half meter. had no effect on mistletoe density (Table 2). CEC Research | https://doi.org/10.21973/N39M1P Fall 2019 3/7 3.2 Effects of Honey Mesquite Spatial Distribution Average neighbor distance had no effect on mistletoe presence (Table 1). However, all other spatial factors we tested influenced the probability that a host mesquite thicket would be infected (Table 1). Hosts that were taller were more likely to have mistletoe present. Hosts