As climate change progresses, riparian habitats of Arizona are rapidly changing and some may convert from perennial to ephemeral flows. Understanding interactions between riparian foundation tree species and their hemiparasites could show how stressors affect the ecophysiology of these species, with implications for ecosystem processes and riparian management. We examined the relationship between tree condition (branch dieback/leaf area/bark quality), site stress (water limitation), and mistletoe (Phoradendron macrophyllum) abundance. Using paired ephemeral and perennial streams, we replicated a decade old survey of mistletoe/host dynamics, while additionally measuring water potentials of host tree species across sites to assess water status. Coupled with these measurements, we quantified the ratio of living to shed mistletoe on four different riparian tree species: Arizona Alder (Alnus oblongifolia), Arizona sycamore (Platanus wrightii), Fremont cottonwood (Populus fremontii), and velvet ash (Fraxinus velutina). We focused on three main hypotheses: 1) annual climate influences mistletoe abundance, 2) greater host-tree water stress is associated with a decrease in the number of live mistletoe, and 3) tree physiological water-use strategies and hydraulic function will influence mistletoe shedding.
Results/Conclusions
Our results revealed three patterns relevant to our hypotheses: 1) mistletoe abundance increased following a high rainfall year compared to a dry year occurring a decade ago (2006), 2) site-level water stress is correlated with more negative water potentials across host trees, resulting in lower mistletoe abundance, and 3) mistletoe occurrence varies among tree species and in relation to tree physiological condition. Mistletoe abundance for the current year was on average 3-times greater across study species compared to the 2006 survey. Significantly more negative midday water potentials at our ephemeral site (W = 105.5, p <0.01) explained the decrease in mistletoe occurrence relative to the perennial site. We also showed that certain host species, such as sycamore, have a great potential to shed mistletoe compared to other species, as they hosted only 10% of the average mistletoe occurrence/per tree and shed >30% more. These results indicate that repeat measurements of mistletoe abundance, as well as the living to shed mistletoe ratio, could be used for assessing riparian health and water availability. Thus, mistletoe (presence and abundance) may serve as a useful indicator species for foundation tree health, thus informing forest and wetland management strategies.