Intraspecific competition for host resources in a parasite
We studied intraspecific competition in desert mistletoe (Phoradendron californicum) co-infections in velvet mesquite (Prosopis velutina) hosts by measuring ecophysiological and elemental traits of parasites and hosts before and after performing a reciprocal removal experiment of mistletoe occupying the same branch. Phoradendron species acquire limited carbon (C) through photosynthesis, and generally maintain high heterotrophy. Host carbon availability may therefore be a limiting resource for mistletoes, resulting in resource competition, especially when individuals are found on the same host branch and in the same xylem stream. We predicted that upon removal from a host branch: 1) distal mistletoes remaining downstream (with respect to xylem) would increase in nitrogen (N) and C uptake (heterotrophy; H) from the host, while reducing gas exchange because xylem transport would no longer be shared with another mistletoe in the same xylem stream, and 2) proximal mistletoes remaining upstream (with respect to xylem) would reduce gas exchange because of improved water status from loss of sinks that normally increase xylem water tension. Finally, lack of removal (controls) would reduce N and H and increase gas exchange relative to the removal treatment. The data come from experimental manipulations and surveys conducted in the Santa Rita Experimental Range located 45 km south of Tucson, Arizona, United States (31.821°N, 110.866°W, 1120 m above sea level). The landscape is predominately a semidesert grassland that is being converted through succession and fire suppression to savanna with woody shrubs, including velvet mesquite 50. Average precipitation is 380 mm and accumulates in two bursts, late summer monsoons and winter rains. Most of the growing season occurs in July through September during the monsoon season. CO2 and water use, elemental content, and spatial and functional traits for each host and parasite were measured prior to leaf flush (April), prior to mistletoe removal when host leaves were present (May-pre removal), seven days after removal (May-post removal), mid to late monsoon season (September), and prior to leaf senescence (November). For more details on how the data were collected and analyses, please see the methods of the published article.