Patterns of mistletoe infection in four <i>Acacia</i> species in a semi-arid southern African savanna

Abstract

In a range of systems, studies on mistletoe distribution on the host plant have documented a number of factors that affect their occurrence and spread (Aukema &amp; Martinez del Rio 2002a, Bowie &amp; Ward 2004, Overton 1996, Reid et al . 1995). These patterns can be determined by host specificity, environmental conditions, host plant characteristics (Martinez del Rio et al . 1995) and the movement patterns of dispersal agents (Aukema &amp; Martinez del Rio 2002a, 2002b). In mistletoe plants, host choice can be considerably influenced by the advantages of interacting with relatively abundant hosts (Norton &amp; Carpenter 1998, Norton &amp; De Lange 1999). Besides the relative abundance of host species, characteristics such as branch size, age and height can have a strong effect on mistletoe attachment resulting in size-related mistletoe infection patterns (Overton 1994). Generally positive relationships between mistletoe infection and host size have been demonstrated worldwide (Donohue 1995, Martinez del Rio et al . 1996, Norton et al . 1997, Reid &amp; Stafford Smith 2000) and they have been interpreted in terms of the preferences by dispersing birds to perch and feed in taller trees (Aukema &amp; Martinez del Rio 2002a) and trees accumulating infections as they age (Overton 1994). Aukema &amp; Martinez del Rio (2002a) reported more frequent perching in taller-than-average trees by the phainopepla ( Phainopepla nitens ), which is the principal disperser of the desert mistletoe Phoradendron californicum . Thus, visits by mistletoe-seed-dispersing birds, and therefore mistletoe seeds received, tend to increase with tree height (Aukema &amp; Martinez del Rio 2002a). Using a simple metapopulation model, Overton (1994) predicted the frequency of parasitized trees to increase with host age. Therefore, assuming that size is a good proxy for age, large trees are likely to be more infected than smaller trees. Reid &amp; Stafford Smith (2000), using experimentally disinfected trees, found that larger trees were disproportionately re-infected with mistletoes. This size–intensity relationship may be used to describe mistletoe infection patterns. However, several previous studies have shown size–intensity relationships to be weak (Aukema &amp; Martinez del Rio 2002a, Donohue 1995, Overton 1994, Reid &amp; Stafford Smith 2000). This indicates that other factors may be important in determining mistletoe infection intensity, including that already parasitized hosts of a specific height are more likely to receive seeds than non-parasitized hosts of the same height or dispersers are likely to be attracted to trees for reasons other than size (Aukema &amp; Martinez del Rio 2002a).