Response of a root hemiparasite to elevated CO 2 depends on host type and soil nutrients

Abstract

Although elevated CO may affect various forms of ecological interactions, the effect of elevated CO on interactions between parasitic plants and their hosts has received little attention. We examined the effect of elevated CO (590 μl l) at two nutrient (NPK) levels on the interactions of the facultative root hemiparasite Rhinanthus alectorolophus with two of its hosts, the grass Lolium perenne and the legume Medicago sativa. To study possible effects on parasite mediation of competition between hosts, the parasite was grown with each host separately and with both hosts simultaneously. In addition, all combinations of hosts were grown without the parasite. Both the parasite and the host plants responded to elevated CO with increased growth, but only at high nutrient levels. The CO response of the hemiparasite was stronger than that of the hosts, but depended on the host species available. With L. perenne and M. sativa simultaneously available as hosts, the biomass of the parasite grown at elevated CO was 5.7 times that of parasites grown at ambient CO. Nitrogen concentration in the parasites was not influenced by the treatments and was not related to parasite biomass. The presence of the parasite strongly reduced both the biomass of the hosts and total productivity of the system. This effect was much stronger at low than at high nutrient levels, but was not influenced by CO level. Elevated CO did not influence the competitive balance between the two different hosts grown in mixture. The results of this study support the hypothesis that hemiparasites may influence community structure and suggest that these effects are robust to changes in CO concentration.

Resource limitation and the role of a hemiparasite on a restored prairie

“…Bardgett et al (2006) found no interactions between R. minor and soil fertility or manure application on aboveground biomass in a mesocosm experiment, but R. minor increased species diversity in lownutrient soil. The results of two-pot culture experiments suggest that the eVect of a parasite was greater in nutrientpoor soils (Matthies and Egli 1999;Davies and Graves 2000), although the latter study did not report results for the interaction, while Cameron et al (2005) showed that R. minor had a larger eVect on competitive abilities of the grass Poa pratensis under high-nutrient levels. Together, these studies, and those of Cameron et al (2009), suggest that eVects of hemiparasites on communities are not straight-forward responses to resource supply in many systems.…”

“…Pot culture experiments indicate that the eVect of a parasite on an individual host is greater under low-nutrient conditions (Gibson and Watkinson 1991;Matthies and Egli 1999; but see Cameron et al 2005), and mesocosm experiments indicate that some community-level eVects decline with soil fertility (Bardgett et al 2006). For parasitism to depress community productivity, the parasite-induced reduction in the growth of hosts must exceed the contribution of the parasite mass (Hautier et al 2010).…”

Parasitic plants: parallels and contrasts with herbivores

“…In the same way, since parasitic plants selectively depress the biomass of preferred host taxa, plant parasitism can alter the competitive balance between preferred and nonpreferred hosts (Gibson and Watkinson 1991;Matthies 1996;Callaway and Pennings 1998; but see Matthies and Egli 1999). As a result of this indirect effect, the species composition of the non-parasitic vegetation may differ between communities with and without parasitic plants (Gibson and Watkinson 1992;Pennings and Callaway 1996;Joshi et al 2000).…”

“…Although hemiparasitic plants also reduce the biomass of their host community, this reduction can occasionally be compensated for, or even exceeded by, the production of the parasite (Marvier 1996;Joshi et al 2000). Thus, the net effect of a hemiparasite on total plant community biomass can be negative, neutral or positive, although negative effects are most typical (Mizianty 1975;Matthies 1995Matthies , 1996Matthies , 1998Marvier 1998b;Matthies and Egli 1999;Joshi et al 2000).…”

“…Since different host species may supply a parasite with different resources, including both nutrients and chemical defenses, a mixture of host species may be superior to a single host alone (Govier et al 1967), much like the fact that "mixed diets" provide better nutrition to animals than "singleitem" diets (Pennings et al 1993;Bernays et al 1994). Experimental analyses of mixed diets show that they benefit some parasitic plant species, but not all (Matthies 1996;Marvier 1998a, b;Kelly and Horning 1999;Matthies and Egli 1999;Joshi et al 2000). Future studies of mixed diets may be helpful in understanding the broad host ranges of parasitic plants versus insects.…”