(1) Parasitic plants, among many other parasitic organisms, are shifting in their geographic distribution in response to global change, establishing novel interactions with susceptible but previously unexposed host plants. Projections of future species assemblages in increasingly changing environments frequently overlook the strong capacity of these parasites to transform ecological systems, and host switching is considered a mere change of partner with no subsequent impact on ecosystem properties.

(2) We explore the ecological consequences of parasite host shifts analysing the growth and reproductive responses of two host trees that share a different interaction-history with the parasitic plant Viscum album: recent in the case of Pinus sylvestris subsp. nevadensis and longstanding in that of Pinus nigra. We quantified growth by measuring primary (bud elongation) and secondary (dendrochronology) tree growth, and reproduction by quantifying cone crops, cone size, seed production, seed weight, seed germination, and emergence success.

(3) Pinus nigra registered a minor effect on reproduction but a major reduction in biomass, clearly reflected in lower primary and secondary tree growth, smaller cones and lighter seeds. On the contrary, P. sylvestris subsp. nevadensis underwent a strong decline of its reproductive capacity with no effects on growth, manifested in smaller cone crops, less seed production, and lower germination and emergence success.

            (4) Synthesis: Mistletoe parasitism unleashes contrasting responses in taxonomically close host species with sharply different consequences for the forest ecosystem. While old hosts tolerate better the effect of parasitism, minimizing the impact on reproduction at the expense of their growth, recent hosts undergo greater effects of parasitism, suffering reproductive collapse with no perceptible loss of growth.  We conclude that the increasing phenomenon of parasite host switching is not just a mere change of partner, but a driver of a new ecosystem organization. These results highlight the importance of considering the specific effects of newly established interactions when predicting future species assemblages, as these can trigger different ecological organizations from those we currently know.