Often dismissed as the parasitic kiss of death, mistletoes turn out to form complex relationships with their trees, sometimes fostering new life.
A spiky mistletoe species on juniper trees attracts birds that spread the trees’ seeds, says Ron J. van Ommeren of Senna Environmental Services in Phoenix. More than twice the number of juniper seedlings sprout in mistletoe-rich patches than in mistletoe-free ones, report van Ommeren and Thomas G. Whitham of Northern Arizona University in Flagstaff.
“This is a fairly novel kind of look,” van Ommeren says. “The general view has been that mistletoes are almost always parasites.” Yet the new analysis raises the possibility that this sap-stealing plant sometimes forms a mutualistic partnership with the tree, the ecologists suggest in the January (number 2) Oecologia.
The mistletoe that the researchers studied, Phoradendron juniperinum, belongs to the same genus as North America’s holiday decoration (SN: 12/23 & 30/00, p. 411: http://www.sciencenews.org/20001223/bob2.asp).
This more jagged-looking cousin bears gooey whitish berries, which hitchhike via birds to juniper trees and start new plants. The mistletoe can kill branches and slow a tree’s growth but doesn’t seem to kill it.
Van Ommeren and Whitham monitored the mistletoe in patches of arid woodland dominated by pion pine and one-seed juniper, or Juniperus monosperma, north of Flagstaff. During the 3 years of the researchers’ study, juniper proved an unstable food source for birds. A boom year offered up to 15 times as many juniper berries as a lean year did. Mistletoes, however, fruited in about the same abundance each year.
In a lean year, patches of woods bristling with mistletoes attracted three times as many major fruit-eating birds, such as Townsend’s solitaires, than mistletoe-free patches did.
All those avian visitors may explain the bumper crop of juniper seedlings in mistletoe-rich patches, suggest van Ommeren and Whitham. The flesh on a juniper fruit hardens into a seed prison if left untouched. Removing that flesh when soft, as a bird’s digestive system does, increases by a factor of 10 a seed’s chance of sprouting, according to earlier experiments.
Van Ommeren cautions that he’s not arguing that mistletoe always benefits juniper. A small clump in a lonely spot might not manage to attract birds.
Also, the mistletoe-juniper relationship might flip-flop between parasitism and mutualism.
Robert E. Bennetts of the U.S. Geological Survey in Gainesville, Fla., has taken an unconventional look at a controversial dwarf mistletoe that can kill trees prized for timber. Bennetts found, however, that ponderosa pine stands with dwarf mistletoes attract more and more-varied birds than mistletoe-free stands do.
Bennetts muses that mistletoe’s image may change as views of fire have–from unremitting scourge to essential ecological force. “There’s a whole bunch of perturbation processes that do a quick change on an ecosystem,” says Bennetts. Until ecologists take a close look at these events and find otherwise, most of them are decried as destructive. He says, “The take-home message is that mistletoes are not a disease.”