Parental care seen in mountain plants.

A big thistle and a monument plant share the mother-of-the-year award from plant ecologists.

Both species die after flowering only once, and the decaying bodies trap soil moisture that boosts survival of the seedlings, report Candace Galen and Anna Wied of the University of Missouri-Columbia. On high, dry mountain slopes, this advantage amounts to parental care, they suggest in the July Ecology.

Plants provide obvious benefits to their seeds, but Galen knows of no other clear demonstration of maternal care for seedlings. She and Wied examined the thistle Cirsium scopulorum and the gentian called a monument plant, Frasera speciosa. "It's probably the most compelling case," Galen says.

Thistles live on "rocky and miserable" mountain slopes in western North America, dry boulder fields with barely an eye-blink of growing season each year, Galen says. Monument plants also live in dry habitats. Both species follow the so-called big bang approach to life. After decades of growing just a tad larger each season, the plants send up a tall, showy head of flowers and then topple over dead. The seeds have no means for dispersal and just stay in the debris.

Soil under that litter is significantly moister, suffering less evaporation, than in nearby exposed spots, the researchers found. Seeds planted by the scientists in both the debris-strewn spots and exposed areas germinated, but moisture then seemed to make a vital difference in survival. Sheltered monument seedlings survived about twice as well as exposed seedlings. Little thistles were about four times more likely to survive in plots scattered with debris than in the bare ground.

The study sheds new light on the big bang lifestyle, Galen notes. "At first, it seems like a stupid thing to do--you could get killed before you reproduce if you wait too long."

However, theorists have pointed out that years with no flowers may minimize the attention of short-lived predators, and the fireworks finale of a huge flower head--something a plant growing in harsh conditions needs several years to work up to--may help attract scarce pollinators. Galen now points out that the big bang approach can also secure the advantages of sheltering offspring without competing with them. Mom isn't going to crowd out the seedlings, because she's already dead.

Postmortem parental care is not unprecedented, notes Elizabeth Lacey, a plant evolutionary ecologist with the University of North Carolina at Greensboro. The male praying mantis can turn into his mate's first prenatal nutrition. Also, plants in fire-adapted ecosystems like longleaf pine and turkey oak communities have to go up in smoke before seedlings can benefit from the bloom of newly available nutrients. In these cases, "provisioning comes from the death of a parent," Lacey notes.

Plant ecologist John Willis of the University of Oregon in Eugene points out that "people usually think of provisioning in terms of seeds." Mother plants essentially pack a lunch for their offspring in the nutrient-rich endosperm tissue inside a seed. He sees the extra help from Galen's moisture boost as "a very strong effect."