Timing is everything: the ecological mystery of harmonious rhododendrons

High up in the Hengduan Mountains, east of the Himalaya in southern China, dozens of different rhododendron species live together in meadows.

The fact that they do this is puzzling: why are there so many similar, but not identical, plants? Don’t they compete with each other for resources, forcing some to adapt or die out?

“There’s this basic idea in ecology of the niche, that a species’ lifestyle, like what it eats and how it fits into the environment, cannot be replicated in the same community,” says Dr Rick Ree, a curator at the Field Museum in Chicago, US.

There are over 30 different species in the Rhododendron genus within in the same area of the Hengduan mountains.

Ree, along with a team of US and Chinese researchers, might have the answer.

“Since there are so many closely-related species of rhododendrons, all living together in these mountains, we wanted to figure out how they were able to co-exist,” says Ree.

Apparently it’s all timing, as the researchers have published in the Journal of Ecology.

The rhododendron species all flower at different points in the year, meaning they don’t compete with each other for pollinators.

“By partitioning that timeline, they can reduce their chances of wasting their pollen and the resources that go into reproduction,” says Ree.

Lead author Dr Qin Li, a researcher at the Field Museum, figured this out by taking a field trip to Mount Gongga, in the Hengduan range, documenting when different rhododendron species bloomed.

“I’d never done fieldwork in southwestern China before, but we were actually pretty close to my hometown in Sichuan province,” says Li.

“With my field assistant and co-author Ji Wang from Sichuan University, I spent more than two months going to more than 100 sites, and we visited each of these sites four times throughout the season.”

The researchers logged a variety of ecological features for 34 different rhododendron species – including size and shape of leaves and flowers, when they bloomed, and where they grew.

After statistical analyses, they found that the thing that differentiated the plants was when they flowered.

“Going in, we had a hunch that timing would be important, but we weren’t super certain,” says Ree.

“It’s kind of conspicuous that there’s a long season where you can see flowers in the Himalaya region – there are some species that put out striking blossoms against a backdrop of a field of snow, and others that wait till the end of the summer.”

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It’s troubling that timing seems to play such a big role in the mountain’s ecology – because it makes the flowers more vulnerable to climate change.

“There’s abundant evidence that the pace of climate change is messing with plants’ flowering times, causing population declines and extinctions,” says Ree.

“The question is, how are plant communities around the globe going to respond? Weather is part of what signals them to blossom, and since climate change affects the weather, it’s likely to shift that competitive landscape.

“When the environment changes, species have three choices: you move, you adapt, or you die. Climate change is accelerating that dynamic.”