Holden Arboretum research sheds light on how climate change will alter plant ecosystems

CLEVELAND, Ohio – Last spring, Katie Stuble was surprised when her daffodils popped out of the ground on the same day as her crocuses.

Never had that happened before. Usually, the crocuses came first and the daffodils at least two weeks later.

But last year was different. It was an extremely warm winter with little snow, which could explain why the daffodils were so eager to get started.

What’s harder to understand is why the crocuses didn’t advance two weeks as well.

For Stuble, who lives in Mentor, it was more evidence that species don’t respond equally to the same triggers, such as air temperature or lengthening of the day, and that as the world heats up, ecosystems will change their natural order.

Some plants will adjust to warming trends brought on by climate change, while others that are programmed differently may lag, denying themselves the life-sustaining resources they have always counted on.

It all falls under a topic called phenological mismatch, a long-recognized consequence of climate change that Stuble has been researching at Kirtland’s Holden Arboretum, where she is director of research and a restoration ecologist.

The forest floor

Some of Stuble’s research is conducted just a short walk from the arboretum’s Long Center for Plant & Environmental Science building — past a grove of ash trees being studied for their potential resistance to emerald ash borer – to a forested area called Bol Woods dominated by maple and beech trees.

Eight years ago, Stuble and others began tracking the emergence of spring wildflowers in Bol Woods to see if, from year to year, they remained in sync with the burgeoning tree canopy above.

When wildflowers emerge from the ground, they only have so much time to soak in energy from the sun before the trees around them leaf out and bathe the forest floor in shade. Stuble wants to better understand how global warming might shorten that window of opportunity.

Her laboratory is the forest floor, which in spring is a mosaic of fragile-looking wildflowers, such as squirrel corn, trout lily and trillium, that pop through the matting of decomposing leaves.

Stuble and her team of volunteers have found that while the wildflowers tend to track climate change — meaning they emerge earlier to greet the warmth – the trees have been more erratic.

At first the trees weren’t responding at all, she said, and then there was a warm year when they leafed out two weeks early.

When compared to the flowers, trees “seem to respond to different signals in different ways,” she said.

Comparing regions

Stuble and the arboretum have been involved in similar projects, including one conducted with four other public gardens around the country that drew similar conclusions about the vulnerability of wildflowers in their early stages of growth.

Chelsea Miller, an ecologist who worked in Stuble’s lab, wrote about it in a blog post on the Holden Forests & Gardens website.

“Because of this short window for growth and reproduction, bracketed on one end by late season frosts and at the other by decreased light availability due to forest canopy closure, spring wildflowers in North American are at particular risk to shifts in phenology,” Miller wrote.

Going to seed

A third experiment at the arboretum that addressed phenological mismatch involved planting seeds in two different indoor chambers. The temperature inside one of the chambers reflected conditions outside, while the other was dialed up three degrees warmer.

Fifteen different species of flowers and grasses, some native and some not, were planted in each chamber, and the order of germination was compared to see if there were differences.

There were. The species that responded earliest went on to become more robust.

Stuble said she expected non-native species to germinate quicker in the warmer chamber, but that wasn’t the case. Two native species — ironweed and a type of goldenrod – were among those that emerged earliest.

Vicky M. Temperton, a fellow ecologist writing in the journal Trends in Ecology & Evolution, cited Stuble’s seed work as a “great starting block for” additional study.

“Recent research has focused a clear spotlight on the role of germination sensitivity to warming, highlighting that earlier germination timing could vary strongly between plant species and hence alter dominance patterns in seasonal vegetation,” wrote Temperton, a professor at Leuphana University in Germany.

The future

The seed experiment shows what scientists believe is likely to happen as the atmosphere warms, but more research is needed to determine those species likely to benefit, Stuble said.

“There will be winners and losers,” she said, “and that’s going to shape what our world looks like in the future.

Continued research will also give society more to ponder as it decides how seriously it wants to reduce the volume of carbon in the atmosphere, she said.

And more tactically, it could help with efforts to restore natural landscapes, such as prairies, by identifying the types of vegetation to include — and in which order they should be planted — to ensure a diverse and thriving plant community going forward.

“You can start to manipulate those sorts of things if you know the rules of the game,” she said.

Peter Krouse writes about the environment for cleveland.com and The Plain Dealer. He can be reached at pkrouse@cleveland.com.