Warm springs and hot summers have opposing effects on tree seedlings under climate change

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Emerging leaves of a sugar maple in spring. (Shutterstock)

Balmy springs may boost tree seedling performance, but hot and dry summers could eclipse such benefits, according to a new study that suggests species survival will depend on the severity of climate change.

Researchers, whose findings were published May 11 in Global Change Biology, predicted how climate change's effects on timing, or phenology, of spring leaf-out in red oaks and sugar maples could affect their growth and survival in deciduous North American forests.

"If shifts in phenology are affecting how well these seedlings are able to survive and grow, then that has important implications for recruitment processes. Recruitment is the process by which a seed becomes a seedling, a seedling becomes a sapling, and so on," said study lead author Ben Lee, who recently earned his Ph.D. from the University of Michigan School for Environment and Sustainability. "If species can't recruit, then they won't be found in that area, eventually."

Organisms use environmental cues such as temperature and day length to regulate the timing of biological processes.

"For plants, this includes everything from flowering to when deciduous species will flush their leaves in spring or drop their leaves in the fall, and when they disperse their seeds," Lee explained. 

But as anthropogenic climate change disrupts temperature cues, plants and animals may shift the timing of these processes. 

"There are literally centuries-long datasets that have shown a lot of these events are shifting in response to some combination of climate drivers — most commonly, changes in temperature," Lee said. "For instance, in Japan, people have been observing the first day of cherry blossom for hundreds of years now, and this year it was the earliest on record out of the entire dataset, which is pretty crazy."

If these events become out of sync with other members of the ecosystem, this could spell trouble for a species. For example, if pollinators emerge at different times than the flowers they pollinate, plants may fail to reproduce. Likewise, changes in the timing of plant processes could affect interactions among plants and shift forest dynamics.

In deciduous forests, tree seedlings are shaded by the canopy for most of the year. These mature trees block light that drives photosynthesis, the process by which plants suck carbon dioxide from the atmosphere and convert it into sugars. Seedlings can temporarily escape the shade of canopy trees by flushing their leaves earlier in spring or dropping their leaves later in fall, and studies have shown that understory plants may make most of their food in these brief windows.

As such, early leafing out has been associated with improved seedling growth and survival, but the mechanisms for this finding had not been identified. In a study published May 9 in Functional Ecology, Lee and his colleague Inés Ibáñez, an associate professor at the University of Michigan School for Environment and Sustainability, looked at how timing of spring leaf flushing affected carbon assimilation and survival in sugar maples and red oaks. 

The researchers planted seedlings of both species in Michigan forests and carefully tracked survival and growth over the year, noting when seedlings produced leaves in spring and dropped them in fall, and the date of canopy closure, when mature trees fully shaded the understory. The duo also estimated carbon assimilation throughout the growing season by measuring photosynthesis rates.

In line with other studies, the researchers found that seedlings assimilated the majority of carbon in early spring before the canopy closed.

"The earlier in spring, relative to canopy closure, that seedlings were able to leaf-out, the more carbon they were able to assimilate via photosynthesis and the more likely they were to survive," Lee explained.

These findings led the researchers to suspect that climate change could have a positive effect on seedling performance due to warm temperatures advancing spring leaf-out and increasing access to light. But on the other hand, hotter and drier conditions expected with climate change could have negative effects on seedlings.

To get a handle on how different effects of climate change collectively affect red oak and sugar maple seedling performance, the researchers used the findings from their first study to build a statistical model under best- and worst-case climate scenarios in 2100.

Under both scenarios, the model predicted that red oak and sugar maple seedlings would leaf-out earlier in relation to canopy closure compared with present day. As such, seedlings would assimilate "substantially more carbon" in the springtime, Lee said.

However, these beneficial effects of extra light in spring should be tempered by hot and dry summertime conditions that are forecast under climate change. Just as people breathe out more carbon dioxide when they exercise, plants lose more carbon through respiration when it's warm, Lee explained.

"Under really hot conditions, that respiration gets out of control and it becomes something that vastly outweighs any positive photosynthesis that occurs," he said. "When that happens, the net amount of carbon that plants are able to assimilate is actually negative."

Considering the effects of both earlier springs and hotter summers, the researchers predict that seedling growth and survival of both species will suffer in the worst-case climate scenario. 

In contrast, the researchers predict that seedling performance under a best-case climate scenario would be similar to present-day conditions because increased carbon assimilation in spring would balance out the loss of carbon in summer.

According to Lee, these findings indicate that early spring leaf-out is "incredibly important" for helping seedlings cope with climate change, and that seedling performance will likely depend on the severity of future climate change. 

However, he added, "The best-case climate scenario is increasingly looking like that will not be the case, since we keep just kind of blowing through global carbon reduction [goals]."

"This suggests, as things are going right now, that seedlings of these two species will experience decreases in overall performance over the next 80 years or so," he added. "The implication is that recruitment will be lower and that slowly these two species will become less prominent and less abundant in these woods."

Red oaks and sugar maples are important trees in long-lived forests throughout North America. 

"Losing them from the woods will not be good. If those species aren't in these systems, it leaves open the door for other species," Lee said. And it could be invasive species that fill those niches, he added. "It's not necessarily that forests won't be there; it's more that forests will look very different."

The study, "Improved phenological escape can help temperate tree seedlings maintain demographic performance under climate change conditions," published May 11 in Global Change Biology, was authored by Benjamin Lee, formerly University of Michigan; and Inés Ibáñez, University of Michigan.

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