Based on previous research, scientists have predicted that plants will be able to adapt to rising global temperatures by changing their seasonal responses. Changing the timing of seasonal processes, such as new leaf growth in spring, flower formation and seed set, was expected to help plants keep pace with climate change. However, a new, global-scale analysis has revealed that changes in these phenological (seasonal) processes are lagging behind the temperature increases, especially in human-dominated landscapes.
“We would commonly expect phenology to adapt very quickly to climate change, but with these findings, we see that there’s probably a limit to how much adaptation we can see, and this limit is affected by human activities,” said study lead author Yiluan Song. “These findings open up a lot of questions to explore in terms of our expected response of plants to climate change.”
Under the guidance of Professor of Kai Zhu, researchers at UC Santa Cruz analyzed satellite remote-sensing data in the Northern Hemisphere at mid- to high-latitudes. This data identified the warm-season growth patterns of plants on the ground. They hypothesized that growing seasons in the Northern Hemisphere would start earlier and last longer in response to increases in temperature. But this is not what they found.
Between 1981 and 2014, the length of the growing season changed more slowly than the average annual near-surface temperatures across a majority of the Northern Hemisphere. In some areas, this mismatch was even more extreme, with growing seasons getting shorter instead of longer.
In addition, the phenological mismatch corresponded with human population density and was particularly pronounced in the most human-dominated landscapes, especially croplands. This finding was supported with additional data from on-the-ground observations by citizen scientists which showed that the mismatch between temperature-related environmental conditions and the timing of initial growth of spring leaves increased with human population density.
While growing seasons in these human-impacted landscapes did not match temperature changes, phenological shifts were actually ahead of temperature changes in wildland areas. Researchers have yet to understand the reasons for these findings
One possibility is that the rate of temperature change may simply be too great for plant phenology to adequately keep up. Prior research shows that plants may be limited by their need to respond to other environmental variables too – like the length of the day and levels of precipitation – and that the species types and compositions of plant communities across parts of the Northern Hemisphere may also shape phenological response.
The findings have significant implications for areas where crops are farmed. In agricultural settings, warming temperatures can accelerate crop development, leading to an early harvest, but adaptive management practices could better match plant phenology with climate conditions to take advantage of longer growing seasons. Additional research will be needed to understand how human activities – like irrigation, selective planting, or land disturbance – might affect climate-phenology coupling and what the long-term impacts of this effect could be.
“While plant phenology has been well-researched, conventional approaches focus on natural systems, looking at forests or grasslands and intentionally excluding the areas with human dominance,” explained study senior author Kai Zhu. “But we are in the Anthropocene. When you look at the land across the earth, a large chunk of it has been modified by humans, and I think this study shows that it would be a mistake to not account for that.”
In addition, changes in the plant growth have potential impacts on environmental processes. “We particularly want to know more about the implications of these new findings on the carbon cycle,” said Zhu. “Plants are currently doing us a great service in taking up a lot of the carbon emissions we put into the atmosphere, but the length of the growing season is an important factor that determines how much carbon they can sequester.”
The researchers hope their findings might help to inform new approaches in the study of plant phenology. They argue that maintaining a stable relationship between climate and plant phenology is crucial for maintaining both biological diversity and ecosystem productivity in the future.
“We anticipate that the degree of mismatch may ultimately depend on how humans intervene,” said Zhu. “If we are smart enough, I think we could probably be adaptive to climate change through better management of plants. We are still learning in that process.”
The study is published in the journal AGU Advances.