Across the United States, trees are shaking off winter and bursting with fresh green leaves. This time-tested sign of spring betrays the fact that the season’s arrival increasingly breaks tradition. And its abnormal arrival is accelerating: This year, spring leaf arrived earlier than average in some parts of the country and later in other parts. A new study in the journal Methods in Ecology and Evolution shows evidence that warming temperatures likely play an important role in this process, with the ability to change bud blooms by nearly three weeks.
One thing that’s become clear is that climate change is altering the timing of spring, study co-author and University of British Columbia associate professor Elizabeth Wolkovich, Ph.D., tells Inverse. Since the 1980s, records clearly show that plants leaf and flower differently — a large change that stems from only a rise of 1 degree Celsius. This study is an attempt to anticipate what will happen in the future so that we can prepare for what’s to come.
“We need much better models to anticipate and cope with the changes we expect in spring leaf-out, given predicted warming of 2 to 4 degrees Celsius by the end of the century,” Wolkovich says.
To investigate this future scenario, they collected branches from 10 different tree and shrub species found in Quebec and Massachusetts during mid-winter, after the plants experienced enough cold days to allow bud-burst in the right conditions. They included the striped maple, honeysuckle, and American beech.
Then they placed the branches in special growth chambers where they were exposed to varying combinations of temperature and daylight and closely watched them until the buds bloomed into leaves.
On average across all the species, the branches kept at warmer temperatures burst bud 20 days earlier compared to the branches kept at cooler temperatures. While warmth had the strongest influence on budburst, daylight also played a role: The samples that received more daylight burst about 12 days earlier compared to those that received less.
Wolkovich notes that based on other work, they thought that some species might not respond to daylight length, but this study shows all of them do. She says this adds a level complexity to predicting future spring arrivals because it means they can’t rely on temperature trends alone.
While on average all species budburst earlier in warmer temperatures, there was some variation across species, with the American beech tree responding the strongest to warm temperatures. Additionally, related individuals, when subjected to the same cues, burst bud at a similar time.
Study co-author and University of Montreal adjunct professor Simon Joly, Ph.D., says this variation across species suggest some plants have the potential to adapt to increasing temperatures in the future — and in turn, these individuals will be the ones most likely to propagate in future environmental conditions. This cues scientists in on what plants are in most need for protection.
“Understanding how temperature, day length, and cold temperature in winter work together to cause bud-burst each spring will become more critical to manage our forests and parks in the future,” Joly says.
He also notes that, while this study focused specifically on leaf openings, it also has implications for allergy sufferers. Flowering in plants, he notes, is also rooted in genes, and flowering reacts strongly to changes in temperature. This will impact when the pollen of the future will strike.
“We can expect the time of year when the pollen is released to change in the future with the increase of ambient temperature caused by climate change,” Joly says.
Other studies have confirmed this as well: In a study published in The Lancet: Planetary Health in March, scientists found that climate change advanced the arrival of spring over the course of 26 years in sites across the Northern Hemisphere. They also noted an increase in pollen loads and pollen-season duration, which they argue clearly links a warming planet to increased pollen.
Pollen triggers seasonal allergies, and a recent study published in PLOS One found that changes in the onset of spring are making allergies worse. That’s bad news for humans — but it’s also our fault that this is happening.
Analyses in many fields of ecology are increasingly considering multiple species and multiple individuals per species. Premises of statistical tests are often violated with such datasets because of the non‐independence of residuals due to phylogenetic relationships or intraspecific population structure. If comparative approaches that account for the phylogenetic relationships of species are well developed and their benefits demonstrated, the importance of considering the intraspecific genetic structure, especially in combination with the phylogenetic structure, has rarely been addressed.
We investigated whether it is beneficial to account for intraspecific genomic relatedness in multi‐species studies. For this, we used a phylogenetic mixed model to analyse first a suite of simulated data and then results from one example ecological study—a budburst experiment where clippings of 10 tree and shrub species were subjected to different temperatures and photoperiods.
We found that accounting for intra-specific genetic structure yields more accurate and precise fixed effects as well as increased statistical power, but more so when the relative importance of the intraspecific to the phylogenetic genetic structure is greater. Analysis of the bud-burst experiment further showed that accounting for intra-specific and phylogenetic structures yields improved estimates of warming and photoperiod effects and their interaction in explaining the time to bud-burst.
Our results show that statistical gains can be made by incorporating information on the intra-specific genomic relatedness of individuals in multi‐species studies. This is relevant to investigations that are interested in intra-specific variation and that plan to include such observations in statistical tests.