Robert Marsham was an English naturalist who lived on an estate in Norfolk, UK until 1797. For sixty-one years, the researcher carefully noted the timing of both plant and animal species in the gardens surrounding his home, Stratton Strawless Hall. That included the first leafing dates of thirteen trees, flowering dates for a variety of other plants, as well as the records of animal occurrences on his property. It was for his painstaking attention to detail that he eventually became known as the “father of phenology,” the scientific study of the ways in which the passing of the seasons affects plants and animals.

After Marsham died, his family picked up where he left off, continuing to monitor the plants and animals of the estate until 1958. Altogether, that makes the Marsham record among the most extensive phenological time-series datasets in the world, spanning a stretch of 222 years.

Now, Adrian Roberts, an Edinburgh-based biomathematician, has returned to the Marsham family’s data. He and his colleagues applied modern statistical techniques, combining the leafing and flowering information with historical temperature records, in an attempt to understand how temperature affects plants’ life cycles. That, they hope, will help them predict how natural communities might respond under a predicted climate change scenario.

Roberts picked out fourteen forest plant events for his analysis. For thirteen of them – hawthorn, sycamore, horse chestnut, elm, birch, rowan, hornbeam, lime, maple, sweet chestnut, beech, oak, and ash – it was the first leafing. For wood anemone, it was the first flowering.

As expected, he found that temperature played an important role in determining the phenology of all species considered by the study. The plants were sensitive to changes in temperature; as the environment shifts from cooler to warmer, the trees respond by growing leaves on their limbs and flowers on their stems. When spring temperatures increase, that process occurs earlier in the year. This phenomenon is known as “spring forcing.” Earlier spring forcing is already a well-known consequence of global warming.

But a sub-set of the species showed a slightly more complex pattern. They took longer to grow leaves in the spring when the previous autumn wasn’t cold enough, regardless of the springtime warmth.

In general, trees have to experience the natural range of cooling and warming in order to produce leaves and flowers. During the cold winter, the trees’ natural physiological processes are halted or slowed to a crawl. Many trees have evolved ways of detecting both the duration of time in which the weather has been cold as well as the “amount” of coldness. And for some species, it is only after sufficient “winter chilling” that the tree can resume proper growth in the spring.

It is likely that the variation in each species’ sensitivities to both spring forcing and winter chilling will mean that forests will look quite different in the future. Those species for whom spring forcing is most important will grow leaves earlier in the year; those for whom the autumn and winter chill is more critical could leaf later in the year. Eventually, a late-leafing species like oak might wind up growing its leaves earlier than an early-leafing species like birch.

That might not seem like a very big deal, but each species evolved in a certain ecological context. Changes in the timing of leafing for some species, for example, could result in changing environments for plants that live in the forest’s understory. Those plants may rely on having more or less shade in order to carry on with their own lives. If a plant’s leafing is accelerated or delayed, that means that the forest floor will become shadier sooner, or stay brighter longer. For many important agricultural trees, an insufficient chill can mean reduced yield and quality of leaves, flowers, and fruits, resulting in a reduced harvest. For species that require the transfer of genetic information between male and female individuals, insufficient chilling can reduce pollination as well, which will also severely impact harvests.

Understanding the ways in which a changing climate impacts the life cycle of a forest is like trying to predict how a row of dominoes will fall, except you don’t know where all the dominoes are placed, nor do you know which domino will fall first, in what direction, or when. “The ecological consequences” of the ways in which individual species respond to our changing climate and how those changes will interact each other, “are not currently known,” write the researchers. But Roberts and his colleagues do have one prediction. “Shifts in the abundance of species and community composition,” they suspect, “will be a more likely long-term outcome than genetic adaptation of species.” In other words, the forests will look quite different to those who are looking closely, even if the trees themselves do not.

It is striking that information on which this study relied is effectively the result of two centuries’ worth of citizen science. Marsham himself may have been a “proper” scientist, but his descendants carried on his hard work for several generations after his death. Even now, citizen scientists can continue to observe the growth of leaves and flowers in plants around the world. Eventually, researchers will be able to apply sophisticated statistical techniques to that crowd-gathered data to evaluate whether Roberts’s predictions about changing forest ecology were correct. – Jason G. Goldman | 11 March 2015

Source: Roberts A.M.I., Christine Tansey, Richard J. Smithers & Albert B. Phillimore (2015). Predicting a change in the order of spring phenology in temperate forests, Global Change Biology. DOI: 10.1111/gcb.12896

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