Reproductive phenologies, biotic interactions, and when things go wrong….

Animals need to synchronise their reproductive phenologies with the seasonal availability of resources, so that they’re reproducing at the time of maximum resource abundance. Plants and insects have advanced their spring phenologies as spring temperature has increased. The animals which rely on them for food need to advance their reproductive phenologies in step, in order to cope with this change.

Migratory birds have been heavily studied in this regard. Some species have not advanced their timing of breeding. Unsurprisingly, European migratory bird species which advanced their spring migration phenology in the past decades have stable or increasing populations, but those which have failed to respond have decreasing populations.

The migratory pied flycatcher, Ficedula hypoleuca, is one example. It has had trouble adapting to climate change because the timing of its migration is controlled by “endogenous rhythms”, such as day length, which are not affected by climate change. The pied flycatcher populations with the largest declines are those where its prey’s population peaks early in the spring – because it hasn't been arriving earlier, it’s missed the time of peak food availability. When prey population peaks later, the birds do better. 

Pied flycatcher pic from BBC

Land surface phenology

Phenology has traditionally used data collected on individual organisms, or species. “Land surface phenology” uses satellites to monitor the phenological status of vegetation from space. This is a different kind of data: satellites provide a continuous record of the pattern of every number of phenological processes as an aggregation of a large area of land.

One of the applications of land surface phenology has been the study of the urban heat island effect, where urban areas are warmer than rural areas. Zhang et al. (2004) used satellite data to look at vegetation phenology in eastern North America. They found that, on average, the growing season lengthened by 15 days surrounding urban areas compared with rural ones. This was because of an earlier onset of green-up by 7 days, and a later onset of dormancy by 8 days. The difference in green-up was a largely function of temperature, which is elevated around urban areas. However, dormancy is more complicated, and probably has more to do with a variety of factors such as water availability and day length. They also found that the difference was detectable up to 10km away from the edge of urban areas.

Greenup (ΔG) comes earlier and dormancy onset (ΔD) comes later around the urban areas of Washiungton DC, New York and Boston. Temperature is elevated around urban areas in spring (ΔLST_1–5), and in autumn/winter (ΔLST_9–12). From Zhang et al. (2004).

A diverse vegetation phenological response to climate change has also been detected using remote sensing. Bunn and Goetz (2006) looked at trends in photosynthesis for 22 years of satellite measurements in the northern circumpolar high latitudes. Warming would be expected to cause vegetation growth, as with the urban heat island effect. The treeline has also been observed to be migrating northwards because of global warming. However, they found that 85% of the areas they studied showed no change. Moreover, of those areas that did show change, tundra areas experienced increases in photosynthesis (“greening”), but boreal forest areas experienced decreases (“browning”). It could be that increases in evaporation in the Arctic is causing drought stress that trees cannot cope with, leading to browning, despite global warming.

Changes in photosynthetic activity in northern circumpolar areas in early and late summer. Red and yellow areas are mostly tundra and show a greening trend. Blue areas are mostly boreal forest and show a browning trend. Most areas show no difference and are lilac. From Bunn and Goetz (2006).

These results are not entirely straightforward: phenology is not limited to the observation that 'spring is coming sooner'! Phenological phenomena - autumn senescence around urban areas, and photosynthetic activity in boreal forests - are not solely a function of temperature. But a complicated world is more interesting than a simple one. :)

Welcome to the Anthropocene

In its 4.6 billion year history, the Earth has always been in a state of flux: various astronomical, geological, and biogeochemical forces have driven dramatic and almost unimaginable changes in the Earth’s environment. I would love to go back in time and see a completely alien Earth with lush tropical forests at the poles about 55 million years ago, or to somehow watch a time-lapse from the moon of huge ice sheets coming and going over the surface of the Earth over hundreds of thousands of years!

Since humans evolved some 200,000 years ago, and as our populations and enterprises have grown, we’ve been leaving more and more of a mark on this world. In fact, the sheer pace and magnitude of recent, human-induced environmental change is so dramatic that it overshadows natural forces we observe in the geological record. This has led scientists to propose a new epoch - that is, coin a new term for our time - the Anthropocene.

The human impact on the environment is not limited to global warming, but that's what grabs everyones attention. What is happening, and will happen in the future, to plants and animals during this time of rapid warming?

There are numerous ways in which species can potentially respond to environmental change. On long timescales, they can evolve. They can go extinct. They can move away, to a more suitable habitat. Or, they can show changes in their phenology.

Phenology is the study of the timing of seasonal plant and animal activity. The oldest phonological records date from AD 705 Japan, and are of the first cherry blossoming. Robert Marsham has been called the founding father of phenology. From 1789 he made meticulous records of, for example, the first frog and toad croaks, first snowdrops, and first turnip flower to appear. He kept these records up for decades, and successive generations of the Marsham family maintained the tradition. Nowadays anybody can submit their observations online. Modern technology is a blessing, letting us amass such large datasets.

Plenty of people have casually observed that spring is coming sooner. In fact, studies have shown that spring has been advancing by 2.3-5.2 days per decade for the last 30 years, and there is good evidence to show that this is caused by recent warming.

In this blog I'll be exploring current research in phenology. Plants and animals are inherently interesting, but the details of scientific papers can sometimes be a bit inaccessible to the layperson (and to students!!!). I hope to learn a lot, and I look forward to showing this blog to my friends and family. Until next time!