Following my last post, I’d like to continue on the theme of why deciduous tree phenology is different from place to place. Last time, I discussed how phenology might be different over relatively small distances, like hundreds of feet, between different microclimates in a forest. However there are also larger spatial trends in the timing of phenology. And, like so many phenological phenomena, they are correlated with temperature.
To understand how urban heat islands affect plant phenology, first it’s important to know what an urban heat island is. Modern cities have grown tremendously in terms of land area in recent decades, and the way we build cities has many effects on the environment. Viewed from above, cities today consist largely of paved surfaces such as roads and parking lots, and roofs. This type of city increases temperature in two ways. First, when rain falls, instead of soaking into the soil and evaporating back into the atmosphere, carrying heat with it, water flows over paved surfaces, into sewers, and ultimately into the city’s wastewater infrastructure. The heat that would have been transferred to the atmosphere instead remains in the city, which becomes hotter as a result. Second, paved surfaces such as asphalt and roofs are commonly dark in color. Darker colors absorb more solar radiation, and things become warmer still. In fact, if you are looking to design a building to result in cooler temperatures, using a more reflective roof will earn you credits toward LEED certification. Green roofs have also emerged as a way to make cities cooler and more pleasant in general.
Cities today still have a lot of dark, paved surfaces, however. The upshot of these effects is that urban areas are, on average, warmer than rural areas, as shown in this image of surface temperature in the greater Boston area. (Red is warmer, blue is cooler.)
Since tree phenology depends to a large extent on temperature, the way humans have engineered the climate by creating cities also has effects on plant phenology. In particular, warmer temperatures lead to an earlier start of trees’ growing seasons, when they sprout leaves. This image, created by David Miller for his project in the Harvard Forest Summer Research Program in Ecology, clearly shows that “Start of season”, or when leaves appear on trees, comes earlier in Boston, and close to Boston, than outlying areas tens of kilometers away, that are less affected by Boston’s urban heat island.
So now you know why spring comes earlier in the city than in the country, and you’ve got plenty of knowledge to impress your family with this holiday season. There are other reasons, though, that phenology can be different from place to place, over even larger spatial scales than urban to rural gradients, and I’ll discuss some of these in my next post.