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. :)