Seasons, sinks and sources

(This topic continues the project I started with this earlier post on seasonal variation.)

Every few years, in southern California, I watch vast areas go up in flames. A small, recent, and well-contained example from August 2009 is shown here:

I make no claim to know if this is more frequent in recent years, but regardless, frequent burning is part of the ecology here. A common land cover called chaparral gets its name from the Spanish word for fire; it's dominant plant, chamise, is also called greasewood and burns easily. It's probably safe to assume that ignition sources have increased with the increased population here (as has fire suppression efforts?).

Less dramatic than the release of stored carbon by fire, is the seasonal transition from a carbon sequestering state (the spring and summer growing season) to a carbon emitting state (in autumn when carbon sequestered in new growth is less than CO2 exhaled by animals).

Chaparral, also called Elfin Forest, in July; the tallest bushes are about 12-14 ft high

Some plants in this photo are: 1) sugar bush (left foreground, low green and flowering; 2) California Everlasting dried short herbs in center--they smell like maple syrup; 3) right foreground, manzanita; 4) center background and still green, chamise; and 5) I'm looking up the tallest bushes that dominate this scene.)

Chaparral in October

I hope these photos photo shows the color and dryness of the region. Though the July photo is noticably greener, it marks the beginning of a dry season that continues through October on till when the winter rains return. (October has been quite a fire season in recent years.)

Whereas I used to watch seasons in terms of plant turnover, change in flowering and color, I now to try to imagine where my observations reside in a biome's cycle between CO2 sink (growth season) and source (decay are dormant season). Thus, I'm intrigued by efforts to quanitify the extent to which the biosphere acts as a sink or emitter of carbon and CO2. This topic has gotten quite lively with a recent paper published in Nature and discussed by RealClimate.

Two years ago I studied a review article and paper in the Jan. 3, 2008, issue of Nature that measured the sink/source effects in terms of zero crossing dates that represent the tipping point between spring-summer growth (spring zero crossing date) and between autumn-winter decay (autumn zero crossing date). I've redrawn and embellished the diagram from the original review.

The blue line shows a yearly cycle of atmospheric CO2 levels. Roughly, January would be on the far left and December on the far right. The orange line is the long term CO2 level that rises a couple parts per million each year. Warmer climate can extend spring by making it start earlier (e.g., earlier snow melt) or end later (e.g., more rainfall). Likewise, warming can make the dryness of late summer start earlier and extend longer. The illustration shows an examination of just the spring zero crossing date. A shift depending one which season (growing or decay) gets longer relative to the other can result in more or less atmospheric CO2.

 Assuming I understood the research correctly, warming (and other consequences of warming, such as changing rainfall patterns) will affect various regions differently. Some will get longer springs, thus longer growing seasons and more CO2 sequestration. Others will get longer Augusts, where growth stops, decay begins, and animal respiration dominates the CO2 cycle.

The paper I read is based on a study about a dozen locations in the northern hemisphere (update 2/21/10; added illustration):

Generally, the sites in North America were showing longer Autums (thus acting as net emitters of CO2) ; and sites in Europe were getting longer Springs (thus acting as net sinks of CO2). It is an ongoing question how regions respond globally, which will bring me to the next article I'm trying to understand, which is on the speed of climate change.

Poisen Ivy on Palomar Moutain in October