Is the Length of the Growing Season Changing in Chesapeake Bay?

Figure 1: A histogram and probability density distribution of the growing season length in the near-shore Chesapeake Bay region from 1901-2010. Note that the shape has an extended right-hand tail, indicating that the growing season length has had a few longer-than-normal periods.

Figure 1: A histogram and probability density distribution of the growing season length in the near-shore Chesapeake Bay region from 1901-2010. Note that the shape has an extended right-hand tail, indicating that the growing season length has had a few longer-than-normal periods.

The primary question driving this collaborative research project is “how will ecosystems within Chesapeake Bay be affected by climate variability and change?” One of the most relevant extreme climate indices in addressing this broad and complex question is the Growing Season Length.

What is the Growing Season Length? And why do we care?

The length of the growing season is just that: the duration of time when most commercial crops are capable of growing successfully. The growing season length could be an especially important economical parameter as it could provide farmers insight to any changes to the best time to plant (or harvest) their crops.

But crops are not the only group of organisms affected by the growing season length. Just think of all the critters which depend on the timing of a certain plant to bloom. For example, many migratory bird species depend of the availability of wild rice in Jug Bay, MD to sustain them on their journey. Wild rice in Jug Bay already has many ecological pressures, such as over-grazing by Canadian Geese or invasive plant species; thus changes to the growing season length could also affect the health of wild rice and by extension the abundance of food in the Atlantic Flyway.

Figure 2: The time series of the annual length of the growing season in the Chesapeake Bay regions of Maryland and Virginia. The dashed blue line is the mean length over the time series of 256.3 days.

Figure 2: The time series of the annual length of the growing season in the Chesapeake Bay regions of Maryland and Virginia. The dashed blue line is the mean length over the time series of 256.3 days.

Agriculturally, the growing season can be defined as the period of time: 1) after the last frost and before the first frost or 2)  when the temperature is above the minimum for a specific plant to germinate.

For this project, we will be using the Climdex definition of growing season length (GSL) for the Northern Hemisphere using daily mean temperatures. Thus, the GSL is the time between the first span of 6 consecutive days above 5°C and the first span after July 1st of 6 consecutive days less than 5°C.

 

This is a lengthy definition, but it makes perfect sense. 5°C, or ~40°F is the minimum temperature at which spring wheat can germinate. So, we can think of the Growing Season Length as the length of time that many important food crops can survive.

Figure 3: A 25-year rolling mean of the growing season length in the Chesapeake Bay region from 1901-2010 (light purple squares).  The dashed red line is the linear regression fit which had a statistically significant coefficient of determination (R2=0.51). The violet curve is a smoothed trend.

Figure 3: A 25-year rolling mean of the growing season length in the Chesapeake Bay region from 1901-2010 (purple squares). The dashed red line is the linear regression fit which had a statistically significant coefficient of determination (R2=0.51). The violet curve is a smoothed trend.

From Figure 2, there is an apparent upward trend in the growing season length of the near-shore Chesapeake Bay region from 1901-2010. This upward trend is further demonstrated by Figure 3 which shows a 25-year rolling mean. While this trend does have a cyclic pattern, a statistically significant linear trend is present.

In other words, since the start of the 20th century, the growing season length has been getting longer. Using the slope of the linear regression, we can approximate that the growing season length has been extended by ~8 days over the last century…that is more than a week longer than in the early 1900’s.

Figure 4: The probability distribution function of the growing season length in the Chesapeake Bay region from 1910-1960 (violet) and 1961-2010 (green).

Figure 4: The probability distribution function of the growing season length in the Chesapeake Bay region from 1910-1960 (violet) and 1961-2010 (green).

The probability density function of the first half of the 20th century (1910-1960) compared to the second half (1961-2010) shows a rightward shift in the mean of the growing season length (Figure 4). In this plot, the mean growing season length for the first 50 year span is 256 days and is shifted by three days to a mean of 259 days for the 1961-2010 period.

While more work will be done with these temperature climate extreme indices, it is apparent that certain trends are emerging and are in line with climate model future projections. A longer growing season length could affect aquatic ecosystems, and hopefully we can help provide some in sight on how these changes might manisfest.

Kari Pohl

About Kari Pohl

I am a post-doctoral researcher at NOAA and the University of Maryland (Center for Environmental Science at Horn Point Laboratory). My work investigates how climate variability and extremes affect the diverse ecosystems in Chesapeake Bay. I received a Ph.D. in oceanography from the University of Rhode Island (2014) and received a B.S. in Environmental Science and a B.A. in Chemistry from Roger Williams University (2009). When I am not busy being a scientist, my hobbies include running, watching (and often yelling at) the Boston Bruins, and taking photos of my cat.
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