Global Change Education
GLOBE Green-up/down Phenology Studies: Green-Up
Plant Phenology – Looking At Your Data
Are the data reasonable?
The first step in looking at plant phenology data is to see if the data seem reasonable and make sense. Is the green-up leaf length always greater than or equal to previous measurements? . Looking at a graph of these data, such as figure 1, makes it easy to check. Notice from the following figure that leaf length on May 22 is less than May 19. Either the leaf measurement on May 19 or May 22 is probably in error.
Another potential problem is illustrated using Student B's data in the following figure.
Notice that there are not enough measurements showing that leaf growth stopped. Has leaf growth reached 100 percent by May 24, or will it continue for weeks? It is impossible to tell unless there at least 3 measurements showing that leaf length has reached a constant.
Student C's data have 2 problems: 1) it is impossible to estimate when budburst occurred because the student did not record at least 3 dates prior to budburst. 2) it is impossible to estimate percent of leaf growth from the data since there are not at least 3 measurements showing that leaf growth has stopped at the end of May.
What do people look for in these data?
In plant phenology data, most interest is in how plants are responding to climate. If the plant is greening-up in a response to warming temperature, growing degree days are sometimes calculated to predict plant green-up.
Growing degree days is the the summation of degrees above a threshold temperature. For example, if we set a threshold temperature of 0 C, then the growing degree days from days having temperatures of 2, 2, 3, 4, 5, and 6 degrees C would be the summation of these, or 2+2+3+4+5+6 = 22 growing degree days.
Different plant species will have a different green-up response to climate. Since plant phenology estimates from remote sensing satellites are primarily from large, natural vegetation areas, it is critical that your plant selected for phenology observations is from natural vegetation.
Student researchers should consider comparing temperatures or precipitation with dates of budburst and leaf growth rates from different schools.
An example of a student research investigation
A student at the school in Waynesboro, PA selects a red maple tree for the Budburst Special Measurement. She observes budburst on 31-March-99. She looks at other GLOBE schools that have used Red Maple for the Budburst Special Measurement and finds that Pashley Elementary School in Glenville, NY observed budburst of red maple on 1-May-1999.
As a simple starting point for her research she hypothesizes that: Budburst was earlier at my school because the climate is warmer here compared to the other school.
Since temperature data have already been collected by the other GLOBE school, she simply needs to get their data and does not need to take further measurements herself. She downloads the temperature data from Pashley Elementary school. For both schools, she creates a data table with minimum and maximum temperatures from January 1 to May 1, 1999. Some days do not have recorded minimum or maximum temperatures. Therefore the next step is to fill in these missing values as averages from recorded temperatures. For example, there was no temperature recorded on Januay 16, 1999 at Pashley Elementary School. The max/min temperature for January 15 was 0/-20 and the max/min temperature for January 17 was 5/-12. Therefore the estimated minimum temperature for Janary 16 is -20 + -12 / 2 = -16 . The estimate maximum temperature for January 16 is 0 + 5 / 2 = 2.5. The missing temperature values are estimated for each school site using this method.
Once the missing temperature values have been estimated, the mean temperature for each day can be calculated as min + max temperature / 2. Then she calculates the degree day value for each school at budburst. For example, at her school, the degree day value was 341 degree-days when budburst occurred on 31-March-1999 (Table 1). A similar table was created from the Pashley Elementary School data and the degree day value was 440 when the red maple burst bud on 1-May-1999. Pashley Elementary School is located in Glenville, NY.
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By plotting her cumulative degree days from both schools, she finds that
budburst at her school occurred on 31-March-1999 when the cumulative degree
days at her school was 341. Pashley Elementary School had only 140 growing
degree days accumulated on 31-March-1999.
This supports her hypothesis that budburst was earlier at her school because
the climate at her school was warmer than the climate at the Pashley Elementary
School.
Look at the figure above. Budburst did not occur at Glenville, NY until there was 440 degree days accumulated. Why did budburst at Glenville, NY not occur at around the same cumulative degree days as at Waynesboro, PA (341 degree days)? One possible reason is that the soil was colder at the Glenville, NY school site and therefore it took more cumulative degree days to heat the soil up to a temperature that is warm enough for budburst. To test this idea, soil temperature data are needed from the two school sites…unfortunately soil temperature data are available only from the Waynesboro school site and not the Glenville school site.
The student develops a second hypothesis: Budburst will require a greater cumulative degree days value at school sites that have later spring budburst. This is because the colder the climate, the colder the soil, and therefore more heat is required to warm the soil to a temperature that favors budburst.
To test this hypothesis, she could download air temperature data and budburst dates from a variety of schools. She then could plot cumulative degree days as a function of date to test her second hypothesis. Or she might download data from sites that recorded soil temperatures and plot the date of budburst as a function of soil temperature.
Looking at the data
Graphing Leaf Growth
- After each green-up observation, graph the leaf length (on the y- or vertical axis) with date (on the x- or horizontal axis). Green-up is complete on the date when leaf length stops increasing (two consecutive observations of same leaf length and leaf length graph plateaus).
Calculating Percent Leaf Growth
- After the completion of leaf growth, calculate the percent of leaf growth for each observation date.
- Divide the length (mm) of each leaf for each observation by the length of the mature leaf (mm) that was measured at the end of your observations.
Example: If the leaf was 10mm long at one observation time and the mature leaf length is 200mm, then the percent of total leaf growth would be calculated as 10mm/200mm X 100 = 5 percent. Do not include leaf stem or petiole in your leaf length measurements.
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The Website for National Science Foundation Grant No. ES1-9910219
Last Modified on May 20, 2002 by Sidney Stephens
