Global Climate Change and Forests

Scientists use mathematical models to predict the growth of trees under different climate conditions. For example, if a scientist wanted to know the effect of a 3-degree change in minimum temperature, he/she might use a forest growth model to see what would happen. The model will give results that may or may not predict what will happen under real world conditions. Consequently, we use the model as a tool to guide future investigations.

You can make your own model to predict the effects of climate change on trees at your school. To do that you will measure bud break and growth in the spring. You will also need to measure daily minimum and maximum temperature (Tmin and Tmax).

Growing Degree Days

Growing Degree Days (GDD) are used to model the growth and development of plants during the growing season. The basic concept is that development occurs when outdoor temperatures exceed a minimum level, or base temperature (Tbase). Base temperatures are determined experimentally. We suggest you start with 60 degrees.

To calcualte GDD, first take the daily average temperature (Tmax-Tmin)/2. If the average temperature is at or below Tbase, then the GDD value is zero. If the average temperature is above Tbase, then the GDD value equals the average temperature minus Tbase. For example, if the average daily temperature is 65 degrees and Tbase is 60, then  the GDD value is 5.

Click here to see the data for the graphs shown below -

Developing your own model

Select a bud from a tree and begin making weekly measurements (in cm). Choose the same day each week to make your measurements. At first you wonít notice any change.

You will also need to record minimum and maximum daily temperatures. You can do this with your own thermometer, or get this information from the newspaper or from the internet.

After bud growth has started, you will want to make graphs of cumulative growth and cumulative GDD and then compare them. Choose a Tbase (we suggest 60) and see if GDD predicts growth. Look to see if the GDD line matches the Growth line in the graphs. If not, either the GDD base should be changed (perhaps 50 or 70), or the method just does not work.

Some questions to answer:

• Which tree species initiates growth the earliest? The latest?
• Is there and advantage to early growth? Disadvantage?
• Do all trees use the same Tbase?
• Add three degrees to your average daily temperature. How would this effect the timing of bud break on your tree?