Ask a Scientist
Submit your questions about plants, phenology, ecology, and more to our scientists and we'll post their answers below.
Read our Frequently Asked Questions and discover what other people are asking about.
Meet Our Scientists and learn how they became involved with plants and phenology.
Email your questions to: firstname.lastname@example.org!
Why do leaves change color in the fall? --3rd Grade Class
Courtney's Answer: This is a great question, because the changing leaves in Oregon can be really eye catching! The primary job of a leaf is to make sugar – just like your brain needs a constant supply of glucose sugar to stay alive, a plant also needs sugar to live. But whereas you and I need to eat to get sugar, a plant's leaves can make sugar with light, water, and a gas in the air called carbon dioxide. The leaves start off looking green because the main chemical in the leaf that makes sugar, called chlorophyll, absorbs blue and red light, but lets green light pass right through. When the days get shorter and the temperature drops in the autumn, there isn't enough light and water available for the plant to keep making sugar. Deciduous trees deal with this problem by breaking down the chlorophyll (it's full of nutrients), and moving those nutrients back into the twigs and stems to save for making next year's crop of leaves. When the green is gone, leaves will sometimes look yellow – these yellow chemicals (xanthophylls) were there all the time – but you can see them now because the green is no longer covering them up. These yellow chemicals are a natural sunscreen. Too much sun is even bad for plants! Some plants will make new sunscreen chemicals in the autumn as well, and for species like Vine Maple in Oregon, these chemicals are red (they're called anthocyanins). So as you can see, there are lots of really interesting reasons why plants change color in the autumn, and why they change different colors!
How much will a tree grow in a year, and how large will it get? --3rd Grade Class
Courtney's Answer: The questions concerning how much a tree can grow in a year, and how large a tree will ultimately get are interesting, and are a little complicated to answer. To start with, you can find out how much a tree grows in a year by measuring its diameter at breast height – breast height is defined as 4.5 feet above the ground – probably about head height for you 3rd graders. People have figured out how to use this diameter number to determine how much a tree weighs, and if you measure a tree's diameter at breast height year after year, you can then calculate how much it grew in a year by subtracting one year's mass number from a previous year's mass number.
How much a tree ends up growing in a year, and how tall it ultimately gets, both depend on a lot of things. Probably most important is what kind of tree it is – that is, what species it is. If you've been to any old growth forests (like the HJ Andrews forest near Blue River to the East of Eugene), you know that Douglas Fir trees grow very tall – sometimes over 250 ft! – but that Juniper trees in the desert East of the Cascades are rarely over 30 ft tall. But even if we're only talking about Douglas Firs, trees might grow at different rates, and reach different heights depending on whether they are growing in sites with lots of water, the right temperature, lots of nutrients (food), lots of light, or lots of pests. For example, if you go for a hike in a valley, you might find that trees are bigger and grow faster on the valley floor where there is more water and nutrients, but grow more slowly and be smaller on the valley slopes. Another example is that a 100 year-old Douglas Fir growing in Oregon could be over 100 ft. tall and be bigger than you could fit your arms around, whereas here in Colorado where I live, a 100 year old Douglas Fir would likely be much shorter and its trunk would be much smaller around. The difference in size is due to the fact that Colorado is much colder and drier than Oregon.