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You Asked: Notice fewer bright leaves this fall? Blame the weather

Boise might not be a magnet for fall leaf watchers like New England or Virginia’s Blue Ridge Parkway, but we have our own bright colors to boast in the high desert. 

But, this year, abrupt changes in temperature could be behind less vibrant colors than Boiseans are used to seeing in the City of Trees. 

While temperatures in September and October were higher than normal, they sharply dropped below average in November. This steep decline into winter, instead of the steady slide downward in temperatures we normally see, likely had an impact on the internal workings of the trees and the various chemicals inside that create the yellows and reds that normally adorn the sidewalk during fall. 

There is no data BoiseDev is aware of correlating swings in fall temperatures with leaf color or historical information collected on if this is an ongoing trend in the Treasure Valley as climate change impacts weather patterns. But, the combination of temperatures, sunlight and when spring starts in Idaho are behind what color the leaves turn and if they drop at all like usual. 

How does the weather impact leaves?

To get some insight into the science of fall leaves, BoiseDev interviewed Boise State Biological Sciences Professor Marcelo Serpe. 

He explained that prolonged periods of cool temperatures below 50 degrees, but still above freezing combined with the bright and sunny days of late September and October brings out the famous colors in the foliage. If you don’t have enough bright days, or the proper mid-range temperatures, it lessens the impact of the colors. 

“Let’s say now, during the early fall that did not happen because it was warmer than usual,” Serpe said. “So, the temperatures during the day when you have sunny conditions are above that range. If they are above 60 or 70 degrees that process will not occur. It will occur, but to a lesser extent so you have less of these pigments. That will be a reason for having less intensity of colors in the leaves.”

Data from the National Weather Service shows a steep decline into below-freezing temperatures, without much time in the mid-range to allow the pigments in the leaves to brighten. Early September had highs of over 100 on some days, where the normal range for temperatures in that period tops out at 86 on September 1. This warm trend continued into October where temperatures stayed warmer than average. 

Things changed drastically in November when temperatures hit below average. While the average temperature at the beginning of the month at the Boise Airport is usually 56 and drops to the low 40s by the end of the month, there were many days in November when the high temperature didn’t leave the 30s. 

Temps impact leaves dropping as well

Abnormal temperatures also impact how leaves fall off of the trees as well.
When temperatures fall below 50 degrees but stay above freezing and the days start getting shorter, leave start producing a specific chemical in the leaves that makes the stem start to separate from the tree. But, if the temperature rapidly drops, that process doesn’t happen and leaves hang onto the trees. 

This can also have an impact on how the trees start storing nutrients for the winter. Serpe said he couldn’t speculate about how this could impact the long-term health of the trees because of the myriad of factors at play, but it does disrupt the plants typical cycle. 

“That green leaf has a lot of nitrogen and phosphorus and all of this process of changing color, during that time the plant can breakdown some of these larger molecules and send the nutrients to other parts of the plant,” he said. “Before they drop their leaves, they will recycle up to 90% of the nutrients present in the leaves and they will be going into the roots or the bark of the plant. The importance of this is it allows more recycling of nutrients present in the leaves.”

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Margaret Carmel - BoiseDev Sr. Reporter
Margaret Carmel - BoiseDev Sr. Reporter
Margaret Carmel is a BoiseDev reporter focused on the City of Boise, housing, homelessness and growth. Contact her at [email protected] or by phone at (757)705-8066.

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