Human perception and autumn colors

This year seems to be the brightest and most spectacular display of autumn colors in a long time. But are we right in thinking so? We often perceive the world differently from it’s physical (absolute) reality. This is easily illustrated by various types of optical illustions or the fact that we do not feel absolute temperatures, only changes in temperature. Not only do our brains play tricks on our senses, they can also fail us when it comes to interpreting long term environmental change.

And this begs the question: can we analyze the intensity of this years autumn colours using a more quantitative approach? More so, was this autumn really as spectacular as most people seem to think?

In order to answer these questions we need to look at some PhenoCam data. Normally we look at the greenness signal, which is mostly driven by the presence of healthy green leaves. In this case however, I focused on how red the leaves were during the autumn. In the figure below you see a time series of canopy redness at the Bartlett Research Forest in New Hampshire. On the left is “RCC” which is calculated the same was as the greenness signal (GCC), but using red instead of green. In this graph you see two high peaks which are caused by leaves turning colour, from green to yellow and red, in the autumns of 2014 and 2015.time_series_rccThe tops of the peaks tell us how intense the colours were at their brightest. We can also calculate the intensity over a period of time by summing up the red intensity values over a period of a couple weeks around each peak. Higher numbers mean a more intense red color for a longer period of time. Below I show peak values (on the left) and the summed values (on the right). autumn_coloursWhen looking at the left graph, you can see that 2014 was actually the best red intensity year since 2007 (red line). However, if you also consider the duration of intense color, then 2015 wins by far (right graph, red line). At Bartlett Research Forest, autumn colors were especially intense and long lasting. This increases the lasting impressions on visitors that it was a spectacular fall. So in this case, our senses weren’t fooled – and we have the data to prove it!

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About Koen Hufkens

I’m a research associate at Harvard University where my time is divided between modelling plant phenology across various biomes in response to climate change, being the lab geek and intrepid open source hardware and software developer.
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