Tuesday, April 14, 2015

Growing Season Length

A couple of weeks ago I saw an article about birch tree health in Fairbanks, and a statement about the growing season caught my eye.  According to the article, the growing season has increased in length by 45% in the past century.  This struck me as a surprisingly large number, so of course I had to investigate.

The chart below shows the annual length of the growing season in Fairbanks, with the beginning and end of the season defined as the last and first occurrences of either a 32 °F frost or a 27 °F freeze.  Of course this is for the official Fairbanks climate station (the airport over most of the history); outlying areas will often have a significantly shorter growing season and may show different long-term trends.  The thin lines show the annual values, and the bold lines indicate the trailing 15-year medians.

It's clear that the growing season has increased in length over the last 85 years, but according to the Fairbanks climate record it's not a 45% increase.  Taking the difference between the minimum and maximum 15-year median values, the greatest increase that we could claim would be 22 days (21%) for the 32 °F threshold or 26 days (20%) for the 27 °F threshold.  I think it's possible that the 45% number refers to the change in total growing degree days within the frost-free season, but I'll have to look at that another time.

As an aside, it's worth noting that the growing season has expanded more on the autumn side than on the spring side.  In fact, the 85-year linear trend in the first frost (32 °F) date shows a remarkable gain of 1.9 days per decade in the autumn, compared to a change of 0.9 days per decade in the spring.


  1. A search yields the definition of Growing Degree Days: http://en.m.wikipedia.org/wiki/Growing_degree-day. It's basically like Heating Degree Days but with the base temperature the temperature where the plant experiences zero growth. The base is usually 50℉. So the 45% increase should look very similar to the increase in hearing degree days.

    I personally find this metric interesting since it assumes a linear one-to-one response of plant growth and temperature which is obviously lacking. Plants are very non-linear in their growth as anyone who doesn't have a green thumb would show you.

  2. It would seem that growing season for Birch (Betula sp.) could be determined by the time between green-up and yellow-up of the leaves. Maybe. I'm not a tree guy, just their slave who cleans up after them.

    There is a mention of a 55* F July isotherm as a limiting factor in their distribution in northern latitudes and altitudes. That range may change as temperatures warm:


    Forest migration is an interesting topic that has impacted Northern latitudes since the Last Glacial Maximum 20K+ years before present. Migration via the distribution of future progeny is an interesting concept: