Groves of Tall Redwoods – Changes over Decades, Centuries, and Millennia

1       Tall Redwood Groves – Recent Changes in Height

Redwood tree height measurement came into its own in the 1990’s.  Skilled researchers and naturalists combined laser rangefinder technology, LiDAR height estimation, hiking and climbing skill, and direct tape drop from the canopy to create a nearly complete inventory of tall redwood trees throughout their range (with the exceptions of Six Rivers National Forest and Headwaters Reserve, which have not been thoroughly assessed). From this it was determined redwoods over 100 meters (328 feet) in height were uncommon, totaling about 2,000 trees.  And redwoods over 350 feet (106.7 meters) in height were very uncommon, totaling about 230 trees.  Each tall tree is remeasured every five years or so, with the tallest trees having more frequent measurements.

 I don’t have direct access to the 15-25 years of longitudinal height data for tall redwoods but through research I was able to find height information for the tallest known trees in the year 2000 as well as their remeasured heights as of 2015.  There were 129 trees over 350 feet on the 2000 list, indicating many more 350-foot redwoods were yet to be identified, particularly in Redwood National Park.  Here are the height changes for these 129 trees in inches growth per year (parks with smaller tree counts are excluded) on the left axis and 2015 height on the right axis (line).

On the chart the trees are grouped by Bull Creek (Humboldt Redwoods along Bull Creek), Eel River (Humboldt Redwoods along Eel River including Rockefeller Loop), Montgomery (Montgomery Woods State Natural Reserve) and Redwood (Redwood National Park).

Height Change (left axis and bars) and 2015 Height (right axis and line)

It is evident in all four areas most trees had height increases over fifteen years.  Nine trees lost height, a few in each area.  Median height growth was about three inches per year.  Over the past few years two of the trees in the data have fallen.

The chart also includes tree height in 2015.  Correlation between 2015 height and height change is 0.53, correlation between 2000 height and height change is 0.02.  So height change was not related to initial starting height.

Here is summary data related to the chart.

Height Changes Summary

This data indicates the canopy of very tall redwood trees increased three feet from 2000 to 2015.   The canopy in 2000 was the result of several thousand years of forest development. Then why did the tallest existing redwood trees increase on average another three feet in height from 2000 to 2015?  Some potential contributing factors:

  • More sun reaching the leaves of edge habitat trees due to cutting of nearby trees from 1860-1979.
  • Fire suppression
  • Increased atmospheric CO2 providing more energy for photosynthesis

2       The Future of These Tall Trees

Based on plot information, 350-foot tall redwoods are between 700 and 2000 years old, with the median age 1180 years.  It is very likely almost all of the current 350-foot tall redwoods will fall over the next 1000 years, being replaced by grow in from trees currently under 350 feet and trees yet to sprout.

Let’s test this against the known 350-foot trees to fall in the last 30 years in Humboldt Redwoods State Park.

Humboldt Redwoods Fallen Trees

It can be expected about ten percent of the tallest redwoods will fall every 100 years, with 3-4 trees falling each decade.

Before falling many trees will lose and regain height over time.  About two thirds of the 350-foot redwoods in study plots have reiterated tops.

3       What is the Maximum Height for Redwoods

The current tallest tree is about 381 feet tall.  There are four trees around 375 feet tall or taller, and all four are gaining height.  Then the data shows the canopy for very tall trees in general is increasing by about one foot every five years.

So will there be 385-foot redwoods in 20 years?  Could be.

Will there be 390-foot redwoods in 50 years, and 400-foot redwoods in 100 years?  Maybe.  The theoretical maximum height based on tree structure and physics has been calculated to be 425 feet.  There is no reliable historic record for a redwood tree over 400 feet in height.  One was measured right at 400 feet about fifty years ago along Wilson Creek by an expert timber cruiser.

Will some second growth Douglas Fir beat all the redwoods and get to 400 feet first?  Maybe, but then again the reliable historic maximum height for Douglas Fir is 393 feet.

Thanks for reading.