Canopy Height Distribution in Old Growth Redwood Forests

Methods

This article is related to an earlier article posted on this web site, “Distribution of Tree Height in An Old Growth Redwood Forest”.   An analysis of recent LiDAR data allows for confirmation and expansion of the analysis in that article.

LiDAR data includes first return and ground points.  From this information height above ground can be calculated, typically at the square meter level using average ground elevation and maximum first return elevation within the square meter.  From this, derivative products can be created, including color height maps and data tables. 

In order to use the LiDAR data for canopy height distribution, I aggregated the results to 20 m x 20 m (400 square meter) tiles.  This is used to approximate crown spacing in old growth redwood forests.  The maximum calculated height above ground within each 400 square meter area is used to create the distribution graphs.  In practice the number of tiles is slightly greater than the number of actual redwood trees, by about twenty percent.  This is due to the same tree being included in two tiles.

Overall Results

Most importantly, redwood groves have a variety of footprints but similar distributions of tree height beyond the median height for the grove.  Some groves are thin wisps along canyon bottoms (think Big Sur) versus others are broad forests filling both sides of large valleys (think Prairie Creek).  But for most groves the very tallest trees in the grove are much higher than average height and the number of trees drops markedly in each ten foot height increment beyond the grove median height.

Redwood forests are amazingly scarce and small.  Coming out of the latest glacial maximum redwoods were few in number, then filled in a shifting northward range that represented just 2,000,000 acres in a 500 miles long by average six miles wide thin rectangle just inland of the Pacific Ocean from southern Oregon to Big Sur. In this range the needed blend of rare winter freezes (as redwoods cannot propagate by seed in areas with regular winter freezes) and regular intraday offshore to onshore summer fog (as fog hydrates the tall crowns of redwoods but lots of sun is also needed) occurs. Today’s remaining 100,000 or so old growth acres cover about the same range but in remnant patches, almost all in parks. 

Second, redwoods grow tall in good habitat throughout their range. Three-hundred-foot redwoods can be found from Big Sur to southern Oregon, and 328-foot (100 meter) redwoods can be found from the Santa Cruz Mountains to the Smith River near Crescent City.  Then the 50 or so tallest, those over 360 feet, can be found in Humboldt Redwoods State Park (about 33), Redwood National Park (about 13), and Montgomery Woods Reserve (about 4).  Prairie Creek Redwoods State Park and Jedediah Smith Redwoods State Park may also each have one or so 360-foot trees.

Third, old growth redwoods follow similar height distributions beyond a midpoint which is much lower than the maximum height in any particular grove.  The median height in a grove is typically 250-275 feet tall, with 300-foot redwoods often common and redwoods over 340 feet relatively rare.  When reviewed at the hectare level, the tallest canopies are around 340 feet median height.

The shortest trees reaching the canopy may be about 150 feet tall, there is no zero point.  This makes the distribution a truncated normal distribution, not a normal distribution. The canopy height distribution does follow a near normal distribution to the right of the canopy height midpoint.

Fourth, the tallest redwoods tend to clump together within groves. Reasons could be local light advantage, a local underground water source, common genetic specificity, or exceptional local soil conditions. The majority of the tallest redwoods have locally tall trees adjacent or nearby. The tallest redwoods tend to grow in groups of two, three, or four, with their tops twenty to thirty feet above the other tree tops in the same area. This is especially true on flats where the canopies are continuous rather than on slopes where the canopies tend to be emergent.

Demonstrated Canopy Height Distribution

The first area assessed are the Bull Creek and Eel River groves in Humboldt Redwoods State Park.

Here the canopy is relatively continuous within groves (in other words very few canopy gaps) with the exception of a couple areas.   This is a LiDAR color height map along Bull Creek in the Tall Tree area (Tall Tree / Rockefeller Redwood is in upper right).  The area shown covers about 500m x 500m.  Yellow is over 80 meters, orange over 90 meters, and red over 100 meters.  Here you can see yellows and oranges dominate, with most tree heights in the 275-300-foot range.  Then here and there are taller trees, which tend to clump together.

HRSP Tall Tree Area Color Height Map of Canopy (Tall Tree is to Upper Right)

This same pattern is repeated over and over in these groves, resulting in this aggregate canopy height distribution map:

Humboldt Redwoods State Park Old Growth Eel River and Bull Creek Canopy Height Distribution

To highlight what is shown, this is the percent of the canopy over given heights:

Over 280 feet    51%

Over 300 feet    35%

Over 320 feet    16%

Over 330 feet    8.5%      

Over 340 feet    3.5%                                

Over 350 feet    1.1%

Over 360 feet    0.2%    

Over 370 feet    0.01%

A top five percent tree is 340 feet tall. 

Then the second area assessed is a drainage well north of Humboldt Redwoods State Park.  Here the canopy is emergent, there are large gaps with lower heights but certainly many areas with tall redwoods, typically following slopes along and above creeks, which provide good soil and some wind protection.  This is a LiDAR color height map of a portion of the valley, covering about a 500m x 500m area. Yellow is over 80 meters, orange over 90 meters, and red over 100 meters.  Here you can see yellows and oranges in groups separated by lower canopy, with most tree heights in the 250-260-foot range.  Then here and there are taller trees. The tall trees are less dense, more spread out, versus Humboldt Redwoods State Park.

Northern redwood park Color Height Map of canopy in creek valley

A dozen similar areas were then aggregated to create this height distribution chart:

Area of Northern Redwood Park Canopy Height Distribution

To highlight what is shown, here is the percent of canopy above certain heights:

Over 260 feet    54%

Over 280 feet    25%

Over 300 feet      7%

Over 310 feet      3%

Over 320 feet    1.2%

Over 330 feet    0.3%      

Over 340 feet    .07%                                

Over 350 feet    .03%

A top five percent tree is 310 feet tall.

Putting HRSP and the northern redwood park area together in a graph of ten-foot height intervals above median height, it is evident both follow a similar distribution up to the tallest trees.  This is likely true when reviewing other old growth redwood areas. 

Height Distribution Ten Foot Intervals Right of Median

Finally this is a chart showing the canopy cross section in the three tallest 100m x 100m (hectares) in Humboldt Redwoods State Park. As noted earlier, the tallest trees tend to grow adjacently and are twenty to thirty feet above the average height in the local (one hectare) area.

Height Distribution HRSP Tallest Hectares

Redwoods became rare coming out of the latest ice age, then old growth redwoods became much, much rarer after commercial harvest.  Then within remaining old growth areas, the tree heights follow a truncated normal distribution, with decreasing numbers of trees for each ten foot increment above the local median. Then the tallest trees, those over 340 feet, are very uncommon and tend to clump together in very specific areas having the most optimal growing conditions.

Thanks for reading.

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