- Author: Rob York
[originally posted at www.foreststeward.com on June 14, 2010]
Article reviewed: Radial growth responses to gap creation in large, old Sequoiadendron giganteum
By R.A. York, D. Fuchs, J.J. Battles, and S.L. Stephens published in Applied Vegetation Science In Press, full text available at the bottom of this page.
The plot line: Large, old giant sequoia trees in a native grove of the southern Sierra Nevada were measured to see if they increased in growth (i.e. to see if they “released”) following harvesting of smaller adjacent trees. The researchers found that giant sequoia trees adjacent to harvested openings grew more than similar trees that were not adjacent to harvested openings. They conclude that, even when giant sequoia trees are massive and very old, they maintain their capacity to increase their growth rates when nearby competing vegetation is removed. The authors suggest that moderate-severity disturbances (i.e. when patches of trees die from fires or harvesting) can increase growth rates (and possibly the overall vigor) of old giant sequoia trees that persist through the disturbance.
Relevant quote: “Despite their great age and massive size, old S. giganteum responded positively and with surprising sensitivity to the creation of adjacent canopy gaps. The response occurred quickly and was sustained for the decade following gap creation.
Relevance to landowners and stakeholders:
Large giant sequoia trees are a sight to behold. Even the most utilitarian of souls can appreciate the beauty of the largest trees on earth. In a state known for its eccentricity, it is fitting that California’s iconic native tree should also be an ecological outlier. It is an outlier not only because of its huge size, but also because of the unique way in which it survives (i.e. it’s “life history strategy”). Tree species can usually either grow very fast OR live a very long time, but not both (think baseball: a player can focus on hitting home runs OR hitting for average, but usually not both). Like with baseball, however, there are a handful of exceptions. Giant sequoia is an extreme exception. Its potential growth rate is much higher than the tree species it lives with, and its potential longevity is much greater by a long shot. It is like a baseball player who is a man among boys, hitting for both power and average. In other words,
Giant sequoia is the Babe Ruth of tree species.
It is no wonder, therefore, that people pay a lot of attention to management activities that influence the iconic giant sequoia (there’s no Giant Sequoia candy bar, but there is an SUV named after it). In this article, the researchers documented that even very large giant sequoia trees respond positively in terms of stem radial growth when competing vegetation is removed. Recently, a lot of attention has been given to the capacity of large trees to grow very fast because large and fast growing trees can have several benefits that are relevant for today’s forest management objectives. Such trees can be more resistant to fire, they can pack on lots of carbon, and they are important for wildlife habitat.
Relevance to managers:
Removing surrounding vegetation (via thinning or burning, for example) in order to release trees is nothing new. But often managers tend to think of tree release in the context of trees in regenerating or young stands. Removing competing shrubs and thinning trees are common ways to make individual trees grow faster when they are young. What seems to be surprising about this research is the fact that even very old and huge trees released much like a sapling or young tree would release. It is sometimes assumed that very old trees are “decadent” and therefore do not have much capacity to release. But in the case of giant sequoia and many other long-lived trees, they remain young at heart and can grow faster when surrounding trees are removed. While this study observed release following harvesting, similar release events have been observed following fires that remove adjacent vegetation.
The authors speculate that the reason for the increased growth is increased water and/or nitrogen availability. The implication is that the root systems of the large giant sequoia overlap with the root systems of the surrounding, smaller trees. Giant sequoia is known to have a two-tiered root system, with the capacity to suck water from both deep sources and widely-spread shallow sources.
The harvests in this case were group selection harvests, where up to ¾ acre areas were cleared of all vegetation. The large giant sequoia right on the edges of the openings were the trees that were measured. It is unclear whether lower intensity thinning would cause the same release, but it would be reasonable to predict that lower-intensity thinning would result in lower-degree release.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
The study was done in one location. If the study had been done in other native grove locations, especially where conditions were wetter, results may have been different.
It would have helped if the authors calculated what the growth response meant in terms of increased carbon gain for the trees. The actual increase in radial growth was on the order of 2 millimeters a year. What does this correspond to in terms of total carbon per year on the whole tree?
It is fair to question whether or not these increased radial growth increases actually correspond with increased vigor. It is true that faster growing trees tend to have a lower likelihood of dying. It could therefore be the case that the releasing trees have greater vigor. On the other hand, the authors bring up the possibility that the trees are simply changing their growth pattern to grow more in stem girth instead of growing taller (as a response to being exposed to wind, perhaps). In other words, it is possible that the trees are re-allocating carbon differently and not increasing the total amount of carbon that is allotted./span>