- Author: Rob York
Article reviewed: Density effects on giant sequoia (Sequoiadendron giganteum) growth through 22 years: Implications for restoration and plantation management
By R. York, K O’Hara, and J. Battles, published in Western Journal of Applied Forestry, vol 28: 30-36
The plot line: This study controlled the number of giant sequoia seedlings in a given area and measured the effect of the different densities on growth through 22 years. The researchers found that giant sequoia can grow very fast when density is low and that it grows very slow when density is high. This is a fairly typical result for most species, but giant sequoia had an exceptionally large difference in its growth under high and low density environments. The researchers relate the results to giant sequoia’s adaptation to growing in recently disturbed and open environments (i.e. it is a “pioneer species”), and make suggestions for managers desiring to alter the way that young giant sequoia forests grow. They conclude that giant sequoia can be “trained” to grow large quickly by thinning or prescribed burning early on and thinning to wide spacing compared to other species.
Relevant quote: “…large stem size can be achieved relatively quickly with low densities, producing large carbon reserves per tree (potentially the largest possible individual tree reserve on the planet) with relatively low risk of loss from fire or disease. Put simply, giant sequoia can be managed for a variety of objectives.”
Relevance to landowners and stakeholders:
This is a traditionally designed experiment applied to a very unique species. Experiments like this are usually designed for species that have commercial value because they can help understand the long-term effects of density management (i.e. planting and/or thinning) on timber production. While giant sequoia has potential to be an important commercial species, it is mostly known for its standing as the largest tree species in the world. Because humans have removed fire- the process that sustains giant sequoia, regeneration has declined within native groves. While some fire has been re-introduced, both the rate of re-introduction and the types of fire often fall short in terms of facilitating giant sequoia regeneration. For vigorous and dense stands of giant sequoia that actually have become established, this study can help inform decisions about whether to alter the development of the giant sequoia stands with further treatments such as thinning or burning.
The relevance for landowners and stakeholders is this paper’s reminder that giant sequoia is “disturbance dependent.” As discussed in previous entries, it needs a pretty large disturbance to the canopy in order to regenerate. Even the new forest made of small giant sequoia is adapted to further disturbances. In managed areas, this can mean thinning or prescribed fire. While giant sequoia is pretty good at competing with other species once established, its growth rate can be severely curtailed if left under high density.
Relevance to managers:
For managers who plant giant sequoia outside of groves and intend on managing it for large size (i.e. for timber, carbon, or assisted migration), the relevance is pretty clear: give it lots of room to grow. This means either planting at low density and controlling competing vegetation, or thinning relatively early. The researchers suggest that the widest spacing used in this study, 20 feet, was the best in terms of growing large trees without losing much in total stand volume. The optimal spacing may have been even wider had an even wider spacing been used. To sustain rapid growth in dense plantations, thinning would be applied around year 10 on a productive site. Sequoias seem to occupy the underground growing space very quickly. Even if crowns are not close to overlapping, it is likely that the roots of adjacent trees are competing heavily for water and nutrients.
For native grove managers, the relevance is to pay attention to the dense stands of giant sequoia that we do have. While more research is needed to find out the effects of burning frequency and severity on these young stands, I believe that fire does have an important role to play in their development (and if fire is not feasible, then thinning). Those who disagree would cite examples of dense giant sequoia stands developing just fine in pure, high-density conditions. But these stands may also be vulnerable to high severity fire and their capacity to be resilient in the face of climate change is uncertain at best. Some are also concerned with fires killing young giant sequoia that can be viewed of as precious given the past lack of regeneration following fire suppression. This and other studies show, however, that giant sequoias can release very quickly from disturbances that lower density. If there are so few giant sequoias present that a prescribed fire could endanger them all in a given area, then the density was probably too low to begin with. I have observed dense patches of giant sequoia surviving moderate intensity fires just fine, with the outer perimeter trees dying but acting like buffers and protecting those trees within the patch.
Another interesting note from this study is the incredible production of branches by giant sequoia. Branches are small but very dense, measured at an average of 17 branches per year. Compare this to ponderosa pine, which is more like 4 to 6 per year.
Critique (I always have one, no matter how good the article is):
It should be noted that the experiment did not include fire as a treatment. So the paper’s discussion of fire used to thin dense giant sequoia stands is speculative. The study also did not include a thinning treatment, so the discussion of thinning is also limited to the extent that planting density effects can be related to thinning. The experiment was also done on a productive site. The results probably would have been different on a lower productivity site.
This study was the brain child of Bob Heald, who I am sure understood that the more interesting results of the study would come along well after he retired. Such is the nature managing or studying forests. The legacy of a forester’s decision lives on well past the forester.