Seeds beget giants

[Originally posted on www.foreststeward.com on Jan 15, 2011]

Article Reviewed: Giant Sequoia (Sequoiadendron giganteum) Regeneration in Experimental Canopy Gaps

By R.A. York, J.J. Battles, A.K. Eschtruth, and F.G. Schurr. Published in Restoration Ecology Vol. 19, 1 (2011) pp. 14-23. Available for open-access.

Plot line: These researchers created canopy gaps within a giant sequoia forest and then planted seedlings of giant sequoia within the gaps. They wanted to see how giant sequoia survived and then grew in different sized gaps (ranging from 1/8 to 1 acre). They also measured how seedlings grew in different positions within the gaps, some positions being shady (near gap edges) and other positions being sunny (near gap centers). They found that, while seedlings usually died if they were not underneath any canopy gap, they only needed the smallest size gap to survive at the same rate as larger sized gaps. Seedlings grew a lot more when gap size was increased to about ½ acre in size, but did not increase when gaps were greater than ½ acre. When planted in an ash substrate following burning, seedlings grew twice as much compared to seedlings planted in unburned soil. They conclude that canopy gaps are necessary for restoring giant sequoia regeneration, and that increasing canopy gap size up to ½ acre can benefit growth (but not necessarily survival).

Relevant Quote: “For giant sequoia and other long-lived species around the world, locally-severe disturbances are an important factor of their persistence and hence restoration.”

Relevance to landowners/stakeholders

Over the past century or so, there have been far fewer giant sequoias reproducing than what would be expected from a self-sustaining population. Fortunately, the primary reason is obvious and in theory should be addressable with restoration programs. The culprit is fire suppression. Giant sequoias regenerate following disturbances (like fire) that kill or remove several trees that are big enough to create a discernable gap in the forest canopy (I like to think of a “gap” as being big enough to allow new trees to regenerate, but not so big that the center of the gap is uninfluenced by the shading and roots of the surrounding trees). A reduction in fires (which used to occur every 12 years or so in the Sierra Nevada) has led to fewer canopy gaps, thus erasing the conditions needed for giant sequoia regeneration.

Restoring fires in order to promote giant sequoia regeneration, however, can be a formidable task. This study suggests that canopy gaps need to be “sweet-spots” of both light and soil moisture in order for giant sequoia to regenerate and then grow well. 1/8 or ½ acre doesn’t sound very big, but creating gaps this big with a fire takes a pretty hot fire- one that might be “out of prescription” if burning near sensitive areas where escapes are unacceptable. On the other hand, there are a lot of good “burn bosses” out there (especially on federal land where giant sequoia are) who seem to be able to conduct fires that are patchy in nature and that can indeed create canopy gaps that are of sufficient size.

Of course, one could create gaps artificially (as was done in this study) with mechanical treatments that remove trees. But this option is often not available because of other competing objectives. On the other hand, fires may not be an option if near areas that are sensitive to smoke or if areas can not accept any risk of escape. Some type of disturbance that creates distinct canopy gaps via the death or removal of several trees is a prerequisite for giant sequoia regeneration. Since giant sequoias live thousands of years, restoration projects should easily be able to “replace” the seedlings that have not been regenerating over the past century.

Relevance to managers

The relationship between gap size and growth is asymptotic. As gap size increases, so does giant sequoia seedling growth. But the benefit of larger gap size diminishes and then levels off. In this study, it didn’t benefit giant sequoia seedling growth to have gaps larger than about ½ acre. This relationship could change in the future as the seedlings grow into the canopy, but a related study of giant sequoia by the same authors have tracked the same asymptotic relationship through 12 years and counting.

Maximizing growth may not be an objective even within the context of restoring giant sequoia seedlings. Survival may be more important than growth if the seedlings will eventually recruit into the canopy. In this study there was no relationship between survival and growth. So seedlings survived as well in the big gaps as they did in the small gaps. But gap presence was necessary for survival. Virtually all seedlings planted underneath the dense canopy died.

The differences between seedlings planted in ash and bare soil are striking. The ash-planted seedlings appear to be men among boys. They are bigger and their color is much better (this can only be appreciated by those managers who are used to looking at giant sequoia seedlings). Another study reviewed earlier found similar results and suggests increased nutrient availability as the reason for the increased growth within ash substrates.

Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:

Although the results are sometimes discussed within the context of using fire to restore giant sequoia regeneration, it should be noted that fires were not used in this study to create the canopy gaps. The ash substrate was created by placing harvest debris into piles and then burning them. While the experimental treatment attempted to mimic the disturbance severity that might be achieved with fire, there will undoubtedly be some differences in giant sequoia regeneration when fires are used to create the gaps. Most notable is that a much higher amount of seed production is expected following an intense fire.

There was only one site used in this study. The results may have been different if the study had been done closer to the edge of the species’ range, or on a different aspect or elevation.

The graph that summarizes the growth response to light and soil moisture is pretty (below), but it should be noted that a surrogate for soil moisture was used. They didn’t measure soil moisture directly, but instead used distance from edge as a proxy. They have other data in the study that suggests this is a reasonable thing to do, but it would have been even more powerful had they measured soil moisture directly.