A week or so ago a large redwood tree growing near my home here in Big Sur lost its top. The wind blew hard and broke off the upper part of the redwood as shown in Photo 1. In many places around Big Sur ancient redwood trees have lost their tops during the winter storms. Some of these tops are more than three feet in diameter and you don’t want to be around when they come crashing to the ground.
I realize, of course, that it is not unusual for large trees to succumb to high winds, but what does seem unusual is that in some groves nearly half of the ancient trees have lost their tops within the past 20 years or so. By all appearances these redwoods have grown healthily together in these groves for three, four, five centuries or more, so why are they suddenly losing their tops?
The answer is not too hard to figure out. Yes, there is acid rain falling in Big Sur (see here, here, and here) and that no doubt has some effect on the redwood ecosystem. But recent changes in land practices, most notably fire suppression, are causing dramatic shifts in the successional status of the redwood forests. In the past the native people set fires that revitalized the soil and kept the young redwoods from crowding out the older ones.
It is easy to see from the remnants of numerous large branches extending the length of the trunks that the ancient redwoods grew up in much more open canopy conditions. Photo 2 shows that healthy redwood groves can still be found here in Big Sur where fire and grazing around the trees has inhibited the spread of young redwoods.
However, in many other places around Big Sur redwood groves that for centuries supported a few dozen mature trees are now being overrun by many hundreds of young redwoods, most of them less than 50 years old (Photo 3). The majority of these young redwoods are literally offshoots from a nearby tree and are fed, in part, by the parent tree.
But the nutrients, water, and light needed to sustain a redwood grove are all in finite supply. So the more the young redwoods grow and prosper, the less resources are available for the older trees. When conditions become too difficult, top decline sets in among the older, larger trees.
Photo 4 shows a couple mature redwoods with the first hints of top dieback. Top dieback is a widely observed phenomenon that probably results from root/nutrient/water dynamics in the soil creating problems in water transport. The trees can’t get water to their highest branches.
Note also in Photo 4 that the redwood trees are starting to show top dieback among the lateral branches just below the tops. At the very top of a tree is the apical meristem tissue. Since the apical meristem is the critical tissue for upward growth, the tree is likely sacrificing some of the branches immediately beneath in order to maintain the flow of resources to the top.
There are wise land owners in Big Sur who are very aware of these problems and are taking measures to thin the redwood groves so to limit the competition from so many young trees. In the event that fires burn, the thinning reduces fuel loads and lessens the intensity of the fires, giving the mature trees a better chance at survival. These and other fire mimicry practices are ensuring that many of the ancient redwoods here in Big Sur will not be dying on our watch.