Restoration of Giant Sequoia: An Experimental Approach to Assessing Restoration Options
It seems unfathomable that the tiny seedlings Rob York sowed among ash piles in a clearing at Whitaker’s Forest could someday grow to be among the largest creatures on earth. Yet these green specks grew into giant sequoias two years after seeds were strewn in canopy gaps. This species of titan tree has stagnated in regeneration efforts for nearly a century. York, along with his graduate advisor, John Battles, is working on unlocking the secrets to growing new giants.
The giant sequoia is known as a long-lived pioneer species. Previous studies have demonstrated it depends on relatively large natural disturbances such as fires and falling trees to create gaps and soil conditions appropriate for the growth of new seedlings. York knew that the current giants in a particular grove developed in the wake of a man-made disturbance; the trees dominating the canopy in Whitaker’s Forest germinated following an incident of intensive logging and burning in the 1870s. By carefully removing canopy trees, thereby creating gaps in the recovering forest, York’s research attempted to mimic natural forest disturbances to grow the next generation of giant sequoias. The question was simple: Which conditions gave the tiny seedlings the best chance of survival and rapid growth?
By sowing seeds and planting seedlings in various surroundings, York identified that giant sequoia seedlings grew better as gap size increased, but that growth did not increase above gap sizes beyond 0.2 hectares. The resource-rich environments in the centers of gaps led to rapid growth of seedlings. York also experimented with soil conditions that might encourage giant sequoia growth. Soil with ash, which might be found after a fire sweeps through a forest and clears gaps in the foliage, increased seedling growth. The differences were subtle, but those slight distinctions in juvenile performance might determine the success of a tree’s lifetime growth. Germination rates are low for giant sequoia — only 50 of 442 seed spots planted grew any seedlings — so these slight changes in conditions could affect the very survival of the species.
Giant sequoia regeneration is an issue in nearly all of the remaining groves in their small range on the western slope of the Sierra Nevada. “Given its extreme longevity, the current giant sequoia population can easily absorb a century of missing regeneration,” York explained of the trees that can grow for 3,000 years, “but the consequences of continued regeneration failures will increase dramatically with time.”
— Rob York’s work has been submitted for publication in Restoration Ecology.