Anza Design & Nursery

As I sit and write, in January of 2023, California is on its third or fourth week of large sequential storms from back to back to back atmospheric rivers. Much of California has received its annual average precipitation levels (and then some) in just the past month. The rain, snow and saturated moisture levels are of course a well needed reprieve following a multi-year exceptional drought cycle. While this past month's deluge is significant, it is still not an unprecedented event in the history of California. It only ranks maybe fourth of fifth in the history of California's recorded weather (think Great Flood of 1862). But it does come on the heels of 3 years of severe drought that was exceptional and unprecedented. This wet, wet winter may still be followed by yet another exceptionally hot and dry summer that could undermine the long-term benefits of this wet winter. And so goes the future of California - the wets may still be wet, if not significantly wet at times, but the hots will also be hotter. And drier. And longer in term. 

Over the years I have come to realize that the California flora is the result of plant species who have learned to cling to survival. Without doubt there are species that thrive under hot and dry conditions by virtue of their fitness for drought. Species like Eriogonum have adapted themselves to the hot and dry, which gives them an upper hand in a warming climate. But others, well, they have figured out how to not die under a progressively hotter and drier climate. They cling to existence by holding a delicate balance between a climate pattern that once was and the hotter and drier climate pattern that has followed. As such, their existence is wanning. To experience them today is to experience a gift from the past - a past long long ago, when the world was much different than today. 

Meet Giant Sequoia (Sequoiadendron gigantea).....the world's largest tree and one with the power stop you in your tracks, to humble your existence, and shift any notion that what we see today is not always how things once were. 

Today, there are roughly 75 giant sequoia groves throughout the western slope of the Sierra Nevada. Most of these groves occur throughout the southern Sierra Nevada. The southernmost groves occur southeast of Porterville while the northernmost grove occurs northeast of Auburn. But the bulk of the groves occur east of Fresno and Visalia in Kings Canyon and Sequoia National Parks. The groves east of Fresno and Visalia are the biggest and include some of the largest individual trees. 

Many of the roughly 75 groves of Giant Sequoia occur along an elevation band that ranges between 4,500 and 6,500 feet in elevation. In the northern portions of the range, the groves trend toward the lower elevations. In the southern portions of the range, the groves trend closer to the 6,500 foot elevation. This band strongly correlates to where sufficient snowfall, mild winter temperatures, summertime moisture, and a deep and loamy soil profile exist. The relatively abundant moisture and low evapotranspiration rates within this band approximate a climatic condition that occurred when Giant Sequoia had a more widespread range.

Today, the range extension for Giant Sequoia is fragmented into a patchwork of groves with somewhat large distances occurring between the northern groves. This fragmented patchwork of groves is believed to be the result of glaciation that occurred during the Pleistocene.

During the Pleistocene, large glaciers existed throughout the higher portions of the Sierra Nevada. Those glaciers scoured much of the downslope terrain. It's suspected the glacial scouring "cut" into what has been assumed to be continuous band of Giant Sequoia (and other conifers) along the western slope of the Sierra Nevada. Over time, as the climate became hotter and drier, successive reproduction diminished and this fragmented band of Giant Sequoia shrank as more vulnerable trees died off. The range of Giant Sequoia contracted into the isolated groves we have today. 

The groves of Giant Sequoia that we have today are suspected to be the contracted result of those stands spared by past glacial activity. The stands that evaded glacial activity were those on broad shelfs between the valleys and canyons cleared by glacial activity. This is one reason the groves are more abundant in the southern Sierra Nevada, where the lower latitude meant many of the glaciers remained at higher alpine elevations. 

The groves of the Giant Sequoia that exist today occur on broad shelfs adjacent to drainage courses. The sites benefit from nearby creeks and streams that are maintained by summer snow melt, which provide a supply of moisture throughout California’s dry summers. This is critical to helping the trees persist under a climate regime that includes less available moisture. 

To persist under a hotter and drier climate means that Giant Sequoia must also survive fire. Giant Sequoia is, in fact, adapted to fire. It carries a thick resinous layer of bark that insulates the tree's vasculature from repeated fire impacts. This means the tree can sustain fire damage without impairing the tree's ability to draw water and nutrients from the soil and environment. Additionally, the very large and healthy girth that gives Giant Sequoia its common name means sections of the tree can sometimes die off without compromising the overall function of the tree. A tree struck by lightning or partially burned by a fire can continue to grow, allowing the tree to endure for thousands of years and therefore gain its huge size. Recent study has suggested the largest and tallest trees function like multiple trees tied into one with sizeable tree limbs acting as a tree in and of itself. 

Giant Sequoia's fire adaptability is showcased by its reproductive necessity for fire. The cones of Giant Sequoia are serotinous, which means they are closed until a certain level of heat is applied. Fire often acts as that heat event. During a fire, the cones open and seed is released into bare soil that has been newly invigorated with nutrients. With the groundplane now cleared of vegetative mass, the seedling can grow relatively free of competition for light and water. Many of these seedlings will not survive due to herbivory, erosion or soil compaction. But some will find that sweet spot, where there is enough water, sun, and minimized disturbance to continue generational succession. But that of course is predicated on a certain delicate balance being maintained....where there is enough of what is needed and a minimum of what is not. It requires the window to that past remains somewhat open.