Do Deciduous Trees Have a Future in Bay Area Urban Forests?

Do you find this a shocking question?

Do you think the answer is an obvious and unequivocal yes?

If so, I'd like to take you on a little journey, from the past, to the present, and into the future.

Today, looking around the urban forests of the Bay Area (and other parts of central California) the sight is rather dire. Trees are seriously stressed virtually everywhere you look.

For one egregious example, most of the remaining southern magnolias appear to be on death's doorstep. For another, the nearly ubiquitous deciduous fall color trees, few of which have any meaningful drought tolerance, have mostly gone into their fall color early, which is drought color. The trees are cutting their losses as they run out of soil moisture in the midst of our current sharp drought.

It's a sobering sight, even for someone who learned his trees during the droughts that have come regularly to California since the mid 1970s. I remember very well the drought lasting from roughly 1988 to 1992. I started planting oak trees all over the Stanford campus in 1989, with my friends at Magic, a local ecological service community.

By 1992, after several years of drought, I remember seeing the native Coast Live oaks starting to look seriously stressed. I still recall the fear I felt watching even the toughest of our oaks look so unhappy.
And this was after having watched even seemingly-indestructible giant Eucalypts succumb to drought and freeze over the preceding years. I also recall watching nearly every European White Birches and Tulip Tree die all over the Palo Alto and Menlo Park area, during this time period.
But the fall of 2021 in the Bay Area seems worse than anything I've seen.

For comparison, I live in Santa Barbara, and travel to the LA and San Diego areas. From direct observation, I can tell you that the urban forests in southern California don't really look like there's a drought. For real.
One of the critical reasons for this is that SoCal has very few deciduous trees. And guess what? Giving up a few weeks of fall color in return for 4 to 6 extra months of green leaves is not a bad tradeoff.
Among the deciduous trees in SoCal, there are quite a few Platanus racemosa, the California sycamore, and most of them look pretty stressed. But what about the red maples and red oaks and ash trees and all the other Oregon-sourced wet zone deciduous tree species that you see virtually everywhere in the Bay Area?
There aren't any in SoCal. Their leaves burn in the heat and dry. And bare branches don't make a whole lot of sense when the sun is bright and temperatures warm in the middle of winter.

Do you know why deciduousness evolved? Cold temperatures (and some other reasons, but this is the biggie). Freezing temperatures cause the water-filled cells of leaves to explode, killing the leaves, and damaging the plant. With deciduousness, the tree drops the leaves to avoid injury.

There's a Chinese pistache I watch in Santa Barbara. It's confused.
It can't tell when to put on leaves, or when to drop them. As with many deciduous species, a cold period is needed to "orient" the plant in its environment. This is well known with deciduous fruit trees requiring "chill hours". Without any meaningful cold temperatures in our subtropical environment, the Chinese pistache loses its way.
Chinese pistache is one of our best drought adapted deciduous trees, so it can take the heat and drought in SoCal, but without cold, it is out of its element. And then there's the months of warm sunshine falling on bare branches.
Along with abundant evergreenness, the trees of SoCal, virtually without exception, display meaningful drought tolerance. The weak trees do not survive there, at least not for long.

Speaking of bare branches, have you noticed that mistletoe mostly infects deciduous trees? If so, have you figured out why? It's because we live in an environment that offers excellent photosynthesis, irregularly, over the winter months.

When was the last time you felt a freeze in the Bay Area? All that lovely warm sunlight falling on bare branch scaffolding creates an ecological niche, which mistletoe are most happy to take advantage of. How often have you seen mistletoe in an evergreen tree?

Continuing our tour, let's go back in time, just a little bit, and think about the forests and woodlands that Spanish settlers laid eyes on when they first came to California. Much of the Bay Area is historical Coast Live Oak woodland. There were Valley Oaks in places, and a variety of riparian trees along creeks, but, really, there were a whole lot of Coast Live Oaks, true evergreen broadleaf trees.
Back in the mid 1990's, Canopy in Palo Alto conducted the OakWell survey, sending volunteers down literally every street in Palo Alto, and identifying every visible native oak, whether in public or in private yards. The results of this survey were eye-opening.

Well more than 80% of all native oaks in Palo Alto were Coast Live oak. Valley oak came in around 15%, and Blue oak only a percent or two. The upshot of this survey is that, on a population basis, the Bay Area was, even historically, an evergreen broadleaf tree climate. Valley oaks certainly do grow, but only on the very best of growing sites. And the Blue oak is on its way out, no matter what people wish.
For more than 40 years, Magic has been planting natives oaks on Stanford lands. In many of those years, we planted all the species present on the lands together, in each tree planting site, meaning Coast Live oak, Valley oak and Blue oak all planted within inches of each other.

We chose this strategy to cover our bases within an unpredictably changing climate, not knowing which species would play out through climate change. And, yes, we were purposely planting with climate change in mind MORE THAN 30 YEARS AGO! Excuse me for yelling, but this is all proceeding in predictable ways that have been visible for decades.

So, using basic CYA reasoning, we planted 'em all, and inadvertently (or not) did a rather profound experiment comparing the species in direct competition. Now, 30 years later, what do we find? There are virtually no Blue oaks that have survived and grown to size. They all get powdery mildew. I won't plant Blue oak anywhere near the Bay Area. I like my trees to grow.

Valley oaks grew, but with vast inconsistency, which I've never found talked about elsewhere. After the experience of growing about 3,000 locally-sourced Valley oaks, we can say that the growth habit and powdery mildew resistance of individual trees varies enormously. Growth habit varies from weeping willow pendulous, to upright and nearly fastigate. This variation could be found even on siblings from the same tree!
So then the big question is, what happens when you plant a Valley oak and a Coast Live oak together, from acorn? In almost all cases, the Coast Live oak literally swallows the Valley oak. Back in 2003, I did hard pruning on some of these combination sites, cutting the Coast Live oaks, often viciously, to try to allow the suppress and outcompeted Valley oak to get above and hopefully overtop the Coast Live oak.

20 years later I can safely report the futility of my attempt. This is physics.
Leaves are solar panels. Valley oak drops its solar panels for roughly 4 months, missing out on up to a quarter of the total photosynthetic potential of the planting location. The Coast Live oak, an evergreen, soaks up that extra solar energy, and turns it into a staggering growth advantage. Plus, Coast Live oak has much better shade tolerance than Valley oak, potentially increasing the advantage, depending on the actual photosynthetic efficiency of the species.

In direct head-to-head competition, the local native evergreen species routed the local native deciduous species. Does this mean that we should be planting Coast Live oak everywhere? I certainly don't think so.
You can read more about this in my section on the Coast Live oak, but a key point regarding Coast Live oak that is rarely, if ever, mentioned, is that, on an overall forest balance, virtually every location in the Bay Area is already fairly substantially overpopulated with Coast Live oaks. In many locations, we have a virtual monoculture of Coast Live oaks. This makes us vulnerable.

Times of great change are a time for diversification, not continued investment in the most common. Especially since that very species, Coast Live oak, isn't looking so hot in SoCal, the climate zones coming to the Bay Area faster than anyone might wish. Check out The Road To Julian, and the Coast Live oak entry, for more on this disturbing trend.

Now let's take a look at the Santa Cruz Mountains. By total tree population percentage, deciduous trees are rare in these mountains, making up only a few percentage points of the total. This is an area vastly dominated historically by evergreen trees.

And now this already evergreen tree climate is warming with climate change, as temperatures now common in SoCal make their way steadily north. So the past was already an evergreen climate, now getting more so by the month and year.
Yet most of the planted urban forests of the Bay Area, especially the publicly-owned ones, are deciduous dominated.

Are there future scenarios in which this deciduous strategy makes sense? Well, yes there are, but it's doubtful you even want to contemplate what's required. Looking at paleoclimatic reconstructions of earth climate over geological time spans, there are a couple of ways to cool the planet suddenly.
One way appears to be continental scale volcanism. Imagine most of India turns into volcanoes (see the history of the Deccan Traps). Fun, huh?

How about the Tunguska Event? A comet remnant came obliquely into earth's atmosphere, and exploded above Siberia, in an event called a cosmic air burst. If the Tunguska event had occurred over New York or Paris or another major global urban center, our world might be vastly different today. But it occurred over one of the least human populated parts of earth, so we missed a learning lesson as a species. Next time might be different.
So we have mass volcanism, cosmic airbursts, and, finally, direct asteroid strikes. You know, the type that is purported to have finished off the dinosaurs? A much smaller cosmic strike is the likely source of the Younger Dryas cooling event roughly 11,000 years ago.

Short of one of these kinds of events, which are unlikely and deeply catastrophic, the planet is warming. And deciduous trees will be less and less relevant to Bay Area urban forests as they succumb to heat and drought.
Luckily, the solution is simple. Plant drought tolerant broadleaf evergreen trees as primary choices, and plant deciduous tree species only sparingly. And how about using some carefully selected conifers? There are cool Eucalyptus and related trees coming back into production, with vastly improved root systems, and much improved species selection thanks to Matt Ritter at Cal Poly SLO.

The future is evergreen, and it's lovely.