A "Natural History Note" From UC California Naturalist's lead scientist, Dr. Cameron Barrows.

“Climate change is the greatest threat to our existence in our short history on this planet. Nobody's going to buy their way out of its effects.” - Mark Ruffalo

The environment is changing. There is a daily cycle governed by the earth's rotation – warmer days and cooler nights. There is a seasonal cycle governed by the tilt of the earth toward the sun – tilting toward the sun gives us summer, away from the sun and it is winter. Within those cycles there can be changes in the jet stream resulting in wetter or drier periods that can last weeks, years, or even decades. Then there are more stochastic changes, earthquakes governed by shifting plate boundaries, floods, and wildfires. These natural changes are fodder for biodiversity, changing opportunities (niches) over space and time for animals and plants to occupy. Within the backdrop of all these changes, changes that we nearly hairless apes have little or no control over, how then do we distinguish changes that are born from a climate change catalyzed by human-caused emissions of greenhouse gasses?

Wildfires are a case in point. We know that for at least the past decade every year the wildfires burn more acres and burn across more months of the year. A Euro-centric perspective holds that fires are bad, destructive, and should be stopped at all costs. That belief has its roots in historical and current patterns of our species inhabiting dense clusters of tinder dry wood-structured homes within villages by forests, as well as the economic value of timber for centuries of home and ship building. Smokey the Bear has waged a decades-long campaign to teach us all about the overwhelming negative impacts of wildfire. Up until recently we have been largely successful at reducing the numbers of acres burned each summer, with a result of increasingly dense forests and chaparral, landscapes where fire control has been ever more difficult to achieve.

In contrast, an Indigenous American perspective sees fire as an essential tool for rejuvenating the landscape, catalyzing new growth of beneficial plants, and supporting wildlife, and probably for preventing larger more catastrophic wildfires. Early Spanish explorers seeing California for the first time described a multitude of fires burning across all the landscapes they could see from their sailing ships. An ecological perspective is more in line with that of the Indigenous people; a patchwork of burned and unburned lands does support higher biodiversity. There are many plants and their pollinators that have evolved as “fire followers”, taking advantage of the open spaces and enriched soils present after wildfires. Some birds, such as black-backed woodpeckers, are burned forest specialists foraging on beetle grubs in fire charred trees, and the snags created by forest fires provide nesting sites for birds, bats, and flying squirrels. Rufous-crowned sparrows are only found in open, sparsely vegetated landscapes. Once shrubs or grasses become too dense, these sparrows are gone. For evolution to have resulted in so many species that either benefit from wildfires or that are restricted to recently burned areas, wildfires fires have clearly been natural ecological process, a regular part of California's natural patterns and rhythms far longer than people have lived here.

The difference today is the timing, location, scale, and intensity of the fires. So, is climate change to blame? Maybe, but it is not that simple. A century of fire suppression, leading to overly dense stands of trees and layers of fuels leading right into the forest canopy could explain scale and intensity. The introduction of non-native invasive grasses (fertilized by the nitrogen in Los Angeles' smog) certainly explains the incursions of wildfire into the deserts, where previously the lack of continuous fuels impeded fires from spreading beyond just a few feet from where, say a lightning bolt truck a Joshua tree. But how about the length of the fire season? The fire season – defined as the time elapsed between the first large fire ignition and last large fire control – went from 138 days in the 1970s to 222 days in the last decade, an increase of 84 days or about two and a half months. The effect of climate change in California and throughout the southwestern states is to increase aridity. Droughts are more common, but even in years or close to average rainfall, higher temperatures mean that regardless of how much rain falls, more and more of that rain evaporates before reaching the roots of trees and shrubs. The plants dry out faster and the burn season becomes longer and longer. That is the climate change effect.

Using width of individual tree rings to estimate the effective levels of aridity, scientists have estimated that the currently level of aridity in the South Western United States exceeds any other period in the past 1300 years. The effect of that aridity goes beyond increasing the length of the fire season. Young trees and shrubs are especially vulnerable to perishing due to heightened aridity; larger plants have greater water storage capacities that can buffer them from moderate periods of drought, whereas the young plants do not, and so more easily succumb to dry conditions. Where this is occurring, we can see stands of what appear to be healthy pines, junipers, Joshua trees, and manzanita, but if there is no successful recruitment of young cohorts, those stands are doomed. Think of a town or village with no children. Using recruitment as a metric is an effective was of identifying whether a particular site is, at least so far, resilient to the effects of climate change.

At its extreme, longer periods of aridity can kill the adult plants as well. My wife and I were recently driving a road in northern Arizona, a road that follows an ecological gradient that starts in a ponderosa pine forest, passing through a pinyon pine-juniper zone, then just junipers, and finally into a grassland. It's a road we have driven a dozen times in recent years, only this time the juniper zone was different. Except for thin strips along the road where the plants benefited from added road-runoff during rainstorms, all the other junipers were dead. Mature plants that were certainly half a century or more old were now dead. Thousands of acres of junipers, all dead. All the ecological interactions between the junipers and birds, insects, and mammals were now severed. That is the climate change effect.

Just this past week I was hiking with a group of community scientists along a north facing slope above 8,000' to a ridge below the +10,000' peak of Mount San Jacinto. North facing slope, high elevations, this should be a place that is resilient to the effect of climate change. We noted many saplings of sugar pine and white fir, the dominant overstory trees there, so good recruitment. Lots of birds and lots of southern sagebrush lizards, all indicating a healthy forest. And it was healthy, but even here we detected subtle indications of recent increases in aridity. The understory shrubs were comprised of currants and gooseberries, and bush chinquapin (a relative of oak trees, one of several species that was probably dropped on this sky island during one of the glacial maxima during the Pleistocene – otherwise ranging throughout the Sierra Nevada and into northwestern California). Most of the understory looked healthy, but we noticed one large patch of chinquapin that was dead – but was it a climate change effect or perhaps a disease? Then we came across a forest opening with a thick stand of currants. Forest openings spawn questions – why no trees? Particularly wet soils maintain meadows of grasslands, or currant thickets and keep conifers from becoming established. Except this this currant thicket had a dozen or more sapling white firs poking through the tangle of berry branches. The firs were likely no more than a decade old, and there were no mature trees of any kind within the opening. One explanation for understanding both the dead chinquapins and the sapling fir invasion is that some of the soils here are becoming increasingly dry. Even at this elevation, even on a north-facing slope, there appear to be climate change effects.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.