Sitting atop the land of the Muwekma Ohlone Tribe, located on the eastern hills of the Santa Cruz Mountains and about five miles southwest of Stanford University’s main campus, is one of its most beautiful classrooms: Jasper Ridge Biological Preserve ('Ootchamin 'Ooyakma). Since 1965, this site, a part of the School of Humanities and Sciences (H&S), has been the center of long-term scientific studies that aim to understand the intricacies of the natural world. 

It was here, during the summer of 2009, that Rodolfo Dirzo, professor of biology in H&S and of Earth system science in the Doerr School of Sustainability, began research into how herbivores (such as deer) feeding on oaks affect the seedlings of these trees. He surveyed the preserve’s landscape to locate pairs of oak seedlings. Once located, he caged one seedling in each pair to protect it from deer herbivory; the other was used as a control. Dirzo established 75 such pairings for three predominant oak species. Then began the long wait to observe nature taking its course. 

At first, a flourishing deer population caused the seedling oak population to diminish – except when seedlings were protected – a main finding in Dirzo’s experiment. A few years later, though, mountain lions returned. “For reasons we don’t quite understand yet, they came back,” said Dirzo, who monitored the oaks and the surrounding vegetation via a combination of cameras and in-person visits. “We went back to those sites and it was fascinating to see the change in vegetation. Ten years after the return of the mountain lions, there was much more abundance and regeneration of the woody plants, including oaks.” 

In the years since that first experiment started, Dirzo’s lab has expanded its suite of studies about oak survival at JRBP and often found a more complicated story than they expected. This includes recent surprising results about the longer-term effects on oaks of those very deer whose populations have flowed and ebbed.

The lab’s work not only adds to our knowledge of these iconic trees but aims to provide possible solutions for improving their prospects. Among the oaks available for study at JRBP, native deciduous species, the valley oak (Quercus lobata) and blue oak (Quercus douglasii), are of particular concern as they are declining more significantly than their native evergreen neighbors, the coast live oak (Quercus agrifolia). That said, all of these species are at risk from various threats, including climate change, pests, disease, and drought. 

The loss of any native oak population would likely have complex and wide-ranging consequences – affecting other plants, soil, animals, and people, too. 

“These plants have significance for the cultural and traditional views and cosmovision of the local people here,” said Dirzo. “We're also losing different ways of being a plant.” 

Plants that protect

Interspersed amongst the tall and dominating oaks that stretch across JRBP’s sprawling, grassy fields are patches of smaller shrubs. Rough with stiff, spindly branches that ascend and spread outwards, these shrubs – called coyote brush – are not as inhospitable as they first appear. For, if one were to look deep between the thick intertwining branches, they might find at the bottom, sheltered from heat and herbivores, a little oak seedling. 

Due to their unique ability to protect oak seedlings growing underneath them, these are sometimes referred to as “nurse plants.” By taking seedlings under their wing, nurse plants protect them from biotic stressors, such as grazing by herbivores, and abiotic stressors, such as extreme heat and drought.

In 2015, two of Dirzo’s postdoctoral scholars, Ramon Perea and Marta Peláez, began an experiment to more closely quantify how nurse plants affect the quality and quantity of oak regeneration. They surveyed areas both underneath and outside 216 coyote brushes, identifying and measuring naturally recruited oaks – the term for plants that have made it past the seedling stage to be “recruited” into the population. They focused on three species: the deciduous valley oak, the deciduous blue oak, and the evergreen coast live oak (Quercus agrifolia).

Then, in 2023, an undergraduate student in Stanford’s Earth Systems Program and Dirzo’s lab, Chrysanthe Frangos, went back to those shrubs surveyed by Perea and Peláez to check how many of the oak recruits had survived and proliferated. What she found was both comforting and concerning: while the evergreen oaks underneath nurse plants flourished to great heights, the deciduous oaks were barely hanging on, with most of them having died out. 

“Because they're deciduous, they have different functional traits and contribute differently to biodiversity than evergreens,” said Frangos, “So their loss is going to significantly reduce ecological and biological diversity.” 

The study did find that, in drought conditions, nurse plants are highly effective in facilitating both deciduous and evergreen oak survival and growth. But under less stressful conditions, they may not have as much of an impact. 

The misunderstood villains 

Scientists and conservationists concerned with the survival of Californian oaks are faced with many potential threats to these species, including climate change, hotter temperatures, severe droughts, and growing herbivore populations.

Perea and Peláez have recently published research that attempts to understand whether all these factors are additive. For this research, they began growing oak seedlings inside greenhouses in 2014, and then mimicked drought and herbivory conditions by controlling watering and periodically clipping off their leaves. They tracked how many trees survived and measured root and leaf growth, as well as other physiological parameters.

Deer at Jasper Ridge Biological Preserve. | Chrysanthe Frangos

Spanning over a decade, these rigorous observations and tricky measurements led to intriguing insights. Herbivores have often been seen as the villains to oaks because they feed on their leaves, preventing them from maturing into adult trees – in line with the findings of Dirzo’s previous caged plants experiment. In contrast to the established paradigm, this research found that mimicking herbivory actually helped oak seedlings survive in drought. In other words, in certain situations, deer were not the villains they have been thought to be. 

Turns out, the researchers determined that deer are the anti-heroes of this story. Oak seedlings have smaller roots that cannot access groundwater. By eating the leaves, deer help reduce the amount of water that evaporates from leaf surfaces. Reserving this water improves the overall health of the seeding and allows it to spend more energy in growing deeper, stronger, and more numerous roots. 

“These herbivores might be detrimental for oak saplings, but our main takeaway was that, at the seedling stage, they can play a positive role too,” said Peláez.

Furthermore, under drought conditions, they found that herbivory can be especially beneficial to deciduous trees’ survival. 

Both these studies reflect the yin and yang balance of nature. Deer can be villains or anti-heroes depending on the situation, while nurse plants can be saviors under complex and nuanced conditions.

“Traditionally, we ecologists have always thought of plant-animal interactions to be negative in nature,” said Dirzo. “This paradigm of always a negative interaction is beginning to be challenged by new research, including the work of these two lab members that shines some light into this complementary view as to how nature operates.”

Droughts and dwindling oaks 

These studies underline the urgent situation of deciduous oaks in the California oak savannas, especially with increasing climate change.

Losing deciduous oaks could have huge ecological impacts on biodiversity, said Frangos. She explained that deciduous oak acorns are tastier to animals and their relatively thin leaves are preferred by deer. This puts blue and valley oaks at higher risk for herbivory, and, ironically, this preference could lead to less nutrition for the animal populations overall, as it prevents acorns from planting. By contrast, evergreen oaks are more drought and heat tolerant, and they have spiky leaves that deter many herbivores, said Dirzo. 

Dwindling deciduous oak populations will also likely affect the chemistry of the soil. Deciduous oaks have a higher carbon-absorbing capacity than evergreens and decompose faster. “The nutrient cycling will be altered dramatically, because the provisioning of nutrients in terms of abundance and speed is going to be very different, depending on what species you have and the combination of the species present,” said Dirzo. 

Based on their findings, the team suggests that land managers run pilot studies, such as clipping leaves off deciduous oak seedlings and securing sustainable deer population levels (which can be achieved by maintaining the populations of their predators, mountain lions), to give the trees the best chance of survival. 

Dirzo’s team is now looking into expanding their research to mix realism and precision. Frangos’s field research involved very real environmental situations, but her results were influenced by a host of variables that operate in nature. Perea and Peláez’s study featured high precision due to the control of the influence of confounding variables achieved in the greenhouse. Now, Peláez is trying to recreate her experiment out in the field, while Frangos is examining how oak seedling survival is affected by smaller mammals like mice and squirrels that are, in turn, affected by nurse plants.

“The ultimate goal of that research is to see where an acorn is safest from any type of herbivory, so land managers dealing with both large herbivores (like deer) and small rodents can get a better idea as to where an acorn might be safest to plant,” Frangos said.

The unraveling of these complex interactions in the field is what makes JRBP so unique. 

“Jasper Ridge has been a fantastic laboratory to study how various and complex natural factors impact oak regeneration,” said Dirzo. 

The preserve has become the setting for collection of a rich bank of knowledge, accumulated by scientists over decades. 

“This research is a good example of how long-term research can accumulate and how new research projects can build on the findings from previous research,” said Tadashi Fukami, JRBP faculty director. “Now new students who come here have this wealth of information that helps them ask newer and more intriguing questions.”