This is how Robin Grossinger, a scientist at the San Francisco Estuary Institute, does field work: He drives around town with a stack of 150-year-old sepia-toned photos from local history books, looking for any landmark - a creek, a 200-year-old oak tree - that might be a match. When he finds one, he swerves his car to the side of the road, races through mud when it's muddy and rain when it's rainy and parking lots when it's necessary to document the evidence with a digital camera, and then races back to the car to drive off before someone starts to wonder what he's doing photographing their strip mall.

"This," he said on a recent field trip, as he pulled the car in next to a massive valley oak, preparatory to the running-photographing-fleeing bit, "is where it gets fun."

Grossinger is a historical ecologist, merging history and ecology to figure out the environments of our great, and great-great-great grandparents. When he compares those landscapes with the present, he turns up surprise after surprise - streams that weren't streams, wetlands where there used to be beaches, thick groves of trees where there used to be plains and plains where there used to be thick groves of trees.

A lot of ecology that was around two centuries ago has been forgotten. Grossinger surveys Bay Area landscapes you think you know, and then tells you what they looked like 200 years ago - what early explorers said about them, how early cartographers drew them and, most importantly, what elements from that early picture could be brought back. Even as he talks, though, Grossinger is not down on urbanization. His projects tend to suggest ways that nature can be realistically reintegrated with the modern landscape, rather than waxing nostalgic.

"It seems like historical ecology is sort of a sentimental exercise, but it's really about understanding the contemporary landscape," Grossinger said. "The landscape you kind of inherit and don't really have the tools to decipher."

Grossinger has led the Estuary Institute's historical ecology program for the past decade, and in that time he has seen the field grow from a curiosity to an imperative before restoration. Bay Area agencies consult him before beginning large projects, and Grossinger and his team have worked all over the Bay Area.

Their most recent major report explores the landscape south of San Jose, as a project for the Santa Clara Valley Water District and the Nature Conservancy. Released last summer, the report found plenty of surprises, the most significant of those having to do with valley oaks.

Grossinger's colleague Alison Whipple, an environmental analyst, started by counting all the valley oaks in the study area - roughly the area along Highway 101 from Morgan Hill to Gilroy - using a 1939 aerial photo and a 2005 aerial photo. She found a decline from 1,976 trees in 1939 to about 1,000 trees in 2005. Because the lone oak on rolling savannah is pretty much archetypal California, and since most of the Santa Clara Valley's development came with Silicon Valley's growth, it seemed as if one could extrapolate back further to maybe, say, a few thousand trees in the 1800s. But that would be very, very wrong. As they were working, Grossinger and Whipple found a few photographs from the late 1800s with dense oak forests and some explorers' accounts that mentioned thick, shady woodland. They decided to do more detective work. This led, late last winter, to one of Grossinger's field trips to find and photograph remnant oak trees (this was an exciting but infrequent event; as with any scientist, most of his job involves office work).

As he prepared to head out, Grossinger walked through his marsh-front Oakland office where a huge pile of maps sat splayed on a 10-foot-by-10-foot table. On top of the pile was a half-empty box of mint-chocolate cookies, at the bottom was the 6-foot-square 1939 aerial survey of south Santa Clara County. In between were maps and photos of every dimension, style and date: recent detail maps, historical photos, a California atlas, an "Images of America" local history book, and a printed marsh plan with "georectify?" scrawled across it in blue marker.

Grossinger pulled the huge 1939 aerial map out from under the pile and traced the route he'd take on his oak-hunting expedition through the South Bay. Whipple sat at her computer nearby, labeling oak trees on an older map, and she had a surprise for Grossinger. They'd both started to suspect that they were going to find evidence of higher tree density in the past, but Grossinger hadn't heard an actual number. Whipple, who was finishing the report, told Grossinger that she had an estimate: Something like 50,000 trees. Grossinger's eyes widened.

"Fifty thousand," he said. "Wow." In fact, in the finished report, the number went to 60,000.

Surprises are Grossinger's job, and the oak tree result - which they've also spun into a research paper with UC Santa Barbara oak ecologist Frank Davis - was a good one. But it's got competition: In his time on the job, Grossinger has compiled an impressive list. The central bay shoreline of a few hundred years ago was made up of lagoons and long, curling white-sand beaches, where native Californians picked strawberries out of sand dunes. The creeks that flow through the South Bay were mostly engineered by people in the last 100 years, and historically fanned out underground to create swampy wetlands. Less than 200 years ago, there were so many octopuses overflowing the bay's tide pools that settlers would walk around grabbing them for dinner.

"Each generation kind of loses the knowledge of what this place was like, and we find ourselves 200 years later with very little knowledge of what was there," Grossinger said. "So very basic things like octopuses in tide pools catch us by surprise."

Walk into the office on any day and you're likely to find something interesting about the Bay Area's past. A few weeks ago it was wetlands near the summit of Mount Diablo, found on a map from the 1880s that Grossinger's colleague Ruth Askevold, a geographic information systems analyst, had matched on her computer. "The cool thing of the day," Grossinger called it, standing at Askevold's double-monitor workstation and watching as she flipped back and forth between a map from 1880 and aerial photos from 1939 and 2005.

"Knowing where wetlands are could be useful," Grossinger said. "You could bring back red-legged frog or tiger salamander habitat. That usually means creating artificial ponds, but if you could do that where there used to be a wetland, that's even better."

That's the main point of historical ecology: To give restoration planners a better idea of what they might try to bring back and what might succeed. Like the oak trees: You don't need to return to undeveloped land to bring back oak trees. You just need to plant the right kind of trees in suburban yards and street medians - the kind of thing that cities wouldn't know to do without the historical context. And then, they suspect, if the oak trees make a comeback, so would a lot of the native species that once benefited from them - meaning that in this case, it wouldn't be just the past, but the future, that would be full of surprises.