In forested regions of the nation, natural regeneration may help cities achieve tree cover goals at the expense of maintaining the desired tree species. A study by U.S. Forest Service scientists published recently in Urban Forestry and Urban Greening showed that on average, 1 in 3 trees in sampled cities were planted while two-thirds resulted from natural regeneration.

However, for newly established, young trees in cities in forested regions, only about 1 in 12 trees (Syracuse, N.Y.) to 1 in 20 trees (Baltimore) were planted.

The lower proportion of naturally regenerated trees in the entire city tree population may be because naturally regenerated trees have a higher mortality rate than planted trees, according to Dave Nowak, a research forester with the Forest Service’s Northern Research Station and the study’s principal investigator. Naturally regenerated trees typically have more competition for the water, light and nutrients that are needed for survival.

“Urban forests change constantly in response to human activities and environmental factors,” said Michael T. Rains, Director of the Northern Research Station. “This study is an example of the work U.S. Forest Service scientists are doing to help cities and counties achieve healthy and sustainable urban forests.”

Most of the cities included in the study are located within forested regions, where environmental conditions favor trees. The study suggests that natural regeneration may be the most cost-effective means to attain desired tree cover levels and associated ecosystem services in forested regions, but relying on natural regeneration may alter the tree species making up a given forest.

The study included Baltimore, Chicago, Hartford, Conn., Los Angeles and Syracuse, N.Y., as well as nine cities in the Canadian province of Ontario.

The percentage of planted trees within the entire tree population ranged from a low of 11.1 percent in Hartford to a high of 89 percent in Los Angeles, according to the study. Results indicated that in cities located in grassland or desert regions, a greater percentage of the urban tree population is planted compared to cities in forested regions. The percentage of planted trees also increases with increased population density and impervious cover.

The factors that affect tree cover also play a role in natural regeneration. Precipitation, air temperatures, surrounding natural vegetation type, and the distribution of land use types in the city all influence how much seed source exists and whether seedlings grow. Land use plays a role in whether seedlings are mowed down or whether paved surfaces prevent seedling establishment, and ultimately affect a city’s need for tree planting to sustain tree cover.

“Before we can say how many trees we need to plant, we need to know how many trees are regenerating naturally and how many are dying,” according to Nowak. “Planting programs to sustain tree cover can be greatly improved with long-term monitoring data to show how the urban forest is changing.”

Researchers evaluated the magnitude of natural regeneration or tree planting across an entire city system by randomly locating 0.04-hectare field plots in each city and recording tree measurements as well as an estimate of whether a sampled tree was planted or occurred through natural regeneration.

Crews based their estimates on surrounding conditions. For example, trees along fence lines or in unmaintained areas were generally classified as naturally regenerated, while trees in maintained lawn areas or street trees were typically classified as planted.

Residential and commercial/industrial areas have the highest proportion of planted trees, 74.8 percent and 61.2 percent respectively, while trees found in parks, cemeteries, golf courses, open space/vacant land, agricultural fields and wetlands are predominately the result of natural regeneration.

The study also evaluated two cities, Syracuse, N.Y., and Baltimore, more closely. An estimated 58 percent of new trees recently established in Baltimore are native species while only 35 percent of new trees in Syracuse are native species, according to the study. Fifty-two percent of the new trees were classified as invasive species in Syracuse.

The mission of the U.S. Forest Service is to sustain the health, diversity, and productivity of the nation’s forests and grasslands to meet the needs of present and future generations.

The agency manages 193 million acres of public land, provides assistance to state and private landowners, and maintains the largest forestry research organization in the world.

The mission of the Forest Service’s Northern Research Station is to improve people’s lives and help sustain the natural resources in the Northeast and Midwest through leading-edge science and effective information delivery.

A vast outcrop of the Arctic Siberian coast that had been frozen for tens of thousands of years is releasing huge carbon deposits as rising temperatures thaw parts of its coastline, a study warned Wednesday.

The carbon, a potential source of Earth-warming CO2, has lain frozen along the 7,000-kilometre (4,400-mile) northeast Siberian coastline since the last Ice Age.

But atmospheric warming and coastal erosion are gnawing at the icy seal, releasing about 40 million tonnes of carbon a year — 10 times more than previously thought, said a study in the journal Nature.

About two-thirds of the carbon escapes into the atmosphere as carbon dioxide (CO2) and the rest becomes trapped in higher layers of ocean sediment.

About half the carbon pool in soil globally is held in permafrost in the Arctic, a region that is experiencing twice the global average of climate warming, said the study led by researchers at Stockholm University.

Earlier this week, US scientists said the sea ice in the Arctic Ocean had melted to its smallest point ever.

The region covered by the Nature study, called Yedoma, is twice the size of Sweden but has been poorly researched because it is so remote.

The finding touches on a vicious circle, or positive feedback in climate parlance.

Under this, man-made warming caused by the burning of fossil fuels releases naturally-occurring stocks of CO2 that have been stored in permafrost since the last Ice Age, called the Pleistocene.

The released gases in turn add to global warming, which frees even more locked-up carbon, and so on.

“Thermal collapse and erosion of these carbon-rich Pleistocene coastline and seafloor deposits may accelerate the Arctic amplification of climate warming,” the paper warned.

The atmospheric leakage from Yedoma is equivalent to the annual emissions of around five million passenger cars, on the basis of average carbon output (five tonnes per year) of vehicles in the United States.

In a separate study also in Nature, researchers in Britain, the Netherlands and the United States used computer models to estimate there could be as much as four billion tonnes of methane under Antarctica’s icesheet.

Methane is 25 times more efficient at trapping solar heat than carbon dioxide.

Before it froze over, the region teemed with life whose organic remains became trapped in sediment later covered by ice sheets.

“Our modelling shows that over millions of years, microbes may have turned this old organic carbon into methane,” which could boost climate warming if released by icesheet collapse, the researchers said in a statement.

The collapse of the Antarctic icesheet is considered an extremely remote scenario by most climatologists, and some studies have suggested that parts of it could be thickening, due to localised increases in snowfall.

Ancient pollen and charcoal preserved in deeply buried sediments in Egypt’s Nile Delta document the region’s ancient droughts and fires, including a huge drought 4,200 years ago associated with the demise of Egypt’s Old Kingdom, the era known as the pyramid-building time.

“Humans have a long history of having to deal with climate change,” said Christopher Bernhardt, a researcher with the U.S. Geological Survey. “Along with other research, this study geologically reveals that the evolution of societies is sometimes tied to climate variability at all scales – whether decadal or millennial.”

Bernhardt conducted this research as part of his Ph.D. at the University of Pennsylvania, along with Benjamin Horton, an associate professor in Penn’s Department of Earth and Environmental Science. Jean-Daniel Stanley at the Smithsonian Institution also participated in the study, published in July’s edition of Geology.

“Even the mighty builders of the ancient pyramids more than 4,000 years ago fell victim when they were unable to respond to a changing climate,” said USGS Director Marcia McNutt. “This study illustrates that water availability was the climate-change Achilles Heel then for Egypt, as it may well be now, for a planet topping seven billion thirsty people.”

The researchers used pollen and charcoal preserved in a Nile Delta sediment core dating from 7,000 years ago to the present to help resolve the physical mechanisms underlying critical events in ancient Egyptian history.

They wanted to see if changes in pollen assemblages would reflect ancient Egyptian and Middle East droughts recorded in archaeological and historical records.

The researchers also examined the presence and amount of charcoal because fire frequency often increases during times of drought, and fires are recorded as charcoal in the geological record. The scientists suspected that the proportion of wetland pollen would decline during times of drought and the amount of charcoal would increase.

And their suspicions were right.
Large decreases in the proportion of wetland pollen and increases in microscopic charcoal occurred in the core during four different times between 3,000 and 6,000 years ago.

One of those events was the abrupt and global mega-drought of around 4,200 years ago, a drought that had serious societal repercussions, including famines, and which probably played a role in the end of Egypt’s Old Kingdom and affected other Mediterranean cultures as well.

“Our pollen record appears very sensitive to the decrease in precipitation that occurred in the mega-drought of 4,200 years ago,” Bernhardt said. “The vegetation response lasted much longer compared with other geologic proxy records of this drought, possibly indicating a sustained effect on delta and Nile basin vegetation.”

Similarly, pollen and charcoal evidence recorded two other large droughts: one that occurred some 5,000 to 5,500 years ago and another that occurred around 3,000 years ago.

These events are also recorded in human history – the first one started some 5,000 years ago when the unification of Upper and Lower Egypt occurred and the Uruk Kingdom in modern Iraq collapsed. The second event, some 3,000 years ago, took place in the eastern Mediterranean and is associated with the fall of the Ugarit Kingdom and famines in the Babylonian and Syrian Kingdoms.

“The study geologically demonstrates that when deciphering past climates, pollen and other micro-organisms, such as charcoal, can augment or verify written or archaeological records – or they can serve as the record itself if other information doesn’t exist or is not continuous,” said Horton.

This study, Nile delta response to Holocene climate variability, was published in the July edition of Geology, and was authored by Christopher Bernhardt, USGS; Benjamin Horton, Penn; and Jean-Daniel Stanley, Smithsonian Institution. Support for the work came from the University of Pennsylvania, the U.S. Geological Survey, and the Smithsonian Institution.

Even though it sounds like science fiction, researchers are taking a second look at a controversial idea that uses futuristic ships to shoot salt water high into the sky over the oceans, creating clouds that reflect sunlight and thus counter global warming.

University of Washington atmospheric physicist Rob Wood describes a possible way to run an experiment to test the concept on a small scale in a comprehensive paper published this month in the journal Philosophical Transactions of the Royal Society.

The point of the paper – which includes updates on the latest study into what kind of ship would be best to spray the salt water into the sky, how large the water droplets should be and the potential climatological impacts – is to encourage more scientists to consider the idea of marine cloud brightening and even poke holes in it. He and a colleague detail an experiment to test the concept.

“What we’re trying to do is make the case that this is a beneficial experiment to do,” Wood said. With enough interest in cloud brightening from the scientific community, funding for an experiment may become possible, he said.

The theory behind so-called marine cloud brightening is that adding particles, in this case sea salt, to the sky over the ocean would form large, long-lived clouds. Clouds appear when water forms around particles. Since there is a limited amount of water in the air, adding more particles creates more, but smaller, droplets.

“It turns out that a greater number of smaller drops has a greater surface area, so it means the clouds reflect a greater amount of light back into space,” Wood said. That creates a cooling effect on Earth.

Marine cloud brightening is part of a broader concept known as geoengineering which encompasses efforts to use technology to manipulate the environment. Brightening, like other geoengineering proposals, is controversial for its ethical and political ramifications and the uncertainty around its impact. But those aren’t reasons not to study it, Wood said.

“I would rather that responsible scientists test the idea than groups that might have a vested interest in proving its success,” he said. The danger with private organizations experimenting with geoengineering is that “there is an assumption that it’s got to work,” he said.

Wood and his colleagues propose trying a small-scale experiment to test feasibility and begin to study effects. The test should start by deploying sprayers on a ship or barge to ensure that they can inject enough particles of the targeted size to the appropriate elevation, Wood and a colleague wrote in the report. An airplane equipped with sensors would study the physical and chemical characteristics of the particles and how they disperse.

The next step would be to use additional airplanes to study how the cloud develops and how long it remains. The final phase of the experiment would send out five to 10 ships spread out across a 100 kilometer, or 62 mile, stretch. The resulting clouds would be large enough so that scientists could use satellites to examine them and their ability to reflect light.

Wood said there is very little chance of long-term effects from such an experiment. Based on studies of pollutants, which emit particles that cause a similar reaction in clouds, scientists know that the impact of adding particles to clouds lasts only a few days.

Still, such an experiment would be unusual in the world of climate science, where scientists observe rather than actually try to change the atmosphere.

Wood notes that running the experiment would advance knowledge around how particles like pollutants impact the climate, although the main reason to do it would be to test the geoengineering idea.

A phenomenon that inspired marine cloud brightening is ship trails: clouds that form behind the paths of ships crossing the ocean, similar to the trails that airplanes leave across the sky. Ship trails form around particles released from burning fuel.

But in some cases ship trails make clouds darker. “We don’t really know why that is,” Wood said.

Despite increasing interest from scientists like Wood, there is still strong resistance to cloud brightening.

“It’s a quick-fix idea when really what we need to do is move toward a low-carbon emission economy, which is turning out to be a long process,” Wood said. “I think we ought to know about the possibilities, just in case.”

The authors of the paper are treading cautiously.

“We stress that there would be no justification for deployment of [marine cloud brightening] unless it was clearly established that no significant adverse consequences would result. There would also need to be an international agreement firmly in favor of such action,” they wrote in the paper’s summary.

There are 25 authors on the paper, including scientists from University of Leeds, University of Edinburgh and the Pacific Northwest National Laboratory. The lead author is John Latham of the National Center for Atmospheric Research and the University of Manchester, who pioneered the idea of marine cloud brightening.

Two new studies by scientists at UC Berkeley provide a clearer picture of why some species move in response to climate change, and where they go. One study, published online in the journal Global Change Biology, finds that changes in precipitation have been underappreciated as a factor in driving bird species out of their normal range.

In the other study, published the journal Proceedings of the Royal Society B, researchers found a sharp decrease in range for the Belding’s ground squirrel, but noted some surprising areas where the species found refuge.

The two studies exemplify the type of research being explored through the Berkeley Initiative in Global Change Biology, or BiGCB, an ambitious effort to better understand and predict how plants and animals will respond to changing environmental conditions by studying how they have responded to earlier periods of climate change.

The first study’s findings challenge the conventional reliance on temperature as the only climate-related force impacting where species live. The authors noted that as many as 25 percent of species have shifted in directions that were not predicted in response to temperature changes, yet few attempts have been made to investigate this.

“Our results redefine the fundamental model of how species should respond to future climate change,” said study lead author Morgan Tingley, who began the research as a graduate student in UC Berkeley’s Department of Environmental Science, Policy and Management. “We find that precipitation changes can have a major, opposing influence to temperature in a species’ range shift. Climate change may actually be tearing communities of organisms apart.”

The findings are based upon data gathered from the Grinnell Resurvey Project, which retraces the steps of Joseph Grinnell, founder of the UC Berkeley Museum of Vertebrate Zoology, in his surveys of Sierra Nevada wildlife from the early 1900s. The resurvey project, which began in 2003, was led by Craig Moritz, former UC Berkeley professor of integrative biology, and his colleagues at the Museum of Vertebrate Zoology.

For the bird study, the researchers included 99 species in 77 historic survey sites in Lassen Volcanic, Yosemite and Sequoia national parks, as well as in several national forests. In the century that has passed since the original Grinnell survey, summer and winter temperatures have increased an average of 1-2 degrees Celsius in the Sierra Nevada. Yosemite experienced the most warming – with average temperatures increasing by 3 degrees Celsius – while parts of Lassen actually got cooler and much wetter.

Among the bird species that moved upslope are the Savannah Sparrow, which shifted upward by 2,503 meters, and other meadow species such as the Red-winged Blackbird and Western Meadowlark. The ones that shifted their range downslope include both low-elevation species like the Ash-throated Flycatcher and Western Scrub-Jay, and high-elevation species like the Cassin’s Finch and Red-breasted Nuthatch.

“Temperature did not explain the majority of these shifts,” said Tingley, who is now a postdoctoral researcher at Princeton University’s Program in Science, Technology and Environmental Policy. “Only when we included precipitation as an explanatory variable did our models adequately explain the movement patterns we observed.”

The researchers found that while rising temperatures tended to push birds to cooler regions upslope, increased precipitation, which is more common at higher elevations, pulled them downslope.

“We believe many species may feel this divergent pressure from temperature and precipitation, and in the end, only one wins,” said Tingley.

Notably, more than half of the bird species in each of the three study regions did not shift their range despite pressures from climate change. “Moving is a sign of adaptation, which is good from a conservation standpoint,” said Tingley.

“More worrisome are the species that have not shifted. How are they adapting? Are they moving, but we just can’t detect it? Or are they slowly declining as environmental conditions gradually become less ideal where they live?”

The answers are complex, as illustrated by the second UC Berkeley paper about range changes for a species of squirrel found in the mountains of the western United States.

In that paper, researchers again used information obtained from the Grinnell Resurvey Project. Through visual observations and trapping surveys conducted throughout the mountains of California, they discovered that the Belding’s ground squirrel had disappeared from 42 percent of the sites where they were recorded in the early 1900s. Extinctions were particularly common at sites with high average winter temperatures and large increases in precipitation over the last century.

“We were surprised to see such a dramatic decline in this species, which is well-known to Sierran hikers and was thought to be fairly common,” said study lead author Toni Lyn Morelli, a former National Science Foundation postdoctoral researcher who was based at UC Berkeley. “In fact, the rate of decline is much greater than that seen in the same region for the pika, a small mountain-dwelling cousin of the rabbit that has become the poster child for the effects of climate warming in the contiguous United States.”

Morelli added that the squirrels are thriving in areas that have been modified by humans. For example, irrigated Mono Lake County Park serves as an artificial oasis that sustains squirrel populations despite otherwise hot and dry conditions in eastern California.

“As predictions indicate that the range of the Belding’s ground squirrel could disappear out of California by the end of the century, these areas might be particularly important for this and other climate-impacted species,” said Morelli, who is now a technical advisor for the U.S. Forest Service’s International Programs in the Democratic Republic of Congo.

Although the Belding’s ground squirrel is widespread, the rapid decline in its distribution is of concern because it is an important source of food for raptors and carnivores. However, the paper suggests that even when climate change causes large range declines, some species can persist in human-modified areas.

“Taken together, these two studies indicate that many species have been responding to recent climate change, yet the complexities of a species’ ecological needs and their responses to habitat modification by humans can result in unanticipated responses,” said Steven Beissinger, professor at UC Berkeley’s Department of Environmental Science, Policy and Management and the senior author on both studies. “This makes it very challenging for scientists to project how species will respond to future climate change.”

Funding from the National Science Foundation, National Park Service, and California Landscape Conservation Cooperative helped support this research.