If trees could talk: Forest research network reveals global change effects

In addition to identifying, mapping, measuring and monitoring trees in the CTFS-ForestGEO study plots, researchers describe the relatedness of trees, track flower and seed production, collect insects, survey mammals, quantify carbon stocks and flows within the ecosystem, take soil samples and measure climate variables like rainfall and temperature. The thorough study of these plots provides insights into not only how forests are changing but also why. Credit: Beth King, STRI

Permafrost thaw drives forest loss in Canada, while drought has killed trees in Panama, southern India and Borneo. In the U.S., in Virginia, over-abundant deer eat trees before they reach maturity, while nitrogen pollution has changed soil chemistry in Canada and Panama. Continents apart, these changes have all been documented by the Smithsonian-led Center for Tropical Forest Science-Forest Global Earth Observatory, CTFS-ForestGEO, which released a new report revealing how forests are changing worldwide.

"With 107 collaborators we've published a major overview of what 59 forests in 24 countries, where we monitor nearly 6 million trees teach us about forest responses to global change," said Kristina Anderson-Teixeira, first author of the report and CTFS-ForestGEO and ecosystem ecologist based at the Smithsonian Conservation Biology Institute.

Many of the changes occurring in forests worldwide are attributable to human impacts on climate, atmospheric chemistry, land use and animal populations that are so pervasive as to warrant classification of a new geologic period in Earth's history -- the Anthropocene, the Age of Humans.

Measuring and understanding the effects of all these changes -- collectively termed "global change" -- are easier said than done. Some of the best information about these global-scale changes comes from CTFS-ForestGEO, the only network of standardized forest-monitoring sites that span the globe.

Since the censuses began at the first site on Barro Colorado Island in Panama in 1981, atmospheric carbon dioxide has increased by 16 percent. The forest sites in the network have warmed by an average of over 1 degree F (0.6 degree C) and experienced up to 30 percent changes in precipitation. Landscapes around protected sites experience deforestation.
The plot network now includes forests from Brazil to northern Canada, from Gabon to England and from Papua New Guinea to China.

In addition to identifying, mapping, measuring and monitoring trees, researchers describe the relatedness of trees, track flower and seed production, collect insects, survey mammals, quantify carbon stocks and flows within the ecosystem, take soil samples and measure climate variables like rainfall and temperature. The thorough study of these plots provides insights into not only how forests are changing but also why.

Climate change scenarios predict that most of these sites will face warmer and often drier conditions in the future -- some experiencing novel climates with no modern analogs. Forests are changing more rapidly than expected by chance alone, and shifts in species composition have been associated with environmental change. Biomass increased at many tropical sites across the network.

"It is incredibly rewarding to work with a team of forest scientists from 78 research institutions around the world, including four Smithsonian units" Anderson-Teixeira said. "CTFS-ForestGEO is a pioneer in the kind of collaborative effort it takes to understand how forests worldwide are changing."

"We look forward to using the CTFS-ForestGEO network to continue to understand how and why forests respond to change, and what this means for the climate, biodiversity conservation and human well-being," said Stuart Davies, network director.

Source: Smithsonian Tropical Research Institute

Colorado's Front Range fire severity not much different than past

A new study indicates present-day forest fires on Colorado's Front Range are not significantly more intense than historical fires. Credit: Glenn Asakawa, University of Colorado

The perception that Colorado's Front Range wildfires are becoming increasingly severe does not hold much water scientifically, according to a massive new study led by the University of Colorado Boulder and Humboldt State University in Arcata, Calif.

The study authors, who looked at 1.3 million acres of ponderosa pine and mixed conifer forest from Teller County west of Colorado Springs through Larimer County west and north of Fort Collins, reconstructed the timing and severity of past fires using fire-scarred trees and tree-ring data going back to the 1600s. Only 16 percent of the study area showed a shift from historically low-severity fires to severe, potential crown fires that can jump from treetop to treetop.

The idea that modern fires are larger and more severe as a result of fire suppression that allowed forest fuels to build up in the past century is still prevalent among some, said CU-Boulder geography Professor Thomas Veblen, a study co-author. "The key point here is that modern fires in these Front Range forests are not radically different from the fire severity of the region prior to any effects of fire suppression," he said.

A paper on the subject was published Sept. 24 in the journal PLOS ONE. The study was led by Associate Professor Rosemary Sherriff of Humboldt State University and involved Research Scientist Tania Schoennagel of CU-Boulder's Institute of Arctic and Alpine Research, CU-Boulder doctoral student Meredith Gartner and Associate Professor Rutherford Platt of Gettysburg College in Gettysburg, Pa.

The study was funded by the National Science Foundation.

"The common assumption is that fires are now more severe and are killing higher percentages of trees," said Sherriff, who completed her doctorate at CU-Boulder under Veblen in 2004. "Our results show that this is not the case on the Front Range except for the lowest elevation forests and woodlands."

One important new finding comes from a comparison of nine large fires that have occurred on the Front Range since 2000 -- including the 2002 Hayman Fire southwest of Denver, the 2010 Fourmile Canyon Fire west of Boulder and the 2012 High Park Fire west of Fort Collins -- with historic fire effects in the region.

"It's true that the Colorado Front Range has experienced a number of large fires recently," said Schoennagel. "While more area has burned recently compared to prior decades -- with more homes coming into the line of fire -- the severity of recent fires is not unprecedented when we look at fire records going back before the 1900s."

In addition, tree-ring evidence from the new study shows there were several years on the Front Range since the 1650s when there were very large, severe fires. The authors looked at more than 1,200 fire-scarred tree samples and nearly 8,000 samples of tree ages at 232 forest sample sites from Teller County to Larimer County.

The study is one of the largest of its kind ever undertaken in the western United States. The team was especially interested in fire records before about 1920, when effective fire suppression in the West began in earnest.

"In relatively dry ponderosa pine forests of the West, a common assumption is that fires were relatively frequent and of low severity, and not lethal to most large trees, prior to fuel build-up in the 20th century," said Veblen. "But our study results showed that about 70 percent of the forest study area experienced a combination of moderate and high-severity fires in which large percentages of the mature trees were killed."

Along the Front Range, especially at higher elevations, homeowners and fire managers should expect a number of high-severity fires unrelated to any kind of fire suppression and fuel build-up, said Schoennagel. "This matters because high-severity fires are dangerous to people, kill more trees and are trickier and more expensive to suppress."

"Severe fires are not new to most forests in this region," said Sherriff. "What is new is the expanded wildland-urban interface hazard to people and property and the high cost of suppressing fires for society."

In addition, a warming Colorado climate -- 2 degrees Fahrenheit since 1977 -- has become a wild card regarding future Front Range fires, according to the team. While fires are dependent on ignition sources and can be dramatically influenced by high winds, the team expects to see a substantial increase in Front Range fire activity in the low and mid-elevations in the coming years as temperatures continue to warm, a result of rising greenhouses gases in Earth's atmosphere.

Source: University of Colorado at Boulder

Mountain pine beetles get bad rap for wildfires, study says

Following wildfires in 2011, a UW-Madison research team studied lodgepole pine trees in the Northern Rocky Mountains to examine whether earlier outbreaks of mountain pine beetles changed the ecological impact of the wildfires. Credit: Turner Lab

Mountain pine beetles get a bad rap, and understandably so. The grain-of-rice-sized insects are responsible for killing pine trees over tens of millions of acres in the Western U.S. and Canada over the last decade.

But contrary to popular belief, these pests may not be to blame for more severe wildfires like those that have recently swept through the region. Instead, weather and topography play a greater role in the ecological severity of fires than these bark-boring beetles.

New research led by the University of Wisconsin-Madison and the Washington State Department of Natural Resources provides some of the first rigorous field data to test whether fires that burn in areas impacted by mountain pine beetles are more ecologically severe than in those not attacked by the native bug.

In a study published this week in the Proceedings of the National Academy of Sciences, UW-Madison zoology professor Monica Turner and her graduate student, Brian Harvey, show pine beetle outbreaks contributed little to the severity of six wildfires that affected more than 75,000 acres in the Northern Rocky Mountains in 2011. They also show that the beetle outbreaks, which occurred from 2000 through 2010, have not directly impacted post-fire recovery of the forests. The study does not, however, address fire behavior, such as how quickly fires spread or how dangerous they are to fight.

While the findings may exonerate the insect scapegoats, they should also help ecosystem managers better respond to changes in the face of climate-driven disturbances, like drought and warmer temperatures.

Large, severe fires are typical in the lodgepole pine forests found throughout the region, even without mountain pine beetle outbreaks. However, as the climate has warmed, outbreaks and big fires have both become more common. The phenomenon of more beetles has meant more dead trees, and some have grown concerned about how beetle attacks and wildfires may interact.

"The conventional wisdom is that a forest of dead trees is a tinder box just waiting to burn up," says Turner, who has long studied the forest landscape of the Mountain West. "There were very little data out there but a lot of concern."

Forests attacked by bark beetles -- which burrow into the bark of lodgepole pines to mate and incubate their larvae -- can seem nothing more than ample kindling for a raging blaze, with their dead wood and dry, reddish-brown needles.

The burrows the beetles carve under the bark of pines, called galleries, choke off water and nutrient circulation in the trees. The trees die and, for the first couple of years, they hold on to their dry, lifeless needles. Scientists call this the "red stage," and some believe these trees could fuel more severe fires.

By year three, most beetle-attacked trees have entered the "gray stage," dropping their once green pine foliage, becoming needleless wood carcasses.

Earlier studies from Turner's group suggested that beetle outbreaks would not lead to more severe fires. But without actual fires, the interaction could not be tested.

However, in 2011, wildfires throughout eastern Idaho and western Montana -- in forests that had experienced varying mountain pine beetle outbreak impacts -- provided opportunity for the research team to begin to answer the question: Do the two disturbances, beetle attacks and wildfire, together change the ecological response of the forest to fire?

Fortunately for the team, among the burned areas studied were pine stands that had not been attacked by beetles. These areas served as controls. Others suffered a range of mortality from the beetles; in some stands, beetles killed nearly 90 percent of the trees prior to wildfire. The fires that raged also ran the spectrum of severity, allowing the researchers to compare a number of variables.

Some study plots comprised mostly live trees, while others contained mostly red-stage or gray-stage trees -- allowing the researchers to assess whether plots with red-stage trees (with dry needles) experienced greater levels of fire severity than plots with mostly gray-stage trees (no needles), as they and others had expected.

The study team examined ecosystem indicators of fire severity, such as how many trees were killed by fire and how much char covered the forests.

Engaging in what Harvey calls "post-fire detective work," in 2012, the scientific team evaluated fire severity in each study plot and stripped sections of bark from over 10,000 trees to determine what killed them, beetles or fire. Beetle galleries can remain visible under the bark even after fire.

As they sifted through the blackened trees and forest floor, the team became covered with ash and soot.

"We looked like coal miners when we were done," says Harvey.

They found that the severity of the outbreak and whether trees were in the red or gray stage had almost no effect on fire severity under moderate burning conditions.

Only under more extreme fire-burning conditions -- when it was hot, dry and windy -- did areas with more beetle-killed trees show signs of more ecologically severe fires, such as more deeply burned trunks and crowns (the part of the tree that includes its limbs and needles). The presence of more gray-stage trees actually had a stronger impact on fire severity than the amount of red-stage trees, to the surprise of the scientists.

Overall, however, Turner says the effects of beetle outbreaks on fire severity took a back seat to stronger drivers -- primarily weather and topography. Fire severity increased under more extreme weather, regardless of pre-fire outbreaks, and forest stands higher in the landscape burned more severely than those at lower elevation as fires moved uphill, building momentum.

"No one says beetle-killed forests won't burn," says Turner. "The data set looks at whether they burn with different severity compared to unattacked forests burning under similar conditions."

The team was also interested in whether beetle outbreaks slowed the recovery of the forests after fires. Lodgepole pines are adapted to fire, containing two types of seed-carrying cones: those that release seeds as soon as they mature and those that require fire to open, blanketing the forest floor with potential new life following a blaze.

By counting the number of post-fire tree seedlings in their plots, the researchers found very little beetle-related impact. Tree seedlings were most numerous where more of the fire-killed trees bore the fire-adapted, or serotinous, cones. Beetle-killed trees likely contributed to post-fire seedling establishment, too, as their seeds remain viable in cones if they are not consumed in fire. Only high-reaching char from tall flames reduced the number of seed-spreading cones.

The scientists emphasize the results may differ in other forest types or with different lengths of time between beetle outbreaks and fire.

"These are both natural disturbances, fire and beetle outbreaks," says Turner. "It's not surprising the ecosystem has these mechanisms to be resilient. What we as people see as catastrophes are not always catastrophes to the ecosystem."

Source: University of Wisconsin-Madison

Climate change not responsible for altering forest tree composition, experts say

Eastern US forest canopy. Credit: Mary Ann Fajvan, West Virginia University, Bugwood.org

Change in disturbance regimes -- rather than a change in climate -- is largely responsible for altering the composition of Eastern forests, according to a researcher in Penn State's College of Agricultural Sciences.

Forests in the Eastern United States remain in a state of "disequilibrium" stemming from the clear-cutting and large-scale burning that occurred in the late 1800s and early 1900s, contends Marc Abrams, professor of forest ecology and physiology.

Moreover, Abrams noted, since about 1930 -- during the Smokey Bear era -- aggressive forest-fire suppression has had a far greater influence on shifts in dominant tree species than minor differences in temperature.

"Looking at the historical development of Eastern forests, the results of the change in types of disturbances -- both natural and man-caused -- are much more significant than any change in climate," said Abrams, who is the Steimer Professor of Agriculture in the Department of Ecosystem Science and Management.

"Over the last 50 years, most environmental science has focused on the impact of climate change. In some systems, however, climate change impacts have not been as profound as in others. This includes the forest composition of the eastern U.S."

To determine how forest tree species have responded to changes in disturbance regimes, temperature and precipitation over long periods of time, Abrams collaborated with Gregory Nowacki, a scientist with the U.S. Department of Agriculture Forest Service office in Milwaukee, on a study of the tolerance and sensitivity of trees to various factors.

"Many ecological phenomena combine to direct vegetation trends over time, with climate and disturbance playing prominent roles," said Nowacki, who received his Ph.D under Abrams. "To help decipher their relative importance during Euro-American times, we employed a unique approach whereby tree species/genera were partitioned into temperature, shade tolerance and pyrogenicity classes and applied to comparative tree-census data."

The researchers compared presettlement -- original land survey data -- and current vegetation conditions in the eastern United States. Early tree surveys chronicle the westward progression of European land acquisition, with some dating back to the 1600s along the East Coast.

In the research, published online in Global Change Biology, researchers analyzed 190 datasets to determine the relative impacts of climate versus altered disturbance regimes for various biomes across the eastern United States. Because the Euro-American period from 1500 to today spans two major climatic periods -- from the Little Ice Age to the current Anthropocene -- the researchers expected vegetation changes consistent with warming.

"In most cases, however, European disturbance overrode regional climate change," Abrams said. "To the north, intensive and expansive early European disturbance resulted in the ubiquitous loss of conifers and large increases of Acer (maple), Populus (poplar) and Quercus (oak) in northern hardwoods, whereas to the south, these disturbances perpetuated the dominance of oak in central hardwoods."

Maple increases and associated mesophication -- the forest growing increasingly dense, cool, shady and moist in the absence of regular fire -- in oak-pine systems were delayed until mid-20th century fire suppression. This led to significant warm-to-cool shifts in temperature in which cool-adapted sugar maple increased and caused temperature-neutral changes in which warm-adapted red maple increased.

"In both cases, these shifts were attributed to fire suppression rather than climate change," Abrams said. "Because mesophication is ongoing, eastern U.S. forests formed during the catastrophic disturbance era followed by fire suppression will remain in climate disequilibrium into the foreseeable future."

Overall, he concluded, the results of the study suggest that altered disturbance regimes rather than climate had the greatest influence on vegetation composition and dynamics in the eastern United States over multiple centuries.

"Land-use change often trumped or negated the impacts of warming climate, and this needs greater recognition in climate change discussions, scenarios and model interpretations," he said.

Source: Penn State

Coexist or perish, new wildfire analysis says: Changing wildfire paradigm from fighting to coexistence

The lightning-sparked Castle Rock Fire burned nearly 50,000 acres in 2007 in the Sawtooth National Forest and adjacent state and private lands surrounding Ketchum, Idaho, in the Smoky Mountains region of the Rocky Mountain range. Credit: Kari Geer, courtesy of the National Interagency Fire Center

Many fire scientists have tried to get Smokey the Bear to hang up his "prevention" motto in favor of tools like thinning and prescribed burns, which can manage the severity of wildfires while allowing them to play their natural role in certain ecosystems.

But a new international research review led by the University of California, Berkeley, says the debate over fuel-reduction techniques is only a small part of a much larger fire problem that will make society increasingly vulnerable to catastrophic losses unless it changes its fundamental approach from fighting fire to coexisting with fire as a natural process.

The paper, "Learning to Coexist with Wildfire," to be published in the Nov. 6 issue of the journal Nature, examines research findings from three continents and from both the natural and social sciences. The authors conclude that government-sponsored firefighting and land-use policies actually encourage development on inherently hazardous landscapes, amplifying human losses over time.

"We don't try to 'fight' earthquakes -- we anticipate them in the way we plan communities, build buildings and prepare for emergencies. We don't think that way about fire, but our review indicates that we should," said lead author Max Moritz, Cooperative Extension specialist in fire at UC Berkeley's College of Natural Resources. "Human losses will only be mitigated when land-use planning takes fire hazards into account in the same manner as other natural hazards, like floods, hurricanes and earthquakes."

The analysis looked at different kinds of natural fires, what drives them in various ecosystems, the ways public response to fire can differ, and the critical interface zones between built communities and natural landscapes. The authors found infinite variations on how these factors can come together.

"It quickly became clear that generic one-size-fits-all solutions to wildfire problems do not exist," Moritz said. "Fuel reduction may be a useful strategy for specific places, like California's dry conifer forests, but when we zoomed out and looked at fire-prone regions throughout the Western United States, Australia and the Mediterranean Basin, we realized that over vast parts of the world, a much more nuanced strategy of planning for coexistence with fire is needed."

Planning for co-existence

If humans choose to live in fire-prone regions, fire must be managed on par with other naturally occurring hazards, the authors argue, and research must seek to understand what factors and outcomes we can and cannot affect.

One common tool is applicable to the vast array of ecological and social science interactions at the critical wildfire/urban interface: more effective land-use planning, along with the regulations that guide it.

The authors recommend prioritizing location-specific approaches to improve development and safety in fire-prone areas, including:

Adopting new land-use regulations and zoning guidelines that restrict development in the most fire-prone areas;

Updating building codes, such as requiring fire-resistant construction to match local hazard levels and encouraging retrofits to existing ignition-prone homes;

Implementing locally appropriate vegetation management strategies around structures and neighborhoods;

Evaluating evacuation planning and warning systems, including understanding situations in which mandatory evacuations are or are not effective;

Developing household and community plans for how to survive stay-and-defend situations; and

Developing better maps of fire hazards, ecosystem services and climate change effects to assess trade-offs between development and hazard.

As an example of positive steps, the report cites new fire danger mapping efforts, including an existing fire hazard severity zone map that guides building codes in California. Produced by the state's Department of Forestry and Fire Protection, the current map does not explicitly incorporate locally varying wind patterns, which influence the worst fire-related losses of homes and lives, but future iterations will include these data.

Fire ecology and climate

The authors underscore that wildfires are a natural part of many ecosystems and can have a positive long-term influence on the landscape, despite people labeling them as "disasters." They can stimulate vegetation regeneration, promote a diversity of vegetation types, provide habitat for many species and sustain other ecosystem services, such as nutrient cycling.

Around the world, the numbers, sizes, and intensities of fires vary greatly. In some ecosystems, big, severe wildfires are natural events and more climate-driven -- by drought or high winds -- so fuel reduction is not a very effective tool in these locations. By contrast, many ecosystems that would naturally experience frequent lower-severity fires may respond to vegetation management aimed at both reducing fire hazard to humans and restoring crucial ecosystem processes. But, the authors agree, where fuel reduction is an appropriate goal, it would ideally be achieved by letting wildfires do their job.

A changing climate will complicate management strategies.

"How should future fire patterns compare to this historical variability? That's the big question," Moritz said.

Describing wildfire as "one of the most basic and ongoing natural processes on Earth," the authors call for a paradigm shift in the way society interacts with it, changing to an approach that achieves long-term, sustainable coexistence that benefits the planet's ecosystems on the landscape scale, while minimizing catastrophic losses on the human scale.

"A different view of wildfire is urgently needed," said Moritz. "We must accept wildfire as a crucial and inevitable natural process on many landscapes. There is no alternative. The path we are on will lead to a deepening of our fire-related problems worldwide, which will only become worse as the climate changes."

Source: University of California - Berkeley

Even in restored forests, extreme weather strongly influences wildfire's impacts

Fire Sweeps Up, South Flank, Rim Fire. Credit: Mike McMillan - USFS

The 2013 Rim Fire, the largest wildland fire ever recorded in the Sierra Nevada region, is still fresh in the minds of Californians, as is the urgent need to bring forests back to a more resilient condition. Land managers are using fire as a tool to mimic past fire conditions, restore fire-dependent forests, and reduce fuels in an effort to lessen the potential for large, high-intensity fires, like the Rim Fire. A study led by the U.S. Forest Service's Pacific Southwest Research Station (PSW) and recently published in the journal Forest Ecology and Management examined how the Rim Fire burned through forests with restored fire regimes in Yosemite National Park to determine whether they were as resistant to high-severity fire as many scientists and land managers expected.

Since the late 1960s, land managers in Yosemite National Park have used prescribed fire and let lower intensity wildland fires burn in an attempt to bring back historical fire regimes after decades of fire suppression. For this study, researchers seized a unique opportunity to study data on forest structure and fuels collected in 2009 and 2010 in Yosemite's old-growth, mixed-conifer forests that had previously burned at low to moderate severity. Using post-Rim Fire data and imagery, researchers found that areas burned on days the Rim Fire was dominated by a large pyro-convective plume -- a powerful column of smoke, gases, ash, and other debris -- burned at moderate to high severity regardless of the number of prior fires, topography, or forest conditions.

"The specific conditions leading to large plume formation are unknown, but what is clear from many observations is that these plumes are associated with extreme burning conditions," says Jamie Lydersen, PSW biological science technician and the study's lead author. "Plumes often form when atmospheric conditions are unstable, and result in erratic fire behavior driven by its own local effect on surface wind and temperatures that override the influence of more generalized climate factors measured at nearby weather stations."

When the extreme conditions caused by these plumes subsided during the Rim Fire, other factors influenced burn severity. "There was a strong influence of elapsed time since the last burn, where forests that experienced fire within the last 14 years burned mainly at low severity in the Rim Fire. Lower elevation areas and those with greater shrub cover tended to burn at higher severity," says Lydersen.

When driven by extreme weather, which often coincides with wildfires that escape initial containment efforts, fires can severely burn large swaths of forest regardless of ownership and fire history. These fires may only be controlled if more forests across the landscape have been managed for fuel reduction to allow early stage suppression before weather- and fuels-driven fire intensity makes containment impossible. Coordination of fire management activities by land management agencies across jurisdictions could favor burning under more moderate weather conditions when wildfires start and reduce the occurrences of harmful, high-intensity fires.

Source: USDA Forest Service - Pacific Southwest Research Station

Seeing the forest for the trees: Youngest trees in a forest tell the biggest story

LSU ecologist Kyle Harms co-authors first study to quantify the process of diversification in forests and likely all other sessile ecosystems. Credit: Louisiana State University

The largest trees in a forest may command the most attention, but the smallest seedlings and youngest saplings are the ones that are most critical to the composition and diversity of the forest overall. While many people gaze up into the forest canopy, scientist Joseph Connell has spent much of his career looking down quite closely at the forest understory. Connell, who is a professor emeritus in the Department of Ecology, Evolution and Marine Biology at the University of California at Santa Barbara, established one of the world's longest, in-depth ecological research studies on the planet. The Connell Plots Rainforest Network has thus far produced a 50-year collection of data on individual trees in Australia's protected rainforests.

"Having such a long-term, detailed dataset is highly unusual. It's the kind of temporal depth we need to answer some of the big questions such as, what are the ecological processes that maintain diversity?" said Kyle Harms, professor in the LSU Department of Biological Sciences and a collaborator with Connell.

Early in his career, Harms was a post-doctoral researcher in Connell's lab at U.C. Santa Barbara. There, he met former fellow post-doctoral researcher and current collaborator Peter Green, who is a senior lecturer at La Trobe University in Melbourne, Australia.

Harms and Green were inspired to use their mentor's dataset; therefore, they devised an analysis to test the long-standing hypothesis that the patterns of composition and diversity among a forest's mature trees are largely set by processes that occur in trees' earliest life stages. Harms ran statistical analyses on 7,977 individual trees across 186 species that were censused in one of Connell's tropical Australian forest plots from 1971-2013.

He repeatedly ran simulation analyses on six tiers of trees based on size in order to predict the expected outcome of diversity at each tier. Then he compared the expected levels of diversity in each tier with the true collected data.

"What we found was that the seedlings are more diverse than the statistical expectations predicted them to be, but the larger trees' levels of diversity were about the same as the predictions" he said.

These results are the first quantitative evidence that the earliest life cycle stages of individual trees are more critical than later stages to the overall relative abundances of mature trees in a forest. Their findings will be published online in the Proceedings of the National Academy of Sciences this week.

The stronger influence of ecological sorting processes operating at the earliest life cycle stages compared to later life stages, which they quantified, also likely occurs in other highly diverse ecosystems with rooted, or sessile, organisms including grasslands, herbaceous plant communities and marine communities of coral.

"I think this is something that is happening broadly in ecosystems across the planet," Harms said.

He and his collaborators' results underscore the importance of support for long-term, in-depth datasets, as well as the need to investigate the early life stages -- for example, the smallest, newly germinated seedlings -- where the most critical processes are occurring.

"I think it helps us understand where to focus in order to really understand the biased sorting processes that create the composition and diversity patterns in the forest overall," he said.

Source: Louisiana State University