News Article | April 27, 2017
The ability of some Western conifer forests to recover after severe fire may become increasingly limited as the climate continues to warm, scientists from the Smithsonian Conservation Biology Institute (SCBI) and Harvard Forest found in a new study published today in Global Change Biology. Although most of these cone-bearing evergreen trees are well adapted to fire, the study examines whether two likely facets of climate change -- hotter, drier conditions and larger, more frequent and severe wildfires -- could potentially transform landscapes from forested to shrub-dominated systems. As part of the study, which was funded by the National Science Foundation, scientists examined conifer forests in the richly diverse Klamath region of northern California and southwestern Oregon. The Klamath region is a botanical hotspot, home to 29 species of conifers and a suite of plant species that exist nowhere else on earth. The researchers sampled sites that burned severely in wildfires between 1987 and 2008. They found that, after fire, hardwood trees and shrubs quickly established by either re-sprouting from surviving root systems or growing rapidly from seeds that persisted in the soil. These plants dominated the vegetation for at least the first few decades after fire. Most conifers, on the other hand, were slow to compete, relying on establishment of new seedlings borne by trees in less severely burned patches or from outside the fire perimeter. As a result, conifers had only a few years to establish before the regenerating hardwoods and shrubs grew dense enough to suppress them. "If they miss that window there's much less chance of successful establishment and their growth will be slower," says study author Kristina Anderson-Teixeira, a forest ecologist at SCBI and the Smithsonian Tropical Research Institute. In fact, the study found that the longer the interval between the fire and the conifer's establishment, the slower the tree's growth. "The Klamath ecosystem is an important transition zone separating the shrubs of the California chaparral from the Pacific Northwest's temperate rainforest," says Jonathan Thompson, a Senior Ecologist at Harvard Forest and co-author on the study. "Our work suggests climate change will push the chaparral north at the expense of the Klamath's existing conifer forests." Because most conifers depend on seed dispersal from surviving trees, larger patches of high-severity fire could put a growing portion of the landscape at risk of poor post-fire conifer regeneration. The study suggests this trend could be even more pronounced because under drier conditions more abundant seed sources are needed to support conifer seedlings at densities sufficient for forest recovery. In addition, previous research by Thompson and others suggests the young, shrub-dominated vegetation that develops after severe fire tends to burn more severely in subsequent fires than older conifer forests, meaning that once severe fire converts a conifer forest to a shrub-dominated system, the non-forested vegetation could be perpetuated almost indefinitely through a cycle of repeated burning. "We see climate change affecting the system from two directions," says Thompson. "First, it is slowing conifer growth, keeping them low to the ground and more vulnerable to future fires for a longer period of time. Second, climate change is making fire more frequent. This phenomenon, which researchers call the 'interval squeeze,' threatens to transform this and other arid, fire-prone forests worldwide." Still, portions of the landscape may be relatively resilient. For example, conifers were able to regenerate in wetter sites, even amid relatively large high-severity patches with few surviving trees. "The Klamath region has supported conifers for thousands of years," says Thompson. "Some patches will surely survive no matter what climate throws at them." The researchers hope these findings could help provide information needed to prioritize management efforts. "Our study helps to identify the places that are at greatest risk of forest loss, where managers could either target management to promote post-fire forest recovery, or accept that we're going to see some degree of landscape transformation in the coming decades and learn to meet ecological objectives under the new climate and disturbance regimes," says Alan Tepley, a forest ecologist with SCBI and the study's lead author. These findings could also be applied in a broader context to other forest ecosystems. "There are concerns for much of the western U.S. and other similar landscapes that under climate change, forests may be less likely to regenerate," says Anderson-Teixeira. "And that can then reduce forest cover on the landscape and result in big losses of carbon storage." According to Anderson-Teixeira, the fate of the Klamath region depends in part on societal carbon emissions, where increased emissions lead to more warming, which ultimately could result in more forest loss. An additional author on this paper is Howard Epstein from the University of Virginia. The study is part of a large collaborative effort that includes the US Forest Service and Portland State University. The Harvard Forest, founded in 1907 and located in Petersham, Mass., is Harvard University's outdoor laboratory and classroom for ecology and conservation, and a Long-Term Ecological Research (LTER) site funded by the National Science Foundation. Its 4,000 acre property is one of the oldest and most intensively studied research forests in the U.S. In addition to studying New England landscapes, research scientists at the Forest study ecosystems around the U.S. and the globe. More information can be found at http://harvardforest. . SCBI plays a leading role in the Smithsonian's global efforts to save species from extinction and train future generations of conservationists. SCBI spearheads research programs at its headquarters in Front Royal, Va., the Smithsonian's National Zoo in Washington, D.C., and at field research stations and training sites worldwide. SCBI scientists tackle some of today's most complex conservation challenges by applying and sharing what they learn about animal behavior and reproduction, ecology, genetics, migration and conservation sustainability. For interviews with a Harvard Forest scientist or contacts for SCBI scientists, contact Clarisse Hart, email@example.com; 978-756-6157.
News Article | January 19, 2016
With another year of record-breaking warmth just over, the passage of a new climate treaty is definitely timely. But even if these new diplomatic and legislative efforts are successful the planet is ensured some amount of long-term warming—enouigh that around 8% of species are likely to go extinct due to climate change. If the warming is greater than current models project, that number could double. In all likelihood, though, those numbers are an underestimate, because there are species that no climate scientists are keeping track of. In fact, there are a lot of them, and we may come to miss them quite a bit if they disappear. Who are these guys? Microbes. There are currently no published studies of how climate change may cause extinction of microbial species. However, there are researchers exploring how warming can alter microbial communities, and their results tend to show that microbial consortia and their functioning are sensitive to climate change. In experiments at the Harvard Forest, for example, long-term artificial warming led to reductions in both total and active microbial biomass as well as to changes in how the soil community behaved1. Even short-term experiments with just a few months of warming have found reductions in growth and functional diversity as well as evidence for local extinctions2. Interestingly, immigration of new bacteria to those communities was unable to rescue them from these effects, indicating that the ease with which microbes are able to move around may not be enough to prevent negative impacts of climate change. An alternative way of studying microbial sensitivity is to transplant intact communities to warmer sites. Recent work doing this with grassland soils in China has also shown loss of soil microbial biomass as well as changes in composition and function3. The microbial community was not resistant to warming—it changed—and it wasn’t resilient—it didn’t recover over time through acclimation to the new environment. These results, combined with a significant reduction in diversity, imply that species were lost from the community. These data were not analyzed to test specifically for local extinction events, however, so while it seems like extinctions were likely we cannot know for sure. Admittedly it can be pretty hard to study threats to microbes. Most of the species we’ve found have not been documented as anything more than a snippet of a genetic sequence, and there are untold numbers out there for which we don’t even have that. At last count, there were 84 named bacterial phyla (one of the broadest taxonomic groupings), but we have cultured representatives for only 12 of those4. It is very hard to experimentally determine sensitivity to changing conditions for microbes that you can’t isolate and grow. The cultured species that we have in cell line collections or biobanks like ATCC and the German equivalent DSMZ focus almost exclusively on type strains rather than capturing the diversity possible within species—so how much variation there is in sensitivity is even harder to study. On the flip side, keeping track of species in wild communities can be notoriously difficult, particularly with rare taxa, so it may be almost impossible to document a real extinction event with certainty. “Maybe we don’t need to worry about microbes,” you might think. “They grow all sorts of places. Heck I can’t even keep my house clean, so I’m sure they’ll be fine.” You wouldn’t be the only one to think this. Microbes as a group (which is not actually a single, evolutionarily related group. I’m talking here about bacteria, archaea, and microscopic eukaryotes, i.e. the vast majority of life on earth) have survived all the mass extinctions (arguably causing at least one of them), and they can be found in the most extreme environments on the planet. But it turns out that most of them are extremely specialized5. So as we lose the habitats they are specialized to—climate change is expected to decimate wide swaths of ice sheets, permafrost soils, mangrove forests, and areas above tree line—it is very likely that species that are specific to these habitats will also be lost. There is at least one group of microbes we can be certain is going to go extinct—all the bugs who are exclusively associated with a species of plant or animal that itself is going to be impacted by climate change. These include the bacteria that make amino acids for insects threatened by shifting phenology, the dinoflagellates that share their photosynthesized sugars with corals dying from ocean acidification, and the nitrogen-fixing bacteria whose legume hosts need cool environments. Host-association is just another form of habitat specialization, and at least 8% of those habitats seem destined for destruction. Even microbes that aren’t necessary for the growth of their hosts will be threatened by losing their partners. Climate change researchers have been calling for a greater inclusion of interactions in models of climate change induced species loss because biotic interactions are an important determinant of species success. Microbe-host interactions are critical for this from the perspective of both parties. While inability to disperse or evolve under new conditions is expected to limit microbial taxa much less than larger organisms, it is not really clear how much these factors will protect microbial species. While high mutation rates, horizontal gene transfer, and short generation times can produce fast evolution in microbes, it can still be hard for microbes to adapt. For example, in Richard Lenski’s long term evolution experiment, E. coli was able to evolve the ability to use a new sugar they couldn’t before6. But, the mutation arose in only one of the populations, and the rate of the necessary mutations arising was just once per trillion cell divisions. Furthermore, many microbes are extremely slow growing—with doubling times in the tens of thousands of years. How much evolution could serve to buffer these taxa from climate change remains totally unknown. Even if microbes can adapt fast enough, entire suites of lifestyles may be lost as the environments that require them disappear. If the great-great-great-great granddaughter of a cell is still around but has evolved a totally new suite of functions, isn’t it fair to say the original “species” has been lost? And in the face of increased variability in climate, the loss of a function like cold tolerance may result in ensured extinction down the line. In recorded history, we know of only two microbial species that have gone extinct. Both were driven out intentionally by humans. These events were met with headlines like Rinderpest, Scourge of Cattle, is Vanquished. Today we mourn the passing of the passenger pigeon—to the extent that there are active attempts to bring them back—but not that of smallpox. Continued efforts to extinguish other pathogenic microbes, like polio and malaria, are lauded and given multi-million dollar campaigns. But in the future, the microbes we lose while we weren’t looking may be missed as much as the American Pika and coral reefs. Microbes play critical roles in biogeochemical cycling, plant productivity, human health, and even climate itself. They produce much of the oxygen we breathe. We cannot begin to predict how the earth would function without 16% of them, and this is in no small part because we haven’t even tried. More research is needed into the likely impacts of climate change on microbes, although it may be too late for many already. Thanks to programs like the Earth Microbiome Project, we may at least one day be able to identify what we have lost, but in the absence of efforts to systematically preserve microbial species diversity repopulation efforts will be impossible. 1. Frey, S. D., Drijber, R., Smith, H., & Melillo, J. (2008). Microbial biomass, functional capacity, and community structure after 12 years of soil warming. Soil Biology and Biochemistry, 40(11), 2904-2907. 2. Lawrence, D., Bell T., & Barraclough T. G. (2016). The effect of immigration on the adaptation of microbial communities to warming. The American Naturalist, 187(2). 4. Youssef, N. H., Couger, M. B., McCully, A. L., Criado, A. E. G., & Elshahed, M. S. (2015). Assessing the global phylum level diversity within the bacterial domain: A review. Journal of Advanced Research, 6(3), 269-282. 6. Blount, Z. D., Borland, C. Z., & Lenski, R. E. (2008). Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli. Proceedings of the National Academy of Sciences, 105(23), 7899-7906.
Archetti M.,Harvard University |
Archetti M.,University of East Anglia |
Richardson A.D.,Harvard University |
O'Keefe J.,Harvard Forest |
Delpierre N.,University Paris - Sud
PLoS ONE | Year: 2013
Climate change affects the phenology of many species. As temperature and precipitation are thought to control autumn color change in temperate deciduous trees, it is possible that climate change might also affect the phenology of autumn colors. Using long-term data for eight tree species in a New England hardwood forest, we show that the timing and cumulative amount of autumn color are correlated with variation in temperature and precipitation at specific times of the year. A phenological model driven by accumulated cold degree-days and photoperiod reproduces most of the interspecific and interannual variability in the timing of autumn colors. We use this process-oriented model to predict changes in the phenology of autumn colors to 2099, showing that, while responses vary among species, climate change under standard IPCC projections will lead to an overall increase in the amount of autumn colors for most species. © 2013 Archetti et al.
News Article | December 20, 2016
Over the past centuries, as we humans have cleared fields for farms, built roads and highways, and expanded cities ever outward, we've been cutting down trees. Since 1850, we've reduced global forest cover by one-third. We've also changed the way forests look: much of the world's woodlands now exist in choppy fragments, with 20 percent of the remaining forest within 100 meters of an edge, like a road, backyard, cornfield, or parking lot. Scientists have studied fragmented forests for decades, mostly to gauge their effects on wildlife and biodiversity. But recently, two Boston University College of Arts & Sciences (CAS) scientists -- Andrew Reinmann (GRS'14), a postdoctoral research associate, and Lucy Hutyra, an associate professor of Earth and environment -- have turned their attention to another issue: the effects of forest fragments on carbon storage and climate change. They found that temperate broadleaf forests, like the stands of red oak common in New England, absorb more carbon than expected along their edges, but they also found that those edges are more susceptible to heat stress. The research, funded by the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and the National Science Foundation, and published in the December 19, 2016 issue of the Proceedings of the National Academy of Sciences, offers some good news and bad news about forest fragmentation. It suggests that while these forests may be more valuable carbon sinks than previously thought, they are also more sensitive to climate change. "Having accurate estimates of what those trees on the edge are doing--how much carbon they're taking out of the atmosphere--is really important when we think about our future climate," says Reinmann, lead author on the paper. The annual atmospheric concentration of carbon dioxide (CO2), a potent greenhouse gas and agent of global warming, has increased by more than 40 percent since the start of the Industrial Revolution and continues to rise. Forests play a critical role as a carbon sink, absorbing about 25 percent of the CO2 emissions we humans put into the sky. Most of our understanding of forest carbon dynamics comes from studying intact rural forests like Hubbard Brook in New Hampshire's White Mountains and Harvard Forest in Petersham, MA, not from studying forest fragments. "When you fragment a forest, you change a lot of the growing conditions of the forest that's left behind," says Reinmann, "but we don't have a very good understanding of how that change affects carbon sequestration and storage." To find out, Reinmann and Hutyra gathered data from 21 fragmented forest plots around Boston, measuring about 500 trees. In eight of those plots, they went a step further, taking sample cores from trees above 10 centimeters in diameter, a total of 420 cores from 210 trees. They used the cores, and other data, to calculate how fast the trees grew. A tree's size and growth rate indicate how much carbon it can absorb and also how much stress it's experiencing. Reinmann and Hutyra found that forest fragments grow faster along the edges than intact forests, absorbing more carbon than expected. "When you create that edge, you essentially are reducing competition and freeing up resources like light, water, and nutrients for trees," says Reinmann, who notes that the effect extends in about 20 meters from the forest edge. Curiously, the finding may hold only for temperate broadleaf forests common in New England, the Appalachians, Canada, and Europe. Amazon rainforest has the opposite effect when fragmented, with lower biomass and less carbon storage along the edges. "Foresters and loggers have known this intuitively for a long time: if you go in and you reduce the competition for resources, the remaining individuals will grow faster," adds Hutyra. "The novel piece of this work was to quantify it across these edges, see how far into the forest it goes, and put it into context with how much this fragmentation matters in a portion of the world--southern New England--that we know is a large net carbon sink." Though this seems like a win for our patchy New England forests, deforestation is still bad for carbon sequestration overall. "When you fragment a forest, the remaining forest can offset a little bit of what was lost, but not completely," says Reinmann. "So it may not be as terrible from a carbon perspective as we thought, but it's still bad." Offsetting this (somewhat) good news is the paper's other finding: these forest edges, more exposed to wind and sun, grow more slowly when stressed by heat. "You lose a lot of carbon benefit in hot years," says Reinmann, who found that the "magic number" for local trees is about 27°C (80.6°F), which corresponds to the average high temperature in July, our hottest month. "But once you get much past that threshold, the trees grow much slower," he says. And the really bad news: if regional temperatures continue to increase at a steady pace, the current carbon benefit offered by forest edges may decline significantly. "If this carbon sink all of a sudden shuts off, our projections for future climate will change," says Reinmann. "So our current understanding and ecological models, which don't account for this, are missing something important." Reinmann and Hutyra are currently expanding the work to study rural forests and are so far finding even larger effects there. They are also hoping to use high-resolution imaging and more precise chemical analyses to look closer at core samples to see how growth and photosynthesis change over days, seasons, heat waves, and other environmental stressors. More data may lead to better models, says Hutyra. "As we continue to more actively manage our landscape, whether it be thinking about agricultural intensification in Brazil or urban expansion in China or sprawling urban development here, the fragmenting of the landscape is ubiquitous. It's likely to stay, if not increase," says Hutyra. "And so quantifying the effects of all this fragmentation is really important for understanding the long-term and short-term ability of forests to continue to take up carbon, and for us to be able to accurately model that to project future climate."
Brooks R.T.,University of Massachusetts Amherst |
Colburn E.A.,Harvard Forest
Journal of the American Water Resources Association | Year: 2011
Effective regulatory protection and management of headwater resources depend on consistent and accurate identification and delineation of stream occurrence. Published maps and digital resources fail to represent the true occurrence and extent of headwater streams. This study assessed the accuracy of mapped origins of "blue-line" streams depicted on U.S. Geological Survey topographic maps, and, if present, the morphological characteristics of unmapped stream segments. We identified 170 mapped stream origins on the Quabbin Reservoir watershed, Massachusetts. Of 30 mapped stream origins, we identified and examined 26 unmapped stream segments above 25, with an average length of 502m. Twenty unmapped tributaries occurred on 10 of the 26 unmapped segments, with an average length of 127m. Wetland reaches occurred more frequently and were larger on unmapped than on mapped stream segments. A significant and complex stream network occurs above most mapped stream origins. For the Quabbin watershed, we estimate that there are 85.8km of unmapped stream upgradient of 314.5km of mapped streams. Reliance on mapped stream networks for regulatory standards allows for the potential disturbance or even destruction of the unmapped stream resources. Jurisdictional regulations and guidelines should be revised so that the occurrence of streams should require field validation. © 2010 American Water Resources Association. This article is a US Government work and is in the public domain in the USA.
Hufkens K.,Boston University |
Friedl M.A.,Boston University |
Keenan T.F.,Harvard University |
Sonnentag O.,Harvard University |
And 4 more authors.
Global Change Biology | Year: 2012
In the spring of 2010, temperatures averaged ~3 °C above the long-term mean (March-May) across the northeastern United States. However, in mid-to-late spring, much of this region experienced a severe frost event. The spring of 2010 therefore provides a case study on how future spring temperature extremes may affect northeastern forest ecosystems. We assessed the response of three northern hardwood tree species (sugar maple, American beech, yellow birch) to these anomalous temperature patterns using several different data sources and addressed four main questions: (1) Along an elevational gradient, how was each species affected by the late spring frost? (2) How did differences in phenological growth strategy influence their response? (3) How did the late spring frost affect ecosystem productivity within the study domain? (4) What are the potential long-term impacts of spring frost events on forest community ecology? Our results show that all species exhibited early leaf development triggered by the warm spring. However, yellow birch and American beech have more conservative growth strategies and were largely unaffected by the late spring frost. In contrast, sugar maples responded strongly to warmer temperatures and experienced widespread frost damage that resulted in leaf loss and delayed canopy development. Late spring frost events may therefore provide a competitive advantage for yellow birch and American beech at the expense of sugar maple. Results from satellite remote sensing confirm that frost damage was widespread throughout the region at higher elevations (>500 m). The frost event is estimated to have reduced gross ecosystem productivity by 70-153 g C m -2, or 7-14% of the annual gross productivity (1061 ± 82 g C m -2) across 8753 km 2 of high-elevation forest. We conclude that frost events following leaf out, which are expected to become more common with climate change, may influence both forest composition and ecosystem productivity. © 2012 Blackwell Publishing Ltd.
News Article | October 26, 2016
Adding warmth predicted in climate-change models destabilized forest ant communities east of the Appalachian Mountains, a possible harbinger of disruption to the broader ecosystem, researchers, led by a Case Western Reserve University biologist, have found. The five-year study in the Harvard Forest of Northeast Massachusetts and Duke Forest in the Piedmont Region of North Carolina suggests the loss of stability makes communities less resilient and slower to rebound when disturbed. The research is published today (Wed. Oct. 26) in the journal Science Advances. "We've had a unique opportunity to look under the hood of how these ant communities function,and how experimental warming affects their overall stability under climate change," said Sarah Diamond, an assistant professor of biology at Case Western Reserve and leader of the study. "We've looked at not only at the direct effects of warming but at indirect effects mediated by altered species interactions," Diamond said. "There's good evidence the altered species interactions are affecting the stability of communities, making them more fragile and susceptible to environmental change." By their numbers, ants comprise more than half the macroinvertibrates in North American forests. Harvard and Duke forests are home to 60 species. The study focused on floor dwellers, which are important to the forest ecosystem as scavengers and seed dispersers. The insects aerate the soil and are regular prey to other animals. The species studied compete for food and nest sites and typically forage within a yard of their nest. The scientists erected in each forest 15 chambers--pens 5.5 yards across, encircled with plastic sheets and left open wide at the top and open enough at the bottom to allow crawling insects to migrate in and out. The researchers installed four nest boxes with Plexiglas tops at the beginning of the experiment and four more halfway through. At nine chambers at each site, heaters incrementally raised the temperature from 1.5 degrees Celsius to 5.5 degrees Celsius above ambient temperature during the study. The researchers took censuses and collected other data monthly--except when snow covered the ground--and built a statistical model called a Markov model. In the unheated chambers, colonies of different ant species were frequently coming and going. Stability for the community as a whole was characterized by near-constant overturning of nesting sites with no vacancy between occupants. "In the heated chambers, thermophilic queens and colonies were moving in and parking," Diamond said. "Other species couldn't take advantage of the nesting spaces, which had the overall effect of making the community less stable and slower to return to equilibrium--in the long run, this may make them susceptible to climate change." Although the forests are separated by about 6.5 degrees in latitude and a mean average temperature difference of 5.8 degrees Celsius, warming destabilized the ant communities in both, the statistical model showed. While loss of community stability translates to longer return times to equilibrium after disturbances, the long-term biological consequences remain an area for future exploration. Within the timescale of the experiment, relative abundance of thermophilic species grew while heat-intolerant species declined in the warmed chambers. At the warmest, southernmost site in Duke Forest, the composition of ant communities already appears to be changing. The researchers are investigating fine-scale interactions among species and correlating them with thermal tolerance. They are also extending the Markov model forward to try to answer whether communities return to equilibrium over a long period of time or never under climate change. Diamond worked with Lauren Nichols, Clint Penick and Robert Dunn of North Carolina State University's Department of Applied Ecology; Shannon Pelini of Bowling Green State University's Department of Biological Sciences; Grace Barber of University of Massachusetts' Department of Environmental Conservation; Aaron Ellison of Harvard University's Harvard Forest; Sara Helms Cahan and Nicholas Gotelli of University of Vermont's Department of Biology; and Nathan Sanders, University of Copenhagen's Center for Macroecology, Evolution and Climate. More information, including a copy of the paper, can be found online at the Science Advances press package at http://www.
News Article | October 27, 2016
Climate change models destabilizing forest ant communities may be a harbinger of bigger issues in the ecosystem. A five-year, multi-university study on the impact of climate change on ant communities in Harvard Forest, Massachusetts and Duke Forest in the Piedmont Region of North Carolina shows that the loss of stability due to climate change makes communities less resilient and slower to rebound when disturbed. “We’ve had a unique opportunity to look under the hood of how these ant communities function, and how experimental warming affects their overall stability under climate change,” Sarah Diamond, an assistant professor of biology at Case Western Reserve and leader of the study, said in a statement. “We’ve looked at not only at the direct effects of warming but at indirect effects mediated by altered species interactions,” she added. “There’s good evidence the altered species interactions are affecting the stability of communities, making them more fragile and susceptible to environmental change.” The researchers erected 15 chambers for each forest and incrementally raised the temperature from 1.5 degrees Celsius to 5.5 degrees Celsius above ambient temperature on nine of the chambers at each site. As a result, the unheated chambers saw colonies of different ant species frequently coming and going with a near-constant overturning of nesting sites with no vacancy between occupants. “In the heated chambers, thermophilic queens and colonies were moving in and parking,” Diamond said. “Other species couldn’t take advantage of the nesting spaces, which had the overall effect of making the community less stable and slower to return to equilibrium—in the long run, this may make them susceptible to climate change.” The results run counter to previous studies that suggest that animal communities will quickly shift to a different, but stable state under climate change. The report also contradicts previous reports that suggest communities will remain resilient even as the Earth warms. Further studies are needed for the long-term biological consequences of climate change.
Oswald W.W.,Emerson College |
Foster D.R.,Harvard Forest
Quaternary Research | Year: 2011
Analyses of a sediment core from Little Pond, located in the town of Bolton, Massachusetts, provide new insights into the history of environmental and ecological changes in southern New England during the late Holocene. Declines in organic content and peaks in the abundance of Isoetes spores indicate reduced water depth at 2900-2600, 2200-1800, and 1200-800 calibrated years before present (cal yr BP), generally consistent with the timing of dry conditions in records from elsewhere in the northeastern United States. The Little Pond pollen record features little change over the last 3000. yr, indicating that the surrounding vegetation was relatively insensitive to these periods of drought. The 1200-800. cal. yr BP dry interval, however, coincides with increased abundance of Castanea pollen, suggesting that the expansion of Castanea in southern New England may have been influenced by late-Holocene climatic variability. © 2011 University of Washington.
Longo G.,University of Buenos Aires |
Seidler T.G.,Harvard Forest |
Garibaldi L.A.,National University of Rio Negro |
Tognetti P.M.,University of Buenos Aires |
Chaneton E.J.,University of Buenos Aires
Journal of Ecology | Year: 2013
Summary: Variation in functional community composition is expected to influence the extent of exotic species invasions. Yet, whether resident functional groups control invasion through their relative biomass (mass ratio hypothesis) or by traits other than biomass (identity hypothesis) remains poorly understood. We performed a 6-year experiment to determine the effects of removing different functional groups on exotic species biomass in a Flooding Pampa grassland, Argentina. Functional groups were defined by life-form (grasses or forbs), phenology (winter or summer) and origin (native or exotic). Removal of each functional group was compared against the removal of an equivalent amount of random biomass. Exotic group responses were monitored over 4 years of continuous removals, and after 2 years of recovery without manipulations. Removal of dominant native summer grasses caused the greatest impact on exotic species and overall community composition. Native summer-grass removal significantly increased exotic grass (120%) and forb (730%) biomass beyond the level (46% and 180%, respectively) expected from deleting a similar amount of biomass at random. Exotic annual grasses showed only a transient increase, whereas exotic forb invasion persisted even after 2 years without removals. Removing subordinate, native or exotic winter grasses, and rare native forbs significantly promoted exotic forbs, but to the same level (300%) as random biomass removals. Total grass removal increased exotic forbs to half the extent expected from adding the effects of single grass group removals. Dispersal limitation and harsh abiotic conditions may constrain exotic forb spread into such heavily grass-depleted patches. Synthesis. The impact of losing a functional group on the magnitude and persistence of invasion reflected its relative contribution to community biomass. Identity attributes other than biomass (e.g. phenological niche) further enhanced the biotic control that dominant native grasses exerted on established exotic species. Our findings highlight the community legacies of past disturbances to dominant functional groups. © 2013 British Ecological Society.