Time filter

Source Type

Harvard, MA, United States

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.

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.

Wang Y.,CAS Institute of Tibetan Plateau Research | Liang E.,CAS Institute of Tibetan Plateau Research | Liang E.,Chinese Academy of Sciences | Ellison A.M.,Harvard Forest | And 3 more authors.
Global and Planetary Change | Year: 2015

The Tibetan Plateau hosts one of the world's highest undisturbed alpine juniper shrublines. However, little is known about the dynamics of these shrublines in response to climate warming and shrub-to-shrub interactions. Since growth of shrubline junipers is limited more by moisture availability than by low temperatures, we tested if upslope advancement of alpine juniper shrublines was constrained by warmer temperatures and related recent droughts. We also evaluated whether facilitation among neighboring shrubs, as inferred from spatial analyses, influenced shrubline dynamics. Three rectangular plots crossing the Juniperus pingii var. wilsonii shrubline were sampled at elevations from 4810 to 4917. m. a.s.l. near the Nam Co Lake, central Tibetan Plateau. Location of each stem and its diameter at the root collar and age were measured. We reconstructed the spatial and temporal shrubline dynamics during the past 350. years using standard dendrochronological methods. Independent, long-term summer temperature reconstructions also were associated with shrub recruitment. Point-pattern analyses were used to characterize spatial patterns of different size classes of shrubs. The three shrublines showed little long-term changes despite ongoing warming; no upward shift has occurred in the past 100. years. Recruitment was negatively associated with summer temperatures and drought occurrence since the 1920s. Spatial patterns were characterized by clustering at local scales and attraction between the different size classes, suggesting facilitation. We conclude that moisture availability limits the recruitment and elevational advance of junipers in this area of the Tibetan Plateau. Dynamics of alpine shrublines are more contingent on positive interactions and local environmental factors than on regional climatic variability. © 2015 Elsevier B.V.

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.

Chaib De Mares M.,University of Groningen | Chaib De Mares M.,Harvard University | Hess J.,Harvard University | Hess J.,University of Oslo | And 6 more authors.
New Phytologist | Year: 2015

The genus Amanita encompasses both symbiotic, ectomycorrhizal fungi and asymbiotic litter decomposers; all species are derived from asymbiotic ancestors. Symbiotic species are no longer able to degrade plant cell walls. The carbohydrate esterases family 1 (CE1s) is a diverse group of enzymes involved in carbon metabolism, including decomposition and carbon storage. CE1 genes of the ectomycorrhizal A. muscaria appear diverged from all other fungal homologues, and more similar to CE1s of bacteria, suggesting a horizontal gene transfer (HGT) event. In order to test whether AmanitaCE1s were acquired horizontally, we built a phylogeny of CE1s collected from across the tree of life, and describe the evolution of CE1 genes among Amanita and relevant lineages of bacteria. CE1s of symbiotic Amanita were very different from CE1s of asymbiotic Amanita, and are more similar to bacterial CE1s. The protein structure of one CE1 gene of A. muscaria matched a depolymerase that degrades the carbon storage molecule poly((R)-3-hydroxybutyrate) (PHB). Asymbiotic Amanita do not carry sequence or structural homologues of these genes. The CE1s acquired through HGT may enable novel metabolisms, or play roles in signaling or defense. This is the first evidence for the horizontal transfer of carbohydrate metabolism genes into ectomycorrhizal fungi. © 2014 New Phytologist Trust.

Discover hidden collaborations