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Baldeck C.A.,Program in Ecology | Baldeck C.A.,Urbana University | Harms K.E.,Smithsonian Tropical Research Institute | Harms K.E.,Louisiana State University | And 19 more authors.
Proceedings of the Royal Society B: Biological Sciences | Year: 2013

Both habitat filtering and dispersal limitation influence the compositional structure of forest communities, but previous studies examining the relative contributions of these processes with variation partitioning have primarily used topography to represent the influence of the environment. Here, we bring together data on both topography and soil resource variation within eight large (24-50 ha) tropical forest plots, and use variation partitioning to decompose community compositional variation into fractions explained by spatial, soil resource and topographic variables. Both soil resources and topography account for significant and approximately equal variation in tree community composition (9-34% and 5-29%, respectively), and all environmental variables together explain 13-39% of compositional variation within a plot. A large fraction of variation (19-37%) was spatially structured, yet unexplained by the environment, suggesting an important role for dispersal processes and unmeasured environmental variables. For the majority of sites, adding soil resource variables to topography nearly doubled the inferred role of habitat filtering, accounting for variation in compositional structure that would previously have been attributable to dispersal. Our results, illustrated using a new graphical depiction of community structure within these plots, demonstrate the importance of small-scale environmental variation in shaping local community structure in diverse tropical forests around the globe. © 2012 The Author(s) Published by the Royal Society. All rights reserved.

Henne P.D.,Program in Ecology | Henne P.D.,University of Bern | Hu F.S.,Program in Ecology | Hu F.S.,Urbana University
Quaternary Science Reviews | Year: 2010

Lake-effect snow is an important constraint on ecological and socio-economic systems near the North American Great Lakes. Little is known about the Holocene history of lake-effect snowbelts, and it is difficult to decipher how lake-effect snowfall abundance affected ecosystem development. We conducted oxygen-isotope analysis of calcite in lake-sediment cores from northern Lower Michigan to infer Holocene climatic variation and assess snowbelt development. The two lakes experience the same synoptic-scale climatic systems, but only one of them (Huffman Lake) receives a significant amount of lake-effect snow. A 177-cm difference in annual snowfall causes groundwater inflow at Huffman Lake to be 18O-depleted by 2.3‰ relative to O'Brien Lake. To assess when the lake-effect snowbelt became established, we compared calcite-δ18O profiles of the last 11,500 years from these two sites. The chronologies are based on accelerator-mass-spectrometry 14C ages of 11 and 17 terrestrial-plant samples from Huffman and O'Brien lakes, respectively. The values of δ18O are low at both sites from 11,500 to 9500 cal yr BP when the Laurentide Ice Sheet (LIS) exerted a dominant control over the regional climate and provided periodic pulses of meltwater to the Great Lakes basin. Carbonate δ18O increases by 2.6‰ at O'Brien Lake and by 1.4‰ at Huffman Lake between 9500 and 7000 cal yr BP, suggesting a regional decline in the proportion of runoff derived from winter precipitation. The Great Lakes snowbelt probably developed between 9500 and 5500 cal yr BP as inferred from the progressive 18O-depletion at Huffman Lake relative to O'Brien Lake, with the largest increase of lake-effect snow around 7000 cal yr BP. Lake-effect snow became possible at this time because of increasing contact between the Great Lakes and frigid arctic air. These changes resulted from enhanced westerly flow over the Great Lakes as the LIS collapsed, and from rapidly rising Great Lakes levels during the Nipissing Transgression. The δ18O difference between Huffman and O'Brien lakes declines after 5500 cal yr BP, probably because of a northward shift of the polar vortex that brought increasing winter precipitation to the entire region. However, δ18O remains depleted at Huffman Lake relative to O'Brien Lake because of the continued production of lake-effect snow. © 2010 Elsevier Ltd. All rights reserved.

Clegg B.F.,Program in Ecology | Hu F.S.,Program in Ecology | Hu F.S.,Urbana University
Quaternary Science Reviews | Year: 2010

Understanding the ecological and socio-economic impacts of climatic warming requires knowledge of associated changes in moisture balance. Reconstructions of Holocene moisture-balance variation offer indispensible baseline information against which recent changes can be evaluated. We analyzed Chara-stem encrustations in the sediments of Takahula Lake, located in the south-central Brooks Range of Alaska, for oxygen and carbon-isotope composition to infer climatic change over the past 8000 years. To help constrain climatic interpretations of the sediment δ18O record, we also analyzed water samples from Takahula and other lakes in the region for oxygen and hydrogen isotope composition. Results show that winter precipitation dominates the water balance of these lakes and that post-input evaporation is a key control of lake-water isotope composition of Takahula Lake. Stratigraphic patterns in Chara-δ18O, supplemented by those in δ13C and sediment lithology, reveal distinct changes in effective moisture (precipitation minus evaporation) over the past 8000 years. Effective moisture was relatively high from 8000 to 5000 cal BP, with marked fluctuations between 6800 and 5000 cal BP. It then decreased to reach a minimum around 4000 cal BP and increased with fluctuations from 4000 to ∼2500 cal BP, followed by a decreasing trend toward the present that was interrupted by a wet Little Ice Age (centered at 400 cal BP). Aridity during the 20th century was among the highest of the entire 8000-year record. At the millennial timescale, the temporal patterns of moisture-balance shifts at Takahula Lake are broadly coherent with those inferred from previous paleoclimate records from the region. The Chara-δ18O values around 5600 cal BP and during the Little Ice Age are up to 5‰ lower than at present and 3.6‰ lower than that of the modern input-water to the lake. These exceptionally low values suggest that factors other than effective moisture must have contributed to the pronounced variations in the Takahula Lake δ18O record. Increased winter precipitation associated with a westerly Aleutian Low position may account for 1‰ of the δ18O decrease. Other factors leading to the 18O-depletion during these periods probably include decreased temperatures, as well as increased lake-ice cover and associated reductions in evaporation. © 2009 Elsevier Ltd. All rights reserved.

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