C. Lehman | University of Minnesota - Twin Cities (original) (raw)

Papers by C. Lehman

Spatial Ecology

Page 249. CHAPTER TEN Habitat Destruction and Species Extinctions David Tilman and Clarence L. Le... more Page 249. CHAPTER TEN Habitat Destruction and Species Extinctions David Tilman and Clarence L. Lehman INTRODUCTION The expansion of humans across the earth has caused a long string of species extinctions (see ...

Quaternary Science Reviews, 2012

Peatland ecosystems store about 500-600 Pg of organic carbon, largely accumulated since the last ... more Peatland ecosystems store about 500-600 Pg of organic carbon, largely accumulated since the last glaciation. Whether they continue to sequester carbon or release it as greenhouse gases, perhaps in large amounts, is important in Earth's temperature dynamics. Given both ages and depths of numerous dated sample peatlands, their rate of carbon sequestration can be estimated throughout the Holocene. Here we use average values for carbon content per unit volume, the geographical extent of peatlands, and ecological models of peatland establishment and growth, to reconstruct the timetrajectory of peatland carbon sequestration in North America and project it into the future. Peatlands there contain ~163 Pg of carbon. Ignoring effects of climate change and other major anthropogenic disturbances, the rate of carbon accumulation is projected to decline slowly over millennia as reduced net carbon accumulation in existing peatlands is largely balanced by new peatland establishment. Peatlands are one of few long-term terrestrial carbon sinks, probably important for global carbon regulation in future generations. This study contributes to a better understanding of these ecosystems that will assist their inclusion in earth-system models, and therefore their management to maintain carbon storage during climate change.

PLoS ONE, 2013

Perennial biomass from grasslands managed for conservation of soil and biodiversity can be harves... more Perennial biomass from grasslands managed for conservation of soil and biodiversity can be harvested for bioenergy. Until now, the quantity and quality of harvestable biomass from conservation grasslands in Minnesota, USA, was not known, and the factors that affect bioenergy potential from these systems have not been identified. We measured biomass yield, theoretical ethanol conversion efficiency, and plant tissue nitrogen (N) as metrics of bioenergy potential from mixed-species conservation grasslands harvested with commercial-scale equipment. With three years of data, we used mixed-effects models to determine factors that influence bioenergy potential. Sixty conservation grassland plots, each about 8 ha in size, were distributed among three locations in Minnesota. Harvest treatments were applied annually in autumn as a completely randomized block design. Biomass yield ranged from 0.5 to 5.7 Mg ha 21. May precipitation increased biomass yield while precipitation in all other growing season months showed no affect. Averaged across all locations and years, theoretical ethanol conversion efficiency was 450 l Mg 21 and the concentration of plant N was 7.1 g kg 21 , both similar to dedicated herbaceous bioenergy crops such as switchgrass. Biomass yield did not decline in the second or third year of harvest. Across years, biomass yields fluctuated 23% around the average. Surprisingly, forb cover was a better predictor of biomass yield than warm-season grass with a positive correlation with biomass yield in the south and a negative correlation at other locations. Variation in land ethanol yield was almost exclusively due to variation in biomass yield rather than biomass quality; therefore, efforts to increase biomass yield might be more economical than altering biomass composition when managing conservation grasslands for ethanol production. Our measurements of bioenergy potential, and the factors that control it, can serve as parameters for assessing the economic viability of harvesting conservation grasslands for bioenergy.

Sexually Transmitted Infections, 2011

Conclusions Rates of STIs are strongly associated with deprivation. Presentation of STI rates by ... more Conclusions Rates of STIs are strongly associated with deprivation. Presentation of STI rates by ethnic categories should be adjusted to take into account the strong interaction between ethnicity and SED. While SED is a key determinant of ill health other cultural influences on sexual behaviour may contribute to STI risk exposure among ethnic groups. The high STI rates seen among black ethnic communities are likely to be the consequence of a complex interplay of cultural, economic and behavioural factors.

This paper presents a parallel algorithm for calculating the eight-directional (D8) up-slope cont... more This paper presents a parallel algorithm for calculating the eight-directional (D8) up-slope contributing area in digital elevation models (DEMs). In contrast with previous algorithms, which have potentially unbounded inter-node communications, the algorithm presented here realizes strict bounds on the number of inter-node communications. Those bounds in turn allow D8 attributes to be processed for arbitrarily large DEMs on hardware ranging from average desktops to supercomputers. The algorithm can use the OpenMP and MPI parallel computing models, either in combination or separately. It partitions the DEM between slave nodes, calculates an internal up-slope area by replacing information from other slaves with variables representing unknown quantities, passes the results on to a master node which combines all the slaves' data, and passes information back to each slave, which then computes its final result. In this way each slave's DEM partition is treated as a simple unit in the DEM as a whole and only two communications take place per node.

ABSTRACT Agricultural regions of the world have vast natural watersheds of lakes, ponds, rivers, ... more ABSTRACT Agricultural regions of the world have vast natural watersheds of lakes, ponds, rivers, and streams. Interwoven are large artificial watersheds of drain tiles and ditches. Just as household plants benefit from drains in their pots, so agricultural crops benefit from drain tiles in their fields. In today's world the drain tiles carry not only water but also fertilizers, antibiotics, pesticides, hormones, and other chemicals into the natural watershed. We analyzed where wetlands and buffers for bioenergy and other applications could be placed on the Minnesota landscape to intercept drain waters and help purify them before they reach the natural watershed.

BioEnergy Research, 2012

ABSTRACT

Science, 2006

Biofuels derived from low-input high-diversity (LIHD) mixtures of native grassland perennials can... more Biofuels derived from low-input high-diversity (LIHD) mixtures of native grassland perennials can provide more usable energy, greater greenhouse gas reductions, and less agrichemical pollution per hectare than can corn grain ethanol or soybean biodiesel. High-diversity grasslands had increasingly higher bioenergy yields that were 238% greater than monoculture yields after a decade. LIHD biofuels are carbon negative because net ecosystem carbon dioxide sequestration (4.4 megagram hectare –1 year –1 of carbon dioxide in soil and roots) exceeds fossil carbon dioxide release during biofuel production (0.32 megagram hectare –1 year –1 ). Moreover, LIHD biofuels can be produced on agriculturally degraded lands and thus need to neither displace food production nor cause loss of biodiversity via habitat destruction.

Quaternary Science Reviews, 2007

A set of simple ecological models accounts well for the cumulative initiation of peatlands throug... more A set of simple ecological models accounts well for the cumulative initiation of peatlands throughout North America in relation to glacial retreat. The most parsimonious form incorporates, first, a delay term to account for the lag during which newly deglaciated land became suitable for peatland initiation and, second, an intrinsic rate of initiation related to the probability of migration and

Computers & Geosciences, 2014

In processing raster digital elevation models (DEMs) it is often necessary to assign drainage dir... more In processing raster digital elevation models (DEMs) it is often necessary to assign drainage directions over flats-that is, over regions with no local elevation gradient. This paper presents an approach to drainage direction assignment which is not restricted by a flat's shape, number of outlets, or surrounding topography. Flow is modeled by superimposing a gradient away from higher terrain with a gradient towards lower terrain resulting in a drainage field exhibiting flow convergence, an improvement over methods which produce regions of parallel flow. This approach builds on previous work by Garbrecht and Martz (1997), but presents several important improvements. The improved algorithm guarantees that flats are only resolved if they have outlets. The algorithm does not require iterative application; a single pass is sufficient to resolve all flats. The algorithm presents a clear strategy for identifying flats and their boundaries. The algorithm is not susceptible to loss of floating-point precision. Furthermore, the algorithm is efficient, operating in O(N) time whereas the older algorithm operates in O(N 3 /2) time. In testing, the improved algorithm ran 6.5 times faster than the old for a 100 x 100 cell flat and 69 times faster for a 700 x 700 cell flat. In tests on actual DEMs, the improved algorithm finished its processing 38-110 times sooner while running on a single processor than a parallel implementation of the old algorithm did while running on 16 processors. The improved algorithm is an optimal, accurate, easy-to-implement drop-in replacement for the original. Pseudocode is provided in the paper and working source code is provided in the Supplemental Materials.

NOTE: This revised addendum addresses issues raised by the Peer Review Panel on December 13, 2007... more NOTE: This revised addendum addresses issues raised by the Peer Review Panel on December 13, 2007 but does not yet incorporate changes made possible by additional funds allocated the Legislative-Citizen Commission on January 15, 2008. A subsequent ...