News

Minnesota scientists probe tree rings for clues to climate history

posted Mar 12, 2015, 8:16 AM by Daniel Griffin


http://www.startribune.com/lifestyle/291303201.html
[2015 image by Glen Stubbe © Star Tribune]

Xiaolu (Grace) Li Master's Thesis Defense

posted Nov 14, 2014, 3:02 PM by Daniel Griffin   [ updated Nov 23, 2014, 2:27 PM by Scott St. George ]

It’s our pleasure to announce the public defense of Xiaolu (Grace) Li’s M.A. thesis in Geography, which is titled “Forward modeling of tree-climate relations across the Northern Hemisphere”.

The defense will be held on Monday, November 24 in Social Sciences 609. We’ll begin promptly at 12:30PM, and everyone is welcome.
Forward modeling of tree-climate relations across the Northern Hemisphere
This thesis uses the Vaganov-Shashkin Model of Tree Ring Formation, a multivariate, nonlinear, mechanism model that directly predicts tree-ring growth using only climate records, to simulate tree-ring formation across the Northern Hemisphere. Previous research has shown the model has skill in reproducing ring widths variability and climate sensitivity at local and regional scales, but its ability to simulate the major geographical differences in tree-climate relationships at a hemispheric scale has not yet been tested. In this study, we ran the model at over 7,000 locations across the Northern Hemisphere, and compared the seasonal climate responses of the simulations against a network of nearly 2,200 real tree-ring width records. We also calculated the predicted dominant factor at each station and used relative growth rates to explain these patterns.
Simulated tree-ring chronologies are consist with the real ones in the seasonality and relative strength of the encoded climate signals, demonstrating that the model has skill in reproducing tree-ring growth response to climate variability in the Northern Hemisphere. Because the simulations were produced using only climate records and the same set of parameters, the fact that the model was able to reproduce major geographical differences in the observations suggests that climate is the primary factor in determining large-scale tree-climate relationships. We also used relative growth rates to show the sequence of events happened in the growing season and the possible mechanism of the climate response of tree rings. We found that temperature dominates growth at temperature-sensitive sites during most of the growing season and that the relative importance of summer and winter precipitation to tree-ring formation can be examined by the relative importance of temperature versus soil moisture at the beginning and end of growing season. Because the model has skill in reproducing ring widths and tree-climate relationships at local, regional and hemispheric scales, we suggest VSM can potentially be used as a low-cost estimator to predict tree-ring response to climate prior to sampling and to forecast long-term changes in tree-climate relationships.

[GEOG8260] Learn how to rock your (science) talk

posted Nov 5, 2014, 7:45 AM by Scott St. George

This spring, I’ll again be offering my graduate seminar on scientific presentations. The course will run a half semester (7 weeks) and be offered twice (back-to-back), so studdents have two chances to fit it into their schedule.

All graduate students, no matter their research speciality, are welcome. Past offerings of the course have drawn students across the entire university system, including natural resource management, geography, geosciences, ecology, aerospace engineering, civil engineering, and more.

I probably won’t teach the course again in 15/16 (and maybe not the year after either), so if you’ve been on the fence in the past, don’t miss this chance.

GEOG8260: The Art of Scientific Presentations (two credits)
The ability to deliver effective and engaging oral presentations is a critical skill for scholars in all disciplines. Unfortunately, despite the importance of clear communication, professional presentations about research are too often confusing, abstract and boring. In this seminar, you'll be introduced to a diverse set of presentation methods and use exercises to apply these techniques to your own work and ideas. By the end of the semester, you will have experimented with a broad range of presentation styles and identified the method or methods that best suits your own style and research subject. More generally, you will have become a more effective communicator, improved your ability to discuss your research with non-specialists, and be better representatives for your discipline, your institution, and your ideas.

Which section should I choose?
In both sections, the contents and goals of the course will be exactly the same but will differ in date and location. Section 001 will be held on Thursdays from 9:30AM to 12:30PM and will run between Jan 22 and Mar 5 (inclusive). This section will meet on the St. Paul campus. Section 002 will also be held on Wednesdays from 2:00PM to 5:00PM, but will convene during the second half of the semester (Mar 26 to Apr 7) on the West Bank of the Twin Cities campus.
Depending on the section, we'll have a different mix of guests, but the content and exercises will be the same, so students are welcome to join either one.

What have students said about this course?
If you're thinking about joining us, I'd encourage you to look over the written feedback provided by students enrolled in previous years. I've posted their comments (complete and without any editing) at: http://www.tc.umn.edu/~stgeorge/Scott_St._George/GEOG8260__Student_comments.html

Webinar: Five Things You Can Do Right Now to Make Your Presentations a Little Bit Better
If you'd like to get a short preview of some of the ideas and approaches we'll review in this course, watch this short webinar I delivered to a seismology research group based in DC. It's available here: http://www.youtube.com/watch?v=V10W_PZVCos

If you have any questions about this class, please send an email to stgeorge@umn.edu.

UMN Dendro is growing

posted Sep 25, 2014, 10:17 PM by Daniel Griffin   [ updated Sep 26, 2014, 1:01 PM ]




Counting myself and our two new Ph.D. students Elizabeth Schneider and Uday Thapa, we're very pleased that our group is expanding. See more on our people page, linked on the left.

















Our facilities are expanding too. We have a new state-of-the-art teaching laboratory, complete with 25 boom arm microscopes and an instructional scope simulcast to several large-screen monitors around the room. We also just added a fourth operational Velmex measuring system and are expanding the sampling equipment and analytical facilities to include a full complement of tools needed for analysis of wood anatomy, cambium phenology, and xylogenesis. Exciting times!

[above] At the 2006 WSL Winter School on Tree-Ring Wood Anatomy, I seized a rare opportunity to use Fritz Schweingruber's personal sledge microtome.

Public lecture on global warming and future drought

posted Feb 3, 2014, 5:41 PM by Scott St. George   [ updated Feb 23, 2014, 8:37 AM ]


We're pleased to announce Dr. Ben Cook will deliver a public lecture on global warming and twenty-first century drought at the University of Minnesota on March 7. 

Ben is a climate scientist at NASA's Goddard Institute for Space Studies in New York City. He's also affiliated with Columbia's Lamont Doherty Earth Observatory in Palisades, New York. Ben has published extensively on the causes and consequences of drought in North America, as well as the projected response of moisture-delivery systems to anthropogenic climate change. He also has first-hand experience developing paleoclimatic records and has used proxies to evaluate the performance of general circulation models. 

If you'd like to meet with Ben during his visit, please contact Scott St. George (stgeorge@umn.edu).

What: The Department of Geography's 'Coffee Hour' lecture series
When: 445 Blegen Hall on the University of Minnesota's West Bank campus
Where: 3:30PM on March 7, 2014

Global warming and 21st century drying
Global warming is expected to increase the frequency and intensity of droughts in the 21st century, but the relative contributions from changes in moisture supply (precipitation) versus evaporative demand (potential evapotranspiration; PET) have not been comprehensively assessed. Using output from a suite of general circulation model (GCM) simulations from version 5 of the Coupled Model Intercomparison Project, projected 21st-century drying and wetting trends are investigated using two offline indices of surface moisture balance: the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI). PDSI and SPEI projections using precipitation and Penman-Monteith based PET changes from the GCMs generally agree, showing robust cross-model drying in western North America, Central America, the Mediterranean, southern Africa, and the Amazon and robust wetting occurring in the Northern Hemisphere high latitudes and east Africa (PDSI only). The SPEI is more sensitive to PET changes than the PDSI, especially in arid regions such as the Sahara and Middle East. Regional drying and wetting patterns largely mirror the spatially heterogeneous response of precipitation in the models, although drying in the PDSI and SPEI calculations extends beyond the regions of reduced precipitation. This expansion of drying areas is attributed to globally widespread increases in PET, caused by increases in surface net radiation and the vapor pressure deficit. Increased PET not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone. This PET amplification effect is largest in the Northern Hemisphere mid-latitudes, and is especially pronounced in western North America, Europe, and southeast China. Compared to PDSI projections using precipitation changes only, the projections incorporating both precipitation and PET changes increase the percentage of global land area projected to experience at least moderate drying (PDSI standard deviation of ≦ -1) by the end of the 21st-century from 12% to 30%. PET induced moderate drying is even more severe in the SPEI projections (SPEI standard deviation of ≦ -1; 11% to 44%), although this is likely less meaningful because much of the PET induced drying in the SPEI occurs in the aforementioned arid regions. Integrated accounting of both the supply and demand sides of the surface moisture balance is therefore critical for characterizing the full range of projected drought risks tied to increasing greenhouse gases and associated warming of the climate system.

Torbenson Thesis Defense, December 5

posted Dec 2, 2013, 7:57 AM by Scott St. George

It's my pleasure to invite you to attend the public defense of Max Torbenson's Master's thesis on Thursday, December 5 at 9:30AM. The defense will be held in Room 614 in the Social Sciences Tower and should be conducted over roughly one hour.

Max's research has focused on the growth dynamics of red fir (Abies magnifica) in the Klamath Network Parks. In his presentation, Max will describe his efforts building a network of new tree-ring width records from this species, and describe how climate influences the growth of this tree within northeastern California and southeastern Oregon's high-elevation forests.

I hope you can join us!

Grad seminar: Become a presentation superstar!

posted Nov 6, 2013, 3:56 PM by Scott St. George

This spring, I'll be offering my graduate seminar on scientific presentations again (for the first time in two years). The course will run for half the semester (7 weeks) and be offered twice (back-to-back), so people will have two chances to fit it into their schedule.

GEOG8260: The Art of Scientific Presentations (two credits)

The ability to deliver effective and engaging oral presentations is a critical skill for scholars in all disciplines. Unfortunately, despite the importance of clear communication, professional presentations about research are too often confusing, abstract and boring. In this seminar, you'll be introduced to a diverse set of presentation methods and use exercises to apply these techniques to your own work and ideas. By the end of the semester, you will have experimented with a broad range of presentation styles and identified the method or methods that best suits your own style and research subject. More generally, you will have become a more effective communicator, improved your ability to discuss your research with non-specialists, and be better representatives for your discipline, your institution, and your ideas.

Which section should I choose?
In both sections, the contents and goals of the course will be exactly the same, but they will differ in location and thematic focus. Section 001 will be held on Thursdays from 9:30AM to 12:30PM and will run between Jan 22 and Mar 12 (inclusive). This section will meet on the St. Paul campus and will emphasize the challenges of communicating physical science. Section 002 will also be held on Wednesdays from 2:30PM to 5:30PM, but will convene during the second half of the semester (Mar 26 to Apr 7). This section will meet on the West Bank of the Twin Cities campus and is intended primarily for scholars in the social sciences and the humanities.
    Depending on the section, we'll have a different mix of guests visiting the course, but the content and exercises will be the same, so students are welcome to join either one, regardless of their own background.

What have students said about this course?
If you're thinking about joining us, I'd encourage you to look over the written feedback provided by students enrolled in previous years. I've posted their comments (complete and without any editing) here.

Webinar: Five Things You Can Do Right Now to Make Your Presentations a Little Bit Better
If you'd like to get a short preview of some of the ideas and approaches we'll review in this course, watch this short webinar I delivered in April to IRIS (a seismology research group based in DC). It's available here.

If you have any questions about this class, please send an email to stgeorge@umn.edu.

Dan Griffin will join the Dendro Center next summer

posted Nov 4, 2013, 11:50 AM by Scott St. George   [ updated Nov 4, 2013, 11:50 AM ]


On behalf of the Dendro Center, I'm happy to announce that Dan Griffin will join the faculty in Department of Geography, Environment and Society next summer. Dan obtained his doctorate in Geography at the University of Arizona earlier this year, and currently holds a post-doctoral research position at Woods Hole Oceanographic Institution in Massachusetts, where he's investigating past and future climate change in Central America.

Dan is an extremely accomplished young scientist. At Arizona, he held a Graduate Research Fellowship from the US Environmental Protection Agency's STAR Program. He's also led or contributed to over a dozen scientific papers on paleoclimatology, water resources, old-growth forests, and several other topics.

We're all very excited that Dan will be joining the Dendro Center in August 2014, and look forward to Minnesota having one of the largest research groups specializing in dendrochronology anywhere in the nation!

The Dendro Center goes to Tucson for AmeriDendro '13

posted May 1, 2013, 1:52 PM by Scott St. George

Two weeks from now, several members of our research group will be traveling down to sunny Tucson Arizona to attend the Second American Dendrochronology conference (known as 'AmeriDendro'). If you'll be attending the meeting and are able to attend one of our talks or posters, please wander over and say 'hi'!

Tuesday May 14
3:30 PM
Chris Crawford
Frost-ring formation in Douglas-fir at the lower forest border in central Idaho
Salon B


Wednesday May 15
6:00 to 9:00PM
Salon E

Max Torbenson
Traumatic resin ducts in Abies magnifica at interior sites along the central Pacific Coast of the United States

Sarah Appleton
Dating the Mid-Holocene History and Glacial Stratigraphy of Wachusett Inlet, Glacier Bay National Park and Preserve, Southeast Alaska


Thursday, May 16
9:20 AM
Salon D
Kurt Kipfmueller
Fire history along a historic travel corridor in Minnesota's Boundary Waters Canoe Area Wilderness

10:40 AM
Salon B
Scott St. George
Widespread absent rings have not occurred in boreal and temperate trees outside the American Southwest

Ancient trees and past climates from Craters of the Moon, Idaho

posted Feb 3, 2013, 7:17 PM by Scott St. George   [ updated Feb 3, 2013, 7:18 PM ]


This Wednesday (February 6), Chris Crawford from the Dendro Center will give the first Quaternary Paleoecology seminar of the Spring 2013 semester. The QP seminars are open to the public and refreshments are available for purchase.

When: 7:30PM, Wednesday February 6, 2013
Where: 1426 North Hythe Street, St. Paul, Minnesota

Annual and sub-annual precipitation reconstructions for the Craters of the Moon lava complex, eastern Snake River Plain, USA.
Christopher J. Crawford, University of Minnesota Center for Dendrochronology

Abstract: The Craters of the Moon (COM) National Monument is a basaltic-lava complex on the eastern Snake River Plain (SRP) in south central Idaho, USA. COM has formed over eight separate eruptions during the Holocene and since the last eruption ~ 2,000 years ago, ancient limber pine (Pinus flexlis) and Douglas fir (Pseudotsuga menziesii) trees have sought refuge on lava flows and weathered cinder cones. Tree-ring widths from these ancient trees provide unique high-resolution paleoclimate archives that can be used to examine past and present climate changes on the eastern SRP. Using instrumental precipitation records, tree-ring widths were transformed into annual and sub-annual precipitation estimates spanning the past 500 years. COM precipitation reconstructions show well known drought and pluvial episodes documented elsewhere with tree-rings across the western US. More importantly, these reconstructions suggest that droughts (pluvials) on the eastern SRP are frequency-dependent, and that prolonged dry (wet) conditions over multiple decades cannot occur without summer-winter precipitation deficits (surpluses).

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