1. Factors affecting regional variation in fall down rates of drought-killed aspen (Populus tremuloides)biomass aspen (Populus tremuloides) biomass in west-central Canada in west-central Canada E.H. (Ted) Hogg Natural Resources Canada, Canadian Forest Service (CFS) Northern Forestry Centre 5320-122 Street, Edmonton, Alberta contact: ted.hogg@nrcan.gc.ca 4 th NACP All-Investigators Meeting Albuquerque, New Mexico 4-7 February 2013

2. Some recent publications: Worrall et al. 2013. Recent declines of Populus tremuloides in North America linked to climate (For. Ecol. Manage.) Williams et al. 2012. Temperature as a potent driver of regional drought stress and tree mortality (Nature Climate Change) Ma et al. 2012. Regional drought-induced reduction in the biomass carbon sink of Canada’s boreal forests (PNAS) Peng et al. 2011. A drought-induced pervasive increase in tree mortality across Canada’s boreal forests (Nature Climate Change) Allen et al. 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests (For. Ecol. Manage.) Van Mantgem et al. 2009. Widespread increase of tree mortality rates in the western United States (Science) Drought-induced forest decline: An emerging global change issue Increase in percentage of scientific papers on forests that have reported on drought & tree mortality (Allen et al. 2010) Updated Oct 2011 Increase in percentage of scientific papers on forests that have reported on drought & tree mortality (Allen et al. 2010) Updated Oct 2011 Updated Oct 2011 Global locations showing recent episodes of drought- and heat-induced tree mortality (Allen et al. 2010)

3. Trembling Aspen (Populus tremuloides) Most widespread tree in North America Most widespread tree in North America (Alaska to Mexico) Aspen forests have high rate of CO 2 uptake Recent episodes of massive tree mortality following severe drought Distribution of aspen decline mapped during 2000-2010 (Worrall et al., 2013, For. Ecol. Manage.) Aspen decline caused by tent caterpillars & drought in Ontario & Minnesota (Candau et al. 2002, forest health reports 2007-2010) “Sudden Aspen Decline” following drought in Colorado (Worrall et al. 2010) Drought-induced aspen mortality in Alberta (Michaelian et al. 2011, GCB)

4. Science question: How will recent increases in drought-induced tree mortality affect the North American carbon cycle? Drought-killed aspen: a major source of future C emissions Michaelian et al. 2011; Huang & Anderegg 2012 Snag breakage & fall down leads to accelerated decay (C losses) Sander 2003, Yatskov et al. 2003, Boulanger et al. 2011 Knowledge gap as to factors affecting fall down transfers of C from drought-killed trees to the forest floor

5. Initiated by Canadian Forest Service in 2000 Initiated by Canadian Forest Service in 2000 Multi-scale study with a hierarchical design Multi-scale study with a hierarchical design (25 study sites, 150 plots) Annual tree-level monitoring in pure aspen stands Annual tree-level monitoring in pure aspen stands (50-90 years old) Severe 2001-2002 drought led to a regional increase in aspen mortality Provided opportunity to observe fall down of dead aspen over a 12-yr period Provided opportunity to observe fall down of dead aspen over a 12-yr period CIPHA study Climate Impacts on Productivity & Health of Aspen CIPHA study region in western Canada 3 stands per site 2 plots per stand 25-30 trees per plot 20 m BERMS Aspen tree-rings

6. Objectives: Measure rate of mass fall-down in aspen trees with known year of death Measure rate of mass fall-down in aspen trees with known year of death Determine factors affecting regional and inter-site variation Determine factors affecting regional and inter-site variation Overview of Methods: Tree assessments in 150 plots (2000-2008, 2012) Tree assessments in 150 plots (2000-2008, 2012) - Living vs. dead, height, dbh, damage by insects & fungi For 1010 aspen that died during 2000-2008: For 1010 aspen that died during 2000-2008: - Estimated original tree biomass as f(dbh, height) using national equations of Lambert et al. (2005) - Assessed 2008 & 2012 status (standing dead, snapped snags & fallen) - Measured height of breakage for snapped snags

7. H tree H snag Snag height ratio: HR snag = H snag / H tree Snag mass ratio:* MR snag = 2HR snag - (HR snag ) 2 Standing snag mass: M snag = M tree x MR snag *Equation derived from stem analyses (10 aspen) at BERMS Old Aspen site showing a linear decrease of stem cross-sectional area with increasing height up the stem Partitioning of mass in dead aspen stems (standing snags vs. snapped fallen crowns) (standing snags vs. snapped fallen crowns) HR snag = 1.00 HR snag = 0.30 MR snag = 1.00 MR snag = 0.51 Fallen snapped crowns: M snapfall = M tree x (1- MR snag )

8. Results: according to stem number Status of N=1010 dead aspen stems (all 23 CIPHA sites) as a function of years since death

9. Results: according to stem mass or C >1 years: Aspen mass fall down follows negative exponential (k = 19% per year for all sites) 0-1 years: Negligible fall down of aspen mass

10. Results: according to stem mass or C %Mass standing Years after stem death High inter-site variation in mass fall down rates Fort Nelson, B.C. k = 5% per year BERMS OA, Sask. k = 19% per year Porcupine Hills, Man. k = 41% per year Years after stem death Slow Fast Average

11. Alberta Sask. Manitoba NWT BC Ontario Factors affecting variation in mass fall down rates Multiple regression analyses: Dependent variable: k Independent variables: Climatic (wind, temp., moisture) Incidence of major insects & fungi Stand characteristics (e.g., age) CIPHA study sites included in the analysis site excluded due to tornado damage in 2007

12. Factors affecting variation in mass fall down rates Positive relationship (R 2 = 0.49) with mean incidence of Phellinus tremulae (decay fungus)

13. Factors affecting variation in mass fall down rates Positive relationship with stand age was nearly as strong (R 2 = 0.46), but older stands tended to have more Phellinus

14. Factors affecting variation in mass fall down rates Boreal forest Prairie Arctic High wind sitesLow wind sites Mean wind speed (m/s) was also a significant positive factor in the regression with stand age

15. Conclusions & implications Drought-affected dead aspen fell down rapidly - regional mean mass fall of 19% per year (after 1 year lag) - 80% fallen within 9 years High inter-site variation (5 – 41% per year) driven by: - incidence of decay fungus (Phellinus) - stand age - blowdown by wind Implications for modelling C losses from drought-killed aspen Implications for aerial & satellite-based mapping

16. Team members & collaborators collaborators Forest health crew leaders Funding (CIPHA study) Climate Change Action Fund Program of Energy Research and Development Mistik Management Ltd., Meadow Lake, Sask. Forest 2020 AAFC – PFRA Administration Canadian Space Agency CFS-NRCan A-base funding Tree-ring analyses CIPHA team Ted Hogg (NoFC) Ron Hall (NoFC) Mike Undershultz (ASRD) Michael Michaelian (NoFC) Trisha Hook (NoFC) Eric Arsenault (NoFC) James Brandt (CFS-HQ) Tony Hopkin (GLFC) Al Keizer (GLFC) Collaborators Craig Allen (USGS) Alan Barr (EC) Pierre Bernier (LFC) Andy Black (UBC) Werner Kurz (PFC) Vic Lieffers (U of A) Dan MacIsaac (CWFC) Juha Metsaranta (NoFC) David Price (NoFC) Dan Rowlinson (OMNR) Taylor Scarr (OMNR) Jim Worrall (US For Serv) and others Mike Michaelian Trisha Hook Mike Undershultz (ASRD) Al Keizer (GLFC) and others Trisha Hook and others Field & laboratory assistance Aerial surveys & remote sensing Mike Michaelian Eric Arsenault Rob Skakun Amy Irvine Oksana Izio Angela Johnson Devin Letourneau Chelsea Martin Sarah Martin Lindsay McCoubrey Pam Melnick Ryan Raypold Erin Van Overloop Mark Schweitzer Dominic Senechal Jessica Snedden Joey Tanney Bill van Egteren Bryan Vroom Cedar Welsh Dave Wieder (PC) and many others Jim Hammond Rick Hurdle Roger Nesdoly (MM) Brad Tomm Jim Weber Marc Berube Natacha Bissonnette Sarah Breen Lindsay Bunn Laura Chittick Brian Christensen Owen Cook Andrea Durand Ray Fidler (PC) Michelle Filiatrault Cathryn Hale Bonny Hood Tom Hutchinson Graduate students Sophan Chhin Miranda Hart and others