Challenges in assessing drought impacts on boreal aspen and mixedwood stands in western Canada

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1 Challenges in assessing drought impacts on boreal aspen and mixedwood stands in western Canada E.H. (Ted) Hogg and Trisha Hook Climate Change Adaptation Project Natural Resources Canada, Canadian Forest Service (CFS) Northern Forestry Centre Street, Edmonton, Alberta contact: Boreal Mixedwoods 2012 Ecology & Management for Multiple Values Edmonton, Alberta June 2012

2 Team members & collaborators 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 Forest health crew leaders Mike Michaelian Trisha Hook Mike Undershultz (ASRD) Al Keizer (GLFC) and others Tree-ring analyses Trisha Hook and others Aerial surveys & remote sensing Mike Michaelian Eric Arsenault Rob Skakun Graduate students Sophan Chhin Miranda Hart and others Field & laboratory assistance 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 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 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

3 Drought-induced forest decline: An emerging global change issue Some recent publications: Ma et al Regional drought-induced reduction in the biomass carbon sink of Canada s boreal forests (PNAS) Peng et al A drought-induced pervasive increase in tree mortality across Canada s boreal forests (Nature Climate Change) Allen et al A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests (For. Ecol. Manage.) Updated Oct 2011 Worrall et al Effects and etiology of sudden aspen decline in southwestern Colorado (For. Ecol. Manage.) Williams et al Forest responses to increasing aridity and warmth in the southwestern United States (PNAS) Phillips et al Drought sensitivity of the Amazon rainforest (Science) Van Mantgem et al Widespread increase of tree mortality rates in the western United States (Science) Global locations showing recent episodes of drought- and heat-induced tree mortality (Allen et al. 2010)

4 Trembling Aspen (Populus tremuloides) also known locally as white poplar Most widespread tree in North America (Alaska to Mexico) Most abundant deciduous tree in the Canadian boreal forest Important ecologically & commercially in pure and mixed-wood stands Aspen is the main native tree in the parkland zone (including farm woodlots) Aspen forests have high rate of CO 2 uptake Distribution of Populus tremuloides

5 Aspen dieback in western Canada Local dieback events prompted concerns in the early 1990s Early studies showed that drought was a major causal factor Hogg & Schwarz 1999: study funded by Mistik Management, Meadow Lake, SK Hogg et al. 2002: study funded by Weyerhaeuser Canada, Grande Prairie, AB Raised questions about potential impacts of climate change Significant warming trend (up to 2 C during the 20 th century) 1998 was a record-warm year with widespread drought 1998 Aspen dieback & decline in Alberta & Saskatchewan, 1990s

6 CIPHA study Climate Impacts on Productivity & Health of Aspen Initiated by CFS in 2000 (Hogg & Brandt) funding from CCAF, PERD & forest industry (Mistik Management) Includes tree-level monitoring in pure aspen stands (50-90 years old) Multi-scale study with a hierarchical design (30 study sites, total of 180 plots) Collaboration with tower-based monitoring of carbon, water & energy fluxes at the Old Aspen site (Barr, Black et al., BERMS & Fluxnet Canada) BERMS Aspen tree-rings trees per plot 20 m 2 plots per stand 3 stands per site CIPHA study region

7 CIPHA plot network CIPHA west network 150 plots CFS-NoFC (Hogg et al.) & Alberta SRD (Underschultz et al.) CIPHA east network 30 plots CFS-GLFC & OMNR (Keizer, Scarr et al.)

8 Methods: CIPHA plot network Annual forest health assessments (2000-present): -defoliation, dieback, mortality, insects & diseases Every 4 years (2000, 2004, 2008, 2012): -tree measurements (height, dbh etc.) -sampling for tree-ring analysis -regeneration Other: leaf area index (litter traps), soil analysis Annual growth & mortality of stand biomass (T ha -1 ) - calculated from plot-based measurements, tree-rings, & national biomass equations (Lambert et al. 2005) Portable device & system for field data acquisition

9 Methods: tree-ring analysis of aspen White tree rings formed during defoliation by forest tent caterpillar Hogg et al. 2002, CJFR Tree-ring sampling Livrée des forêts (Malacosoma disstria) Aspen defoliation by forest tent caterpillar

10 Method for tracking changes in moisture: Climate Moisture Index (CMI) Hogg (1997) Agric. For. Meteorol. 84: CMI = P PET (units in cm/year) P is mean annual precipitation includes water input as both rain and snow PET is annual potential evapotranspiration potential loss of water vapour from a well-vegetated landscape when soil moisture is not limiting, as estimated from mean daily maximum and minimum temperature for each month P PET runoff Note: The CMI provides a simple index for assessing moisture variation & drought severity in remote forested regions where long-term climate data are typically limited to temperature and precipitation

11 Aspen growth index (A ) Results: Tracking past changes in growth Tree-ring analysis showed dramatic changes in regional aspen growth since 1951 Major factors affecting growth: Drought (CMI) Insect defoliation (D) Severe drought years Severe defoliation years CIPHA sites included in analysis Regression equation: A = CMI CMI CMI CMI CMI D, r 2 =0.697 From Hogg et al CJFR

12 The exceptional drought of Worst drought in >80 years across a large area of AB & SK Led to massive mortality of aspen forests Conifers and urban forests also affected Drought-damaged aspen leaves (2002) Aerial view of mortality in the parkland (2004) Drought-caused dieback resembling fire effects Drought-affected area showing severe dieback of aspen forests

13 Dry CMI Moist The exceptional drought of Mapping & analysis of the CMI showed the severity & extent of the drought where dieback & mortality was recorded long-term mean CMI Climate Moisture Index (CMI) in drought survey area for 12-month periods ending 31 July of each given year Drought ( ) Drought survey area DRY CMI MOIST Climate Moisture Index (CMI) ANUSPLIN interpolation of CMI by Price, Siltanen & McKenney

14 % Mortality Relative biomass of dying aspen Results: drought impacts on aspen mortality Regional aspen mortality tripled & has since remained higher than normal Hogg et al. 2008, CJFR Average % mortality of aspen biomass 1.0 y = e x 5 4 Drought ( ) 0.8 R 2 = Dry Moist Year Minimum CMI ( ) Larger size classes of trees were affected in areas with severe drought: Normal stand-thinning leads to mortality of smaller, suppressed aspen (mean biomass ca % of the stand average) Drought-related mortality affects all size classes nearly equally (mean biomass ca % of the stand average)

15 Change in live biomass (T ha -1 y -1 ) Results: changes in aspen biomass From CIPHA plot measurements ( ) & tree-ring analysis Drought Growth Mortality CIPHA sites included Hogg et al. 2008, CJFR

16 Change in live biomass (T ha -1 y -1 ) Results: changes in aspen biomass The drought led to a multi-year collapse in biomass increment Drought Growth Mortality Net change CIPHA sites included Hogg et al. 2008, CJFR

17 % aspen stems affected Results: Contributing factors Increased damage by wood-boring insects Insect defoliation (localized outbreaks) Role of fungal pathogens & decay fungi These agents may have amplified & prolonged the drought s impact Damage by Saperda (wood-boring insect) 30 Stem damage in CIPHA plots Borers Phellinus Peniophora 5 Drought Repeat photography of CIPHA plot showing stand breakup

18 Results: Estimation of dead aspen biomass in severely drought-affected area Michaelian et al. (2011), GCB Spatial interpolation of dead aspen biomass Estimated 45 Mt of dead aspen biomass based on spatial interpolation = 20% of total aspen biomass in survey area Ground plot measurements

19 Areas with exceptional, recent droughts where the CMI was the lowest on record during Maps based on climate interpolations using the BioSim program

20 Areas showing signs of significant drying since 1998 Based on preliminary, interpolated CMI values using BioSIM dcmi dcmi = mean CMI in minus mean CMI in

21 Recent multi-species decline in Alberta Waterton National Park, 2010 (multi-species forest decline) White spruce mortality Red Deer river valley 2010 Balsam fir mortality (widespread) Urban forest decline, Edmonton, 2011 Birch mortality (widespread)

22 Challenges in assessing drought impacts on mixed-species stands The CIPHA study is focused on pure aspen stands Single tree species with a clonal growth form Boreal aspen stands are usually even-aged Relatively easy to scale up from tree-rings & stand measurements Mixedwood stands pose additional challenges Drought impacts may vary according to tree species Differences in tree species susceptibility to specific insects & diseases Greater unevenness of ages and/or heights across species Stand dynamics are much more complex (e.g. competition for light)

23 Mixedwood stands dominated by aspen (Populus tremuloides) & white spruce (Picea glauca) Major upland forest type in western Canada Important ecologically & commercially Not as complex as other mixedwood types (2 species)

24 Comparison of aspen & white spruce growth responses near Whitehorse, Yukon (Hogg & Wein 2005, CJFR) Conducted tree-ring analysis at 12 sites Co-dominant aspen and white spruce Relatively open stands due to dry climate No evidence of past insect outbreaks Ideal for comparing species responses to climate 1958 Takhini burn near Whitehorse, Yukon Old mixedwood stand (white spruce aspen) near Whitehorse, Yukon

25 Detrended growth (A ) Mean area increment (cm 2 year -1 ) Summary of results (Hogg & Wein 2005, CJFR) Aspen and spruce had very similar patterns of growth variation (r = 0.86 for detrended growth) Growth of both species was strongly governed by variation in moisture

26 Detrended growth (A ) Mean area increment (cm 2 year -1 ) Summary of results Major drought years (Hogg & Wein 2005, CJFR) Aspen and spruce had very similar patterns of growth variation (r = 0.86 for detrended growth) Growth of both species was strongly governed by variation in moisture Droughts in mid-late 1990s led to growth collapses of more than 50% Suggests that the same model of drought impacts on growth could be applied for both species

27 Mean % area increment DRY <---- CMI ----> WET White spruce growth near Edmonton 30 Major drought years Climate Moisture Index (CMI) at Edmonton 20 Exceptionally dry during Led to a prolonged growth collapse (Hogg et al., unpublished data) White spruce near Edmonton (3 stands near Stony Plain) White spruce (N=22)

28 Mean % area increment DRY <---- CMI ----> WET White spruce growth near Prince Albert, SK Exceptionally wet during Severe drought Climate Moisture Index (CMI) at Nisbet Forest (BioSim) Led to a 3-fold increase in growth (Hogg et al., unpublished data) White spruce near Prince Albert (6 stands, Nisbet Forest & Duck Lake) White spruce (N=42) Wet years

29 Detrended growth growth Mean increment (cm 2 / y) White spruce growth in mixedwood stands (Hogg & Schwarz 1999, NOR-X-359) Mixedwood forests near St. Walburg SK (3 stands) Aspen and spruce had very different patterns of growth variation (r = 0.05 for detrended growth) Spruce growth increased in years of severe aspen defoliation by forest tent caterpillar (FTC) Growth releases of spruce Aspen (N=39) White spruce (N=12) FTC defoliation (aspen)

30 Mean % area increment DRY <---- CMI ----> WET Mixedwood stands near Fort McMurray Overall growth declines in both aspen & white spruce Major drought years Climate Moisture Index (CMI) at Fort McMurray Recent, severe droughts ( & ) Different growth patterns due to impacts of different insects Mixedwood forests near Fort McMurray (6 stands) Mixed forests east of Fort McMurray during a simultaneous outbreak of spruce budworm (SBW ) & forest tent caterpillar (FTC) (Photo by M. Michaelian, 26 June 2008) Aspen (N=25) FTC defoliation (aspen) White spruce (N=24) SBW defoliation (spruce)

31 Climate-related browning of aspen & poplar across >400 km in NE Alberta, July 2011 Photo by Mike Vassal, Wood Buffalo National Park Aerial view S of Fort McMurray AB, July 2011 Photo by Tom Hutchinson, Alberta SRD

32 Summary and conclusions Drought has had a major impact on aspen forests across western Canada CIPHA provides an example of how to assess drought impacts in pure stands Mixedwood stands pose additional challenges, even in the simplest case of two species (aspen-white spruce) having similar growth responses to drought Pest outbreaks (e.g., insect defoliation) can strongly confound growth-climate relationships (including growth releases of non-host tree species) Knowledge gap on factors affecting mortality in different tree species Despite these challenges, research on mixedwood forest responses to drought is needed for: - quantifying drought impacts on growth & yield, including carbon cycling - guiding alternative forestry practices for adapting to climate change

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