Effects of a forest tent caterpillar (Malacosoma disstria Hbn.) outbreak in the boreal forest of western Quebec

Transcription

1 Effects of a forest tent caterpillar (Malacosoma disstria Hbn.) outbreak in the boreal forest of western Quebec Julien MOULINIER PhD student Julien.Moulinier@uqat.ca François LORENZETTI Yves BERGERON Edmonton, June 19 th 2012

2 Forest succession in southern boreal forest FTC Time 1 st cohort 2 nd cohort 3 rd cohort ( Bergeron et al. 1999, Harvey et al. 2002) In Canada, 20 million ha were burned between 1997 and 2007, whereas over 120 million ha were disturbed by insect outbreaks during the same period (Natural Resources Canada 2010).

3 Forest tent caterpillar Most important defoliator in deciduous and mixedwood boreal forests Trembling aspen is the preferred host species (Peterson and Peterson, 1992) Epidemic insect with cycles of ~ 9-13 years in Quebec and Ontario (Cooke and Lorenzetti 2006) Large scale disturbance ( , 6 outbreaks in Quebec, ~ km 2 ) (Cooke et al. 2009) Important lost of harvestable volumes (growth reduction, mortality)

4 Forest tent caterpillar and Gap dynamics Canopy transition Recruitment and succession (Kneeshaw and Bergeron 1998; Cumming et al 2000; Hill et al. 2005). Gap size and species functional traits (Bergeron, 2000). Gap of < 200 m2 favor shade tolerant species (McCarthy, 2001)

5 Objective Recent studies have documented gap dynamics following FTC outbreaks (Man and Rice, 2010; Moulinier et al, 2011) However, the effects of FTC disturbance on stand dynamics in southern boreal mixedwood forest have not been described yet. How FTC outbreak severity affects successional pathway of deciduous and mixedwood stands dominated by trembling aspen in northwestern Quebec?

6 Last outbreak in western Quebec : the most important in this region since 1950 s in terms of duration, severity and extent Aerial survey (MRNF): ~ ha Defoliation classes: - 0: no defoliation - L: light, <25% - M: moderate, 26 to 65% - S: severe, 66 to 100% Outbreak portrayal: (years of moderate-severe defoliation) - no defoliation, 30% - 1 year: 38% - 2 years: 24% - 3 years: 7% - 4 years: 1%

7 Selected stands characteristics Field measurements were realized during summers Deciduous Forest type Mixedwood 20 Nb of stands % (PePe) Basal area (aspen) 50-60% (PeR) Clay Soil deposit Clay (aspen) Age (aspen) (conifers) Nb of severe years 0,1,2,3 Defoliation gradient Annual mean rate 6.5 to 65%

8 Canopy gap measurements Deciduous Forest type Mixedwood 14.4 km Cover transects 7.4 km 4.4 km Gap transects 2.4 km 728 Total nb gaps Gaps sampled 112 Stand Gaps Transects Plots 400m 2 Line-intercept transects - Canopy: presence/absence (canopy opening rate) - Stand: density, basal area, regeneration density - Gap: length, width, area, nb gap makers, regeneration density

9 Method Regeneration (aspen) Suckers (<1 m), Saplings (1-2 m) and Poles (2-4 m) Regeneration (conifers) Seedlings (<0.5 m) Saplings (0.5-2 m) and Poles (2-4 m) Regeneration density calculated for aspen and balsam fir Apical growth of balsam fir was quantified along the gradient of defoliation intensity in gaps of different size and under forest cover Statistics Defoliation effect: - canopy opening, gap size, gap makers ~ linear regressions - frequency of gap size distribution ~ Kolmogorov-Smirnov Defoliation and gap size effects: - regeneration density ~ mixed models, model selection, model averaging

10 Canopy opening Deciduous Mixedwood Defoliation intensity

11 Mean gap area Deciduous Mixedwood

12 Deciduous Relative frequency of gap size distribution NS ** ** n=32 n=117 n=50 n=33 Mixedwood NS NS NS NS NS

13 Gap makers: mean number/gap Deciduous Mixedwood Increase the number of gap makers Modification of deadwood stand attributes

14 Trembling aspen regeneration vs defoliation Deciduous Mixedwood 0-1m 1-2m 2-4m 0-1m 1-2m 2-4m Increased recruitment of [1-2 m] class with outbreak severity Improvement of suckering and understory growth conditions No significant effect of defoliation on aspen recruitment Conifers effect? (competition for light, soil conditions)

15 Trembling aspen regeneration vs gap area Deciduous Mixedwood 0-1m 1-2m 2-4m 0-1m 1-2m 2-4m Large gaps improve recruitment of [1-2 m] and [2-4 m] classes Better understory conditions for suckering, growth and survival No significant effect of gap size on aspen recruitment Conifers effect? (Competition)

16 Balsam fir regeneration vs defoliation and vs gap area Mixedwood 0-0.5m 0.5-2m 2-4m 0-0.5m 0.5-2m 2-4m Increased recruitment of [0-0.5m] and [0.5-2m] classes Effect on seeds production? Survival? Growth? No effect of gap size, fir more abundant under forest cover Effect of spatial distribution of mast trees inside vs outside gaps?

17 Conclusion FTC outbreaks constitute an important agent of gap formation in deciduous as well as mixedwood stands. Low intensity : do not really influence stand dynamics. Moderate intensity : modify stands structure by increasing aspen mortality - Deciduous: low recruitment of aspen create open stands with uneven structure. - Mixedwood: recruitment of fir change more rapidly canopy stand dominance from aspen to fir. Severe intensity : strongly affect aspen mortality and stand structure - Deciduous: recruitment of a new cohort of aspen should restore closed aspen canopy through time. - Mixedwood: recruitment of a second cohort of fir should accelerate canopy stand transition from MW-aspen stands to MW-fir. Landscapes disturbed by FTC represent a mosaic of even and uneven stands struture.

18 Stand dynamics after a severe FTC outbreak Deciduous C1 C2 C3 Longer deciduous phase and «rejuvenated» forest cover Delayed change in canopy composition Delayed availability of both deciduous and softwood harvestable volumes Time 1 st cohort 2 nd cohort 3 rd cohort Mixedwood Faster forest succession Faster canopy conversion Harvestable softwood volumes more rapidly available C1 C2 C3

19 Tanks to Field assistants, Dominique Herman and Caroline Trudeau, Logistic assistance, Marie-Hélène Longpré and Danielle Charron, Statistical advices, Marc Mazerolle, And all partners,