Post-fire recovery in black spruce forests Analysis Summary October 2007 Jill Johnstone

Intensive Sites 32 forest sites + 5 treeline sites Range of moisture * severity levels Seed and seedling applications in 2005 Initial responses surveyed in 2006

Site moisture –drainage class –mid-season soil moisture –soil texture Fire severity –visual severity rank –pre-fire organic layer from adventitious roots Post-fire seedbeds –residual organic layer depth –cover of seedbed types –% cover of organics>3 cm –composite burn index (CBI) Post-fire recruitment –seedling densities –transplant growth Field measurements

surface fire severity organic seedbeds pre-fire organic depth site moisture landscape position Fire and Site Effects on Seedbeds: Hypotheses Assess using Structural Equation Modeling

Univariate correlations

Fire and Site Effects on Seedbeds: Model Specification severity index (1=low, 5=high consumption) pre-fire organic depth (adv. root height in cm) drainage index (1=subhygric, 6=xeric) elevation (m) Organic Seedbeds Composite Burn Index post-fire organic depth (cm) % cover of bare organics

Fire and Site Effects on Seedbeds: Model Results severity index pre-fire organic depth (adv. roots) drainage index elevation Organic Seedbeds Composite Burn Index post-fire organic depth organic seedbed cover Model χ 2 =13.5, p=0.2 (no significant lack of fit) R 2 =0.96 dashed lines are ns

Summary: Fire Effects on Seedbeds Organic soils dominate seedbeds where: –Pre-fire organic layers were thick –High elevation restricts surface fire severity Variations in fire severity: – ~50% of the variation in organic seedbeds Expect impacts of organic seedbeds to be strongly controlled by fire severity

post-fire organic seedbed conifer recruitment site moisture landscape factors Site and Seedbed Effects on Tree Recruitment: Hypotheses Note: Seed availability is fixed for seeded plots and is not included in the model deciduous recruitment

Seedling counts

drainage elev. resid.org BS WSLP conifer TA PB deciduous Used residual organic layer depth (sqrt transformed) to represent seedbed effects (and fire severity) Site and Seedbed Effects: Model specification Seedling counts of black spruce, white spruce, lodgepole pine (highly intercorrelated, r>0.8) Seedling counts of trembling aspen and paper birch (moderately correlated, r=0.7)

drainage elev. resid.org BS WSLP conifer TA PB deciduous Model χ 2 =19.2, df=15, p=0.2 (no significant lack of fit) R 2 =0.57 R 2 = Site and Seedbed Effects: Results Presenting standardized regression coefficients and sum of squared correlations for predicted seedling counts

Drivers of forest regeneration Differential sensitivity of functional groups –Deciduous highly sensitive to post-fire seedbeds –Conifers more responsive to site moisture Important role of fire severity in potentially tipping the balance between deciduous and conifer dominance

post-fire organic seedbed seedling growth landscape factors site moisture Seedling Growth: Hypotheses Note: Possibility for reciprocal effects between tissue N and seedling growth tissue nitrogen

post-fire organic seedbed black spruce height growth elevation site moisture index Seedling Growth: Results Note: Possibility for reciprocal effects between tissue N and seedling growth tissue % nitrogen R 2 = 0.73 R 2 =

Seedling Growth Low severity Negative response to increasing organic layer depths Appears to mediated by N availability –need to clarify causal effects on soil N as opposed to tissue N Generally consitent responses among species Greatest growth at driest sites High severity

Resilience of Black Spruce Forests Low severity Black spruce self-replacement –More poorly drained sites –Sites with thick organic layers (cool & moist) Potential switch to deciduous dominance –Where high burn severity exposes mineral soils –More frequent with climate warming? High severity

Thank you everyone!