How do forest ecosystems respond to environmental change?

Resilience & Ecosystem Feedbacks Dominant species Recruitment Interactions Competition Functional traits Disturbance

Resilience Feedbacks Black spruce Low nutrient cycling Cold soils Conservative growth Mosses accumulate Aerial seed bank Recruitment on organic soils

Typical successional trajectories Self-replacement of black spruce Asexual regeneration of understory Little compositional change after disturbance

Deep burning fires can shift successional trajectories Is there a threshold of organic soil consumption that has predictable effects on regeneration pathways?

Recruitment declines with organic depth 2-yr recruitment from seeding experiments (Alaska/Yukon) n=4 to 16 (total plots = 60)

Species differences lead to strong effects on post-fire dominance Seedling Density Aboveground Biomass Burned spruce forest Alaska 40,000 ha burn 8 yrs post-fire n=19 stands

Can we detect thresholds across real landscapes? Alaska 2004 fires 3 fire complexes 90 black spruce sites

Regression Tree pseudo-R 2 =0.65 Deciduous vs. Spruce Recruitment Natural seedling densities 2 yrs post-fire

Fire & regeneration thresholds Residual organic layer determines seedbed quality Differences in species sensitivity lead to strong composition effects Increased fire severity => crossing threshold of residual organics => shift in successional trajectory

Disturbance & climate interact to alter black spruce resilience tundrablack sprucedeciduous dynamic equilibrium directional change

Resilience Feedbacks Species dominant Recruitment Interactions Organic seedbeds Conservative growth Low nutrient cycling Mosses accumulate

Critical Research Moving deeper in time –What are the longer term consequences of variations in fire severity? Understanding space –Which parts of the landscape are vulnerable to shifts in trajectories, and which aren’t? Can we test anticipated changes?

Climate response dynamics Key feedbacks support resilience Do threshold responses measure the depth of resilience? At the landscape scale, is this the primary pathway for climate change response?