Past tree influence and prescribed fire mediate biotic interactions and community reassembly in a grassland-restoration experiment

1. Woody plant encroachment of grasslands is occurring globally, with profound ecological
consequences. Attempts to restore herbaceous dominance may fail if the woody state is resilient
or if intervention leads to an alternate, undesirable state. Restoration outcomes often
hinge on biotic interactions – particularly on priority effects that inhibit or promote
community reassembly.
2. Following experimental tree removal from conifer-invaded grasslands, we documented
substantial variation in community reassembly associated with the changing abundance of the
native clonal sedge Carex inops L.H. Bailey subsp. inops. We explored possible mechanisms
for this variation, focusing on the nature and timing of interactions between the meadow
community and Carex and on how past tree influence and prescribed fire mediate the
outcomes of these interactions.
3. Meadow species increased after tree removal, but less so in burned than in unburned
plots. Carex expanded dramatically after fire, particularly where past tree influence had been
greater.
4. Meadow species and Carex developed an increasingly negative association over time; preemption
was reciprocal, but offset in time and space. Meadow species inhibited Carex
through vegetative recovery in areas of limited or recent tree influence, irrespective of fire.
Carex inhibited meadow reassembly in areas of greater tree influence, but only with burning.
5. Synthesis and applications. Tree removal and fire imposed across a range of altered meadow
states yielded varying outcomes, reflecting biotic interactions and species’ regenerative
traits that inhibited or promoted reassembly. Fire tended to destabilize the remnant meadow
community and, in areas more degraded by encroachment, stimulated release of Carex, which
inhibited reassembly. Knowledge of the context dependence of biotic interactions can enhance
the effectiveness of restoration by establishing the bounds within which treatments produce
desirable or undesirable outcomes.
Key-words: burning, competition, inhibition, meadow restoration, pre-emption, priority
effects, species’ interactions, tree encroachment