Active interventions accelerate native plant recolonization following agricultural abandonment

A key challenge in restoration planning is predicting how passive and
active processes may best be used together to promote successful landscape
recovery. While passive recolonization of native plant communities can
drive recovery, this process is often slow, and success is variable.
Active restoration practices, such as reinstating historical disturbance
regimes, may accelerate recolonization. However, few studies have
empirically tested how specific active interventions can be used to
facilitate recovery across heterogenous landscapes. We conducted a 13-year
field experiment monitoring the recolonization of longleaf pine savanna
ground-layer communities in landscapes recovering from agricultural use in
the southeastern USA. Across 12 sites, we established pairs of 1-ha plots
in untilled, fire-suppressed remnant savannas and adjacent pine
plantations previously under agriculture (recovering areas). Each pair was
randomly assigned a restoration treatment: canopy thinning in the remnant,
the recovering area, both, or neither (control), combined with either low
or high frequency of prescribed fire. In each recovering area, we measured
the spread of remnant indicator plant species along a 100 m transect
extending away from the remnant. We found that remnant savanna indicator
species can slowly recolonize recovering areas after 70 years without
active interventions. However, high fire frequency and canopy thinning in
the remnant or recovering area resulted in 3-4× more remnant indicator
species in recovering areas. These patterns were modified by seed
dispersal mode: species with vertebrate and short-distance (e.g., by
gravity or ants) dispersal modes spread further into recovering areas with
combined thinning and burning treatments whereas species dispersed by wind
were rare regardless of treatment. Synthesis and applications: Our results
show that combined canopy thinning, and prescribed burning can accelerate
natural recolonization in savanna systems. However, additional measures
(e.g. seed addition) may be needed for weak-dispersing species. These
findings highlight the utility of integrating passive processes with
active interventions to conduct efficient landscape-scale restoration.