Data and code from: Diverging restoration pathways for overstory and understory communities in a Mediterranean-climate riparian ecosystem

Few restoration studies have long-term longitudinal data to evaluate
recovery trajectories, which typically vary among different aspects of
ecosystem structure and composition. We used repeat surveys to evaluate if
the trajectory of vegetation structure and community composition in
restored and reference forests converged from recovery of restored forest
or degradation of reference forest, and whether patterns differed for
forest overstories and understories. We measured vegetation communities
9–16 years after prior surveys and 21–30 years after planting in 11
restored and 8 reference forest sites along a 100-km span of the
Sacramento River, California. We used generalized linear mixed models and
non-metric multidimensional scaling to evaluate convergence in overstory
structure, understory cover and understory composition. Restored forest
structure became more similar to reference forests over time, converging
on basal area of ca. 20 m2 ha-1 around 25 years and densities of stems
around 35 years. In contrast, overstory species composition remained
distinctive, primarily due to the different initial tree species
composition and limited overlap in new species recruitment in restored and
reference forests. Restored understories had lower native and higher
exotic cover than reference sites in both surveys. Restored understory
composition became more similar to reference sites as shade-intolerant
exotic species cover declined. However, understory composition in restored
sites remained distinct from reference sites due to the increased woody
cover, primarily from the invasive, shade-tolerant shrub Rubus armeniacus,
which increased from 2 to 23 percent mean site cover between the two
surveys. Path analysis showed that in restored forests, R. armeniacus
cover was negatively related (R = -0.51) to native understory cover,
suggesting that it impedes forest recovery. Our results suggest that this
system has converged with reference forest in overstory structure but not
yet in overstory or understory composition.