Beyond species richness and community composition: Using plant functional diversity to measure restoration success in jarrah forest

Aim: The importance of restoring ecosystem functions to native systems
that have been degraded, damaged or destroyed is increasingly recognised.
Yet few studies have measured the effect of restoration efforts on
ecosystem functioning or the functional diversity (FD) that underpins it.
Here we assessed change in FD of restored assemblages one to 25 years
after the onset of post-mine restoration. Location: Northern Jarrah
(Eucalyptus marginata Donn ex Sm.) Forest bioregion of south-western
Australia. Methods: Functional richness, evenness, divergence and
dispersion were derived from five plant functional traits relevant to
community reassembly. Effects of three explanatory variables (i.e., age,
year restoration was initiated, and time since fire) on six response
variables (i.e., four FD indices, species richness, and compositional
similarity to nearby reference forest) were analysed using linear mixed
models for a dataset with repeated measures of plots through time (n= 810
plots), and linear models for a sub-set of one-time measures of different
aged assemblages (i.e., space-for-time approach; n= 490 plots). Results:
Functional evenness and functional dispersion increased with age, while
functional divergence and functional richness decreased with age.
Functional dispersion increased with time since fire, while functional
richness decreased with time since fire. Species richness decreased with
age, but at 25-years, species richness was comparable to that observed in
reference forest. In contrast, similarity showed no relationship with age
of restored forest, and at 25-years, similarity of restored forest to
reference was low compared with similarity of reference forest to itself.
Three of four FD indices had not reached those of reference jarrah forest
25-years after restoration had been initiated. Conclusions: Reassembly of
FD suggests importance of environmental filtering and high functional
redundancy. A longer time frame may be needed to assess FD of restored
assemblages, and in the meantime, species richness is not an adequate
surrogate of FD.