Light quality and spatial variability influences on seedling regeneration in Hawaiian lowland wet forests

Tropical forest understories tend to be light-limited. The red-to-far-red
ratio (R:FR) is a useful and reliable index of light quality and its
spatial variability can influence competition between native and
non-native seedlings. While percent light transmittance has been
quantified in some Hawaiian lowland wet forests (HLWF), no information
exists on how the spatial distribution of understory light varies in
relation to species invasion, or if patterns of seedling regeneration and
light are linked. We measured the R:FR of light in the understory to
assess light quality in three HLWF forest types: native-dominated,
partially-invaded, and Psidium cattleyanum- (strawberry guava) dominated
to quantify light quality in the understory (0-50 cm height). We also
identified relationships between light quality and native and non-native
seedling presence, diversity, and abundance. Together, these data can help
to determine the restoration potential of HLWF. Linear mixed-effect
modeling showed that native-dominated forests had significantly greater
R:FR than P. cattleyanum-dominated forests, demonstrating a transformation
in the light environment with increased invasion. Heterogeneity in R:FR
varied more across sites than among forest types. In both native-dominated
and partially-invaded forests, there were more native seedlings in the
low-quality R:FR (0.0 – 0.40) category and fewer in the medium- (0.41 –
0.70), and high- quality (≥ 0.71) light categories than would be expected
by chance, and there were no native seedlings in the P.
cattleyanum-dominated forests. Native-dominated forests had greater
species richness and abundance of native seedlings than the
partially-invaded forests, likely due to propagule availability. However,
the spatial clustering of seedlings, the mismatch of native seedlings in
light environments less suitable, and a considerable proportion of open
high-quality microsites, highlights that conditions are not optimal for
native species in HLWF in the long-term. Synthesis and applications The
native-dominated and partially-invaded forests still hold conservation
value, despite variation among sites. Seedling additions could be targeted
to different R:FR environments and at different spatial scales, but the
lack of a strong relationship between R:FR and seedling number suggests
that other factors besides light quality should be considered in seedling
enrichment or other management activities.