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.热带森林林下生境普遍存在光照受限的问题。红光与远红光比值（red-to-far-red ratio, R:FR）是评估光质的可靠有效指标，其空间异质性可影响本地与外来幼苗的种间竞争。尽管已有部分研究针对夏威夷低地湿润森林（Hawaiian lowland wet forests, HLWF）量化了光照透射率，但目前尚无关于林下光照空间分布如何随物种入侵发生变化，亦或是幼苗更新格局与光照是否存在关联的相关报道。本研究针对三类HLWF林分——本地物种主导林分、轻度入侵林分以及草莓番石榴（Psidium cattleyanum）主导林分——开展林下光照R:FR测定，以评估其光质特征，测定高度设定为林下0~50 cm区间。同时本研究还分析了光质与本地及外来幼苗的出现率、多样性及丰度之间的关联。上述数据集可助力评估夏威夷低地湿润森林的修复潜力。线性混合效应模型分析结果显示，本地物种主导林分的R:FR值显著高于草莓番石榴主导林分，表明随着入侵程度加剧，林分光照环境发生了显著改变。R:FR的空间异质性在样地间的差异大于林分类型间的差异。在本地物种主导及轻度入侵林分中，低光质R:FR区间（0.0~0.40）内的本地幼苗数量显著高于随机预期值，而中光质（0.41~0.70）与高光质（≥0.71）区间内的本地幼苗数量则显著低于随机预期；草莓番石榴主导林分中则未发现本地幼苗。本地物种主导林分的本地幼苗物种丰富度及丰度均高于轻度入侵林分，这一现象可能与繁殖体可获得性有关。但幼苗的空间聚集分布、本地幼苗与不适宜光照环境的错配现象，以及大量开放的高光质微生境，均表明从长期来看，夏威夷低地湿润森林的生境并非本地物种的最优生长条件。综合与应用 尽管样地间存在差异，但本地物种主导及轻度入侵林分仍具备保护价值。幼苗增补工作可针对不同R:FR区间的生境及不同空间尺度开展，但由于R:FR与幼苗数量之间未呈现显著关联，因此在开展幼苗定植或其他管理措施时，还需考虑光质以外的其他影响因素。