'Secondary hemiepiphytes' - A concept under scrutiny

热带植物的生物多样性不仅在物种数量方面令人印象深刻，而且在结构和生命形式多样性方面也令人印象深刻。热带森林的复杂性和多样性在很大程度上是由于在温带森林中很少甚至完全没有的生命形式的存在，例如维管附生植物、勒死或非勒死半附生植物、木本和非木本攀援植物，但这些类群的生态学仍然知之甚少。所谓的“次生半附生植物”（SH）就是一个很好的例子，这种植物的定义是在地面发芽，在树干上上升，茎的下部枯死，导致与地面的所有联系断绝。尽管关于热带森林生态学的现代教科书通常将热带雨林描述为五个机械依赖的植物类群之一，但“次生半附生植物”的概念几乎完全基于轶事证据。少数发表的实验报告和自己的初步数据表明，被归类为SH的植物并没有失去与土壤的联系，而是不定根取代了茎作为导水系统。这可能仅仅是缺乏二次生长的攀援者的水力限制的结果：几乎所有所谓的SH都是单子叶。拟议的研究现在将严格解决藤本植物研究人员和附生植物研究人员都忽视的问题。基于对两种热带森林中松香植物群落的定量时空分析和对根系连接的全面普查，我们将研究与“正常”藤本植物和附生植物相比，松香植物的萌发、建立和重要功能方面。如果我们能证明所有被研究的地表植物都持续依赖于土壤根接触，那么地表植物的整个概念很可能是模糊了生物学现实，次生半“附生植物”这个术语是一个不必要的，甚至是误导性的用词。或者，至少在少数物种中验证Kress的概念将成为植物生长策略的一个令人兴奋的方面的证据。

Tropical plant biodiversity is not only impressive in terms of species numbers, but also in terms of structural and life form diversity. Much of the complexity and diversity of a tropical forest is due to the occurrence of life forms that are quite rare or even altogether absent in temperate forests, e.g. vascular epiphytes, strangling or non-strangling hemiepiphytes, or woody and non-woody climbing plants, but the ecology of these groups is still little understood. A case in point are so-called ‘secondary hemiepiphytes’ (SH), plants defined by germination on the ground, ascent on tree trunks and dieback of the lower portion of the stem leading to severance of all connections with the ground. Although modern text books on tropical forest ecology typically describe SH as one of five mechanically dependent plant groups, the concept of ‘secondary hemiepiphyte’ is almost entirely based on anecdotal evidence. The few published experimental reports and own preliminary data suggest that plants categorized as SH do not lose connection with the soil, but rather that adventitious roots replace stems as water conducting systems. This could simply be a consequence of hydraulic limitations in climbers that lack secondary growth: almost all so-called SH are monocots. The proposed study will now rigorously address a problem that has been neglected both by liana researchers and epiphyte researchers. Based on a solid quantitative spatio-temporal analysis of SH communities in two tropical forests with a thorough census of root connections, we will investigate germination, establishment, and important functional aspects of SHs as compared to ‘normal’ vines and epiphytes. If we can show that all studied SH continuously depend on soil root contact, the entire notion of SH is probably blurs biological reality and the term secondary hemi-‘epiphyte’ is an unnecessary and even misleading misnomer. Alternatively, validating Kress´ notion for at least a few species would be evidence for an exciting facet of plant growth strategies.