Van Steenis’ Rheophyte of the World book

[Nat]

So…today, I want to discuss about this after reading Van Steenis’s Rheophytes of the World.

This is just my understanding of it so, I might make a lot of mistake.
In this book, Van Steenis coined the term “Rheophyte” which means plants growing in fast flowing river. He explained that to be a rheophyte, the plant must have elastic stems, and he also measured drags on the different leaf shapes.

For trees and shrubs it usually look very noticeable with the same leaf shape see: Homonoia riparia, Ficus squamosa, Ficusischnopoda, Ixora salicifolia, Augusta rivularis etc.. For Araceae: most Bucephalandra, Anubias, Homalomena humilis, Many Schismatoglottis species, Ooia, Pipthospatha, lots of Cryptocoryne (lots of it in crispatula group). Other plants: Aponogetonmadagascariensis, Microsorum pteropus, Carex thomsonii etc.

Okay, so there are two types:
1. Facultative Rheophyte: It can grow on land, and in slow flowing water but greatly preferred flowing water habitat.
2. Obligate Rheophyte: Plants that need flowing water to grow e.g. Hydrostachys and Podostemaceae.

In his book, he omitted a lot of aquatic plants however, for example; Vallisneria and Potamogeton not sure why, but I assumed that these are generalist.

 

[Nat]

Later, the botanist; Peter C. Boyce: distinguished subcategory of rheophytes being:
1. Steenisian rheophytes
2. Xerorheophytes
3. Extreme rheophytes
4. Vegetative rheophytes
5. Heterodox rheophytes

Not sure how to explain it, but see this paper, page 505-510:
https://www.nparks.gov.sg/sbg/resea...1_s2_y2019/71_s2_24_y2019_v71s2_gbs_pg497.pdf

 

[Nat]

Personally, I have a mixed feeling on how we defy it, but I want to know what you think first.

 

[Koan]

omitted a lot of aquatic plants however, for example; Vallisneria

It looks like Boyce refers to these as Hydrophytic rheophytes

1. Hydrophytic rheophytes — flaccid, permanently submerged aquatic herbs with usually floating, often strap-shaped, leaves.

but I want to know what you think first.

What a cool paper

Can you clarify what is the question you're asking for discussion?

 

[Nat]

Can you clarify what is the question you're asking for discussion?

I feel that there might be an issue with how we define whether a plant is a rheophyte or not.

For example, in Thailand, we have Homonoia riparia, a rheophytic tree. According to Van Steenis, it would be considered a facultative rheophyte.

However, I've noticed that although this species can grow in fast-flowing torrents, it also occurs in brackish-water habitats where the water current is considerably slower. This makes me question where we should draw the boundary. If a plant can tolerate both strong currents and relatively slow-moving water, how do we distinguish it from a more generalist species that simply happens to occur in rheophytic habitats?

The definitions proposed by both Van Steenis and Peter Boyce seem to create a rather binary distinction: a plant either qualifies as a rheophyte or it doesn't.

But in reality, other than those obvious torrent species btw, it seems to be a continuum between specialised rheophytes and generalist plants that can tolerate flowing water.

 

[Tim Harrison]

Ecological classifications like the Raunkiær system, are imperfect. They are functional models meant to simplify nature, but because nature rarely fits into neat boxes, these systems come with major limitations. Which you are identifying here. So a plant species can belong to several categories, sometimes at the same time.

Plants often exhibit enormous environmental plasticity, snd adapt to changing conditions. For example Ficus cataractarum can grow upright as a tall shrub during periods of low water, and be classified as a phanerophyte.

But it can also grow horizontally in fast flowing water pinning its stems flat against rocks to avoid being torn away. In this instance it could both be described as a rheophyte and a chamaephyte. Or a rheophytic chamaephyte.

To complicate matters further you wouldn't necessarily be wrong to add a third category, lithophyte. Then our Ficus becomes a rheophytic litho-chamaephyte

So even if the plant is literally set in stone the Raunkiaer system perhaps isn't.

It's just a foundational tool used in plant ecology and plant geography to analyze how different vegetation communities adapt to their environments. It's why modern ecologists treat the Raunkiær system as a flexible baseline rather than an absolute law.