Why rainforest species have buttress roots

Eric Frei

For some time I have sat on the fence as to why rainforest trees have large buttress roots.

There is no shortage of information online that says three particular things.

1. That soils are shallow so trees need wide spreading roots
2. That soils are nutrient poor so a wide spreading root system is required to mine the shallow organic layer
3. That due to a shallow wide root system buttresses are required for stabilty

Here's a huge list of URL's that say that above.

The Plants of the Rainforest

Why do rainforest trees have buttress roots

Plant Adaptations

What type of amazon rain forest trees have giant buttress roots? - Yahoo! Answers

Trees: Magnificent Structures of the Natural World

Buttress root - Wikipedia, the free encyclopedia

Plants of the Tropical Rainforests

Why rainforest soils are generally poor for agriculture

Why rainforest does not regenerate after total clearing

However I am not totally sold. I have spent enough time wandering many kms of the Lamington National Park rainforest to see the trees and the soil. To see huge trees blownover and get a decent look at the soil profile. I see land slips that expose the soil.

First of all I think there's a terminology issue, "shallow soils". To the lay person familiar with the urban soils it likely means no depth and hard rock or shaley clay beneath. You know, the typical soil we encounter in the yard, dig a hole and there's 100mm of topsoil and crap beneath that'll break your fork, but lots of trees grow especially eucs. But I think the term "shallow soils" is used to define the humus layer (that 100mm thick richer organic layer) at the top of the soil profile.

I see in the rainforest physically deep soils, you could easy dig a 1m deep hole. So soils are not shallow in the sense of hitting rock or something below.

As far as nutrient content goes without recent soil tests we'd really not know but I think it would be reasonable to assume that rainforest soil would be better for nutrients, organic matter, fungi and microbes than an open field or urban soil. One of the reasons it is reported rainforest soils are poor is due to the high rainfall continually leeching away the nutrients, but the nutrient supply from the decomposing forest floor is abundant and quickly replenished. I think it is possible that the soil is actually deeper in the context of biota.

So keep going down this track even if I am wrong .... if the soils are physically deeper and there is more biota at a greater depth then why the buttress roots?

I believe that due to the high moisture content of the soil there is less soil cohesion and skin friction between the soil and the tree roots. Certainly judging by the failures I have seen it would be easier pulling out a sinker or striker root from a saturated clay soil than a dry one. I think one of factors for buttress roots is definately stability in a sodden soil.

Large eucalyptus (E regnanas) grow in a temperate forest in Victoria and Tasmania. They attain heights greater than rainforest species and are the worlds tallest flowering tree. They have rainfall equivalent to that of rainforests but they do not have the buttress roots.

Some eucalyptus like the flooded gum (E grandis) also grow in rainforests, they do not have buttress roots.

So with these thoughts I decided to pursue the soil angle further and discovered I'm not the first to do this with some interesting research as follows.

Factors influencing species in tropical lowland rainforest: Does soil matter?

http://www.dpi.nsw.gov.au/__data/ass...ainforests.pdf

The Importance of Rainforests

Determinants of Location and Limits of Rainforest

I think that last link is the best and I'll quote from it, it is also Australia focused:-

Now I must say although I have been to Fraser Island it was for a fishing expedition and not trees so I cannot talk about the rainforest species there, if there is species with buttress roots or not.

Being a sand island it is fair to say a few things:-

1. Nutrient leeching would be faster due to soil porosity
2. Skin friction for roots would be higher so more stability
3. Roots would be deeper due to larger air content of the soil

So if there is trees there which are also found in the loamy/clay rainforests do they have as prominent buttresses?

A key point is that I do not believe rainforest trees have buttress roots due to shallow or infertile soils. I believe they have buttress roots for stability on a sodden slippery soil.

So what are your thoughts on why rainforest trees have buttress roots? Please read all of the above before answering.

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Eric

I am inclined to burst your bubble about deep rainforest soils, well in part at least.

In 1992 I was the leading hand field technician on a rainforest research project looking at life in the rainforests of central and mid-southern Queensland. We had field study sites at 20 odd sites from just north of Nambour to one near Rockhampton on limestone soils. You should really call these site closed forest - they still have buttress roots on the dominant canopy trees so I figure the point is relevant. At each site we collected data for five days, then moved on to the next one. Repeated the exercise six months later for seasonal contrast.

My task in the first two days was to dig the sampling holes, 300mm dia. 400-500mm deep. The site at Erimbula NP near Agnes Water was a nice mature sand dune so it was deep soft digging despite an abundance of roots thru the soil profile.

On the other sites the soils became partly rocky within 300mm depth. A few in natural hollows had a deeper sediment to dig thru but most were simply rocky. I think this is why I was selected for the job because I had a lot of shovel experience as a labourer and the scientists and senior techs didn't like the crow bar and shovel situation. Great job for the whole year so I didn't mind 15 holes per trip for 20 trips. 600 samples.

The other thing to share is the shortage of roots below 250mm depth. Once I had dug below about 250mm there were fewer roots. Organic darker horizon often fading by 100mm depth most feeder roots here. Bigger roots to 250mm. soil and dirt below 250mm.

Let you work thru this, you seem to be doing a good job thinking it over.

I'll have a dig around here later to see if I can scan some old pics.

RM

Eric Frei

You've burst nothing ..... you didn't state why you think rainforest trees have buttress roots.

You really believe the soils of plains and dry sclerophyll forest are that different and that the humus layer is deeper than 100mm?

Are buttress roots required for a spreading root system to absorb nutrients? Many trees have wide spreading absorbing roots, but not buttress roots. So why don't they need buttress roots?

In this picture it's a typical look at soil and roots anywhere in Brisbane.



A closed system suggests that nutrients come from within the system. An open system suggests that they can come from other places.

So in large open fields, plains, grass plains like around Hay where is the outside source? I see only rivers flooding and fires being the major difference. I would say that they are of poorer soil quality but the rainfall is less.

I think the key point is that Australian soils are the world's oldest and most depleted, they are poor soils in general.

So in an evolution sense if soils are poor why then did rainforest species develop buttress roots? Stability is my bet.

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Brent Ferris

I haven't read the articles yet, and I am not familiar with rainforest species or landscapes, but are you saying that most (if not all) rainforest trees have buttress roots, but other species eg eucalyptus can grow in the rainforest, but it is not their natural habitat? If so, the buttressing roots would make most sense for sodden soils and stability, as a natural adaptation. Will read the articles as more time becomes available -- look forward to the info.

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JayJay

Question: Why Rainforest Trees have buttressing roots?


Buttress roots = area of interest being the base of trunk and/or commonly referred to as the root crown.


Great question which has me thinking …


What I believe to date BUT still learning …

oSoil properties influence species composition
oButtressing is a feature of support and somewhat genetically determined

Therefore:

Trees exist in a state of internal and external flux with age playing a defining role on buttressing characteristic’s. The dynamic interactions of the various tree parts are correlated through growth control regulators and resource allocations. From moment to moment, I believe a tree is attempting to solve a series of simultaneous biological equations based on age and climatic influences.

Conclude:

Trees do not grow at random; but rather in predictable ways, following strict principles…….. with important difference in phylogeny and ontogeny existing between the root system, and soil constraints.


Honestly I need to give this more thought.

NB: Eucalyptus regnans have a solid trunk taper and buttressing based on my climbing experience but not as pronounced as let’s say a Fig tree in an urban soil arrangement.

rangitata

Personally I think that the formation of buttress root systems is an adaptive trait related to maintaining tree stability within the typically saturated soils that are typically found in rain forest conditions. Also I found this link that puts forward another theory within the abstract (haven't been able to locate the full text) that hints towards the buttress root systems controlling water flow and soil erosion on the ground surface.

(Buttresses of tropical rainforest trees influence hillslope processes - Herwitz - 2006 - Earth Surface Processes and Landforms - Wiley Online Library

I agree with JayJay that it is a probably a genetically predetermined trait, but also put forward the idea from my observations that the formation of buttress systems is typical found on species with a spreading decurrent canopy structure regardless of the growing conditions, thus again it possibly relates back to maintaining tree stability.

Eric Frei

Great point.

The blue quondongs have excurrent form but they have decent buttresses.



One particular spot I consult at has a grove of huge quandongs at the valley floor in almost a creek bed. In heavy rain they are in water and can have up to 1m of water up the trunk, running water. They have decent buttresses.

So when we compare the form of that to the form of say E regnans or E grandis .... the eucs have large spreading lateral branches, so does the quondong. Both species are apically dominant (excurrent form). But the quondong has a very pronounced buttress root system.

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Jeff Darby

Here is a tree in suburbia which has developed buttress roots, you can see by the picture this tree is on the verge and it's a small verge at that. I've been meaning to take some photos for you all to take a look at and today the opportunity arouse...

Here are some of the caps I found under this tree...and yes the photos are off my phone.



notice the ribs on the caps...

Here is a photo of the tree...



And here is closeups of the lower trunk...



looking from either side...



This is the only tree of this species I have noticed like this in the whole area, so for me I feel it is a stability adaption.

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Joshua Rugg

nice BUTTress

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Garry Brockley

The way i look at the rainforest trees butressess are generally these trees are forced higher by competition, but are kept narrow also by competing species, so from an engineering point of view a wider fluted base is much stronger for support than a cylindrical pole rising from the earth.
it,s the difference between a tripod and a single stake the tripod is much stronger against winds and maybe allows the tree to force more structural roots out when there is so much close competition.
just my tuppence worth.

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