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In my previous question, it was quite well established that tanks were basically superior to walkers in every way.

So, lets even the playing field. I've been contemplating several ways of doing so, but in honor of this fortnights topic challenge, I'll start with plantlife.

Would it be possible for a certain type of plant, which, if growing rampant in the wild, would cause tanks to become impractical while allowing bipedal mecha to become dominant?

This anti-tank plant can be entirely fictional. However, try to keep it realistic - as in, try to ensure that plant fiber/material can actually do what you propose.

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  • $\begingroup$ Wondering if there has ever been examples of tanks trying to make it through extremely dense jungle, such as a rain-forest or such. I'd imagine they could get "tied up" with all the vegetation, but I could be wrong. Mechs could use a large blade to slash their way through. $\endgroup$ – DoubleDouble Feb 23 '15 at 18:32
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    $\begingroup$ My grandpa told me that during WW2, part of his job was to weld some of those defenses pictured below ("dragon's teeth" I think) onto the fronts of the tanks to use as bulldozers against the french hedgerows, which were remarkably difficult for tanks to navigate. So maybe a think enough tangled mass of brush would be tank-deterrent. $\endgroup$ – Josiah Feb 24 '15 at 1:43
  • $\begingroup$ @Josiah Wow. Props to your grandad for being in WW2! $\endgroup$ – Feaurie Vladskovitz Feb 24 '15 at 4:09
  • $\begingroup$ I am hardly a WW2 expert but I have read accounts that hedgerows often served as effective barriers because they could contain (and even unintentionally conceal!) walls made of things such as brick. Definitely want to fact check me on this, however. $\endgroup$ – user1445967 Feb 24 '15 at 19:13
  • $\begingroup$ How about thinking about this another way round. In our world and technology, bipedal machines don't make sense, because they are hard to make. Yet humans are bipedal. The reason is that biologically we use linear motors called muscles whilst our machines use rotary engines. If early on in your world's development they produced muscle like machines which are powerful and cheap, then bipedal machines would make sense. However it would also mean almost every piece of tech in this world would be different. $\endgroup$ – Aron Feb 25 '15 at 1:05

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Perhaps the simplest method would be to have your trees grow at 45 degrees (or more) to the vertical. The end result would be similar to the defences employed on many beaches during WWII. enter image description here

From the front the tank is actually pushing the base of the tree deeper, from behind the tank rides up the tree and beaches itself, and from the sides the trees are too thick to permit passage.

While this would be completely impenetrable to tanks, a walker could use its weight and superior position (applying the force straight down rather than horizontally from the ground) to crush the trees flat as it walks.

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    $\begingroup$ This doesn't stop the tanks from going in the other direction though. Maybe you can consider "caltrop trees"? $\endgroup$ – March Ho Feb 23 '15 at 11:47
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    $\begingroup$ @MarchHo: You're on to something there. Czech hedgehog is a growth pattern that could be rationalized, and tanks do not like them. A walker could step over... $\endgroup$ – DevSolar Feb 23 '15 at 12:32
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Perhaps there are many common plants (grasses and bushes) which secrete a thick gum-like fluid when squeezed or crushed which causes the tracks on tanks to clog and seize (particularly with dirt and plant matter adhering).

Walkers are unaffected except for occasional problems with the ankle joint, and build-up on their soles which eventually need cleaning.

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  • $\begingroup$ Same idea, but the opposite, they could secrete a material which causes the tracks to be incredibly slick instead, causing the tank's tracks to "spin in place" against the smallest of obstacles. The walker's higher "ground pressure" (or cleats) could make it less affected. $\endgroup$ – DoubleDouble Feb 23 '15 at 18:45
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    $\begingroup$ @DoubleDouble Couldn't a tank get past that obstacle by using high-friction treads or treads that hook slightly into the ground? $\endgroup$ – Kevin Feb 24 '15 at 8:26
  • $\begingroup$ @Kevin probably, but if the environment setup is flexible I'm sure a wet muddy surface, after enough fluid, could still just cause the tank to dig itself in until it gets stuck. The walker would have to struggle like a person trying to walk in mud that sinks to their knees - but it should make it. $\endgroup$ – DoubleDouble Feb 24 '15 at 15:52
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Very high grass could be enough to make tanks the lesser option. Any high and dense plant life would:

  • Show tank tracks: When a tank moves through high grass, it will flatten the grass, and create very clear tracks that can be seen for days afterwards. Seen from the air, each tank would leave a nice trail pointing to it's current location. Tanks would be unable to hide, which can be a large tactical disadvantage. Walkers tracks are much smaller, and could actually be completely hidden by the grass.
  • Block the view from inside the tank. A walker could be high enough te be above the grass. There could be solutions for this off cource (a periscope to look above the grass).
  • Hide shallow streams. This could be a problem if these streams are too high for the tank to cross. Walkers could have high enough legs to cross streams of several meters deep.
  • Hide other nasty terrain features. Tanks can get stuck in chasms and boulder fields. This is a big problem if you can't detect them early enough. A sudden 2 meter drop might be invisible in high grass, and could be devastating to a heavy tank. Walkers could have an advantage here, if they have enough legs, they can feel and compensate for these obstacles. Being higher above the ground, walkers could also have better a more efficient downward radar to detect underlying terrain.
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So, let us begin by listing out the few key differences between tanks and walkers, so we can then know what to engineer our plants to attack.

  • Tanks are lower, making them more stable, with a low center of gravity
  • Tanks are easier to armor, because it is simply a box, with fairly low surface area compared to a walker
  • Tanks can carry much heavier guns, also because it is stabler

Short Tanks

Well, this should be quite easy to attack, you can simply have the fauna constantly spewing out some form of thick, denser-than-air gas. This gas would then play havoc with the tank's short stature, and the crew would then be unable to see anything. And that is with the gas simply being there, you could easily than also make it toxic, corrosive, or radioactive or something else.

Thickly armored tanks

Tanks are at the very simple concept, they are simply METAL BAWKSES, with plenty of important things on the inside, such as ammo, crew and the engine. In real life, an armor piercing shell hitting a tank only creates a fairly, relatively tiny hole, but such a hole is enough to stop a tank dead, as the shrapnel would then fly about on the inside and hollow it out. So, now you create plants with tentacles, coated in super acid. Let this super acid cut through anything your people can create, so heavy armor tanks do nothing against that. Meanwhile, walkers, which also tend to have quite a number of important things in a small compartment, have the advantage of being more agile, as well as being more "holey", as for the same amount of weight as a tank, they tend to be bigger. Not only that, an acid penetration to the arm of a walker simply means no more arm, not no more walker.

Tanks have big guns

Well, this should be quite easy to attack. The forest can be also extremely overgrown, meaning that it is not easy to maneuver large and heavy weapons around. Another thing to note might be that the super acid tentacles may be fairly fragile, and can be easily destroyed with mild gunfire, so submachine guns akimbo may be more effective at fending off acid tentacles than a tank cannon.

Well, that's all the stuff I can think of now, I am sure I will be able to add more later

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  • $\begingroup$ Where is the realism? $\endgroup$ – Ismael Miguel Feb 24 '15 at 18:01
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With enough determination and firepower, a tank can get through or around any type of vegetation. However, there are many types of vegetation which will slow them down, or otherwise hinder their movement.

Hedgerows like in Normany

In World War II, getting through Normandy with tanks became a real issue. A special type of tank called a Rhino had to be created to break through them. These tanks basically had large metal prongs in the front which were designed to destroy the hedges. A walker could squeeze their way through, or leap over these with ease.

Kudzu

Kudzu is a thick invasive vine that literally grows over everything it comes in contact with. It can grow over entire trees and make it difficult to navigate the terrain. It can also hide other obstacles such as large boulders, etc. A walker can cut a narrow path, and would be able to navigate around any hidden obstacles.

Pine Barrens

The Pine Barrens in New Jersey is a thickly wooded area with many trees that are close together. This covers a huge area and would take a long time to go around with a tank. It would take several crews with chainsaws to cut a path through wide enough for a tank. This kind of terrain would barely slow down a smaller size walker.

Cypress Forests

Cypress trees grow in wetlands, and therefore this terrain would nearly be impossible to have a tank go through unless the water was shallow enough for them to pass through. The roots of the trees are large and exposed. A walker could hop from tree root to tree root.

Rice Paddies, Cranberry Bogs, Corn Fields, Plantations, etc.

A walker could walk right through one of these and only cause minimal damage. A tank would flatten all of the crops and completely destroy them.

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    $\begingroup$ Appropriately-designed tanks (e.g. the Chinese Type 63) would be able to overcome your Cypress problem, though it does limit you to rather small tanks. $\endgroup$ – Compro01 Feb 23 '15 at 20:37
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    $\begingroup$ If you're already in a war zone, I'm not sure how concerned you will be with flattening crops. I'm also having trouble thinking of a situation where sending your army through wetlands is a good idea, walkers or no. $\endgroup$ – KSmarts Feb 23 '15 at 21:42
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    $\begingroup$ +1 for the bocage country in Normandy. "the bocage was made for the sniper and the man who lay in wait ... with a "panzerfaust" (Chester Wilmot, The Struggle for Europe). Tanks had to evolve to meet the new threat; Rhinos were made from Sherman tanks in field workshops. $\endgroup$ – Brian Drummond Feb 24 '15 at 12:57
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The wise plant does not stand like the oak against the storm, but bends like the grass...

Rather than fight a tank strength on strength and losing, attack the weakness of the tank. Tank engines need air intakes. Men inside need air intakes. If the plant is fragile, and emits lots of pollen or dust, the air intakes will suck that up right into the filters, which will clog. The tank then needs to stop, clean or replace the filter, and start again.

A dozen feet later, it all happens again.

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    $\begingroup$ Wouldn't a tank be able to have a raised air intake? $\endgroup$ – JonTheMon Feb 24 '15 at 14:39
  • $\begingroup$ This would interfere with the turret gun, would collide with trees and overhead wires, and depending on other tanks might not even help much. $\endgroup$ – Oldcat Feb 24 '15 at 19:23
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Big trees will certainly impede your tanks. You can't knock over a redwood with a tank. However, you can usually trash trees with artillery fire, and (non-redwoods) with regular fire.

You'd want them to be fast growing, and you'd want them to be thick, but not so thick that walkers can't get through. (or thin/slenderish Mecha).

Trees also make good tank traps, if you have rebels / low-tech combatants. See Finns in the Winter War.

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I agree that plants acting like traditional tank traps would be a pain for tanks, however it might be worth considering their impact on walkers a bit more. Plants like brambles can grow in very thick tangled messes quickly. You'll need some special, possibly unrealistic, reason why spacing between plant 'obstacles' can't just be decreased causing increased pressure on the Mechas thinner legs. Also Mechas have more extruding parts to get tangled compared to tanks. But what's possibly more important is whether these Mechas will have any advantage over a tanks which can fly for short distances, though I realise you might have some specific reason for not wanting machines to fly.

What about something like high up within the trees of a giant forest? Then your mecha can act something like a monkey swinging between the branches. You'll have the advantage over flying machines by being able to hang on to branches without expending fuel (or engine noise), whilst tanks won't be able to bridge the gaps between branches.

Basically you want rough/irregular terrain so that tanks can't pass, and enclosed space so that flying becomes either hazardous or not worthwhile. A tight network of caves might also work.

(Sorry for the naff English, very tired)

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  • $\begingroup$ Give the walkers retractable "snowshoes" and they could walk on top. $\endgroup$ – Josiah Feb 24 '15 at 19:02
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Short, fat trees that grow in a checkerboard pattern and do not burn well. A walker can sidestep through them, a tank would have a major problem getting through them. Sure, you can blast but that would become very inefficient to go any distance that way.

As for how trees can do that, think of things like Aspens--that spread via roots.

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Like most people who answered, I think that a plant should be rather obstrusive to the tank. I doubt it's likely to actually destroy it. I got one idea I haven't seen here and it's also very far out there. Entirely fictional if you will.

Think of an overgrown Flytrap. It would snap around the tank and make it stuck. The crew would have to unman the vehicle and carefully clear the 'maw' of that flytrap with tools which is time consuming and leaves them rather vurnable. While a walker would also get stuck in such a plant, his weapon pods would be still free due to his size. He could use some sort of flamethrower to just burn the plant with as much as a few scorched marks on his armor. Skilled pilots could even use their still free, second leg to pry the maw open again.

One reason I haven't found so far. Traces! We were talking about movement the whole time. What about reconnaissance? A mech could maneuver through dense vegetation with crushing a small tree here and there. A tank would leave a very obvious lane where he went through. Considering they maybe got some leading vehicle with clearing equipment (like the bulldozer shield mentioned before) they would also have to move in a line. That kind of formation is very vulnerable to flanking and leaves less armored support vehicles wide exposed.

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In addition to the usual barrier type plants people are suggesting, I remember the army in the desert had problems with their tanks breaking down. Although they had filtering, the desert sand was so fine it was making its way through and clogging the air intake filters which quite quickly caused total tank failure as their engines stalled.

A plant that gave off a large cloud of pollen or other fine materiel as it was crushed or touched could do the same damage, especially if it was more sticky than desert dust. A tank covered in pink pollen could be quite a sight :)

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  • $\begingroup$ There's no reason a walking machine be any more resistant to this than one using treads and otherwise having the same technology base. $\endgroup$ – smithkm Feb 28 '15 at 20:01
  • $\begingroup$ @smithkm possibly, but I'd expect a tracked tank to have a much lower centre of gravity - ie the engine is near the ground where this could be a problem, not up in the air where the density (plus stronger dispersal winds) would not affect it. $\endgroup$ – gbjbaanb Mar 1 '15 at 11:52
  • $\begingroup$ Assuming that there really were enough of a difference in pollen density, it would be far simpler to just raise the air intakes of the tank on snorkels than to raise the entire hull on legs. $\endgroup$ – smithkm Mar 1 '15 at 17:06
  • $\begingroup$ @smithkm seriously? I think you're over-thinking this. $\endgroup$ – gbjbaanb Mar 2 '15 at 8:46

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