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In my world the Human Empire had always been in a constant struggle against nature itself. One of the Empires foes is a parasitic/symbiotic collective of sentient plants.

Basically treants but humanoid sized and much more nimble, with some larger specialized units sprinkled in. In defensive battles they would mostly use ambush and guerrilla tactics as they can hide themselves extremely well in forests.


In my specific case the plant based species would have:

  • Mana infused bark and wood...
  • ... allowing them to move them as if they were muscles ...
  • ... but making them require to reroot every few hours in their home lands to recharge their mana stores.
  • A node based network that functions as means of long distance communication as well as enriching the land with mana. These nodes can be destroyed to drive back their influence but are fairly bulky.
  • Non-verbal collective intelligence, constantly sharing information telepathically between individuals within a 50m radius. (The collective is intelligent, the individual stupid.)

The humans have at their disposal:

  • Roman empire level military organization
  • Late medieval metallurgy
  • No access to magic

Edit: For clarification: the collective is intelligent enough to learn and to adapt different military maneuvers over time.

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  • $\begingroup$ Is there any advantages the humans even have? Make that the focus of their military strategy. If not, then maybe tone down your treemen. In real life, guerrilla tactics are next to impossible to stop without winning the hearts of the public so that the guerrilla fighters can no longer hide in them as well as acquire supply/soldiers through the civilian population. If the treemen can hide in the trees and not need to supply, then why would the humans even be fighting in the forest? If the treemen come out of the forest, then the humans can fight them in the open without guerrilla warfare. $\endgroup$ – A. C. A. C. Feb 26 '18 at 17:15
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    $\begingroup$ editing the question to (partially) invalidate some of the answer is not really appreciated. $\endgroup$ – L.Dutch - Reinstate Monica Feb 26 '18 at 17:20
  • $\begingroup$ @L.Dutch It's not really editing the question as much as clarifying it. $\endgroup$ – API-Beast Feb 26 '18 at 17:35
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    $\begingroup$ If the edit invalidates answers, then the question was written very poorly to begin with. $\endgroup$ – Aify Feb 26 '18 at 18:55
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    $\begingroup$ Instead of heads on pikes, place chairs and bookshelves and wardrobes on the battle plain in front of your castle to strike terror into your foes' hearts. $\endgroup$ – workerjoe Sep 11 '18 at 15:26
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Thankfully, humans are great at destroying forests so we can look at some real life examples for inspiration:

Fighting the plants themselves: In this world, a soldier is basically a lumberjack, and lumberjacks are pretty good at taking down trees. Although the trees in this world are more mobile than in real life, the principles are the same: axes and chopping.

The soldiers would need to focus on either repeated hits to the plants' thick woody sections to remove chunks and create areas that the plant might stress and snap on its own, or try to take off thinner 'limbs' in a single hard chop. Chopping at grassy, viney masses will likely just get the soldiers' axe stuck.

Fighting the plant's environment: Humans have been eroding ecosystems as a side effect of their activities for millennia. Now they have a reason to do so! Since these plants need to recharge in their home environment for a bit, if the humans can figure out where the plants live they can start destroying it from afar. This question goes into some options for killing vegetation:

  • Diverting rivers away from the area kill/dry out the plants. This has the added benefit of making fire an option!
  • Diverting rivers into the area to 'drown' the plants and erode the soil.
  • 'Salt the Earth', either with actual salt from some sort of seawater aqueduct, or various other chemicals that the plants can't use for nutrients.
  • Invasive species, such as termites, carpenters ants, woodpeckers if you also want to annoy the plants. Even non-native(and non-sentient) plants that can spread easily in the environment and starve the enemy of resources will work well.
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Burn and Salt the Earth.

During a dry period the human army Draws them a few kilometres away from their forest and get into a fight. A secondary force mostly of conscripts will enter the forest and set fire to it as others shovel salt out onto the ground.

When the trees realize what's happening it will be too late, they can't root in the ground as the flames, hot ash or salt will hurt and even kill them, so they can't rest and they have two tired but still very capable armies to deal with. It might not be a massacre but it won't end well for the trees.

If they can't draw the trees out, send the army in to attract attention and have the second group burn and salt the Earth in a nearby part of the forest. Rinse and repeat as needed.

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Look at the Lord of the Ring. How did they attacked the Ent?

Fire is always a good weapon against plants: shower your foes with fire arrows from a distance, cut their way back setting fire to the ground.

If they are hiding in the woods, set the woods on fire.

Also, modify ballistae so that you can launch rotating bladed discs to cut short some of their branches.

Patrol the path they have to take to refuel their mana, and again use fire on them. If they are short of mana they have less fighting spirit.

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    $\begingroup$ You can burn dry wood easily, yes, but living and very likely humid wood is a entirely different matter. You could have some sort of Greek fire, but the effect of that would be mostly localized. Also of course the collective isn't stupid and would develop some kind of counter measure against more naive attempts to burn down their territory. $\endgroup$ – API-Beast Feb 26 '18 at 16:42
  • $\begingroup$ @API-Beast, mind that relative primitive farmers nowadays still burn down living forests to make space for their crops. $\endgroup$ – L.Dutch - Reinstate Monica Feb 26 '18 at 17:05
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    $\begingroup$ Our modern forests don't have armies fighting to protect them though. $\endgroup$ – API-Beast Feb 26 '18 at 17:08
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    $\begingroup$ @L.Dutch: If the forest could move out of the way, it would be a lot harder to spread a forest fire. $\endgroup$ – Giter Feb 26 '18 at 17:12
  • $\begingroup$ @L.Dutch But you have to constantly add fuel to burn the forest, which would be suicide, since they could shoot at you. And if these things live in a forest, they will have some sort of defence set up there. $\endgroup$ – Nuloen The Seeker Feb 26 '18 at 17:17
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Create a Trojan Termite!

That is, an enormous metal horse with a thin layer of wood on the outside. Fill it with starving termites and then roll it down the hill to meet the enemy.

As the treants surround the odd contraption in an attempt to figure out what it is and if it is dangerous, the termites chew through the thin veneer and pour out onto them.

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There are a couple of approaches.

First you don't want the trees growing too close to the city/castle, so making the land difficult to root in key. Put rocks and salt in a nice wide ring around an area.

Second, if certain forests are suspected of being especially dangerous use a parasite to weaken them, something like a pine beetle should do. Although that could make high quality wood harder to acquire so probably keep that as last resort.

In a pinch though, burning tar or oil would be the way go as it's nice and sticky so can burn the three for a while, if nothing else it'll force them to disperse or risk forest fire

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There were always good weapons against plants - a decent fire. Early flamethrower prototypes like greek fire, makeshift napalm-like liquids, explosions - all af these shall do the trick. Something like Agent Orange to force trees to drop their foliage, the only problem is that the first chemical herbicide was invented in the 20th century - however, consistent plant attacks could push your nation's science forward. You can also use acid to destroy land fertility or grown trees. It is said that formic acid is the natural herbicide. By the way, melee combat is highly useless as the living wood can not be efficiently damaged with medieval weapons. So you should consider range combat with various chemicals.

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  • $\begingroup$ Hm, about melee, the plants of course would play to their strength and try to close the distance, if the humans only focused on ranged or artillery attacks they would need some defense against melee charges. $\endgroup$ – API-Beast Feb 26 '18 at 17:14
  • $\begingroup$ Ditches and trenches usually slow down enemy attacks. You can also set a wall of fire to slightly decrease the enemy numbers. Anyway living plants are tough against such undeveloped civilization. $\endgroup$ – nameless noob Feb 26 '18 at 17:23
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Triffid gun!

http://www.imfdb.org/wiki/Day_of_the_Triffids_(1981),_The#Triffid_Gun

day of the triffids!

You can't discuss anti-plant weaponry without a shoutout for the Triffid Gun. In the book, this thing shot radial saw blades like Frisbees.

Haven't read Day of the Triffids? Get you to the library! You are in for a treat!

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  • $\begingroup$ I'm not sure they could have made the triffid gun using "Late medieval metallurgy". $\endgroup$ – RonJohn Sep 11 '18 at 13:29
  • $\begingroup$ @RonJohn - really the triffid gun just shoots a sharp discus. The ancient greeks could have made those. Actually a field-event style discus would work better against triffids than a radial saw blade because a discus has more mass behind the edge. Just paint the unsharpened place where you are supposed to grab and do it by hand. That would be awesome for an anime - the shirtless discobolous whirling and hurling as his green enemies fall around him. $\endgroup$ – Willk Sep 11 '18 at 13:49
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Burn them!

You mentioned that they had bark, etc.. Bark is flammable, and so is wood (and other tree material). If you set them on fire, they will rapidly burn, and any attempt made by the trees to attack the humans can be turned against them by luring them towards other trees which will also catch fire.

An unpleasant side effect of this is that everything is now on fire.

Eat them alive!

Termites are notorious for being very destructive to wood. You can "breed" termites and then release them into the wild. There are also a multitude of other diseases and insects which destroy trees; here is an article which describes many of these.

One issue with this is that every tree will get infected as it's very hard to contain pests, diseases more so.

Do what humans do best!

If these are trees, they can be cut down, and that is what we will do. Presumably, these trees are made of wood; if so, we can cut them down when most vulnerable and use them for structures, maybe even for tree-destructing structures. This benefits your society and also gets rid of trees.

Humans are also very good at ruining the environment. The anti-tree people can try and cause massive damage to the environment (e.g. desertification, diverting/polluting rivers, salting the soil, etc.) preventing trees from growing.

Preventing tree attacks

If these trees are remotely similar to real trees, then they cannot take root in certain conditions. Most trees cannot stay in very sandy or rocky ground, and cannot survive in salt water. These can all be utilized to prevent the trees from attacking your fortress.

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    $\begingroup$ +1, but I disagree that having everything on fire would be unpleasant. I'm of the opinion that rather than labeling some people as pyromaniacs, we should label the rest of them all as pyrophobic. $\endgroup$ – Renan Feb 26 '18 at 21:04
  • $\begingroup$ Some trees have evolved countermeasures against fire, though...(cork, for instance) $\endgroup$ – Shalvenay Feb 27 '18 at 12:41
  • $\begingroup$ That's true, without more info about the structures these trees have there's not much we can say about this $\endgroup$ – Adrian Zhang Mar 1 '18 at 4:25

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