Were WW1 trenches always in a devastated landscape?

The question can be interpreted in one of two ways: 1) Were there advantages to digging trenches in "already" devastated landscape? or 2) Did the process of digging trenches by itself"devastate" the landscape. Of the two, the latter is more probable.

Just the act of digging trenches will "carve up" the landscape. Of course you might do this by plowing fields, building irrigation ditches, etc. But as we will see, building trenches is not only a different operation, but has a different fundamental purpose.

The purpose of trenches is to protect men who are fighting. In the "modern" era, this means "shooting." Specifically tons of bullets and (artillery) high explosive. The idea was that the men inside the trenches would be killed at a slower rate than men attacking from the outside across open field, particularly when barbed wire was installed in front of the trenches. These trenches were usually built for "imminent" attacks that occurred most of the time, not "hypothetical" attacks, at some distant point in the future.

There may have been some advantages in building trenches in "devastated" (broken) ground. But the fighting that accompanied trench building caused most of the devastation even if there were none previously.

Summary: Trenches were often dug in un-wrecked countryside, but attacks on, and defence of those trenches, created the familiar scenes of devastation.

The entrenched Western Front of WWI developed as a consequence of the Race to the Sea. This phase of the war lasted from mid-September to mid-October 1914, as the Allied and German armies repeatedly tried to go round each other's western flank, and were stopped every time. This only ended when the front line reached the English Chanel, with both armies spread out along the front line from there to the Swiss border.

With WWI technology, artillery was devastating to troops in the open, but almost ineffective against troops in trenches. So the troops on both sides had to dig in to hold their positions, and they had to hold their positions to prevent their armies being outflanked.

The traditional answer to this was cavalry, who could move faster than infantry, and thus outflank them. However, machine guns and barbed wire would stop cavalry very effectively, allowing a line to be held against both infantry and cavalry attacks with far fewer troops than would have been necessary in the nineteenth century. That meant that the armies of both side could actually defend a front line hundreds of miles in length.

The trench lines were originally dug through countryside that wasn't utterly devastated, although digging several lines of trenches made a fair mess. What wrecked the countryside was the artillery bombardment that had to be done to suppress defending troops before each attack, was done by the defending side during each attack to kill advancing troops, and had to be available and demonstrable everywhere along the line to prevent surprise attacks. The ever-present mud had two sources:

• The destruction of the drains and ditches that farmers normally use to control water on their arable land.
• In parts of Western Belgium, the surface layers are impermeable clay, which prevents water draining away. This land is only normally used for forestry, but trenches went through there. For more details, this paper by Peter Doyle & Matthew R. Bennett: "Military Geography: Terrain Evaluation and the British Western Front 1914–1918". The diagrams on p. 11 illustrate the geology systematically. This can produce effects much like Rasput**sa.

Most of the technique of trench warfare was developed by the French army during the winter of 1914-15, and has been universally practiced since then whenever the conditions force it on an army. Armies hate it, but it is sometimes unavoidable.

Poison gas attacks with the primitive gasses of WWI would theoretically have made the devastation worse, by killing all the plant life, but it's doubtful that this was particularly noticeable.