Have them encounter a Pumice Desert

The type of dirt can change the overall environment drastically regardless of weather.

I was up in Crater Lake a few years ago and as we drove from the still snow covered Alpine we suddenly found ourselves in a vast Pumice Desert. The otherwise forested region came to an abrupt halt where the soil turned to sand for miles. Nothing can really grow there, as the entire valley had been filled thousands of years ago with pumice rock up to 300 feet deep.

If you are looking for the ABSOLUTE SHORTEST way to change a forested biome into a desert, go with a drastic change in soil.

Have them encounter a Pumice Desert

The type of dirt can change the overall environment drastically regardless of weather.

I was up in Crater Lake a few years ago and as we drove from the still snow covered Alpine we suddenly found ourselves in a vast Pumice Desert. The otherwise forested region came to an abrupt halt where the soil turned to sand for miles. Nothing can really grow there, as the entire valley had been filled thousands of years ago with pumice rock up to 300 feet deep.

If you are looking for the ABSOLUTE SHORTEST way to change a forested biome into a desert, go with a drastic change in soil. And if you want to keep it warmer than the immediate area, change its rock color to something more like the Shasta Lava Beds. And to further increase the temperature there year-round, lower the elevation of this desert to below sea level.