It is not necessary in all forests but in some dense forests this happens because there are natural magnets or magnetic rocks that easily attract the needle of the compass towards it. This acts as a shield to the earth’s magnetic field. That means, earth’s magnetic field will not be able to influence the magnetic compass correctly within the forest. 

Why the Magnetic Compass is not Working in the Middle of the Forest?

There are several factors that can affect the accuracy and functionality of a magnetic compass in the middle of a forest. Let's explore these factors in detail:

Magnetic Interference:
In a dense forest, there are abundant sources of magnetic interference that can disrupt the accuracy of a compass. These sources include:

1. Trees: The high iron content in tree trunks can interfere with the Earth's magnetic field, leading to inaccuracies in compass readings.

2. Rocks and Minerals: Certain rocks and minerals found in the forest, such as magnetite, can generate their own magnetic fields, causing disturbances in the compass needle.

3. Metal Objects: Man-made metal objects like fences, tools, or abandoned structures can create localized magnetic fields that can interfere with the compass reading.

Declination:
Declination refers to the angular difference between true north (the North Pole) and magnetic north (the direction a compass needle points). In different locations around the world, the declination can vary. If you are in a forest where the declination is significant, the compass readings may be inaccurate unless you adjust for the declination.

Limited Visibility:
In a dense forest, the thick canopy of trees can obstruct the visibility of the sky. A magnetic compass relies on aligning with the Earth's magnetic field, which is influenced by the North and South poles. Without clear visibility of the sky, it becomes challenging to establish an accurate reference point for the compass.

User Error:
Sometimes, the compass may not work in the forest due to user error. It is possible that the compass is not held correctly, or the user may not be familiar with the proper use of a compass. In such cases, it is crucial to have a basic understanding of compass usage and orientation techniques.

Conclusion:
In the middle of a forest, the functionality of a magnetic compass can be compromised due to various factors, including magnetic interference from trees, rocks, and metal objects, declination, limited visibility of the sky, and user error. It is important to be aware of these limitations and consider alternative navigation methods or use additional tools like GPS devices or topographic maps to ensure accurate direction finding in forested environments.