The reason why transplanted pine trees should not be watered immediately is primarily related to their root system and overall health. Here are the potential hazards of watering them right away:

Root Shock: Transplanting can cause stress to the tree, and immediate watering may exacerbate root shock. Allowing the roots to adjust to their new environment before adding water can help mitigate this stress.

Waterlogged Soil: Newly transplanted trees may not have established root systems to absorb excess water. Overwatering can lead to waterlogged soil, which can suffocate the roots and promote root rot.

Disruption of Soil Structure: Immediately watering can disrupt the soil structure around the roots, leading to compaction. This makes it harder for roots to grow and access nutrients.

Nutrient Leaching: Excessive watering can wash away essential nutrients from the soil, depriving the tree of what it needs for healthy growth.

Pest Attraction: Too much moisture can attract pests and diseases that thrive in damp conditions, potentially harming the tree.

To promote healthy establishment, it is often recommended to wait a few days before watering a transplanted tree, allowing the roots to settle and adapt.