Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue.

One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book "Wind Energy - The Facts". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence.

Open land also makes physical access to the site easier. That's particularly important during construction, when very very long vehicles carry each blade (40-60 metres long) in one piece.

Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, decreases availability, decreases the capacity factor, and decreases the life expectancy of the turbine. So, higher costs, lower revenue.

One of the ways we measure the impact is the surface roughness coefficient $z_0$. Here are the figures from the book "Wind Energy - The Facts". As you can see, forest and woodland has a much higher $z_0$ than open farmland - and that means higher turbulence.

Open land also makes physical access to the site easier. That's particularly important during construction, when very very long vehicles carry each blade (40-60 metres long) in one piece.