Use DropDownList1.SelectedItem.Text to get the selected value.
Try like this:
if( DropDownList1.SelectedItem.Text =="Select")
{
DropDownList1.Items.Clear();
DropDownList1.Items.Insert(00, "Null");
DropDownList1.SelectedIndex = 0;
}
else
{
}
or
if (ddltrees.SelectedItem.Text == "Select")
{
}
else
{
}