Question 193126
again  given  the  statement

IF  a  tree  has  green  leaves,,,   THEN    it  produces  oxygen

The  tree  DOES  have  green  leaves  ,,,  then  it  does  follow  that  it  produces  oxygen.

HOWEVER,  the  initial  statement  cannot  be  just  an  observation,  it  must  define  causality.

For  instance,  the  tree  has  green  leaves,  it  is  in  my  front  yard,  it  provides  shade,  the  birds  like  it,  AND  it  produces  oxygen.   No  causality  is  stated  therefore,  we  do  not  know  if  any of  these  conditions  lead  to  it  producing  oxygen.  

 Your  problem  appears  to  be  an  observation  rather  than  a  conditional  statement,  therefore  we  have  no  proof  it  is  true.