[R-sig-ME] Changes of estimate sign when interaction effects are highly significant

[MORAN LOPEZ, TERESA]

Hi everyone,
I am trying to evaluate the effects of WUE and drought on trees  
growth. I am running a mixed model in which my dependent variable is  
growth and my fixed effects are WUE, drought and their interaction. My  
random variables treeID nested in year. When the interaction among the  
two variables is not included, the effects of WUE on growth is  
negative and the effects of drought negative which is very intuitive.  
However, the best model includes the interaction (which is highly  
significant), and then the sign of the main effects change, now both  
drought and WUE have a positive effect on growth! And the sign of the  
interaction is negative...
Does anyone know what could be happening? Does this mean that when we  
have a very significant interaction between two fixed effects they  
cannot be interpreted separately?
Thanks