I think the interpretation of helicity as "the linkage and knottedness of vortex lines in the flow" is not that relevant in a turbulent flow.

If you want to know more about effects of helicity on 3D turbulence, I suggest you read papers by Biferale and coworkers (Physical Review Letter 2012 and Journal of Fluid Mechanics 2013), or more older papers by André et Lesieur (JFM 1977).

Basically, helicity cannot be created in a homogeneous turbulence if not present initially. So if you have helicity initially in your turbulent flow, the effects on the decay of energy are quite complex, but simply, helicity decays as well.

For forced turbulence, this is much more complex: depending on which scales you are forcing (large or small) you can have appearance of inverse cascades of kinetic energy. With complex forcing, rotation, you have others effects such as the flow becoming 2D. I am not an expert in forcing for helical flows, but I know the effects are multiple.

Also, in the work of Biferale and coworker, it is shown that specific interactions between helical modes of same sign (Decimated Navier Stokes equations) can generate inverse cascades as well.

There is so much to say about helicity ! Maybe you should start reading the few papers I suggested. Hope this helpful

There exists a competition between two effects:

(a) self-organization, e.g., formation of large cells and helical flows (spontaneous spin-up); and

(b) errosion of cells by vorticity filaments (enstrophy).

Forcing, on the one hand, promotes the self-organization vortical cells, and acts as sources of vorticity filaments (enstrophy) on the other. 

if the forcing is weaker, including negative forcing, (a)>(b); and

if the forcing is stronger, (a)