I'm a member of another forum like this, only it is a specific business. Let's say, for example it's a Real Estate forum and they also publish an online weekly newsletter that contains 4-5 related to the business.  

They asked me to provide an interview-style bio, which I was paid for and it was published a few weeks ago. I have recently learned that the forum owner will be asking me if I would like to write some articles for his newsletter. There was no contract involved when I wrote the bio, and I don't believe there have been any contracts between him and any of his other contributors. I have almost a dozen ideas for articles that I would like to submit. However, once he publishes them and pays be (with no contract), who owns those articles? Can I submit them elsewhere or will he now own them? 

Any help is appreciated. 

I've been a professional writer for almost 25 years.  Every time you work for someone, make sure you go over payment and rights (along with the specs of what they need written).  Even if it's not in the contract, you need to ask and make sure it's in writing.  I've found that many times employers don't even understand what "rights" are or how they work, so you may even have to educate your employer at times.  Every place is different, with some wanting exclusive ownership, some needing it only one time, etc.  It just depends, and it can be a deal-breaker.  Personally, I rarely sell "all rights", as I make a lot of money on reprint sales.  Some people, who write highly technical material, don't worry about it as they know they won't ever re-sell that piece again.  So, it really depends, but you need to educate yourself and have policy in place.