"HI Nick, I have a property with native trees (ie. Hackberry) that hasn't been managed, and the hackberry have spread, casting a lot of shade. I want to add more diversity, so should I cut some of the smaller hackberries down, stump grind or girdle the trees, and add seed to the newly open spaces where the trees have been removed. How long do I have wait before seeding since there will be a lot of hackberry tree roots in the vicinity? Thanks!"
Dense stands Hackberry would not have been a natural condition in European pre-settlement times for most of the lands in the Midwest. Native American set fires would have kept fire sensitive trees like hackberry at bay balancing it out in the ecosystem allowing significant amounts of light to the ground. Without annual fires these fire sensitive native trees can tend to fill in quite densely, and this lack of light to the woodland floor is a systemic problem across much of the Midwest with both native and non-native trees. So, you are likely correct we need to first address the shade problem by conducting a selective thinning to open the canopy up a bit. These are often called "timber stand improvements" or TSI in forestry. You will likely need that additional lite to gain woodland wildflower diversity.
Without the use of herbicide, you have several ways to control the resprouts. However, my favorite is simply just cutting and removing the existing overstocked trees/sapling then weed whack the resprouts multiple times per year before they become woody. Click here to see this recent article I wrote up explaining this process in more detail, I describe this process for honeysuckle, but it can be used on virtually any unwanted woody species.
I would suggest not overextending yourself, know that you can stay on top of whatever you tackle. I have phased in this process in my yard. Committing to no herbicide use mean a little more elbow grease and possibly phasing the project over several years to ensure you have the capacity to stay on top of the resprouts and the establishment maintenance the native seed will need.
I would suggest seeding as soon as you have set the correct light level for any particular phase as weeds will also take advantage of the light just the same as native seed. Give you native seed the same opportunity as weeds, get that seed in there immediately.
Although stum grinding is effective at controlling stumps, I would not recommend stump grinding in most circumstances as the large machinery can compact the soil and cause unnecessary disturbance causing weeds to be more prolific than without. Plus it can be expensive.
If you do not have any oak trees in there, you may wish to plant a few oaks in the open areas to create a more natural oak community if you are here in the Midwest.
Creating a more light regime in the ecosystem allows for a more diverse woodland floor layer so we can support woodland pollinators like bumble bees.
I would be happy to chat a bit more about your particular problem, you can register for a free chat here so I can better understand your particular problem and give you a more tuned in solution.