This could be approached in (at least) two ways; perhaps including both is better than just one or the other.
Firstly, the management plan is a good place to summarise what, in broad terms, is known about the site, which in turns helps to identify where the information gaps are – what don’t you know that you need to know in order to meet the site’s objectives? This could include anything from species inventories or population trends and water quality or other hydrological factors, to patterns of visitor access or public attitudes to the site.
Secondly, many protected areas have been the focus of environmental research for many years. It is useful to document the main projects, or at least subject areas, that have been undertaken, in particular any long term datasets that may require further survey and/or the need to keep records safe and available to those who might make use of them.