Meserve hybrids are crosses between the English (I. aquifolium) and Japanese super-hardy holly (I. rugosa). They were obtained in the US in 1950's with the goal of creating a plant looking as English holly, though far more hardy. Various cultivars eventually made it to some natural areas, like the one in this case, though so far there aren't any documented cases of naturalization. This massive clump could originate from multiple young plants, among which there could be both male and female. When nurseries sell holly seedlings of unknown sex, they recommend planting a few together for this purpose.