Spatial distribution of Penstemon grandiflorus (Nutt) and Geomys bursarius in a fragmented oak woodland in Minnesota, USA

We tested a method to evaluate the likely vulnerability of a species to the fragmentation of open area that occurs in oak savannas and woodlands in the absence of fire. This method is based on detailed analyses of the current spatial distribution of the species and does not depend on data obtained from long-term monitoring. We used this method to describe and analyze the spatial distribution of an herbaceous perennial, Penstemon grandiflorus (Nutt.) and pocket gophers, Geomys bursarius, in an oak savanna and woodland in east-central Minnesota (USA) that had not been burned for more than 30 years. In the absence of fire, woody canopy encroached into the open area of the savanna at an average rate of 7 cm year(-1) during this period. This increase in woody canopy resulted in a 50% reduction of open area and transformed the savanna landscape into a woodland containing 62 openings of different sizes. Analysis of the current spatial distributions of P. grandiflorus and G. bursarius showed that both species were primarily restricted to open areas and that the abundance of both declined rapidly in the woody edges surrounding openings. Occupancy rates for both species were significantly lower in small openings than in large openings. Both species were absent from most openings less than 100 m(2), while G. bursarius always occupied openings greater than 328 m(2) and P. grandiflorus always occupied openings greater than 670 m(2). The percent area that was occupied in openings was also positively correlated with opening size for both species. In the case of G. bursarius, a decline in opening size of an order of magnitude was associated with a 50% decline in the relative area occupied in openings. In the case of P. grandiflorus, an order of magnitude decline in opening size was associated with a decline of relative area occupied of nearly 60%. These results indicate that both species are likely to be vulnerable to further decline and fragmentation of open area in the study site.