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 t o the fragmentation of open area that occurs in oak savannas and woodl ands in the absence of fire. This method is based on detailed analyses of the current spatial distribution of the species and does not depen d on data obtained from long-term monitoring. We used this method to d escribe and analyze the spatial distribution of an herbaceous perennia l, 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, wo ody 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 canop y 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 ed ges surrounding openings. Occupancy rates for both species were signif icantly lower in small openings than in large openings. Both species w ere absent from most openings less than 100 m(2), while G. bursarius a lways occupied openings greater than 328 m(2) and P. grandiflorus alwa ys 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 si ze of an order of magnitude was associated with a 50% decline in the r elative area occupied in openings. In the case of P. grandiflorus, an order of magnitude decline in opening size was associated with a decli ne of relative area occupied of nearly 60%. These results indicate tha t both species are likely to be vulnerable to further decline and frag mentation of open area in the study site.