Pathways in old-field succession to white pine: Seed rain, shade, and climate effects

Trees slowly colonize old fields on sandy outwash in the prairie-forest ecotone of the north-central United States, and in the absence of fire, succession is expected to proceed toward oak woodland. We analyzed whether a case of unusually rapid and spatially extensive invasion by white pine (Pinus strobus) could be explained by the presence of specific temporal or spatial opportunity windows suitable for such invasion. We tested whether the invasion was temporally restricted to the period immediately after abandonment or to periods of favorable climate, and whether it was spatially restricted to areas of high seed rain or high forest-edge shade. White pine invasion into the field occurred in two waves separated from each other by a 1987-1989 drought period. The first wave (1980-1985) occurred during a period of average climate and led to the establishment of dense sapling patches in shade near forest edges. The second wave (1991-1994) occurred during a period of high precipitation and cooler than normal temperature, and resulted in colonization of the unshaded field center. In addition to the two temporal windows, white pine invasion occurred within two spatial windows: in areas highly sheltered by forest edge and in areas receiving high white pine seed rain. Overall these windows produced three different successional pathways: (1) a slow, creeping white pine invasion into highly shaded areas with low seed rain near forest edges; (2) a rapid, discrete-step invasion in areas where seed rain was abundant enough to overcome mortality in lower shade and where early arrivals facilitate filling in by later arrivals; and (3) a deferred invasion in the field center where low seed rain and lack of shade allowed the persistence of a grassland stage until favorable climate resulted in a white pine recruitment pulse. Temporal variation in climate can accelerate or decelerate any of the three successional pathways.