The changing relation of landscape patterns and jack pine budworm populations during an outbreak

The influence of landscape patterns on ecological processes is generally ac knowledged, but often difficult to quantify. The objective of our study was to quantify the relation of jack pine budworm (Choristoneura pinus pinus) population levels to both the abundance of jack pine (Pinus banksiana) and of jack pine stand edges in the landscape. The 450000 ha Pine Barrens regio n, located in northwestern Wisconsin, USA, experienced a severe jack pine b udworm outbreak from 1990 to 1995. We calculated landscape indices on a lan dcover map derived from Landsat TM satellite imagery. Landscape indices wer e calculated on circular buffers (0.5; 0.75, 1, 1.5, and 2.5 km radius) cen tered on 143 budworm population sampling points for which annual budworm co unts were available. Edge density was normalized for the proportion of jack pine in the landscape using random maps as a standard. Correlations betwee n landscape patterns and budworm populations varied over time: proportion o f jack pine showed strongest positive correlation with budworm population l evels up to the peak of the outbreak (1993). Edge density exhibited positiv e correlation up to the peak of the outbreak, but negative correlation in t he subsequent years as the outbreak declined. This may suggest that pollen- bearing male cones. which are more abundant along edges, support higher bud worm populations in the initial phase of the outbreak, but stronger predati on on budworm along edges subsequently reduces populations. We provide insi ght into previously inconclusive results on the relation of jack pine budwo rm population density to jack pine stand edges. The effects of landscape pa tterns, such as edge density, may vary not only in magnitude, but also in d irection, being positive and negative during different phases of an insect outbreak. Therefore, caution should be taken in relating landscape patterns to process at either a single scale or point in time.