Hemispherical digital photographs offer advantages over conventional methods for quantifying pathogen-mediated changes caused by infestation ofPhytophthora cinnamomi

Abstract

Estimates of canopy closure using hemispherical digital images, canopy cover using a point-intercept method, ground cover, number of plant species and total plant species cover were used to quantify the impact of Phytophthora cinnamomi in disease centres in Banksia shrubland, Banksia woodland and Eucalyptus marginata forest biomes of the South-West Botanical Province of Western Australia. Estimates of the time required to capture and analyse data and correlations were used to contrast the five methods. For canopy closure or cover, hemispherical digital images were on average, seven times faster to capture and analyse than the point-intercept method. For both canopy closure and ground cover, mean values from three point samples were not significantly different to mean values from transects across the disease front. Differences in canopy closure and ground cover between biomes and disease obtained from point sampling showed the same trends as transects. Canopy closure differed significantly between biome, being greatest for E. marginata forest, least for Banksia shrubland and intermediate for Banksia woodland. Ground cover was greatest for Banksia shrubland, least for E. marginata forest and intermediate for Banksia woodland. For all biomes there was significantly less canopy closure in diseased than adjoining healthy areas. There was no significant difference in ground cover between diseased and adjoining healthy areas of Banksia shrubland, but significantly less ground cover in diseased than adjoining healthy areas in the Banksia woodland and E. marginata forest. Canopy closure and ground cover are likely measuring different aspects of the impact of P. cinnamomi and, therefore, both should be measured when determining pathogen-mediated changes. Advantages of using hemispherical digital images over the other methods for determining temporal and spatial changes associated with P. cinnamomi infestation are discussed.