SELECTING INDICATOR TRAITS FOR MONITORING LAND USE IMPACTS: BIRDS IN NORTHERN COASTAL BIRCH FORESTS

Coastal birch forests in northern Norway are currently being converted to nonnative spruce plantations. Since such land use changes may cause large alterations in the birch forest ecosystems, indicators suitable for detecting and monitoring impacts need to be pursued. In this study, we used a three-table ordination method (RLQ analysis) to identify species traits in bird assemblages indicating changes in environmental attributes at stand and landscape level. RLQ analysis has the advantage of linking species traits directly to environmental attributes, hence detecting the major ecological impacts of land use change, as well as a set of species traits sensitive to these alterations. Birds and environmental attributes were sampled in six forest types in 12 different landscapes. To assess whether species traits were relatively consistent geographically and temporally, two different geographic regions and years were also included in our study design. We found the species traits investigated here to be promising indicators of land use impacts. In particular, ground nesters, cavity nesters, flycatchers, and long-distance migrants were adversely affected by conditions in elderly spruce plantations. Also, species that forage on trunks and in deciduous foliage, as well as species that eat larvae and flying insects, were related to forests with a deciduous overstory. Despite the variety of species traits reflecting ecological changes at the stand level, only the species sensitive to the reduction in area of rich birch forests responded to fragmentation effects at the landscape level. These effects of land use changes appeared to be consistent in time (years) and space (geographic regions). We argue that knowledge about trait–environmental linkages is essential when selecting indicators for monitoring land use impacts. To best separate human effects from natural fluctuations, major environmental attributes that vary naturally in time and space should also be monitored in conjunction with the indicator species traits. Finally, trait–environmental linkages could form the basis of making predictions about changes in biological and functional diversity in response to land use changes. These predictions could subsequently be evaluated in monitoring programs.