Environmental correlates of the conservation value of boreal headwater streams and their riparian forests

Headwater streams and their riparian forests are considered as biodiversity hotspots and key habitats for the conservation of aquatic and terrestrial biota [1, 2]. However, most previous research has focused on single organism groups and studies providing a more comprehensive assessment of the conservation status of both stream and riparian fauna and flora do not exist. We used data ranging from near-pristine to strongly modified boreal headwater streams and their riparian forests to assess which in situ and larger scale environmental factors are the best determinants of the overall conservation value of sites. We then evaluated validity of the selected variables in predicting the conservation value of 15 independent test sites. Specifically, we asked whether GIS-derived environmental variables can provide more (or equally) accurate predictions of the conservation value compared to more laborious in situ variables. Our data comprised species-level information on stream-dwelling bryophytes, diatoms and macroinvertebrates and riparian birds, vascular plants, bryophytes, polyphores and insects (nematocerans, trichopterans, and coleopterans). We calculated the overall conservation score (0-1) for each study site by averaging standardized i) total species richness, ii) rarity-weighted species richness (RWR) and Pielou’s evenness (J’) (calculated for each organism group separately) and total number of endangered taxa (summed across all 10 organism groups). Next, we used hierarchical partitioning to evaluate the independent contribution of 25 in situ environmental variables and 19 GIS-variables in determining the conservation status of the study sites. Among the in situ variables, the cover of coniferous trees, amount of decaying wood and forest shading contributed most (28%, 25%, and 11%, respectively) to the conservation score. Drainage intensity (20%), average tree diameter (18%; 100 m buffer) and a number of forest ditches draining into the stream (17%) were the best GIS predictors. Both in situ and GIS variables had, however, only moderate (23.7% and 24.6%, respectively) prediction success when the selected environmental variables were fitted to independent data. Our results therefore suggest that although it is possible to identify environmental variables strongly related to conservation value of a site, using this information for predictive purposes remains challenging. This might be due to fact that different biodiversity facets tend to show variable responses to environmental gradients, resulting in no net change in the overall conservation score regardless of alteration of site integrity. Nevertheless, our study underlines the importance of the protection of mature riparian forests and avoidance of peatland drainage to maintain headwater streams and their riparian surroundings as key habitats for biodiversity conservation. 1Marczak L.B. et al. 2010. Ecol Appl., 20, 126-134. 2Finn D.S. et al. 2011. J. N. Am. Benthol. Soc., 30, 963-980.