Managed adaptation of upland streams using tree planting

Managed adaptation is a strategy to reduce the impacts of climate change on ecosystems. Of particular interest to WTT, the planting of riparian woodland is advocated widely to provide shade and hence reduce the warming of temperate streams, but of course there are other benefits as well such as increasing retention of water within the catchment (to reduce potential flooding) or increase bank stability (and hence reduce soil ingress / bank erosion). Leaf litter has also long been recognised as a vital energy source to many rivers and streams.

In a recent study of upland Welsh streams, a team of researchers from Cardiff University has investigated whether four types of riparian management, including those proposed to reduce potential climate change impacts, might also affect the community composition of macroinvertebrates, and how they feed and what they might functionally contribute within the wider food web.

Perhaps surprisingly, the type of riparian land use (native deciduous, non-native conifer, grassland pasture, or moorland) had only small effects on invertebrate taxonomic composition, and their diet appeared to comprise roughly 50:50 food derived from within the river and food derived from outside (i.e. leaf litter), irrespective of land management type.

Nevertheless, streams draining the most extensive deciduous woodland had the greatest stocks of coarse particulate matter derived from leaf litter and greater numbers of shredding detritivores. Moreover, the biomass of macroinvertebrates in deciduous woodland streams was around twice that in moorland streams, and lowest of all in streams draining non-native conifers. The full study is available here

These results suggest that planting deciduous riparian trees along temperate headwaters as an adaptation to climate change can also modify macroinvertebrate function, increase their biomass and potentially enhance resilience by increasing the energy input to the food web where cover is extensive (>60m riparian width).

So, all good news for trout then? Well apparently it’s not quite that simple as the same team have earlier published results suggesting that Welsh upland catchments with predominantly deciduous trees do not appear to harbour increased density and biomass of salmonids per se. However, there was a marked negative effect on these parameters in streams flowing through non-native conifers. Far better then to use native, and moreover, locally adapted trees when rehabilitating or managing the riparian strips of streams and rivers.