Over a four-month summer period, we monitored how forest (Pinus sylvestris) and heather moorland (Calluna spp. and Erica spp.) vegetation canopies altered the volume and isotopic composition of net precipitation (NP) in a southern boreal landscape in northern Scotland. During that summer period, interception (I) losses were relatively high, and higher under forests compared to moorland (46% of gross rainfall (GR) compared with 35%, respectively). Throughfall (TF) volumes exhibited marked spatial variability in forests, depending upon local canopy density, but were more evenly distributed under heather moorland. In the forest stands, stemflow (StF) was a relatively small canopy flow path accounting for only 0.9-1.6% of NP and only substantial in larger events. Overall, the isotopic composition of NP was not markedly affected by canopy interactions; temporal variation of stable water isotopes in TF closely corresponded to that of GR with differences of TF-GR being -0.52 ‰ for δ2 H and - 0.14 ‰ for δ18O for forests and 0.29 ‰ for δ2 H and -0.04 ‰ for δ18O for heather moorland. These differences were close to, or within, analytical precision of isotope determination, though the greater differences under forest were statistically significant. Evidence for evaporative fractionation was generally restricted to low rainfall volumes in low intensity events, though at times subtle effects of liquid-vapour moisture exchange and/or selective transmission though canopies were evident. Fractionation and other effects were more evident in StF but only marked in smaller events. The study confirmed earlier work that increased forest cover in the Scottish Highlands will likely cause an increase in interception and green water fluxes at the expenses of blue water fluxes to streams. However, the low energy, humid environment means that isotopic changes during such interactions will only have a minor overall effect on the isotopic composition of NP.
- Forest hydrology
- boreal forest