Macroinvertebrate-wood associations during decay of plantation pine in New Zealand pumice-bed streams: stable habitat or trophic subsidy?

Extensive areas of production pine forest in New Zealand haw been planted in the North American native Pinus radiata. We investigated the use of pine large woody debris (LWD) by aquatic invertebrates in central North island spring-fed streams with pumice beds to provide an improved basis for managing LWD inputs following logging. Invertebrate faunas in early summer were dominated by Ephemeroptera and Diptera on inorganic substrates, and by these groups and Trichoptera (predominantly Pycnocentria funerea; Conoesucidae) on wood. Densities of total invertebrates and P. funerea, Eukiefferiella sp. (Diptera), and the ephemeropterans Coloburiscus humeralis (Coloburiscidae), Zephlebia dentata, and Austroclima sepia (both Leptophlebiidae) were significantly higher on wood than on inorganic substrates in summer. These dominant species showed varying preferences for wood at different stages of decay. Austroclima sepia appeared to prefer wood at early to intermediate stages of decay P. funerea and Z. dentata preferred wood at intermediate to advanced stages of decay, and C. humeralis and Eukiefferiella preferred severely decayed wood. Pycnocentria funerea larvae excavated feeding grooves 1-2 mm deep along LWD, and gut analyses of larvae collected in summer confirmed ingestion of pine wood. This material dominated the gut contents of large larvae collected from wood at intermediate to advanced stages of decay. Stable isotope analyses of potential C sources and selected wood-dwelling invertebrates discriminated between pine wood and other types of allochthonous organic matter, and indicated that some larvae could derive substantial proportions of their nutrition from pine wood at certain times of year. in a laboratory experiment, P. funerea larvae produced significantly more fine particulate organic matter from wood at advanced stages of decay than from less-decayed wood or controls (PVC tubes) over 8 and 26 d. However, growth rates did not differ significantly between wood-decay treatments. Our findings 1) indicate that wood in pumice-bed streams enhances habitat for lotic invertebrates, and 2) suggest invertebrate community succession as wood passes through different stages of decay. Some predominantly xylophagous species, such as P. funerea, appear to be responding partly to enhanced food resources, indicating that inputs of pine woody debris can provide a trophic subsidy to pumice-bed streams in production forest environments.