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

Extensive areas of production pine forest in New Zealand haw been planted i n the North American native Pinus radiata. We investigated the use of pine large woody debris (LWD) by aquatic invertebrates in central North island s pring-fed streams with pumice beds to provide an improved basis for managin g LWD inputs following logging. Invertebrate faunas in early summer were do minated by Ephemeroptera and Diptera on inorganic substrates, and by these groups and Trichoptera (predominantly Pycnocentria funerea; Conoesucidae) o n 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 sign ificantly higher on wood than on inorganic substrates in summer. These domi nant species showed varying preferences for wood at different stages of dec ay. Austroclima sepia appeared to prefer wood at early to intermediate stag es of decay P. funerea and Z. dentata preferred wood at intermediate to adv anced stages of decay, and C. humeralis and Eukiefferiella preferred severe ly 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. Sta ble isotope analyses of potential C sources and selected wood-dwelling inve rtebrates discriminated between pine wood and other types of allochthonous organic matter, and indicated that some larvae could derive substantial pro portions of their nutrition from pine wood at certain times of year. in a l aboratory experiment, P. funerea larvae produced significantly more fine pa rticulate organic matter from wood at advanced stages of decay than from le ss-decayed wood or controls (PVC tubes) over 8 and 26 d. However, growth ra tes did not differ significantly between wood-decay treatments. Our finding s 1) indicate that wood in pumice-bed streams enhances habitat for lotic in vertebrates, and 2) suggest invertebrate community succession as wood passe s through different stages of decay. Some predominantly xylophagous species , such as P. funerea, appear to be responding partly to enhanced food resou rces, indicating that inputs of pine woody debris can provide a trophic sub sidy to pumice-bed streams in production forest environments.