Willow Habitat Research Articles

Introduced willows can become invasive pests in Australia

Although willows (Salix spp.) are much appreciated for their various benefits, concern has grown over the past decade about their invasive natural spread in the water courses of southeastern Australia. The main environmental effects include obstruction and diversion of streams and hence erosion, extensive displacement of native vegetation with loss of biodiversity, and reductions in the quantity and quality of water. So far, only a few thousand kilometres of streams have been infested badly; that is less than 10% of potential willow habitat. Except for some of the S. cinerea infestations, it is still possible and worthwhile to control the willows in Australia. The following biological features help to explain the spread and provide a basis for control. 1. The original cause of spread has been importation and planting, usually as cuttings of just one or two clones at a time. 2. The bases of the small branches of most of the tree willows in Australia are easily broken. These willows thus tend to spread by broken branches taking root in wet areas downstream. The shrub willow species are far less fragile and thus unlikely to spread by broken branches. 3. Male and female flowers are usually on separate trees. Female trees will usually produce viable seed with pollen from a male of the same species or of any other species of its botanical group (i.e. either tree willows or shrub willows), provided that the trees occur within pollinating distance of each other (up to at least 1000 m) and flower at the same time (most do). 4. In Australia, most of the resulting seedlings are hybrids and are able to grow vigorously and to breed with each other and with both their parents. 5. Except for S. cinerea and S. purpurea, regeneration from seed is virtually restricted to more or less bare sediment that is kept wet for weeks or months from the time of seed shed (about October/November) because the seed lives for only 1-7 weeks when dry, and germinates in about one day when wet, and because the tiny seedling needs much light and has slow root growth. 6. Seed is easily carried by wind for more than 1 km, and some travels up to 50 or even 100 km. Transport of seed or live branches by streams also serves to spread willows, but is less effective, and only downstream. 7. Probably the main barriers to the survival of seedlings are lack of suitable seed bed, rising or rapidly falling water levels, and floods that uproot or bury the seedlings. 8. Conditions suitable for the establishment of large numbers of seedlings probably occur in most southeastern streams at perhaps 5 to 20 year intervals. Major disturbances, such as wildfire or the collapse of a swamp may also promote massive regeneration from seed. National and regional strategies for willow management are being developed and implemented. They range from prohibitions on the importation, sale, and planting of nearly all willows to the total eradication of the most aggressive species; S. nigra and S. cinerea. Regional strategies aim to eliminate naturally regenerated willows and to reduce their sources to manageable levels. That means, 1. Control fragile willows where their broken branches are very likely to take root, and 2. Keep seeding females to minimal numbers and at least 3000 m away from suitable seedbed, and (preferably) keep males at least 2,000 m from compatible females. In the coming centuries, alien willows are in a position to change much of the natural environment in Australia and elsewhere.

Influences of Herbivory and Water on Willow in Elk Winter Range

Elimination of large predators and reduced hunter harvest have led to concerns that an increasing ell (Cervus elaphus) population may be adversely affecting vegetation on the low-elevation elk winter range of Rock Mountain National Park, Colorado, USA. Beaver (Castor canadensis) and their impoundments also have declined dramatically (94%) in the same area over the past 50 years coincident with a 20% decline in willow (Salix spp.) cover. From 1994 to 1998, we studied vegetation production responses of willow communities to elk herbivory and water availability. We estimated willow production by measuring current annual growth of shrubs in 9.3-m 2 circu lar plots, and we measured herbaceous production by clipping vegetation within 0.25-m 2 circular plots. Elk her bivory suppressed willow heights, leader lengths, annual production, and herbaceous productivity of willow corn munities. Water impoundment had a positive effect on herbaceous plant production, but little effect on shrubs, possibly because water tables were naturally high on the study sites even without beaver dams. Nevertheless, the winter range environment previously included more riparian willow habitat because of more stream area (47-69%) due to larger beaver populations. Elk herbivory appears to be the dominant force determining vegetation pro ductivity in willow sites, but the effects may he exacerbated by lowered water tables. Fewer elk or protection from browsing, and water enhancement for <10 years along with management to encourage elk movement away from willow communities, could possibly work as strategies to reestablish sustainable willow communities.

Stopover Ecology and Habitat Use of Migratory Wilson's Warblers

The conservation of long-distance migratory songbirds is complicated by their life-history characteristics and the spatial scales that they traverse. Events during migratory stopovers may have significant consequences in determining the population status of migratory songbirds. Using Wilson's Warbler (Wilsonia pusilla) as a focal species, we investigated effects of age and sex on stopover parameters during spring and fall migration. Variation in stopover patterns was predominantly sex related in spring and age related in fall. In spring, males appeared at study sites earlier than females, and they carried higher fat stores and had lower recapture rates and higher rates of mass gain. In fall, migratory passage overlapped temporally among age and sex classes, but young birds had lower fat stores, higher recapture rates, and longer stopovers than adults. Habitat use differed between spring and fall migration and was affected by the combination of age and sex. Wilson's Warblers appeared to prefer native willow habitat during spring migration. Agriculture field/edge habitats may have represented "sink habitats" because birds in these habitats tended to be immatures and had a lower rate of fat deposition and longer stopovers. The data suggest that immatures were particularly vulnerable to habitat disturbances at stopover sites during fall migration, probably because of their lower social status and inexperience with long-distance migration. We conclude that knowledge of intraspecific variation in migration strategies is important for effective management of stopover habitats for migratory songbirds.

Possums (Trichosurus vulpecula) on Hawke's Bay farmland: Spatial distribution and population structure before and after a control operation

Over 6.5 years, 723 possums were live‐trapped and marked in swamp, willow, and farmland habitats on approx. 350 ha of two adjoining sheep farms in Hawke's Bay, New Zealand. About equal numbers of males and females were trapped in each habitat, but males were recaptured more often than females. Eighty‐five percent of the possums were trapped along the margins of swamp and willow habitats that comprised only 6% of the study area. The swamp habitat contained at least 8.8 possums ha‐1, the willow habitat 16.7 possums ha‐1, and the farmland habitat 0.13 possums ha‐1. Possums generally moved less than 200 m between successive recaptures, and there were few trap‐revealed movements between habitats. Possums on part of one farm comprising 23 ha of swamp and willows were then poisoned with cyanide in bait stations. Two hundred and fifty‐eight possums—including 142 previously marked animals—were found dead, an estimated 90% of the population. Fifty‐nine percent of the marked possums were poisoned within 100 m of their most recent trapping point, and 85% within 200 m. Proportionately more adult possums were killed on the first night's poisoning and more juveniles on subsequent nights. After the poisoning, the surviving possums showed very varied responses to the reduced population density, ranging from no change in location or range of movements to changes in location, range of movements, or both.

Use of Landsat TM Imagery in Determining Important Shorebird Habitat in the Outer Mackenzie Delta, Northwest Territories

Landsat Thematic Mapper (TM) imagery was examined to determine important habitats for shorebirds in the outer Mackenzie Delta, Northwest Territories. In June and July 1991 and 1992, 89 ground plots (200 X 200 m) in different habitats were censused for breeding shorebirds. Habitat type in ground plots was determined by observation and compared to the type identified at the site by an unsupervised Landsat classification technique. The most common species of shorebirds breeding in the area were red-necked phalaropes (Phalaropus lobatus) and common snipe (Gallinago gallinago), followed by semipalmated sandpipers (Calidris pusilla), stilt sandpipers (C. himantopus), pectoral sandpipers (C. melanotos), whimbrel (Numenius phaeopus), Hudsonian godwits (Limosa haemastica), lesser golden plovers (Pluvialis dominica), and semipalmated plovers (Charadrius semipalmatus). Long-billed dowitchers (Limnodromus scolopaceus) were rarely seen. Most species were concentrated in areas of low-centre polygons, sedge, and "low terrain" upland tundra (damp and tussocky). However, snipe were most common in dense willow habitat, and semipalmated plovers were found breeding only on sparsely vegetated gravel. Average density of breeding shorebirds in low-centre polygon or "pure" sedge habitat was 82 pairs per sq km in 1991 (SD=73.8), and 49 in 1992 (SD=49.5). Although the Landsat TM imagery analysis used here correctly identified habitat types near the original, intensively surveyed ("ground-truthed") area, it often misidentified habitats at some sites 10 to 30 km away, probably because of irregular flooding and subtle year-to-year differences in water levels in the active outer delta, and edge habitats too narrow to be distinguished by the satellite imagery. However, the technique can identify potential shorebird habitat roughly, and at least eliminate obviously unsuitable areas in large regions of the Arctic.

Willows in the environment

SynopsisThe ecological role of native willows is described in terms of the diverse structure of the species involved, the wide range of plant communities they form and the large numbers of invertebrates associated with them. The conservation importance of the genusSalixis discussed along with comments on the necessary management to retain willow habitats.

Temporal patterns of foraging activity in some wood warblers in relation to the availability of insect prey

I monitored the temporal pattern of diurnal feeding activity in several wood warbler (Parulidae) species and concomitantly recorded the numbers of active (flying) insects in 2 willow habitats in the western United States. At one site the temporal relationship between the density of active and inactive (nonflying) insects was investigated. The diurnal patterns of insect and bird activity were inversely related and each pattern was significantly nonuniform throughout the day; the wood warblers were largely inactive during the middle of the day when insects were most active.