CONCLUSIONES

reaches was in advanced decay states. Similar decay states in L WD in both vegetation types may reflect recruitment from a previous rainforest vegetation in reaches now bordered by sclerophyll vegetation. Generally sclerophyll forests were lower in catchments surveyed than rainforest and many sclerophyll reaches were observed to have been subject to logging previously. Harmon et al. ( 1 986)suggest that species composition, age and condition of riparian vegetation ultimately determine the type and volume of wood that enters rivers and many authors agree that the higher density of older larger trees (such as those found in undisturbed forests) will contribute greater wood volume to rivers compared to logged areas or younger forests (Bryant, 1986; Murphy & Koski, 1 989; Nakamura & Swanson, 1 993). These researchers suggest that lower standing stocks ofL WD surveyed in logged and early successional forests reflect the greater mobility of smaller L WD from younger vegetation delivered to reaches and the reduction of input from natural tree mortality. In addition, logged forests undergo a large reduction in overall recruitment ofL WD to adjacent reaches relative to old growth forests (Bryant, 1985; Murphy & Koski, 1 989; McHenry et al., 1998) and many researchers suggest that rapid timber rotations will eliminate L WD unless adequate buffer

strips are left along streams (Murphy et al. , 1986; Andrus et al. , 1988). It is likely that these factors have also contributed to lower L WD standing stocks measured in native sclerophyll vegetation than temperate rainforest in this study.

The vast majority ofL WD of native origin surveyed was in an advanced stage of decay, regardless of riparian type. This indicated that the majority of wood had been in the channel fer a long time and in reaches bordered by vegetation other than rainforest was probably the legacy from the previous vegetation. Uniformity in decay also suggests that wood in each reach was probably delivered at the same time as a result of disturbance, probably episodic in nature. Mechanisms ofL \VD input to streams can be segregated into chronic and episodic

delivery processes (Bisson et al., 1 987; Berg et al., 1 998). Chronic mechanisms include the

regular introduction of wood as a result of natural tree mortality or gradual bank undercutting. These processes tend to add small amounts of wood at frequent intervals. In contrast, episodic inputs result from catastrophic windthrow, fire or severe floods. These events occur

infrequently but can add large amounts of wood to the channel (Fetherston et al., 1 995). The

uniformity in advanced decay classes among reaches and the small amounts of earlier decay classes among native L WD suggests that episodic events are likely to have been the m�or

I

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i

delivery mechanism ofLWD into the reaches surveyed although as lower amounts of wood in

early decay stages were surveyed, there is evidence that ongoing recruitment of small amounts ofL WD occurs on a frequent basis.

l'

In many willowed reaches surveyed, willow L WD was absent and the small amount of willow

!

;_

L WD surveyed was in a comparatively early stage of decay. No L WD was recorded at 65% of sites visited. At the majority of these sites the riparian overstorey had been removed and the vegetation consisted of grass. In addition, grassed sites devoid of woody species had the lowest L WD standing stocks of all the vegetation types surveyed.

f

The low standing stocks of willow L WD surveyed in willow-lined reaches were not

�J

surprising. Willows are deciduous and in the northern hemisphere deciduous forests are

1:

thought to produce less L WD than coniferous forest (Hannon et al. ) 1986) and willow wood i _{l· has been shown to have faster decay rates than Australian native tree species (Lloyd}

et al. ) 1991). Triska (1984) documented historical accounts of small willow debris occupying

interstices between larger debris of other species. These findings suggest that willow species

may deliver smaller wood to rivers instead of larger limbs. In this study, large amounts of small willow woody debris (less than 1 0 em diameter) were commonly observed in densely

I!

willow-lined reaches. These were also aggregated against larger wood of native origin or

:\

;

living willow trees growing in the channel. Large dead willow debris was rarely encountered

/.;

in this study and, instead, partially severed living willow limbs were commonly observed with

I.

vertical shoots. I suspect that willow L WD contributions to Tasmanian rivers are relatively

':; small as larger l�mbs rarely fonned independent units within the stream channel.

; Both willowed and grassed reaches may also have lower or negligible standing stocks of

. L WD as a consequence of "desnagging" for flood mitigation purposes. Both riparian types

:�! were common in lowland areas developed for agriculture where removal of L WD is common

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· 1

practice. In addition, grassed reaches have no local delivery of woody debris into the channel

1

from the lack of adjacent vegetation. The combination of this with human removal ofL WD

In summary, disturbed reaches have lower debris volumes. In the majority, most of the wood volumes surveyed were remnant L \VD in an advanced stage of decay from previous forest stands. In lowland areas particularly in willow-lined and grassed reaches, L \VD delivery appears to be low or negligible. In the long term, many existing L \VD standing stocks in

surveyed reaches will diminish as no recruitment from disturbed or cleared riparian vegetation will take place. Given the importance of L \VD as fish and macroinvertebrate habitat,

particularly in lowland rivers, the ecological implications of reduced L \VD standing stocks is a loss of key habitat for aquatic fauna in many lowland rivers (Wallace & Benke, 1 984; Benke

et al., 1985; O'Connor, 1 992).

Relationships between invertebrates and LWD

L \VD supported a dissimilar fauna from benthic habitats in all comparisons. Benthic habitats were characterised by a suite of taxa and a diversity of feeding modes. Generally benthic taxa

in native reaches were those commonly associated with faster flowing high quality water (e.g.

Ulmerchorema rubiconum (Hydrobiosidae), Eusthenia spectabilis (Eusthemiidae), Sclerocyphon secretus (Psepheniidae), and Agapetus spp.(Glossomatidae)) while taxa in benthic habitats in grassed and willowed reaches were those more tolerant of silty slower flowing conditions. The fauna associated with L \VD had a distinctive xylophagous component particularly in native reaches. This fauna was commonly made up of elmid species ( Notriolus

tasmanica, Simsonia tasmanica ) and some mayflies ( Garinjuga sp. AVl and Atalophlebia

spp.) that may b� facultative in their feeding mode but have been known to include wood in their diets in a previous study (Chessman, 1 986). Other xylophagous elmids found on L \VD

were Simsonia leai, Coxlemis novemnotata, and Notriolus quadriplagiatus although these

'i taxa were not dominant in the elmid fauna. S. leai was found in both benthic and wood

habitats and C. novemnotata was only found in the grassed reach of the Liffey River. Not all elmid taxa sampled were xylophagous as both adult and larval forms of the scraper

Austrolimnius spp. were more common in benthic habitats. Some taxa characteristic of L \VD

! in this study appeared to have a superficial association with L \VD using wood surfaces for

other purposes than food. Philoreithridae ( Kosrheithrus remulus, Philoreithrus sp. AV2 and

·· Austreithrus spp.) were commonly associated with wood in this study and were most likely

using stable wood substrates as egg deposition sites as suggested in other studies (Anderson,

1982; Tank & Winterbourn, 1 995). Other common taxa on L WD may alternatively be using wood surfaces for pupation (trichopteran pupae), attachment (Alloecella grisea) or simply as resting positions or prey ambush sites (Eusthenia spectabilis and Nousia spp.). Many of the taxa associated with L WD occurred exclusively on wood habitats or were rarely found in benthic habitats in this study. The distinctive L WD fauna and the high degree of dissimilarity between wood-dwelling and benthic fauna found in this study supports claims by many researchers that L WD is an important, distinctive and sometimes preferred habitat for many taxa (Anderson et a/. , 1 984; O'Connor, 1991).

Wood has often been found to support higher secondary production ofmacroinvertebrates than benthic substrates (Wallace & Benke, 1 984; Benke et a/., 1 98 5; Smock et a/. , 1 985). In Australia, O'Connor (1 992) found a significant proportion of total macroinvertebrate densities and total macroinvertebrate biomass on woody debris and Lloyd et a/. (1991) found woody debris supported a distinctive fauna compared with muddy substrates and water plants. Most researchers attribute these trends to the greater stability and habitat complexity of wood in rivers with finer, less stable benthic substrates; thus wood offers more sites for attachment,

cover and food gathering activities (Benke et a/., 1 984; Harmon et a/., 1 986; O'Connor, 1 99 1 ;

Hax & Golladay, 1 998). In this study, although no quantitative comparisons were made

between benthic and wood habitats in relation to substrate preferences, L WD in the grassed reach in the Liffey River and the native reach in the North Esk river had significantly higher densities of macroinvertebrates on native L WD than on native wood sampled in other riparian reaches in each river. Both these reaches had finer substrates of gravels and sands than the larger substrate grades in other reaches. This may suggest that a higher number of animals show a preference for woody substrates over the least stable benthic substrates surveyed in this study, although it is likely that macroinvertebrate preferences are also related to L WD wood type and influences of differing riparian vegetation on surfaces of wood substrates.

· Many researchers agree that the selection of wood as a substrate and its use by stream

'! invertebrates appear to be based on wood quality (amount of microbial conditioning and/or softness), texture, and wood species (Dudley & Anderson, 1 982; Anderson et al. , 1 984). Few '! attempts have been made quantitatively to study the use of specific wood types by

macroinvertebrates, although Kaufman & King (198 7), McKie & Cranston (1 998)and Dudley

& Anderson (1987) all found that many macroinvertebrates generally prefer hardwoods over

softer, pulpy wood.

�·facroinvertebrate populations in willowed reaches preferred native L \VD to willow L \VD. In willowcd reaches, macroinvertebrate density, taxon number and the densities of filterers, collectors and predators was higher on native wood than willow wood in both rivers, although macroinvertebrate community composition was similar on each wood type. There were no differences between the punkiness of native and willow wood types sampled. Native wood was more decayed than willow wood substrates and had more complex surface texture than willow L WD, which was firm, lacked pits and grooves and was comparatively smooth in texture. These differences indicate that the higher surface complexity of native debris

represents a potentially more heterogeneous habitat than the smooth finn substrate of willow wood and surface complexity may be an important determinant of macro invertebrate use by increasing niches and refuge and correspondingly, the numbers of invertebrates and taxa that

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use complex wood.

With the notable exception ofMagoulick (1998), most researchers have found increased densities and number of taxa on structurally more complex wood surfaces, and they suggest that surface effects influence the numbers of aquatic animals that can be supported by that substrate (Anderson et al., 1984; Dudley & Anderson, 1987; O'Connor, 1991; McKie &

Cranston, 1998). Magoulick (1998) investigated willow wood colonisation by invertebrates

and found differe�ces in wood hardness and condition were important in structuring stream macro invertebrate conditions rather than wood texture. However he used willow wood which was considerably harder than native L WD sampled in this study and therefore likely to have

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