The floodplain biota is affected by several aspects of stand structure and condition and by flooding. The structural characteristics of the forests, especially the much-modified “pole” versus “spreading tree” forms, and the removal of fallen timber have demonstrable effects on floodplain vertebrates [Mac Nally and Horrocks, 2007]. Avian assemblage structure is likely to change substantially because different birds species respond differently to stand structure, the availability of shrubs, etc. For example, the encroachment of invasive Tamarix spp. into Populus-Salix forests in the southwestern United States, associated with increases in groundwater depth due to pumping, led to declines in species richness and the number of unique species on the floodplain [Brand et al., 2008]. Vertebrates appear to respond to river red gum stand condition, but flooding per se is indirectly important, probably through increasing the availability of prey invertebrates [Mac Nally and Horrocks, 2008].
 Small mammals and birds in these floodplain forests prefer higher loads of fallen timber than are typically available in managed forests [Mac Nally et al., 2002b; Lada et al., 2007]. Although not yet teased apart experimentally, this response is likely to be related to increased cover, shelter from predation, and higher availability of food, especially of invertebrates [Mac Nally and Horrocks, 2007]. We manipulated woodloads in thirty 1 ha plots, and monitored them prior to and after the manipulations [Mac Nally, 2001]. Bird assemblages were much affected by fallen-timber loads, with clear differences in avian composition at sites with loads exceeding 40 t ha−1 compared with sites with lower timber loads [Mac Nally and Horrocks, 2007]. The densities of a near-threatened bird species, the brown treecreeper (Climacteris picumnus), were substantially higher in all treatments with timber loads ≥40 t ha−1 (Figure 6), which was possibly due to a higher availability of invertebrate prey.
Figure 6. Densities (mean ± SE) of the brown treecreeper (Climacteris picumnus) in a mesoscale experiment involving relocation of fallen timber among treatments [Mac Nally, 2006]. There were four replicate 1 ha plots for all treatments except the cleared treatment (n = 6), and data are for pre- (open bars) and postmanipulation (hatched bars) surveys. Pre- and postmanipulation means were different for all treatments apart from cleared 20 t ha−1 and the control.
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 Some vertebrate species appear to respond not only to fallen-timber loads but to flooding and the availability of large trees, which have a much higher probability of bearing hollows upon which the species rely [Horner et al., 2010]. The pole-oriented management in the eastern Murray River floodplains has produced stands that are largely bereft of tree hollows, with follow-on effects on hollow-dependent fauna [Horner et al., 2010]. Tree hollows develop from large, essentially horizontal branches [Vesk et al., 2008], which are deliberately removed by current forest management practices to improve sawlog production [NRC, 2009a].
 The most common small-mammal species on river red gum floodplains, the yellow-footed antechinus (Antechinus flavipes), a small marsupial carnivore (30–80 g), appears to respond to three factors (Table 5). Antechinuses were more likely to be trapped (a measure of abundance, [Lada et al., 2008a]) at locations with higher fallen-timber loads, around areas of recent flooding, and where there are more large, hollow-bearing trees (Table 5). Females did not produce young in sites with ≤20 t ha−1, and the most breeding was at sites with the highest fallen-timber loads (80 t ha−1) [Mac Nally and Horrocks, 2008]. The population dynamics of A. flavipes appear to be strongly related to the extent and duration of floods [Mac Nally and Horrocks, 2008]. Population surges in A. flavipes probably occur because of elevated availabilities of food. Flooding appears to cause irruptions of both abundances, and biomass of beetles that contribute to higher food availability for the ground-feeding, insectivorous antechinuses [Ballinger et al., 2005, 2010]. These increases in beetles are greater in areas that are inundated for longer periods. For example, the number of beetles was six times higher in extensively flooded sites compared with unflooded sites, while biomasses were almost 100-fold higher (Table 6). Females of A. flavipes have as many as 15 young in a season (mean = 10.4 ± 2.2 SD), which is consistent with a life history of opportunistic, explosive breeding, so populations are capable of responding rapidly to fluctuations in resource availability. Therefore, when floods generate high invertebrate flushes, antechinus survivorship is high and densities increase sharply.
Table 5. Dependence of Trapping Rates (Genders Combined) of the Yellow-Footed Antechinus on Site Characteristics on the Murray River Floodplaina
|Predictor Variable||Coefficient ± SDb||Change in Capture Odds (95% CI)c|
|Distance to floods (km)||−0.45 ± 0.19||−36% (−56, −8)|
|Fallen-timber load (t ha−1)||0.31 ± 0.07||+36% (19, 56)|
|Numbers of large trees (≥ 60 cm)||0.17 ± 0.07||+19% (3, 36)|
Table 6. Abundances and Biomasses of Beetles in Areas of No, Medium, and Extensive Flooding on the Barmah Floodplain in 2001 [Ballinger et al., 2005]a
|Variable, Treatment||N (Sites)||Mean (Range)|
|Abundanceb|| || |
| No flooding||7||8 (6–11)|
| Medium flooding||8||14 (11–17)|
| Extensive flooding||9||47 (38–57)|
|Biomassc|| || |
| No flooding||7||35 (19–63)|
| Medium flooding||8||75 (43–132)|
| Extensive flooding||9||2981 (1808–4915)|
 Birds respond directly to river red gum stand condition [sensu Cunningham et al., 2009b]. We looked at four measures of avian response: (1) the total number of species recorded over five visits; (2) the total records of all species over five visits; (3) the number of species showing any breeding activities; and (4) total breeding activity summed over all species. We used quantile regression for the median [Koenker, 2010] because of the non-normality of the data. After taking into account the influence of other covariates (e.g., numbers of tree hollows, fallen-timber loads), stand condition (scored on a scale of 0–15) had a positive effect for the four response variables. The estimated slopes (25% and 75% credible intervals) were: (1) total number of species recorded, 0.70 (0.15–1.31); (2) total records of all species (i.e., the numbers of individuals of all species seen over all visits), 4.00 (0.38–5.77); (3) number of species showing any breeding activities, 0.33 (0.20–0.63); and (4) total breeding activity summed over all species, 0.60 (0.10–2.46).
 The yellow-footed antechinus also responds to stand condition. We used data from extensive surveys of 268 sites in Barmah, Millewa, Gunbower, Koondrook, and Perricoota Forests [Lada et al., 2007, 2008a, 2008b, 2008c]. The captures per trap-night of females were positively associated with stand condition [quantile regression: 0.005, bounds = (0.001–0.014)], and even more strongly for the rare females that reach a second breeding season [quantile regression: 0.007, bounds = (0.006–0.010)]. Males, all of which die after breeding, were negatively associated with stand condition [quantile regression: –0.013, bounds = (–0.022, 0.0002)], which is consistent with their ejection from the home ranges of territorial females and their extensive searching for mates during the short breeding season [Lada et al., 2007].
6.2. Floodplain Understorey Plants
 The drying climate is likely to exacerbate forest dieback and lead to landscape-scale changes in the population viability of trees and forest structure. The processes responsible for forest dieback (reduced flooding and soil salinization) may result in a change in understorey plant assemblages. At Wallpolla Island, in the more xeric west of the Murray River floodplains (Figure 1), the richness of native understory plants was strongly negatively associated with plant area index, a measure of canopy coverage (Table 7). Flooding had a pronounced positive effect on native plant richness, more than doubling the number of species compared to unflooded sites, while there was a weaker negative effect of increasing canopy cover (Table 7).
Table 7. Predictors of Species Richness of Native Understory Plants at the More Xeric, Westerly Wallpolla Island Determined by Bayesian Model Averaging [Raftery et al., 1997]
|Predictor Variablea||Pr (Inclusion)b||Estimated Coefficient (± SE)c|
|Plant area index||1.00||−0.19 ± 0.05|
|Live basal area||0.20||−0.01 ± 0.04|
|Percent live basal area||0.26||−0.02 ± 0.04|
|Tree density||0.16||−0.01 ± 0.02|
|Flood||1.00||0.71 ± 0.09|
 Structural heterogeneity among stands is a strong predictor of species diversity at the patch-scale [Connell, 1978; Petraitis et al., 1989]. Manipulating forest structure through silvicultural thinning reduces tree mortality, and enhances wildlife habitat quality and aboveground carbon storage [Horner et al., 2010]. Therefore, combinations of thinning and restoration flooding treatments should create a larger variety of forest structures that will enhance plant species richness and functional diversity.