Correlación de Datos Experimentales y Estadística del Ajuste

Photo: Jake Gillen

In contrast, on the periphery of this floodplain adjacent to marginal dunes Eucalyptus coolabah typically occurs as sparse but healthy fringing woodland. This fringing pattern is also evident in the more mesic areas of the study area upstream where Eucalyptus coolabah is often accompanied by sparse Bauhinia gilva, a species more characteristically located along the well watered levee banks of regularly flooded channels.

Roberts (1993) also noted the presence of healthy Eucalyptus coolabah at the base of dunes during a demographic study of Coolibah in the lower reaches of Cooper Creek in the vicinity of the Birdsville track and a site within the Strzelecki Desert. Interestingly, a similar phenomenon has been noted in the distribution of Eucalyptus largiflorens, a floodplain species sharing similar characteristics and ecological requirements with Eucalyptus coolabah, occurring within the Murray-Darling Basin. In this instance healthier specimens were observed growing on dunes, in contrast to unhealthier, more stressed individuals on the adjacent clay floodplain soils (Taylor et al., 1996). Similarly, Roberts (ibid) also noted that individuals on the heavier clay soils of floodplain and inter dune swale displayed greater signs of die-back and stress.

What process or processes may be driving these apparent anomalous observations?

The extensive death of Eucalyptus coolabah along the Northern Overflow could be attributed to a number of possible causes, including disease, anoxia associated with extended periods of inundation or (conversely) lack of available soil water following extended periods of drought, or osmotic stress induced by toxic levels of salts. Possibly these trees represent a cohort established following a rare and extended period of flooding of this typically xeric outer margin of the Cooper Creek floodplain. The dispersal and establishment of Eucalyptus coolabah

is dependent upon flood events (Roberts, 1993, Roberts et al., 2000).

To become established and persist in the outer xeric regions of the floodplain would require an extended period of suitable soil moisture conditions. The clustered flooding events associated with a sequence of La Niña events over several years would provide such an opportunity. Eucalyptus coolabah would be distributed widely in the landscape and become

established on a range of soil types (Puckridge et al., 2000). Subsequently when climatic conditions revert to the norm, extended periods of drought prevail and soil moisture deficit sets in, particularly in the outer regions of the floodplain. Those individuals established on the heavy clays of the floodplain proper would be severely compromised. Being rarely flooded, these inherently saline soils accumulate salts in the upper surface as a result of capillary rise associated with extended periods of high potential evaporation resulting in severe osmotic stress (Overton et al., 2006). To further compound this situation, the matric potential of these clay soils reduces freely available soil water (Russell, 1980). As previously discussed (Chapter 4), these clays also effectively swell and seal following large local rainfall events, forming an impermeable barrier to the infiltration of water (Maroulis et al., 2007, Costelloe et al., 2009).

In contrast, those individuals established on the sandier soils of the floodplain outer margin would experience different edaphic conditions. The ability of an individual to extract soil water varies with the soil texture (matric potential) and soil water solute concentration (osmotic potential). Unlike clays, sands with a coarser structure facilitate ready infiltration of water from rain or flood flushing salts from the profile. In addition, higher temperatures in the summer months expand the sands, increasing the infiltration rate (Evenari, 1985a). The coarser structure of sands also renders soil water more readily available to plants. The inverse texture hypothesis (Noy-Meir, 1973) proposes that soils of sandy coarse texture particularly in the arid zone tend to support more woody and mesophytic vegetation than finer textured heavy clays (Schwinning et al., 2004, Curran et al., 2009). The converse applies in more mesic climatic zones, hence the term „inverse texture hypothesis‟.

Recent research in the fields of geomorphology, sedimentology and groundwater hydrology conducted within the broader region provides insight into the possible processes supporting the above observations.

Firstly, chronostratigraphic work in south-west Queensland has provided valuable insight into the nature and sequence of the sedimentation of Cooper Creek (Maroulis et al., 2007, Nanson et al., 2008). Figure 8-9a shows the relation of contemporary aeolian dunes to the contemporary floodplain clay drape and the buried palaeo channels and associated coarse fluvial sands. Of great interest in the context of this study is the juxtaposition of aeolian dunes directly upon the coarser fluvial sands below and the attenuation in depth of the floodplain clays at the base of aeolian dunes.

Secondly, a recent study of water use by Eucalyptus coolabah on the Diamantina and Neales Rivers (Costelloe et al., 2008) proposed that three strategies were employed by the species to cope with typically high groundwater and soil water salinities:

1. Preferential growth in zones of most frequent flushing by infiltrating streamflow (typically the bank-tops of channels);

2. Naturally induced limitation of water use through evolved low transpiration rates; and

3. The ability to extract water at very low osmotic potentials (tolerance of saline groundwater)

One of the prime motivations for Costelloe‟s study was that “nothing is known of the sources of water used by this species under natural field conditions” (ibid, p52).

A possible and plausible reason for the observations of Eucalyptus coolabah in the xeric regions of the floodplain is that the junction between the mud drape and aeolian sand superimposed upon the palaeo fluvial coarse sands below could be a significant groundwater recharge zone. A postulated conceptual model of this process is shown in Figures 8-9b, c).

Figure 8-9 Schematic conceptual model of Eucalyptus coolabah distribution

The high infiltration rates following large local rainfall events, typically experienced during summer, and less common extended flood events would both flush soluble salts and, due to the hydraulic connectivity of the sands, recharge groundwater systems (Jolly et al., 2008).

This situation would support the „preferential growth zones „model proposed by Costelloe et al. (2008).

8.2.4.2 Insights from analyses

Both the CCA and GLMM analyses show the significance of soil organic content in the distribution and abundance of Eucalyptus coolabah and other typically riverine species. This finding presents a conundrum, expressed succinctly; “Like everybody else I could see that the

vegetation affects the soil and the soil affects the vegetation, the very circulus vitiosus that I was trying to avoid” (Jenny, 1980)pxi

Jenny was struggling to deal with the nature of plant–soil relationships in the context of deriving his well known soil-forming factor equation: s (soil properties) = f (climate (cl), organisms (), topography (r), time (t).

However when considered in light of the observations discussed above this apparent circular plant-soil-plant conundrum takes on a different perspective. Eucalyptus coolabah, once established on a particular floodplain soil type, will gradually begin to moderate both the microclimate beneath its canopy (reduction of radiation, temperature, wind, evaporation) and edaphic characteristics, both chemically (input of organic matter and resulting organic acids) and physically (structurally via root development) (Bruelheide and Udelhoven, 2005). Additionally, acting as a „bioengineer‟, Eucalyptus coolabah compounds this feedback process by intercepting and trapping wind and floodborne sediments and nutrients, contributing to continuing modification of soil characteristics (Schlesinger and Pilmanis, 1998, Van Breeman and Finzi, 1998, Ehrenfeld et al., 2005, Mueller et al., 2008). The end result of this self- reinforcing process is the creation of new niches for new species which then further contribute to soil modification and organic accumulation (Plate 8-3).

In this context Eucalyptus coolabah, a long lived perennial species is playing a highly significant ecological role in the floodplain landscape: that of a facilitator directly influencing emergent nurse/protégé plant association relationships. It is now recognized that there is an apparent relationship between facilitation and environmental gradients: the harsher the environment the more evident the facilitation process (Fowler, 1986, Lortie and Callaway, 2006, Brooker et al., 2008). Facilitation, the converse of competition, has been generally defined by Bruno et al. (2003) who state that, “Facilitative or positive interactions are encounters between organisms that benefit at least one of the participants and cause harm to neither” (Bruno et al., 2003).

In its role as a facilitator, Eucalyptus coolabah expands the fundamental or physiological niche for a number of species, extending their realized or ecological niche to areas of the floodplain that would be otherwise unsuitable (Figure 8-10).

Figure 8-10 Effect of facilitation

Functioning in this capacity Eucalyptus coolabah appears to be acting as an „umbrella‟ species (Burgman et al., 2005) supporting and sustaining many other plant and animal species within the floodplain landscape.

The source sink model considers that the geographic distribution of a species is composed of two areas: source areas where population growth is positive, and sink areas where population growth is below replacement level. Population of sink areas is both dependent upon and maintained by dispersal from source areas (Pulliam, 2000). This model has relevance within the floodplain environment where a strong mesic to xeric gradient exists. In the floodplain context the more reliably watered/flooded areas of the landscape act as refugia (Stafford Smith and Morton, 1990) or source areas, sustaining populations during periods of extended drought that normally persist. The rare periods of extensive clustered flooding, driven by a sequence of La Niña phases must then be viewed as critical in the dispersal and establishment of long lived perennial species such as Eucalyptus coolabah into the more xeric sink areas of the floodplain landscape. This conceptual process is depicted in Figure 8-11.

Plate 8-3 Accumulated organic material beneath Eucalyptus coolabah channel