Impacts of multiple stressors on ecological dynamics of river red gum (Eucalyptus camaldulensis) Denhn. forests on the Murray River floodplain

2017-02-02T02:50:10Z (GMT) by Horner, Gillis John
Riverine ecosystems already under stress from human actions, such as land-use change, river regulation and excessive surface and groundwater extraction, now are experiencing a profound, chronic, additional stress through climate change. Forest mortality, due to water extraction and salinization, is likely to be exacerbated by climate change-induced droughts. In south-eastern Australia, the extensive river red gum (Eucalyptus camaldulensis Dehnh.) forests provide one of the world’s most dramatic examples of forest dieback. Reduced water availability has created conditions that are unfavourable for the seedling regeneration and adult survival. I used 42-year tree growth dataset to examine the effects of initial stand density on mortality, tree growth and structure of floodplain forests. I showed that a dramatic increase in mortality in the high-density river red gum stands coincided with a steep temporal gradient of drying (imposed by a warming climate, river regulation and reduced groundwater availability), while mortality remained little changed in lower-density treatments. Lower-density planting also produced stands with higher median and maximum stem diameters than higher–density stands. Early thinning also had a pronounced effect on forest structure, tree growth, habitat quality and aboveground carbon (AGC) storage rates of river red gum stands on the eastern reaches of the Murray River floodplain. Thinning improved habitat value by producing hollow-bearing trees, while the unthinned treatment produced none. Moderately thinned stands had the highest aboveground carbon storage rate (4.1 t C yr-1) and the highest aboveground carbon stocks (200.2 ± 9.6 t C ha -1) after 42 years, while the unthinned treatment had the lowest carbon storage rate (1.6 t C yr-1) and an intermediate level of aboveground standing carbon (165.1 ± 31.1 t C ha-1). I showed that flooding and tree canopy cover affected the composition and native richness of understorey plant assemblages on semi-arid and temperate reaches of the Murray River. Sites with the highest canopy cover had the lowest native richness, whereas sites with low to intermediate canopy cover had the highest native species richness. Flooding more than doubled the predicted number of native species compared to unflooded sites. Seedling survival is likely to be a critical process limiting population viability of dominant floodplain tree species in many water-limited river basins. My survival experiment (including 960 planted seedlings) showed that flooding had a pronounced positive effect on regeneration, increasing survival by a factor of 8 (in the absence of livestock feral and native grazing). Grazing and sediment salinity had strong negative effects on seedling survival at both flooded and unflooded sites. Positive effects of flooding (on survival) largely were nullified by grazing and sediment salinity. My results highlight the importance of initial stand density and seedling survival as key determinants of the structural development of floodplain forests. Variation in flooding, grazing and sediment salinity are strong environmental filters, controlling short-term establishment and long-term population viability. Given the extensive dieback of mature trees from river regulation and salinization, there is an urgent need to enhance seedling regeneration, the future forest. Managing these filters correctly probably will enhance regeneration and structural development of floodplain forests. I recommend that environmental flooding and early thinning of developing stands be considered as part of a broader management strategy to enhance regeneration, carbon storage, wildlife habitat and plant-community diversity.