saline sites
4.5 Salinity trials
4.5.1
Seedlings
Salt water results
Mean height ± SE of the 60 Leptospermum lanigerum seedlings at the start of
experiment was 28.3 ± 0.87 cm. After 14 days the 20 L. lanigerum seedlings in salt
water were dead (Figure 4.31). Measurements were to be conducted at 4 weeks to compare to the other treatments but with 100% death rate at two weeks no further measurements were made.
Figure 4.31. Leptospermum lanigerum seedlings were all dead after 2 weeks submersion
in 100% saltwater.
Control and brackish treatments compared
After 4 weeks no significant difference between the control (fresh water) and brackish treatments was found. Mean % difference in height change ± SE for the control treatment was 33.2 ± 3.5 and for brackish treatment was 31.5 ± 2.6 (t1,35 = - 0.38, P = 0.71).
After eight weeks three of the L. lanigerum seedlings in brackish water had died. The
mean percent change in growth from the start of the experiment for the 20 control seedlings was 30.74 ± 2.73 cm and for 17 remaining seedlings in the brackish experiment 28.4 ± 1.78. While 100% of the control seedlings survived the 8 weeks waterlogging, the brackish experiment had a 15% loss. However the mean growth rate of the remaining seedlings in the brackish experiment was only marginally lower than the control group (Figure 4.32).
Figure 4.32. Leptospermum lanigerum seedlings showing a high tolerance for brackish
water after 8 weeks.
4.5.2
Lawn sward
Control and brackish treatment
At the end of the eight-week experiment the sward species had grown vigorously in both the control (Figure 4.33) and brackish (Figure 4.34) treatments with no observed difference in the cover or mean height of the main lawn sward species (Selliera radicans, Schoenus nitens and Eryngium vesiculosum). After eight weeks in the
control (fresh) and brackish treatments S. radicans was between 4 and 6 cm in height
and 9.6 cm respectively) and S. nitens between 14 – 25 cm (mean 16.8 cm and 16.9
cm respectively).
Salt water treatment
Lawn swards given the saltwater treatment showed considerable signs of stress after 4 weeks (Figure 4.35), with over 50% of the plants in each pot dead and those still alive not healthy. A few specimens of S. nitens and S. radicans had survived. After eight
weeks there were half a dozen yellow/green leaves on S. radicans remaining in two
Figure 4.33. Control lawn swards after 8 weeks, note a Leptospermum lanigerum
seedling had germinated and continued to grow in left rear bucket.
Figure 4.34. Lawn swards appear healthy and with considerable new growth after 8 weeks inundation in brackish water.
Figure 4.35. Lawn swards after 4 weeks in 100% salt water treatment.
4.6
Discussion
This chapter set out to investigate if weather patterns had changed in Tasmania over the last thirty years and if so whether the extent of marsupial lawns had been affected. Rainfall data, ring counts, field observations, aerial photographs and salinity trials were used in the process of answering these questions.
4.6.1
Rainfall patterns
Since the late 1970s the Northern, Flinders Island, East Coast and the Southeast regions of Tasmania have experienced unusually dry conditions with these four regions recording the lowest rainfall in the last 60 years in 2006. In these regions, rainfall recordings at the closest weather station to field sites visited averaged a 13% decline, and four regions had a significant decline post 1978 compared to rainfall pre 1978. In addition the high rainfall events, generally occurring in the winter months appear to be lessening in intensity or not occurring at all. High rainfall events replenish ground water and often inundate grazing lawns.
Two regions, the West Coast and the Derwent experienced an increase in the median rainfall post 1978. However, even in these regions the high rainfall events did not match the pre 1978 figures. With consecutive dry years and fewer high rainfall events, it was hypothesised that lawns in the north, east and southeast regions would have little herbaceous growth to attract grazers to ‘mow’ down invading woody species; while high rainfall and saline areas would maintain lawns.
4.6.2
Age of woody invasion from ring counts
Ring counts of invading trees and shrubs confirmed that woody encroachment had occurred over the last 20-25 years with the three oldest trees recorded in the northern region (24 years), east coast region (21 years) and southeast (21 years). The mode for each group of saplings in the 5 x 5 m plots was useful for illustrating the process by which woody species invade a lawn, i.e. that a pioneer species successfully establishes and then colonises an area around it with a cohort of similarly aged seedlings.
The two most common woody invaders were Leptospermum and Melaleuca spp. The
weight of Leptospermum and Melaleuca seeds means they are unlikely to land far
from the parent plant or float when the lawn is inundated. Those seedlings that successfully establish some distance from the parent plant may be due to chance; landing near an obstacle to grazing. This perhaps explains some of the results where the third plot (the one furthest from the bush edge) had the oldest specimen, as was the case at Mt William NP (site 1, plot 3) and at Bruny Island (Table 4.2). Melaleuca ericifolia can colonize by suckering and again it would be expected that the oldest
seedlings/saplings are closest to the parent plant. However, at Mt William site 2 (Table 4.2), the oldest Melaleuca ericifolia specimen was found in the middle plot,
not the plot adjoining the bush, so this generalisation has some exceptions.