Monitoring the health of Walker Swamp’s River Red Gums reveals interesting patterns of growth and flowering
During mid-summer for the last six years, a group of keen volunteers and NGT staff have gathered at NGT’s Walker Swamp Restoration Reserve for two days of field work, to assess the condition of River Red Gums across the reserve. Mid-summer is chosen for these surveys as it is around the time that most of the River Red Gums in the region are flowering or have recently formed fruit (gumnuts).
Each year the same 100 trees are assessed following a method developed by Soutor et al. (2009). Information on reproductive effort and new leaf growth forms an important part of the assessment process used to quantify the condition of the trees. This year over a dozen volunteers enjoyed perfect conditions to walk around the reserve, learning the survey method and assessing each tree (Figure 1).
In addition to assessing each tree’s overall condition, the data collected also provides an insight into annual growth patterns in the canopy, reproductive effort, changing patterns of insect activity on trees, levels of stress and responses to changing environmental conditions. With six years of data, we are starting to see some interesting responses to the changing conditions across the Walker Swamp landscape, following the removal of the Blue Gum plantations and restoration of a more natural water cycle. The overall condition score for each tree is determined using a formula that takes into account the crown extent and density, the presence and condition of epicormic growth (from under the bark), the level of reproduction, the amount of crown tip-growth, leaf die-off and damage, the abundance of mistletoe and the condition of the bark (see Souter et al. 2009).
Following three years of relatively marked changes to tree condition during and after the removal of the Blue Gum plantation and restoration of more natural water cycles across the Reserve, the condition of the trees appears to have settled into a relatively stable pattern over the last three years. This follows the death of two very stressed trees recorded in 2018 and 2019, and the slow recovery of the remaining trees that were in extremely poor and very poor condition.
However, despite the appearance of stability, a number of underlying patterns in the overall condition of the trees have become apparent. The scientific study of the timing of certain periodical biological events, such as flowering or fruiting is called ‘phenology’, noting that we are witnessing the River Red Gums at Walker Swamp experiencing annual cycles in patterns of growth that are consistent for a very large proportion of the trees.
The two-year reproductive cycle continues into its sixth year.
Those who have followed our previous blogs on these surveys will remember that a two year cycle was noticed in the reproductive response of trees across the region after four years of monitoring. This pattern has now continued into the sixth year of monitoring (as shown in Figure 3) with ‘abundant’ reproductive activity (where buds, flowers or fruit (gum nuts) dominates the appearance of the tree), or ‘common’ reproductive activity (where buds, flowers or fruit (gum nuts) are clearly visible across the canopy), being recorded in 70% to 90% of the trees every second year (2019, 2021, and 2023).
In the interim years (2018, 2020, and 2022) either no reproduction or ‘scarce’ reproduction was recorded in over 80% of the trees. Reproduction is considered absent if no buds, flowers or fruit are observed in the canopy when scanned with binoculars. and ‘scarce’ if, following scanning with binoculars, low levels of buds, flowers or fruit are noticed.
The reproductive effort seen in the River Red Gums is not connected to the overall Relative Condition Index (RCI) of the trees.
This pattern in biennial reproductive effort of the trees is very strong but it bears no relationship to the condition of the trees in which it is recorded (Figure 4). The same spread of RCI is observed in the trees irrespective of the level of reproductive activity.
The biennial cycle observed in reproductive activity is not the only one present in these data
In addition to the reproductive cycle, there is also a biennial cycle in the level of crown (tip) growth recorded in the River Red Gums (Figure 5).
However, crown growth has been seen to peak in alternate years to the recorded peaks in reproductive effort, with ‘abundant’ (effect dominates the appearance of the tree) or ‘common’ (effect is clearly visible across the canopy) levels of crown (tip) growth being recorded in over 80% of the trees every second year (2018, 2020, and 2022). Although not as clear cut in the interim years (2019, 2021, and 2023) either no reproduction or ‘scarce’ reproduction was recorded in over 45%, 75% and 85% of the trees.
A comparison of the percentage of River Red Gum trees in each year with either common or abundant reproductive effort, versus the percentage of trees with common or abundant crown tip growth (Figure 6), shows a significant (F1,4=7.591, p=0.05*), strong negative relationship (R=-0.809, R2=0.665) across all six years of data.
The cluster of points in the top left of the graph represent data points from the high crown tip growth years of 2019, 2021 and 2023. The green dot for example is from 2018 when the reproductive effort was relative low (only 19% had common or abundant buds, flowers and fruit), but the crown tip growth was high (80% common or abundant). Those points loosely clustered in the bottom right of the graph are from the high reproductive years of 2018, 2020, and 2022. The red dot for example is from 2021 when the reproductive effort was relative high (91% had common or abundant buds, flowers and fruit), but the crown tip growth was low (31% common or abundant).
So, from the data collected so far, it appears that a large proportion of River Red Gums at Walker Swamp have a two-year cycle in growth. In one year they appear to put a large amount of energy and nutrients into growth of new foliage and little into their reproductive effort. Then in the subsequent year they put a large amount of energy and nutrients into reproductive effort and little into the growth of new foliage.
This does raise interesting questions… for example, why is there such a strong bias with 80 to 90% of the trees in lock step – simultaneously putting large efforts into reproduction one year and leaf growth the next? How is this apparent synchronisation achieved?
If the trees were behaving independently of each other you might anticipate that there would be randomised patterns of some trees showing more reproductive effort one year and others showing more in the next, with around 50% in each group. Perhaps they are in lock step, because of some extreme environmental event causing them to synchronise and then subsequently follow a bienniel pattern until it is potentially reset by a subsequent extreme event?
Unlike in the rest of Australia, River Red Gums in the south of Victoria and adjacent SA (where they occupy large tracts of undulating hills and plains on heavier soils, including the volcanic plains westwards from Geelong) are not always associated with existing floodplains and channels, often found spread across the landscape. Consequently they may have different phenologies from those observed in River Red Gums growing along the inland river systems of Australia. Keatley et al. (2021) found that River Red Gum in the Barmah Forest on the River Murray floodplain showed evidence of flowering more intensely every second year. Jensen et al. (2008) found that for River Red Gum, there is peak seed fall in alternate summers, with trees in the same community on opposite cycles, ensuring that some flower and seed each year. Conversely in a 30-year study of eight Eucalypt species in the Maryborough area, Hudson and Keatley (2017) found that the River Red Gum was one of only two that did not have biennial cycles.. Biennial cycles or ‘on-off’ years have been noted by apiarists from at least the early 1900s but often with the caveat of being location dependent (Beuhne 1922, Rayment 1916).
The River Red Gums at the Walker Swamp Restoration Reserve have retained a biennial cycle in flowering over six years of monitoring. Perhaps, the most interesting insight after six years is that this biennial cycle, with a heavy period of reproduction every two years, appears to alternate with a year in which new leaf growth dominates the canopy.
References
Beauhne, F. R. (1922) The Honey Flora of Victoria. Melbourne Department of Agriculture
Hudson, I. L. & Keatley, M. R. 2017. Singular Spectrum Analytic (SSA) decomposition and reconstruction of flowering: signatures of climatic impacts. Environmental Modeling and Assessment, 22, 37-52.
Jensen, A. E., Walker, K. F., & Paton, D. C. (2008, April). Smart environmental watering: getting most benefit from scant flows for floodplain trees (River Murray, South Australia). In Proceedings of Water Down Under 2008 Conference (pp. 15-17). Melbourne: Engineers Australia.
Keatley, M.R., Bren, L.J., & Hudson, I.L. (2021) The historic flowering behaviour of river red-gum and black box in a flooding forest. Austral Ecology 46(4):640-652.
Rayment T. (1916) Money in Bees in Australia. A practical treatise on the profitable management of the Honey Bee in Australia. Melbourne. Whitcombe and Tombs Ltd.
Souter, N. J., Watts, R. A., White, M. G., George A. K. & McNicol, K. J. 2009. Method manual for the visual assessment of lower River Murray floodplain trees. River Red Gum (Eucalyptus camaldulensis). February 2009. DWLBC Report 2009/25. Adelaide: Government of South Australia, through Department of Water, Land and Biodiversity Conservation.
