Root Tip Growth and the Presence of Leaves Affect

©2021 International Society of Arboriculture

Arboriculture & Urban Forestry 47(4): July 2021 133

Arboriculture & Urban Forestry 2021. 47(4):133–149https://doi.org/10.48044/jauf.2021.014 &URBAN FORESTRY

ARBORICULTURE

Scientific Journal of the International Society of Arboriculture

The forests of coastal and montane south-eastern Australia are dominated by eucalypts and other mem-bers of the Myrtaceae family. Many of these tree spe-cies possess both lignotubers and epicormic buds. Eucalyptus obliqua L’Herit. is widely distributed across south-eastern Australia, attaining heights of 45 m to 90 m and exceeding 3 m in diameter at breast height (Boland et al. 1984). The species grows in a range of fire-prone habitats and has a number of different eco-typic populations (Ashton 1981; Beadle 1981) that are stress-tolerant and fire-resistant with adaptations, such as sclerophyllous leaves, thicker bark than many similar eucalypts, epicormic buds, and, in most popu-lations, lignotubers (Costermans 1981; Nicolle 2006).

Epicormic shoots develop from epicormic buds that grow just below the bark on woody stems and branches (Meier et al. 2012; Moore 2015) and occur

INTRODUCTIONDormant buds are significant assets for those who manage urban trees. The shoots produced from axillary, epicormic, and lignotuberous buds are significant parts of stress recovery responses in a diverse range of tree species (Moore 2015). Depending on the severity of the stress experienced, the usual order of regenera-tion is from axillary and epicormic buds higher up in the branches of the canopy, then epicormic buds on lower branches, then epicormic buds on the trunk, and finally lignotuberous buds. Lignotuberous buds can be seen as a last resort adaptation that allows recovery when production of shoots from other sources of buds has failed or is limited. The manage-ment of shoots produced after a variety of stresses may require arboricultural intervention if well-structured and safe trees are to develop (Moore 2015).

Root Tip Growth and the Presence of Leaves Affect Epicormic and Lignotuberous Shoot

Development and Survival of Stressed Eucalyptus obliqua L’Herit. Seedlings

By G.M. Moore

Abstract. The shoots produced from axillary, epicormic, and lignotuberous buds are significant parts of stress recovery responses in many tree species. The production of either epicormic or lignotuberous shoots does not guarantee survival of a tree, as the mortality of shoots is high. This research investigated the relationship between root tip growth and shoot production and survival after stress and its implications for urban tree managers. Seedlings of Eucalyptus obliqua L’Herit. were stressed by decapitation or different levels of heat stress at temperatures ranging from 40 °C to 100 °C for 2 to 128 minutes, as well as combinations of the two stresses. While the temperatures are not as high as those experienced in a forest fire, the stresses imposed can inform plant responses to stress such as fire. Lower temperatures and shorter durations were often sub-lethal, and decapitation, to the same extent as heat killing of plant tissues, elicited similar levels of epicormic and lignotuberous shoot growth. The root systems of the seedlings were inspected to determine whether the root tips were healthy, and selected root tips were monitored to deter-mine if and when they had resumed growth. Survival rates of epicormic and lignotuberous shoots were enhanced by the presence of healthy leaves. The recommencement of growth after stress by the development of epicormic or lignotuberous shoots was preceded by root tip growth, which emphasises the importance of a healthy root system. Managing for the best soil conditions possible during and immediately after stress may be a key to successful shoot production and tree recovery.

Keywords. Decapitation; Epicormic Shoot; Heat Stress; Lignotuber; Root Tip Growth.

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134 Moore: Root Tip Growth and the Presence of Leaves Affect Epicormic and Lignotuberous Shoot Development

in a diverse group of tree species. Lignotubers (basal burls) are modified stem structures that are reservoirs of large numbers of dormant buds (Carr et al. 1984; Burrows 2002). They occur in many members of the Myrtaceae, and in eucalypts, lignotubers develop from axillary buds, usually at the first three leaf nodes at the base of seedlings at or just below the soil sur-face (Chattaway 1958; Carr et al. 1984; Molinas and Verdaguer 1993; Mibus and Sedgley 2000; Clarke et al. 2013). Subsequent divisions are downwards, and as the stem of the tree increases in diameter, the ligno-tuber is incorporated into the trunk largely below-ground (Mullette 1978; Moore 1982), where it is well-protected by the insulating properties of soil from the high temperatures that occur during forest fires (Jacobs 1955; Chattaway 1958; Parsons 1968; Carrodus and Blake 1970; Mullette 1978; Moore 1982; Clarke et al. 2013).

Lignotubers contain many bud traces (Carrodus and Blake 1970), and trees subjected to multiple epi-sodes of stress may produce hundreds of lignotuber-ous shoots over several decades (Chattaway 1958; Burrows 2002). Lignotuberous buds, which exhibit developmental plasticity, can also form roots that replace damaged root systems (Clarke et al. 2013). When the upper parts of the tree are destroyed, the potential of the lignotuber to produce shoots is real-ised as part of recovery processes (Colak et al. 2009). Lignotubers may act as storage organs when they contain large amounts of carbohydrates (Carrodus and Blake 1970; Bamber and Mullette 1978; Varner et al. 2009).

The production of epicormic or lignotuberous shoots does not guarantee survival of a tree, as the mortality of the shoots is very high. For Eucalyptus obliqua, there is an average mortality rate of up to 50% for epicormic shoots, and between 50% and 100% for lignotuberous shoots, depending on the level of stress imposed (Moore 2015). These responses are relevant to arborists whose role in post-fire management in peri-urban parts of cities has expanded (Moore 2011). However, they also provide insight into shoot pro-duction after other stresses, such as insect defoliation or the severance of tree trunks by vehicles.

In some ways, the effect of fire and heat on plants is to cause an effective defoliation and/or decapita-tion, which subsequently causes a reduction in root carbohydrate reserves when starch is converted to sugar (Parker 1970; Wargo et al. 1972; Clarke et al.

2013). High temperatures can cause the degradation of cellular constituents such as lipids and plant pig-ments (Bär et al. 2019), and there is the possibility of translocated heat injury, where heat damage to one part of a plant produces inhibitors that affect develop-ment in other parts (Yarwood 1975; Bär et al. 2019). The production of phaeophorbide (pheide) from the degradation of chlorophyll has been known for a long time (Aubry et al. 2020), but more recently its effects on other metabolic moderators such as jasmonic acid, ethylene, and abscisic acid (ABA) have become clearer (Kim et al. 2018; Kuai et al. 2018). These hor-mones and inhibitors have a role in senescence but may also inhibit shoot initiation after stress (Aubry et al. 2020). Decapitation and defoliation also reduce root dry weight and growth but enhance leaf produc-tion (Bruce 1956; Madgwick 1975; Hodkinson and Baas Becking 1978; Moore 2015). These responses can impact the production and survival of shoots pro-duced subsequent to fire and other stresses.

Both epicormic and lignotuberous buds usually have access to a store of carbohydrates and a supply of water and nutrients from an established root sys-tem. This is a rapid response system that allows the restoration of photosynthetic capacity in the case of serious damage to the foliage and trunk. In warmer parts of the world, with longer growing seasons, epi-cormic and lignotuberous shoots can grow at rates exceeding 4 m per annum under good conditions (Moore 2015). The production of shoots from dormant buds is often explained in terms of access to carbohy-drate reserves moderated by hormones, particularly auxins and cytokinins (Tworkoski et al. 2006; Ras-mussen et al. 2009; Rasmussen and Hunt 2010; Meier et al. 2012; Clarke et al. 2013; Aloni 2015).

The disruption in hormone concentration may be triggered by defoliation, fire, pruning, or sudden expo-sure to higher levels of light (Moore 1998, 2015; Meier et al. 2012; Aloni 2015). During wildfires, there is an effective defoliation or decapitation that removes some or all of the aboveground parts of the plant, impacting hormone levels (Meier et al. 2012; Clarke et al. 2013; Aloni 2015). Often, however, the root system remains undamaged due to the insulating properties of soil, and so root-sourced hormones are still present (Clarke et al. 2013). Root tip involvement in overall plant coordination and response is well-established (Fromm and Eschrich 1993; Baluska et al. 2004; Aloni 2015; Karban 2015), but the involvement of the root system




(decapitation) of damaged tissues either immediately or a week (delayed) after the heat treatment. It was hypothesised that:

4. after heat stress, plant responses and/or chemicals produced as a result of the stress could inhibit or delay recovery, and that immediate removal of damaged tissues might result in earlier shoot initiation.

The second experiment involved “parallel” decapita-tion of seedlings, where the decapitations “paralleled” the amount of stem tissue and number of leaves that were left undamaged after heat treatments. It was hypothesised that:

5. an element of the response to high temperatures was due to the killing or removal of stem tissue and foliage.

The third experiment, which left different numbers of intact leaves on seedlings, was undertaken to examine the hypothesis that:

6. the more healthy foliage a seedling retained after decapitation or heat stress, the more likely it was to produce epicormic or lignotuberous shoots that survived and grew.

The nodal positions and mortality of shoots produced and the rates of seedling growth were recorded after treatment. The relationships between epicormic and lignotuberous bud development, shoot mortality, root tip growth, and the presence of intact leaves were investigated.

MATERIALS AND METHODSE. obliqua seedlings were grown from seed (Daylesford, Victoria source) for 8 months under ideal greenhouse conditions, where the temperature within the green-house was held at 23 ± 6 °C with a daily temperature variation of ± 3 °C. Seedlings used in experiments were approximately 300 mm high, with a stem calli-per above the lignotuberous swelling of 3 mm to 4 mm.

Trees were grown in specially designed containers so that root systems could be inspected daily to deter-mine whether the root tips were healthy, and selected root tips were monitored for growth and to determine if and when they had resumed growth after treatment. Trees were grown in a pasteurised, soil-based medium so that soil insulation properties were retained. The containers were made from PVC pipe with a diameter of 70 mm cut into 200-mm lengths with a smooth inner surface (Figure 1). At the base, 4 lugs were cut

in dormant bud development has been less researched (Meier et al. 2012; Clarke et al. 2013).

There is a long and extensive literature on the effects of high and low temperatures on leaves, seeds, fruits, pollen, and whole plants, but there are few reports on the effects of high temperatures on roots (Levitt 1972; Precht et al. 1973; Hood et al. 2018). Root tissues cease functioning in nutrient absorption and water uptake at about 40 °C (Precht et al. 1973), and it is generally assumed that root cells and tissues are killed over the range of 50 to 60 °C that kills plant cytoplasm (Levitt 1972; Busse et al. 2005; Bär et al. 2019). One of the reasons why there is little informa-tion on lethal root temperatures is that, under natural circumstances, temperatures are unlikely to be high enough to do damage. Soil is such a good insulator that at depths of 25 mm to 50 mm, soil temperature varies little, depending on soil type, organic matter, and moisture content, even in forest fires (Beadle 1981; Preisler et al. 2000; Busse et al. 2005). A ques-tion that arises in relation to shoot mortality is the role played by a tree’s root system in the production and successful growth of shoots.

This paper investigates the responses of E. obliqua seedlings subjected to two forms of stress, both of which left the root system intact but which caused dif-ferent levels of damage to the aboveground parts of the trees. The first experiment involved exposure to temperatures ranging from 40 to 100 °C for different durations. While the temperatures are not as high as those experienced in a forest fire, the stresses imposed can inform plant responses to stress such as fire. It was hypothesised that:

1. apart from affecting aboveground parts of the plant, stress at this level would affect roots;

2. there would be an interaction between root growth and the initiation of new lignotuberous or epicormic shoots in plants that recovered from the stress imposed;

3. because root tissues were not directly damaged by the treatments, root growth, if disrupted by the level of stress imposed, would resume prior to shoot production, and that if there was no resumption of root growth, there would be no shoot growth.

High temperature stress produces chemicals such as phaeophorbide that may inhibit shoot development (Kim et al. 2018; Kuai et al. 2018; Aubry et al. 2020). Heated seedlings were subjected to the removal




damage was clear), and the third group of 10 seed-lings were left intact after treatment.

Thermocouples were placed in the ovens to ensure consistent, non-fluctuating temperatures and in the grow-ing media of the containers to record temperatures (Moore 2015). This enabled the temperatures and durations at which E. obliqua seedling root apices were killed to be determined and also made it possible to ensure that in other experiments, container media temperatures did not reach levels that would damage root apices. The death of plant tissue was determined using plant impedance ratios according to the method of Moore (1979), which indicate the impact of a treat-ment on cellular membranes. Killing of plants at 80 and 100 °C was very rapid, and given the limited sup-ply, plants were not sacrificed by heating for the lon-ger durations above 32 and 16 minutes, respectively.

The growth of at least 2 root tips per container at the outer surface of the root mass were measured for all control and treated plants using callipers, measuring back from the tip to a small, black, water-insoluble ink mark on the root that did not wash off when seed-lings were irrigated and would not contaminate the growing medium. The mark also allowed the identifi-cation of root tips to be measured from week to week.

Experiment 2Seedlings were treated in “parallel” decapitations, where similar proportions of the stem and leaves to those killed by heat treatment were removed. The degree of killing of seedling leaf and stem tissues was deter-mined using the methods of Moore (2015). In these experiments, seedlings simulating 100 °C for 2 min-utes, 80 °C for 8 minutes, and 60 °C for 32 minutes were decapitated 20 mm above the potting medium (Figure 2). The 80 °C 2-minute and 4-minute simula-tions were decapitated 120 mm and 80 mm above the soil medium, and for the 60 °C 2-, 4-, 8-, and 16-minute simulations, seedlings were decapitated 20 mm, 40 mm, 80 mm, and 120 mm from the shoot apex (Figure 2).

Experiment 3Groups of 7 seedlings were decapitated so that 0, 2, 4, or 8 leaves were left upon the stem of plants to deter-mine whether the number of intact leaves left on a plant had an effect on shoot production and survival. The decision to leave different numbers of leaves was based on experiments 1 and 2, which showed that for seedlings heat treated at 60 and 80 °C for longer dura-tions, all leaves were killed, but for milder treatments,

into the pipe and bent inwards to support an inverted cap with drainage holes that formed the base of the container. The base of the container could then be placed over a piston that allowed the content of the container to be pushed upwards for inspection with-out damaging or disturbing the roots growing on the surface (Figure 1).

Experiment 1Groups of 30 seedlings were subjected to a stress treat-ment that involved heating the aboveground parts of the seedlings in Qualtex® ovens (Qualtex, Murarrie, QLD, Australia) at temperatures of 40, 60, 80, and 100 °C for durations of 2, 4, 8, 16, 32, 64, and 128 minutes (Moore 2015). One group of 10 seedlings that had been heat stressed were subjected to immediate decapitation, and another group of 10 seedlings had a delayed decapitation (after 7 days when the extent of

Figure 1. Diagram showing the specifically designed contain-ers allowing daily root tip inspection to determine whether the root tips were healthy and when selected root tips had resumed growth.




10 replicates were used in experiments. For experi-ment 1, there were 24 temperature treatments, 3 decap-itation treatments, and 10 replicates; for experiment 2, there were 6 levels of decapitation and 10 replicates; and for experiment 3, there were 5 levels of leafiness and 7 replicates. With large numbers of treatments and replicates, data were analysed using analysis of vari-ance (ANOVA), general linear model. P-values of less than 0.05 were considered significant, and the least significant difference (LSD) was calculated.

RESULTSExperiment 1Investigation of root responses, testing hypotheses 1 and 2, showed that root apices were killed quickly at 80 and 100 °C, but survived 60 °C for 8 minutes (Fig-ure 3). These data are for root apices, not older, wood-ier roots. Even when roots are not directly affected by high temperatures, damage to the aboveground parts of the plant impacts seedling root systems, as after

2, 4, and 8 leaves survived. Based on these earlier experiments and results, plants decapitated so that no leaves were left on the stem were compared with plants heated at 60 °C for 16 minutes, and plants left with 2 leaves were compared with plants heated at 60 °C for 8 minutes.

For all experiments, after treatment, rates of height increment, the time of production, nodal position, and mortality/survival of new epicormic and lignotuber-ous shoots were recorded for a period of 10 weeks. Oven dry weights were obtained by removing the parts of the plants produced after treatment and drying material in the Qualtex® ovens at 80 °C until the weight stabilised. The height, habit (single leader and num-ber of branches), and leaf number of treated seedlings were compared with untreated controls of the same age for 6 months after treatment to determine whether there were any long-term differences.

Depending on the availability of seedlings of the appropriate size, a minimum of 7 and a maximum of

Figure 2. Diagram showing the levels of decapitation that were imposed to “parallel” heat stress treatments.




either heat and/or decapitation, root tissues often responded by a change of colour and growth rate. While the milder treatments altered neither, for severe treatments, within 2 days of treatment root tips turned brown, and root tip growth ceased (Table 1, Figure 4). Similar colour responses were seen for both high temperature and for complete decapitation treatments.

The recommencement of growth after significant stress through the development of either epicormic or lignotuberous shoots was preceded by the resumption of root tip growth. Often for milder treatments, root

tip growth commenced 2 to 3 days before epicormic or lignotuberous shoot growth could be detected, but for some of the more severe treatments, the delay in shoot initiation was longer (Table 1). Root tip growth was significantly lower for all treated plants than for controls in the early weeks after treatment, but after 10 weeks, the root growth of plants heated at 40 °C and 60 °C was not significantly different from con-trols, while root growth of plants heated at 80 °C was significantly lower (Figure 4). The growth rates of root tips heated at 40 °C and 60 °C did not differ

Figure 3. Killing of white root tip apices of Eucalyptus obliqua seedlings when soil temperatures reached 60 and 80 °C for different durations. No root tips in the control or plants heated at 40 °C died. Number of replicates (N) = 8 for all treatments; the vertical bar shows LSD (P < 0.05) = 4.60.

Table 1. Time taken for root tips to brown and for the resumption of root tip growth and shoot initiation after heat treatment and heat treatment plus immediate decapitation. Number of replicates (N) = 10 for all treatments. Different letters in a column denote significant difference for all treatment data (P < 0.05).

Treatment Time taken for browning Time taken for resumption Time taken for shoot of root tips (days) of root tip growth (days) initiation (days)

Control No browning n/a n/a40 °C for 2 minutes 2-5 12-14b 14-16a40 °C for 2 minutes and decapitation No effect Immediate 5-8b60 °C for 2 minutes 2-3 3-4a 4b60 °C for 2 minutes and decapitation No effect Immediate 3b60 °C for 4 minutes 2-4 8-12b 14-21a60 °C for 4 minutes and decapitation No effect 1-3a 5-10b60 °C for 8 minutes 2-4 10-13b 14-22a60 °C for 8 minutes and decapitation 2-3 3-5a 8-12b80 °C for 2 minutes 2-3 12-15b 14-17a80 °C for 2 minutes and decapitation 2-3 6-10ab 8-11b




Figure 4. Growth rates of root tips of Eucalyptus obliqua seedlings over a 10-week period after heat treatment for 2 minutes at 40, 60, and 80 °C. Number of replicates (N) = 10 for all treatments; the vertical bar shows LSD (P < 0.05) = 1.13.

significantly at any point in the recovery period, and both were significantly greater than the 80 °C treatment after week 7. If root tips did not recommence growth after the imposition of stress, then epicormic and ligno-tuberous shoots did not develop or survive, and plants died. Shoot production from buds coincided with resumption of root tip growth. Interestingly, when the stems of heated plants were sectioned, they too often showed a brownish colour not found in healthy plants.

The decapitation of heated plants which removed heat-damaged tissues and which tested hypotheses 3 and 4 showed that immediate decapitation of dam-aged tissues resulted in greater height increment after heating than heated-only plants, significantly so for the 40 and 60 °C treatments (Figure 5). Plants that were decapitated immediately after heating behaved more like decapitated than heated plants. However, if decapitation was delayed for a week, plants showed a decreased height increment compared to heated plants, but differences were not significant (Figure 5).

Experiments 2 and 3The decapitation of plants to the same extent as heat killing of plant tissues in experiment 2, which tested

hypothesis 5, elicited similar levels of epicormic and lignotuberous shoot production and height growth (Figure 6). Decapitated plants with more leaves left intact in experiment 3, which tested hypothesis 6, showed higher rates of epicormic shoot, lignotuber-ous shoot, and overall plant survival (Table 2).

In all experiments, shoots were produced from the highest nodal positions available after treatment. Nodal positions 1 to 3 are considered to be lignotu-berous, while positions 4 and above are epicormic. Lignotuberous shoots were only produced when most, if not all, epicormic buds were killed by heat treatment or were removed by decapitation (Figures 7 and 8). The production of new shoots after treatment showed that shoot production may take from 3 to 22 days, depending on the severity of the heat stress imposed (Table 1). Only 1 shoot was produced by seedlings that were subjected to treatment at 100 °C, and none survived (Figure 8), so this treatment is not further represented in the results or discussion. Shoot production by decapitated plants was similar to that of heated plants, with shoots being produced at the highest remaining nodal positions (Figure 9), but it was quicker than for heated plants, and removal of




for plants heated at 60 °C for 4 and 8 minutes (Figure 11). Immediate removal of damaged tissues by decap-itation resulted in a slight increase in dry weight gain, but delaying decapitation by 7 days after heating reduced dry weight production (Figures 11 and 12). For the most severe heat treatment at 80 °C, there was a significant decrease in dry matter production after immediate and delayed decapitation (Figure 12).

heat-damaged tissue decreased shoot production time (Table 1). Production of epicormic or lignotuberous shoots is no guarantee of survival, as many shoots subsequently died (Figure 10).

After the 10-week recovery period established in these experiments, dry matter production for the 2 most severe treatments was significantly reduced com-pared to the control, and there was a significant differ-ence between delayed and immediate decapitation

Figure 5. Height increment of Eucalyptus obliqua seedlings over a 10-week period after heat treatment and heat treatment and decap-itation, either immediately or after a delay of 1 week. Number of replicates (N) = 10 for all treatments; the vertical bar shows LSD (P < 0.05) = 2.40.

Table 2. The average number of lignotuberous shoots produced, the proportion of shoots that died, and the proportion of plants that died after decapitation leaving different numbers of leaves intact. Number of replicates (N) = 7 for all treatments. Different letters in a column denote significant difference (P < 0.05).

Number of leaves left intact Average number Proportion of shoots Proportion of plants of shoots produced that died (%) that died (%)

0 4.6a 54.2a 30.0c2 5.3a 38.6b 10.0b4 4.9a 29.6c 0a8 2.0b 50.0a 0a




Figure 6. Height increment of Eucalyptus obliqua seedlings over a 10-week period after heat treatment and parallel decapitation as described in Figure 2. Number of replicates (N) = 10 for all treatments; the vertical bar shows LSD (P < 0.05) = 2.40.

Figure 7. The range of nodal positions of shoot production by Eucalyptus obliqua seedlings over a 10-week period after heat treatment at 60 °C. Positions 1 to 3 are considered to be lignotuberous shoots, and positions 4 and above are epicormic. Number of replicates (N) = 10 for all treatments.




Figure 8. The range of nodal positions of shoot production by Eucalyptus obliqua seedlings over a 10-week period after different heat treatments for 2 minutes. Positions 1 to 3 are considered to be lignotuberous shoots, and positions 4 and above are epicormic. Number of replicates (N) = 10 for all treatments.

Figure 9. The range of nodal positions of shoot production by Eucalyptus obliqua seedlings over a 10-week period after decapitation. Positions 1 to 3 are considered to be lignotuberous shoots, and positions 4 and above are epicormic. Number of replicates (N) = 7 for all treatments.




Figure 11. Dry matter production over a 10-week period by Eucalyptus obliqua seedlings heated at 60 °C and decapitated either imme-diately or after a delay of 1 week. The lower horizontal scale shows leaves remaining on decapitated-only plants. Number of replicates (N) = 10 for all treatments; bar displays LSD (P < 0.05) = 0.75.

Figure 10. Mortality of shoots produced by Eucalyptus obliqua seedlings over a 10-week period after different levels of heat stress. Number of replicates (N) = 10 for all treatments; the vertical bar shows LSD (P < 0.05) = 4.80.




either epicormic or lignotuberous bud development, growth of root tips resumed. The resumption of root tip growth usually commenced 2 to 7 days before signs of shoot initiation were evident in the aboveground parts of seedlings. There was no recovery in seedlings when root tip growth did not recover, which is consistent with theories of plant communication and the integration of root and shoot systems (Fromm and Eschrich 1993; Baluska et al. 2004; Karban 2015).

Seedlings were used for these experiments, but the meristematic cell division and differentiation responsible for apical shoot and root tip growth have been reported to be similar in studies on mature and juvenile tissue development (Sawchuk and Scarpella 2013). Since responses to phytohormones occur at a cellular level, the responses of the roots tips of mature trees are likely to be similar as those reported for seedlings (Nakamura et al. 2012; Koepke and Dhingra 2013). However, the large root systems of mature trees may have a greater resilience to some stresses than those of seedlings. Greater size can provide a greater reserve of carbohydrates, and larger roots may have a greater degree of insulation from increased soil temperatures simply due to size. Large roots may also contain and transport more water, which could

DISCUSSIONThe killing of root tips at 60 °C for 8 minutes suggests that the assumptions of earlier researchers about lethal high temperatures being in the plant cytoplasmic killing range of 50 to 60 °C are valid (Precht et al. 1973; Busse et al. 2005; Bar et al. 2019). These young, white root tips have neither the protective coverings nor the sclerophyllous tissues that allow the stems and leaves to withstand higher levels of stress. The good insulating properties of soil were confirmed by this research, as it was difficult to develop methods that would achieve high root temperatures (Preisler et al. 2000; Busse et al. 2005).

In heataffected seedlings, the tissues of both the root tips and sectioned stems turned brown, which may be indicative of the presence of a phenolic compound. In plants that resumed growth after treatment, the brown colour did not persist and was lost before the recommencement of root growth, which was an early indicator that growth was about to resume. The presence of the brown colour was not necessarily associated with tissue death, and future investigation of the degree of discolouration and tissue mortality would be worthwhile. Similar browning occurred in seedlings that were completely decapitated. Before recovery of the upper parts of the seedlings from

Figure 12. Dry matter production over a 10-week period by Eucalyptus obliqua seedlings heated for 2 minutes at different tempera-tures and decapitated either immediately or after a delay of 1 week. Number of replicates (N) = 10 for all treatments; the vertical bar shows LSD (P < 0.05) = 0.75.




dry matter production. The decapitation treatment a week after heat treatment is an added stress when seedlings are in the process of recovery, perhaps explaining the negative response. The arboricultural implication of these results is that in managing the urban forest, every effort must be made to avoid dou-ble dosing already stressed trees with another stress.

Immediately after treatment where the apical bud has been killed by heat or removed, there is a loss of apical control, and epicormic or lignotuberous shoots develop rapidly from nodal position 1 to 3, free of apical hormonal control (Meier et al. 2012). As shoots develop over the 10-week recovery period, apical control is reestablished, and the rapidly growing plants develop a classic conical shape consistent with the theories of apical dominance and control (Dun et al. 2006). While these experiments demonstrate that some of the effects of heat treatments can be explained in part by decapitation, there seem to be other factors at play. The immediate removal of heat-damaged tis-sue after heating resulted in enhanced height incre-ment, increased dry weight, and earlier commencement of shoot production, all of which are consistent with an inhibitor affecting seedling growth and the concept of translocated heat injury, in which heat damage to one part of a plant results in the pro-duction of inhibitors that are translocated to other parts, where they affect growth and development (Yarwood 1975; Bär et al. 2019). Four sensors have been suggested as triggers for heat stress responses (HSRs), including a plasma membrane channel for an inward calcium flux, a histone sensor in the nucleus, and unfolded protein sensors in the endoplasmic reticulum and cytosol, which activate HSR genes, leading to enhanced thermo tolerance, but the mecha-nisms of action are still unclear (Mittler et al. 2012). Immediate decapitation would remove inhibitors or phytotoxic degradation products and circumvent HSR, while delaying decapitation for a week would not.

It can be postulated that the roots hold the key to whole plant recovery. In some heat treatments, tem-peratures and/or durations were insufficient to directly damage lignotuberous tissues, but seedlings died. Sections of these plants showed undamaged lignotu-berous tissues and, in a few cases when roots failed to resume growth, it was discovered that there had been stem damage at the growing medium surface, below the lignotuber, and so whole plant recovery was impossible, as all lignotuberous and epicormic buds

increase the time taken before tissue injury occurs (Rasmussen et al. 2009; Varner et al. 2009; Clarke et al. 2013; Maringer et al. 2016; Bär et al. 2019).

The fact that root tip growth commenced before epicormic or lignotuberous bud development empha-sises the importance of a healthy root system. The arboricultural management principle of prevention being better than cure can be applied to stressed trees. Proactively managing for the best soil conditions possible prior to, during, and immediately after stress may be a key to successful shoot production and tree recovery. If the use of mulch, irrigation, decompac-tion, or a combination of treatments before and imme-diately after stress can enhance root tip survival, then a more rapid shoot development and higher rate of shoot survival may be achieved, enhancing the chances of tree survival.

The rapid rates of height increment and reestab-lishment of a leader after treatment were expected from earlier work (Moore 2015) and are largely explained by the removal of apical dominance (Rasmussen et al. 2009; Rasmussen and Hunt 2010), which is grad-ually reestablished over the recovery period of 10 weeks. The rapid rates of shoot growth allow for the reestablishment of photosynthetic capacity after defo-liation from stresses, such as fire, drought, and heavy grazing. Increases in height growth rates with some significant reductions in dry matter production after treatment suggest that the height increases were due to internodal elongation. This is consistent with hor-monal involvement in shoot production and can be interpreted as allowing the plants in their natural environment to resume competitive growth.

Trees that retain healthy, intact leaves after heat stress or decapitation had higher rates of shoot and overall plant survival. The production of photosyn-thate by leaves may be partly responsible for this improvement, but the differences were not signifi-cant, and this aspect of plant response require further research. For arborists managing stressed or damaged trees, parts of trees bearing healthy foliage should be left intact for as long as possible to allow more suc-cessful shoot production as part of post-stress recov-ery, even if they have to be removed later to ensure sound tree structure or human safety.

The dry weight data suggest that a double dose of stress (heat and delayed decapitation) reduces dry matter production, and that early removal of heat-damaged tissue both accelerates recovery and increases




some healthy foliage after being stressed are more likely to recover through successful shoot production, and so premature removal of trees or parts of trees bearing healthy foliage should be avoided during poststress cleanup activities.

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had been killed. Such plant deaths may be due to a failure of roots to resume growth or the effective heat girdling of the stem below the lignotuber, depriving roots of resources for growth.

These results suggest that before deciding on whether a tree is likely to survive after a severe stress, the condition of the root crown and roots should be ascertained if possible. If the tissue below the lignotuber is killed in young trees, there may be no buds available for recovery, and if the tissue of the root crown has been killed, then again, there may be no capacity for recovery. Such situations may occur after a bushfire or in an urban context when a young tree suffers trunk tissue necrosis under the lignotuber due to mower or line trimmer/brush cutter damage.

In the aftermath of stresses such as fires or floods, particularly in urban and periurban sites, soils are often overlooked. In the cleanup after such events, heavy vehicles are often used to clear debris and silt to make the sites safe and usable. The vehicles can compact soil, but so too can vibrations from smaller items of equipment (Trowbridge and Bassuk 2004; Watson 2006; Hascher and Wells 2007). This may lead to a second dose of stress being placed upon already seriously stressed specimens. Tree managers should ensure that soil conditions are optimised after stressful events and do their utmost to minimise inadvertent damage to root systems to enhance tree recovery through successful shoot growth.

CONCLUSIONWhile shoots may be initiated at more than one nodal position, the highest available healthy epicormic bud usually develops into a dominant shoot that reestablishes a single leader habit for seedlings. Lignotuberous buds at nodal positions 1 to 3 only develop when there are few, if any, epicormic buds available. Healthy root systems are crucial to overall tree health and vigour. After experiencing high levels of stress, the production of lignotuberous and epicormic shoots by trees depends on the resumption of root tip growth. Urban forest managers need to ensure that there are good soil conditions for the resumption of growth and avoid further stressing the tree by compacting soils in poststress cleanup operations. For trees of high historic, botanical, landscape, or cultural value, the removal of dead tissue may allow earlier and more successful shoot production, and the added cost could be justified for such significant trees. Trees that retain




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épicormiques et lignotubères. Les systèmes racinaires des semis furent inspectés afin de déterminer si l’extrémité des racines était en bonne condition alors que les pointes de certaines racines furent surveillées pour déterminer à quel moment elles avaient repris leur croissance, le cas échéant. Les taux de survie des pousses épicormiques et lignotubères ont été améliorés par la pré-sence de feuilles en santé. Après un stress, la reprise de croissance, par le développement de pousses épicormiques ou lignotubères, a été précédée par la croissance de l’extrémité des racines, ce qui fait valoir l’importance d’un système racinaire en santé. Favoriser la meilleure gestion possible des sols pendant et immédiatement après un stress peut s’avérer une clé pour la réussite de la généra-tion des pousses et la récupération des arbres.

Zusammenfassung. Die Triebe, die aus axillären, epikormi-schen und lignotuberösen Knospen gebildet werden, sind bei vie-len Baumarten ein wichtiger Bestandteil der Stressreaktion. Die Produktion von epikormischen oder lignotuberösen Trieben garantiert nicht das Überleben eines Baumes, da die Sterblichkeit der Triebe hoch ist. Diese Forschung untersuchte die Beziehung zwischen dem Wachstum der Wurzelspitzen und der Sprosspro-duktion und dem Überleben nach Stress und deren Auswirkungen auf städtische Baumpfleger. Sämlinge von Eucalyptus obliqua L’Herit. wurden durch Enthauptung oder verschiedene Stufen von Hitzestress bei Temperaturen von 40 °C bis 100 °C für 2 bis 128 Minuten gestresst, ebenso wie Kombinationen der beiden Stress-arten. Obwohl die Temperaturen nicht so hoch sind wie bei einem Waldbrand, können die auferlegten Belastungen Aufschluss über die Reaktionen der Pflanzen auf Stress wie Feuer geben. Niedri-gere Temperaturen und kürzere Dauern waren oft subletal, und die Enthauptung löste in gleichem Maße wie die Hitzeabtötung von Pflanzengewebe ein ähnliches Ausmaß an epikormischem und lignotuberem Sprosswachstum aus. Die Wurzelsysteme der Sämlinge wurden inspiziert, um festzustellen, ob die Wurzelspit-zen gesund waren, und ausgewählte Wurzelspitzen wurden über-wacht, um festzustellen, ob und wann sie das Wachstum wieder aufgenommen hatten. Die Überlebensrate von epikormischen und lignotuberösen Trieben wurde durch das Vorhandensein von gesunden Blättern erhöht. Der Wiederaufnahme des Wachstums nach Stress durch die Entwicklung von epikormischen oder lig-notuberösen Trieben ging ein Wachstum der Wurzelspitzen voraus, was die Bedeutung eines gesunden Wurzelsystems unterstreicht. Das Management für die bestmöglichen Bodenbedingungen während und unmittelbar nach dem Stress kann ein Schlüssel zur erfolgreichen Triebproduktion und Erholung des Baumes sein.

Resumen. Los efectos producidos a partir de brotes axilares, epicómicos y lignotuberosos son partes significativas de las respues-tas de recuperación del estrés en muchas especies de árboles. La producción de brotes epicómicos o lignotuberosos no garantiza la supervivencia de un árbol, ya que la mortalidad de los brotes es alta. Esta investigación investigó la relación entre el crecimiento de la punta raíz y la producción y supervivencia de brotes después del estrés y sus implicaciones para los administradores de árboles urbanos. Plántulas de Eucalyptus obliqua L’Herit. fueron estresadas por el descope a diferentes niveles de estrés térmico, a temperatu-ras que van desde 40 °C a 100 °C durante 2 a 128 minutos, así como combinaciones de las dos tensiones. Si bien las temperatu-ras no son tan altas como las experimentadas en un incendio for-estal, las tensiones impuestas pueden informar a las respuestas de

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G.M. Moore (corresponding author)School of Ecosystem and Forest SciencesUniversity of MelbourneBurnley Campus500 Yarra BoulevardRichmond, [email protected]

Conflicts of Interest:The author reported no conflicts of interest.

Résumé. Les pousses issues des bourgeons axillaires, épicor-miques et lignotubères sont des composantes significatives de la réponse au stress chez de nombreuses espèces d’arbres. La géné-ration de pousses épicormiques ou lignotubères n’assure pas la survie de l’arbre, car la mortalité de ces pousses est élevée. Cette recherche a examiné le rapport entre la croissance de la pointe des racines et la production ainsi que la survie des pousses après un stress et ses implications pour les gestionnaires d’arbres urbains. Des semis d’Eucalyptus obliqua L’Herit. ont été stressés par décapitation ou par différents niveaux de stress thermique à des températures variant de 40 °C à 100 °C pendant une durée de 2 à 128 minutes, ainsi que par des combinaisons de ces deux stress. Bien que les températures ne soient pas aussi élevées que celles rencontrées lors d’un incendie de forêt, les contraintes imposées peuvent renseigner sur les réactions des plantes à un stress de cette nature. Les températures plus basses et les durées plus courtes n’entrainaient souvent pas la mort tandis que la décapita-tion, au même titre que la destruction par chaleur des tissus végé-taux, a suscité des niveaux semblables de croissance de pousses




lignotuberosos se vieron reforzadas por la presencia de hojas sanas. La recuperación del crecimiento después del estrés por el desarrollo de brotes epicómicos o lignotuberous fue precedido por el crecimiento de la raíz, que hace hincapié en la importancia de un sistema radicular saludable. La gestión de las mejores condiciones del suelo posibles durante e inmediatamente después del estrés puede ser una clave para la producción exitosa de brotes y la recuperación de árboles.

las plantas al estrés, como el fuego. Las temperaturas más bajas y las duraciones más cortas eran a menudo sub-letales, y el descope, en la misma medida que la matanza por calor de los tejidos de las plantas, provocó niveles similares de crecimiento de brotes epicómicos y lignotuberosos. Los sistemas radiculares de las plantas fueron inspeccionados para determinar si las puntas de las raíces estaban sanas, y se monitorearon las puntas de raíz seleccionadas para determinar si habían reanudado el crecimiento y cuándo lo hicieron. Las tasas de supervivencia de brotes epicómicos y

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