European Journal of Forest Research

, Volume 133, Issue 1, pp 165–175

Survival, growth and physiological status of Acacia disparrima and Eucalyptus crebra seedlings with respect to site management practices in Central Queensland, Australia

Authors

    • Environmental Futures Centre, Griffith School of Biomolecular and Physical SciencesGriffith University
    • Faculty of Science, Health, Education and EngineeringUniversity of the Sunshine Coast
  • T. J. Blumfield
    • Environmental Futures Centre, Griffith School of Biomolecular and Physical SciencesGriffith University
  • Z. H. Xu
    • Environmental Futures Centre, Griffith School of Biomolecular and Physical SciencesGriffith University
Original Paper

DOI: 10.1007/s10342-013-0755-5

Cite this article as:
Hosseini Bai, S., Blumfield, T.J. & Xu, Z.H. Eur J Forest Res (2014) 133: 165. doi:10.1007/s10342-013-0755-5

Abstract

An improved understanding of important ecophysiological mechanisms underpinning tree water and nutrient use is necessary for developing and testing effective revegetation and restoration techniques in disturbed landscapes. A field trial was established in Central Queensland, Australia, to evaluate tree ecophysiological response to site management methods, including site preparation, herbicide application versus top soil removal (scalping) and fertilisation versus non-fertilisation. The influence of site management practices on plant survival, growth and foliar ecophysiological traits of Acacia disparrima (M. W. McDonald and Maslin) and Eucalyptus crebra (F. Muell.) seedlings was investigated within 22 months following tree planting. There was no difference in the survival of A. disparrima and E. crebra in response to the site preparation. However, there was a significant difference in growth response with both species showing greater mean periodic height gain in the herbicide areas compared to the scalped areas. Plant growth and survival of both species were unaffected by fertilisation, regardless of site preparation treatment. We suggest that the effects of fertiliser may have been masked by drought conditions experienced by seedlings in the first 6 months after planting. Neither site preparation nor fertilisation affected the leaf-level ecophysiological traits of seedlings, including foliar total N, photosynthetic capacity, instantaneous water-use efficiency, carbon isotope composition and stomatal conductance, irrespective of species. Scalping was more effective than herbicide application to suppress weeds and reduced the costs of site preparation and maintenance. Surprisingly, scalping had no impact on plant survival and foliar ecophysiological traits. However, it should be noted that the scalping may not be a sustainable practice in plantation establishment with short rotations where organic matter levels may not have a chance to recover between disturbances.

Keywords

Acacia disparrimaEucalyptus crebraEcophysiologyRevegetationSite management

Introduction

Tree cover is diminishing in Australia at a rate of between 0.5 and 2.5 % every year (Gibbons and Boak 2002; Saunders et al. 2003). The consequences of this loss are soil salinity (Barrett-Lennard 2002), erosion (Zuazo and Pleguezuelo 2008) and biodiversity decline (Lamb et al. 2005), which have led to increased ecosystem vulnerability under global climate change conditions. Revegetation schemes are part of the response to forest decline and are often a mandatory condition to infrastructure projects and mining operations. Revegetation decreases the risk of biodiversity loss (Lamb et al. 2005) and enhances carbon (C) sequestration (Laganiere et al. 2010; Wang et al. 2011), which could help to alleviate some of the negative impacts of global warming. One of the major challenges associated with revegetation schemes is to develop efficient and cost-effective methods for site establishment (Hobbs and Norton 1996).

The crucial factors in revegetation establishment are considered to be site preparation, weed control and fertilisation, each of which has an associated cost. All these factors may alter soil moisture, temperature and nutrient availability and consequently affect the early establishment success of revegetation ecosystems (Davis et al. 1999). Herbicides are considered to be the least-expensive method when compared with other techniques to control weeds, which include mulching, grubbing and cover crops (George and Brennan 2002). However, this technique is expensive when applied repeatedly before and after vegetation establishment. In the majority of revegetation schemes, the aim is to reduce the number of site visits to the absolute minimum as each successive visit gives an additional cost burden. There is also a growing concern in terms of environmental damage associated with herbicidal toxicity, particularly to soil organisms (Subhani et al. 2000; Ahemad and Saghir Khan 2009), and it may negatively impact young seedlings through misapplication and overspray. Despite issues associated with the use of chemicals, herbicide application is still commonly used in site preparation and weed control (Flint and Childs 1987; McDonald and Fiddler 1993; George and Brennan 2002; Graham et al. 2009; Ibell et al. 2010, 2013). Other, equally effective methods are needed to reduce the reliance on herbicide application.

Top soil removal or scalping is an alternative method to control weeds in revegetation sites (Harper et al. 2008; Graham et al. 2009). This treatment has been used for natural revegetation (Yildiz et al. 2007; Man et al. 2009), plantation (MacDonald and Thompson 2003) and grass establishment in woodlands (Cole et al. 2005) for a variety of species. The main issues associated with scalping include exposure of subsoil, which is less fertile compared with topsoil, and hardening of the soil surface after rain (Cole et al. 2005). On the other hand, scalping removes weeds and some of the soil seed bank, exposes mineral nutrients and enhances light availability. It may increase available soil moisture and temperature (Spittle house and Childs 1990). Such benefits could improve the early establishment of seedlings. Scalping can have conflicting effects on plant growth and survival, including positive (Yildiz et al. 2007), negative (Flint and Childs 1987; Gradowski et al. 2008) and no effect (MacDonald and Thompson 2003; Man et al. 2009). MacDonald and Thompson (2003) argued that pre-planting scalping may accelerate water loss depending on soil type and consequently negate the beneficial effects of weeding through scalping. In Western Australia, scalping has been applied within planting rows for direct seeding. However, there is little information available on the use of scalping to clear broad lanes for controlling weeds in the inter-row, as well as the planting row, as a viable alternative to herbicide application.

Fertilisers are applied to maintain proper growth and survival for seedlings (Forrester et al. 2010a). However, the response to fertilisation differs (Davis et al. 1999; Marcar et al. 2000; Graciano et al. 2005; Scowcroft et al. 2007) depending on species, site characteristics, water availability and site preparation practices (Forrester et al. 2010a). For instance, if the site is fertile (Bird et al. 2000), additional fertilisation may have no effect on plant growth. Despite contradictory observations, researchers have reported that the efficiency of fertilisation may increase when combined with weeding and site preparation, resulting in increased growth and survival (Marcar et al. 2000; Scowcroft et al. 2007). However, there is limited information on the outcomes of combining fertilisation with weed suppression in a revegetation ecosystem.

Fast-growing Eucalyptus spp. are suitable for use in mixed species plantings along with leguminous species such as Acacia. This combination of species has shown superior growth compared with monocultures (Forrester et al. 2006; Richards et al. 2010). Eucalyptus spp. mixed with Acacia spp. increased plantation productivity even under water deficits (Forrester et al. 2010b) and improved nitrogen (N) and phosphorous (P) cycling of the plantation (Forrester et al. 2006), leading to enhanced soil fertility (Khanna 1997). When legumes are part of the species mixture, there is evidence of accelerated C sequestration due to improved soil organic C (SOC) (Resh et al. 2002). Mixed species plantings incorporating legumes could decrease potential costs of fertilisation due to the leguminous N fixation contribution to soil N (Forrester et al. 2010b).

It has been well documented that leaf-level N is strongly correlated with photosynthetic capacity (Rosati et al. 1999; Huang et al. 2008a), which is largely influenced by environmental conditions including nutrient and water availability in soil (Huang et al. 2008a, b). One of the driving factors of photosynthetic capacity is CO2 concentration at the carboxylation site (Ci), which itself is influenced by stomatal conductance (gs). Plant CO2 uptake is closely linked to water and nutrient availability. Water limitation enhances stomatal closure influencing photosynthetic capacity and restricting nutrient flow from roots to shoots. A given amount of water loss through stomata due to CO2 uptake for photosynthesis is described as water-use efficiency (WUE) (Orchard et al. 2010); plants with low WUE may suffer from lower growth. Therefore, it is expected that scalping may decrease plant growth compared to herbicide application due to lower water and nutrient availability as a result of the top soil removal.

Whilst there have been many studies that have investigated the effects of weed control and fertilisation on growth and/or survival (Fleming et al. 1996; MacDonald and Thompson 2003; Graham et al. 2009), few have reported the ecophysiology of young seedlings, particularly under field conditions. Concurrent investigation of these factors may provide a better understanding of plant growth response to environmental conditions and may help to predict plant performance under different revegetation schemes. In a complementary research, we showed that weed recovery following scalping did not exceed more than 50 % of the ground cover. In contrast, multiple applications of herbicide failed to control weed competition as evidenced by 100 % weed recovery shortly after herbicide application (Hosseini Bai et al. 2012). Given that the knowledge of the effects of scalping on plant growth and ecophysiology is scant, this study aims to investigate plant growth, survival and ecophysiological responses to scalping compared to plants in the herbicide areas.

Materials and methods

Site description

The site was located at Stanwell (23°31′24″ S, 150°18′14″ E), approximately 25 km southwest of Rockhampton, Central Queensland, Australia. Before tree planting, the site was used to grow grapes, and afterwards it was turned to a poor-quality pasture. The soil was a sandy loam including 22 % clay, 12 % silt and 66 % sand content, and the pH was 5.7. Soil organic carbon (SOC) was 1.90 %. The average maximum monthly temperature 28 °C, from 23 to 34 °C, and rainfall 1800 mm were recorded for the period of this study (from June 2009 to April 2011) using a weather station installed at the site (Fig. 1).
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Fig. 1

Rainfall and maximum mean daily temperature during the period of study

The experiment was designed in a randomised complete block split plot with four blocks and four replicates per treatment in each block; in total, 32 plots were established. The plots were 12 × 12 m consisting of six planting rows, and on average, eight tube stock plants were planted in each row with approximately 1.5 m space between two plants. Each plot contained approximately 48 seedlings. Each planting row was ripped to a depth of 30 cm prior to tree planting. Three different species, A. disparrima M. W. McDonald and Maslin, E. crebra F. Muell. and Melalucaquinquenervia (Cav.) S. T. Blake, were randomly planted in each plot. However, in this study, only A. disparrima and E. crebra were investigated.

Site management practice used in this trial included two site preparation techniques (herbicide application and scalping) and two fertilisation regimes (with and without fertilisation). In the herbicide plots, glyphosate (present as potassium salt at 500 g L−1) diluted to 5 g L−1 was applied on two occasions before plantation establishment within 60 days (April and June 2009), and the follow-up spray after tree planting was a monocotyledon-specific herbicide called Verdict™ 520 with the active ingredient of Haloxyfop applied at the rate of 150 mL ha−1. In the scalped areas, approximately 10 cm of top soil was removed prior to the vegetation establishment and exported from the experimental site. Fertiliser used in this trial was a slow-release NPK (21–5–12 %) fertiliser and applied at the establishment of the plots in June 2009. The fertiliser was placed around each individual plant, approximately 44 g per seedling.

Survival, growth and gas exchange measurements

Survival and plant height were recorded at three sampling periods in December 2009, August 2010 and April 2011, 6, 14 and 22 months after the plot establishment, respectively. All seedlings for all plots were counted for growth and survival, in total, 32 plots containing an average of 30 seedlings of both plant species. The mean periodic height gain (MPHG) was calculated by subtracting height of plants at two consecutive sampling periods. Gas exchange was determined using three north-facing leaves of five plants per plot of each species using a portable photosynthesis system (Model LI-6400, Li-Cor) maintaining a constant CO2 concentration of 380 μmol mol−1 and a blue–red light-emitting diodes (Model 6400-02B) adjusted at photosynthetically active radiation (PAR) 1,400 μmol s−1. The flow rate was adjusted in 500 μmol s−1, and chamber humidity was kept about 60 %. In total, 320 seedlings were used to measure their gas exchange. All measurements of photosynthetic capacity at PAR 1,400 μmol s−1 (A1400), CO2 concentration at the carboxylation site (Ci) and stomatal conductance (gs) were taken on sunny days between 09:00 and 12:00 in December 2009 and August 2010. Instantaneous water-use efficiency (iWUE) at leaf level was determined as A1400/E (μmol mmol−1) (Farquhar and Richards 1984).

Foliar total N concentration (TN), N isotope composition (δ15N) and C isotope composition (δ13C)

After measuring gas exchange, all three fully expanded leaves of each plant, which were used to measure gas exchange, were collected. The leaves were kept in separate paper bags and transferred to the laboratory. Samples were then oven-dried at 50 °C to a constant weight and ground to fine powder by a Rocklabs™ ring grinder, and the homogenised powder was transferred into 8 × 5 mm tin capsules for analysis using an isotope ratio mass spectrometer (GV Isoprime, Manchester, UK) to determine foliar TN, δ15N and δ13C as reported previously (Xu et al. 2000, 2003).

Statistical analysis

Analysis of variance (ANOVA) was conducted to detect the plant response to site preparation methods, fertilisation regimes and sampling period for each species. The Tukey HSD test at P < 0.05 was used to determine a comparison between treatment means. Statistics software (version 8) was used for all the statistical analyses.

Results

Survival

The survival of both species was not significantly affected by site preparation or fertilisation with survival of A. disparrima, ranging from 59.8 to 76.4 %, whereas that of E. crebra varied from 83.5 to 86.6 % (Fig. 2). Survival of A. disparrima was significantly lower than that of E. crebra (P < 0.05).
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Fig. 2

Survival of Acacia disparrima (white columns) and Eucalyptus crebra (black columns) seedlings in August 2010 in response to the herbicide + fertilisation (HF+), herbicide + non-fertilisation (HF−), scalping + fertilisation (SF+) and scalping + non-fertilisation (SF−) in a revegetated ecosystem located at Stanwell, Central Queensland, Australia

Mean periodic height gain (MPHG)

Both A. disparrima and E. crebra had significantly higher MPHG in the herbicide area than in the scalped area at both sampling periods (P < 0.05; Table 1). Fertilisation did not affect plant MPHG at both herbicide and scalped areas, irrespective of species. Interaction between site preparation and fertilisation was not significant. Our data suggested a steady increase in plant height gain, irrespective of the species throughout the period of study, in all treatments (P < 0.05, Table 1).
Table 1

Mean periodic height gain (MPHG) of Acacia disparrima and Eucalyptus crebra seedlings in response to different management practices in a revegetated ecosystem located at Rockhampton, Central Queensland, Australia

MPHG (cm)

Site preparation

Fertilisation

Aug/2010–Dec/2009

Apr/2011–Aug/2010

A. disparrima

 Herbicide

Fertilised

67.7 (3.9)a

120 (5.3)a

Non-fertilised

82.7 (5)a

108 (5)a

 Scalping

Fertilised

43.9 (5)b

106 (7.8)b

Non-fertilised

43.5 (3.8)b

104 (6.6)b

E. crebra

 Herbicide

Fertilised

46.2 (2.5)a

86.7 (3.8)a

Non-fertilised

39.1 (2.7)a

91.5 (4)a

 Scalping

Fertilised

28.3 (1.9)b

68.4 (3.9)b

Non-fertilised

23.4 (1.7)b

67.0 (3.7)b

Means followed by the different lower-case letters for each species demonstrate difference in main factor (site preparation) at the level P < 0.05. Each paired bold mean in the same row indicates the significant difference in the sampling period at the level P < 0.05. Parentheses present mean standard errors

Foliar total N (TN) and δ15N

In both December 2009 and August 2010, there was no significant difference in foliar TN of A. disparrima and E. crebra between the site management practices, including site preparation and fertilisation (Tables 2, 3). Foliar TN of both species was significantly lower in August 2010 compared to December 2009, regardless of site management practices (P < 0.05, Table 2).
Table 2

Foliar total N (TN) and N isotope composition (δ15N) of Acacia disparrima and Eucalyptus crebra seedlings in response to different management practices in a revegetated ecosystem located at Stanwell, Central Queensland, Australia

Parameters

TN (%)

δ15N (‰)

Site preparation

Fertilisation

Dec/2009

Aug/2010

Dec/2009

Aug/2010

A. disparrima

 Herbicide

Fertilised

2.02 (0.06)

1.80 (0.1)

2.2 (0.2)

0.8 (0.3)

Non-fertilised

1.98 (0.03)

1.65 (0.06)

2.6 (0.4)

-0.2 (0.3)

 Scalping

Fertilised

1.94 (0.03)

1.63 (0.12)

1.9 (0.3)

1.6 (0.6)

Non-fertilised

1.85 (0.03)

1.65 (0.07)

2.7 (0.3)

1.0 (0.5)

E. crebra

 Herbicide

Fertilised

2.07 (0.05)

1.58 (0.06)

2.8 (0.2)

2.4 (0.4)

Non-fertilised

2.02 (0.04)

1.70 (0.08)

3.2 (0.3)

2.8 (0.5)

 Scalping

Fertilised

2.07 (0.03)

1.77 (0.07)

2.4 (0.2)

3.2 (0.3)

Non-fertilised

1.95 (0.03)

1.44 (0.06)

3.6 (0.4)

3.1 (0.3)

Means followed by the different lower-case letters for each species demonstrate difference in main factor (site management) at the level P < 0.05. Each paired bold mean in the same row indicate the significant difference in the sampling period at the level P < 0.05

Table 3

Probability from ANOVA for mean periodic height gain (MPHG), foliar total N (TN), N isotope composition (δ15N), photosynthetic capacity (A1400), stomatal conductance (gs), instantaneous water-use efficiency (iWUE), C isotope composition (δ13C) and internal CO2 concentration (Ci)

Sources

df

Variables

MPHG

TN

δ15N

A1400

gs

iWUE

δ13C

Ci

A. disparrima

 Site preparation

1

<0.0001

0.266

0.287

0.287

0.988

0.214

0.791

0.137

 Fertilisation (F)

1

0.938

0.453

0.872

0.695

0.988

0.604

0.826

0.494

 Sampling period

1

<0.0001

0.001

0.0003

<0.0001

<0.0001

<0.0001

0.546

0.001

 Site preparation × F

1

0.725

0.741

0.598

0.440

0.509

0.784

0.801

0.850

 Site preparation × sampling period

1

0.004

0.871

0.161

0.311

0.676

0.201

0.015

0.653

 F × sampling period

1

0.075

0.982

0.082

0.795

0.887

0.794

0.09

0.967

 Site preparation × F × sampling period

1

0.109

0.546

0.972

0.995

0.553

0.958

0.920

0.480

E. crebra

 Site preparation

1

<0.0001

0.632

0.429

0.410

0.607

0.407

0.292

0.284

 Fertilisation (F)

1

0.34

0.172

0.181

0.508

0.557

0.918

0.289

0.327

 Sampling period

1

<0.0001

<0.0001

0.712

0.001

<0.0001

<0.0001

<0.0001

<0.0001

 Site preparation × F

1

0.648

0.059

0.730

0.858

0.693

0.398

0.623

0.759

 Site preparation × sampling period

1

0.308

0.971

0.457

0.173

0.199

0.510

0.792

0.319

 F × sampling period

1

0.089

0.899

0.354

0.548

0.636

0.702

0.705

0.571

 Site preparation × F × sampling period

1

0.355

0.178

0.360

0.908

0.831

0.760

0.784

0.725

Species

1

<0.0001

0.826

<0.0001

0.089

0.137

0.293

0.027

0.751

 Species × sampling period

1

0.119

0.185

0.009

<0.0001

0.210

<0.0001

<0.0001

<0.0001

Bold probabilities present significance at the level P<0.05

There was no significant difference in foliar δ15N for either species in response to the site preparation or fertilisation in both sampling periods (Tables 2, 3). Foliar δ15N of A. disparrima was significantly lower in August 2010 compared to December 2009 (P < 0.05, Tables 2, 3), but there was no significant difference in foliar δ15N of E. crebra in August 2010 compared to the December 2009. Interaction between plant species and sampling month was significant (P < 0.05, Table 3).

A1400 and instantaneous water-use efficiency (iWUE)

In both sampling periods, there was no significant difference in foliar A1400 of A. disparrima and E. crebra between site management practices (Fig. 3a, b; Table 3). A1400 of both species was significantly higher in August 2010 compared to December 2009. Interaction of A1400 between species and season was also significant (P < 0.05; Table 3).
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Fig. 3

A1400 and instantaneous water-use efficiency (iWUE) of Acacia disparrima (a, c) and Eucalyptus crebra (b, d) seedlings in response to the herbicide + fertilisation (HF+, black columns), herbicide + non-fertilisation (HF−, grey columns), scalping + fertilisation (SF+, white columns) and scalping + non-fertilisation (SF−, hatched columns) in summer (December 2009) and winter (August 2010)

Instantaneous water-use efficiency (iWUE) of both species was affected neither by the site preparation methods nor by fertilisation (Fig. 3c, d), and iWUE of both species in all site management practices significantly increased in August 2010 compared to December 2009 (P < 0.05; Table 3). Interaction between species and sampling month was significant, regardless of site management practices (P < 0.05; Table 3).

Foliar C isotope composition (δ13C), CO2 concentration at the carboxylation site (Ci) and stomatal conductance (gs)

In both sampling periods, there was no significant difference in δ13C of A. disparrima and E. crebra between all site management practices (Fig 4a, b; Table 3). The δ13C of A. disparrima did not differ significantly between August 2010 and December 2009, regardless of the treatments (Table 3). Interaction between site preparation and sampling month was significant in the δ13C of A. disparrima. In contrast, δ13C of E. crebra was significantly lower in August 2010 compared to December 2009 in all treatments (P < 0.05; Fig. 4a, b and Table 3). There was a significant interaction between species and sampling period (P < 0.05; Table 3).
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Fig. 4

Foliar C isotope composition (δ13C), CO2 concentration at the carboxylation site (Ci) and stomatal conductance (gs) of Acacia disparrima (a ,c, e) and Eucalyptus crebra (b, d, f) seedlings in response to the herbicide + fertilisation (HF+ , black columns), herbicide + non-fertilisation (HF−, grey columns), scalping + fertilisation (SF+ , white columns) and scalping + non-fertilisation (SF−, hatched columns)

Foliar CO2 concentrations at the carboxylation site (Ci) of both species were not significantly different between all site management practices at both sampling periods. The Ci significantly increased in August 2010 compared to that of December 2009, regardless of the treatments and species (P < 0.05, Fig. 4c, d). There was a significant interaction between species and sampling period (P < 0.05; Table 3). Stomatal conductance (gs) was not significantly different between the site management practices for either species (Fig. 4e, f) though gs was significantly higher in August 2010 than in December 2009 for both species, irrespective of treatments (P < 0.05).

Discussion

In our experiment, there was no significant difference in plant survival as a response to the site preparation or fertilisation. Other researchers have reported that weed competition may increase mortality in young plants due to the competition for limited resources (Rey Benayas et al. 2003, 2005). Weed coverage was significantly higher in the herbicide area than in the scalped areas (100 and 50 %, respectively) (Hosseini Bai 2012). However, the scalped area had significantly lower soil moisture compared to the herbicide area (Hosseini Bai et al. 2012). Therefore, whilst plants in the herbicide area may have benefitted from higher soil moisture availability, plants in the scalped area had the benefit of less competition.

Our results were also consistent with those who observed no effect of fertilisation on survival of three species of eucalypts in a plantation in Western Australia (Ritson et al. 1991) and on A. stenophylla and E. camaldulensis in a plantation in New South Wales, Australia (Marcar et al. 2000). Fertilisation may affect growth rather than plant survival. The survival of acacia, for all site management practices, was significantly lower than that of Eucalyptus, suggesting that Acacia may be more susceptible than Eucalyptus to the drought conditions that occurred during early seedling establishment. Environmental conditions in the first growing season are important factors influencing plant survival, and drought has been shown to increase plant mortality. Weed competition and environmental stresses, including low water availability, increased the mortality of Quercus spp. in a plantation in Spain (Rey Benayas et al. 2005).

Plants in the herbicide areas had stronger MPHG than those of the scalped plots, regardless of sampling period (P < 0.05; Table 1) and irrespective of species. MacDonald and Thompson (2003) and Man et al. (2009) used spot scalping to regenerate a mixed species by planting Picea glauca and Pinus banksiana in Canada. Those authors found that spot scalping did not increase plant growth. Scalping may accelerate water loss compared to herbicide application (Flint and Childs 1987) and also negatively impact soil organisms due to reduced soil organic matter (SOM) following top soil removal (Mallik and Hu 1997; Hosseini Bai et al. 2012). In our study, the scalped areas were significantly drier than the herbicide areas (Hosseini Bai et al. 2012) due to higher exposure to sun. The significantly lower SOM and soil microbial activity in the scalped areas reported in a previous study (Hosseini Bai et al. 2012) indicate lower soil fertility, partly a result of the treatment itself but may also have been caused by the slower development of plants in the scalped areas compared to the herbicide areas with consequently lower nutrient cycling.

Despite the fact that the growth differences between herbicide and scalped areas were lower over time, still both A. disparrima and E. crebra showed significantly higher growth rate in the herbicide areas compared to those of the scalped areas. Plants in scalped areas showed faster growth than those of the herbicide areas, and difference in height gain of plants in the scalped and herbicide areas declined over time (Table 1). Scalping may decrease plant growth as soil is more exposed to sun and consequently higher soil temperatures, exacerbating water availability problems (Castro et al. 2002; Gunter et al. 2009). There was an indication of soil fertility improvement in the scalped areas 3 years after tree planting particularly in the soil profile (unpublished data), suggesting incorporation of plant litter in soil over the period of time may accelerate height gain in the scalped areas. Such an improvement in plant growth in the scalped areas may suggest that the differences in plant growth between herbicide and scalping would be insignificant over time.

Fertilisation did not influence plant growth in both herbicide and scalped areas. Weeding decreases potential competitors and fertilisation reduces nutrient deficiency, both of which may improve the growth of eucalypts as indicated by other studies (George and Brennan 2002; Forrester et al. 2010a). Otsamo et al. (1995) investigated the height gain of A. mangium with respect to herbicide application with and without fertilisation (NPK) in Indonesia 6 years following plantation establishment and observed an increase in the height of A. mangium in herbicide with fertilisation compared to the herbicide without fertilisation. In their study, the fertiliser was applied on two occasions, 1 and 18 months, after tree planting. Our study site received average 800 mm rain per year and had two distinctive dry (June–September) and wet (December–March) seasons. The site was predominately dry within first 6 months following tree planting (Fig. 1), and the occasional high-intensity showers (occurred at months 5 and 6 following tree planting, Fig 1) generated runoff, rather than infiltrating the soil profiles, which may suggest that the plants may not have been able to access the fertiliser at the critical phase of growth.

Lack of significant response of leaf-level ecophysiological traits to either site preparation or fertilisation was inconsistent with a study that reported an improvement in leaf-level ecophysiological traits, including foliar N and gas exchange in the presence of weed control and nutrient inputs through mulching and/or fertilisation, which led to reduced competition and improved nutrient and water availability (Pinkard 2003; Huang et al. 2008a). Our study was consistent with other studies that found no effect of weed control and/or fertilisation on leaf-level ecophysiological traits despite the fact that growth was affected (Munger et al. 2003; Allen et al. 2005; Eyles et al. 2012). Eyles et al. (2012) investigated foliar N and gas exchange of a seven-month-old E.globulus and showed that foliar photosynthesis was not affected by weed control despite the superior growth of plants in the weed control areas. Increased leaf area and canopy size in the presence of weed control and/or fertilisation may be responsible for greater light capture and growth gain of plants (Munger et al. 2003; Allen et al. 2005; Eyles et al. 2012). In our study, a significant higher specific leaf area was observed in the herbicide areas compared to the scalped areas at both species (data not presented), which may be responsible for plant superior height gain in the herbicide areas.

Although ecophysiological traits of both species were not responsive to the site management practices, there was a significant response in ecophysiological traits to the sampling period. A1400 and iWUE were higher in August 2010 compared to December 2009. Our results for photosynthesis were consistent with other studies undertaken in Australia, including eucalypts in a plantation measured in December and photosynthetic capacity varied between 6 and 8 μmol m−2 s−1 in Queensland, Australia (Huang et al. 2008a), and in a mixed species of 50A:50E, A. mearnsii and E. globulus had on average 10 and 14 μmol m−2 s−1 maximum photosynthesis, respectively, measured in September in Victoria, Australia (Forrester et al. 2012). We believe that air temperature was one of the driving factors for A1400 in the sampling periods. The average daily temperature in December 2009 was between 33 and 35 °C, which may have limited photosynthesis. Research has shown that this temperature can decrease photosynthesis significantly (Prior et al. 1997). However, in August 2010, the average daily air temperature was 24 °C, providing an optimum temperature for photosynthesis. The experimental site was also very dry in December 2009, soil moisture was less than 2 % at both herbicide and scalped areas, and no significant difference in soil moisture was observed between herbicide and scalped areas in December 2009 (Hosseini Bai et al. 2012). Dry condition and high temperature in December 2009 can explain the fairly low A1400 in this season. An increase in iWUE in August 2010 for both species compared to December 2009 may be due to improved nutrient availability in August 2010. Most of the weeds in the research area are annual with considerable weed die-back occurring in the colder, drier winter months, which may have increased resource availability in August 2010 affecting iWUE. Research has shown that increased nutrient availability increases iWUE because photosynthetic capacity improves whilst transpiration remains unchanged (Hobbie and Colpaert 2004; Forrester et al. 2010b).

There was also a significant interaction between species and sampling period for A1400, iWUE and δ13C (P < 0.05; Table 3). In December 2009, A1400 and iWUE of E. crebra were higher than those of A. disparrima for all site management practices. In December 2009, plants were under stress and struggled with drought conditions and high temperature, low nutrient availability and weed competition (Fig. 1). This suggests that E. crebra may be able to alter the strategy of water use with drought stress, using this factor as a tool of adaptation especially when the climatic condition is uncertain. In contrast, A. disparrima showed greater A1400 and iWUE than E. crebra in August 2010 when the weather conditions, especially temperature, were gentler and plants had outgrown the weeds.

No significant response in plant water-use efficiency to the site preparation for either plant species was indicated by both δ13C and iWUE. The δ13C is used to indicate time-integrated impacts of the treatments on plants (Xu et al. 2000, 2003; Choi et al. 2005; Huang et al. 2008a,2008b), because this value is linearly linked to Ci/Ca ratio, where Ci and Ca are the partial pressure of CO2 in the leaf and atmosphere, respectively (Farquhar et al. 1989). The Ci itself is influenced by stomatal conductance (gs), which can be regulated by water availability; thus, these two factors could drive the values of δ13C, and lack of significant response of plant species to leaf-level Ci and gs for both site preparation practices may suggest that water used by plants irrespective of the treatments was similar (Huang et al. 2008a).

It has been well documented that foliar isotope N composition (δ15N) is sensitive to soil N availability because plants uptake N from soil (Ibell et al. 2013; Huang et al. 2008a; Xu et al. 2000). Soil δ15N enrichment suggests N loss from the ecosystem because most mechanisms, including leaching, microbial activity and volatilisation discriminate against the heavier N isotope, and therefore, the soil 15N signals tend to be enriched due to N loss (Vitousek et al. 1992). Our previous study showed no significant difference in soil δ15N between scalped and herbicide areas, leading to the conclusion that both treatments were subject to N loss (Hosseini Bai et al. 2012). Lack of significant difference in foliar δ15N in response to scalping and herbicide application at both species reflected soil N availability in plant tissue.

Conclusions

Scalping did not have any implications for plant survival and leaf-level ecophysiological traits despite removing the most biologically active layer of the top soil. However, plant growth was superior in the herbicide areas for both species. It should be noted that scalping may not be a sustainable practice in plantation establishment with short rotations where SOM levels may not have a chance to recover between disturbances. The effects of fertilisation were also negligible on plant growth, survival and leaf-level ecophysiological traits for either species. Drought was considered to be the main reason for the plants’ inability to access fertiliser at this early establishment phase. It is interesting to note that despite the lack of early weed control in the herbicide treatment, seedlings in this treatment showed increased growth responses compared to the scalping treatment, suggesting that these tree species, especially A. disparrima, have some capacity to tolerate a certain level of weed competition, at least during early establishment.

Acknowledgments

This research was supported by Powerlink QLD through the provision of a full-time research scholarship. We thank Ms. Marijke Heenan and Dr. Elizabeth Gordon for their technical supports and Dr. Fangfang Sun, Dr. Lu Shumbao, Dr. Yichao Rui and Mr. Kadum Abdullah for their assistance in the field work. Z. H. X. received the funding support from the Australian Research Council.

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© Springer-Verlag Berlin Heidelberg 2013