Managing productivity and drought risk in Eucalyptus globulus plantations in south-western Australia

Forest Ecology and Management 259 (2009) 33–44

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Managing productivity and drought risk in Eucalyptus globulus plantations in south-western Australia Donald A. White a,*, D. Stuart Crombie b, Joe Kinal c, Michael Battaglia a, John F. McGrath b, Daniel S. Mendham a, Scott N. Walker d a

CSIRO Sustainable Ecosystems, Private Bag 5, Perth, WA 6913, Australia Forest Products Commission, Level 1, 117 Great Eastern Highway, Rivervale, WA 6103, Australia Department of Environment and Conservation, Del Park Road, Dwellingup, WA 6213, Australia d Astron Environmental Services, PO Box 426, Leederville, WA 6903, Australia b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 July 2009 Received in revised form 22 September 2009 Accepted 22 September 2009

More than 2.5 million ha of Eucalyptus globulus are now planted across the globe including approximately 500 000 ha in southern Australia. In this region average annual rainfall has declined since 1960 and this trend is predicted to continue in the coming decades. E. globulus is a premium species for paper manufacture and grows well under moderate seasonal water stress. The traits that underpin this rapid early growth also make the species vulnerable to prolonged water stress. We established nitrogen rate and nitrogen-by-stocking experiments in five 2-year-old E. globulus plantations along a climatic gradient in south-western Australia. We measured volume growth, predawn leaf water potential and leaf area index over 7 years or until the plantations were 9 years old. These data were used to explore the relationship between growth and water stress, to understand the mechanistic basis for the relationship and to identify best-bet management strategies for E. globulus plantations in southern Australia. Nearly all of the variation in volume growth rate between sites could be explained by a combination of climate wetness index and soil depth. There was a significant growth response to nitrogen at two low rainfall and one high rainfall site. There was no growth response to nitrogen on sites where total soil nitrogen in the top 0.1 m of soil was more than 1.9 mg g1 and a very rapid increase in relative growth response below this threshold. The observed growth response to nitrogen was associated with an increase in water stress and on at least one site increased mortality. Matching the supply of nutrients to demand will maximise the growth at any site but this may increase the risk of drought death at waterlimited sites. This will be exacerbated if forecasted changes in the climate of southern Australia are realised. Thinning to 600 stems ha1 significantly reduced the level of water stress experienced by E. globulus in Western Australia without significantly affecting end of rotation stand volume compared to unthinned stands. These results indicate that for a range of sites in south-western Australia a final stocking density of 600 stems ha1 coupled with application of fertiliser to maximise growth will minimise risk without sacrificing any of the site potential. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Leaf water potential Thinning Stocking density Nitrogen Climate change Sustainable production Growth efficiency Leaf area index

1. Introduction Eucalyptus globulus Labill. is a globally important plantation species. Approximately 2.5 million ha have now been planted (Louppe et al., 2008), primarily in temperate regions with winter dominant rainfall including the Iberian Peninsula, California and southern Australia. In southern Australia the area of the E. globulus estate has increased rapidly since 1995 to more than 500 000 ha

* Corresponding author. Tel.: +61 8 9333 6693; fax: +61 8 9387 8991. E-mail address: [email protected] (D.A. White). 0378-1127/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2009.09.039

(Parsons et al., 2006). This includes 250 000 ha in south-western Australia where the rainfall distribution is winter dominant; more than 80% of the annual rainfall occurs between May and October (Gentilli, 1989). In south-western Australia rainfall since 1960 has been 20% lower than the 1900–1960 average and further reduction is predicted over the coming decades (Li et al., 2005). In these environments, soil stored water is important for buffering plantations against the effects of summer drought (Butcher and Havel, 1976; Butcher, 1977; Edwards and Harper, 1996). E. globulus grows rapidly under moderate water stress. Physiological characteristics that facilitate this growth include maintenance of a high leaf area to sapwood area ratio (White et al.,

D.A. White et al. / Forest Ecology and Management 259 (2009) 33–44

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experiments established across a climatic gradient in southwestern Australia.

1998) and relatively high leaf conductance during drought (White et al., 1999). These traits render E. globulus vulnerable to prolonged or extreme water stress (White, 1996); significant areas of E. globulus plantations in south-western Australia died during the drought of 1994–1995 after which soil depth was included as a key criterion in site selection. Continued cultivation of E. globulus in Mediterranean environments will require an understanding of the relationship between growth and drought risk and a capacity for managing this through tree breeding or silvicultural techniques, including thinning and management of site fertility. Varying stand density either at planting or by thinning is one silvicultural option for moderating the effects of seasonal drought in E. globulus plantations. Thinning or reducing planting density will increase the volume of soil available to each tree and can reduce the amount of water stress experienced by retained trees (Butcher and Havel, 1976; Donner and Running, 1986; Cregg et al., 1990; Breda et al., 1995). In the overwhelming majority of published studies, diameter and volume growth of individual trees is greater at lower stand density (e.g. Ginn et al., 1991; Schonua and Coetzee, 1989; Goncalves et al., 2004). This has been attributed to increased radiation interception per tree and reduced water stress, resulting in a lengthened growing season (Butcher and Havel, 1976; Laurent et al., 2003). Laurent et al. (2003) argued that thinning was a mechanism for drought-proofing stands of Picea abies L. (Karst.). Many studies show that, although thinning or reduction in stocking density increased tree size, they may reduce yield at the stand-scale (Schonua and Coetzee, 1989 in a review of studies in Eucalyptus plantations). However, although there is a strong, positive relationship between stand density and stand volume growth in wet or summer rainfall dominated environments (Schonua and Coetzee, 1989), there is good evidence that total water supply constrains the stand-scale response to increased stand density in water-limited environments (Butcher and Havel, 1976; Hasenauer et al., 1997; Will et al., 2001). Lower stocking may therefore reduce the risk of mortality in water-limited environments without compromising total volume production. In south-western Australia nearly all E. globulus plantations are established on ex-agricultural land where available nutrients, particularly phosphorous and nitrogen, have accumulated (O’Connell et al., 2004). However, these soils are inherently infertile and available nitrogen and phosphorous have halved over the first rotation of E. globulus (Mendham et al., 2004). Where nitrogen or phosphorous are limiting, their addition increases forest growth (Cromer and Williams, 1982; Bennett et al., 1997; Smethurst et al., 2003). It is therefore likely that in second and subsequent rotations, managing nutrient supply will be critical to sustaining yield and may offer a mechanism for managing the relationship between production and drought risk. This paper tests the hypothesis that there is a predictable relationship between growth and the development of water stress in E. globulus plantations that may be managed by thinning and the application of nitrogen. To do this we quantify the effect of thinning and application of nitrogen fertiliser on temporal patterns of growth, plantation water status and growth efficiency (the ratio of volume growth to leaf area index, Waring et al., 1981) in five

2. Materials and methods 2.1. Site descriptions All of the measurements reported were made in five E. globulus plantations (see Fig. 1 for locality map). At each site, nitrogen rate and nitrogen-by-stocking experiments were established at age 2 years. Three of the sites, at Scott River, Wellstead and Boyup Brook were planted in July 1996 and were selected to represent the climatic range of the E. globulus plantation estate in south-western Australia (Table 1). Additional sites, at Narrikup and Perup, were planted in 1997 and represent intermediate climate wetness (Table 1). At Perup, seedlings were planted 2 m apart in rows separated by 5 m (1000 stems ha1, Table 1) while, at all other sites, planting rows were 4 m apart (1250 stems ha1). Prior to planting, rows were ripped to a depth of 0.5 m, weeds were sprayed with glyphosate for 1 m either side of the planting line. Immediately after planting, N (40 kg ha1) P (10 kg ha1) and K (15 kg ha1) were applied in tablet form to each tree. Soil depth varied from 5 m to more than 8 m at Scott River, Boyup Brook and Narrikup and was greater than 8 m throughout Wellstead and Perup (see note on how this was determined under next section). All sites had a sandy A-horizon, and showed evidence of laterite in the top 2 m with clay sub-soils. The site at the Scott River was selected as it matched the soils at the other

Fig. 1. Map showing the location of the five field sites in south Western Australia and average annual rainfall and evaporation from 1901 to 2000.

Table 1 Site descriptions including latitude, longitude, year of planting, year of treatment, average annual rainfall (mm), climate wetness index (CWI) and stocking density at the time of treatment. Name

Latitude

Longitude

Stocking density (stems ha1)

Annual rainfall (mm)

CWI

Scott River (SR) Wellstead (WS) Boyup Brook (BB) Perup (PR) Narrikup (NK)

340 1700 340 3500 330 3800 330 4000 340 3700

1150 2900 1180 3300 1160 3400 1160 2800 1160 0800

1145 1131 934 948 1020

1082 591 620 750 720

0.88 0.38 0.43 0.51 0.54

S S S S S

CWI, ratio of annual rainfall to annual potential evaporation.

E E E E E

D.A. White et al. / Forest Ecology and Management 259 (2009) 33–44 Table 2 Total soil N (mg g1), organic C (mg g1), electrical conductivity (Ec, mS m1) and pH (0–0.1 m) at the five sites. Site

Total N (mg g1)

Organic carbon (mg g1)

C:N

Ec (mS m1)

pH

Scott River Wellstead Boyup Brook Perup Narrikup

1.1 2.0 0.8 1.9 2.6

32.6 12.9 15.3 46.7 30.3

29.6 6.5 19.1 24.6 11.7

3.47 3.24 2.51 5.22 5.61

4.3 4.0 4.5 4.5 4.2

four sites but was not typical of the extensive, shallow podsolised duplex soils of this area, which are subject to winter water logging. In August 2000, soil cores were collected to 0.1 m depth from five locations in each of the untreated plots at each site. The samples from each plot were pooled and sub-sampled. Total nitrogen was determined by a micro Kjeldahl method (McKenzie and Wallace, 1954), organic carbon was determined by the technique of Walkley and Black (1934) and pH was measured following equilibration of a 1:5 soil: 0.1 M CaCl2 solution (Rayment and Higginson, 1992). Soil salinity was assessed as the electrical conductivity of a 1:5 soil to water extract (Rayment and Higginson, 1992) (Table 2). 2.2. Experimental design Between July and October 1998, when the trees were approximately 2 years old, a nitrogen rate and nitrogen-bystocking experiment were established at each of Scott River, Wellstead and Boyup Brook. Treatments in the nitrogen-by-stocking trial included three stocking rates (300, 600 and 1250 (i.e. unthinned) stems ha1) and two rates of nitrogen application (0 and 250 kg N ha1 year1) in a factorial design giving six plots in each of three replicate blocks. The stocking treatments are subsequently referred to as low (300 stems ha1) and medium stocking (600 stems ha1) or unthinned (1250 stems ha1) and the nitrogen treatments as either fertilised (250 kg N ha1 year1) or un-fertilised (0 kg N ha1 year1). In the nitrogen rate trial, N was applied at four rates (0, 45, 125, 250 kg N ha1 year1) to unthinned plots at all sites. At Scott River and Wellstead an additional 400 kg N ha1 year1 treatment was included. The 0 and 250 kg N ha1 year1 unthinned plots were common to both experiments giving a total of 9 plots per replicate block at Scott River and Wellstead and 8 plots per replicate at Boyup Brook. A year later, in 1999, similar pairs of experiments were established at Narrikup and Perup, but without the 600 stems ha1 stocking treatment. Each plot was 40 m  40 m or 10 rows  20 trees (8 rows  20 trees at Perup) with an internal measurement plot of 20 m  22 m. The average stocking at the time of treatment was 1145 stems ha1 at Scott River, 1131 stems ha1 at Wellstead, 934 stems ha1 at Boyup Brook, 1020 stems ha1 at Narrikup and 1048 stems ha1 at Perup (Table 1). Soil depth was determined at opposite corners of each growth plot using a drill rig. 2.3. Treatments and site management Nitrogen was applied as urea in split applications; half of the designated annual rate was applied in early spring (September– October) and the other in autumn (April–May). At all sites nitrogen application was commenced when the plantations were 2 years old and continued for 7 years. Coincident with the first application of nitrogen, phosphorous (100 kg P ha1), potassium (125 kg K ha1), magnesium (1.0 kg Mg ha1 as MgSO47H2O), manganese (3.3 kg Mn ha1 as MnSO4H2O), zinc (2.3 kg Zn ha1

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as ZnSO47H2O) and copper (1.3 kg Cu ha1 as CuSO45H2O) were applied to all plots. All fertiliser was broadcast by hand. Low and medium stocking plots were thinned at age 2 years in September of either 1998 (Scott River, Wellstead and Boyup Brook) or 1999 (Perup and Narrikup). All felled trees were removed from the trials at the time of thinning to prevent preferential immobilization or leaching of nitrogen in the thinned plots. If significant weed growth was evident in early spring of any year it was controlled using glyphosate herbicide. All coppice regrowth from stumps in the thinned treatments was sprayed with glyphosate approximately 2 months after thinning. Before the stands were treated, the diameter of all trees was measured and basal area calculated. These data were used to quantify any systematic within-site variation in productivity and arrange blocking of the experiments. At Boyup Brook, stand basal area decreased systematically from the bottom of the slope to the top and the replicate blocks were arranged to account for this variation. No systematic variation in pre-treatment basal area was evident at any of the other sites. 2.4. Stand growth The height and diameter of all trees were measured annually in either September or October. Initially, height (h) was measured using either a telescopic or sectioned height pole. When the trees grew too tall for this to be practical, a Digital Vertex III Hypsometer (Haglof, Sweden AB) was used to measure height. Diameter was measured at breast height (1.3 m above ground) and all trees were marked at the position where diameter (d) was first measured so that subsequent measurement could be made at the same height above ground. Measurements commenced in either 1998 (Scott River, Wellstead and Boyup Brook) or 1999 (Perup and Narrikup) and ended in 2004. An additional measurement was made just prior to harvest in December 2005 at Scott River, February 2006 at Boyup Brook and June 2006 at Wellstead. The conical volume over bark (v, m3) of each tree was calculated using Eq. (1).

v¼

 p  d  h 2 h 12 100 h  1:3

(1)

Standing volume (V, m3 ha1) was calculated using Eq. (2), V¼

n 10000 X vi A i¼1

(2)

where vi was the volume of the ith tree in each plot and A was the ground area (m2) of the measurement plot. 2.5. Predawn leaf water potential After each annual growth measurement, the trees within treatments were ranked according to cross-sectional area and allocated to five size classes, each of which contributed one fifth of the stand basal area. In each plot, one tree was selected at random from each size class and these trees were used for predawn leaf water potential measurements until the next set of growth data were collected. At Scott River, Wellstead and Boyup Brook, predawn leaf water potential (Cmax) was measured regularly between December 1998 (soon after the treatments were imposed) and June 2004. At Perup and Narrikup measurements commenced in November 1999. Every year Cmax was measured in early September at the end of the wettest period at the experimental sites, and then at intervals varying from one to three months until the beginning of the autumn rains which was usually between late March and May.

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Just before dawn on each measurement day, one leaf was cut from each of the five measurement trees in all plots using a singleedged razor blade. Each leaf was wrapped in cling film and placed in a portable refrigerator until all the leaves from that site were collected. Predawn leaf water potential (Cmax) of all sampled leaves was measured using a Pressure Chamber (PMS, Portland, Oregon, USA) after Scholander et al. (1965) and using precautions described by Turner (1981). The plot average of the five leaves was used in subsequent analysis. 2.6. Leaf area index Plant area index (PAI) was measured periodically using an LAI2000 Plant Canopy Analyser (Li-Cor Inc., Lincoln Nebraska, USA) in two-sensor mode. The remote sensor was set up in a large (>100 m) clearing as close as possible to the plantation and concurrent measurements made with a second sensor at the corner of each plot. A 2708 mask was placed on both the remote and in plantation sensors. Leaf area index (L) was calculated using the calibration of the LAI-2000 developed for E. globulus by Hingston and Galbraith (1998) (Eq. (3)). The parameters of this calibration are identical to those observed by Battaglia et al. (1998) for E. nitens Maiden (Deane and Maiden) and Battaglia et al. (1998) also reported that the calibration was suitable for stands with stocking density in the range 600–1400 stems ha1. L ¼ 1:51PAI

(3)

climate wetness index (ratio of rainfall to potential evaporation) for the study sites over the measurement period from 1998 to 2006. 2.9. Data analysis The effect of nitrogen application and thinning stand density on measured and calculated variables was tested using one- (nitrogen rate trial) and two-way (nitrogen-by-stocking trial) analysis of variance (Genstat Version 11). Linear regression was also used to develop relationships between MAI, growth efficiency and the climate wetness index (CWI). 3. Results 3.1. Growth and climate wetness index At all sites the fastest growth was observed in unthinned plots fertilised with 125 kg N ha1 year1 (e.g. Scott River, Fig. 3). If Wellstead was excluded from the data, a linear relationship with climate wetness index (CWI) explained 99% of the variation in mean annual increment (MAI) of this treatment at the end of the study; age 9 years at Scott River and Boyup Brook and age 7 years at Perup and Narrikup (Fig. 2). The soil at Wellstead was at least 20 m deep and during the experiment the trees depleted the soil water store to at least 14 m. Available soil water was limited to the top 8 m of the soil profile at the other sites.

2.7. Current and mean annual volume increment, annual growth efficiency and cumulative water stress integral

3.2. The effect of nitrogen on growth and predawn leaf water potential (Cmax)

The current annual volume increment (CAI, m3 ha1 year1) and mean annual volume increment (MAI, m3 ha1 year1) were calculated for each full year between growth measurements using Eqs. (4) and (5),

Application of nitrogen significantly increased standing volume (V, m3 ha1) at Scott River, Wellstead and Boyup Brook but not at Narrikup or Perup. At Scott River and Boyup Brook no additional increase in V was observed for rates of N application greater than 125 kg ha1 year1 (Fig. 3, Table 3). At Scott River the increase in V

CAI ¼ V i  V i1

(4)

Vi i

(5)

MAI ¼

where Vi was standing volume in the ith year, t was stand age (years) at the end of that year. The stand growth efficiency (E) was calculated for each measurement year as the ratio of the current ¯ for that year annual increment to average leaf area index (L) (Eq. (6)). Growth efficiency defined in this way is sometimes called crown vigour index and was originally proposed by Waring et al. (1981). E¼

CAIi ; L¯

(6)

For each interval between predawn leaf water potential measurements the water stress integral (Sc, Myers, 1988) was calculated as:   ðc þ ciþ1 Þd  Sc ¼  i (7)  2 where Ci and Ci+1 were predawn leaf water potential for consecutive measurements and d was the number of days between measurements. For each year between growth measurements the cumulative water stress integral was calculated as the sum of SC for all intervals between measurements during that year. 2.8. Climate wetness index We obtained daily rainfall and potential evaporation data from SILO (Jeffrey et al., 2001). These data was used to calculate the

Fig. 2. Mean annual increment calculated at the end of the rotation (age 9 years) for Scott River (SR), Wellstead (WL), Boyup brook (BB) and at age 7 at Perup (PR) and Narrikup (NK) as a function of average annual climate wetness index (CWI = rainfall/potential evaporation) from planting to the time of measurement. The line is a linear relationship fitted by regression to the data for all sites except Wellstead.

D.A. White et al. / Forest Ecology and Management 259 (2009) 33–44

Fig. 3. Mean annual increment from age 2 to 9.5 years (just before harvest) for 5 rates of N application (0, 45, 125, 150 and 400 kg N ha1 year1) in the unthinned plots at Scott River.

due to application of N remained significant throughout the study from the first measurement in September 1999 (3 years old, 1 year after the first application) to the last measurement in December 2005. At Boyup Brook the increase in V due to application of nitrogen did not become significant until September of 2001 (5 years old, 3 years after the first application). At Wellstead the increase in V was only significant at the final measurement in June 2006 (10 years old, 8 years after the first application, Table 3). Just prior to harvest at the end of the first rotation standing volume was, respectively, 90% and 66% higher in the 125N treatment than the un-fertilised plots at Scott River and Boyup Brook (Table 3). The percentage response to nitrogen in volume and leaf area index at age 8 years (7 at Perup and Narrikup) was plotted against total soil nitrogen for the top 0.1 m of soil as proposed by Smethurst et al. (2003) and E. globulus responded to nitrogen on sites where total soil nitrogen was less than 1.9 mg g1 (Fig. 4). The application of nitrogen significantly decreased predawn leaf water potential (Cmax) during summer at Scott River and Boyup Brook (Fig. 5) but not at the other sites. At Scott River nitrogen significantly decreased Cmax during every summer after 1999–2000 and minimum values were lower than 2.4 MPa in February 2002 (Fig. 5a). The effect of nitrogen on Cmax was not as pronounced at Boyup Brook as at Scott River and only became significant after the summer of 2001–2002 (Fig. 5b). The cumulative effect of nitrogen application on summer values of Cmax at Scott River and Boyup Brook was reflected in significant increases in the annual cumulative water stress integral (SC) at

37

Fig. 4. Percentage increase in stand volume (V) and leaf area index (L) of the 125 kg N ha1 treatment compared to the 0 kg N ha1 treatment plotted against total nitrogen (mg g1) in the top 0.1 m of soil. Volume and leaf area response were calculated at age 7 years for all sites. Data labels indicate variable (V or L) and site.

Table 4 Annual water stress integral (SC, MPa d) by annual rate of N application for all measurement years at Scott River and Boyup Brook. In years where N significantly (p < 0.05) affected SC, data are labelled either with a * (Scott River) or y (Boyup Brook). Site

Scott River

Treatment

0N

250N

Boyup Brook 0N

250N

Age in years 3 4 5 6 7 8

195 242* 256* 190* 224* 298*

206 284* 437* 564* 552* 501*

345 218 386 350y 218y 278

314 224 431 433y 406y 470

both sites (Table 4). At Scott River SC for the fertilised treatment was more than twice that of the unfertilised treatments after age 6 years. The relative difference decreased in later years while remaining significant. The effect of nitrogen on SC was less pronounced at Boyup Brook than at Scott River but was nonetheless significant (Table 4). Higher rates of mortality were observed over the rotation in the fertilised, unthinned treatment than for any other treatment at Scott River (Table 5). It is important to note that soil depth in two of the unthinned, fertilised plots at Scott River was approximately 5 and 7 m (determined from drilling

Table 3 Mean standing volume (V, m3 ha1) by annual rate of N application and site at the ages 5 and 9 years. Numbers within columns with different superscripts are significantly different (p < 0.05). Site

Scott River

Wellstead

Boyup Brook

Age at measurement (years)

5

9.4

5

9.5

5

9.8

Rate of N application (kg ha1 year1) 0 45 125 250 400

73a 94b 137c 135c 136c

125a 185b 238c 211c 238c

116a 106a 125a 113a 122a

211a 194a 223ab 205a 242b

46a 48a 76b 63ab

83a 129b 138b 128b

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At Scott River and Wellstead the current annual increment of stands thinned to 600 stems ha1 exceeded, occasionally significantly, the growth of unthinned stands for a period of 4 years after thinning or until the stands were 6 years old. After age 6 years, stands thinned to 600 stems ha1 had similar V and MAI (Fig. 6a and b) to unthinned plots at both sites. This remained true until the trees were harvested at the end of the rotation. This convergence of MAI for medium stocking density and unthinned plots was not observed at Boyup brook (Fig. 6c). At all sites predawn leaf water potential (Cmax) was lower during summer in the unthinned plots than in plots thinned to either 300 or 600 stems ha1 and this persisted for 4 years after thinning. Data for Scott River (high rainfall) and Wellstead (low rainfall with deep soil) are plotted in Fig. 7 to illustrate the effect of thinning at age 2 on development of water stress in these plantations. 3.4. Annual growth efficiency and cumulative water stress integral

Fig. 5. Time course of predawn leaf water potential (Cmax) for the 0N and 250N unthinned treatments at (a) Scott River and (b) Boyup Brook. Error bars are LSD’s from analysis of variance.

logs and subsequent measurement of soil water content – data not shown) and most of the observed mortality occurred in these plots. 3.3. The effect of stocking density on growth and predawn leaf water potential Although the interaction between nitrogen and spacing was occasionally significant for all of standing volume (V), current annual increment (CAI), mean annual increment (MAI) and leaf area index (L), these effects were transient and for the most part the effect of nitrogen and spacing were additive. At all sites, volume growth, expressed as standing volume (V) or mean annual increment (MAI, Fig. 6), was significantly lower for stands thinned to 300 stems ha1 than stands thinned to 600 stems ha1 or left unthinned (Fig. 6 – MAI by stocking).

For all treatments at all sites growth efficiency (E) peaked at between age 2 and 4 years and declined thereafter. Growth efficiency (E) was greatest in thinned, fertilized plots; for the fertilised, 600 stems ha1 plots peak values were 1.045  103 m3 wood m2 leaf area at Boyup Brook, 1.31  103 m3 wood m2 leaf area at Scott River and 1.35  103 m3 wood m2 leaf area at Wellstead. By the end of the rotation growth efficiency was less than 0.7  103 m3 wood m2 leaf area for all treatments at all sites and any significant treatment effects had disappeared. Growth efficiency (E) was significantly greater in fertilised compared to unfertilised plots at Scott River to age 4 years. After age 6 years this effect was reversed but no longer significant (Fig. 8a). The application of nitrogen did not significantly affect E at any of the other sites including Wellstead and Boyup Brook (Fig. 8). The growth efficiency was significantly increased by thinning at all sites (Fig. 9). At Scott River E of 600 stems ha1 plots was significantly greater than for unthinned plots for 5 years after thinning. Growth efficiency was greater in lower density stands and was significantly different between 300 stems ha1 and unthinned plots at all five sites (Fig. 9). Annual growth efficiency (E) was negatively correlated with SC for all sites (Fig. 10). As the trees grew they experienced progressively higher SC (Table 4) and this was associated with a decline in E. When only fertilised treatments were included a linear relationship with SC explained 82% of the variation in E at Scott River (Fig. 11). Thinning at age 2 years delayed this increase in SC and the associated decline in E by between 1 and 2 years compared to unthinned controls (Fig. 11). 4. Discussion At all of the five sites available water was a major determinant of productivity. For fertilised stands, MAI at age 9 (Scott River and Boyup Brook) or 7 years (Narrikup and Perup) was predicted by a linear relationship with climate wetness index (r2 = 0.99, p < 0.01).

Table 5 Actual stocking density for all treatments in the stocking density by nitrogen experiments at Scott River and Boyup Brook, both immediately after thinning in 1998 and 6 years after thinning in 2004. Stocking

N rate

300 300 600 600 1200 1200

0 250 0 250 0 250

Scott River

Boyup Brook

1998

2004

% Reduction

1998

2004

% Reduction

312 340 611 625 1166 1104

305 312 604 596 1122 890

2.2 8.2 1.1 4.7 3.7a 19.4b

298 264 548 493 979 958

263 243 541 452 868 847

11.7 7.8 1.2 8.3 11.3 11.8

D.A. White et al. / Forest Ecology and Management 259 (2009) 33–44

Fig. 6. Mean annual increment over time for the 300 stems ha1, 600 stems ha1 and unthinned plots (all 250N) at all sites.

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D.A. White et al. / Forest Ecology and Management 259 (2009) 33–44

Fig. 7. Time course of predawn leaf water potential for the low density, medium density and unthinned treatments (all fertilised) at (a) Scott River and (b) Wellstead. Error bars are LSD’s from analysis of variance.

Wellstead was an exception and annual growth exceeded the value predicted by this relationship by more than 6 m3 ha1 year1. The most likely explanation for this disparity is the extra water stored in the soil (>1200 mm) at Wellstead compared to the other sites. This is certainly consistent with the comparatively muted seasonal variation in predawn leaf water potential at Wellstead compared to either Scott River or Boyup Brook (Figs. 5 and 7). Thinning and the application of nitrogen affected both volume growth and water status in E. globulus at five sites established across a climatic gradient in south-western Australia. At Scott River and Boyup Brook, application of nitrogen increased the standing volume at the end of the rotation by as much as 100%. This increase in volume growth was associated with increased mortality at Scott River and decreased predawn leaf water potential (Cmax) at both sites. As noted earlier, the higher mortality in the fertilised plots at Scott River may be at least partially attributable to shallower soils in two of the three plots. At Scott River and Wellstead, the two most productive sites in this experiment, thinning to 600 stems ha1 reduced water stress and drought risk without any reduction in stand volume at age 9.5 years. These results clearly demonstrate that it is possible to manage the relationship between growth and drought risk in E. globulus plantations growing in a Mediterranean type climate. Importantly this can be achieved without yield penalty by matching nutrient supply with demand and reducing stocking density to minimise risk. Although available water was a major determinant of stand growth (Fig. 2), nitrogen application significantly increased

Fig. 8. Average growth efficiency over time for the fertilised and unfertilised plots at Scott River, Wellstead and Boyup Brook. Error bars are LSD’s from analysis of variance.

D.A. White et al. / Forest Ecology and Management 259 (2009) 33–44

Fig. 9. Average growth efficiency over time for the 300 stems ha1, 600 stems ha1 and unthinned plots at all sites. Error bars are LSD’s from analysis of variance.

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Fig. 10. Growth efficiency plotted against the annual water stress integral for a range of N  stocking treatments at Scott River, Wellstead and Boyup brook.

standing volume (V) at Scott River and Boyup Book where the total soil nitrogen in the upper 0.1 m of soil was, respectively 1.1 and 0.8 mg g1. Smethurst et al. (2004) found that total soil nitrogen explained about 70% of the variation in relative growth response to nitrogen of 16 E. nitens plantations in Tasmania and suggested that it might provide a practical, operational tool for predicting response. In this study, the threshold for growth response to nitrogen addition occurred at a soil nitrogen concentration between 1.9 and 1.1 mg g1 which is lower than the threshold of 2–3 mg g1 for substantial volume increase in E. nitens (Smethurst et al., 2004). Smethurst et al. (2004) observed that such calibrations were likely to be situation specific. Interestingly,

Fig. 11. Growth efficiency at Scott River at age 4, 5, 6 and 7 years old for fertilised E. globulus at 300 stems ha1, 600 stems ha1 and 1200 stems ha1.as a function of annual water stress integral. Data labels show age, stocking density and the line is a linear regression fitted to the full data set (E = 2.30–0.0037(SC), r2 = 0.82).

the responsiveness to fertiliser below the critical concentration was much greater in this study than Smethurst et al. (2004) observed in Tasmania. In an earlier study O’Connell et al. (2003) showed that available nitrogen was approximately halved during the first rotation of E. globulus in south-western Australia. Importantly in the same soils there was no detectable change in total soil nitrogen and the change in available nitrogen was attributed to a coincident change in the quality of the soil organic matter. Although total soil nitrogen was a reasonable indicator of potential growth response to fertiliser in this study the results of O’Connell et al. (2003) suggest that this will need to be combined with some measure of available nitrogen to predict potential growth response in second and subsequent rotations. At both a high (Scott River) and a low rainfall site (Boyup Brook), faster growth in response to nitrogen was associated with significantly lower predawn leaf water potential. At Scott River, this effect was statistically significant in every year of this study. Although the decline in available nitrogen and phosphorus in the first rotation (O’Connell et al., 2003) offers potential for increasing yield through fertility management (O’Connell et al., 2004) this must be traded off against the increased risk of drought death due to the increased water use of faster growing plantations. To understand properly the relationship between nitrogen status and drought risk we must consider the mechanisms by which the growth response to nitrogen is mediated. At Scott River, Boyup Brook and Wellstead, the application of nitrogen significantly increased volume growth (Table 3) and this was associated with an increase in leaf area index at these sites (Fig. 4). At the stand-scale, nitrogen fertiliser caused an increase in wood production per unit leaf area which was statistically significant at Scott River (Fig. 8). Thus nitrogen fertiliser increased growth both through an increase in leaf area and an increase in the amount of wood production per unit leaf area. The relationship between leaf area index (L) and growth is usually observed to yield declining benefits at high L (Smethurst et al., 2004). This is because radiation interception is characterised by a negative exponential relationship with L, in a manner described by Beer’s law (Saeki, 1960). The application of nitrogen has been shown to increase rates of photosynthesis on a foliage area and mass basis for many conifer species (Teskey et al., 1995) and the general relationship between carbon assimilation and nitrogen (Reich et al., 1997) has formed the basis of nitrogen, light optimisation models of canopy production (Field et al., 1983; Hollinger, 1996; Sheriff and Nambiar, 1991). In eucalypts, which have an indeterminate growth habit, the increase in foliar nitrogen concentration and photosynthetic capacity on an area basis is often transient and rapidly followed by an increase in leaf area and resultant dilution of nitrogen (Cromer et al., 1981; Cromer et al., 1993; Pereira et al., 1994; Attiwill and Adams, 1996). In Eucalyptus plantations the major influence of nitrogen nutrition on stand productivity is through increased leaf area and light interception. A strong relationship between radiation interception and growth has been observed for a number of species (Linder, 1985) including E. globulus (Landsberg and Hingston, 1996). Smethurst et al. (2003) observed that the main effect of nitrogen fertiliser in E. nitens was to increase leaf area index and reported a strong relationship between leaf area index and current annual increment. They argued that the upper boundary of this relationship represented the potential light use efficiency of the species expressed in terms of wood production at the stand-scale. While increased leaf area index in plantations will increase radiation interception and growth (Linder, 1985; Landsberg and Waring, 1997) it will also increase transpiration (Hatton et al., 1998). This increased water use results in lower leaf water potential as was observed in this study. Thus, increasing yield

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through improved nutrition brings increased risk of drought death. It has been predicted that rainfall in south-western Australia will continue to decline (Li et al., 2005). If this prediction is realised it will be particularly important to have a mechanism for managing drought risk other than variation in nutrient supply. At Scott River (a high rainfall site) and Wellstead (a low rainfall site), thinning to 600 stems ha1 at age 2 years did not decrease the end of rotation standing volume at age 9.5 years. Thinning caused an initial decline in volume but thereafter the thinned stand grew more quickly until, by age 8, the volume of stands thinned to 600 stems ha1 was the same as for unthinned stands. However, at Boyup Brook we did not observe convergence between the standing volume of thinned and unthinned stands. Although we do not have the data to explore this further, it is possible that growth at Boyup Brook was limited by factors in addition to water and nitrogen. In contrast to our observations for E. globulus growing at both Scott River and Wellstead, many previous studies have shown an increase in stand yield for stocking densities up to twice that of the unthinned plots in these experiments (Schonua and Coetzee, 1989). This has been observed in a range of species and environments including E. camaldulensis Dehnh., E. pellita F. Muell. and E. urophylla S.T. Blake in southern Brazil (Bernardo et al., 1998), E. regnans F. Muell. in southern Australia (Cremer et al., 1984), Pinus contorta Douglas (Sullivan et al., 2005) P. taeda Linnaeus (Harms et al., 1994), Pseudotsuga menzeissii (Mirb.) Franco (Mitchell et al., 1996) and Betula papyrifera Marsh. (Simard et al., 2004). Although a positive relationship between stand density and stand volume growth has been consistently observed in wet or summer rainfall dominated environments (Schonua and Coetzee, 1989), in water-limited situations there is good evidence that total water supply constrains the stand-scale response to increased stand density (Butcher and Havel, 1976; Hasenauer et al., 1997; Will et al., 2001; Woodruff et al., 2002; Blevins et al., 2005). Also, in south-west Western Australia, thinning studies in both P. pinaster Ait and P. radiata D. Don showed unequivocally that thinning is a valuable mechanism for drought-proofing plantations that produces more valuable trees without reducing volume growth at the stand-scale (Butcher and Havel, 1976; Butcher, 1977). Results in this study suggest that the optimum stand density or carrying capacity for E. globulus plantations in southwest Western Australia is somewhere between 300 and 600 stems ha1. In reality, determining the optimum density will require a full economic analysis of growing, harvesting and processing of plantations thinned to different stocking densities. Thinning significantly reduced predawn water potential and cumulative water stress and increased stand-scale growth efficiency for several years. There has been a substantial amount of research to understand the mechanisms for increased growth of individual trees and to understand the stand-scale response to thinning or variation in planting density. Thinning instantaneously exposes the lower crowns to more radiation than before thinning. This increases photosynthesis low in the crown and improves the carbon balance of this part of the trees (Ginn et al., 1991; Laurent et al., 2003; Tang et al., 2003; Gravatt et al., 1997). This results in an increase in leaf growth low in the crown and retention of lower crown foliage. The increase in leaf area (Landsberg and Waring, 1997) further increases the radiation interception which feeds forward to increased carbon assimilation and tree growth (Will et al., 2001). At the same time, the water balance of thinned stands is altered for the stand and for trees within the plantation. Interception is less (Stogsdill et al., 1992), soil evaporation is greater and each tree has access to a larger volume of soil stored water (Breda et al., 1995). As a result, trees in thinned stands experience less water stress (Donner and Running, 1986; Aussenac and Granier, 1988) and are able to grow for longer through the dry

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season (Cregg et al., 1990). The increase in leaf area and the supply of soil stored water results in a change in the pattern of sap flux density within stems (Medhurst et al., 2002) and an increase in tree transpiration (Whitehead et al., 1984; Morikawa et al., 1986; Medhurst et al., 2002). At the stand-scale, transpiration is at least temporarily reduced (Whitehead et al., 1984) which further improves the water status of the retained stand. All of this results in an increase in the growth efficiency of thinned stands (Waring et al., 1981; Waring, 1983). We observed a reduction in water stress compared to unthinned controls which resulted in improved growth efficiency and faster stand growth from the first year after thinning at Scott River and Wellstead, Narrikup and Perup. 5. Conclusions We have demonstrated that the relationship between growth and drought risk may be manipulated using a combination of thinning and fertiliser addition (in this case nitrogen). Matching the supply of nutrients to demand will maximise the growth at any site, but this may increase the risk of drought death at waterlimited sites. Pre-commercial thinning to 600 stems ha1 significantly reduced the level of water stress by E. globulus in Western Australia, but did not significantly decrease end of rotation stand volume compared to unthinned stands. In Mediterranean southwestern Australia, thinning coupled with good plantation nutrition is a sound strategy for drought-proofing plantations while maximising growth and financial returns. This has been demonstrated for both E. globulus (this study) and P. pinaster and P. radiata (Butcher and Havel, 1976). Over the coming decades, as rainfall declines in the important Mediterranean climate plantation regions around the world, this strategy may become increasingly important for optimising the trade-off between production and drought risk. Acknowledgements This paper includes data from a number of experiments in Western Australia. We thank Albany Plantation Forests Limited, Great Southern Plantations, Hansol PI, Timbercorp, WA Plantation Resources, the WA Department of Environment and Conservation and the Forest Products Commission for financial and in-kind support. Individual landowners including Grant Muir, Russell and Pattie Leighton generously gave access to their properties over 8 years. The experiments were established and maintained and data measured, collated and analysed by a large number of people in addition to the authors including: Jeff Galbraith, Stan Rance, Tammi Short, Gary Ogden, Shelley McArthur, Robin Bowles, Richie Fairman, Rob Hill and Robert Archibald. Shelley McArthur also completed all laboratory analyses of soil properties. References Attiwill, P.M., Adams, M.A. (Eds.), 1996. Nutrition of Eucalypts. CSIRO, Collingwood, Australia, 440 pp. Aussenac, G., Granier, A., 1988. Effects of thinning on water stress and growth in Douglas-fir. Can. J. For. Res. 18, 100–105. Battaglia, M., Cherry, M.L., Beadle, C.L., Sands, P.J., Hingston, A., 1998. Prediction of leaf area index in eucalypt plantations: effects of water stress and temperature. Tree Physiol. 18, 521–528. Bennett, L.T., Weston, C.J., Attiwill, P.M., 1997. Biomass, nutrient content and growth response to fertilisers of six year old Eucalyptus globulus plantations at three contrasting sites in Gippsland, Victoria. Aust. J. Bot. 45, 103–121. Bernardo, A.L., Reis, M.G.F., Reis, G.G., Harrison, R.B., Firme, D.J., 1998. Effect of pacing on growth and biomass distribution in Eucalyptus camaldulensis, E. pellita and E. urophylla plantations in south-eastern Brazil. For. Ecol. Manage. 104, 1–13. Blevins, D.P., Prescott, C.E., Allen, H.L., Newsome, T.A., 2005. The effects of nutrition and density on growth, foliage biomass, and growth efficiency of high-density

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