CSIRO PUBLISHING Functional P lant Biology, 2013, 40, 400-408 http://dx.doi.org/10.1071/FP12204

The anatomical basis of the link between density and mechanical strength in branches

Nadia S. SantiniA, Nele SchmitzB/C, Vicki BennionA and Catherine E. LovelockA/D

AThe School of Biological Sciences, The University of Queensland, St Lucia, Qld 4072, Australia, laboratory for Plant Biology and Nature Management, Vrije Universiteit Brussel, Brussels 1 050, Belgium. cLaboratory for W ood Biology and Xylarium, Royal Museum for Central Africa, Leuvensesteenweg 1 3, 3080 Tervuren, Belgium. DCorresponding author. Email: [email protected]

Abstract. Tree branches are important as they support the canopy, which controls photo synthetic carbon gain and determines ecological interactions such as competition with neighbours. Mangrove trees are subject to high wind speeds, strong tidal flows and waves that can damage their branches. The survival and establishment of partly depend on the structural and mechanical characteristics of their branches. In addition, mangroves are exposed to soils that vary in salinity. Highly saline conditions can increase the tension in the water column, imposing mechanical stresses on the xylem vessels. Here, we investigated how mechanical strength, assessed as the modulus of elasticity (MOE) and the modulus of rupture (MOR), and density relate to the anatomical characteristics of intact mangrove branches from southeast Queensland and whether the mechanical strength of branches varies among mangrove species. Mechanical strength was positively correlated with density of mangrove intact branches. Mechanical strength (MOE) varied among species, with Avicennia marina (Forssk.) Vierh. branches having the highest mechanical strength (2079 ± 176 MPa), and Rhizophora stylosa Griff, and Bruguiera gymnorrhiza (L.) Savigny ex Lam. and Poiret having the lowest mechanical strength (536.8 ± 39.2 MPa in R. stylosa and 554 ± 58.2 MPa in B. gymnorrhiza). High levels of mechanical strength were associated with reductions in xylem vessel lumen area, pith content and bark content, and positively associated with increases in fibre wall thickness. The associations between mechanical strength and anatomical characteristics in mangrove branches suggest trade-offs between mechanical strength and water supply, which are linked to tree growth and survival.

Additional keywords:fibres, intact branches, modulus of elasticity, modulus of rupture, xylem vessels.

Received 9 July 2012, accepted 17 November 2012, published online 11 January 2013

Introduction mechanical strength (Jacobsen et al. 2007; Chave et al. 2009). The wood of branches performs vital functions during the life of a The density of intact branches is not determined by the wood tree and also determines important ecosystem properties (Chave (xylem) characteristics only. The density of intact branches is also et al. 2009). Wood density is an important characteristic defining dependent on all the cell tissues that comprise the intact branch the mechanical properties of the wood and its performance, segment, such as the pith, composed of parenchyma, and the bark, such as resistance to breakage during high winds (Curran et al. composed of phellem, phloem and cambial cells (Sterck et al. 2008; Niklas and Spatz 2010) and susceptibility to insects and 2006; Onoda et al. 2010). pathogens (Augspurger and Kelly 1984). The wood density also The structure and function of the tissues that constitute the influences the physiological functions of branches because of different parts of a plant (i.e. main stems, branches androots) vary, wood's role in water transport (Prattet al. 2007). For instance, corresponding to the distinct hydraulic and mechanical demands high wood density has been found to decrease vessel implosion of these parts (Schweingruber et al. 2008). Studies of wood by reducing the mechanical stresses associated with the negative structure and function in plants have widely focussed on the pressure in the water column during drought (Hacke et al. 2001 ; wood of main stems because of its importance in forestry. Fewer Jacobsen et al. 2005). studies have addressed the relationship between the structure Wood density in branches is dependent on the anatomical and mechanical strength of intact branches (but see van Gelder characteristics of the wood such as xylem vessels and fibres et al. 2006; Sterck et al. 2006; Onoda et al. 2010), but intact (Jacobseni/al. 2005; Prestone/al. 2006). Inangiosperms, wood branches affect tree growth and survival. Branches are involved density has been observed to decline with decreasing vessel area in leaf deployment and expansion of canopies (e.g. when and thus the space for water transport (Prestonet al. 2006), and to competition for light with neighbouring trees occurs; e.g. increase with increasing fibre wall thickness, which provides Henriksson 2001). In addition, branches are more vulnerable

Journal compilation © CSIRO 2013 www .publish. c siro. au/j oumals/fpb Mechanical strength in mangrove branches Functional Plant Biology 401

to wind stresses than main stems (Whigham et al. 1991; van positions that are higher in the intertidal zone. When rainfall Gelder et al. 2006). runoff is limited, soil pore water that is high in the intertidal zone Here, we studied the mechanical strength, density and becomes highly saline due to evaporation of the seawater. Highly anatomy of intact canopy branches (including the xylem, the saline conditions are conditions similar to drought and can result pith, and the bark) in mangroves from southeast Queensland. in an increase in the tension of the water column within the xylem; Mangroves in this study were selected to include species with this tension can impose mechanical stresses on the xylem vessels typical and atypical secondary growth. Typical secondary growth (Jacobsen et al. 2005). refers to a single cylindrical cambium that produces phloem In this work we assessed: (1) the relationship between externally and xylem internally, whereas atypical secondary mechanical strength, the density of intact branches and growth displays successive cambia (i.e. forming consecutive anatomy in mangroves; (2) whether the mechanical strength of bands of xylem and phloem; Schweingruber et al. 2008). branches varies among mangrove species, and (3) whether Species with typical secondary growth for this study were mechanical strength and the density of intact branches varies three members of the family Rhizophoraceae, Rhizophora betweenH. marina specimens from the high and the low intertidal stylosa Griff., Bruguiera gymnorrhiza (L.) Savigny ex Lam. zones. and Poiret, and Ceriops australis (C.T. White) Bailment T.J. Sm. and J.A. Stoddart; and Aegiceras corniculatum (L.) Blanco Materials and methods from the Myrsinaceae (Shi et al. 2005). In addition, the most widely distributed mangrove across the Indo-West Pacific shores, Site description and sample collection Avicennia marina (Forssk.) Vierh., exhibits atypical secondary Our study sites were located in southeast Queensland, where the growth. Including the pith and bark in this study was important mean air temperature is 21°C (a mean minimum temperature because (1) we were interested in the mechanical properties of of 15°C and a mean maximum temperature of 25°C) and the intact branches, not just the density or the mechanical strength of mean rainfall is 1150mmyear_1. Our study sites were in the the xylem tissue, and (2) species varied in their wood anatomy, Tinchi Tamba Wetlands Reserve (27.23°S, 153.02°E) and with the inclusion of phloem within the wood ofA. marina. the Point Halloran Reserve (27.56°S, 153.28°E), 35km to the Therefore, inclusion of the bark in other species provided an south of the Tinchi Tamba site. These sites were chosen because appropriate comparison. they contain five of the most abundant mangrove species in Mangroves grow on the coasts of tropical and subtropical southeast Queensland (Table 1). shores. Mangroves are subject to high wind speeds, strong tidal At each site and on the basis of elevation relative to mean sea flows and waves. In Queensland, wind gusts can reach 90 km IT1 level, we defined two intertidal zone habitats: low and high during thunderstorms and >200 km IT1 during cyclonic activity intertidal. We assessed the salinity of soil pore water at each (Australian Bureau of Meteorology 2012) and can cause intertidal zone from each site. We collected pore water samples at structural damage to branches of trees (e.g. Paling et al. 2008). 30 cm depth using a suction device (McKee et al. 1988) and Therefore, tree establishment and survival under different analysed them with a handheld refractometer (W/ATC 300011, environments partly depend on the structural and mechanical SPER Scientific, Scottsdale, AZ, USA). characteristics of their branches (van Gelder et al. 2006; Curran We collected canopy branches from each species in each et al. 2008). intertidal zone (low and high intertidal) during July 2010; see In addition to wind and wave forces, mangroves are exposed to sample sizes and details of collection dates in Table 1. We tested soils that vary in salinity. Mangrove trees that grow low in the for differences between species in mechanical strength (modulus intertidal area are more frequently inundated than trees growing in of elasticity (MOE) and modulus of rupture (MOR)) and the

Table 1. Characteristics of the mangrove species from southeast Queensland, including growth form, position along the intertidal zone, pore water salinity and sample size The first collection was performed on 20 July 2010 and the second collection was performed on 30 July 2010. The samples from the first collection were used to measure the modulus of elasticity (MOE), modulus of rupture (MOR) and density of intact branches. The samples from the second collection were used to measure the MOE, MOR and density of intact branches, and xylem and branch characteristics, ppt, parts per thousand

Site Species Growth Intertidal Salinity n First Second Total (n ) form zone (ppt)±s.e. collectionin) collectionin)

Point Halloran Avicennia marina Tree Low 32 ±1 5 10 - 10 Bruguiera gymnorrhiza Tree 25 5 30 Rhizophora stylosa Tree 17 6 23

A. marina Shrub High 48 ±2 5 21 - 21 Ceriops australis Shrub 10 5 15 Tinchi Tamba A. marina Tree Low 32±2 5 14 5 19 Aegiceras corniculatum Shrub 31 4 35 R. stylosa Tree 10 3 13 A. marina Shrub High 44 ±3 5 17 5 22 C. australis Shrub 11 5 16 402 Functional Plant Biolog\> N. S. Santini et al.

density of intact branches as described in the data analyses Onoda et al. 2010). We determined the mass of water displaced by section. We made a second collection of branches to assess the the green volume ofthe intact branches, then dried the subsamples relationship between mechanical strength, the density of intact in an oven at 60°C for 5 days(until constant weight was attained. branches and anatomical characteristics! see details in Table 1). F inally, we calculated the density of intact branches as the ratio of We chose straight branches with diameters of 8-10 mm and dry mass over green volume following Chaveet al. (2006). lengths that were 20 times larger than branch diameter to minimise shear during the measurements (Onoda et al. 2010). Measurement of xylem characteristics After cutting the branches, we covered them with a moist To assess the anatomical characteristics of the xylem, we made paper towel and stored them in sealed plastic bags inside an transverse sections of 25 pm thickness with a Reichert-Jung insulated container. Branches were then stored in a refrigerator sliding microtome (Reichert-Jung, Fleidelburg, Germany). We at 8°C until laboratory analyses of mechanical strength were photographed the sections using a BX61 microscope (Olympus, performed (within 2 days of sample collection). After analysis Tokyo, Japan). From the photographs, we measured 100-200 of the mechanical properties of the branches, we stored two xylem vessels and 100-200 fibres for each stem. subsamples of each sample (~7 cm length each) for assessment We calculated the total vessel lumen area (XA) considering the of the density of intact branches and anatomical characteristics. vessel area (VA) and the vessel density (VD), following Lewis For anatomical characteristics, we analysed xylem characteristics (1992), where a and b are the short and the long axes of each vessel (fibre wall thickness and xylem vessel diameter; for A. marina, (Eqn 4, Eqn 5 and Eqn 6). we also assessed the proportion of phloem per layer of wood as described in Santini et al. 2012) and branch characteristics (the XA = VA VD ( 4 ) proportion of branch diameter that comprised pith and bark). with vessel area

Measurement of mechanical properties VA = nabO.25 ( 5 ) We measured the MOR and the MOE of the intact branches and vessel density using an electromechanical testing machine (INSTRON 5584, Chicago, IL, USA). The mechanical properties of branches can Vessels VD = ( 6) be described by measuring their MOR and their MOE. The MOR measures the maximum load that branches can resist, depending Fibre wall thickness was calculated as double fibre wall on their circular cross-sectional area (Eqn 1, Eqn 2) (Gere and thickness dividing the total distance by two. Goodno 2009). We measured proportion of wood comprised of phloem F maxLR for A. marina. The proportion of phloem per growth layer was MOR = - (D 41 calculated as the width of the phloem band divided by the total width (xylem + phloem) of the growth layer. where Fmax is the maximum load (in N), L is the span length (m), R is the radius o f the stem (m) and / refers to the second moment of Measurement of branch characteristics area (m4) of a stem with a circular cross-section, with: For pith and bark measurements, we first smoothed the branch kR transversal surface with a blade, removing any protrusions. The (2 ) I = ■ proportions of pith and bark were determined as the width of the pith and the mean width of the bark (measured at three different The MOE indicates how much a branch can bend and it is points around the branch circumference) divided by the total defined as the slope of the linear elastic region of load(F) in N and branch diameter. deflection (5) in mm (Eqn 3). We measured all xylem and branch anatomical characteristics FÜ using Image Pro Plus ver. 5.0.1 (MediaCybemetics, Rockville, MOE = ( 3 ) MD, USA). 4875' For each branch, we performed a three-point bending test. Data analyses For this test, a vertical force was applied at 25mmmin -1. To assess the variation in the MOE, MOR and density of intact A representation of the three-point bending test can be found branches (a) among species within the same intertidal zone in the supplementary online material (Fig. SI, available as and (b) between A. marina specimens occurring in two Supplementary Material to this paper). In addition, in order to different positions in the intertidal zone, we performed a facilitate comparison with other studies in the literature (e.g. Kruskal-Wallis one-way ANOVA, followed by a Dunn's Jacobsen et al. 2007; Pratt et al. 2007), equations to calculate the post-test. We assessed the relationships among the MOE, MOE of xylem tissue (i.e. branches excluding the contributions MOR and density of intact branches, and the anatomical from pith and bark) are also available in the supplementary online characteristics using Pearson correlation tests based on mean material (Methods section). values of species to avoid pseudoreplication (Crawley 2007). We performed these analyses with the software Prism ver. 5.0 Measurement of density of intact branches (GraphPad Software, La Jolla, CA, USA). Additionally, we used We measured the density of intact branches from the 7 cm long general linear models, where species were nested factors by using subsamples of the collected branches (van Gelder et al. 2006; the glm function with the R software package (The R Foundation Mechanical strength in mangrove branches Functional Plant Biology 403

for Statistical Computing; Free Software Foundation, Boston, Relationship between mechanical strength MA, USA). Further stepwise multiple regression analyses and and xylem characteristics principal component analyses (functionprcomp) to investigate Assessment of the relationships among MOE, the density of the contribution of anatomical characteristics to the variation in intact branches and xylem vessel lumen area of mangroves the MOE, MOR and wood density were performed with the R found that increases in MOE were associated with decreased software. Independent variables used for the multiple regression xylem vessel lumen area (Fig. 3a). In addition, the MOE and analyses were not correlated and there were no significant density of intact mangrove branches increased as fibre wall differences in the MOE, MOR and density of intact branches thickness increased (Fig. 3b, Table 2). Fibre wall thickness within species from different sites. The MOE, MOR and density varied from 2.40 ± 0.13 pm to 5.47 ± 0.16 pm, with fibre walls of intact branches were log-transformed to normalise the from C. australis being the thickest of the species examined data. Additionally, D’Agostino-Pearson normality tests were (Table 3). Additionally, the results of linear regression analyses performed to confirm that the data for the xylem vessel lumen between the MOE of xylem tissue alone and xylem characteristics area, fibre wall thickness, pith content and bark content were remained similar to the results of the MOE from intact branches normally distributed. and xylem characteristics (see Fig. S6). In A. marina, in which phloem is an important component of the wood, we found the MOE to decrease with the proportion Results of phloem (expressed as a percentage) per wood layer (Fig. 4). For branches of all species, we found a high correlation among the MOE, MOR and density of intact branches. Both the MOE and MOR increased with increasing density in intact branches Relationship between mechanical strength (Fig. la, b). Additionally, the MOE and MOR were strongly and branch characteristics correlated (Fig. le). Because of the strong correlation between MOE decreased as the proportion of pith increased (Fig. 3c). MOE and MOR, we only present the results of MOE in the We found that the proportion of stem that was pith varied following paragraphs (see Figs S2 and S3 for analyses of MOR). significantly among species. The pith comprised 13.3 ±0.3% We found the MOE of branches to be variable among species of the branch diameter in A. corniculatum to 47.2 ± 3 % in growing in the same intertidal zone. For instance, low in the B. gymnorrhiza (Table 3). Finally, we evaluated how intertidal zone, MOE values varied from 536.8 =b 39.2 MPa in variations in MOE were explained by proportion of bark of the R. stylosa and 554 =b 58.2 MPa in B. gymnhorriza to 2079 ± 1 7 6 stem diameter. We found that MOE declined as the proportion (MPa in A marina (Fig. 2a). In the high intertidal zone, A marina of bark increased (Fig. 3d). Bruguiera gymnorrhiza exhibited and C. australis exhibited a similar MOE (Fig. lb). Similar to the highest proportion of bark per stem diameter (9.26 ± 0.69%) MOE, the density of intact branches varied among species of our studied species (Table 3). from the same intertidal zone (Fig. 2c, d). Additionally, the Multiple regression analyses of the MOE over the intact MOE of xylem tissue alone was generally higher than that of branches of all species found that xylem vessel lumen area intact branches (P< 0.05), but the species remained in the same and fibre wall thickness explained 87% of the variation in order as for the MOE of intact branches (see Fig. S5). MOE. Similar to MOE, the multiple regression model for The tail A. marina growing in the low intertidal zone had MOR found that 89% of the variation could be explained by higher MOE values than A marina specimens growing in the high xylem vessel lumen area and fibre wall thickness. Finally, 88% of intertidal zone (Fig. 2a, b). However, the density values of intact the variation in density of intact branches was explained by pith branches in A. marina from the high intertidal zone were similar to content and fibre wall thickness (Table 2). A principal component those ofA. marina from the low intertidal zone (Fig.2c, d). analysis was done to visualise the relationships among MOE,

r2 = 0.90, p = 0.01 r2 = 0.99, p < 0.0001 g>1.0 0.4 0.5 0.6 0.7 0.4 0.5 0.6 0.7 2.6 2.8 3.0 3.2 Density of intact branches (g cm-3) Density of intact branches (g cm-3) Log modulus of elasticity (MPa)

Fig. 1. The relationship between (a) log modulus of elasticity (MOE) and the density of intact branches, (b) log modulus of rupture (MOR) and the density of intact branches and (c) log MOR and log MOE. Symbols represent different mangrove species: Avicennia marina from the low and the high intertidal zones (filled circles), Aegiceras corniculatum (open diamonds), Bruguiera gymnorrhiza (open upside- down triangles), Ceriops australis (squares) and Rhizophora stylosa (filled triangles). Each point represents the mean ± s.e. (n = 30-72). Values of r2 and P are based on Pearson correlation tests. 404 Functional Plant Biology N. S. Santini et al.

p < 0.05 et al. 2009). Allometric scaling theory predicts that larger lumen areas allow higher rates of transpiration and photosynthetic (b) 2500- carbon gain, which supports large canopies and high growth rates (e.g. Enquist et al. 1999; King et al. 2006). Therefore, decreases in xylem vessel lumen area result in increased — 2000- mechanical strength but potentially at the cost of reduced growth in mangrove branches. In contrast to stems and 8 1500 . branches, the trade-off between mechanical strength and vessel area appears to be absent in roots. Pratt et al. (2007) cn 1000- found that increases in vessel area do not explain decreases in root mechanical strength, suggesting that hydraulic demands may be decoupled from mechanical strength in some organs. Mechanical strength increased with the wall thickness of the fibres (Fig. 3b). Fibres provide structural support in the secondary wood of angiosperms (Jacobsenet al. 2007; Schweingruber et al. 2008). Additionally, thick fibre walls have been proposed to provide mechanical strength to the xylem vessels under negative (d) pressure (Jacobsen et al. 2005). The thickest fibres of all species ? E were found in the wood ofC. australis, which grows high in the o 05 intertidal where soil pore water is hypersaline. In our study sites, C. australis occurred at saline concentrations of 41-50 parts per .c thousand (ppt) but has been observed to persist in salinities of co C0 over 90 ppt (Ball 1998). In this species, which tends to occur in -0 0.4 - landward, hypersaline environments, thick fibre walls may o 0 5 protect the xylem from implosion under extreme negative c pressure, although this remains to be tested experimentally. o 0.2 - A previous study found that fibre wall thickness increased with increasing salinity (up to 23 ppt, which does not reach the lower end of the range of pore water salinity in our study) in 0.0 the mangrovei, corniculatum (Sun and Lin 1997). In our study, .N g we did not observe significant increases in fibre wall thickness S in A. marina specimens growing in more saline high intertidal environments compared with trees growing low in the intertidal zone, although mean fibre wall thickness was higher in the high compared with the low intertidal zone. The salinity of our Fig. 2. (a) The modulus of Elasticity (MOE) of branches from the dominant high intertidal habitat was 11-19 ppt higher than in the low mangrove species in southeast Queensland from the low intertidal zone and intertidal site, which may not have been a large enough difference (b) high intertidal zone, (c) The density of intact branches from the dominant mangrove species in southeast Queensland from the low intertidal zone and to result in significant differences in fibre wall thickness. (d) the high intertidal zone. Values are means and s.e. (n = 30-36). Different Moreover, increases in fibre wall thickness may not vary letters indicate that the means were significantly different (P<0.05). The linearly with salinity (or with water potential in the xylem) and pointed line indicates an additional f-test between the MOE of Avicennia variation in fibre wall thickness with salinity may vary among marina branches from the low and the high intertidal zones and the density of species with differing levels of salinity tolerance. For instance, intacta, marina branches from the low and the high intertidal zones; P < 0.05 the wood ofAvicennia germinans (L.) L. had thinner fibre walls indicates that the means (72 = 3141 ) were significantly different. (-3.5 pm) in less saline sites of 12.6 ppt and thicker fibre walls (-5pm) in higher salinity sites of 19.9 ppt, but the relationship MOR, the density of intact branches and the anatomical between fibre wall thickness and salinity was not linear (Yáñez- characteristics of mangrove branches (see Fig. S4). Espinosa et al. 2009). The mechanical strength ofA. marina, which has successive Discussion cambia, was negatively correlated with the proportion of the wood per growth layer that comprised phloem (Fig. 4). Relationship between mechanical strength and xylem The proportion of phloem in wood was higher in branches characteristics from the high intertidal and therefore branches from trees in We found the density of intact branches to be a good indicator of the high intertidal zone had low mechanical strength compared mechanical strength (MOE and MOR) in mangrove species from with the branches of trees from the low intertidal zone southeast Queensland. Mechanical strength decreased with (Fig. 2a, b). The decline in mechanical strength could be increases in xylem vessel lumen area (Fig. 3a). Therefore, the due to the phloem bands, which may create areas of weakness area of wood allocated to xylem accounted for a large part of the between the xylem fibre matrix, thereby lowering the mechanical variation in mechanical strength (64%), as has also been reported strength in A. marina branches in high intertidal, saline habitats. in other studies (Wagner et al. 1998; Jacobsen et al. 2007; Chave The presence of higher proportions of phloem per wood layer in Mechanical strength in mangrove branches Functional Plant Biology 405

(a) (b)

r2 = 0.64, p < 0.0001 r2 = 0.32, p < 0.0002 3.5-

3.0-

05 CL

2.5-I ---- 'o 5x10' 1 6 X 104 7x10' x 104 9x 10' 2 3 4 5 6 Xylem vessel lumen area (jam2) Fibre wall thickness (jam)

(c) (d) r2 = 0.60, p < 0.0001 r2 = 0.22, p < 0.002 TD 3.5 O E O) o —I

3.0

2.5 10 20 30 40 50 4 6 8 10 Pith content (%) Bark content (%)

Fig. 3. Relationship between (a) log modulus of elasticity (MOE) and xylem vessel lumen area, (b) log MOE and fibre wall thickness, (c) log MOE and pith content (expressed as total %) and (cl) log MOE and bark content (expressed as total %) of intact mangrove branches. The lines represent the linear regressions where: (a) Log MOE = -2.14 x IO-5 xylem vessel lumen area + 4.53, ( b) log MOE = 0.16 fibre wall thickness + 2.38, (c) log MOE = -2 .1 6 x IO-2 pith content+3.64 and (d) log MOE = -8 .8 6 x IO-2 bark content + 3.53. Confidence intervals (95%) for the slope and the intercept of the regression lines are: (a) slope: (-2.69 x IO“ 5 to -1.59 x IO“5), intercept: (4.14-4.91); (b) slope: (0.083-0.24), intercept: (2.03-2.72); (c) slope: (-2.7 x IO“2 to -1.60 x IO“ 2), intercept: (3.46-3.81);(d) slope: (-1.40 x IO“ 1 to -3.3 x IO“2), intercept: (3.21-3.84). Symbols represent different mangrove species: Avicennia marina from the low and the high intertidal zones (filled circles), Aegiceras corniculatum (open diamonds), Bruguiera gymnorrhiza (open upside-down triangles), Ceriops australis (squares) and Rhizophora stylosa (filled triangles). Each point represents the mean ± s.e. (n = 4-10).

A. marina high in the intertidal zone may be important in its role is composed of parenchyma tissue with thin cellulose walls. for salinity tolerance (Robertet al. 2011), but higher proportions The pith is responsible for various metabolic processes of phloem per wood layer reduce the density of intact branches including storage of water, synthesis (hormones, enzymes and and mechanical strength, thereby suggesting a trade-off between pigments), photosynthesis, respiration and communication with higher salinity tolerance and reductions in mechanical strength the vascular system of the trees via the pit structures of the cells forA. marina. (Pfanz and Aschan 2000; Schweingruber et al. 2008). The pith tissue is an important component of branches, compared with Relationship between mechanical strength and branch main stems, in which the xylem tissue is more abundant characteristics (Schweingruber et al. 2008). The negative correlation between Over all the species in our study, we found the mechanical pith and mechanical strength in mangrove branches may indicate strength of intact branches to decrease with an increasing that supporting metabolic activity within the vascular system has proportion of pith in the wood (Fig. 3c). Pith in woody plants associated costs of reducing mechanical strength. 406 Functional Plant Biolog\> N. S. Santini et al.

Table 2. Stepwise multiple regression models for log modulus ofAegiceras corniculatum exhibited higher mechanical strength elasticity (MOE) and xylem and branch characteristics, log modulus and density in their intact branches than R. stylosa and of rupture (MOR) and xylem and branch characteristics, and density ofB. gymnorrhiza1 which may provide enhanced resistance to intact branches and xylem and branch characteristics in mangroves wind and water forces (tidal flows and waves) in A. marina from southeast Queensland and A. corniculatum as well as resistance to the high tension in Only characteristics that contributed significantly to the models are shown; ,P<0.05, s.e. <0.01 the water column that can occur in highly saline environments (such as those in which C. australis occurs). In contrast,R. stylosa and B. gymnorrhiza, the branches of which had the lowest y - dependent variable Estimate /-value (original) mechanical strength and density, are potentially more likely to be damaged during cyclones or storms when high wind speeds Log MOE (MPa)A and waves occur. Intercept 0.74 73.4 Bardsley (1985) compared the degree of damage in 13 Fibre wall thickness (jim) 0.92 362 mangrove species after the Kathy devastated Vessel lumen area (pm ) 2.40 ; 10“ 186 the McArthur River mangrove forests in the , Fibre wall thickness (pm) x vessel - 1.11 ; 10“ -335 lumen area (pm ) Australia. Bardsley (1985) recorded strong winds of 185kmh_1 and wind gusts of up to 232kmh~1 from the Log MOR (MPaf adjacent Meteorological Station on Centre Island before the Intercept -2.61 -11.9 anemometer was severely damaged. She found that branches Fibre wall thickness (pm) 1.31 23.7 ofR. stylosa were particularly sensitive to damage by cyclonic Vessel lumen area (pm- ) 4.73 : 10“ 16.8 Fibre wall thickness (pm) x vessel -1.63 : 10“ -22.5 winds. In contrast, she found branches of A. marina to be less lumen area (pin- ) damaged by the wind forces associated with Cyclone Kathy. The low mechanical strength ofR. stylosa and B. gymnhorriza Density o f intact branches (g cm '/ branches may have an important role in tree survival during severe Intercept 0.36 16.1 weather events. For example, cyclone damage to species in the Fibre wall thickness (pm) 0.07 2.12 Fibre wall thickness (pm) x pith content (%) -1.78 ; 10“ -10.7 Rhizophoraceae is often fatal, not because of breakage of the main stems but because of damage to the canopy and defoliation AFinal model, r = 0.87. (Baldwin et al. 1995). Flowever, in contrast to the branches, the BFinal model, r =0.89. main stems ofR. stylosa and B. gymnhorriza exhibit different cFinal model, r =0.88. anatomical characteristics (e.g. lower ratios of pith area to xylem area, data not shown, N. S. Santini and N. Schmitz, unpubl. data), Furthermore, the proportion of bark in the branches in our which would increase their density and therefore their mechanical study was loosely negatively correlated with density of intact strength compared with branches. branches and mechanical strength (Fig. 3 cl). Niklas (1999) found The main stems of the genera Rhizophora and Bruguiera are that bark contributed positively to mechanical strength in old considered the most important species for timber in a range main trunks. In our study, bark did not contribute positively to of countries. The main stems of these genera exhibit higher mechanical strength, probably because bark in young branches wood densities (e.. B. gymnorrhiza with 0.699 g cm-3 and comprises thin tissue with low lignin content; however, in older Rhizophora apiculata BÍ. with 0.770 g cnT3) than smaller stems, bark can be thicker with a higher lignin content and, branches (Komiyama et al. 2005). The size of branches and consequently, be mechanically stronger (Wainhouseet al. 1990). stems and their associated changes in anatomical characteristics (such as changes in the pith area : xylem area ratio and fibre wall thickness) with age may be important when considering Mechanical strength of branches varies among the use of mangroves for timber and tree responses to severe mangrove species weather events. The branches of the studied mangrove species had a range Although, in general, the density of intact branches was a good of mechanical strengths. Avicennia marina, C. australis and indicator of mechanical strength, branches of A. marina trees

Table 3. Xylem and branch characteristics of intact canopy branches of the dominant mangrove species from southeast Queensland Species are ordered from low to high mechanical strength. Values are means and standard errors. Different letters indicate that means were significantly different ( P< 0.05 )

Species Xylem characteristics Branch characteristics Fibre wall thickness Total vessel lumen area Pith content Bark content (|im)±s.e. (pm: )±s.e. (%)±s.e. (% )is.e.

Rhizophora stylosa 2.40±0.13a 0.083 ± 0.005a 38.5 i l a 5.1 i0.3ac Bruguiera gymnorrhiza 4.09 ± 0.23d 0.082±0.011a 47.2 i 3b 9.3 i0 .7 d Avicennia marina (high intertidal) 4.05±0.27cd 0.046 ± 0.002b 22.8 i 2cd 4.0i0.2ce Ceriops australis 5.47 ± 0.16b 0.068 ±0.007ab 18.9 i lde 4.2i0.2ae Aegiceras corniculatum 3.99±0.14c 0.058 ±0.007ab 13.3 i0 .3 e 6.9 i 0.8b Avicennia marina (low intertidal) 4.18±0.26d 0.062 ±0.002ab 18.8i0.7cde 4.6i0.2ce Mechanical strength in mangrove branches Functional Plant Biology 407

who helped to improve this manuscript: Dr Tim R. Mercer, Lucy Hurrey and Bianca A. Santini. We thank Graham Ruhle from The School of Mechanical 05 û_ and Mining Engineering (UQ), and Robert Gould, Richard Webb, Robyn Webb, Wendy Armstrong and Kay Hodge from the Centre for Microscopy and Microanalysis (UQ) for assistance during the laboratory work. 'o 3.4- CD References O cn Augspurger C, Kelly CK (1984) Pathogen mortality of tropical tree 3.2- seedlings: experimental studies of the effects of dispersal distance, TO 3 seedling density, and light conditions. Oecologia 61, 211-217. E O) doi: 10.1007/BF00396763 o 3.0- _i Australian Bureau of Meteorology (2012), ‘Australian Bureau of Meteorology home page.’ (Commonwealth of Australia: Canberra). Available at: http://www.bom.gov.au [Verified 22 November 2012] 25 30 35 Baldwin AH, Platt WJ, Gathen KL, Lessmann JM, Rauch TJ (1995) Hurricane Phloem %/Layer damage and regeneration in fringe mangrove forests of Southeast Florida, USA. Journal of Coastal Research 21, 169-183. Fig. 4. Relationship between log modulus of elasticity (MOE) and phloem Ball MC (1998) Mangrove species richness in relation to salinity and percentage per growth layer. The line represents the linear regression water logging: a case study along the Adelaide River floodplain, where log MOE = ^1.0 x IO-2 phloem% of layer+4.47; n= \0. Symbols Northern Australia. Global Ecology and Biogeography Letters 7, represent Avicennia marina branches from the low intertidal zone (filled 73-82. doi: 10.2307/2997699 circles) and A. marina branches from the high intertidal zone (open circles). Bardsley KN (1985) The effects of Cyclone Kathy on mangrove vegetation. In ‘Coasts and tidal wetlands of the Australian monsoon region’. (Eds KN Bardsley, JDS Davie, CD Woodroffe) pp. 167-183. growing in the low intertidal zone, where tidal forces are usually (Australian National University North Australia Research Unit: Darwin) stronger than higher in the intertidal zone, exhibited higher Chave J, Muller-Landau HC, Baker TR, Easdale TA, Steege H, Webb CO mechanical strength values than branches of A. marina trees (2006) Regional and phylogenetic variation of wood density across growing high in the intertidal zone, but the density of intact 2456 neotropical tree species. Ecological Applications 16, 2356-2367. doi: 10.1890/1051-0761 (2006)016[2356:RAPVOW]2.0.CO;2 branches in both intertidal zones was similar (Fig. 2 a-d). Chave J, Coomes D, Jansen S, Lewis S, SwensonN, Zanne A (2009) Towards Differences in mechanical strength between A. marina a worldwide wood economics spectrum.Ecology Letters 12,351-366. specimens from the low and the high intertidal zones could be doi:10.1111/j.1461-0248.2009.01285.x due to a reduction in the inclination angle of microfibrils in the low Crawley MJ (2007) ‘The R book’. (John Wiley & Sons: Chichester) intertidal zone (Déjardin et al. 2010). Curran TJ, Gersbach LN, Edwards W, Krockenberger AK (2008) Wood density predicts plant damage and vegetative recovery rates caused by Conclusions cyclone disturbance in tropical rainforest tree species of north Queensland, Australia. Austral Ecology 33,442-450. doi:10.1 111/j. 1442-9993.2008. Overall, mechanical strength varies with the density of intact 01899.x branches in mangrove species from southeast Queensland. The Déjardin A, Laurans F, Arnaud D, Breton C, Pilate G, Leplé JC (2010) higher mechanical strength and density of intact branches were Wood formation in angiosperms.Comptes Rendus Biologies 333, mainly explained by reductions in xylem vessel lumen areas, 325-334. doi: 10.1016/j.crvi.2010.01.010 which may incur costs through reducing water transport, carbon Enquist B, West G, Chamov E, Brown J (1999) Allometric scaling of gain and growth. In addition, increases in the mechanical strength production and life-history variation in vascular plants. Nature 401, and density of intact branches were associated with increases in 907-911. doi:10.1038/44819 fibre wall thickness and with lower pith content (where storage of Gere JM, Goodno BJ (2009) ‘Mechanics of materials.’ (Cengage Learning: Stamford) water and synthesis of hormones, enzymes and pigments occur). Hacke U, Sperry J, Pockman W, Davis S, McCulloh K (2001) Trends in The mechanical strength ofA. marina varied with position in the wood density and structure are linked to prevention of xylem implosion intertidal zone: mechanical strength decreased with high levels of by negative pressure. Oecologia 126, 457-461. doi:10.1007/s0044201 phloem in the wood of branches from trees growing in the highly 00628 saline high intertidal zone. Therefore, there may also be a trade­ Henriksson J (2001) Differential shading of branches or whole trees: survival, off between mechanical strength and salinity tolerance in this growth, and reproduction. Oecologia 126, 482^186. doi: 10.1007/ species. The influence of wood traits such as fibre length and ray S004420000547 characteristics on mechanical strength remains to be tested in Jacobsen AL, Ewers FW, Pratt RB, Paddock WA, Davis SD (2005) Do xylem mangrove branches. fibers affect vessel cavitation resistance? Plant Physiology 139,546-556. doi:10.1104/pp.104.058404 Jacobsen AL, Agenbag L, Esler KJ, Pratt RB, Ewers FW, Davis SD (2007) Acknowledgements Xylem density, biomechanics and anatomical traits correlate with water This work was supported by the Mexican Council of Science and Technology, stress in 17 evergreen shrub species of the Mediterranean-type climate The Secretary of Public Education (Mexico), the School of Biological region South Africa. Journal o f Ecology 95, 171-183. doi: 10.1111/ Sciences at the University of Queensland (UQ) and by the Southeast j.1365-2745.2006.01186.x Queensland Climate Adaptation Research Initiative. We are thankful to King DA, Davies SJ, Tan S, NoorNSMD (2006) The role of wood density and Dr Julieta Rosell for her advice during the development and completion of stem support costs in the growth and mortality of tropical trees.Journal this study. We thank Dr Ruth Reef for fieldwork assistance, and the readers o f Ecology 94,670-680. doi:10.1111/j. 1365-2745.2006.01112.x 408 Functional Plant Biology N. S. Santini et al.

Komiyama A, Poungpam S, Kato S (2005) Common allometric equations for Santini NS, Schmitz N, Lovelock CE (2012) Variation in wood density and estimating the tree weight of mangroves.Journal of Tropical Ecology 21, anatomy in a widespread mangrove species. Trees 26(5), 1555-1563. 471—477. doi:10.1017/S0266467405002476 doi : 10.1007/s00468-012-0729-0 Lewis A (1992) Measuring the hydraulic diameter of a pore or conduit. Schweingruber FH, Borner A, Schulze ED (2008) ‘Atlas of woody plant American Journal o f Botany 1158-1161.19, doi: 10.2307/2445215 stems. Evolution, structure, and environmental modifications’. (Springer: McKee KL, Mendelssohn IA, Hester MW (1988) Reexamination of pore Jena) water sulfide concentrations and redox potentials near the aerial roots Shi S, Huang Y, Zeng K, Tan F, He H, Huang J, Fu Y (2005) Molecular ofRhizophora mangle and Avicennia germinans. American Journal o f phylogenetic analysis of mangroves: independent evolutionary origins Botany IS, 1352-1359. doi: 10.2307/2444458 of vivipary and salt secretion. Molecular Phylogenetics and Evolution Niklas KJ (1999) The mechanical role of bark. American Journal o f Botany 34, 159-166. doi: 10.1016/j.ympev.2004.09.002 86, 465^169. doi: 10.2307/2656806 Sterck FJ, Van Gelder A, Poorter L (2006) Mechanical branch constraints Niklas KJ, Spatz H (2010) Worldwide correlations of mechanical properties contribute to life-history variation across tree species in a Bolivian forest. and green wood density. American Journal o f Botany 1587-1594.91, Journal of Ecology 94, 1192-1200. doi:10.1111/j.l365-2745.2006. doi: 10.3732/ajb. 1000150 01162.x Onoda Y, Richards AE, Westoby M (2010) The relationship between stem Sun Q, Lin P (1997) Wood structure of Aegiceras corniculatum and its biomechanics and wood density is modified by rainfall in 32 Australian ecological adaptations to salinity. Hydrobiologia 352, 61-65. woody plant species.NewPhytologist 185,493-50 1.doi: 10.1111/j. 1469- doi: 10.1023/A: 1003092906969 8137.2009.03088.x van Gelder H, Poorter L, Sterck F (2006) Wood mechanics, allometry, and Paling El, Kobryn HT, Humphreys C (2008) Assessing the extent of life-history variation in a tropical rain forest tree comunity.New mangrove change caused by in the eastern Exmouth Phytologist 171,367-378. doi: 10.1111/j. 1469-8137.2006.01757.x Gulf, northwestern Australia. Estuarine, Coastal and Shelf Science 11, Wagner KR, Ewers FW, Davis SD (1998) Tradeoffs between hydraulic 603-613. doi:10.1016/j.ecss.2007.10.019 efficiency and mechanical strength in the stems of co-occurring species Pfanz H, Aschan G (2000) The existence of bark and stem photosynthesis of chaparral shrubs. Oecologia 117,53-62. doi:10.1007/s004420050631 and its significance for the overall carbon gain: an eco-physiological and Wainhouse D, Cross DJ, Howell RS (1990) The role of lignin as a defence ecological approach.Progress in Botany 62, 477-510. against the spruce bark beetle Dendroctonus micans: effect on larvae Pratt R, Jacobsen A, Ewers F, Davis S (2007) Relationships among xylem and adults. Oecologia 85,257-265. doi:10.1007/BF00319411 transport, biomechanics and storage and roots of nine Rhamnaceae species Whigham DF, Olmsted I, Cabrera Cano E, Harmon ME (1991) The impact of of the California chaparral.New Phytologist 174,787-798. doi: 10.1111/ Hurricane Gilbert on trees, litterfall and woody debris in a dry tropical j. 1469-8137.2007.02061.x forest in the northeastern Yucatan Peninsula. Biotropica 23,434^141. Preston AK, Cornwell KW, DeNoyer JL (2006) Wood density and vessels doi: 10.2307/2388263 traits as distinct correlates of ecological strategy in 51 California coast Yáñez-Espinosa L, Angeles G, López-Portillo J, Bárrales S (2009) Variación range angiosperms. New Phytologist 170,807-818. doi: 10.1111/j. 1469- anatómica de la madera de Avicennia germinans en la laguna de La 8137.2006.01712.x Mancha, Veracruz, México. Boletín de la Sociedad Botánica de México Robert E, Schmitz N, Boeren I, Diiessens T, Herremans K, De Mey J, Van de 85,7-15. Casteele E, Beeckman H, Koedam N (2011) Successive cambia: a developmental oddity or an adaptive structure? PLoS ONE 6, e l6558. doi:10.1371/joumal.pone.0016558

www.publish . c siro. au/j oumals/ípb