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Ann Bot. Nov 2011; 108(7): 1337–1345.
Published online Sep 21, 2011. doi:  10.1093/aob/mcr225
PMCID: PMC3197453
Is leaf dry matter content a better predictor of soil fertility than specific leaf area?
J. G. Hodgson,1* G. Montserrat-Martí,2 M. Charles,3 G. Jones,3 P. Wilson,4 B. Shipley,5 M. Sharafi,6 B. E. L. Cerabolini,7 J. H. C. Cornelissen,8 S. R. Band,4 A. Bogard,3 P. Castro-Díez,2 J. Guerrero-Campo,2 C. Palmer,3 M. C. Pérez-Rontomé,2 G. Carter,3 A. Hynd,3 A. Romo-Díez,9 L. de Torres Espuny,10 and F. Royo Pla10
1Station House, Leadmill, Hathersage, Hope Valley S32 1BA, UK
2Departamento Ecología Funcional y Biodiversidad, Instituto Pirenaico de Ecología (CSIC) Aptdo. 202, E-30080 Zaragoza, Spain
3Department of Archaeology, The University, Sheffield S1 4ET, UK
4Unit of Comparative Plant Ecology, Department of Animal and Plant Sciences, The University, Sheffield S10 2TN, UK
5Département de Biologie, Université de Sherbrooke, Sherbrooke, Québec, Canada
6Department of Biology, Faculty of Sciences, University of Mazandaran, Babolsar, Iran
7Unità di Ecologia Vegetale e Fitogeografia, Dipartimento di Biologia Strutturale e Funzionale, Università degli Studi dell'Insubria, Via J.H. Dunant, 3, 21100 Varese, Italy
8Department of Systems Ecology, Faculty of Earth and Life Sciences, VU University, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
9Institut Botànic de Barcelona, Parc Montjuïc, Av. dels Muntanyans s/n 08038, Barcelona, Spain
10Grup de Recerca Científica ‘Terres de l'Ebre’, C/ Rosa Maria Molas, 25 A, 2n B,43500 Tortosa Spain
*For correspondence. E-mail j.hodgson/at/sheffield.ac.uk
Received February 16, 2011; Revisions requested March 25, 2011; Accepted July 12, 2011.
Background and Aims
Specific leaf area (SLA), a key element of the ‘worldwide leaf economics spectrum’, is the preferred ‘soft’ plant trait for assessing soil fertility. SLA is a function of leaf dry matter content (LDMC) and leaf thickness (LT). The first, LDMC, defines leaf construction costs and can be used instead of SLA. However, LT identifies shade at its lowest extreme and succulence at its highest, and is not related to soil fertility. Why then is SLA more frequently used as a predictor of soil fertility than LDMC?
Methods
SLA, LDMC and LT were measured and leaf density (LD) estimated for almost 2000 species, and the capacity of LD to predict LDMC was examined, as was the relative contribution of LDMC and LT to the expression of SLA. Subsequently, the relationships between SLA, LDMC and LT with respect to soil fertility and shade were described.
Key Results
Although LD is strongly related to LDMC, and LDMC and LT each contribute equally to the expression of SLA, the exact relationships differ between ecological groupings. LDMC predicts leaf nitrogen content and soil fertility but, because LT primarily varies with light intensity, SLA increases in response to both increased shade and increased fertility.
Conclusions
Gradients of soil fertility are frequently also gradients of biomass accumulation with reduced irradiance lower in the canopy. Therefore, SLA, which includes both fertility and shade components, may often discriminate better between communities or treatments than LDMC. However, LDMC should always be the preferred trait for assessing gradients of soil fertility uncoupled from shade. Nevertheless, because leaves multitask, individual leaf traits do not necessarily exhibit exact functional equivalence between species. In consequence, rather than using a single stand-alone predictor, multivariate analyses using several leaf traits is recommended.
Keywords: Ellenberg numbers, functional traits, leaf density, leaf nitrogen, leaf size, leaf thickness, relative growth rate (RGR), shade tolerance, variation in trait expression
A key element controlling function in terrestrial ecosystems is soil fertility, and this factor is an important environmental driver of the fundamental trade-off in leaves involving the rapid acquisition of nutrients and conservation of resources within well-protected tissues (see Lambers and Poorter, 1992; Reich et al., 1992; Garnier and Laurent, 1994; Grime et al., 1997; Wright et al., 2004; Shipley et al., 2006). The ensuing ‘worldwide leaf economics spectrum’ (Wright et al., 2004) is accepted as a key axis of specialization between soft, mesomorphic leaves in productive habitats and tough, fibrous ones in unproductive habitats. Plant growth rate, palatability to unspecialized herbivores, nutrient recycling and agricultural yield all scale along this ‘worldwide leaf economics spectrum’ (Grime et al., 1997; Díaz et al., 2004; Wright et al., 2004; Hodgson et al., 2005b).
Two easily measured predictors of this important ecological axis have emerged, leaf dry matter content (LDMC; dry leaf mass/water-saturated fresh leaf mass) and specific leaf area (SLA; leaf area/leaf dry mass) or its reciprocal, leaf dry mass per unit area (LMA) (see Cornelissen et al., 2003; Poorter et al., 2009). The soil fertility indicator of choice is SLA and, for example, Westoby's (1998) leaf–height–seed (LHS) plant ecology strategy scheme in which SLA alone defines the fertility axis is still widely accepted (e.g. Laughlin et al., 2010; De Frenne et al., 2011). The use of SLA is, however, problematic. As Witkowski and Lamont (1991) emphasize and Wilson et al. (1999), Craine and Towne (2010) and Kitajima and Poorter (2010) subsequently remind us, SLA is the product of two variables, leaf tissue density (LD) and leaf thickness (LT): leaf tissue density is closely related to leaf dry matter content (LDMC) (Shipley and Vu 2002), leading to eqn (1).
A mathematical equation, expression, or formula.
 Object name is mcr225eqn1.jpg
(1)
The inclusion of LT as a component of SLA in eqn (1) presents both a practical and a theoretical dilemma. Although LT has been investigated in detail by Vile et al. (2005), it is not frequently measured. It is not included in the handbook of methods for measuring functional traits for the global flora (Cornelissen et al., 2003) or in many international databases (e.g. Kleyer et al., 2008). Moreover, its ecological relevance is suspect. To the best of our knowledge, LT has never been recommended as an independent predictor of soil fertility and growth rate. Rather, its lower and upper extremes appear to identify habitat shade and leaf succulence, respectively (Vendramini et al., 2002; Lusk et al., 2008), and particularly with respect to growth responses at different light intensities there are important issues of plasticity. Sun leaves have a thicker lamina and lower SLA: the SLA of shade leaves can be twice that of sun leaves on the same plant (see Popma and Bongers, 1988; Cornelissen, 1992; Dong, 1993). Moreover, sun leaves exhibit higher net rates of photosynthesis, contain a greater quantity of photosynthetic apparatus (with attendant physiological adaptations) and increased gaseous exchange is facilitated by a higher density of stomata (Bolhàr-Nordenkampf and Draxler, 1993; Terashima et al., 2001).
Because LT impacts in this way on the expression of SLA, Witkowski and Lamont (1991), Wilson et al. (1999) and Craine and Towne (2010) prefer the use of LDMC. It is even easier to measure than SLA and is also routinely measured as an important general predictor of the ‘the worldwide leaf economics spectrum’ (e.g. by Kleyer et al., 2008). In addition, while SLA loses most of its biological interpretation in species whose photosynthetic organs do not have the typical planar form, LDMC remains well defined.
While we find the arguments pioneered by Witkowski and Lamont (1991) compelling, they have not been generally followed. The common interpretation of SLA as an indicator of soil fertility amongst ecologists represents a paradox. If LT is not strongly associated with soil fertility, shouldn't LDMC be the preferred functional trait in field studies as an indicator of adaptation to fertility? Moreover, if LT reflects important environmental (light or water) gradients, shouldn't SLA be deconstructed into its component parts and both LDMC and LT be included as independent functional traits?
This paper attempts to answer the above questions by assessing the relative merits of SLA and LDMC as predictors of rapid growth and soil fertility. First, using a data set of >2000 species, we confirm some generally accepted relationships [i.e. the approximate correspondence between LD and LDMC and that SLA is an inverse function of LDMC and LT; see eqn (1)] and assess whether these relationships are numerically identical in disparate ecological and taxonomic sub-sets of the data. We additionally examine the range of values and consider the importance of a fourth variable, leaf size (LA). Like the three leaf variables above, LA also affects construction costs (Givnish, 1976; Niinemets et al., 2007). Secondly, we attempt to validate each leaf attribute as a predictor of soil fertility. Using correlations, we identify the extent to which leaf attributes predict relative growth rate (RGR), leaf nitrogen content and Ellenberg ‘Nitrogen Figures’ (Ellenberg et al., 1992). These three characteristics are widely used in studies of soil fertility and related processes. Thirdly, we consider the ecological implications of SLA being a function of both LDMC and LT [eqn (1)]. In particular, since LT and SLA both vary between sun leaves and shade leaves, we assess variation in SLA, LDMC and LT in relation to different combinations of soil fertility and shade. Finally, on the basis of these analyses, we discuss the relative merits of SLA and LDMC as predictors of soil fertility.
Data sources and measurements
SLA [leaf area (mm2)/leaf mass (mg)], LDMC (100 × dry mass of leaf/saturated mass of leaf), LT (mm, thickness measured between main veins) and maximum leaf size, LA (mm2) were abstracted from a large functional data set currently being prepared for publication. The measurement procedures are described in detail in Charles et al. (1997) and Wilson et al. (1999). They conform to the general recommendations of Garnier et al. (2001) and Cornelissen et al. (2003). In addition, because leaf volume was not measured, LD (mg mm−3) was estimated indirectly from eqn (1) as 1/(SLA × LT). To achieve numerical parity with LDMC, values of LD were multiplied by 100 prior to analysis. LT for leaves with prominent veins underestimates average leaf thickness and, had more time and resources been available, LD would have been determined more directly by measuring leaf volume as, for example, in Shipley and Vu (2002).
Data for SLA, LDMC and LT were obtained from the Unit of Comparative Plant Ecology, Department of Archaeology (Sheffield University), Instituto Pirenaico de Ecología (Zaragoza) and the senior author, and relate to plant material collected from unshaded habitats in Northern and Southern Europe and to a lesser extent the Near East and the Canary Islands. Values for leaf nitrogen concentration (leaf N) also originate from the database (see Hodgson et al., 2005a, 2010 for details). Maximum RGR values (RGRmax, g g−1 d−1) were abstracted from Grime and Hunt (1975) and Vile et al. (2006), and mean RGR values (RGRmean) from Cornelissen et al. (1996).
In most analyses, species were classified into ecological groupings on the basis of Grime et al. (2007), unpublished vegetation surveys and observed field distribution in Central England. Many Mediterranean species were similarly grouped according to Royo Pla (2006). For broad European analyses, phytosociologically related Ellenberg ‘Nitrogen Figures’, or soil fertility preference classes on a 1–9 scale, were abstracted from Ellenberg et al. (1992). ‘Nitrogen Figures’ are associated in the theoretically expected manner with growth and resource allocation studies (Thompson et al., 1993) and are strongly correlated with both field measures of soil productivity (Schaffers and Sykora, 2000) and foliar concentrations of nitrogen and phosphorus in the field (leaf N, r = 0·73, n = 91, P < 0·001; leaf P, r = 0·74, P < 0·001; leaf nutrient data from Thompson et al., 1997). Ellenberg ‘Light Figures’ have also been used. Nomenclature follows, preferentially, Stace (1997), but, where necessary, Tutin et al. (1964–80).
Analyses
After checking frequency distributions for normality and variance heterogeneity, it was necessary to present LDMC and LD as their square root, and to log10-transform the remaining variables prior to statistical analysis. Regressions all relate to the Type II of Warton et al. (2006). Except where otherwise stated, the remaining statistical tests were performed using SPSS for Windows™ (Version 14·0).
After exploring simple relationships between individual leaf traits and between leaf traits and other ecological variables, two additional more complex analyses were carried out. First, to examine how SLA varies with respect to different combinations of LDMC and LT, median values were used to divide ‘LT/LDMC space’ into quadrants, and the position of ‘SLA contours’ within this space was calculated from eqn (2a) in the Results section. Secondly, to determine the extent to which the relationship between leaf attributes and Ellenberg ‘Nitrogen Figures’ (N) changes with the typical light environment of the species, analyses of covariance (ANCOVAs) were conducted based upon groups defined by Ellenberg ‘Light Figures’ (L) using those species for which we had values for SLA, LT, LDMC and the two Ellenberg indices. In all, three ANCOVAs were performed, one for SLA, one for LT and one for LDMC, using the lm function of the R statistical package (R. Development Core Team 2008).
Range of values for SLA and other leaf traits
The range of values for each measured leaf trait is identified in Table 1. Perhaps significantly, our highest recorded value for SLA, 122·8 mm2 mg−1, was for the woodland herb, Ceratocapnos claviculata.
Table 1.
Table 1.
Range of values of specific leaf area [SLA; leaf area (mm2)/leaf mass (mg)], leaf dry matter content (LDMC, 100 × dry mass of leaf/saturated mass of leaf), leaf thickness (LT, mm) and maximum leaf size (LA, mm2) within the database
The level of correspondence between estimated LD and LDMC
Our estimate of LD explained 56 % of the variation in LDMC within the data set (Table 2; Supplementary Data Fig. S1, available online). However, LD and LDMC did not show a 1:1 relationship, and in all but one case the regression had a statistically significant constant term (Table 2). Furthermore, while LD and LDMC patterned similarly for the floras of the UK and the Canary Islands, the regression equations showed small but statistically significant differences between habitat, life history and taxonomic groupings (Table 2).
Table 2.
Table 2.
Correspondence for contrasted geographical, life history, habitat and taxonomic groupings between (A) estimated leaf density (LD, 100 mg mm3) and LDMC and (B) SLA and the product of LDMC and LT
Interrelationships between SLA, LDMC, LT and LA
LDMC and LT, in separate analyses, explained 29 and 35 %, respectively of the variation in SLA within our data set (P < 0·001; n = 1914; Supplementary Data Fig. S2A, B). Moreover, because these two components of SLA were very weakly negatively correlated (R2 = 0·02; P < 0·001; n = 1858), LDMC multiplied by LT was an even more effective predictor of SLA within the full data set, explaining nearly three-quarters of the observed variation [Supplementary Data Fig. S2C; eqn (2a)].
A mathematical equation, expression, or formula.
 Object name is mcr225eqn2.jpg
(2a)
The corresponding equation generated for LMA, with the same level of statistical significance, was
A mathematical equation, expression, or formula.
 Object name is mcr225eqn3.jpg
(2b)
A consequence of eqn (2a) is that one value of SLA can arise from various combinations of LT and LDMC. This is illustrated by an ordination of SLA values in ‘LT/LDMC space’ (Fig. 1).
Fig. 1.
Fig. 1.
A tentative ecological classification of species (×) in relation to LT and LDMC. Median values are used to divide ‘leaf thickness/leaf dry matter content space’ (dashed lines) into quadrants and the position of five labelled ‘SLA (more ...)
The negative relationship between SLA and LDMC × LT was a consistent feature within the data set (Table 2). However, as with the relationship between LD and LDMC, the exact numeric details differed significantly between some ecological and taxonomic sub-sets (Table 2). Unlike LDMC and LT, LA was not a good predictor of SLA (Supplementary Data Fig. S2D). Only 2 % of the variation in our database was explained.
SLA, LDMC, LT and LA as predictors of RGR, leaf N and Ellenberg ‘Nitrogen Figures’
Our field-measured SLA and LDMC were both statistically significant but weak predictors of experimentally derived RGR abstracted from the scientific literature (Table 3). R2 values were slightly higher for LDMC than for SLA: Central England, LDMC R2 = 0·185, SLA R2 = 0·152 (lower by 0·032); Mediterranean succession, R2 = 0·500, SLA R2 lower by 0·280; Western European woody species, R2 = 0·270, SLA R2 lower by 0·008. Correlations between RGR and LT and LA were fewer and less consistent (Table 3). Similarly, SLA and LDMC both strongly predicted leaf N (SLA R2 = 0·412; LDMC R2 = 0·323; Table 3) but only weakly identify Ellenberg ‘Nitrogen Figures’ (SLA R2 = 0·082; LDMC R2 = 0·070; Table 3). Here, however, R2 values were slightly higher for SLA. Correlations for leaf N and Ellenberg ‘Nitrogen Figures’ with LT and LA were also highly statistically significant, but in three out of the four cases had a lower R2 than those for SLA and LDMC (leaf N, LT R2 = 0·133; LA R2 = 0·141; Ellenberg ‘Nitrogen Figures’, LT R2 = 0·018; LA R2 = 0·197; Table 3).
Table 3.
Table 3.
Pearson r correlations comparing relative growth rate (RGR; g g−1 d−1), leaf nitrogen content (leaf N; % dry mass of leaf) and Ellenberg ‘Nitrogen Figures’ with SLA, LDMC, LT and LA
Variation in LDMC, LT and SLA in relation to soil fertility and light intensity
The relationship between SLA and Ellenberg ‘Nitrogen Figures’ (N) changed with the typical light environment of the species (Fig. 2A). Although log(SLA) did vary with Ellenberg N (F1,1129 = 100·35, P < 0·001) its average values also differed between the groups defined by Ellenberg L (F8,1129 = 14·22, P < 0·001). Most importantly, the slope between SLA and Ellenberg N differed significantly (F7,1084 = 3·66, P = 0·001) across the different levels of Ellenberg L. As the typical light environment of a species increased, the slope decreased (i.e. SLA increased with increasing values of Ellenberg N for species typical of shaded environments but the relationship disappeared in those species typical of the most open habitats (Fig. 2A). In contrast, in a comparable analysis using LDMC as the dependent variable (Fig. 2C), although √LDMC also varied strongly and negatively with Ellenberg N (F1,129 = 123·70, P < 0·001) and the intercepts did vary slightly with levels of Ellenberg L (F8,1129 = 2·42, P = 0·01), we detected no significant differences in the slopes across the levels of Ellenberg L (F7,1129 = 1·6, P = 0·11). In other words, as the typical level of soil fertility of a species increased, its LDMC value decreased by the same amount independently of its typical level of light intensity. With log(leaf thickness) as the dependent variable, the LT–Ellenberg N relationship was sensitive to light levels (F7,1057 = 2·85, P = 0·006) and even changed direction (Fig. 2B). At the highest light level, leaf thickness increased with increasing soil fertility but at the lower light levels leaf thickness decreased with increasing soil fertility.
Fig. 2.
Fig. 2.
Ellenberg ‘Nitrogen Figures’ plotted against (A) SLA, (B) LT and (C) LDMC for 1073 species. Lines show the linear regressions of each dependent variable against Ellenberg N for subsets of species having different Ellenberg ‘Light (more ...)
Properties of the database
Since the range of values of SLA, LDMC, LT and LA in our data set were wide, the database has the ecological breadth appropriate for comparisons of functional traits. For instance, the 5th–95th percentiles for SLA (7·7–40·6 mm2 mg−1, n = 2044) were similar in range to the 3–33 recorded for the world flora (Poorter et al., 2009). Moreover, our study, with in excess of 2000 species, had the potential to identify patterns and differences that studies with fewer taxa might have failed to detect.
Differences in the expression of leaf traits between ecological and taxonomic groupings
Plants, their organs, tissues and component cells all multitask, and the leaf is no exception, being simultaneously a light collector, an energy conversion facility and a short-term energy and nutrient storage facility. Moreover, succulents have specialist water storage tissue. Furthermore, some wetland plants have aerenchyma and, even between dryland species, there is variation in the proportion of leaf air space (Niinemets, 1999; Castro-Díez et al., 2000). This will cause deviations from a strictly 1:1 relationship between LD and LDMC. Additionally, in some species, all mechanical support is provided within the tissues of the leaf itself; in others the petiole and/or stem may also contribute, generating a higher SLA and a lower LDMC than would otherwise be expected. Therefore, as emphasized by Vernescu and Ryser (2009) and Bonser et al. (2010), leaf traits do not necessarily share exact functional and structural equivalence between species.
This theoretical construct agrees with our own results. The relationship between LD and LDMC appears arithmetically inexact, but this is expected (Shipley and Vu, 2002): in consequence, that between LDMC × LT and SLA is not exact either (Table 2). Although regression equations were of the same general form for all groupings, slopes and intercepts of the set of regressions (LD vs. LDMC, and LDMC × LT vs. SLA) differed significantly for contrasted ecological and taxonomic groupings (Table 2).
In these circumstances, how valid is it to use LDMC and SLA as stand-alone ecological predictors? We urgently need to identify the extent to which the expression of other ecological factors and structural features interact with and modify the traits that we routinely use to assess soil fertility. To these ends, the studies of Garnier and Laurent (1994), Van Arendonk and Poorter (1994), Niinemets et al. (1998), Roderick et al. (1999), Vernescu and Ryser (2009) and others need to be extended so that the interrelationships between leaf structure, leaf function and trait expression are more fully understood.
Relationships between LD, LDMC, LT and SLA
For reasons identified above, traits must be used with caution in all analyses. Nevertheless, in broad-brush meta-analyses, robust statistical relationships can be identified. Although the caveats above apply, we confirm the positive relationship between LD and LDMC described by Shipley and Vu (2002). Moreover, the equivalence between LD and LDMC appears robust [see eqn (1)] because otherwise LDMC × LT could not have accounted for nearly three-quarters of the observed variation in SLA [Supplementary Data Fig. S2C; eqn (2a)]. However, the key element of our early analyses was to confirm the pioneering work of Witkowski and Lamont (1991), Wilson et al. (1999), Kitajima and Poorter (2010) and Craine and Towne (2010), who all identified that SLA is a compound trait, a function of LDMC and LT. As a result of this duality, species differing markedly in leaf form and function can have the same SLA value (see Vernescu and Ryser, 2009) and, as illustrated in Fig. 1, very different combinations of LT and LDMC regularly generate similar values of SLA. Moreover, in a paper on the leaves of tropical trees, Kitajima and Poorter (2010) identified variation in dry mass density and tissue level mechanical strength as loading strongly on principal component analysis (PCA) axis 1 (with LDMC as a component variable) while lamina thickness loaded strongly on PCA axis 2. SLA (as LMA) was an important component of both axes. In summary, and as illustrated from a more physiological viewpoint by Niinemets (1999), SLA is a compound trait responding to two underlying traits (LD and LT) that have different functional roles.
The origins of SLA's popularity
That SLA is a function of LDMC and LT was already well established (Witkowski and Lamont, 1991) before SLA became the ecological trait of choice for identifying soil fertility. So, where does this popularity of SLA originate? We suspect that the key reason has been historical inertia. RGRmax was considered a key indicator of soil fertility (i.e. ‘stress’) by Grime (1979), but RGR is difficult and time-consuming to measure. However, the classical decomposition of RGR [eqn (3); see, for example, Evans, 1972)] shows that it is partly a function of SLA, an easily measured variable that does not require sequential harvests.
A mathematical equation, expression, or formula.
 Object name is mcr225eqn4.jpg
(3)
where RGR, NAR and LMF are relative growth rate (g g−1 d−1), net assimilation rate (or unit leaf rate; g cm−2 d−1) and leaf mass fraction (g g−1), respectively: this equation, used routinely in growth analysis, equates to eqn (1c) in Shipley (2006) and is similarly expressed in Poorter et al. (2009).
Tilman's (1988) influential book identified biomass allocation to leaves as a key determinant of RGR, and RGR was then found to be tightly correlated with SLA in the pioneering interspecific study of Poorter and Remkes (1990). Furthermore, although Shipley (2002) found otherwise, SLA has been identified as more important than NAR as a predictor of RGR (Poorter and Remkes, 1990; Poorter and van der Werf, 1998; Veneklaas and Poorter, 1998). SLA is also regarded as a key element of the ‘worldwide leaf economics spectrum’ of Wright et al. (2004). Thus, SLA appeared to ‘have it all’. Not only was it a key determinant of growth rate in the laboratory, but SLA also defined crucial ecosystem processes in the field. This view, that SLA was an effective stand-alone predictor of soil fertility and growth rate, was formalized within the LHS plant ecology strategy scheme of Westoby (1998).
Casting a shadow over SLA
From the outset, the potential impacts of shade (and its LT component) on the expression of SLA have not been adequately assessed. In the case of growth analysis, there were at first technical constraints. The best artificial lighting available provided only low irradiance, and many early experiments were effectively carried out in shaded conditions. For example, the classic ecological growth rate study of the British flora (Grime and Hunt, 1975) was performed at only ‘24 % of the mean daily value for similar latitudes to Sheffield’, while Poorter and Remkes (1990) used only 300 µmol photons m−2 s−1. In this context, it may be significant that the strength of the relationship between RGR and SLA appears to decline at higher light intensities (Poorter, 1999; Shipley, 2006), where, coincidently, the interspecific range of LT is likely to be greater. If light intensities had been uniformly higher in all early work, the possible impact of LT on the expression of growth rate may have been more fully explored with the ‘classic’ eqn (3) recast as
A mathematical equation, expression, or formula.
 Object name is mcr225eqn5.jpg
(4)
Superimposed upon this technical problem in relation to growth rate has been an ecological one. Species of deep shade typically constitute only a small fraction of the total flora (e.g. in Ellenberg et al., 1992) and have generally been poorly represented in ecological studies. Often, therefore, impacts of shade on the expression of SLA are not immediately apparent. Take, for example, Table 3, where a similar capacity to predict Ellenberg N was identified for both SLA and LDMC. When the typical light environment of each species was factored into the analysis, this broad equivalence disappeared. A typical light environment was now identified as an important co-variant (Fig. 2). The relationship of SLA with Ellenberg N changed with the typical light environment of the species (Fig. 2A). As a result, SLA became clearly identified as a flawed predictor of soil fertility. This failure of SLA was because one of its component variables, LT, is in many respects a shade index (Fig. 2B). In contrast, since it was negatively correlated with Ellenberg N and did not change with the typical light environment of the species (Fig. 2C), LDMC was the better, more consistent predictor of soil fertility.
SLA appears to be a hybrid ‘soil fertility–shade’ index. In open habitats with lower above-ground biomass and less shading within the canopy, it may predict, as it is supposed to, soil fertility and position in the ‘worldwide leaf economics spectrum’ of Wright et al. (2004). On the other hand, in productive sites with higher biomass and canopy density, there may be an additional elevation of the SLA value because of the presence of inherently thin-leaved, more shade-tolerant species. Such species are not just low-growing subordinates. Often, they are major contributors to the biomass: canopy dominants typically possess several overlapping layers of leaves and there is potentially a high degree of self-shading (see Niinemets, 2007). It appears that leaf morphological plasticity plays a more important role in determining foliar adaptation to high irradiance, whereas at low light biochemical and anatomical changes are of similar magnitude (Niinemets et al., 1998). In this context, we urgently need to identify the point of inflection where high SLA ceases to identify the potential for fast growth and high biomass accumulation but begins instead to signify slow growth under chronically light-deficient conditions. Until this has been achieved, there is a strong case for the additional use of an independent index of shade tolerance to make the prediction of soil fertility using leaf traits less ambiguous.
The ‘worldwide leaf economics spectrum’ of Wright et al. (2004) was identified using multivariate analyses. It will robustly inform ecologists for many years to come. However, the ideal, a predictor of soil fertility that works in all field situations, is problematic. Traits do not necessarily exhibit an exact functional equivalence between taxa (see Stebbins, 1974; Bonser et al., 2010), or even necessarily between habitats. Therefore, rather than a stand-alone, ‘flag-ship’ predictive trait, multivariate analysis of trait matrices, with SLA included (e.g. Jones et al., 2000; Hodgson et al., 2005a, b), is perhaps the best way forward. In the event of limited availability of data, we strongly recommend that the advice of Wilson et al. (1999) is followed. Rather than SLA, LDMC, whose link with growth rate and soil fertility is less ambiguously identified, should be the preferred predictive trait in studies relating to soil fertility.
ACKNOWLEDGEMENTS
A considerable quantity of the data used in this project was collected during projects funded by NERC (UK) and Comisión Interministerial de Ciencia y Tecnología (Spain). We thank Phil Grime, Simon Pierce, Bill Smith and two anonymous reviewers for constructive criticism of an earlier version of this manuscript.
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