Allocation of nitrogen and carbon in leaves of Metrosideros polymorpha regulates carboxylation capacity and δ¹³C along an altitudinal gradient

Abstract

1. Metrosideros polymorpha (O’hia), the dominant tree species in Hawaiian forest ecosystems, grows from sea level to treeline (2500 m). Consistent changes in its morphology and anatomy occur along this altitudinal/temperature gradient. Patterns of variation in photosynthetic gas exchange, leaf nitrogen content, nitrogen‐use efficiency, δ13C, and morphological and anatomical characteristics were determined across the elevational gradient. In addition, on‐line carbon isotope discrimination studies of high and low elevation M. polymorpha were performed. 2. Observed trends with increasing altitude were: (1) progressively higher carboxylation efficiency, leaf N content on an area basis, leaf mass per unit area (LMA), less negative foliar δ13C and (2) progressively smaller leaf size. Net CO2 assimilation (A) expressed on an area basis, leaf dry mass and N content per leaf remained relatively constant along the gradient. 3. Foliar δ13C became less negative with increasing elevation (– 30‰ at low elevation to – 24‰ at high elevation) and was strongly correlated with foliar N and LMA. Foliar δ13C was also correlated with variations in the ratio of intercellular to ambient partial pressure of CO2 (pi/pa), as determined by field gas‐exchange studies. 4. Results from on‐line fractionation experiments suggested that the relatively large internal resistance to CO2 diffusion did not differ between high and low elevation populations, despite differences in LMA. Less negative values of δ13C at high elevations and corresponding lower values of pi/pa were associated with increased carboxylation efficiency and N content on a unit leaf area basis. 5. Two major homeostatic responses in M. polymorpha plants along elevational/temperature gradients were observed: (1) maintenance of similar photosynthetic rates per unit leaf surface area despite suboptimal conditions for CO2 assimilation at high elevation and (2) similar N content per leaf despite lower soil N availability at high elevations. These homeostatic mechanisms allow M. polymorpha to maintain a relatively high level of growth‐related activities at high elevation, despite limiting environmental conditions.

Large-scale climatic and geophysical controls on the leaf economics spectrum

“…6A). This finding agrees with field studies on elevation gradients suggesting that community-scale adaptations to decreasing temperature are more strongly expressed in foliar structure than in N concentration (40,41). Notably, decreasing forest canopy LMA in the Andes is known to occur in conjunction with decreasing canopy compositional and functional diversity (21,42).…”

Augmentation de la capacité photosynthétique avec l’altitude: mesures d’échanges gazeux à pressions partielles de CO2 ambiante et constante

“…In the literature, most studies that have examined photosynthetic capacity (A max ) over altitudinal gradients have kept temperature, humidity and light level constant in order to compare populations, but either (i) did not mention how they managed the CO 2 variable (Cordell et al, 1998;Rundel et al, 2003), or (ii) used constant CO 2 molar fraction (ppm or µmol mol −1 or µbar bar −1 ) instead of constant partial pressure (GonzalezReal and Baille, 2000;Yin et al, 2004). These studies found results similar to those of Treatment A in our study: no significant variations of A max along the gradient (Cordell et al, 1999;Körner and Diemer, 1987;Kumar et al, 2006). However, a few studies have showed either slight increases in A max with increasing elevation (Friend et al, 1989;Premoli and Brewer 2007) or a decrease (Kao and Chang, 2001;Zhang et al, 2005).…”

“…The main geophysical drivers along altitudinal gradients from an ecological point of view are temperature, air pressure, precipitation and radiation (Körner, 2008). Indeed, morphological, phenological and physiological changes allow trees to maintain a relatively high level of growth-related activity despite increasingly constraining environmental conditions towards high elevation (Cordell et al, 1999). The main physiological traits that control the carbon uptake and water loss in plants and generally vary according to elevation are photosynthesis and stomatal conductance.…”

“…However, variations in plant performance with altitude are difficult to predict, partly because of the complexity of the geophysical effects of altitude and partly because of biological responses to these changes. Indeed, maximum rates of CO 2 assimilation in plants from different elevations, measured at ambient CO 2 partial pressure, have been found to be equal (Benecke et al, 1981;Cordell et al, 1999;Körner and Diemer, 1987), lower (Kao and Chang, 2001;Slatyer and Morrow, 1977;Zhang et al, 2005), or higher at high elevation (Premoli and Brewer, 2007).…”

The relevance of seed size in modulating leaf physiology and early plant performance in two tree species

“…Since seed reserves provide a large part of first-year seedling' nitrogen (Villar-Salvador et al 2010), a positive effect of seed size on leaf WUE is expected if seedlings from bigger seeds (i.e. with more reserves) have leaves with more nitrogen (Brookes et al 1980;Tripathi and Khan 1990), higher nitrogen concentration (Khurana and Singh 2004), and thus higher carboxylation capacity relative to stomatal conductance to water vapour (Cordell et al 1999). On the other hand, the microsite of seed dispersal affects leaf WUE in relation to the abundance of resources, such as water and light (Ehleringer and Cooper 1988).…”