Variations in Amazon forest productivity correlated with foliar nutrients and modelled rates of photosynthetic carbon supply

Abstract

The rate of above-ground woody biomass production, W(P), in some western Amazon forests exceeds those in the east by a factor of 2 or more. Underlying causes may include climate, soil nutrient limitations and species composition. In this modelling paper, we explore the implications of allowing key nutrients such as N and P to constrain the photosynthesis of Amazon forests, and also we examine the relationship between modelled rates of photosynthesis and the observed gradients in W(P). We use a model with current understanding of the underpinning biochemical processes as affected by nutrient availability to assess: (i) the degree to which observed spatial variations in foliar [N] and [P] across Amazonia affect stand-level photosynthesis; and (ii) how these variations in forest photosynthetic carbon acquisition relate to the observed geographical patterns of stem growth across the Amazon Basin. We find nutrient availability to exert a strong effect on photosynthetic carbon gain across the Basin and to be a likely important contributor to the observed gradient in W(P). Phosphorus emerges as more important than nitrogen in accounting for the observed variations in productivity. Implications of these findings are discussed in the context of future tropical forests under a changing climate.

Figure 1.

Rainforest site locations used in this study. Black and grey symbols denote N- and P-limited sites, respectively, according to Domingues et al. [18] parametrization (equation 2.2).

Figure 2.

Variability of foliar and on (a) area basis, (b) dry-weight basis and (c) their N : P ratio. Data taken from Fyllas et al. [7].

Figure 3.

Relationships of foliar N and P (on dry-weight and area basis) [7] and soil P content [5] against (a) observed stem growth (W_P), (b) basal area growth (ΔB) [6] at 33 sites. Filled and open symbols correspond to N- and P-limited sites, respectively, according to equation (2.2). The triangle symbols correspond to sites with no available soil phosphorous data.

Figure 4.

Relationships of simulated (using the ‘min{N : P}’ relationship under four model configurations) against (a) observed stem growth (W_P), and (b) basal area growth (ΔB) [6] at 33 sites. Each of the four model configurations correspond to a column: (i) with invariant and , assuming (ii) N limitation, (iii) P limitation and (iv) both N and P limitation across all studied sites. Filled and open symbols correspond to N and P limited sites, respectively, according to equation (2.2). The triangle symbols correspond to sites with no available soil phosphorous data.

Figure 5.

Model evaluation of simulated gross primary productivity (), and leaf respiration (R_C) using available observations derived from (a) eddy correlation and (b,c) bottom-up. Error bars for eddy correlation measurements correspond to uncertainty [35] and correspond to standard error for bottom-up estimates. Data sources for each site are described in the methods.

Figure 6.

Relationships of simulated against (a) observed stem growth (W_P) and (b) basal area growth (ΔB) [6] at 33 sites. Model configurations correspond to assumption of (i) N limitation (using [N]_A only) and (ii) P limitation (using [P]_A only) across all studied sites. Filled and open symbols correspond to N- and P-limited sites, respectively, according to equation (2.2). The triangle symbols correspond to sites with no available soil phosphorous data.

Figure 7.

Comparison of top of the canopy V_cmax derived from gas exchange measurements at four rainforest sties in the Amazon Basin (crosses, error bars correspond to one standard deviation), and estimated in this study using the ‘min{N : P}’ relationship (circles). Data sources from Brazilian sites are Caxiuana from Vale et al. [46], Manaus from Carswell et al. [45] and Tapajos from Domingues et al. [47]. The Bolivian site LFB-02 is courtesy of T. F. Domingues, unpublished data.