Increased Needle Nitrogen Contents Did Not Improve Shoot Photosynthetic Performance of Mature Nitrogen-Poor Scots Pine Trees

Abstract

Numerous studies have shown that temperate and boreal forests are limited by nitrogen (N) availability. However, few studies have provided a detailed account of how carbon (C) acquisition of such forests reacts to increasing N supply. We combined measurements of needle-scale biochemical photosynthetic capacities and continuous observations of shoot-scale photosynthetic performance from several canopy positions with simple mechanistic modeling to evaluate the photosynthetic responses of mature N-poor boreal Pinus sylvestris to N fertilization. The measurements were carried out in August 2013 on 90-year-old pine trees growing at Rosinedalsheden research site in northern Sweden. In spite of a nearly doubling of needle N content in response to the fertilization, no effect on the long-term shoot-scale C uptake was recorded. This lack of N-effect was due to strong light limitation of photosynthesis in all investigated canopy positions. The effect of greater N availability on needle photosynthetic capacities was also constrained by development of foliar phosphorus (P) deficiency following N addition. Thus, P deficiency and accumulation of N in arginine appeared to contribute toward lower shoot-scale nitrogen-use efficiency in the fertilized trees, thereby additionally constraining tree-scale responses to increasing N availability. On the whole our study suggests that the C uptake response of the studied N-poor boreal P. sylvestris stand to enhanced N availability is constrained by the efficiency with which the additional N is utilized. This efficiency, in turn, depends on the ability of the trees to use the greater N availability for additional light capture. For stands that have not reached canopy closure, increase in leaf area following N fertilization would be the most effective way for improving light capture and C uptake while for mature stands an increased leaf area may have a rather limited effect on light capture owing to increased self-shading. This raises the question if N limitation in boreal forests acts primarily by constraining growth of young stands while the commonly recorded increase in stem growth of mature stands following N addition is primarily the result of altered allocation and only to a limited extent the result of increased stand C-capture.

Keywords: Jmax; Pinus sylvestris; Vcmax; arginine; optimality; phosphorus; photosynthesis; resource-use efficiency.

Figure 1

Photosynthetic capacity vs. needle nitrogen and phosphorus content per surface area (N_a and P_a) in Pinus sylvestris. Variation in (A) maximum carboxylation rate (V_cmax) at 25°C with N_a, (B) V_cmax at 25°C with P_a, (C) maximum electron transport rate (J_max) at 25°C with N_a, and (D) J_max at 25°C with P_a. Control plot (C) = open symbols, fertilized plot (F) = filled symbols, squares = current-year needles, circles and dashed lines = 1-year-old needles.

Figure 2

Relationship between predicted and measured maximum carboxylation rate (V_cmax) and maximum electron transport rate (J_max) at 25°C in 1-year-old Pinus sylvestris needles based on (i) needle nitrogen content per unit area, N_a, (N: open circles, dashed lines) and (ii) N_a and needle phosphorus content per unit area, P_a, (N, P: filled circles, solid lines). Sub-figures: V_cmax on the (A) fertilized and (B) control plots; J_max on the (C) fertilized, and (D) control plots. Dotted lines, non-significant relationships.

Figure 3

Variation with daily mean shoot-incident photosynthetic photon flux density (Q) in (A) needle N allocation per long-term mean incident light intensity (N_a/Q), (B) light-use efficiency (LUE), (C) nitrogen-use efficiency (NUE), and (D) phosphorus-use efficiency (PUE) of 1-year-old Pinus sylvestris shoots within the fertilized (filled circles, solid lines) and the control (open circles, dashed lines) canopies in August 2013.

Figure 4

(A) Cumulative percentage of incident photosynthetic photon flux density (Q) measured adjacent to the continuously measured Pinus sylvestris shoots at the fertilized (F, filled symbols) and the control (C, open symbols) plots, and (B) the daily mean integrated shoot-scale net photosynthetic performance (A_n) in relation to daily mean Q at F and C observed in August 2013. The regression line and statistics in (B) are shown for the pooled data.

Figure 5

Modeled vs. observed net photosynthetic performance (A_n) of Pinus sylvestris shoots at the fertilized (F, filled symbols) and the control (C, open symbols) plots in August 2013. The regression line and statistics are shown for the pooled data.