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.
Highlights
Many studies, since the mid-1900’s, have shown how addition of nitrogen (N) to boreal, coniferous forests results in sharply increased basal area increment rates (Tamm, 1956; Brix, 1971, 1983; Linder and Axelsson, 1982; Axelsson and Axelsson, 1986; Linder, 1987)
The use of N as a scaling factor is supported by observational data showing photosynthetic capacity variation in response to foliar N contents (e.g., Evans, 1989; Reich et al, 1995; Kellomäki and Wang, 1997; Niinemets et al, 2001; Han et al, 2004; Wyka et al, 2012), and by theoretical calculations indicating that canopy-scale photosynthesis is maximized when N is allocated proportionally to photosynthetic photon flux density (Q) within the canopy (Field, 1983; Farquhar, 1989)
The current study provides additional evidence of the importance of partitioning shifts by showing that photosynthetic performance per unit needle area was not enhanced in response to the N fertilization and, increased canopy-scale C uptake could only account for a portion of the observed aboveground primary production (ANPP) difference
Summary
Since the mid-1900’s, have shown how addition of nitrogen (N) to boreal, coniferous forests results in sharply increased basal area increment rates (Tamm, 1956; Brix, 1971, 1983; Linder and Axelsson, 1982; Axelsson and Axelsson, 1986; Linder, 1987). The use of N as a scaling factor is supported by observational data showing photosynthetic capacity variation in response to foliar N contents (e.g., Evans, 1989; Reich et al, 1995; Kellomäki and Wang, 1997; Niinemets et al, 2001; Han et al, 2004; Wyka et al, 2012), and by theoretical calculations indicating that canopy-scale photosynthesis is maximized when N is allocated proportionally to photosynthetic photon flux density (Q) within the canopy (Field, 1983; Farquhar, 1989) This proportionality is not observed in natural canopies and various explanations have been offered for why the theoretically optimal pattern does not emerge. While modeling studies have shown optimal N allocation to have the ability to considerably increase the plant-scale C uptake in some cases (Hirose and Werger, 1987; Hollinger, 1996), the predicted differences in C uptake between observed and optimal N allocation patterns were small for hemi-boreal Picea abies growing under N-rich conditions (Tarvainen et al, 2013)
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