Modelling the limits on the response of net carbon exchange to fertilization in a south‐eastern pine forest

Abstract

Using a combination of model simulations and detailed measurements at a hierarchy of scales conducted at a sandhills forest site, the effect of fertilization on net ecosystem exchange ( NEE ) and its components in 6-year-old Pinus taeda stands was quantified. The detailed measurements, collected over a 20-d period in September and October, included gas exchange and eddy covariance fluxes, sampled for a 10-d period each at the fertilized stand and at the control stand. Respiration from the forest floor and aboveground biomass was measured using chambers during the experiment. Fertilization doubled leaf area index (LAI) and increased leaf carboxylation capacity by 20%. However, this increase in total LAI translated into an increase of only 25% in modelled sunlit LAI and in canopy photosynthesis. It is shown that the same climatic and environmental conditions that enhance photosynthesis in the September and October periods also cause an increase in respiration The increases in respiration counterbalanced photosynthesis and resulted in negligible NEE differences between fertilized and control stands. The fact that total biomass of the fertilized stand exceeded 2·5 times that of the control, suggests that the counteracting effects cannot persist throughout the year. In fact, modelled annual carbon balance showed that gross primary productivity ( GPP ) increased by about 50% and that the largest enhancement in NEE occurred in the spring and autumn, during which cooler temperatures reduced respiration more than photosynthesis. The modelled difference in annual NEE between fertilized and control stands (approximately 200 g C m − − − − 2 y − − − − 1 ) suggest that the effect of fertilization was sufficiently large to transform the stand from a net terrestrial carbon source to a net sink.

Scaling up of Carbon Exchange Dynamics from AmeriFlux Sites to a Super-Region in the Eastern United States

“…Furthermore, uncertainties in describing the non-stationarity and vertical inhomogeneity in physiological parameters (e.g., in photosynthesis calculations) may overshadow any improvements gained by resolving this 2-way interaction. While the well-mixed assumption may be defensible for some canopy types, it is too simplistic for forested ecosystems, where experimental evidence suggest that vertical variations in excess of 50 ppm for CO2 concentration and 3 degrees or more for air temperature occur inside the canopy volume during day time conditions (Lai et al, 2002a;Siqueira and Katul, 2002). Because the vertical variations in mean scalar concentration profiles are not random within the canopy, the well-mixed assumption may inject systematic biases in modeling scalar sources, sinks, and fluxes.…”

Forest and shrubland canopy carbon uptake in relation to foliage nitrogen concentration and leaf area index: a modelling analysis

“…Net carbon gain in response to fertiliser application was less than 5% for Pinus radiata [36] or not detectable for Pinus elliottii [7]. When L in young Pinus taeda was doubled following application of fertiliser, canopy A increased by only 50% and canopy R d was increased by 100% [26].…”

“…From late summer (March) onwards, net carbon uptake was reduced because of decreases in photosynthesis associated with lower irradiance but continued rates of respiration. This emphasises the important Carbon uptake, leaf area and nitrogen 533 contribution of net carbon uptake in spring and early summer for tree growth [26]. There may be less carbon available for growth in summer and winter when canopy photosynthesis is more offset by respiration or, at sites elsewhere, when other environmental influences, for example drought, limit photosynthesis [34,49].…”

“…This is accompanied by an increase in the rate of photosynthesis, A, at the leaf and canopy scales, but the size of this response is usually much less than the effects on L [33,48,49]. Further, the combined effects of increases in self-shading, and rates of night-time respiration associated with increased foliage area [42], may result in only small increases in net carbon uptake at the canopy scale [7,26,36].…”

Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

“…V 0cmax25 is derived from Lai et al [2002] where V cmax at the top of canopy at 28°C was measured as 84.5 umol/m 2 /s for the same stand, and k n is the nitrogen extinction coefficient which was also taken from Lai et al [2002]. V ccmax25 is total canopy carboxylation capacity at 25°C, which will be dynamically distributed between sunlit and shaded leaves.…”