Modelling forest response to increasing CO₂ concentration under nutrient‐limited conditions

Abstract

The growth rates of woody plants depend on both the rate of photosynthetic carbon gain and the availability of essential nutrients. Instantaneous carbon gain is known to increase in response to increasing atmospheric CO2 concentration, but it is uncertain whether this will translate into increased growth in the longer term under nutrient‐limited conditions. An analytical model to address this question was developed by Comins & McMurtrie (1993, Ecological Applications 3, 666–681). Their model was further tested and analysed. Manipulation of various assumptions in the model revealed its key assumptions and allowed a more confident prediction of expected growth responses to CO2 enrichment under nutrient‐limited conditions. The analysis indicated that conclusions about the CO2 sensitivity of production were strongly influenced by assumptions about the relationship between foliar and heartwood nitrogen concentrations. With heartwood nitrogen concentration proportional to foliar nitrogen concentration, the model predicted a strong response of plant productivity to increasing CO2 concentration, whereas with heartwood nitrogen concentration set constant, the model predicted only a very slight growth response to changing CO2 concentration. On the other hand, predictions were only slightly affected by: (1) assumptions about the extent of nitrogen retranslocation out of senescing roots and foliage or wood during heartwood formation; (2) the effects of nitrogen status on specific leaf area or (3) leaf longevity; (4) carbon allocation between different plant parts; or (5) changes in the N:C ratio of organic matter sequestered in the passive pool of soil organic matter. Modification of the effect of foliar nitrogen concentration on the light utilization coefficient had only a small effect on the CO2 sensitivity for pines. However, this conclusion was strongly dependent on the chosen relationship between single‐leaf photosynthesis and leaf nitrogen concentration. Overall, the analysis suggested that trees growing under nitrogen‐limited conditions can respond to increasing atmospheric CO2 concentration with considerable increases in growth.

Effects of elevated CO2, O3 and K deficiency on Norway spruce (Picea abies): nutrient supply, content and leaching

“…We suggest therefore that the increased net photosynthetic carbon gain resulting from COg enrichment (see Barnes et al, 1995a) might have been reflected in a substantially increased rate of carbon input into the rhizosphere, which, combined with an increase in root size (Pfirrmann, 1992), stimulated microbial activity and increased CECeff. It remains to be established whether similar effects occur in the field in response to prolonged CO2 enrichment because longer-term exposure to elevated CO2 has been shown to increase the C/N ratio of litter through increases in carbon-based secondary compounds such as lignin and tannins (Eajer, Bowers & Bazzaz, 1992), and this might reduce decomposition and nitrogen mineralization processes, leading to a progressive immobilization of nutrients in the soil (Kirschbaum et al, 1994).…”

“…However, there is growing recognition that this response can only be sustained where other resources, particularly nutrients, are available to support the extra growth induced by CO2 enrichment (Kirschbaum et al, 1994;Gundersson & Wullschleger, 1994;Petterson & McDonald, 1994;Barnes et al, 1995a). One factor influencing the nutrient content of tree foliage is the rate at which cations are leached from the canopy (Tukey, 1970).…”

Tree and forest functioning in an enriched CO2 atmosphere

“…However, most models are still sensitive to key input parameters and assumptions, so that sensitivity analysis of model outcomes must be made to indicate the probability range of the predictions. One ecosystem model suggests that increases in tree C storage under elevated CO # might eventually be reduced through nutrient limitations if N is stored in pools of slow turnover such as wood (Kirschbaum et al, 1994). However, Thornley & Cannell's (1996) model suggested that coniferous forests increase N acquisition in response to elevated CO # and hence sustain increased net primary productivity despite decreasing N supply and\or leaf area index.…”

“…Models incorporating the effects of elevated CO # on production or ecosystem physiology have increased greatly in complexity and realism since a previous review (Long & Hinchin, 1991), and it is now possible to employ experimental results from plantlevel studies toward estimating regional and global processes in a future, higher CO # environment (Kirschbaum et al, 1994 ;Woodward, Smith & Emmanuel, 1995) with appropriate scaling (Jarvis, 1995). However, most models are still sensitive to key input parameters and assumptions, so that sensitivity analysis of model outcomes must be made to indicate the probability range of the predictions.…”

MAESPA: a model to study interactions between water limitation, environmental drivers and vegetation function at tree and stand levels, with an example application to [CO<sub>2</sub>] × drought interactions

“…For example, although the model has often been used to estimate effects of elevated atmospheric [CO 2 ] (C a ) on canopy performance (e.g. Kirschbaum et al, 1994;Medlyn, 1996;Kruijt et al, 1999;Luo et al, 2001), the model cannot be used to investigate elevated [CO 2 ] impacts in drought conditions because it is not possible to calculate how elevated [CO 2 ] modifies soil moisture.…”