CO2 fertilization plays a minor role in long-term carbon accumulation patterns in temperate pine forests in the southwestern Pyrenees

Abstract

Isolating the long-term fertilization effect of CO2 from other climate-and site-related effects on tree growth has been proven a challenging task. To isolate long-term effects of [CO2] on water use efficiency at ecosystem level, we used the FORECAST Climate forest model, calibrated for Scots pine (Pinus sylvestris L.) forests in the southwestern Pyrenees, growing at a Mediterranean montane site and at a continental subalpine site. Future climate scenarios (RCP 4.5 and RCP 8.5) were generated using a battery of six climate models to estimate daily values of temperature and precipitation in a 90-year series. A factorial experiment was designed to disentangle the importance on C pools of three growing limiting factors (nitrogen limitation, climate (temperature + precipitation) limitation and atmospheric CO2 concentration). The relative importance of each factor was quantified by comparing the scenario with the limitation of each individual factor turned on with the non-limitation scenario. Positive CO2 fertilization due to improvement in water use efficiency was detected by the model, but its quantitative impact improving tree growth was minimum: its average increase in ecosystem C pools ranged from 0.3 to 0.9%. At the site with cooler climate conditions (continental), the main limitation for tree growth was climate. Such limitation will be reduced under climate change and the ecosystem will store more carbon. At the site with milder climate conditions (Mediterranean), N availability was the main limiting factor albeit modulated by water availability. Such limitation could be reduced under climate change as N cycling could accelerate (higher litterfall production and decomposition rates) but also increase if droughts become more frequent and severe. In addition, the magnitude of the uncertainty related to climate model selection was much more important than CO2 fertilization, indicating that atmospheric processes are more important than tree physiological processes when defining how much carbon could be gained (or lost) in forests under climate change. In conclusion, due to the small changes in forest C pools caused by variation of atmospheric CO2 concentrations compared to changes caused by other growth limiting factors (nutrients, climate), reducing uncertainty related to climate projections seems a more efficient way to reduce uncertainty in tree growth projections than increasing forest model complexity.

Ecosystem services provision by Mediterranean forests will be compromised above 2℃ warming

“…Whereas increasing atmospheric CO 2 concentrations might directly promote forest productivity and growth (e.g. Keenan et al, 2011), this effect will be strongly modulated by increasing temperatures and drought conditions (Bussotti et al, 2014; Doblas‐Miranda et al, 2017; Lo et al, 2019; Penuelas et al, 2011). This might partly explain why median projected trends for the ‘materials and assistance’ category were more negative for scenarios below the Paris Agreement (Figure 2b), under which CO 2 forcing is smaller.…”

Modelling Forest Ecosystems: a crossroad between scales, techniques and applications

“…(2017) simulated competition for nutrient, light and water in mixed conifer-broadleaf forests, whereas in this issue Lo et al (2019) demonstrated the low impact of increased CO2 availability in the productivity of pine forests. Both works used FORECAST Climate.…”

Reliable predictions of forest ecosystem functioning require flawless climate forcings

“…For this reason, they represent promising prospects for understanding and predicting forest ecosystems functioning changes. Studies have already tried to predict trends of forest productivity (Madani et al, 2018;Prislan et al, 2019;Tei et al, 2017), carbon storage (Gustafson et al, 2017;Lindner et al, 2014;Lo et al, 2019) or C, H2O and N dynamics (Dong et al, 2019) in the decades to come.…”