Paper Reviewed
Lu, K., Chen, N., Zhang, X., Wang, J., Wang, M., Khan, S., Han, C., Zhang, C., Wang, S., Wang, L., Gao, W., Liu, Y. and Zhao, C. 2019. Increased drought and atmospheric CO2 positively impact intrinsic water use efficiency but do not promote tree growth in semi-arid areas of northwestern China. Trees 33: 669-679.

In a recent paper Lu et al. (2019) examined sixty years of climate and tree-ring data to investigate the impact of rising atmospheric CO2 and drought on the growth and water use efficiency of trees growing in the Xinglong Mountain National Nature Reserve, which reserve is located at the intersection of the Qinghai-Tibet, Loess and Neimenggu-Xinjiang plateaus. And what did their examination reveal?

Climate data over the past six decades indicate this semi-arid site has experienced rising temperatures and declining precipitation, which combination of factors has led to long-term drought over the period 1955-2014, evidenced by the downward linear trend of the Palmer Drought Severity Index displayed in Figure 1a. Not surprisingly, the sustained drought in this already semi-arid region has had a negative effect on tree basal area increment (BAI) as shown in Figure 1b, albeit the negative effect has reversed direction since 1980, inducing a positive trend in BAI since that time (green shading) that is returning to values witnessed at the beginning of the record.

With respect to trends in intrinsic water use efficiency (iWUE), Lu et al. report that this parameter has increased by 32% since 1955. And as for the cause of that increase, they estimate 98% of it is due to the approximate 23% concomitant rise in atmospheric CO2, which CO2 increase had a large reducing effect on stomatal conductance.

As for the future, it is very likely that as the CO2 concentration of the atmosphere continues to rise, its growth-enhancing and water-saving effects will increase to the point of fully mitigating the negative impacts of drought on the growth of these trees. And, of course, those negative impacts may be alleviated a lot sooner if climate conditions causing the drought are eased. Nevertheless, this study provides additional field proof in the world of nature of the reality of the benefits of rising atmospheric CO2 on plant water use efficiency.

Figure 1. Trends in the Palmer Drought Severity Index (PDSI, left panel), basal area increment (BAI, center panel) and intrinsic water use efficiency (iWUE, right panel) for the Xinglong Mountain National Nature Reserve over the period 1955-2014. Source: Adapted from Lu et al. (2019).