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How (not?) to estimate changes in the suitability of days for plant growth

Posted by colinprentice on 11 Jun 2015 at 10:47 GMT

Implications of global environmental change for the growth of plants (whether as natural ecosystems, managed forests, or crops) are a hot topic. Key aspects, such as the control of leaf canopy temperatures, the ability of plants to acclimate to high temperatures, and the effectiveness of CO2 "fertilization" and water saving by plants at high CO2 under different environmental conditions, are incompletely understood. There are obvious concerns, for example, for regions that are undergoing increasing drought and where this trend is projected to continue, and about the major geographic changes in global agriculture that will be necessary if "high-end" climate change projections come about.

This paper attempts to cut through these complexities by means of an empirical analysis of net primary production (NPP) derived from satellite observations. The results are dramatically more pessimistic than previously published analyses obtained with Earth System models, which have many of their own uncertainties and problems. However, there are good reasons to suspect that these new results are strongly biased towards a "glass half empty" interpretation of the evidence. In summary:

1. The specific method adopted implicitly assumes that climate combinations that are rare today are unsuitable for plant growth. So for example, projected future increases in temperature and rainfall together might create novel environments that would be suitable for plant growth, but the method assumes they are not suitable. This approach produces some strange but presumably significant side-effects, such as an upper temperature threshold that is below the optimum for photosynthesis as observed in many species.

2. The quantity analysed, called MODIS NPP, is not a measurement of NPP. It is a model of NPP, that has previously been criticized for the way in which it treats plant respiration, which tends to exacerbate the modelled effects of warming on NPP.

3. The approach assumes that any positive effects of CO2 concentration on growth or water use by plants do not exist. This is one extreme position on a continuing controversy. The authors refer to an "over-emphasis" on CO2 fertilization in current Earth System models, but they do not present evidence for their view which is, as far as I know, impossible to reconcile with the continuing and strong uptake of anthropogenic CO2 by the land.

Colin Prentice

No competing interests declared.

RE: How (not?) to estimate changes in the suitability of days for plant growth

belinda_medlyn replied to colinprentice on 11 Jun 2015 at 21:42 GMT

Thanks Colin, you have very clearly articulated the concerns I had with this article. Here is the link to my previous comment on the temperature dependence of respiration in the MODIS NPP algorithm

One further comment is that although the authors of this article made their data available for download, it is not in a form that is usable. I was attempting to recreate Figure 1 from their data to find out exactly what the upper temperature threshold was (it seemed surprisingly low). However, the columns in the dataset are given labels like "VAL1", "VAL2" and "VAR" with no explanation or units. The readme sheet just says "These are the data used in Figure 1" with no further explanation. It is important to provide sufficient information along with data to enable their correct interpretation.

No competing interests declared.