Have we drastically underestimated the productive capacity of plants?
August 28, 2014
By Chad Kruger
A new paper published in Environmental Science & Technology (DeLucia et al., 2014) suggests that scientists have drastically underestimated the earth’s theoretical potential to produce biomass – by as much as 2 orders of magnitude! That’s going to take a minute to wrap my mind around.
The rationale of the authors is that most analyses of global Net Primary Productivity – or NPP (the amount of biomass produced around the world annually) – assume that natural ecosystems represent the upper threshold of potential productivity. However, we know that this is not necessarily true, as varietal development, fertilizer enrichment, agronomic management, and supplemental irrigation (moving water from one part of an ecosystem to another increase water use efficiency) can all contribute to managed ecosystems (e.g. agricultural fields) that are considerably more productive than native ecosystems. Andy McGuire recently pointed out that for much of the irrigated western US, agricultural yields are much higher than would be expected given the characteristics of the soils produced by native vegetation.
But two orders of magnitude? That’s phenomenal and leaves me thinking two things:
First, I’m not sure I really want to live on a planet where we focus on maximizing global NPP. While it may theoretically be possible to see this kind of increase in productivity, such increases would likely result in challenges to sustainability of an equivalent magnitude. Many of our current agricultural sustainability concerns are caused by such things as varietal development, fertilizer enrichment, agronomic management and supplemental irrigation. For instance, biomass yields for corn in the arid west are often an order of magnitude greater than biomass productivity in shrub steppe, but we also add upwards of an order of magnitude more water and fertilizer than is present in the native ecosystem. Isolating this degree of human intervention to a few million acres in the west gives us plenty of challenges to deal with. Thinking about ocean to ocean management of our landscape should be humbling.
Second, this analysis provides the first rigorous global quantification (even if it is very simplistic) that gives some credible basis for some of the more outlandish claims for terrestrial carbon sequestration potential (e.g., Alan Savory’s grazing systems). One of the most frequent criticisms of these claims are that it wouldn’t be possible to sustain productivity increases sufficient to drive soil carbon sequestration rates long enough to achieve the change Savory and others have suggested possible. Restoring soil carbon to “native levels” has often been assumed to be the theoretical maximum. However, we know that the primary driver of soil carbon increases is biomass inputs – so even an order of magnitude more biomass produced should translate into significantly greater carbon sequestration than might currently be assumed.
Whether this “new theoretical maximum NPP” is plausible, much less desirable at a global level, it does give me pause to think about whether targeted management of certain landscapes (e.g. rangelands) do hold a much greater potential to produce biomass and sequester carbon than has been assumed to date.