An email about estimating productivity


I had a student ask me something along the lines of:

How do I calculate woody productivity in a woodland plot? I have seen others calculate the difference in standing biomass between censuses as net primary productivity. Is this right?

In our lab group we use a lot of repeat inventory tree growth data, from plots in forests, savannas, and woodlands, to understand rates of biomass dynamics and land-atmosphere carbon exchange. I wrote quite a long email in response, so I thought I would share it here:

There are lots of subtly different approaches to quantifying woody “productivity”. The literature is not always totally precise in their definitions of productivity, which can make it all a bit confusing.

Generally, Net Primary Productivity (NPP) is defined as the atmospheric carbon uptake by vegetation after subtracting the CO2 emitted via plant respiration, i.e. GPP minus autotrophic respiration, where GPP is Gross Primary Productivity, the rate of atmospheric carbon uptake by plants over a given time period.

Crucially, NPP doesn’t account for carbon losses from the system via the mortality and subsequent decomposition of woody stems, i.e. heterotrophic respiration, or loss of carbon from woody stems due to disturbances like fire.

So, to measure NPP you can’t simply calculate the difference in living woody biomass between censuses. This calculation of living woody biomass change would be more readily termed Net Biome Productivity (NBP).

Instead, to measure NPP you would have to sum all the woody growth that has occurred within the census interval, including woody stems which have died within the census interval. This is not straightforward, as in our fire-prone system in Bicuar sometimes trees sometimes die and burn away within the census interval. Feasibly, trees may also germinate, grow to sufficient size to be measured, and die within a single census interval. There are ways to estimate this unmeasured productivity, e.g. Talbot et al. (2014 ), and Kohyama et al. (2019 ).

The next thing to think about then, is whether you actually want to measure NPP (the rate of woody growth in living stems), or NBP (the rate of ecosystem-level woody biomass change, including mortality losses). I will leave you to consider this.

I have written R code to estimate both NPP and calculate NBP , using the methods in Kohyama et al. (2019). Let me know if you would like me to send it over.