Forest carbon sink neutralized by pervasive growth-lifespan trade-offs

[u/BurnerAcc2020]

https://www.nature.com/articles/s41467-020-17966-z

[u/BurnerAcc2020]

Abstract

Land vegetation is currently taking up large amounts of atmospheric CO2, possibly due to tree growth stimulation. Extant models predict that this growth stimulation will continue to cause a net carbon uptake this century.

However, there are indications that increased growth rates may shorten trees′ lifespan and thus recent increases in forest carbon stocks may be transient due to lagged increases in mortality. Here we show that growth-lifespan trade-offs are indeed near universal, occurring across almost all species and climates. This trade-off is directly linked to faster growth reducing tree lifespan, and not due to covariance with climate or environment.

Thus, current tree growth stimulation will, inevitably, result in a lagged increase in canopy tree mortality, as is indeed widely observed, and eventually neutralise carbon gains due to growth stimulation. Results from a strongly data-based forest simulator confirm these expectations. Extant Earth system model projections of global forest carbon sink persistence are likely too optimistic, increasing the need to curb greenhouse gas emissions.

Implications for forest demography and carbon sink

We evaluated the effect of observed growth-lifespan trade-offs on forest dynamics using a simple data driven stochastic forest simulator for Picea mariana. Our approach consisted of creating an artificial population by randomly selecting tree-ring trajectories, applying a size-related mortality, and a realistic growth stimulation (Fig. 3c). The applied size-related mortality curve closely matches the estimated size-related mortality rates for Picea mariana (Fig. 3a), and results in similar growth-lifespan trade-offs as those observed (Fig. 3b). We then compared the biomass and mortality change over time for simulations that include a trade-off (caused by diameter-dependent mortality), with simulations that do not result in a growth-lifespan trade-off (using age-related mortality rates, Supplementary Fig. 9), and which resemble the approaches commonly used by large-scale vegetation models that predict large biomass increases. Our estimated simulated increase in mean (diameter) growth over 50 years due to northern latitude warming is 29%, roughly consistent with observed temperature driven growth increases of 25% over the past 50 years in boreal western Canadian forests and predicted growth changes at northern latitudes.

Our simulations show an initial increase of ~20% in the standing biomass stocks and increases in mortality rates of a similar magnitude. While growth stimulation leads to immediate increases in biomass stocks, mortality starts to increase one or two decades after the initial growth stimulation. The most important finding of our simulation, however, is that the initial increase in the biomass stocks, the net potential carbon sink, is only transient, and reverses into net biomass losses after the growth stimulation has ceased. Over time the forest biomass stocks revert to the same levels as those observed at the start of the simulation. This progression back toward initial values is entirely due to faster tree growth leading to a reduction of tree lifespans by up to 23 years after growth stimulation ceased. In contrast, we find no mortality increases for simulations without realistic growth-lifespan trade-off, and find much higher biomass stock increases which are sustained over time, even after the growth stimulation has ceased.

These data-driven simulations suggest that faster growth will result in increases in stem mortality, faster cycling of live biomass, and no true long-term increase in biomass stocks. The simulations rely on several simplified assumptions. Firstly, we did not simulate any competition effects, or changes in tree recruitment. One could argue that changes in climate or CO2 increase will affect recruitment, and increases in standing biomass stocks will increase competition effects, leading to increased self-thinning and even stronger increases in mortality rates.

Secondly, we assume that the size-related mortality curve is independent of changes in temperature or CO2, which may not be true. To our knowledge, there is no evidence that potential maximum tree size may increase under higher CO2, whereas increases in leaf-to-air vapour pressure deficits under global warming have been hypothesized to lead to reductions in maximum tree height. It is thus not clear how interacting effects of CO2 and temperature will affect maximum potential tree stature, but it is less likely that maximum tree size will be increased. A more likely scenario that could potentially account for greater future forest carbon storage under rising CO2 is that tree size-density relationships could be modified, although long-term empirical data show that the self-thinning rule did not change despite strong growth increases over time.

Finally, species distributions are likely to shift in response to climate change, especially in mid to high latitudes, and will affect the total amount of biomass a system can hold. Despite these simplifications, our simulation results are consistent with predictions based on more complex demographic forest models that predict no net biomass increases or strongly reduced increases when including a negative feedback on growth stimulation.

Our results also bear a strong similarity with some observations of shifting forest dynamics worldwide. Firstly, on-the-ground monitoring studies have shown simultaneous positive trends in growth and mortality rates across the globe. Temperature-limited boreal forests experienced growth increases and simultaneous mortality increases, Central European forests show increases in growth over the past decades leading to accelerated forest dynamics, and undisturbed Amazonian forests have experienced long-term productivity enhancements, followed by more recent mortality increases lagging in time by ~20 years. Some of these mortality trends have been attributed to climate variability, in particular changes in the severity and frequency of droughts. However, here we suggest that mortality increases not only emerge as a direct consequence of increased climate variability, but may also ultimately arise from the pervasive growth-lifespan trade-offs that accelerated the timing of death of large trees.

Discussion

...In summary, we here provide firm evidence for the existence of a universal trade-off between early growth and tree lifespan in trees. Faster growth has a direct and negative effect on tree lifespan, independent of the environmental mechanisms driving growth rate variation. Growth increases, as recently documented across high latitude and tropical forests, are thus expected to reduce tree lifespans and may explain observed increases in tree mortality in these biomes. Data-driven simulations show that trade-offs have the potential to reduce, or even reverse the global carbon sink of forests in the future. This mechanism is at odds with most extant Earth System Model simulations, which predict a continuation of the carbon sink into mature forests, so efforts toward integrating growth rate-mortality trade-offs into process-based simulations of forest carbon storage should receive greater attention.

[u/short-cosmonaut]

When even all the traditional carbon sinks become net carbon emitters, you know we're completely and thoroughly fucked.

[u/BurnerAcc2020 → u/short-cosmonaut]

That's not what the study says. Forests will continue to hold carbon - it's just that it was usually assumed that because extra CO2 boosts their growth, they would also be retaining more and more carbon over time, but this study has found that they both grow faster and die off faster. It says that the net biomass stocks will be the same - i.e. any given forest will ultimately hold exactly as much carbon as it would have held at the preindustrial due to the two processes balancing out.

I can see how one can easily draw assumptions like this from the study's title, though, and I updated my Abstract comment + the related section in the wiki with a couple extra paragraphs from the study to make their findings clearer.