Global Greening: As CO2 Levels Rise, Forests Grow Faster

How much? Well, according to Prof. Pretzsch et al., intimate that 'the rise in temperatures' and 'extended growing seasons' may be the causes

Meet Professor Hans Pretzsch, formerly at the Technical University of Munich, Germany, and one of the leading forest ecologists of his generation (according to his faculty profile, he was born in 1957).

Back in 2014, a very interesting paper was published in Nature, which bears the quite concise title ‘Forest stand growth dynamics in Central Europe have accelerated since 1870’. It is apparently available in full (and if it doesn’t work for you, please drop me a line by email), and we shall go through that publication today.

Here cometh the abstract, and please note that I’ve removed the references, added emphases, and commentary (in squared parentheses) here and there:

Forest ecosystems have been exposed to climate change for more than 100 years, whereas the consequences on forest growth remain elusive. Based on the oldest existing experimental forest plots in Central Europe, we show that, currently, the dominant tree species Norway spruce and European beech exhibit significantly faster tree growth (+32 to 77%), stand volume growth (+10 to 30%) and standing stock accumulation (+6 to 7%) than in 1960. Stands still follow similar general allometric rules, but proceed more rapidly through usual trajectories. As forest stands develop faster, tree numbers are currently 17–20% lower than in past same-aged stands. Self-thinning lines remain constant, while growth rates increase indicating the stock of resources have not changed, while growth velocity and turnover have altered. Statistical analyses of the experimental plots, and application of an ecophysiological model, suggest that mainly the rise in temperature and extended growing seasons contribute to increased growth acceleration, particularly on fertile sites.

Oh, look, would you have guessed? Sure, NASA has been confirming ‘global greening’ for some time, at least since 2016, if this piece entitled ‘Carbon Dioxide Fertilization Greening Earth, Study Finds’ or the below short video clip are any indication:

So, Pretzsch et al. are providing statistical analyses underwriting these ‘findings’ by NASA. Let’s see what this is all about, shall we?

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Introduction

The lifespan of many tree species is over several hundred years; therefore, knowledge regarding tree and forest stand dynamics, and long-term impacts due to environmental change is largely incomplete. Retrospective tree ring analyses can only partly close this knowledge gap, as this method indeed offers insights into tree growth, but not into stand dynamics. Forest inventories primarily assess managed forests, where the influences of climate change and thinning might be coalesced, and difficult to differentiate. [line break added]

Models are frequently used as a means to circumvent data collection and subsequent analyses. However, modelling is no substitute for underlying field data, and the full potential of any modelling approach is only fulfilled when feedback between modelling studies and empirical analyses are achieved. A unique source of information, however, was provided by long-term experimental plots established in approximately 1872, the founding year of the International Union of Forest Research Organisations. These plots, which were surveyed 10–20 times until the present day, provide the longest existing time series data on forest stand dynamics available, approximately 140 years. Originally, the study stands were established to examine stand growth principles, but not for growth trend analysis.

We chose Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) as the study species. These taxa dominate Central Europe’s forests occupying 30%, that is, a total area of 14 × 10⁶ ha of all forest areas. The plots selected for this study represent pure, even-aged stands, which were established by planting or seeding. Site conditions varied broadly, and soils ranged from dry silty sands to moist deep silts. Since the first site observations and records in 1870, the plots were maintained under continuous scientific control, and surveyed on a single tree basis. Therefore, investigators excluded plots and reports impacted by disturbances, including storms or bark beetle infestations. We included only unmanaged, or at most moderately thinned, but always fully stocked plots. This selection resulted in a unique survey data set from 36 spruce and 22 beech plots.

Based on these data we show that both species currently exhibit significantly faster tree growth, stand volume growth and standing stock accumulation than still in 1960 and the decades before. Self-thinning lines remain constant, while growth rates increase indicating the stock of resources have not changed, while growth velocity and turnover have altered [so, basically, while humans are massively polluting the area and air, trees are growing faster and bigger]. This means stands [another word for ‘plot’] still follow similar general allometric rules, but proceed more rapidly through usual trajectories. As we can demonstrate, this results in stands currently having lower tree numbers per unit area than past stands at the same age. Our data also reveal that the growth acceleration is stronger on fertile sites, which is supported by scenario runs with an ecophysiological growth model.

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Intermission: Why Does the Earth Get Greener?

Basically, this has to do with the ‘pores’, or ‘mouths’ of plants—known as stomata—that ‘breath’ in ambient air don’t need to be as open as under earlier conditions of less CO2.

Most plants would ‘breath’ in ‘air’ during the day, and they do so via these stomata. CO2 being a key reactant in photosynthesis, most plants ‘open and close their stomata during the daytime, in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration’.

Utilising a process called ‘transpiration’, water, which is typically taken up by the roots of most plants, is the key here. ‘When water uptake by the roots is less than the water lost to the atmosphere by evaporation plants close small pores called stomata to decrease water loss.’ Closing the stomata ‘slows down nutrient uptake and decreases CO2 absorption from the atmosphere’, which means that plants become less metabolically active, perform less photosynthesis, and grow less.

As temperatures rise and growing seasons get longer, with warmer air holding more water vapour (i.e., warmer air is more humid, as per the Clausius-Clapeyron relation), this seemingly small change is a huge contributor here: for every 1 °C (1.8 °F) rise in temperature, the water-holding capacity of the atmosphere increases by about 7%.

In short: higher temperatures signify a more humid atmosphere, which causes plants to close their stomata less long/often/frequently. Hence, higher temperatures and water vapour content of the atmosphere drive plant growth.

As an aside, here’s a piece from the WEF (I know…) ringing the alarm bells about desertification and calling for ‘desert skin graft’. You may contrast it with this piece in Popular Mechanics reporting on a new study indicating that desertification may be due to the Earth’s axial tilt (wobble) that makes deserts ‘green’ or dry every roughly 21,000 years. You can read that study here.

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Results by Pretzsch et al. (2014)

First, we pooled our data and compared them to standard yield tables. Yield tables, common forestry tools that tabulate stand growth age dependently, were developed primarily from 1795 to 1965. Yield table data were derived from long-term plot field survey data; and thus they served to represent past growth conditions in this comparison. We found stand growth rates and standing stocks after 1960 exceeded the yield table ranges by 50–100%, which called the validity of yield table range data into question, and suggested essential changes in stand dynamics.

There is an illustration that I’m not reproducing here, but it does show yields being literally ‘off the charts’.

For the same period our plot surveys spanned, we compiled available data on environmental variables reported to drive forest growth dynamics. For Central Europe, forest environmental and growing conditions exhibited significant changes since the first experimental forest plots were established in 1870. During the addressed period, the atmospheric CO2 concentration rose from 295 p.p.m. in 1900 to approximately 390 p.p.m. in 2010. This means an increase of more than 30% within nearly one century. Wet N-deposition increased by 0.5–1.0 kg ha⁻¹ per decade. Throughout Central Europe, average total N-deposition increased from approximately 2.5 kg ha⁻¹ per year in 1900 to more than 9 kg ha⁻¹ per year in the first decade of the twenty-first century. [line break added]

Global average temperature has increased by roughly 0.7 °C within the twentieth century. During the same period, the average temperature in Europe has risen by 0.95 °C. In Germany, the mean annual air temperature increased by 1.0 °C during the twentieth century; and the sum annual precipitation increased by 9% during the same period. However, the annual distribution varied. During the winter months, precipitation increased by 19% during the last century, and rainfall in summer decreased by 3% on average. If only the second half of the twentieth century is measured, summer precipitation shows a 16% reduction.

In addition, the rise in atmospheric CO2 concentration, the higher N-deposition and the increase in air temperature were two to three times higher in the second half of the twentieth century compared with the first half. However, the strong temperature increase during the last 50 years reported for all of Europe, but was not reported for Germany. The annual mean temperature increase for 1950–2000 was equal to the century average at 1.0 °C; only winter temperatures showed a higher increase in the second half of the twentieth century. Higher temperatures will also extend the growing season. Menzel and Fabian reported the average annual growing season has been extended by 10.8 days, since the early 1960s. Chmielewski and Rötzer also found the vegetation period was lengthened between 0.6 and 6.3 days per decade in different European natural regions during 1969–1998. Based on temperature data from the four climate stations used in this study, the length of the growing season, defined as the number of days annually with temperatures above 10 °C, was calculated for the last 110 years. Averaged over the climate stations, the growing season was extended by 22 days. The main increase, however, was detected over the last 50 years.

This suggests notable wood volume growth rate increases at the stand level over the last 100 years, coinciding with an increase in resource supply (CO2, N), together with an extended growing season accompanied by changes in other climatic variables. These observations justified statistical analyses of growth trends, and model-based examination of the underlying mechanisms.

This, dear readers, is how science is done: observation leads to analyses, which then begs ‘model-based examination of the underlying mechanisms’.

First, we employed linear mixed models (LMMs) to determine whether the most important stand characteristics were dependent on only stand age, or also on calendar year…Under the environmental conditions of the year 2000, any given mean diameter was attained following stand establishment up to more than one decade earlier than its counterpart in 1960 or before. Stand volume growth, expressed as periodic annual increment of volume (PAIV) changed from 1960 to 2000 by, respectively, 10% and 30% for Norway spruce and European beech…A consequence of accelerated stand development was a more rapid decrease in stand tree number N per unit area and change in tree mortality rate. A comparison between 1960 and 2000 showed a 17% decrease in Norway spruce tree number and a 21% decrease in European beech. We did not detect a significant change in Norway spruce mortality rate, however, European beech exhibited a −17% change.

Pretzsch et al. also ran a series of additional tests to check for confounding variables. What they found, however, is that all their observations were mostly confirmed by subsequent modelling—and it breaks down, basically, to considerations that may be colloquially expressed as ‘the bigger the creature/tree, the fewer per measured area there are’.

Present forest stands grow more rapidly, and accumulate a given standing volume earlier than comparable stands did a century ago. Regression results suggest the stands grow along a self-thinning line similar in slope and growth levels as in the past but, pass more rapidly through this usual trajectory. Consequently, the identified growth trend was primarily based on a changed relationship between tree size and growth. Remarkably, the change in the relationship was manifested only in the curve’s level and not in its slope, the allometric coefficient…

We showed for stands of this age, stand characteristics changed from 1960 to 2000. Both years (1960 and 2000), as well as stand age (75 years), were inserted in the statistically fitted model functions to quantify the relative changes. Although tree height increased marginally, mean tree diameter and most notably mean tree volume increment showed accelerated change. Beech stand volume growth (+30%) and standing volume accumulation (+7%) proceeded significantly faster, so that tree number at a given age was already 21% lower than in the past. As the species’ self-thinning lines showed no significant upward shift during the whole time span covered by our data, decreased beech mortality rate (−17%) cannot be attributed to delayed mortality. Rather, it can be explained by the fact that in even-aged stands, mortality rate decreases continuously with stand age under steady-state conditions. Seemingly, owing to the more rapid growth, decreased mortality rates are reached significantly earlier than five or more decades ago…

The fact that spruce and beech grew more rapidly, but stands still followed similar self-thinning became most obvious by 32–77% increased mean tree volume increments over time, an upward shift in growth-size allometry by 25–57%, and continued self-thinning. Our findings that self-thinning remained constant, while growth rates increased indicated the stock of resources have not changed, while the growth velocity and turnover have altered over time. Note, that changes in the reported order of magnitude are relevant for forest ecology and management.

There is a whole, detailed sub-section on site variation (soil quality), which boils down to the following results:

On fertile sites, the observed environmental change patterns resulted in increased growth acceleration, facilitating defined forest stand sizes, standing stock and developmental stages decades earlier than in the beginning of the twentieth century. In contrast, on sites with mineral nutrient limitations, environmental changes accelerated growth to a lesser extent…current increased temperatures and extended growing season contributed more to growth acceleration when site mineral nutrient supply was greater.

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From the Discussion by Pretzsch et al. (2014)

Studies in long-term ecosystem dynamics and change should consider past and present anthropogenic and natural causes. Long-term time series analyses on ecological processes and climate were applied to better understand ecosystem behaviour in the American Southwest. Based on tree ring analysis, Swetnam et al. revealed an unprecedented ramp in tree growth since the mid-1970s, and attributed the observations to recovery from a 1950s extreme drought period, anomalous warming and mild wet winters associated with El Niño events. The study did not completely rule out anthropogenic effects, such as CO2 enrichment.

The increased stand growth revealed in our study surprisingly occurred during the period when acid rain (1970–1990) and drought episodes (1976 and 2003) suggest decreased productivity should have occurred. One possible explanation for these results includes acid rain, after long-distance transport, only affected rather restricted areas of the Central European highland mountain tops (for example, Ore Mountains, Black Forest, Bavarian Forest or Bohemian Forest), but rarely the lowland forests, where the experimental areas used in this study are located. The 1976 and 2003 droughts were the most severe in Europe’s recent climate history. However, both droughts were rather short lived, and an upward growth trend occurred immediately following each drought. Longer drought periods, as expected under future climatic conditions, might cause much longer lags in tree growth.

Here’s a hypothesis for the ages: it may very well be that the future holds longer droughts, but this is quite literally impossible to know.

Kahle et al. provided evidence for growth trends at the tree level, however, our approach showed the relevance of growth trends for stand level productivity. A broad scientific community was made aware of such trends in the 1990s, however, to date, statistical inference beyond the case study level was missing. Our data, which covered an observational time span of more than one century, and even included records from subsequent stands at the same locations, revealed statistically significant growth changes from past to present. These unique time series analyses and results reflected how climate change actually modified the various components of forest stand dynamics. Tree and stand development are driven by resources rather than mere age, and because of increased resource availability, both aged faster than in the past. As predicted by allometric theory, the stands passed along continuous self-thinning lines, which reflected site carrying capacity. However, to date, because of increased growth rates, stands achieved defined sizes, standing stock and stand development stages significantly earlier than in the past. In other words, an average tree exhibited accelerated growth, but at any given average tree size, the maximum tree packing density per unit area did not change.

This is followed by what I think is arguably the killer argument here:

Our simulation results were consistent with biosphere model evaluations in response to climate variability, nitrogen and CO2 concentration changes. Despite high variability in published models, overall results exhibit congruencies. The dependencies between carbon, water and nitrogen cycles as depicted in the model simulations are also obvious in empirical studies. For example, boreal Norway spruce growth was not increased due to higher CO2 concentrations, unless nutrients were supplied…enhanced CO2 concentrations also affect nitrogen availability and plant water supply, whereas the effect of elevated CO2 concentrations on forest stand growth is still unclear, especially under long-term conditions…

Because our findings were based on continuously unthinned or at most moderately thinned forest stands, but always fully stocked experimental stands, progress in silvicultural practices can be excluded as cause for the observed growth trend. In the primarily intensively thinned stands from routine forest practices in Central Europe, the positive effects of thinning on tree and stand growth might even contribute to a climate change-induced growth trend. Tree breeding can also be excluded as a cause for observed growth trends, as on our research plots in many cases subsequent stands at the same location are of the same genotype. Other relevant species in this area, including Sessile oak (Quercus petraea L.) and Scots pine (Pinus sylvestris L.) dominate on less fertile sites than those typically stocked with Norway spruce and European beech, so that the benefit of the additional resource supply and growth acceleration might be even higher. The increased growth rate, harvest and standing stock accumulation can be expected to heighten the carbon turnover rate in Central European forests.

I’ll stop here, because you get the gist. Much of this section is actually stuffed with forest management implications and the like, but in all this is, I think, an important puzzle piece as it highlights, in no uncertain terms, the possible impacts of warmer temperatures on temperate forest zones.

If you’d now compare these results to the price tag I mentioned in yesterday’s posting—100b euros for planting trees, according to a financial services ‘expert™’—this is mind-boggling. We may, at that point, ask who would obtain such a large sum for (pretending to) plant these trees? Bill Gates, for once, came out strongly against doing so…but still, there would certainly be a ton of money to be ‘made’…

So, all told, a quite interesting read, and it provides strong empirical evidence for global greening, which dovetails nicely with the observations made by NASA.

Now, if we could only get more people to know about such papers…