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The State of Earth's Terrestrial Biosphere:
How is it Responding to Rising Atmospheric CO2 and Warmer Temperatures?

Continental-Scale Analyses of Terrestrial Productivity: North America - Eastern USA

We begin our investigation of plant growth responses to rising atmospheric CO2 and temperature in the Eastern United States with the model-based study of Pan et al. (2009), who examined "how changes in atmospheric composition (CO2, O3 and N deposition), climate and land-use affected carbon dynamics and sequestration in Mid-Atlantic temperate forests during the 20th century." This they did by modifying and applying "a well-established process-based ecosystem model with a strong foundation of ecosystem knowledge from experimental studies," which they validated "using the U.S. Forest Inventory and Analysis (FIA) data." And what did they find?

For previously harvested and currently regrowing forests, the calibrated model produced the following percentage changes in net ecosystem productivity (NEP) due to observed changes in N deposition (+32%), CO2 (+90%), O3 (-40%), CO2 + O3 (+60%), CO2 + N deposition (+184%), and CO2 + N deposition + O3 (+138%), while corresponding changes in NEP for undisturbed forests were +18%, +180%, -75%, +78%, +290%, +208%. In addition, the results of Pan et al. revealed that "the 'fertilization' effect of N deposition mainly stimulates carbon allocation to short-lived tissues such as foliage and fine roots," but that "the 'fertilization' effect by elevated CO2 likely enhances more sustainable carbon storage such as woody biomass (including coarse roots)."

In discussing the future implications of their findings, the four USDA Forest Service scientists say they indicate that "the change in atmospheric composition, particularly elevated CO2, will gradually account for more of the carbon sink of temperate forests in the Mid-Atlantic region," and they opine that "such a significant 'fertilization effect' on the forest carbon sequestration could eventually result in a 'greener world' after a long period of chronic change in atmospheric composition and cumulative impact."

Crossing the threshold into the realm of real-world data, Westfall and Amateis (2003) used mean height measurements made at three-year intervals over a period of 15 years from dominant stands of loblolly pine plantations growing at 94 locations spread across the southeastern United States to calculate a site index related to the mean growth rate for each of the five three-year periods. It was their expectation that the index would increase monotonically if growth rates were being enhanced above normal by some monotonically-increasing factor that promotes growth. The results indicated, in the words of the researchers, that the "mean site index over the 94 plots consistently increased at each re-measurement period," which would suggest, as they further state, that "loblolly pine plantations are realizing greater than expected growth rates." And it should be added that the growth rate increases grew larger and larger with each succeeding three-year period.

As to what could be causing the monotonically increasing growth rates of loblolly pine trees over the entire southeastern United States, Westfall and Amateis named increases in temperature and precipitation in addition to rising atmospheric CO2 concentrations. However, they report that a review of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study. They also suggested that if increased nitrogen deposition were the cause, "such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots." Hence, they tended to favor the ever-increasing aerial fertilization effect of atmospheric CO2 enrichment as being responsible for the accelerating pine tree growth rates.

Writing as background for their research, McMahon et al. (2010) state "there are indications that forest biomass accumulation may be accelerating where nutrients and water are not limiting," citing the work of Myneni et al. (1997), Lewis et al. (2004), Lewis et al. (2009b), Boisvenue and Running (2006), Delpierre et al. (2009), Salzer et al. (2009) and Chave et al. (2008); and, therefore, they felt it important to further investigate the subject because of the great significance such growth portends for the planet's carbon balance and the future course of potential CO2-induced global warming.

Using unique datasets of tree biomass collected over the prior 22 years from 55 temperate forest plots with known land-use histories and stand ages ranging from 5 to 250 years (which were derived from knowledge of when the stands had begun to regrow following major disturbances such as significant logging, various natural disasters that had decimated large patches of trees, or the clearing of trees to make room for agriculture that was ultimately abandoned), McMahon et al. "estimated biomass change, while controlling for stand regeneration" (see Figure 21). This they did within various parts of a temperate deciduous forest in the vicinity of the Smithsonian Environmental Research Center, Edgewater, Maryland (USA) by comparing recent (the prior 22 years or less) rates of biomass accumulation of the various stands with rates predicted for those age intervals by the overall growth function derived from the combined data of all of the stands. Then, last of all, they compared their findings with "over 100 years of local weather measurements and 17 years of on-site atmospheric CO2 measurements."

Figure 21. Graphic showing the relationship between above-ground biomass (AGB) and stand age of multiple-censused forest plots in a temperate deciduous forest in and near the Smithsonian Environmental Research Center in Edgewater, MD. From this plot it is seen that younger trees are growing faster than older ones. Adapted from McMahon et al. (2010).

The results of this analysis revealed that "recent biomass accumulation greatly exceeded the expected growth caused by natural recovery," noting that in stands younger than 50 years the observed rate increase was generally at least one-third of total growth, and that in older stands it typically was "the majority of growth," even though past experience and the ensemble relationship of growth vs. age derived from the totality of their data suggest that "old forests should grow very little as they approach equilibrium."

As for what could have caused the tremendous recent increases in forest plot growth rates detected by the Smithsonian scientists, they say that "increases in temperature, growing season [which is largely driven by temperature], and atmospheric CO2 have documented influences on tree physiology, metabolism, and growth," and they state that these global-change factors -- the magnitudes of which rose significantly over the course of their study -- may well have been "critical to changing the rate of stand growth observed across stands."

Further north, Capers and Stone (2011) "studied a community in western Maine, comparing the frequency and abundance of alpine plants in 2009 with frequency and abundance recorded in 1976," while noting that "the 2009 survey was designed to provide a fair comparison with that of 1976," which was conducted and described by Stone (1980). In doing so, the two researchers found that the 2009 survey "provided evidence of the increasing importance of woody plants - both trees and shrubs - in the alpine community" (Figure 22), commenting that "the most widespread tree species increased dramatically." In addition, they say they "recorded an increase in total species richness of the community with the addition of four lower montane species that had not been recorded previously." And in another important positive finding, they say they "found no evidence that species with high-arctic distributions had declined more than other species."

Figure 22. The relative frequency five functional plant groups from surveys taken in 1976 and 2009 in a Northeast United States alpine plant community. The relative frequency of trees increased by a significant 14%, whereas the changes in forbs and graminoids were only marginally significant. Adapted from Capers and Stone (2011).

In discussing their findings, Capers and Stone state that the changes they recorded "are consistent with those reported in tundra communities around the world." And although there is some concern that the observed increase in species richness could ultimately turn out to be temporary if alpine species were to disappear because of competition from the new species appearing on the scene, they state that "species losses resulting from competition have not typically been found with rising richness in high alpine areas, possibly because newly arriving species occupy different microhabitats," citing the work of Walther et al. (2005).

Lastly, in a somewhat different study, working within and around Baltimore, Maryland, USA, Ziska et al. (2004) characterized the gradual changes that occur in a number of environmental variables as one moves from a rural location (a farm approximately 50 km from the city center) to a suburban location (a park approximately 10 km from the city center) to an urban location (the Baltimore Science Center approximately 0.5 km from the city center). At each of these locations, four 2 x 2 m plots were excavated to a depth of about 1.1 m, after which they were filled with identical soils, the top layers of which contained seeds of naturally-occurring plants of the general area. These seeds sprouted in the spring of the year, and the plants they produced were allowed to grow until they senesced in the fall, after which all of them were cut at ground level, removed, dried and weighed.

Along the rural to suburban to urban transect, the only consistent differences in the environmental variables Ziska et al. measured were a rural to urban increase of 21% in average daytime atmospheric CO2 concentration and increases of 1.6 and 3.3°C in maximum (daytime) and minimum (nighttime) daily temperatures, respectively, which changes, in their words, "were consistent with most short-term (~50 year) global change scenarios regarding CO2 concentration and air temperature." In addition, they determined that "productivity, determined as final above-ground biomass, and maximum plant height were positively affected by daytime and soil temperatures as well as enhanced CO2, increasing 60 and 115% for the suburban and urban sites, respectively, relative to the rural site."

In light of these observations, the three researchers say their results suggest that "urban environments may act as a reasonable surrogate for investigating future climatic change in vegetative communities," and those results indicate that rising air temperatures and CO2 concentrations tend to produce dramatic increases in the productivity of the natural ecosystems typical of the greater Baltimore area and, by inference, probably those of many other areas as well.

In considering each of the studies discussed above, it would appear that late 20th-century increases in air temperature and atmospheric CO2 concentration did not negatively affect plant communities in the eastern United States. In direct contrast, in fact, local and regional productivity there was significantly enhanced; and there is little reason not to believe such enhancements will continue throughout the foreseeable future.

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