Greening of the Earth: Forests

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McMahon et al. (2010) note “there are indications that forest biomass accumulation may be accelerating where nutrients and water are not limiting,” citing Myneni et al. (1997), Lewis et al. (2004), Lewis et al. (2009a), Boisvenue and Running (2006), Delpierre et al. (2009), Salzer et al. (2009), and Chave et al. (2008). They further investigate the subject because of the great significance such a phenomenon would hold 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 past 22 years from 55 temperate forest plots with known land-use histories and stand ages ranging from five 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.” They did this within various parts of a temperate deciduous forest in the vicinity of the Smithsonian Environmental Research Center, Edgewater, Maryland (USA) by comparing recent (last 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. Finally, they compared their findings with “over 100 years of local weather measurements and 17 years of on-site atmospheric CO2 measurements.”

The three researchers report “recent biomass accumulation greatly exceeded the expected growth caused by natural recovery,” noting that in stands younger than 50 years the observed increase was generally at least one-third of total growth, and 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 “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 state “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 these global-change factors—the magnitudes of which rose significantly over the course of their study—may have been “critical to changing the rate of stand growth observed across stands.” Such findings and this explanation provide additional important evidence for the greening of the Earth phenomenon that is transforming the face of the planet as the air’s CO2 content continues to rise.

Further north, satellite studies based on Normalized Difference Vegetation Index (NDVI) data have produced conflicting trend estimates, ranging from greening to browning, for the boreal forest zone of Canada. In an attempt to resolve this disagreement, Alcaraz-Segura et al. (2010) argue that a significant factor not considered in past studies is fire history. If points in time are compared before and after a fire, for example, NDVI will be seen to have decreased, but not for the reason for which the data were sought. Similarly, trends that begin right after a fire will show increasing NDVI that is unrelated to climate factors, as the vegetation recovers from fire. Hence, they use this latter phenomenon of vegetation recovery after fire as a ground-truth test for two satellite datasets.

Working with GIMMS satellite data that represent 64-km2 cells and newly available CCRS data that represent 1-km2 cells, the five researchers used an algorithm shown to detect recent fires to correctly classify pixels as burned or not-burned, comparing the performance of the two datasets for detecting trends. They found the GIMMS data were unable to properly detect increases in NDVI over time in burned areas compared to the CCRS data, and that GIMMS data are thus a poor choice for this type of study. The CCRS data, on the other hand, detected strong greening in burned areas (as expected) and a weaker but consistent greening in unburned forest areas over 1996 to 2006. As a result, Alcaraz-Segura et al. suggest (1) the widely-used GIMMS data may have produced false results in other studies and should be used with caution, (2) satellite data need to be better calibrated with ground data before use, and (3) the greening of the Canadian boreal forest is probably real for the most recent decades.

Although many high-latitude regions may indeed be experiencing greening due to increases in the air’s CO2 content, as well as concomitant warming that allows crops and forests to grow where it has previously been too cold for them to survive, some researchers worry about Earth’s tropical regions, where they claim just a little extra warming may spell disaster for local forests. Consequently, in a thorough review of the scientific literature on this important subject, Lewis et al. (2009b) evaluated tropical forest inventory data, plant physiology experiments, ecosystem flux observations, Earth observations, atmospheric measurements, and dynamic global vegetation models, which “taken together,” in their words, “provide new opportunities to cross-validate results.”

The five researchers note both theory and experiments suggest over the past several decades “plant photosynthesis should have increased in response to increasing CO2 concentrations, causing increased plant growth and forest biomass.” And they did indeed find “long-term plot data collectively indicate an increase in carbon storage, as well as significant increases in tree growth, mortality, recruitment, and forest dynamism.” They also state satellite measurements “indicate increases in productivity and forest dynamism,” and five Dynamic Global Vegetation Models, incorporating plant physiology, competition, and dynamics, all predict increasing gross primary productivity, net primary productivity, and carbon storage when forced using late-twentieth century climate and atmospheric CO2 concentration data. In addition, they state “the predicted increases in carbon storage via the differing methods are all of similar magnitude (0.2% to 0.5% per year).”

“Collectively,” they conclude, “these results point toward a widespread shift in the ecology of tropical forests, characterized by increased tree growth and accelerating forest dynamism, with forests, on average, getting bigger (increasing biomass and carbon storage).” Far from being the bane of the Earth’s tropical forests, twentieth-century increases in air temperature and atmospheric CO2 concentration—which have returned these meteorological parameters to more normal post-Little Ice Age values—have been a great boon to the trees of the tropics.

Commenting on this state of affairs, Gloor et al. (2009) reiterated that “large-scale changes in forest dynamics are currently occurring in Amazonia (Phillips and Gentry, 1994; Phillips et al., 2004), and that an increase in aboveground biomass has occurred, with increases in mortality tending to lag increases in growth (Phillips et al., 1998; Baker et al., 2004a,b; Lewis et al., 2004).” However, they state this conclusion has been challenged recently by an overzealous application of the “Slow in, Rapid out” dictum, which recognizes that forest growth is a slow process, whereas mortality can be dramatic and singular in time, such that sampling over relatively short observation periods may miss these more severe events, leading to positively biased estimates of aboveground biomass trends, when either no trend or negative trends actually exist.

To test this claim, Gloor et al. statistically characterized “the disturbance process in Amazon old-growth forests as recorded in 135 forest plots of the RAINFOR network up to 2006, and other independent research programs, and explore the consequences of sampling artifacts using a data-based stochastic simulator.” They found “over the observed range of annual aboveground biomass losses, standard statistical tests show that the distribution of biomass losses through mortality follow an exponential or near-identical Weibull probability distribution and not a power law as assumed by others.” In addition, they state “the simulator was parameterized using both an exponential disturbance probability distribution as well as a mixed exponential-power law distribution to account for potential large-scale blow-down events,” and they report “in both cases, sampling biases turn out to be too small to explain the gains detected by the extended RAINFOR plot network.”

Gloor et al. therefore conclude their results lend “further support to the notion that currently observed biomass gains for intact forests across the Amazon are actually occurring over large scales at the current time, presumably as a response to climate change,” which in many of their earlier papers is explicitly stated to include the aerial fertilization effect of the historical increase in the air’s CO2 content.

In another of several contemporaneous studies, Silva et al. (2009) describe Araucaria angustifolia as “an indigenous conifer tree restricted to the southern region of South America that plays a key role in the dynamics of regional ecosystems where forest expansion over grasslands has been observed.” Working with various types of tree-ring data obtained from such trees growing in both forest and grassland sites in southern Brazil, they compared changes in intrinsic water use efficiency—iWUE, defined as the ratio of the rate of CO2 assimilation by the trees’ needles to their stomatal conductance—with concomitant historical changes in temperature, precipitation, and atmospheric CO2 concentration over the past century.

They found “iWUE increased over 30% in both habitats” over the past several decades, and “this increase was highly correlated with increasing levels of CO2 in the atmosphere.” Tree growth, however, remained rather stable, due to lower-than-normal precipitation and higher-than-normal temperatures, which would normally tend to depress the growth of this species—Katinas and Crisci (2008) describe A. angustifolia as being “intolerant of dry seasons and requiring cool temperatures.” Therefore, Silva et al. conclude the “climatic fluctuations during the past few decades,” which would have been expected to have been deleterious to the growth of A. angustifolia, seem to have had their growth-retarding effects “compensated by increases in atmospheric CO2 and changes [i.e., increases] in iWUE.”

Also dealing with the expansion of forests onto grasslands were Springsteen et al. (2010), who write, “woody plant expansion within grassland ecosystems is a worldwide phenomenon, and dramatic vegetation shifts from grassland to savanna/woodlands have occurred over the past 50–100 years in North America,” while noting one of the chief factors that has contributed to this phenomenon is believed by many to have been the increase in the air’s CO2 content, as suggested by Archer et al. (1995), Polley (1997), Bond and Midgley (2000), and Bond et al. (2003). They also note once shrublands are established, they tend to persist for several different reasons, one of which is a type of feedback phenomenon referred to as islands of fertility, which “occurs when resources accumulate in soils beneath woody plants due to litterfall, interception of wet and dry deposition, nitrogen fixation, and animal droppings,” as described by Schlesinger et al. (1990), Archer et al. (1995), Reynolds et al. (1999), and Lopez-Pintor et al. (2006). They report “changes in soil attributes under woody vegetation have been documented in the arid grasslands of the southern Great Plains, including increases in soil carbon and nitrogen,” citing Reynolds et al. (1999), Hibbard et al. (2001, 2003), McCulley et al. (2004), Schade and Hobbie (2005), and Liao et al. (2006).

For their part of this massive undertaking—while working at the USDA-ARS Northern Great Plains Research Laboratory near Mandan, North Dakota (USA)—Springsteen et al. examined near-surface (upper 15 cm) soil biogeochemistry along a 42-year (1963–2005) chronosequence that encompassed grassland, woodland, and grassland-woodland transition zones in a northern Great Plains grassland, in order to determine the influence of woody plant expansion on soil carbon and nitrogen contents. They found total soil carbon content rose by 26 percent across the chronosequence from grassland to woodland within the 0–15 cm soil depth, and total soil nitrogen content rose by 31 percent. The rate of woody shrub expansion from 1963 to 1988 (25 years) was ~1,800 m2 per year at their study site, whereas from 1988 to 2005 (17 years) it was ~3,800 m2 per year, or a little more than double the initial rate, as the greening of the Earth accelerated to keep pace with the accelerating increase of the air’s CO2 content.

In the U.S. mid-Atlantic region, Pan et al. (2010) 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.” They modified and applied “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.” 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 percent), CO2 (+90 percent), O3 (-40 percent), CO2 + O3 (+60 percent), CO2 + N deposition (+184 percent), and CO2 + N deposition + O3 (+138 percent), while corresponding changes in NEP for undisturbed forests were +18 percent, +180 percent, -75 percent, +78 percent, +290 percent, and +208 percent. The model results also revealed “the ‘fertilization’ effect of N deposition mainly stimulates carbon allocation to short-lived tissues such as foliage and fine roots,” but “the ‘fertilization’ effect by elevated CO2 likely enhances more sustainable carbon storage such as woody biomass (including coarse roots).” The four USDA Forest Service scientists state their findings indicate “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 conclude “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.”

Martinez-Vilalta et al. (2008) used tree-ring data from the Catalan Ecological and Forest Inventory “to study the temporal variability of Scots pine stem radial growth (period 1901–1997) across a relatively large region (Catalonia, NE Spain) situated close to the southern limit of the distribution of the species.” This inventory “included a total of 10,664 plots randomly distributed throughout the forested area of Catalonia,” where Scots pine was present in 30.2 percent of the plots and was the dominant tree species in 18.4 percent of the plots. The inventory “showed an overall increase of 84% in Scots pine BAI [basal area increment] during the twentieth century, consistent with most previous studies for temperate forests.” The scientists state “this trend was associated with increased atmospheric CO2 concentration,” which they interpret as “a fertilization effect.” In addition, over the same time period, the five researchers note “there was also a marked increase in temperature across the study region (0.19°C per decade on average),” and they report “this warming had a negative impact on radial growth, particularly at the drier sites,” but “its magnitude was not enough to counteract the fertilization effect.”

Cole et al. (2010) introduce their study of the subject by noting that quaking aspen (Populus tremuloides Michx.) is a dominant forest type in north-temperate, montane and boreal regions of North America,” stating it is, in fact, “the most widely distributed tree species on the continent.” They also note that aspen—and related poplars—are “quintessential foundation species (Ellison et al., 2005), shaping the structure and function of the communities and ecosystems in which they occur (Whitham et al., 2006; Schweitzer et al., 2008; Madritch et al., 2009).” This being the case, they attempted to determine how this keystone species may have responded to the twentieth-century increase in atmospheric CO2 concentration.

The four researchers collected branches from 919 trees after their leaves had dropped in the fall, obtaining samples that represented 189 genets or clones (five trees per clone) at 11 sites distributed throughout three regions of Wisconsin (USA). The sampled trees ranged from five to 76 years of age and came from second-growth unmanaged forests south of the areas defoliated by forest tent caterpillars in 1980–1982, 1989–1990, and 2001–2002. In addition, they recorded trunk diameter at breast height for each sampled tree, which parameter, in their words, “is very highly correlated with total biomass in aspen,” citing Bond-Lamberty et al. (2002).

The Minnesota and Wisconsin scientists determined “age-specific ring width increased over time,” and “the greatest increase occurred for relatively young trees, so that young trees grew faster in recent years than did young trees several decades ago.” They found, for example, that during the past half-century the growth of trees 11–20 years old rose by 60 percent. In addition, they observed “rising CO2 causes ring width to increase at all moisture levels, apparently resulting from improved water use efficiency,” so “the overall increase results from historical increases in both CO2 and water availability.” And when they separated the impacts of the two factors, they found “the effect of rising CO2 had been to increase ring width by about 53%,” as a result of “a 19.2% increase in ambient CO2 levels during the growing season, from 315.8 ppm in 1958 (when CO2 records began) to 376.4 ppm in 2003.”

Cole et al. state “the magnitude of the growth increase uncovered by this analysis raises the question of how much other major forest species have responded to the joint effects of long-term changes in CO2 and precipitation.” Indeed, there is reason to believe many other tree species may have experienced similar large growth stimulation, particularly in light of the analysis of Tans (2009), who demonstrated the Earth’s land surfaces were a net source of CO2 to the atmosphere until about 1940—primarily due to the felling of forests and the plowing of grasslands to make way for expanded agricultural activities—but from 1940 onward the terrestrial biosphere had become, in the mean, an increasingly greater sink for CO2 and had done so even in the face of massive global deforestation, for which it apparently more than compensated.


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