Carbon Storage Within an Eastern Massachusetts Patch Forest

Introduction

Forests, whether in the form of coniferous or hardwood structure, serve to sequester large amounts of carbon through photosynthesis. Storage of such carbon within the trees above ground, along with the sequestration of such carbon below ground within the soils, aid in removing carbon from the atmosphere. Because carbon in the atmosphere generally is in carbon dioxide form and such form is a greenhouse gas, the removal of atmospheric carbon by forests on a global basis aids in the maintenance of temperature within the planet. Changes in forest cover, therefore, may alter the ability for the planet to continue such maintenance of atmospheric temperature (Winjum et al., 1998). Hardwood and coniferous forests differ in photosynthetic methods, thereby differing in carbon storage methods. Generally, temperate hardwood trees increase carbon uptake during the summer months, where ample sunlight and warmth allows for their broad-leaves to capture such CO2 and sunlight to create sugars. However, when temperate winters arrive, the trees retract their chlorophyll and shed their leaves, thus halting photosynthesis all together. In contrast, coniferous trees contain needle-like leaves which remain on the trees year-round, hence the name “evergreen.” While the leaves do stay on the trees themselves year round, the general small surface area of the leaves often decreases exposure to sunlight, which decreases photosynthetic rates. However, the trees continue to photosynthesize even within winter months due to their ability to retain leaves. On average, temperate forests store 377 tons of carbon per hectare in above ground biomass while cool dry temperate forests had 176 tons of carbon per hectare (Keith et al., 2009), which greatly helps remove carbon from the atmosphere. Ultimately, the main objective of this experiment is to understand the variation between carbon storage levels, both as aboveground biomass and within soil, of coniferous and hardwood forests. In doing so, researchers could plan a more accurate carbon budget through better understanding of forest management, shifts in climate, and predict future problems which may arise from alterations of forest cover.

Materials and Methods

The tested environment was located northwest of Boston, near the city of Belmont, Massachusetts. Field habitat was predominantly temperate deciduous forest with a variation within ages of trees. While the majority of trees were hardwood, various isolated plots also included coniferous species. Diversity within the forest was substantially higher for hardwoods when compared with conifers. Dominant species within such environment for the former included Quercus rubra and Acer rubrum. In contrasts, the predominant coniferous tree species within the tested plots was Pinus strobus. Additionally, the forest was moderately fragmented by walking and running trails, though the trails themselves were simply cleared dirt paths. Parts of the forest also had large, meadow-like clearings with relatively low number of trees.

The experiment itself involved the separation of various forest areas into two plot types: hardwood and coniferous. As their names suggested, hardwood plots consisted largely of hardwoods while coniferous plots consisted largely of coniferous tree species. Flags were placed in the corners of the plots, which were measured to be 12 meters by 12 meters. Next, we utilized a DBH (diameter at breast height) tape to measure every single tree larger than five centimeters in diameter within the plots and followed by identifying the tree species and correlating the DBH to the species. Diameters were then recorded. This was repeated again for another plot type. Two of each type of plot were eventually measured. Finally, these measurements were utilized for calculation of aboveground biomass.

With augers (we used 1.25cm diameter), we obtained organic horizon soil cores. Note that we took separate samples from the coniferous and hardwood plots specifically. These samples were then bagged and taken into the lab to be dried (drying period of one week). Dried soils were eventually measured for their masses and also their volumes.

Within the laboratory, we calculated the average aboveground biomass with a predetermined formula for each specific individual tree species within our plots (Ravindranath & Ostwald 2007; Siccama et al., 1994; Tritton & Hornbeck 1982; Whittaker et al., 1974). We then calculated the data to the predetermined carbon storage percentage of each particular individual tree species and standardized the resulting aboveground carbon storage biomass to the area of each of the plots (144m2 in our case). The individual numbers of total aboveground biomasses were then combined into two particular tables, one which expressed coniferous total aboveground carbon biomass and another which expressed hardwood total aboveground biomass. Then, the numbers of total aboveground carbon biomass from the same plot types were averaged and then plotted against the other plot type (Hardwood versus Coniferous) with error bars which revealed standard error.

Finally, we took the dried soil samples and measured their mass and depth. Along with the radius, we calculated the volume of the soil samples. These numbers, standardized to cubic meters, were then used to calculate the carbon mass per unit volume for each soil core (g C/m3) with predetermined percentage carbon within the forest plot types (Lamlom & Savidge 2003).

Results

The average aboveground carbon storage of hardwood and coniferous plots differed dramatically. Mean total carbon for tested coniferous forests was 85,560.1 ±2203.108g C/m2 and 36.1% higher than hardwood forests at 62886.4 ±3203.685g C/m2 (Figure 1). In contrast, soil storage was fairly consistent between the averaged carbon values with hardwood forests at 0.0598 ±0.008748g C/m3 and coniferous forests at 0.0638 ±0.009205g C/m3. Similarly, the great crossover within the standard error also suggests a close proximity of the soil carbon values (Figure 2). Total aboveground biomass also varied between the two particular plots. Tested hardwood forest plots had a total of 125,772.86g C/m2 (Table 1) while tested coniferous forest plots had a 25.5% differential increase of total carbon at 157,883.21g C/m2 (Table 2).

Discussion

Our original, main objective was to compare the differences between the amount of stored carbon in aboveground biomass and in soils between hardwood and coniferous forests. We hypothesized that coniferous forest, perhaps due to their more balanced, yearly rates of photosynthesis, would most likely have a higher amount of carbon storage. Our data partially supported such claim, at least in the aboveground stored carbon biomass. Averages expressed that carbon from coniferous forest plots measured 85,560. 1g C/m2, which was over 36% higher than 62,886.4g C/m2 for hardwood forests (Figure 1). In this particular situation, the standard errors failed to cross, suggesting the data does express a level of significance. This may be the result of a coniferous adaptation to low nutrient soils and burns within ranges of some coniferous species, including Pinus strobus. As a result of the species’ resistance to moderate levels of burns and low nutrient soils, conifers often readily uptake and store such carbon when available, which can make them sequester such carbon in higher amounts (DeLuca & Aplet 2008).

In contrast to our hypothesis, the soil content appeared to express similar amounts of stored carbon. The hardwood forest soils averaged at 0.0598g C/m3 and the coniferous forests at 0.0638g C/m3. Great crossover within standard error bars suggest that such difference is negligible and that there is not great significance between the two data points (Figure 2). Such similarity may actually be the result of similar amounts of soil sequestration between the two different forest types. However, past research suggests otherwise. Analysis of colder region ecosystems in the early 21st century suggested that soil net primary productivity and carbon sequestration ratios were significantly higher for evergreen groves than deciduous groves (Gower et al., 2001). Perhaps, a better explanation is the high presence of hardwood species within and coniferous plots, which may have caused a “mixing” of soil carbon concentrations.

The total calculated aboveground carbon storage, however, expressed different results. Hardwoods made up 89% of the tested hardwood plots (Table 1). However, despite a lower number of coniferous trees within the coniferous plots when correlated to the number of hardwoods in hardwood plots, the coniferous trees still contained a higher amount of aboveground storage. Another particular important aspect to note is that P. strobus, the only tree within the coniferous plot, made up only 58% of the measured trees within those particular plots. The other 42% consisted of hardwoods (Table 2). This suggests that conifers, at least in Eastern Massachusetts, are able to sequester a 25.5% total higher amount of carbon than neighboring hardwoods (Tables 1 & 2). Such maybe the result of the conifers’ tolerance to drought stresses, which include evaporative water loss through carbon uptake. Research in other North American regions revealed similar reasoning and expressed fairly similar results. For example, conifers’ photosynthesis and nutrient uptake and storage are often possible during warmer winter days. Likewise, dry spouts during the summer severely limit deciduous trees in carbon storage and photosynthesis. In situations such as these, conifers, being more adapted to dryer environments and lower nutrient availability, are able to sequester carbon through periods of high evaporative demand at a much better rate than deciduous trees (Waring & Franklin 1979).

While precision was best maintained, errors may have resulted from the fact that we tested coniferous groves situated in the middle of a mostly temperate hardwood forest. Here, strong coniferous groves still contained large numbers of hardwood trees (Table 2) which may have skewed data. Additionally, unlike the Gower et al.’s research, our work was on forests within temperate areas, not boreal. Next, the presence of a dry summer may also skew data. The past winter’s high precipitation levels and past summer’s low precipitation levels mimicked the temperate forests of the Pacific Northwest, which was the main focus of Waring an Franklin’s work. Such variation within seasonal variables may have decreased carbon storage in hardwoods due to evaporative stress, thus skewing the data. Indeed, some studies actually suggest that faster growing hardwoods sequester larger amounts of carbon than many coniferous trees (Dewar & Cannell 1992). Yearly trials, along with testing during varied seasons, may aid in negating such problems. Measuring also the carbon concentrations within the atmosphere, fluxes of such concentrations (secondary variables of error), and experimental analysis of a predominantly coniferous forest and correlation of results to our temperate hardwood forest data may improve accuracy and exclude problems (aforementioned soil “mixing”), thus helping in explaining variation within the carbon pools expressed by the experimental data.

Ultimately, the correct research and application of carbon storage levels within forests are crucial for maintaining ecosystem diversity and health, both of which may shift as the climate changes. With proper understanding of carbon storage variations within hardwood and coniferous trees, scientists may gain a better understanding in the necessary steps for short and long term conservation, land use, and habitat preference in mitigating the problems which may arise as a result of anthropomorphic climate change. Finally, correct application of actions as a result of carbon sequestration comprehension may allow researchers to provide information to the public, municipal offices and the government as to perhaps which particular habitat should take protective precedence in conserving the biological integrity of the planet.

Figures and Tables

Literature Cited

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