Estimation of carbon storage in a secondary tropical forest at the CIEE Sustainability Center, Monteverde, Costa Rica



Study zone

This study was conducted on the CIEE San Luis campus, in the monitoring plots that were first established in 1997 and 2014 by Diana and Milton Lieberman on the grounds of the San Luis Ecolodge and Biological Station (University of Georgia, Campus Costa Rica) and purchased by CIEE in 2019. The site has 50 hectares of forest in the upper regions of the San Luis Valley at 1100 m altitude35. It is classified as a humid Premontane forest according to Holdridge and is part of the Arenal-Monteverde protected area36.37. The property borders the Monteverde Cloud Forest Reserve and Children’s Eternal Rainforest and is home to a unique assemblage of flora and fauna from the Cloud Forest above and the Dry Forest below. In Costa Rica, the living area of ​​the Premontane rainforest faced rapid deforestation in the mid to late 1900s, with large areas converted to pasture. As a result, what remains of the forests is only surpassed by the tropical dry forest as the country’s most altered and reduced area of ​​life.38.

The study area is based on two pre-established plots on campus, Camino Real and Zapote. The two plots are 100 m × 100 m (1 ha) divided into 25 sub-plots each. Camino Real is a plot along the Camino Real Trail with its southwest point at 10 ° 16.800 ′ N, 84 ° 47.952 ′ W. It was established on a section of secondary forest south of the main campus with steps of DHP as of 2014. The Zapote plot is on the north side of campus, near the Zapote Trail, and its northwest corner is located at 10 ° 17.321 ′ N, 84 ° 47.800 ′ W. Historically, a large part of the property was used for coffee, guava, bananas and grazing. land until the 1970s. However, the area inside the Zapote plot has been less intensively used and looks more like primary forest compared to the Camino Real plot on the lower slopes of the property near Rio Alondra, as shown in39 Figure 3.

figure 3

Campus plan and location of the Zapote and Camino Real plots. Plan of the campus outlined in red, the forest areas corresponding to Zapote in blue and Camino Real in green. (2018 Google, Image © 2019 Maxar Technologies).

Methods

Biomass assessments, carbon capture and sequestration, is an active area of ​​study and several methods have been published30,39,40,41,42,43,44,45,46,47,48,49. In this study, the estimate of carbon stock was derived directly from tree dasometric variables by measuring tree attributes of field samples, such as diameters (DBH), tree heights, and using a allometric equation. All the measurements of this study were carried out in accordance with the national standard of Costa Rica INTE / DN 03: 2016 Methodology for the quantification and the declaration of the removals of greenhouse gases resulting from the forestry activities (National standard INTE / DN 03: (2016)50. This is the official standard for the development of inventories for the Carbon Neutrality Program and the Protocol for the Establishment and Measurement of Permanent Sample Plots in Natural Forest.39.

Experimental design

Each one-hectare plot was divided into 25 sub-plots and then into 10 20 m by 20 m sub-plots designated for sampling (Fig. 4). This design represented 40% of the two one-hectare plots studied and 1.6% of the total forest area (20 plots each of 0.04 ha / 50 ha-1= 1.6%).

Figure 4
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The schematic representation of the subplots chosen for this study are the shaded subplots.

Field reconnaissance

The plots, Zapote and Camino Real, were first set up in 2014. Therefore, before going into the field, it was necessary to look at the previous data collected. This included a list of trees with a DBH ≥ 10 cm, their assigned number, species, previously recorded diameter measurements and conditions of the trees along with their coordinates. When recognizing if a tree needed a new tag; for example, if the tag was missing, broken, fallen, or covered by the growth of a tree, a new tag was created and attached to the tree for future reference.

The northwest corner of the Zapote plot was identified by GPS, then a compass was used to locate the following subplots and verified with the permanent map of the plot. Where possible, markers visible at the corners of the subplots were located. In each subplot, the tree tag numbers and referenced datasheet were used to verify that the tree was in the assigned subplot. As most of the tree measurement locations were not marked, the tree was numbered, DBH measured (1.3m from the base of the tree) and marked with paint for future reference.

The southwest corner of the Camino Real plot was identified by GPS and then a compass was used to locate the following subplots. Additionally, the tree tags in Camino Real had the given subplot number (1 to 25) on the tag, which also helped find the correct location of the plot. The tree measurement locations at Camino Real were well marked with paint and the DBH was checked.

Shaft measurement procedure

The measurements for this study were carried out during the months of June to August 2019. All trees measured had a DBH ≥ 10 cm and the height of at least 10% of the trees in the plot.

The height of the trees was determined using a measuring stick (telescopic cylinder for heights) to obtain an overall average height of the trees in the given subplot. If a tree was above the post (40ft, 12.1m), either the height was estimated or the average height inside the subplot was used. For each tree, its condition was recorded according to the conditions specified in Sánchez-Monge39.

If a tree looked like a new recruit, its DBH was measured to see if it was actually ≥ 10 cm. A new recruit was marked with colored tape with a new tree number, which was assigned as the next number in the subplot according to the datasheet. Then the number, diameter, condition and location of the tree based on its proximity to other trees were recorded.

If a tree could not be found based on the coordinates on the datasheet, the condition code of 16 (tree not found) would be written. If a previously measured tree had died, it would be recorded as dead and not measured.

CO calculation2

The following allometric equation, Eq. (1), was used to calculate the aerial carbon stored in the plots:

$$ { text {CO}} _ {2} { text {e}} = { text {DBH}} ^ {2} * ( pi {/} 4) * h * { text {FF} } * { text {CF}} * { text {BSW}} * { text {BEFa}} * { text {BEFg}} * 3.67 $$

(1)

To estimate the amount of carbon captured, CO2e, a conversion factor of one ton of forest biomass to about 0.5 ton of carbon, was used.

Basal area is DBH2* (Ï€ / 4), and h is the height of the tree.

However, a shaft is not a perfect cylinder, so several factors must be applied according to the IPCC guidelines, “fit factors” are51:

  • FF is the “shape fit” factor which takes into account the change in diameter with respect to height. The FF varies among tree species but, if the species could not be identified, the IPCC allows an FF of 0.7 in forest trees, which means that the average diameter is 70% of the DBH. .

  • CF is the carbon fraction which represents the estimated volume of carbon stored in the trunk and is considered to be about 50%, or 0.5.

  • BSW is the basic specific gravity which refers to the weight of dry wood in a tree. It varies according to the species and is linked to the density of the wood. If the tree species is unknown, the IPCC recommends using 0.551.

  • BEFa and BEFg refer to the volume of branches and leaves (BEFa) and roots (BEFg). For aboveground biomass, the estimate for both was 1.2 trunk volume, calculated by basal area multiplied by height.

Finally, 3.67 is the ratio of the weight of carbon in carbon dioxide. This means that one tonne of carbon equals 3.67 tonnes of CO2 (obtained on the basis of the molecular weights of CO2 and carbon, from 44/12).

Extrapolation to 50 hectares

To estimate the total amount of carbon captured, a Google Earth image of the CIEE campus was used to estimate how much forest was similar to Zapote, a mature patch, and how much resembled Camino Real, a seemingly more disturbed patch. All areas of dense trees were defined as more mature, while areas with more pasture and sparse growth were defined as younger, corresponding to Zapote and Camino Real, respectively. Figure 3 shows a campus map with mature areas in blue and less mature areas in green. It was concluded that the landscape similar to Zapote was 22 ha and that similar to Camino Real was 28 ha.


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