Comparing the size range distributions of Acacia collinsii in a burned and non-burned area of Palo Verde Park

What research question could link conditions specific to Palo Verde with our course themes of global environmental change and habitat fragmentation?

Our group became interested in the large burned-over areas within the park. In his presentation to our group, OTS director Eugenio Gonzalez had discussed these areas. Although Palo Verde is the driest region of Costa Rica (1000 – 1500 cm/year) these fires are not of natural origin. Rather, they are set by humans who: a) use fire to drive game for hunting purposes (the majority of fires) and b) set fires to secure employment as firefighters. Do these fires change park habitat in measurable ways?

Park ranger Manrique Montes took us on a tour of a burned area. He pointed out an invasive specie of grass, Hyparrhenia rufa, commonly known as African Star Grass. This specie rapidly colonizes burned areas with two main effects: a) the grass accumulates a large amount of dry organic material increasing the likelihood and severity of further burns, and b) the thick mat of dry organic material selectively prevents seeds of some tree species from reaching the soil, inhibiting their germination.

Post fire environments have been shown to offer certain advantages to seedlings including increased space, increased resources (ash, as well as soil nutrients released by heat and pH changes), and an increase in the carbon: nitrogen ratio. It is important to note that plant populations respond to fire differently (Bond and van Wilgen, 1996). For the purposes of our study we chose trees that are known to occur in both a burn and a non- burn area. We hoped to measure a seedling response to a burn as a "pulse" of growth on a growth distribution graph comparing a burn and a non-burn area. We expected that the growth in the burn area would show an initial increase as a result of increases in space and resources. Over time as the favorable factors are mitigated (space is filled and nutrients are consumed) growth in the burned area would slow to the level of the burned area.

A burned area will show a "pulse" of growth associated with the increased space and resources available after the burn. This pulse will show up as a bump in a graph of the size range distribution of a tree specie when compared to a size range distribution of the same specie in a non-burn area.

Methodology

We sampled the growth distribution for three tree species, comparing each species pattern in a burned area to the pattern in a nonburned area. With the assistance of park personnel, we selected two sites along the same road (see figure 1) at Palo Verde. The sites were approximately 0.5km apart. One site experienced a burn approximately 11 years ago while the other had not been effected by a recent burn. We ran two 50m transects at each site. Each transect ran perpendicular to the road in opposite directions at 120 and 300 degrees. We sampled from 1 meter on either side of transect identifying, mapping and measuring individuals of three species of trees that were known to be found in both areas: Acacia collinsii, Cochlospermum vitifolium (poro poro), and Cordia collococca (muneca).

burned area

nonburn area

The Trees Poro Poro was identified by its simple palmate leaf that hangs down like small umbrella. In addition it had a red petiole and smooth shiny bark. We thought the poro poro looked similar to a maple leaf. Muneca was identified by its whorled waxy leaves with a wide petiole base. Acacia was identified by its feather like compound pinnate leaf with opposite leaflets. A tree with similar leaf structure was also found in the transect. We distinguished the acacia by the presence of thorns and ants. For this reason the data from the first 25m of transect one regarding acacia may be flawed. We initially were going to collect data on guacimo instead of acacia but due to inexperience in distinguishing guamico visually and the abundance of the acacia we decided to collect data on acacia instead.
	Guacimo	Acacia collinsii

	Cochlospermum vitifolium Poro poro	Cordia collococca Muneca

As each tree was identified we measured the circumference at 1 m. If a branch was at the one meter height we measured right below the branch. If the tree consisted of two or more trunks the sum of the trunks was recorded. Trees smaller than the measuring limitations of the cloth tape measure were recorded as less than 3mm in circumference. Trees were assigned a number. Tree locations were mapped and circumferences recorded.

Click on a thumbnail to view the enlarged image.

Scotty identifies guacimo

Dave helps Scotty ID muneca

Ran starts the transect

Scotty measured the circumference

Data collection along the transect

Ran and Marilynn recording the Data

Analyzing the data with Evan

Analysis:We only found two Poro Poro and one Muneca tree in our census of the four sites. Therefore, we decided to not use either of these trees in our concluding statements. All our analysis was based on our data from the Acacia tree. From the data we found fifty-two Acacia trees in the non-burned area and six Acacia trees in the burned area with a circumference of three-mm or less. Our data shows that for trees larger than the twelve-mm circumference we found two or less individuals of any given circumference.

Conclusion: The observable differences of trees that were three millimeters or less in the burned area as opposed to the nonburned area was quantitatively substantial. The germination, or very early development of Acacia seems to be affected, for the worst, in the burned area. In addition our group observed a greater amount of grass in the burned area. Is the grass a result of the fire? And does the grass play a role in the growth of Acacia, or are shade and soil factors. The conclusion, however, shows no detection of a pulse of Acacia growth in the burned area.




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