Plants receive nutrients from air, water, and soil. Chemicals move throughout the air, water, and soil, and then into plants. Plants absorb 14 nutrients from the soil. Plants have a disadvantage when faced with unfavorable conditions, such as poor soil conditions. They cannot move quickly to a new environment when conditions are unfavorable. When soil or rainfall conditions are not optimum for plant growth, plants must adapt to thrive. When conditions are not favorable for plants, one way they may adapt is to increase the percentage of their root structure compared with the rest of the plant’s aboveground structure. They may grow their roots deeper and spread them out farther away from the main root stem.

The scientists in this study were interested in how trees growing in similar soil types, but living in different rainfall conditions, adapt so they can thrive. To understand such tree adaptation, the scientists compared the soil’s chemical content with the tree leaves’ chemical content. Studying the chemical content of soil and leaves helps scientists understand how plants adapt to their environment.

Karst is a geochemical landform from which caves and sinkholes are formed (figures 1 and 2). Karst is made up of limestone and other soluble rocks. Limestone erodes easily, especially when groundwater or rainfall is slightly acidic. As limestone erodes, water seeps into the ground and further erodes the limestone underneath. This erosion forms sinkholes, caves, and underground streams and lakes.

Karst landscapes are found worldwide, covering about 20 percent of Earth’s land surface. All of Florida, for example, is karst (figure 3).

Karst landscapes are an important source of drinking water, provide wildlife habitat, and provide outdoor recreation opportunities such as caving. In this study, the scientists were interested in learning about forests growing on karst in Puerto Rico.

Puerto Rico is an island in the Caribbean and is located in Earth’s tropical region (figures 4 and 5).

The northern region of Puerto Rico experiences a lot of rainfall. The southern region of Puerto Rico, in contrast, is semi-arid, an area with light rainfall (figure 6).

About 27 percent of Puerto Rico contains karst landscapes (see “Thinking About the Environment”). Karst is found in both Puerto Rico’s northern wet region and its southern dry region. Karst soils are shallow, and most karst soil is built from the leaves that fall from forest trees (figure 7). Soils in Puerto Rico’s southern dry karst forests build slowly in the crevices between the pavement sections (figures 8 and 9).

The scientists in this study wanted to compare the chemistry of soils and leaves in Puerto Rico’s northern wet and southern dry karst forests. The northern wet karst forests receive an average of between 1,000 and 1,500 mm of rainfall every year. The southern dry karst forests receive an average of less than 1,000 mm of rainfall every year. The scientists wondered whether southern dry karst forests, since they grow in a semi-arid region, use water more efficiently than northern wet karst forests.

The scientists collected leaves from trees growing in both the northern wet and southern dry karst forests. Leaves from the northern wet karst forest were collected from trees on the mogote tops. Leaves from the southern dry karst forest were collected from trees growing on the ridges and pavement. All leaves were collected from tree canopies.

Recall that the scientists were also interested in the differences in how trees use water in northern wet and southern dry karst forests. After the scientists collected the leaves, they dried the leaves in an oven for 3 to 7 days and then finely ground the leaves. The scientists used specialized equipment to determine which chemicals were found in the leaves collected from the two karst regions. Measuring the chemical content of leaves enabled the scientists to compare leaf chemistry with the chemical content of the soil. This comparison helped the scientists to determine how trees growing in similar soils, but different environmental conditions, adapted to those conditions. The scientists tested leaves for the presence of:

• Carbon
• Nitrogen
• Phosphorus
• Sulfur
• Potassium
• Calcium
• Magnesium
• Aluminum
• Manganese
• Iron

The scientists were also interested in stomatal conductance. The word stomatal comes from the fact that leaves have small openings, called stomata, through which gases enter and exit the leaf (figure 10). You can see in Figure 10 that carbon enters the leaf as carbon dioxide gas.

Carbon atoms have different atomic masses. Carbon atoms are either Carbon 12, a lighter atom, or Carbon 13, a heavier atom. Carbon 12 atoms are the most plentiful on Earth, making up 98.93 percent of all carbon atoms. When plants have plenty of water, they open their stomata completely. The plant enzyme responsible for the uptake of CO2 in most plants readily takes up the more plentiful Carbon 12 atoms entering the leaf, preferring Carbon 12 atoms to the heavier Carbon 13 atoms.

When plants have a limited supply of water, the stomata do not open completely. In this case, the plant enzyme responsible for the uptake of CO2 will take up all available carbon atoms. Plants in water-restricted environments, therefore, may be more likely to have a greater amount of Carbon 13. The scientists measured the proportion of Carbon 13 to that of Carbon 12 in the leaf tissues. The ratio of Carbon 13 to Carbon 12 (Carbon 13/Carbon 12), therefore, is related to how well a plant efficiently uses water.

For the most part, leaves were similar in Puerto Rico’s two karst regions. Leaves from the northern wet and southern dry karst forests contained similar amounts of magnesium, phosphorus, and nitrogen. Leaves in both the northern wet and southern dry karst forests contained large amounts of calcium. The scientists also found that leaves in both Puerto Rico’s northern wet and southern dry karst forests are limited in phosphorus. Leaves in the southern dry karst forests, however, contain more potassium than leaves in the northern wet karst forests. The scientists believe this difference is because higher rainfall in the northern wet karst region leaches potassium from the soil.

The scientists found that leaves in the southern dry karst forests contained more carbon-13 than leaves in the northern wet karst forests. Leaves that use water more efficiently contain higher levels of carbon-13. Therefore, the leaves from the southern dry karst forests use water more efficiently, meaning that leaf stomata do not completely open, and less water vapor exits the leaves.

The soils in northern wet and southern dry karst forests were similar. Soils from both regions contained limited phosphorus. Low phosphorus levels limit the growth of karst forests in Puerto Rico. The soil in both regions contained high amounts of calcium. Calcium aids plants by holding cell walls together. The southern dry karst forest soils contained more potassium than northern wet karst forest soils. Northern wet and southern dry karst forests can primarily be distinguished by the amount of rainfall they receive.