Where Do Young Mesquite Trees Get Their Mass?

Building Trees in a Dry Place

At a desert seed bank, workers save seeds of native plants. They also raise young mesquite trees for nearby restoration areas. Before planning supplies, the workers investigate where the material in new roots, stems, and leaves comes from. Their practice notebook contains two studies. These are small studies of young plants, not measurements of full-grown desert trees.

In the first study, workers grow six mesquite seedlings together in one container for twelve weeks. They provide ordinary air, light, and water that keeps the roots moist. A cover keeps soil from blowing or spilling out. Workers collect fallen leaves with the plants. Nothing is added to the soil except water. The water contains too little dissolved mineral material to explain the plants' mass gain.

The six seedlings have an estimated starting dry mass of 12 grams. Workers get this estimate by drying a separate, closely matched set of seedlings. At the end, an adult harvests and dries the study plants, including their roots. Their final dry mass is 62 grams. The dry soil starts at 2,400 grams and ends at 2,398 grams. The container's mass is not included. Dry mass excludes water stored in the plants or soil.

The second study compares four new groups over six weeks. Each group begins with six seedlings of the same kind and similar starting dry mass. N1 and N2 receive normal air. L1 and L2 receive air with much less carbon dioxide, a gas that leaves take in. Adult-operated equipment controls the gas supply. The groups receive matching soil, light, temperature, airflow, and watering. Their roots stay moist. The graph reports gains in dry plant mass.

Other scientific studies provide another kind of evidence. Using special tracing methods, scientists have followed material from carbon dioxide and water into newly made plant parts. Finding a material inside new parts tells more than observing that a plant grows better when a supply is available.

Roots take in water and small amounts of mineral nutrients. Leaves take in carbon dioxide. Mineral nutrients remain important even though plants need relatively small amounts. Light supplies energy for building plant parts; it is not a substance added to their mass. The notebook does not measure the exact shares of new material supplied by air and water.