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Paul Scowcroft

Pacific Southwest Research Station

My favorite science experience was discovering why hanging a sheet of shade cloth on the east side of native ohia tree seedlings protected them from frost damage. At 6,500 feet elevation, where forests have been turned into cattle pasture, temperatures can easily fall below freezing. Shade cloth is a piece of cloth that lets some sunlight through but also provides some shade.

We wondered if the shade cloth caused the leaves to warm slowly in the morning. This slow warming would be similar to treating frostbite by slowly warming the fingers. We attached electrical wires to the underside of leaves. These wires enabled us to record the leaves’ temperature over time. Some of the leaves were fully exposed to the sky. Some were shielded from the morning sun by shade cloth hung on the east side of the seedling. Others were located within a forest of 5-year-old koa trees.

We found that exposed seedlings experienced lower temperatures for a longer time than shade cloth-shielded or koa-protected seedlings. The shade cloth blocked part of the night sky and prevented heat loss into space. The koa trees blocked even more of the sky. In both cases, the results were warmer leaves. Even though the temperature difference was only 2 to 3 degrees, it reduced frost damage. Under koa trees, heat loss was so low that ohia seedlings showed almost no damage and kept growing throughout the winter. From this experiment, we learned that after ice crystals puncture the cell walls of leaves, the cells die. It doesn’t matter how slowly frozen leaves are warmed!