Big Trees: A Bank for Soil Bugs

Legacy trees maintain soil and litter microarthropod abundance and assemblage organization in managed secondary redwood forests

Photo by Miguel Vieira, Flickr Creative Commons
Photo by Miguel Vieira, Flickr Creative Commons

Legacy trees, old-growth trees left standing in second-growth redwood forests, could serve as a habitat refuge for terrestrial microarthropods, miniscule bugs that live in the forest floor and maintain healthy soils, not to be confused with the bigger arthropods like spiders and bees. Dr. Michael Camann, Karen Lamoncha and Laura Hagenhauer have found substantially more and a wider variety of the soil bugs underneath these so-called legacy trees than beneath surrounding second-growth trees.

Dr. Camann and team undertook this study to complement Mary Jo Mazurek and William Zielinski’s study of mammal use of legacy trees, in which they presented evidence that the leftover giants serve as mini-reserves for bats, birds, and the four-footed inhabitants of second-growth redwood forests. Now that 95% of the ancient forest has been cut and conservation energy is shifting to restoring old-growth characteristics to second-growth forests, these legacy trees could very well be the base from which biodiversity spreads. According to Dr. Camann, microarthropod diversity is as important to protect as mammal diversity. While they feed, the bugs aerate and mix soil, regulate the population size of soil fungi and microbes, and shred organic material. In addition to preparing the ground for plants, arthropods are a food source for higher-order creatures, such as ants and salamanders.

The research team compared numbers of microarthropods under legacy trees with those under comparable second-growth trees on private lands and in Jackson State Demonstration Forest. After sampling the soil extending from tree bases to the outermost spot a drop might drip from tree leaves, Dr. Camann’s team brought the samples back to their Humboldt State University lab and, through microscopes, identified as many bugs as they could in four days.

They found several promising results. First, leaf litter—the topmost and least decomposed layer of forest soil—was much thicker in the samples taken from legacy trees. This is important because leaf litter is good habitat for microarthropods. It very well may be the reason for their higher diversity and abundance under legacy trees. Dr. Camann’s team also noticed that mites and springtails made up the overwhelming majority of soil bugs sampled. This finding is consistent with sampling from in tact, unlogged old-growth forests and implies healthy soil. Of all microarthropods, mites and springtails contribute most to soil decomposition.

No regulatory protections exist for legacy trees that have escaped harvest on commercially managed timberlands. Due to harvest rotations, it’s unlikely that forests will return to their former old-growth state on their own. Maintaining diverse and abundant microarthropod populations in legacy tree soils may make the transition to old-growth conditions much smoother.

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