Written by Bill Chapman, Soil Scientist, Cariboo Forest Region; Marty Kranabetter, Assistant Soil Scientist, Prince George Forest Region

"The highest levels of terrestrial diversity anywhere on earth occur in the soils of our temperate forests"
Andrew Moldenke, Entomologist, Oregon State University

Introduction

The forest soil holds an amazing variety of life: everything from microscopic mites to small mammals. The importance of this below-ground ecosystem is just beginning to be appreciated, but what we have learned so far has shown that forest soils are very much alive, dynamic and incredibly complex.

Most of the organisms that live in the forest soil have barely been studied, but two of the largest groups that have received more attention are soil arthropods (insects and insect-like things) and soil fungi. There are, however, several other large groups of organisms in the soil.

Arthropods  

Many people are familiar with the food chain concept of the forest, in which plants supply food for herbivores, such as leaf-eating insects, which then cycles through predators, such as birds.

These same kinds of food chains exist on a microscopic scale in the soil, but with many times the diversity of species found aboveground.

Research into soil fauna diversity in the interior of British Columbia was initiated in 1992 (Battegelli and Berch, MoF); one year's sampling in mature forests found 18 types (families and orders) of arthropods and three types of worms.

Moldenke has reported 75 species of mites per square metre of soil. The population estimates of these fauna are staggering. In the North and Central Interior, for example, the number of arthropods ranges on average from 350,000 to almost 700,000 individuals per square metre.

As shown in Table 1, these fauna exist as many types: mites (Acari), springtails (Collembola), fly larvae (Diptera), worms (Lumbricidae), and snails (Gastropoda), to name a few.

Our understanding of the function of the soil ecosystems to which these animals belong is virtually non-existent.

There are estimated to be 48,000 to 60,000 species of soil arthropods in North America (estimates of around 35,000 have been made of BC), of which only 53% have been identified.

Approximately 80% of the springtails have been identified, but the number and kinds of species are essentially unknown.

Only approximately 17% of the mites have been identified to the species level. For example, out of a small sampling of thirty-four mites from two sites in BC, only eight could be identified to species.

In comparison to other species in BC, approximately 2500 species of vascular plants, 450 species of birds, 100 species of mammals, 20 species of reptiles and 20 species of amphibians have been classified. In these groups, there is very little that has not yet been identified.

The life histories and ecology of the small proportion of identified arthropod types have not been well studied. Many functional groups are represented: detritivores, wood channelers, bacteriavores, fungivores, parasites, and predators.

Many soil species can act in more than one capacity. We can only speculate on some of the interactions between arthropod groups that regulate important processes of the soil, and ultimately, the forest.

Ecosystem Role of Soil Arthropods

One of the key soil processes in which arthropods participate is nutrient cycling. The fieldwork conducted to date in the northern interior shows that the most abundant fauna are fungivores, such as mites and Collembola.

These indirectly affect nutrient cycling by grazing on the fungal biomass. In doing so, nutrients are released from the microbial biomass and made more available for plant uptake.

Grazing can also reduce the rate of organic matter decomposition by keeping microbial biomass levels low. This may be important in conserving nutrients. The types of fungal and bacterial species can also be affected by the feeding of arthropods.

The production of faecal matter is also an important component to nutrient cycling. Converting litter into faecal matter changes their chemistry and shape, which seems to change their availability to microbial decomposition.

This relationship is complicated by the apparently different properties of faecal pellets, depending on whether they originated from micro- or macrofauna, or from different species.

The movement of soil fauna leads to the transport of organic matter and microbes through the soil. This may prove to be one of the most critical roles of soil arthropods.

On a large scale, some of the macroarthropods can move between soil horizons, the rhizosphere, and coarse woody debris.

On a small scale, arthropods move between pores and aggregates that can redistribute organic matter to and from the rhizosphere.

The effect of this movement on nutrient cycling may be especially important for otherwise immobile nutrients, such as phosphorus.

Habitat Types

The forest soil ecosystem is comprised of several habitat types, each of which would likely have unique arthropod communities.

The mineral soil and forest floor provide the most widespread habitat. The forest floors of the temperate and boreal forests are unique substrates in that they are massive accumulations of essentially biodegradable substrates or, in other words, food.

This enormous accumulation of food has decomposition tempered by environmental and biological factors, which leads to one of the largest and most complex habitats in the world.

Within the forest floor are variations in site factors that lead to the formation of mor, moder and mull type forest floors. The faunal species, populations, and activity seem to increase along this gradient.

The presence of coarse woody debris in forest ecosystems provides a unique and ecologically important habitat for soil fauna.

Another microsite is the rhizosphere, which is the soil surrounding feeder roots. This is a very microbially active zone, with large amounts of exudates from the roots that create a unique environment.

In addition, most plants within the forest develop a zone of influence that can alter populations. For example, a Shepherdia plant with nitrogen-rich leaf litter can be expected to have different organism populations within its zone of influence than would, for example, an understorey spruce.

Fungal mats (see below-Fungi) are uniquely hostile environments to organism growth. Soil factors, such as pH and nutrient availability, vary with depth, substrate type, precipitation, and age. These factors create an almost limitless variety of microsite, and complements of organisms, including arthropods, have evolved to occupy them all.