CHAPTER 1
Soil Foundations

OVERVIEW

The forests of the world are shaped by the soils that support them, and the soils are in turn shaped by the trees. Soils change substantially across landscapes, in response to changes in the parent materials in which soils form, to differences in water flow, and the responses of plants and the rest of the ecosystem biota. Differences in soils lead to large differences in the species composition and growth rates of forests, largely mediated by the supply of water, nutrients, and sometimes oxygen in the soils. One of the most characteristic features of forest soils is the O horizon, the commonly occurring uppermost soil layer composed of fresh litter, decaying litter, roots, and soil organisms. The tapestry of our current understanding of forest soil comes from threads that reach far back into nineteenth century, with great detail added from the advances of science in the twentieth century. The broad variety of soils and forests across dimensions of space and time can be investigated with three standard questions: what's up with this soil, how did it get that way, and what will it likely be in the future?

EVERYTHING IN FORESTS AND FORESTRY CONNECTS TO SOILS

A century ago, Aaltonen (1919) published a classic assessment of Finnish forests, and he concluded that understory regeneration of pine trees was limited when overstory trees were present. The apparent inhibition of small trees by large trees extended too far away from the large trees for the suppression to be about shading, and Aaltonen concluded that the key issue must be about competition for soil resources rather than for light (Figure 1.1). If Aaltonen was right, then trenching small plots to cut off competition for soil water and nutrients with surrounding trees should stimulate the regenerating trees. This is exactly what happened in tests around the world: removing belowground competition for water and nutrients leads to large increases in growth of understory plants even when shaded by healthy overstory canopies. Competition for light is of course important for plants too, but the classic forestry idea of “shade tolerance” is often as much about soil resources as about light (Coomes and Grubb 2000). Ancient ideas about single resources limiting the growth of trees need to be left in the past. The production of most plants and ecosystems is limited by more than one resource, and almost always limited by one or more resources found in soils.

Figure 1.1 The regeneration of pine seedlings in Finland was inhibited by mature trees, at distances that were too great to be competition for light. Aaltonen (1919, after the description from Kuuluvainen and Ylläsjärvi 2011) concluded that the issue must be competition for soil resources. Experimentation beneath full canopies of loblolly and shortleaf pine trees in Duke Forest showed that removing competition for soil resources by trenching plots led to huge growth responses by understory herbs and tree seedlings (center left, before trenching, center right, four years after trenching; after Korstian and Coile 1938). Shade is much deeper under mature canopies of Norway spruce, giving the impression there is only enough light to support an understory of mosses. However, trenching a small plot within the dark stand of Norway spruce in Bavaria, Germany, led to prolific growth of understory vascular plants,

(Source: lower photo, from Christian Ammer)

just like the classic study of Fabricius (1927) who found trenching under a spruce stand more than tripled growth of understory oak and hornbeam seedlings.

FOREST SOILS HERE ARE DIFFERENT FROM OVER THERE

Across Union County, South Carolina in the southeastern United States, plantations of loblolly pine differ in growth rates by more than twofold. About 15% of the land can grow less than 5 m3 ha−1 year−1 of wood, and about 15% can grow more than 9 m3 ha−1 year−1 (Figure 1.2). The climate doesn't vary across the County, so what drives the differences in ecosystem productivity? Of course, soils provide the key explanation. The soils across the County vary in capabilities of supplying water and nutrients to support tree growth. Some of these variations result from factors that influence soil development over very long periods of time, such as the original parent material and position on slopes (near the top where water and nutrients are removed, or near the bottom where water and nutrients accumulate). As with most aspects of soils, changes in some factors (such as parent material or water supply) cascade with follow‐on changes in soil chemistry, the biotic community, and the growth of plants.

Figure 1.2 Across 100,000 ha of Union County, South Carolina in the southeastern United States, the growth rate (productivity) of wood in loblolly pine plantations differ by more than twofold. The climate is the same for all locations, so the differences in growth rate result from differences in soils.

(Source: from data of the Natural Resource Conservation Service (2017))

CURRENT FOREST GROWTH IS LIMITED BY SOIL RESOURCES ACROSS REGIONS

Just as productivity differs across locations, the productivity within a single location may shift upward or downward if soil conditions change. At the sub‐continent scale of the southeastern United States, the growth of loblolly pine plantations differs by more than twofold from one region to another (Figure 1.3). The spatial pattern of growth differences connects to temporal patterns too: the ability of soils to support tree growth can change over time, in response to active or passive management. Estimates of the climate‐determined potential productivity across the region can be compared with the observed rates, and the differences between these two suggest that soil‐related factors restrain growth by 15–20% across the region. Silvicultural practices can move forest growth toward the climatic potential, or indeed reduce growth further if done poorly.

Figure 1.3 The accumulation of stemwood in loblolly pine plantations in the Southeastern United States ranges from about 300 to 600 m3 ha−1 of wood volume at age 25 (upper graph). If soil fertility did not limit growth, many locations would show much higher productivity (middle graph). Across much of the region, current limits on soil fertility reduce forest stem biomass by about 60–90 m3 ha−1 (a reduction of 15–20%; lower graph). These graphs based on illustrative simulations using the 3PG model, provided by Jose Alvarez‐Munoz.

SOILS SHAPE FORESTS, AND FOREST SHAPE SOILS

The two forests in Figure 1.4 are growing only 50 m apart, along the Mendocino Coast in California, USA. The tall forest has fast‐growing Douglas‐fir, redwood and Bishop pine trees. The short forest has slender stems of Bishop pine, Bolander pine, and pygmy cypress. What ecological factors explain such huge differences in vegetation? Differences in weather and climate would not be important, as the forests experience the same conditions. Some other possibilities that might come to mind include forest age (perhaps one type of forest is much older), the influence of recent fires (perhaps the short forest was a tall one before a fire?), or a legacy of logging. Given the focus of this book, it's not surprising that the key differences are in the soils. The tall forest grows on a deep soil with good properties for supplying oxygen and nutrients to the trees. The short forest is on a soil with a cemented layer that restricts water movement, and the high water table impairs the supply of oxygen for roots and the soil biotic community (Figure 1.5).

Figure 1.4 These two forests are part of the Ecological Staircase in the Jughandle State Park along the California coast. Differences in soils lead to differences in vegetation and tree sizes.

(Jenny 2015; Source: photo on left from Ron Amundson, on right from Sarah Bisbing)

Figure 1.5 The soils under the tall trees (left) is well drained, with good supplies of oxygen for tree roots and the soil biotic community. The soil that supports only short trees (right) developed an iron‐cemented layer that keep water perched high in the profile, restricting oxygen, root growth, and nutrient cycling rates.

(Jenny 2015; Source: photos by Ron Amundson)

But why are the soils so different? Have the soils been developing for different periods of time, or did they develop from different initial rock types? Or is it possible that they could be the same age, developed from the same initial materials, and some other factor explains these differences? The development of soils always involves complex, interacting processes. An understanding of chemistry, physics, biology and ecology is required to understand how soils develop in different ways, and how these differences shape forests. In this case, the key feature that led to such different sorts of soils was the location relative to the edge of a terrace; water drained away better the locations supporting tall trees, and water flow was more restricted back from the terrace edge where only the short trees can grow.

The comparison of these two adjacent sites also illustrates a key feature of forest soils, and a key challenge in studying these systems. As the forests and...