Environmental controls

Dokuchaev (see citations in Crocker, 1952), Jenny proposed that soils are formed by dynamic or ‘active’ external inputs (e.g. erosion, immigration of biota) and passive internal factors (e.g. parent material, topography).

Time, or the age of the site, is the third important factor. Time incorp-orates the cumulative effect of all active processes at a given site. The best-known elaboration of these active and passive variables includes cli-mate (cl ), organisms (o ), topography (r ), parent cli-material ( p ) and time (t ) together in Jenny’s equation where soil formation (s ) is a function of all the variables:

s = f (cl, o, r, p, t).

Each variable has been considered dependent or independent of the other variables (Matthews, 1992) yet time, having no direct impact on soil formation, is often separated such that all other processes are a function of time:

s = f (cl, o, r, p) dt.

The robustness of this approach is demonstrated by its persistence in nearly identical form for many decades (Amundson & Jenny, 1997). Other useful modifications have explicitly included human land-use practices and fire ( Wali, 1999a). The unmet challenge is to adequately test this the-ory under field conditions. The interdependence of each variable means that removing one while keeping the others can only be approximated, even when permanent plots are monitored over time. Chronosequences (see Chapter 1) are more frequently used to study primary succession than permanent plots, but the use of chronosequences for this purpose requires that factors affecting soil formation at various sites of different ages have remained constant over time. This is obviously unlikely due to the spatial heterogeneity of site variables such as parent material and topography and the temporal heterogeneity of the influx variables of erosion and biota (Stevens & Walker, 1970; Pickett, 1989). Rates of soil formation can be approximated when site ages are carefully established (as for many glacial moraines; Matthews, 1992), but determining the rel-ative importance of each soil-forming variable remains unrealistic. We will now examine both broad environmental parameters and more local physical properties that impact soil formation.

4.2 Environmental controls

In this section we present the passive or endogenous site characteristics (parent material and topography) and the active or exogenous variables

(erosion and biota) that determine how soils form in primary succession.

First, however, we discuss the broadest of all variables, i.e. the climate, in which soil formation occurs.

4.2.1 Climate

Clements (1928) proposed that, within a regional climate, plant com-munities would reach a stable endpoint or climax. He also proposed that soil development, altered by plant influences (reaction) and climate, would reach a stable equilibrium. However, Clements recognized that heterogeneity in soils at a finer spatial scale than regional climates could result in particular endpoints on particular soil types (edaphic climaxes).

Although the emphasis now is on local variation rather than regional sim-ilarity (a dynamic view that emphasizes disturbance and disequilibrium over homeostasis; see Chapter 3), climate does determine the overall rate and direction of soil formation.

Temperature and water availability are perhaps the most important as-pects of climate that influence soil formation. Temperature extremes limit plant enzyme function for photosynthesis and the presence of water in liquid form. Too much water reduces soil oxygen levels for root respi-ration whereas too little water limits decomposition, mineralization and many physiological functions, including photosynthesis. Animals are lim-ited in similar ways. Many organisms have temperature optima between 15 and 35^◦C. Extreme temperatures are frequently encountered in pri-mary succession, where cold (e.g. glacial) or dry (e.g. desert) conditions limit soil formation and plant establishment. Under such conditions, the biotic influence on soil formation is minimal (Matthews, 1992), although many organisms have adapted to such conditions and have temperature optima of less than 15^◦C or high levels of drought tolerance. However, the ability of some microbes to survive in nearly all terrestrial habitats leads to the potential for primary succession in most habitats (although it might be limited to microbial succession; see section 4.4.2).

Both the amount and timing of precipitation are very influential in soil formation. High levels of precipitation result in leaching of nutrients, as on tropical volcanoes ( Whittaker et al., 1989) that can slow soil devel-opment. However, in cooler yet rainy climates such as in river valleys (Tonkin & Basher, 2001) and on glacial moraines (Sommerville et al., 1982) in southern New Zealand, soils develop rapidly. In one river valley that received c. 10,000 mm of precipitation per year, spodosols formed within 500–1500 yr (Tonkin & Basher, 2001). In contrast, low levels of precipitation combined with high temperatures can result in net upward

4.2 Environmental controls · 91 movement of solutes and precipitates that form surface crusts of salts, as in deserts (Smith et al., 1997). Intermediate (mesic) levels of precipita-tion and moderate to warm temperatures result in high rates of plant productivity and decomposition, which is often correlated with rapid soil organic matter accumulation, as on subtropical landslides (Zarin &

Johnson, 1995a,b; Walker et al., 1996). Soils at the relatively warm and moist coastal site of Glacier Bay, Alaska, developed 50–100 yr faster than at a cooler and drier site 160 km inland (Klutlan Glacier, Canada; Jacobsen

& Birks, 1980). A comparison of rates of soil formation in primary seres in six habitats throughout the world further supports more rapid for-mation in warm and wet than in cold and dry climates (Birkeland et al., 1989). Decomposition is most accelerated by changes in temperature and moisture such as occur during wet–dry or freeze–thaw cycles (Taylor &

Parkinson, 1988).

Soils in primary seres are initially entisols (azonal or undeveloped). Soils typical of the region (e.g. spodosols in cool, wet climates or aridisols in arid regions) may eventually develop on the primary site over successional time, but any of the environmental controls discussed below can direct soil development in unique directions.

It is at the scale of the microclimate that soil formation is ultimately reg-ulated, because local conditions must be favorable for soil biota to func-tion. However, conditions that promote net primary production (mesic, warm, aerated) also increase decomposition and mineralization rates (see section 4.3.5 on N). Therefore, in warmer regions there is more organic matter loss and less carbon accumulation in the soil. In addition, warm sites are often too dry for optimal functioning of soil biota. These coun-teracting influences make it difficult to predict the effects of both local and larger (e.g. global warming) influences on soil.

4.2.2 Parent material

The substrate that remains following a severe disturbance is obviously an essential feature that directs the process of soil formation through its physical and chemical properties. Primary surfaces encompass a wide va-riety of textures and levels of stability and fertility (Fig. 2.1). The surfaces have either been transported to the site or modified in situ. Transported substrates result from deposits by wind (aeolian, e.g. dunes), water (e.g.

floodplains, deltas, glacial moraines) or gravity (e.g. rock scree, landslides, mine tailings, soft benthic surfaces). These substrates are generally unsta-ble and soil formation may be accelerated when colonizing plants begin

to stabilize the surface. Volcanic activity provides a variety of new sub-strates of varying stability (pahoehoe lava> a’a lava > lahars > pumice >

ash; del Moral & Grishin, 1999). When the added layer is shallow or per-meable, surviving organisms in established soil layers below can influence succession (for example, gophers bring buried soils to the surface of new tephra on Mount St. Helens; Andersen & MacMahon, 1985). Substrates that are altered in situ can result from scouring (some glacial surfaces, river beds), compaction or scraping (roads, river beds), draw-down (lake shores) or erosion (cliff faces). Subsequent stability (see section 4.2.4 on erosion) is related to texture: rocks are less likely to re-slide than is clay, sand is less likely to blow away than is silt, gravel is less likely to wash away than is sand.

The chemical composition of the parent material can affect patterns of soil formation. Neutral to slightly alkaline surfaces (e.g. limestone) break down faster into soil than acidic surfaces (e.g. granite) or very al-kaline surfaces (e.g. sodium-rich marine clays), owing to conditions that favor biotic weathering by soil microbes (see section 4.3.4 on pH) or abiotic weathering. Some surfaces are rich in minerals (e.g. serpentine:

magnesium; gypsum: calcium sulfate) that support unique flora and form distinctive soils. Physical characteristics that affect soil formation and plant colonization include particular weathering characteristics (granite exfoli-ates, slate or mica come off in sheets) and porosity (water drains through a’a lava but puddles in cracks of pahoehoe lava). Differences attributable to parent material may be temporarily obscured in primary succession by the impacts of land use such as cultivation and grazing (Puerto & Rico, 1994).

4.2.3 Topography

Steep slopes are unstable and offer fewer microsites for seed retention than flat surfaces. On an Illinois (U.S.A.) mine tailing, flatter surfaces supported higher nodulation rates of the shrub Alnus, presumably owing to more favorable water conditions (Dawson et al., 1983). Concave microsites in volcanic tephra on Mount St. Helens had higher nutrient levels after 7 yr than convex surfaces (Zobel & Antos, 1991). Slope and aspect (compass direction of the slope) combine to affect surface temperature and many other soil properties. Surfaces will be hotter and drier if they face toward the equator and may experience three times as much evapotranspiration as pole-facing slopes (Le Hou´erou et al., 1993), higher rates of decompo-sition and mineralization (Gerlach, 1993), and lower plant productivity

4.2 Environmental controls · 93 (Viereck et al., 1983). For succession on mined lands in North Dakota (U.S.A.), Wali (1999a) demonstrated that south (equator)-facing slopes had lower accumulation rates of both N and C than north-facing slopes.

Soil development was equally rapid on gentle and steep slopes in a high rainfall area in New Zealand, suggesting that topography is not always important (Basher, 1986). Similarly, slope had no effect on species distri-bution on Alaskan landslides (Lewis, 1998). The impacts of local surface textures on succession are discussed in section 4.3.1.

4.2.4 Erosion

Steep surfaces made of fine particles are most likely to erode (see section 2.2.1 on erosion). Eroded surfaces are likely to continue to lose soil until the angle of repose is reached or other forces (e.g. plant coloniza-tion) stabilize the surface. However, subsequent forces (e.g. earthquakes, road cuts, floods, vegetation removal) can cause the whole surface to erode anew (secondary erosion). On a landscape scale, secondary ero-sion is dramatically represented by the lahars that were deposited around Pinatubo volcano in the Philippines when rains eroded initial tephra deposits (Newhall & Punongbayan, 1997). Stabilized pumice deposits have been eroding rapidly owing to recent quarrying on the island of Lipari (Italy; R. del Moral, pers. obs.). Active glacial moraines, recently devoid of the stabilizing ice and subject to melt waters from the retreat-ing glaciers, are very unstable substrates (Matthews, 1999). Instability is also a constant factor on floodplains (Malanson, 1993). Only fast-growing species are likely to colonize unstable surfaces such as floodplains ( Johnson et al., 1985; Walker et al., 1986), dunes (Sykes & Wilson, 1990) and volcanic tephra (Chiba & Hirose, 1993). High plant productivity ac-celerates stabilization through root growth and aboveground interception of wind-blown or water-borne particles.

On local scales, erosion can vary within a disturbance type. Landslide surfaces include the upper slip face that is very unstable and likely to resist any soil formation, the middle chute that is frequently scoured to bedrock or mineral soil, and the lower deposition zone where organic de-bris mixed with subsoil can support relatively rapid colonization and soil formation (Lundgren, 1978; Smith et al., 1986; Adams & Sidle, 1987;

Walker et al., 1996). Similarly, dunes are composed of areas of active erosion and relative stability that influence colonization and succession (Moreno-Casasola, 1986). McLachlan et al. (1987) determined that both plant and animal biomass were positively associated with sand stability

Table 4.1. Plant and animal biomass (g m⁻²) in five vegetation zones on dunes in South Africa

Zones 1 and 2 (slip face and windward slope) from Fig. 4.1 are combined into dune slopes in this table. The other zones in Fig. 4.1 are 3 (pebble corridor), 4 (Sporobolus), 5 (Gazania) and 6 (Psoralea). Biomass increases with increasing stability from left to right.

Dune Pebble Sporobolus Gazania Psoralea

Component slopes corridor zone zone zone

Aboveground plants 0.0 0.0 20.6 61.7 394.8

Belowground plants 0.0 0.0 50.3 241.1 752.9

Orthoptera 0.0 0.0 0.3 0.3 0.8

Coleoptera 0.6 0.1 0.8 7.5 5.2

Lepidoptera 0.0 0.0 0.4 2.4 2.3

Homoptera 0.0 0.0 0.0 0.0 1.4

Data from McLachlan et al. (1987).

on coastal dunes in South Africa (Table 4.1). Six distinct topographic zones with unique vegetation were identified (Fig. 4.1). Biomass in-creased with succession for about 6–7 yr, after which the vegetation was buried. Tielb ¨orger (1997) delineated seven plant communities on dunes in the Negev Desert (Israel) that were related to surface stability. She pro-posed but did not test that increased stabilization paralleled successional development. Highest plant cover and species richness were found on the most stable surfaces (Fig. 4.2). Thus plants require a certain level of surface stabilization in order to establish and further stabilize the surface as they grow and spread.

At microsite scales, small erosive channels can displace seeds and seedlings, resetting succession repeatedly. Roads or trails can act as ef-fective erosion channels, funneling runoff, carving deep incisions in the landscape and delaying soil formation, particularly in cold habitats where water accumulates when the permafrost melts and thermal properties of the surface are altered (Kom´arkov´a & Wielgolaski, 1999). Because soil removed by wind or water is up to five times richer in organic matter than the soil left behind (Allison, 1973) and loss of developing topsoils reduces water-holding capacity, nutrients and soil organic matter, the effects of erosion on soil formation are especially destructive (Pimentel & Harvey, 1999). Global effects of aeolian transport of wind-eroded materials are discussed in sections 4.3.6 and 4.3.7.