Tuatara: Volume 19, Issue 3, August 1972
Concepts in Vegetation/Soil System Dynamics — II. Post Steady-State
Concepts in Vegetation/Soil System Dynamics
II. Post Steady-State
The Following Paper is a sequel to a review (Park, 1970) that examined the multitude of terms and concepts pertaining to the state of climax, stability, maturity and steady-state. Steady-state was preferred as a term of comparative stability that may be preceded and succeeded by states of comparative instability — ‘pre-steady state’ and ‘post-steady state’ respectively.
The steady-state has been defined as a temporary state of dynamic equilibrium in an open system. Any open system is continually directional in time (Bray, 1958). Therefore, theoretically, no one stage can a priori be used as a reference point in the sense that ‘climax’ has traditionally been used (Clements, 1936; Whittaker, 1953, 1957). Kovda (1933), referring to this open system, stated that all soils are subject to constant progressive change and constant self-development, independent of their stage of self-development and independent of external conditions. Rowe (1961a) objected strongly to the common concept of vegetation development proceeding to a pre-determined endpoint or ‘climax’.
A ‘post steady-state change’ can be thought of with reference to the vector field in n-dimensional space (Goodall, 1962; Lewontin, 1969; Whittaker, 1969) as simply a positional shift, or series of shifts, of the points in the vector field away from a relatively stable position. Odum (1959) considered that after a period of disequilibrium, another steady-state is attained. Bray (1958) and Whittaker (pers. comm. to Becking, 1968) implied that stages subsequent to a steady-state will be characteristed by a steady increase in entropy (energy disorder), decreased biomass, organisational structure and negentropy. Likewise, a state of imbalance will develop in the energy system, nutrient cycling and population dynamics.page 106
Sanders (1969) developed the Stability-Time hypothesis to explain patterns of diversity in marine benthic communities under increasing physical stress. It is applicable to a vegetation/soil system where environmental stress factors, operating with increased intensity or effect, give rise to post-steady-state situations.
‘Where physiological stresses have been historically low, biologically accommodated communities have evolved. As the gradient of physiological stress increases, resulting from increased physical fluctuations, or by unfavourable physical conditions regardless of fluctuations, the nature of the community gradually changes from a predominantly biologically accommodated, to a predominantly physically controlled community. Finally, when the stress conditions become greater than the adaptive abilities of the organisms, an abiotic condition is reached. The number of species diminish continuously along the gradient.’ (Sanders, 1969) Similarly, Wright (1959), discussing post steady-state changes in tropical vegetation and soils on old stable sites, considered that with increasing age, weathering and leaching reduce the nutrient supply within root range. Accordingly, ‘gross-feeding’ species gradually give way to ‘frugal-feeding’ species.
Rode (1949) stressed the importance of the effect of catastrophic changes such as sudden forest removal and other catastrophic phenomena of drought, rapid temperature changes, wind and fire. Rode also discussed post steady-state soil evolution as part of a gradual process, e.g., podzolisation. These gradual processes bring about changes in the composition and the structure of the vegetation, which in turn result in further changes in the composition and properties of the soil. These will occur so long as the entire process proceeds under unchanging conditions of the macro-climate, topography and ground water level. Rode defined ‘reversibility’ and ‘irreversibility’ in the evolution of these soils. Similar changes in the vegetation which Rode called irreversible can prevail under environmental conditions conducive to nutrient losses. According to Rode, ‘The inevitable loss of mineral substances is the real cause of the completely irreversible evolution of soils, and consequently of vegetation also in a definite direction. The existence of such a cause eradicates the possibility of the establishment under such conditions of a ‘climax’ as a system of dynamic equilibrium, i.e. endowed with a closed cycle in the migration of substances.’
Rode's ideas were based on extensive work on iron and humuspodzols.
Bryan and Teakle (1949) introduced the concept of ‘pedogenic inertia’ defined as the continuation of a soil-forming process, for example podzolisation, despite environmental changes inimical to it. In a study which they claimed supported Bryan and Teakle's concept, Walker and Adams (1959) and Walker (1965) analysed pedogenic sequences from basalt and greywacke in North Auckland. They page 107 demonstrated that once the stage of soil development is reached where on basalt, laterisation leaves only sesquioxides and on greywacke, podzolisation leaves only secondary silica, then it is virtually impossible to reverse the soil formation process unless erosion occurs.
This erosion then initiates a new cycle of soil formation. At these states of protracted laterisation or podzolisation, which Walker (1965) defined as senility, carbon is thought to be the last element to decline, following the restriction of photosynthesis in the vegetation/soil system, and causing decreasing litter returns. Walker (1965) and Stevens (1968) emphasised the essential role of phosphorus in an ecosystem and indicated its ultimate removal from the system. Phosphorus is one of the major elements in soil that must be supplied almost entirely by the parent material. Stevens (1968) demonstrated that very low nutrient levels, of phosphorus in particular, existed for approximately 10,000 years on podzolic soils formed from loess on outwash terraces on the West Coast, South Island. Jenny (1941) asked if there was, in fact, a degree of degradation beyond which reversion becomes impossible.
Clements (1916, 1936) used the term ‘post-climax’ as well as using numerous other prefixes to accommodate ‘climax’ vegetation that was divergent from the ‘climatic climax’ ideal. Whittaker (1953) termed the vegetation changes subsequent to steady-state as ‘retrogressive’. He defined a retrogressive change in vegetation as a change involving decrease in one or more characteristics of ‘mature’ or ‘climax’ communities such as ‘maximum diversity, productivity, soil maturity, stability’. Unlike much of Whittaker's (1953) review, this was a very static argument, almost in the tradition of Clements (1916, 1936). It treated the climax as an entity in itself rather than a temporary state in an open system of continual change. Whittaker distinguished between retrogression of a community and retrogression of a particular community parameter. He stated that the decision as to whether a given change is retrogressive may be necessarily subjective.
Godwin (in Whittaker, 1953) referred to post steady-state change as ‘deflected development’, in an analysis of the effect of severe grazing on the productivity of a community. The inference was that the direction of development under grazing is different from what it would be otherwise. Whittaker (1953) still considered this as retrogressive.
The term ‘deterioration’ was used by Dimbleby (1952, 1962) to refer to floristic and soil changes occurring under heath vegetation that was once forest. These changes are quite reversible according to Dimbleby who defines their reversibility as ‘regeneration’. The term ‘regeneration’ in relation to ‘deterioration’ is of questionable value unless some accretion of fresh soil material (e.g. Vucetich and Pullar, 1963) can be established. ‘Retrogression’, ‘reversible’ and ‘post-climax’ can be similarly criticised in that they imply a trend page 108 backwards from some a priori ‘pre-determined endpoint’ (Rowe, 1961a) in vegetation/soil system development.
In terms of the concept of ‘steady-state’, which has been qualified previously, the term ‘post steady-state’ is used to maintain consistency in usage and meaning. Similarly to steady-state, it is an alternative to a large number of loosely defined terms explaining the same situation. ‘Post steady-state’ change is therefore defined as any change within a period of disequilibrium following a temporary state of dynamic equilibrium in an open system.
Post Steady-State Changes Within The Forest/Podzol System
This review is restricted to research on forest vegetation and forest podzols in the cool super-humid environments of the montane-subalpine belt and the cool temperate-subarctic regions. The processes of development of forests and podzols have, in the main, been studied under the a priori assumptions of ‘climax’ and ‘maturity’ respectively.
In this review, the two groups of post steady-state conditions that were distinguished by Rode (1947) are discussed. They are:—
Those created by progressive developments within the forest/podzol system such as ‘overmaturing’ of even-aged dominant trees, iron pan formation, loss of nutrients, etc.
Those created by environmental stress factors, including wind, introduced browsing animals, clear-felling, thinning, fire and drought.
Most of the latter group are anthropogenic in origin and therefore have been mainly studied by silviculturists. The results of these studies are equally applicable to ‘natural’ post steady-state processes.
(a) Post steady-state change created by development within the forest/podzol system
The development of forest vegetation on podzol soil beyond a steady-state can be interpreted in terms of changes in the podzolisation process leading to gleying and waterlogging. As reported in the literature, these changes may occur for two reasons: on one hand, the formation of gleying and a mildly or completely impervious illuvial horizon, and on the other, the loss of nutrients. Both processes may occur together. In the latter, plants exacting in their nutrient requirements will be physiologically weakened (Russell, 1961), giving way to those that are less exacting (Wright, 1959). Crocker and Dickson (1952) in a study of a chronosequence on glacial moraines suggested that the ‘inevitable process of soil degradation’ following podzolisation would eventually allow species of lower nutrient requirement to dominate if a seed source was available. Among these species are usually many acidophilous shrubs (Ericaceae), grasses, herbs, sedges, page 109 rushes and sphagnum mosses; the latter often causing waterlogging (Rode, 1947; Zach, 1950). Rode refers to Tanfil'ev (1911) who thought that the primary cause of waterlogging was the incoming of hydrophitic species, such as sphagnum moss, conducive to soil water accumulation.
The Russian and Scandinavian literature contains many references to the theme that waterlogging of podzolised soils under forest is a ‘normal’ part of ecosystem development in the northern coniferous forests in the absence of external environmental factors. Sibertsev (1895) termed the successive changes from podzolised forest soils the ‘sod-podzolic process’. Sod-podzolic soils contain a well developed A1 horizon with abundant roots forming the ‘sod’. Muir (1961), reviewing podzol soils, referred to a process of sod-formation whereby, in the ‘opening up’ of a closed-canopy forest with podzolic soils, a grass-herb cover may become so dominant as to exclude regeneration of the woody vegetation. This theory of non-regeneration was formulated by Vil'yams (1940). Despite subsequent criticism of the theory, recent Russian pedology has recognised the sod-podzolic soils as a separate group. The main point of the postulated process is that in contrast to the thick mor of the forest podzol, the herb-grass litter undergoes a melanising mull type of decomposition leading to increased humus content and increased base-status of the mineral soil.
Tiurin (1933) recognised a developmental series of five stages in podzolisation: cryptopodzolic, weakly podzolic, medium podzolic, strongly podzolic and podzols. ‘Sod-podzolisation’, under grassland within a forest zone could occur at any of these stages. Kononova (1951) showed a distinct difference in the proportions of humic and fulvic acids in the podzolic and sod-podzolic soils despite Rode's (1944) conclusion that, chemically, there was no strict division between the two soils. Gorshenin (1961) used the prefix ‘turf-’ in the sense of ‘sod’, describing ‘turf-podzols’ as widespread in the upland, Southern Taiga of Southern Siberia. Podzols and turf-podzols were classified on a regional dynamic basis as separate soil types.
The conversion of forest podzol soils to ‘non-forest’ humus-gley-podzol soils has been described and identified in part by Keller (1927), Sukatchev (1928), Katz (1929), Rigg (1940), Muir and Fraser (1940), Wilde (1940, 1953), Zach (1950), Wright (1951) and Mackney (1961). The non-forest vegetation was described as bog, muskeg, heath, coniferous scrub and grassland on humus-, turf-, or gley-podzols. Katz considered that any bog vegetation which was developed from forest was in a state of ‘disequilibrium’. He placed emphasis on progressive endogenous development of vegetation. Wilde noted gleying, depletion of bases, and increased water-holding capacity under a thick raw forest litter, which adversely affected transpiration of trees, and lowered soil aeration to below critical levels for most tree species. Zach (1950) outlined regional forest ‘deterioration’ in South East Alaska as the result of encroachment page 110 of ‘muskeg’; an expanse of land occupied by bog development (Dansereau, 1957), on flat and gentle slopes. Surplus water, the result of low evapotranspiration in a low radiant energy environment inhibited decay of organic matter causing waterlogging. The cool, wet climate was mild enough, however, for vegetative growth.
In a study of swamping processes from upland coniferous forest, Pierce (1953) found that species composition and regeneration rate was largely determined by the content of electrolytes in water, represented by the supply of available nutrients. Pierce correlated the slow rates of tree growth with a deficiency of dissolved O2 and low redox potentials. Aeration and nutrient supply in the ground water was unfavourable to natural regeneration of most upland tree species. Any reproduction was of acidophilous shrub species. Pierce (1953) also studied the nature of Sphagnum in processes of swamping from forest. Sphagnum was found to concentrate nutrients and retard ground water movement. Rigg (1940) considered Sphagnum to be associated with the death of forest trees. Recent work by Raid (1965) on sod-podzolic soils under heath vegetation found close relationships between soil moisture, climatic factors and ground water levels. Satterlund (1961) found a very significant inverse relationship between forest growth and ground water in forest sites affected by swamping processes.
Lutz and Chandler (1947) discussed in detail the soil ‘deterioration’ which resulted from pure forest stands. They stated that the rate of podzolisation is greatest in pure stands of a species whose litter is low in bases. An example of this in New Zealand would be silver beech (Nothofagus menziesii) (Wright, 1951) or mountain beech (N. solandri var. cliffortioides) (J. Wardle pers. comm.). Lutz and Chandler noted that many misconceptions about this aspect of soil deterioration have been based on the long-term silviculture of pure spruce stands in Saxony.
Ludi (1923) described a situation where self-maintaining heath replaced forest. In this context, Whittaker (1953) considered that ‘seral’ forest communities could be replaced by ‘climax’ communities of lower growth form. This is how Cockayne (1928) and Robbins (1952) interpreted the change from rimu-rata/tawa forest to tawa forest.
Wright (1959) described vegetation/soil system ‘deterioration’ on old stable terraces in British Honduras, British Guiana and Brazil. At a certain stage the soils start to become really low in nutrients and the even forest canopy becomes ragged, with emergence of the crowns of those species best able to tolerate the ‘deteriorated’ soil conditions. The under-storey gradually degenerates and is successively dominated by broadleaved trees, then palms and eventually tall grasses. As the soils become poorer with iron pan or clay pan formation the scattered large trees are replaced by smaller trees of species tolerant of swamp or semi-swamp conditions. Throughout the page 111 ‘deterioration’ sequence there is a general development of an acid surface litter. Only when rainwater is held perched above the sub-soil pan layers, does a deep layer of organic residue accumulate. Some species bring about soil deterioration at a faster rate than others. The rate of the process is also accentuated in soils derived from quartz-rich materials. Wright emphasised the individual successional effect of various species, for example Podocarpus and Dacrydium spp., on podzolisation. Often in island vegetation there may be an insufficient range of species in the flora which are adapted to a range of soil development. For example, in Western Samoa, old, stable, very low nutrient soils are, under natural conditions, still occupied by ‘gross-feeding’ forest trees, much reduced in height and more spindly in form than normally. In the absence of low nutrient-demanding woody species windfalls are occupied by stable areas of grassland.
Post-podzol changes involving gleying
Until recently it was believed that each kind of soil was formed by one process (Rode 1947). Thus ‘podzolisation’ was a process that formed ‘podzols’. The podzolisation process has been defined as one that occurs under humid temperate forest environments in which the strong acidity is the result of the weak-base nutrient cycle and humic acids in the accumulating organic matter (Lavkulich, 1969). Under this system, sesquioxides, organic and clay colloids are translocated, and various theories have been put forward to explain why and how these substances are moved. The result was a zonal ‘podzol’ (Taylor and Pohlen, 1962).
Recently, gross-processes such as podzolisation have been largely discounted. It is now considered that the course of soil development is a function of the ‘relative rates’ of a large number of component reactions involving —
The continual accumulation of parent materials.
The continual differentiation of horizons within the profile.
As long as the relative rates of these processes do not change, a soil should continue to age along a given course (Jenny, 1941; Simonsen, 1967; Lavkulich, 1969) towards the condition of steady-state (Nikiforoff, 1959). When a change of relative rates of processes or reactions exceeds critical limits (Lavkulich, 1969), or is effected by environmental stress factors, a change in the course of development is initiated. Development of a compact argillic B horizon, or a rise in the water table, of a podzol can restrict permeability and/or induce reducing conditions and gleying with concurrent changes in base status, vegetation and rates of other processes.
Much of the literature on the effect of impeded drainage and ground-water comes from the research on hydrological sequences of podzols, gleyed podzols and their variants in the United Kingdom page 112 and elsewhere (Glentworth and Dion, 1949; Crompton, 1952; Crampton, 1963, 1965). This research was carried out in soils from freely drained parent materials under cool superhumid conditions with a considerable excess of rainfall over evapotranspiration. Gleying, defined by Mitchell et al. (1968) as simply the reduction of ferric oxide to the ferrous state under anaerobic conditions induced by waterlogging, is the dominant process involved. The nature of the gleying process throughout a typical sequence from a well-drained ‘iron podzol’ to a peat moss under a progressively rising water table was described by Russell (1961). Briefly, the iron podzol is converted first to an iron humus podzol, then to a humus podzol. During these stages, the A1 horizon thickens at the expense of the A0, usually due to the melanisation by organic matter from grasses and herbs replacing trees (Kononova, 1961). There is an increase in organic matter at the top of the B1 horizon accompanied by gleying below the B. With increased gleying and higher water tables, the A0 thickens, the A1 A2 and B thin, and the translocation of humus and sesquioxides becomes less, forming a peat podzol, then a gleyed-peat and finally a ‘peat moss’.
The mechanism of the gleying process was attributed by Bloomfield (1951, 1954, 1959) to plant degradation products and certain bacteria. These extract and mobilise iron compounds within the low pH range occurring in iron-pan podzols. These iron compounds produce the essential gley colour of greys rather than the ochreous mottlings which are a secondary feature (Bloomfield, 1962).
Crompton (1952) and Crampton (1965) considered that once surface reducing conditions were established, gleying developed rapidly and iron-pans could form within 100 years or less. This is much faster than the time required for iron-pan formation under ‘normal’ podzolising conditions (Franzmeier et al., 1963; Stevens, 1968).
The reducing environment from which humus iron-pans develop is formed at the soil surface by a saturated mottled layer of litter and roots (Crompton, 1956). Crampton (1963) considered that accumulation of ferrous material was related to areas of aeration at the capillary fringe between air-filled and water-filled pores and the direction of water movement; either from above, from below or laterally. Crompton (1952) suggested that gleying accelerated weathering. This was disputed by Mitchell et al. (1968) who considered that only certain compounds were translocated.
Gleys in shallow soils, overlying a bedrock surface or an iron pan, are usually very leached and are consequently particularly low in pH and clay content. This is particularly so in areas characterised by zonal gleying (Wright and Miller, 1952; Crompton, 1956; Gibbs, 1959; Crampton, 1963).
Gibbs (1959) described gleying as a dominant zonal process in soil development in the Tararua Range, where the rainfall is above page 113 100 inches and the percolation rate is insufficient to cope with the supply of water on slopes up to 30°. With decreasing rainfall or increasing evaporation, the zonal gleys grade into gleyed and podzolised yellow brown earths. Gibbs considered that topography and climate are the chief factors responsible for gleying. Gleying in lowland environments is intrazonal, being controlled by topographic ground water, whilst in upland areas climate is more important, forming zonal gleys.
Crompton (in Stevens, 1968) distinguished between surface-water and ground-water gleys. The former are caused by the relative impermeability of some part of the soil profile itself and the latter are associated with the saturation of the profile from below by regional or perched ground-water tables. Thus the peaty gleyed podzol (humus-gley podzol) is the result of surface-water gleying, beneath a saturated humus layer, while a gley podzol is attributed to ground-water gleying, related to a topographic water-table.
Post steady-state change created by environmental stress external to the forest/podozol system
Rode (1947) made reference to the ‘swamping process’ which appears to follow clear-felling in northern coniferous forests on podzolised soils. He considered that such swamping, or waterlogging, is temporary, with renewed forest growth producing a drying-out of the soils, and a return of the soil-forming processes to their ‘normal’ course. Sudden forest removal and other catastrophic phenomena such as drought, wind, rapid temperature change, Rode described as significant factors in the evolutionary processes of soil formation beyond any steady-state. Rowe (1961b) clarified the importance of environmental stress on forest vegetation considered to be ‘climax’ in a criticism of the work of a German phytosociologist, Plochman, on stand structure and species succession in the north-west coniferous forest of Alberta. Plochman considered that the ‘climax’ forest was cyclic and self-maintaining with continual developing, aging and renewing phases. Rowe's critique was based on long-term familiarity with the forests which, he said, are disturbance forests usually maintained in youth and health by frequent windfalls and fires to which all species with the exception of fir are well adapted. Where decadent forests do exist (Rowe, 1961b), they do not rejuvenate through any inevitable cycle of renewal and development, but rather tend to remain open and unhealthy with a dense shrub-component, awaiting the rejuvenating action of fire, flood and wind. A later statement of Rowe (1961b) suggested that the state is soil-fertility based.
As early as 1887, Muller outlined the effects of forest removal on podzol soils with a more humus. He found decomposition to be hastened and nitrification and biological activity to be favoured by the absence of a forest canopy. Keller (1927) and Gorshenin (1961) page 114 described regional ‘swamp’ formation in felled or burnt areas. Muir (1961) referred to a grass/herb phase in cutover-forest areas, and described the work of Tiurin (1936) who demonstrated increased fertility in forest soils, converted to agriculture, in both the A1 and A2 horizons and increased pH, depth and humus content of the A1 horizon. Tiurin also demonstrated a marked increase in soil moisture in the first 10-15 years after clear-felling and noted a subsequent process in which soil moisture levels fell. In similar situations, Wittich (1930) found increased pore volume in sandy soils. Bethlahmy (1962) studied the effects on soil moisture in the first year following clear-felling of forest, by comparison with adjacent unmilled sites. Sampling was at four soil depths, every month from late spring to early autumn. During the dynamic periods of soil water depletion and recharge the soil in the unmilled site required twice the quantity of water to attain field capacity as the clear-felled site. The duration of the dynamic periods in the unmilled forest site was likewise twice that of the clear-felled site. Wilde (1953) suggested that, in terms of a rise in the water-table leading to ‘water-logging’, clear-felling of forest had a greater effect on podzolised soils than on non-podzolised soils. Both McDonald (1955) and Lutz and Chandler (1947) stressed the need to avoid generalising on the equally untenable views that clear-felling of forest is beneficial or harmful.
Data of relevance to canopy opening in forest has emerged from forestry studies on plantation thinning. Watterston and Dyer (1964) demonstrated changes in the soil as a result of thinning spruce forests. Initial thinning stimulated increased stem diameter growth, but any thinning above 30% of the original basal area caused extreme fluctuations in ground water. The effects of thinning were compared with structural deterioration of the forest. Similar inferences can be drawn from the work of Kresl (1967) who formulated a relation expressing the effect of forest gaps resulting from thinning, on the amount of precipitation reaching the ground. Bray and Struik (1963) found from a study of glacial advance into Douglas Fir (Pseudotsuga menziesii) forest that if some trees were removed by glacial action, the remaining marginal trees responded by a very rapid increase in stem diameter growth. The effect of thinning on under-storey vegetation was studied by Pase and Hurd (1957) in stands of Ponderosa pine. They showed a significant curvilinear relationship between openings and under-storey productivity, of little increase in under-storey productivity as basal area decreased from 200-140 sq. ft. per acre but a rapid increase, particularly in grasses, below 140 sq. ft. per acre. Total under-storey herbs and crown cover showed a similar relationship, with an associated continuum of species replacement in the under-storey. Recent work by Anderson et al. (1969) evaluated the response of herbaceous plants to forest canopy opening in page 115 coniferous forest. Canopy opening was found to control the quantity of both radiant energy and precipitation reaching the litter layer. These both influenced soil moisture recharge at the soil surface.
The effect of periodic wind-damage in causing structural deterioration in forest has received little attention. Heinselman (1957) studied wind-caused mortality in black spruce (Picea mariana) forest on peaty, poorly aerated ‘swampland’ soils. Ground vegetation included Sphagnum spp. and ericaceous shrubs. Stands on shallow peat, less than 1ft. deep, were susceptible to windthrow, due to shallow root systems. Sixty-five per cent. of stand volume loss was attributed to wind. Heinselman considered that wind-caused mortality tended to increase with intensity of partial cutting with decreased residual basal area and with increased stand and site ages. The effect of wind on over-mature even-aged forest was discussed by Oshima et al. (1958) and Iwaki and Totsuka (1959) in an area of subalpine fir forest (Abies spp.) in Japan. The authors offered little explanation for the formation of strips of dead standing trees. They inferred that the process was obviously dynamic, but not necessarily due to stress in the soil component of the system. There was a progressive movement of the dead tree strips upslope and rapid regeneration of fir forest within them. The most likely explanation was wind destruction of the canopy of the over-mature even-aged forest at one point, followed by rapid enlargement of the canopy gap to a ‘strip’ of dead trees. There was close correlation between direction of prevailing wind, slope direction and occurrence of dead strips. Wind turbulence and structure in forests was studied by Geiger (1955), Gloyne (1968) and Bull and Reynolds (1968). The former two described the rapid decrease in most forests of mean wind speed, below the canopy/atmosphere interface.
The effects of warmer and drier conditions since late last century on Nothofagus forests in South Chile and Argentina were summarised by Auer (1966). The response to drought has been debility and absence of tree regeneration at the forest-steppe margin, permanence of large non-forest areas and changes in forest composition. Sphagnum appears in drying bogs, but quickly disappears under the influence of drought and wind. Auer termed this ‘retrogressive’.
As discussed above, the usual post-podzol steady-state changes involve a vegetational change from forest to heathland or grassland. However, Dimbleby (1952, 1962) discussed quite contrary changes in the vegetation/soil system resulting from anthropogenic forest removal 2,500 years ago on the Yorkshire moors. Heathland podzols with thin iron pans are, in cases, re-afforested by birch (Betula pubescens) stands. As the birch stands develop, the raw humus of the heath vegetation becomes activated and eventually destroyed. A deep A1 horizon develops in the mineral soil and the iron-pan becomes partially destroyed. At the soil surface, in particular, there is an increase in pH. The whole process, which Dimbleby called page 116 ‘soil regeneration’, takes about 60-100 years. From pollen data Dimbleby related the presence of the heathland podzol systm to Bronze Age anthropogenic influences. He considered that the soil regeneration is re-establishing the brown forest soil that 2,500 years ago covered the Yorkshire moors, under birch forest. The birch forest is very favourable to grasses and herbs, which are normally absent from heathland or coniferous forest. The soil changes which Dimbleby ascribed to birch forest establishment appear to be very similar to the changes resulting from ‘sod-podzolisation’ (see Muir, 1961). Kononova (1951) considered that differences in organic chemical constituents between strongly podzolic-forest soils and sod-podzolic post-forest soils, were attributable to the presence or absence of grass ground vegetation.
There is some support for Dimbleby's thesis in the work of Griffith et al. (1930) who traced a sequence of soil ‘degeneration’ and ‘regeneration’ on abandoned pastures in New England. The colonising species, white pine (Pinus strobus) was associated with raw humus formation and podzolisation. After 80 years there was a mixed hardwood forest on the site that ‘re-developed the brown forest soil’. Fisher (1928) considered that a mull profile could develop from such podzols in only 15-20 years. Research in Sweden, Switzerland and U.S.S.R. has shown similar results (Dimbleby, 1952).
Mark (1958) made an extensive review and analysis of processes of ‘bald’ formation in the Appalachian Mountains. He defined a ‘bald’ as an area of naturally occuring treeless vegetation located on a well-drained site below the climatic tree line in a predominantly forested region. Two types of bald can be distinguished: the ericaceous heath-balds and the herbaceous grass-balds. Mark stated that most of the numerous theories explaining the origin or maintenance of balds were not acceptable because of failure to distinguish between the separate problems of bald origin, maintenance and extension. Mark considered that cooling climate subsequent to the Climatic Optimum 4,000-5,000 years B.P., and extending to the present day, appears to have caused the formation of a bald-susceptible zone along the ecotone between hardwood and spruce-fir forests and at the potential ecotone on the mountains from which spruce-fir forest had been eliminated. At the Climatic Optimum (5,000 B.P.) spruce-fir forest was restricted to an altitude 300 to 1,000 feet higher than its present lower limit; thus causing the elimination of this forest from many peaks. In the bald-susceptible zone, seedling re-establishment of forest species is very slow or nil, giving rise to an herbaceous or heath vegetation derived from the regional flora and later by adventive species. Maintenance and extension of the balds depends on relative severity of the environment, lack of a spruce and fir seed source, possible elimination of spruce and fir biotypes best adapted to invasion into warmer and drier lower altitude areas, and recent grazing and browsing pressure. The only obvious differences in soil profile page 117 between bald and adjacent forest sites were the relatively thick litters of the forest profiles, greater soil depth under the balds, and slightly darker coloured A1 horizons in the balds. Similar comparisons were made by Mark (1958) for soil moisture tension in the upper soil during one summer. The forest sites showed uniformly very low moisture tensions compared with the fluctuating values at the bald sites. Only at the bald sites did soil moisture tension exceed 1 bar.
Post steady-state changes in the forest/podzol system in New Zealand
In New Zealand, the only areas in cool superhumid climates where progressive post steady-state charges within the forest/podzol system occur are throughout the Westland-Fiordland district and elsewhere in a few sites near timber-line or on flat sites in high rainfall areas. In Fiordland, Wright and Miller (1952) stated that under a rainfall of 150 inches or more per annum, the normal processes of soil development produce podzols, gley podzols or zonal peat. They found evidence of accelerated leaching under species, for example silver beech (Nothofagus menziesii), rimu (Dacrydium cupressinum) and kamahi (Weinmannia racemosa) that in other parts of New Zealand maintain more of a nutrient balance in which the soil does not deteriorate markedly.
Gibbs et al. (1953) divided the high terrace soils of the West Coast, in which podzolisation was advanced, into two groups.
Podzolic soils — semi-mature to sub-mature stages.
Podzol soils — mature stage to gley podzol.
A separate classification by Cutler (1960) considered these terrace soils in two main categories: —
The podzolised yellow-brown earths of the well-drained rolling and hilly land.
The gley-podzols of the poorly drained lower-lying land.
The processes of vegetation/soil system development are dominated by topography and water throughout, as is clear from the summarised sequential classification of Cutler which in terms of vegetation/soil system terminology can be interpreted as follows:—
Rimu/rata/kamahi forest//podzolised yellow brown earths on rolling to hilly slopes.
Rimu/kamahi forest//gleyed, podzolised yellow brown earths on undulating and concave slopes.
Rimu forest//gley podzols on higher flats.
Rimu/silver pine forest and manuka scrub//peaty gley podzols, on lower flats.
Open sphagnum and rushland//peats and gley on low-lying depressions.
These soils developed on terraces formed of 24 to 30 inches of loess over gravels. The gley podzols, peaty gley podzols and peats page 118 are typical of ‘pakihi’ land which is very extensive on flat terraces in Westland. The term ‘pakihi’ strictly means an open clearing in forest and is applied to vegetation of low ericaceous, epacridaceous and manuka shrubs, umbrella fern (Gleichenia circinata), grasses, herbs, sedges, rushes and sphagnum moss (Rigg, 1951). Rigg considered that pakihi formation is similar to the development of heath, muskeg, bog or swamp vegetation from coniferous forest. Apart from the general acidophilous nature of such vegetation only the sphagnum/peat system of some areas of pakihi is comparable with overseas situations.
In the past, pakihi comprised innumerable small swampy non-forest clearings with many gradients of structural and compositional change to adjacent forest (P. Wardle, pers. comm.). Most present-day pakihi, however, is extensive moorland created by man's destruction of rimu-silver pine forest and rimu-kamahi forest (Vucetich, 1960; Chavasse, 1962) under which the gley-podzols developed. In Westland National Park, the vegetation of ‘natural’ pakihi at about 1,000 ft. it characterised by a very poor flora with both its exclusive and subalpine components (Wardle, 1964, pers. comm.). Similar features characterise low-lying pakihi areas in the Urewera Range, at 2,000 ft. (pers. obs.). A paleo-climatic view of pakihi formation was that of Holloway (1954) who postulated that the open ‘natural’ pakihis were more extensive during a warm wet period about 1200-1300 A.D. He suggested that the pakihis were subsequently invaded by silver pine (Dacrydium colensoi), then by rimu, whilst progressively ‘drying out’. In terms of the amptitude of climatic change envisaged by Lamb (1965) the precipitation differences influencing soil water levels were not as great in the last 1,000 years as Holloway implied. Holloway claimed that the present climate is unfavourable for rimu regeneration on higher land. Cutler (1960) stated that sites of current and active rimu regeneration (gley podzols) and non-regeneration (gleyed and podzolised yellow brown earths and peats) are climatically too similar to justify the assumption of climatic change. However, the same sites are pedologically very different.
Wright (1951) demonstrated a high negative relationship between deteriorating soil conditions in gley podzols and failure of beech regeneration, in the silver beech and mountain beech forests of south-west Fiordland. Wright inferred a sequence of forest ‘deterioration’ on terraces in soils from loess-like material, in which the water table is rarely more than 12 in. from the surface. The sequence was, briefly, from silver beech forest, which despite an over-all youthful age, contained a progressively higher proportion of dead and dying trees, to Dacrydium spp. bog forest, followed by progressive burial of the soil profile by sphagnum peat.
Very little is known of the inter-relationships of podzols, gley podzols, peaty gley podzols and peats and forest growth. Cutler (1960) has described litter mineralisation as at a minimum in drier page 119 podzol sites and at a maximum in the gley podzol-peaty gley podzol sites. Incorporation of humus within the mineral soil decreases from the drier to the wetter sites (cf. Kononova, 1951). As Cutler said, there is a need for careful chemical and physical analysis of such a soil sequence.
Stevens (1968) studied a chronosequence of moraine and terrace sites aged from 22,000 B.P. to the present in South Westland. He considered that well differentiated gley podzols formed from virgin parent material in 5,000 years. Soils were first podzolised, then gleyed; the course of soil development being ‘directly and completely correlated with the advent and growth of various vegetation types’. Each soil component appeared to attain equilibrium at different rates and within different periods of time. Organic carbon, nitrogen and cation exchange capacity increased rapidly for 12,000 years then slowly declined. There were also losses in exchangeable Ca and Mg, organic P and non-occluded inorganic P after this time. Stevens interpreted these changes as a trend towards ‘ultimate soil degradation after 12,000 years’. Twelve thousand years was considered the appropriate date of final physical communition of stones and gravel whereupon no further materials from fresh rock could enter the system. (See also Walker and Adams, 1959; Walker, 1965.)
A study of soil physical properties of gley podzols and peaty gley podzols was made by McDonald (1955) who attempted to establish whether or not water-logging due to observed raised water tables was consequent on forest removal. He found no statistically significant differences between clear-felled and unmilled sites for moisture contents (on either weight or volume basis) or any other soil physical property. The wetter conditions on cleared land he attributed to surface water on the water-logged soil, whereas inside the forest, the soil was covered with a thick spongy litter. McDonald considered that 10-20 years after clear-felling was sufficient time to produce a change in soil moisture. There was no evidence to show that non-regeneration of forest trees in clear-felled sites was due to adverse changes in soil moisture content or other physical properties.
Although lacking statistical significance of differences, McDonald's results certainly showed the expected nature of a difference between the clear-felled and forest sites. His data is of minimal use in that it tells nothing of the processes of change, such as at what stage did any change occur to the steady-state forest soil. McDonald only replicate-sampled one forest site and one non-forest site at four locations, and with only one non-saturation sampling period had little control over the essential field nature of the soil physical properties. (Cf. Stewart and Adams, 1969.) Understanding a soil process requires not only a large number of replicate samples from one site but also a large range of different sampling sites representing a sequence of change. Ecologically, it is the nature and trend of a process that is page 120 important, rather than a statistically significant difference between two points separated in space and/or time.
On Chatham Island the usual vegetation types associated with development of non-forest from forest — podocarp forest, manuka scrub and tussock-grassland — are absent. Their various places in the developmental sequence have been taken over by Dracophyllum forest. Wright (1959) described soil formation on stabilised dunes under Dracophyllum forest in a cool, damp and windy climate. The Dracophyllum litter is particularly slow to break down and deep layers of forest peat build up, adding greatly to the rate of soil leaching. Podzol characteristics have developed very rapidly in the permeable sand. On gentle slopes the iron-pans inevitably cause bog conditions to develop.
In a study of post steady-state changes Park (1971) demonstrated close gradient relationships between the vegetation and soils in a forest/podzol system in the Tararua Range. A near-threshold steady-state consists of depauperate vegetation and soils of extremely weak base-status undergoing gleying and reduced aeration.
Changes from the steady-state silver beech forest/gley podzol form gradients of decreasing stature of the vegetation, increasing numbers of non-forest species and wetter, less aerated and more organic soils. The progression of post steady-state changes is through silver beech woodland//humic gley podzol, mixed shrub/fernland//humic gley podzol and sedge mossland//humic peaty gley podzol. The gradients of change are not simply linear because environmental factors, mainly topographic, complicate the relationship.
Numerous references have been made to the effects of widespread environmental stresses on forest vegetation and podzol soils in New Zealand (cf. Molloy, 1969). Holloway (1954) stated that much of the South Island forest vegetation was climatically ‘maladjusted’. In general, podocarp forest, mainly situated on warm, moist sites, was thought to be not regenerating, being replaced by beech forests, which were considered to require colder and drier sites. Holloway's hypothesis was supported by Wardle (1963, 1964) who related a ‘regeneration gap’ in podocarp species from ecologically discrete environments to widespread climatic change during the last 700 years. Holloway (1954) also considered that, from pedological evidence, fire was responsible for the ‘final decay and breaking up’ of the forest/podzol system in a climatically induced period of non-re-establishment. Following the fires, the sites were occupied by tussock grassland vegetation and soils. Earlier, from pedological evidence, Raeside (1948) had postulated a climatic change affecting a change from forest to grassland vegetation. Nicholls (1957) also attributed lack of podocarp regeneration in upland Taranaki to effects of climatic change. He disputed Cockayne's (1927) hypothesis that lack of regeneration in rimu forest was part of a linear succession to tawa forest.page 121
Cumberland (1962) attached many misunderstandings to the Holloway-Raeside hypothesis. He argued that a climatic change, if it existed at all, either was not great enough to affect forest vegetation or it came after fires had transformed the forest to grassland. Molloy et al. (1963) on sub-fossil evidence confirmed the widespread removal of forest vegetation by fire in the eastern South Island. Molloy (1964) considered that ‘profile degradation’ of forest podzol soils in upland Canterbury dated from climatic deterioration since the post-glacial optimum between 3,000 and 5,000 years B.P. This degradation was first accentuated by widespread Polynesian fires and then by early burning and grazing under European influence.
Although it is not difficult to demonstrate changes with time in the vegetation/soil system it is difficult to demonstrate the existence of a state of stability. Once this state is recognised the problem then is to define it in terms of the open system concept whereby vegetation and soils are subject to constant progressive change and constant self development. In such a system stability must be relative and temporary and will only exist in parts of the system at any one time as vegetation and soils are polythetic systems in which any parameter is part of a continuum.
The chronosequence approach to vegetation and soil analysis (Stevens and Walker, 1971) relies to a large extent on accurate time-control and consequently is restricted to successional studies tracing vegetation and soil development on newly-formed geomorphic surfaces. Most authors are in agreement that in succession, parameters gradually taper to a state of minimal change over a long period of time. Description of this state of comparative stability has varied between ‘terminal’ and ‘non-existent’; it is probably best defined using the term steady-state meaning a temporary state of dynamic equilibrium in an open system.
In studies of forest ecosystems the steady-state* is invariably assumed to be the forest that is maintaining both healthy structure and constant composition overlying a soil with which it has an adequate nutrient balance and from which advanced gleying, swamping and loss of essential elements are absent. On this basis any development beyond the forest ecosystem can be termed post steady-state.
Although there are many references to post steady-state development of this type there is a lack of studies of soil and/or vegetation sequences beginning with a steady-state system and measuring gradients of change. In cool, superhumid environments the change from a forest//podzol system to heathland and herbaceous vegetation over gley podzols and peats has been consistently demonstrated, page 122 particularly in the northern latitudes of the coniferous zone where the conversion of forests to ‘muskeg’ is a natural regional process.
In general, there are close relationships between deteriorating forest structure, increasing species diversity and increasingly wetter, less aerated and more organic soils. The understanding of the processes involved requires the sampling of many ecosystem parameters from a wide range of sites of different ages; from a newly formed geomorphic surface to sites where heathland and herbaceous vegetation have replaced forest. It is the lot of the student of vegetation and soil dynamics that such an ideal is so unique that it is probably non-existant.
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* In preference to the untenable concept of ‘climax’.