Resource gradients

Erosion by running water is one of the dominant features shaping the surface of the Earth. As water moves from highlands to lowlands, it dissolves minerals from rocks and erodes soil. In some world drain-age basins, the mass of eroded sediments exceeds 1 tonne per km² per year (Milliman and Meade 1983). The yield of sediment varies with annual precipitation and land use (Judson 1968). As a conse-quence of steady erosion and deposition, depressions in the land-scape are repositories of accumulated organic matter and mineral nutrients. Deposition rates in the order of 20 to 3000 cm per 1000 years are suggested by palynological studies of English landscapes, with some of this being peat produced in situ (Walker1970). Rozan et al. (1994) suggest that rates of deposition in a floodplain in eastern North America were below 10 cm per 1000 years prior to this century but increased to nearly 1 m per 1000 years as humans modified the landscape. This is close to the range of deposition rates reported for coastal deltas (Boesch et al.1994).

Erosion and deposition produce gradients at a wide range of scales. In the Great Smoky Mountains, for example, rich deciduous forests develop in fertile valleys called coves, while open oak and pine forests establish on dry ridges (Whittaker 1956). Sediments from uplands are deposited in valleys to form the rich soils associated with alluvial wet prairies, marshland, and swamps (e.g., Sioli1964, Davies and Walker1986). In peatlands, strong fertility gradients are associated with the degree to which running water, as opposed to rainwater, maintains the water table, and this gradient produces striking changes in the species found. The peatlands fed by ground-water (fens) have higher pH and nutrient concentrations than those fed by rainwater (raised bogs) (Gorham1953, Glaser et al.1990, Glaser 1992). At even more local scales, depressions in old fields contain soil with more water and nutrients than the ridges between them (Reader and Best1989). Gradients of soil fertility typify a wide range of habitats from montane forests to wetlands to old fields. To focus upon small resource patches, or to assume environmental homogeneity, denies one of the most natural aspects of landscapes. Therefore, more atten-tion needs to be placed upon gradients and the way in which resources and organisms become distributed along them.

Figure 3:15 Spatial

heterogeneity of soil nitrogen in a 12 12 m plot of sandy soil in a Minnesota prairie (after Tilman1982).

The following illustrates the wisdom on resource gradients avail-able in a traditional forestry source such as Wilde’s (1958) treatise on forest soils:

The modifying influence of the local physiographic conditions deserves particular attention in classification of forested soils in rolling or hilly topography. Such soils usually provide striking illustrations of the effects of the three major edaphic factors: water, aeration, and nutrients [Figure3.16].

The top and upper slopes of a ridge or a mound represent elements of so-called positive topography. Soils overlying this portion of the relief receive the least rain water and are subject to denudation, which decreases the supply of available nutrients. Consequently, such soils often support forest stands of low rate of growth. On the other hand, the negative topography of lower slopes receives run-off water and fertiliz-ing products of erosion – mineral colloids, humus, and soluble salts. As the result of this enrichment of the soil, forest stands increase their height growth, or, as foresters say, ‘‘Trees level the relief with their crowns.’’ With the descent to a depression the water accumulates in excess, and the growth of trees drops abruptly in accordance with the rapidly decreasing soil aeration.

(p. 248)

In heavily developed landscapes, where pastures have replaced forests, similar resource gradients can be found. Again, erosion from upper slopes produces a soil resource gradient (Figure 3.17).

Superimposed upon this, however, are grazing pressures that are most intense in the valleys. At a finer scale, local sites of erosion (eroded pasture) produce adjacent sites with deposition (very oligo-trophic pasture). These local gradients are also superimposed upon the longer gradient running down the elevation gradient.

Overlaid upon such erosional patterns are changes in soil organic matter and types of mycorrhizal associations. Figure 3.18 shows some of the major types of mycorrhizae, and Figure 3.19 sum-marizes their distributions along elevation gradients. Regrettably,

Figure 3:16 Effects of topography on moisture (W), aeration (A), and nutrient content (N) of soil and the resulting growth of trees. Line X1–X2 delineates positive topography, subject to denudation, and negative topography, subject to deposition of eroded materials (from Wilde 1958).

the wetland habitats in valley bottoms are not included in this figure, but evidence to date suggests that flooding reduces both the abun-dance of soil fungi (Kalamees1982) and the occurrence of mycorrhi-zae (Rickerl et al.1994).

Figure 3:17 Typical sequence of grassland communities on a slope.

Community names are vernacular and based on nutrient status and grazing intensity. The main sequence runs from ‘‘Meadow’’ in the bottom of valleys to ‘‘Eroded pasture’’ higher up the slope. The distribution patterns of the types

‘‘Strong grazing,’’ ‘‘Moderate erosion,’’ and ‘‘Strong erosion’’ are superimposed on the main sequence, or occur occasionally;

here arrows indicate the intervals where they usually appear (from Puerto et al.1990).

ARB USCULAR

MYCORRHIZA ECT OMYCORRHIZA

MYCORRHIZAIN ORCHIDS MS

SP EH HN

A

V IHC IHN

MYCORRHIZA IN ERICALES

ar butoid type

Basidiomycetes er icoid type

Ascomycetes

monotropoid type

Basidiomycetes Basidiomycetes

Va sc ular plant s

and Br y oph yta

Zygomycetes

Gymno sper mae

Angio sper mae

Basidiomycetes Ascomycetes Zygomycetes

Figure 3:18 A schematic overview of the different forms of mycorrhizae and their symbiotic partners. MS, Mycelial strands;

EH, external hyphal mantle; HN, Hartig net; IHN, intercellular hyphal net; IHC, intracellular hyphal complexes; V, fungal vesicle;

A, arbuscule; SP, spore (from Larcher2003).

The following resource gradient data come from a slightly spe-cialized example of the foregoing general pattern. Even in wetlands, which may seem to have relatively uniform topography, one can find similar gradients. They stretch from infertile sandy shores, from which waves are eroding nutrients, to sheltered fertile bays, where silt, organic matter, and nutrients accumulate (Figure6.26). At this large scale (illustrated by, but not restricted to, wetlands), one can reasonably discuss resource gradients, because all major resources (N, P, K, Mg) are distributed along a common gradient. They are in low availability on sandy or eroding shorelines; they are in high availability wherever sediments accumulate. In other cases, vast sand plains create extensive areas of wet but infertile conditions (Figure3.20).

What happens if one examines resource patterns progressing from a large (landscape sized) scale to a small (quadrat sized) scale?

Table3.7provides the opportunity to move down this scale, from the top matrix (marshes in eastern North America) to the bottom matrix (a single sedge meadow). In the top matrix, the soil samples are from wetlands ranging from the highly fertile (e.g., Typha marshes and floodplains) to highly infertile sand or gravel shorelines where insectivorous genera such as Drosera and Utricularia are common.

As well, both organic matter and silt and clay content of the soil were positively correlated with nitrogen and phosphorus levels.

Similar patterns occur within a single wetland (Table 3.7(b)). At the lower-most scale (Table3.7(d)), still larger than the small piece

Figure 3:19 Diagrammatic presentation of the postulated relationship between latitude or altitude, climate, soil and mycorrhizal type, and development of the vegetative mycelium associated with mycorrhizae (from Lewis1987).

of prairie shown in Figure 3.15, many of the correlations among resources have become negligible.

As it shall become evident, assumptions about the appropriate scale of enquiry have a big impact upon one’s view of competition.

The assumption of nearly homogeneous environments and trade-offs among resources (e.g., Tilman 1982) may be quite appropriate for a patch such as Figure3.15or perhaps a single sedge meadow, whereas the assumption of resource gradients and trade-offs between stress tolerance and resource competition (e.g., Keddy 1989) seem more appropriate to larger scales of enquiry. Further, these different scales treat resources in entirely different ways. At the small scale, resource availability is seen to be under biological control, with plants creating

( a)

(b) Figure 3:20 Infertile habitats.

(a) Wave washed shorelines like this one in Axe Lake, Ontario, are similar to hill tops in that they are habitats from which nutrients are eroded. The fine particles with attached nutrients are deposited elsewhere, thereby creating the strong resource gradients typical of terrestrial and wetland vegetation (Keddy1981b); (b) sand plains in the southeastern United States have been leached by winter rainfall for millions of years and support vegetation such as this pitcher plant bog (from Peet and Allard1993).

depletion zones around their roots. At the large scale, resource avail-ability is viewed as a consequence of erosion and topography, with plants interacting with one another for access to different sections of this resource gradient. Perhaps greater attention to the distribution of resources in nature would allow one to better decide which scale of investigation is appropriate to a particular set of circumstances.

Table 3.7. Resource gradients in wetlands from the large scale (top) to the small scale (bottom). Note that patterns (as indicated by the size of correlation coefficients) fade as the scale becomes smaller (after Keddy2001).

(a) Marshes in north eastern North America (Gaudet1993, her Table1.2)

% Organic P N K Mg pH

Standing crop 0.77 0.76 0.66 0.58 0.67 0.28

% Organic 0.77 0.57 0.50 0.51 0.47

P 0.72 0.56 0.66 0.13

N 0.53 0.63 0.02

K 0.70 0.28

Mg 0.14

(b) One wetland complex in the Ottawa River watershed (Gaudet1993, her Table1.4)

% Organic P N K Mg pH

Standing crop 0.74 0.80 0.69 0.76 0.69 0.45

% Organic 0.80 0.61 0.66 0.62 0.61

P 0.62 0.82 0.59 0.46

N 0.68 0.53 0.18

K 0.64 0.35

Mg 0.72

(c) One vegetation zone of the St. Lawrence River (Auclair et al.1976a, their Table1)

% Organic P N K Mg pH

Standing crop 0.34 0.29 0.38 0.49 0.17 0.21

% Organic 0.27 0.37 0.75 0.59 0.18

P 0.01 0.48 0.33 0.55

N 0.39 0.32 0.14

K 0.43 0.38

Mg 0.12

(d) Carex meadow, St. Lawrence River (Auclair et al.1976b, their Table1)

% Organic P N K Mg pH

Standing crop 0.13 0.02 0.02 0.22 0.23 0.11

% Organic 0.39 0.30 0.52 0.17 0.14

P 0.26 0.18 0.21 0.03

N 0.24 0.26 0.04

K 0.16 0.01

Mg 0.52