Cascading Effects of Deforestation on Ecosystem Services Across Soils and Freshwater and Marine Sediments

Phillip Ineson, Lisa A. Levin, Ronald T. Kneib, Robert O. Hall, Jr., Jan Marcin Weslawski, Richard D. Bardgett, David A. Wardle, Diana H. Wall, Wim H. van der Putten, and Holley Zadeh

The full assessment of the impacts of ecosystem management and disturbances on the provision of ecosystem services would be a comparatively simple process if ecosystems were totally self-contained and independent. Simple monitoring, observation, and assessment within a system would inform us of the implications of our management activities, with the benefits and costs being simply quantified and traded. In that simple case, increases in yield could be weighed against biodiversity loss, the ability of an ecosystem to buffer against flooding could be traded for loss of its amenity value, and so on. However, this simplistic approach is clearly unrealistic, because "secondary" effects to other systems are an almost inevitable consequence of any adopted management policy or disturbance.

One simple and seemingly objective means of assessing the full economic and cultural value of ecosystems to humans is to consider the provision of ecosystem services (Daily et al. 2000). The ecosystem services of terrestrial, freshwater, and marine soils and sediments have been developed thematically in previous chapters of this volume. With our increasing appreciation of all the previously unquantified benefits that ecosystems have for humans, there is also an increasing requirement for techniques to formalize the impacts of human activities on these services. As outlined in previous SCOPE reports (Wall Freckman et al. 1997; Wall et al. 2001a; Wall et al. 2001b) there is a need to be able to extend this approach to interconnections between ecosystems, to quantify how

Deforestation Effects Pictures
Figure 9.1. Overview of the potential impacts of deforestation on ecosystem services provided by cascading domains, showing the linkages between domains

altering the management of, or inflicting a major disturbance on, one system can have "downstream" effects on the provision of ecosystem services in other interconnected, but spatially separated, ecosystems and domains.

We have chosen to focus here on deforestation as an example of one such potentially interconnected series of cross domain impacts, because this issue has already been investigated from the perspective of each of the three domains (terrestrial, freshwater, marine) but these have never been formally linked together. Figure 4.1 in Chapter 4 shows the overall nature of these cascading links as a result of deforestation, while Figure 9.1 summarizes the approach used in the current evaluation; the objective is to strengthen our understanding of the importance of the links between the domains rather than simply emphasize the within-domain impacts.

However, such an analysis could be almost infinite if one considered all types of forests and all implications for terrestrial and aquatic species, their interactions, biodiversity, trophic food chains, and so on. Therefore, we have limited the scope of the discussion to deforestation of temperate systems and to those aspects directly linked to provision of ecosystem goods and services (Figure 9.1). As ecologists specifically interested in the biodiversity of soils and sediments, we have also focused on the role of biodiversity in ecosystem function, considering how changes in these biological parameters directly affect humans through the provision of ecosystem services. In providing this specific example, we utilize a structured and logical template for assessing the cascading effects of management on one ecosystem through other, spatially separated, domains.

Forests and Forest Soils: Deforestation Impacts

Despite the very diverse nature of forest soils, some gross generalizations about the structure and biological activity can be traced back to the original distinction between mull and mor made by Muller in 1887. The careful examination of soil humus forms, initially advocated by Muller (1887) and further developed by such researchers as Kubi-ena (1953) and Topoliantz et al. (2000), has reconfirmed the crucial interaction between soil parent material, faunal activity, and plant litter quality in determining the development of soil organic matter and in the value of the concepts of mull and mor humus.

Soils with mull humus typically form on calcium-rich, moderately or well-drained clayey parent material, generally beneath grassland and deciduous forest. The mull humus form is often mildly acidic, due to leaching of base cations (e.g., calcium) down the soil profile, and is characterized by intimate mixing of the surface organic and upper mineral A horizon. This mixing results from a high abundance and activity of soil biota, especially earthworms, and also leads to enhanced rates of decomposition, nutrient availability, and plant growth.

In contrast, in coniferous forests and other systems that produce poorer litter quality, there is an obvious build-up of organic matter at the soil surface: the so-called mor humus. These mor humus forms are typically acidic in nature and are characterized by low rates of decomposition and plant nutrient availability, differing both biologically and chemically from the mull form. Biologically, it is typical for the microbial biomass of these mor soils to be strongly dominated by fungi (rather than bacteria), and the fauna are characterized by high numbers of microarthropods (mites and Collembola), but with an absence of earthworms. Chemically, these humus forms are quite different: mull has a lower fulvic acid fraction, with a higher concentration of calcium and higher base exchange capacity in the humic fraction. Mor soils, by contrast, are characterized by less completely degraded organic matter, with an abundance of lignin and hemi-cellulose, resulting in a more fibrous texture. Both base cation content and exchange capacities tend to be much lower in mor than in mull soils.

Although providing a generalized starting point for describing humus forms in forest soils, it must be appreciated that many inter-grades and variations from these basic forms exist. For example, Kubiena (1953) extended these divisions by describing a series of important common transition forms, particularly that of "moder" humus, which superficially resembles a mor in having a sharp distinction between organic and

MOR —► MODER —► MULL

Ratio of bacteriaffungal importa noe increasing

Decreasing impoftanoe of organic N aid P transfers

Ectomyconrhizae dominate -*■ Eclo- and arbuscular mycoirhizae

Increasing orga no-mineral complexes

Increasing bioturbalion

Decreasing importance

a) mites, GoUembola a) insect larvae a) earthworms, termites b) enchytnaeids b) miles, Collernbola b) myriapods. insect larvae c) insect larvae c) enchytraeids c) isopods d) myriapods d) myriapods d) miles. Coltembola e) isopods e) earthworms, isopods

Figure 9.2. The relationship between humus types and fauna in forest ecosystems (adapted from Wallwork 1970).

Figure 9.2. The relationship between humus types and fauna in forest ecosystems (adapted from Wallwork 1970).

mineral horizons, but contains large numbers of insect larvae and some restricted species of acid-tolerant earthworms. Thus, the differing humus forms in forest soils are a reflection of biological activity. Some of these differences are summarized in Figure 9.2, which has been modified from Wallwork (1970).

Forests have a number of key and unique features that determine the development of soils and associated biota. First, forests are among the most productive terrestrial ecosystems in the world, with both high rates of annual net primary productivity and the greatest standing biomasses; global terrestrial net primary production is estimated at around 60 PgC a-1, of which 30 percent is accounted for by forests (see IPCC 2001; Saugier et al. 2001; Gower 2002). The input of organic matter into the soil occurs both as root input and aboveground litter fall, and the fate of these different litter materials is influenced both by their physical placement into the soil and by their intrinsic differences in quality. In particular, the input of woody remains to the forest floor is important both as a substrate and habitat for decomposers, and it has been argued that the input of plant material from "k strategy" plants may be reflected in the "r or k strategy" of the soil population (Heal & Ineson 1984). It has been also argued that an increase in the abundance of litters rich in lignin and more recalcitrant molecules requires a more complex decomposer community.

Forest soils provide an important habitat for a wide range of soil fauna, but forest carbon turnover is actually dominated by microbial activity, with carbon accumulation on the forest floor being the resultant between net primary productivity (above- and below-ground) and decomposition. The microbial community of the upper soil layers in forests is dominated by fungal-based rather than by bacterial-based food webs, which are highly capable of degrading the more recalcitrant components of the incoming litter and are less sensitive to the acidic conditions that frequently develop in the organic rich upper soil horizons (see Chapters 2 and 5, this volume). Fungi tend to be more resistant to drought, and they are well adapted to the changing microclimate in the litter layer through the production of both asexual and sexual sporing structures. However, the sensitivity of my-corrhizal fungi colonization to summer drought in grassland soils has recently been convincingly demonstrated by Staddon et al. (2003). Basidiocarps (reproductive bodies) produced by both saprotrophic and mycorrhizal fungi are an important direct ecosystem product, being a prized food resource across a wide range of human cultures.

Abiotically, one of the most important effects that deforestation and land-use change have on water draining from catchments is in sediment losses. Particulate loading into streams can be conveniently split into two divisions: suspended load and bed-load. Suspended load is the sediment in transport, which is buoyed up by the movement and flow of the water and includes particles from clay through to fine sand. Typically, this is the material that can be collected in a "bucket" or "gulp" sampler, which can have a marked effect on the turbidity of the water. In contrast, the bedload is the particulate matter that rolls down the stream while remaining in contact with the channel bed, and which is actually quite difficult to quantify.

Bonin et al. (2003) collected fine benthic organic matter from a number of catchments in the United States and concluded that previous timber harvesting had a major impact on the sediment load and sediment biological activity at the base of the catchments. Indeed, these researchers found that the resulting fine organic material had higher mineralizable and extractable nitrogen when derived from the less-disturbed sites, and microbial activity (as represented by a wide assortment of assay techniques) in the harvested catchments tended to be greater. More dramatically, the historic felling of forests in New Zealand has been linked to major landslides that, in turn, have contributed significantly to the development of sediment in depositional basins both within the freshwater and nearshore and offshore zones of coastal systems (see Glade 2003, and below). A less obvious but more insidious problem resulting from deforestation is associated with the construction and materials associated with road building. When considered in terms of changes to the management of a forest, road-building practices have the potential to result in major changes to soils and to the freshwater ecosystem (Motha et al. 2003) at many temporal and spatial scales (see also Chapter 8, Lavelle et al.).

The provision by forests of ecosystem goods and services has been discussed throughout this volume, and a brief summary of the impacts of deforestation within the terrestrial domain is provided below. It should be noted, however, that these impacts are necessarily generalized, since specific site factors such as the scale, location, residue, and post-disturbance vegetation regime may be critical in controlling impacts. One extreme illustration of this is provided in Figure 9.3, which emphasizes the point that the spatial distribution of tributaries, even with an equivalent catchment area, can impact a receiving marine domain in very different and idiosyncratic ways, depending on details such as branching structure and number of lakes within the river system.

1. Food provision. Although at one time terrestrial forests were a major source of human nutrition, the development of agriculture has meant that forests no longer occupy such a central role. However, as a habitat for birds and mammals, forests currently represent an important source of meat for those human cultures still dependent on hunting, while even societies that rely heavily on agriculture still value the edible fungi and fruits growing wild in forests. Forest felling can have major effects on forests as sources of foods, increasing access for hunters of larger mammals and increasing the provision of forest margins and glades suitable for wild boar (Brownlow 1994). With respect to game birds, the traditional view is that the creation of forest edges increases the number of game birds, but this has been challenged by Temple and Flaspohler (1998). Thus, many of the effects of deforestation on human nutrition are positive, since the provision of appropriate forest clearings, edges, and rides can encourage, for example, species of mammals and fungi not found in intact forests, and may actually improve the habitat for specific mammals. Indeed, Termorshuizen and Schaffers (1991) suggested that clear-cutting was a significant factor in increasing fungal abundance due to the provision of disturbed, less polluted, soil layers.

2. Carbon sequestration. Soil biota are a crucial determinant of carbon sequestration and mixing in forest soils, with greater carbon accumulation in mor than mull systems. However, carbon stored in mor humus may be more vulnerable to deforestation than carbon held in mull humus, since the stabilization of organic matter onto minerals is more pronounced in mull soils. Earthworm activity is specifically associated with the intimate mixing of soil organic matter and minerals, which has major consequences for the sequestration of carbon. A large component of carbon storage in forest floors is maintained in trunks and woody debris, and the impacts of deforestation on carbon storage depend on how much of this material is left after felling. However, the net effect of deforestation sometimes results in soil carbon loss, largely because canopy removal results in a greater water and heat flux to the soil with increased decomposition together with a reduction of litter inputs after the initial felling.

3. Water quantity. Reduced rooting, together with an associated drop in mycorrhizal fungal biomass and less canopy interception, leads to an increase in soil moisture content and greater runoff. Water yields can be significantly greater in streams draining felled catchments (see below).

Figure 9.3. Diagrammatic representation showing how the physical location and scale of deforestation is important in controlling impacts on freshwater and marine sediments. The top panel represents a forest system with selective cutting, and the bottom panel represents a forest with clear-cutting. The level of intensity of the disturbance can impact freshwater and marine sediments, and the cascading effects of sedimentation, quite differently. For in-depth discussion of spatial and temporal scale, see Chapter 8.

Figure 9.3. Diagrammatic representation showing how the physical location and scale of deforestation is important in controlling impacts on freshwater and marine sediments. The top panel represents a forest system with selective cutting, and the bottom panel represents a forest with clear-cutting. The level of intensity of the disturbance can impact freshwater and marine sediments, and the cascading effects of sedimentation, quite differently. For in-depth discussion of spatial and temporal scale, see Chapter 8.

4. Water quality. Deforestation exerts a major influence on decomposition processes, largely through the changes in leaf fall litter, heat, and water flux to the soil. The removal of tree root activity, together with the large underground litter input from roots, reduces water lost through evapotranspiration and also provides considerable nutrient and detrital input. In a review of the impacts of forest management practices on forest carbon stores, Johnson (1992) concluded that there were no consistent general trends of carbon change after harvesting, although lower soil carbon stocks occurred if the harvesting was followed by intense burning or cultivation. Although a few studies have shown large net losses, most studies show no significant change. Yanai et al. (2003) has highlighted the technical difficulties associated with detecting forest carbon store changes. Increases in dissolved organic carbon (DOC) and sediment load, together with increased concentrations of nitrate (see Binkley 2001), and cations have been frequently observed and experimentally verified. There is generally a transient increase in soil ammonium concentrations after felling, but this often fails to manifest as a change in stream waters. A central feature here is the process of nitrification, which not only serves to produce the highly mobile anion NO3- but may also result in acidification under inappropriate management (Neal et al. 1998). The organisms associated with nitrification in acid soils are poorly understood, yet modern molecular techniques are demonstrating that the characteristic populations of these autotrophic bacteria differ between domains and habitats within domains. Increased stream water concentrations of Al3+ are also a consequence of felling on acidic soils, potentially having a deleterious impact on both soil and aquatic biota.

5. Trace gas production. Standing forests are often major sites of CH4 oxidation within a landscape, and forest removal is often linked to a reduction in the capacity of soils to remove this important greenhouse gas. The mechanisms behind these changes are poorly understood, but are associated with the soil bacteria specifically known to carry out the process. In contrast, soil waterlogging associated with felling may result in increased methanogenesis, associated with the anaerobic decomposition of organic matter. Wetter soils, with a greater NO3 concentration and more available carbon from the felling residue, all interact to increase denitrification rates, with N2O being a common product under the acidic conditions found in forest soils. A significant quantity of dissolved N2O may end up in the streams, being subsequently de-gassed farther downstream.

6. Recreation. Felled forest is both visually and recreationally less valuable than standing forest, with hikers, horse riders, and day-trippers preferring intact forest stands with good road and ride access. Other users, such as hunters, have slightly different preferences and prefer some open space within the forest. However, overall, recreation is reduced in recently felled forest. A summary of changes in ecosystem services and consequences for the outputs moving into linked freshwater systems is provided in Table 9.1, section A.

Freshwater Ecosystems: Deforestation Impacts

Impacts of deforestation on stream assemblages and ecosystem processes are well known, but they have not been considered explicitly within the context of the ecosystem services provided by stream benthos. There is a wealth of observations and experimental data on the impacts of deforestation on the physical, chemical, and biological characteristics of streams, potentially leading to changes in provision of ecosystem goods and services by the freshwater system. However, it must be recognized that not all deforestation is the same, and the impacts are closely linked to the actual management practices used in the felling process. The extent of the area and actual location of deforestation, whether principally in the upper or lower reaches of the catchment, also have a major influence on potential freshwater impacts (see Figure 9.3).

Streams receive increased sediment loads following tree removal (often from road building), which alters geomorphology because depositional areas contain a higher proportion of fine sediments (Platts et al. 1989; Waters 1995). The chemical composition of groundwater flowing from soils to streams can change radically with increased solutes such as nitrate, base cations, and lower pH (Likens et al. 1970). Water temperatures may increase because of the loss of overhanging vegetation (Johnson & Jones 2000) and the considerable reduction of leaf litter (Webster et al. 1990) during the short period before regrowth. Wood inputs will decrease (Bragg 2000), which alter stream geomorphology, since wood may play an essential role in determining riffle-pool structure and sediment storage in streams (e.g., Montgomery et al. 1996).

Stream biota can play a strong role in providing such ecosystem services as juvenile fish production, nutrient transformation, and recreation (see Covich et al., Chapter 3; Wall et al. 2001). Altering these biota via forest removal may affect some of the ecosystem services that streams provide, with reduced litter and wood inputs combined with increased sediment and solute loads, resulting in changes in stream biota. Invertebrate assemblage structure shifts from large shredder-dominated assemblages to increased scrapers (Gurtz & Wallace 1984), though there can be higher secondary production of macroinvertebrates well after the disturbance (Stone & Wallace 1998). Responses to logging are habitat specific; depositional areas may have lower abundance of invertebrates following logging, presumably from increased sediment, while bedrock reaches may experience higher invertebrate density due to increased light (Gurtz & Wallace 1984).

Responses of fish populations and production to deforestation are complex. Increased algal production from higher light input may increase fish production, largely as a result of higher invertebrate production (Bilby & Bisson 1992). However, long-term loss of wood inputs will reduce habitat diversity and fish biomass in streams, despite the short-term increases in light and algal productivity. For example, streams in British Columbia had four to ten times higher salmonid biomass when woody debris was abundant (Fausch & Northcote 1992). Sediment can fill spawning gravel, lowering salmonid recruitment success (Waters 1995).

Deforestation can alter ecosystem services provided by the stream benthos. If the stream has migratory salmonids, there may be strong negative effects on recruitment of smolts (Table 9.1, section B) and much of the decrease of salmon populations in coastal streams has been attributed to sediment impacts from deforestation (review in Waters 1995). Thus, removal of forest production may dramatically affect the stream's ability to recruit salmon, which will lower overall productivity, and, in turn, harm recreational fisheries.

Deforestation may also decrease the stream's capacity to remove nutrients from the water column, resulting from both abiotic and biotic changes. Increased nutrient concentrations may saturate biotic uptake (Dodds et al. 2002), which may proportionally increase loads of dissolved inorganic nutrients. Loss of coarse woody debris may also lower nutrient uptake and storage (Bilby & Likens 1980; Valett et al. 2002), and

Table 9.1. Summary of consequences of deforestation on ecosystem services in terrestrial, freshwater, and marine domains with emphasis on soil and sediment biodiversity.

A rank from ■

-3 (strong disservice) through 0 (neutral) to +3 (strong

service) is

given for each good or service,

indicating its value to human societies.

A: Terrestrial

B: Freshwater

C: Marine

Ecosystem

Imports to

Imports

Service

Rank Biotic

Abiotic Fresh Water

Rank

Biotic

Abiotic

to Ocean

Rank Biotic

Abiotic

Food

production

— 1 mushrooms

PH

-3

deteriorated

sediments

? increased

salinity

spawning

cover gravel

sediment

increased

grounds

builds

nursery

grounds

hunting

soil texture

whirling

increased

declines

higher

disease

temperature

in filter

turbidity

(tubificids)

feeding

bivalves

nuts

moisture

shredders

increased

more

decline

primary

nutrients

production

honey soil fertility light loss of insect taxa honey berries soil fertility light

Carbon sequestration

-3 increased de- increased composition temperature loss of insect taxa algae/deposit feeders increased increases sediment in deposit structure feeders marine seep more DOC

marsh locked up vegetation other fauna increased increase moisture (see narrative)

Water quality -3 nitrifying loss of -3

bacteria structure decomposers sediment mobilization increased no3

reduced roots reduced fungal hyphae loss of leaf litter sediment mobilization increased no

decreased so4"

increased doc, dop, dom base cations increased pH decreased increased

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  • Rebekah
    What are the effect of deforestation on the ecosystem?
    8 months ago

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