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Terrestrial Ecosystems

Two linked processes are of particular potential significance in biogeochemical cycling in terrestrial ecosystems: 1) the cycling of carbon including its capture (photosynthesis), storage (biomass and soil organic matter content) and release (plant and soil organism respiration) and 2) the cycling of mineral nutrients such as nitrogen upon which plant production and ecosystem productivity are dependent. The exchange of trace carbon and nitrogen gases between terrestrial systems and the air also is dependent on these processes. This section on terrestrial ecosystems reviews actual and potential effects of UV-B (direct and indirect) on carbon and mineral nutrient cycling and trace gas exchange.

    Since the 1994 UNEP report (Zepp et al. 1994, 1995), there has been a considerable increase in the number of studies on biogeochemical cycling in terrestrial ecosystems. Current studies include a program in which standard litter (birch leaves) is being decomposed under standard conditions in UV-B enhancement experiments in natural heathlands from the Mediterranean to the high Arctic (Paul and Moody personal. comm.), in collaboration with UV-B exclusion experiments in southern Argentinean heathlands (Caldwell and Balares pers comm). Other on-going projects include UV-B regulation of Sphagnum fuscum production and decomposition in the sub Arctic (Bj` rn et al. 1998), decomposition in Dutch coastal ecosystems (Rozema et al. 1997a) and photodegradation of plant litter in high Arctic ecosystems (Johanson et al. pers comm). Current studies on UV-B impacts, and in combination with other treatments such as drought, fertilization, and perturbation on the growth and community structure of limestone grassland will also give information on some aspects of biogeochemical cycling (C.Thorpe and J.P. Grime pers. comm.)

Carbon Capture and Storage

Although many studies, but not all (Tosserams and Rozema 1995, Gehrke et al. 1996), suggest that photosynthesis and plant growth can be reduced by elevated levels of UV-B radiation, data on plant biomass and rates of net primary productivity, i.e. the standing stock of living matter expressed as g dry weight per unit ground area, and growth expressed as g dry weight per unit ground area per unit time, are difficult to find for natural ecosystems and forests. However, a review (Rozema et al. 1997a) suggests that there is little impact of UV-B radiation on the primary production of terrestrial ecosystems. Although the length of individual Sphagnum fuscum shoots was reduced by 20% within one growing season (Gehrke et al. 1996), changes in biomass or primary productivity per unit ground area were not detected (C. Gehrke pers. comm.).
 
 Fig. 5.2 Summarized relationships between UV-B, plant development, decomposition and carbon and mineral nutrient cycling. "+" denotes measured overall increases in processes, "-" denotes measured overall decreases. Signs are concensuses of never more than 4 studies while those in square brackets are inferences only. Data from Gehrke et al. 1995; Klironomos and Allen, 1995; Newsham et al. 1997a, b; Rozema et al. 1997 a; Gwynn-Jones et al. pers. comm.; Johanson et al. pers. comm.

Data for carbon storage in soils and plant litter are not available. This is primarily due to the practical difficulties of running experiments over time scales sufficiently long to allow high UV-B treated plant material to senesce naturally and be decomposed under experimental conditions. However, it is clear that the impact of increased UV-B on carbon storage will result from a balance between the impacts on plant productivity and decomposition of litter.

    The latter process will be determined by direct photodegradation of litter exposed to elevated UV-B, indirect UV-B generated changes in the tissue quality of living plant tissues exposed to elevated UV-B and direct UV-B impacts on the mortality and growth of fungal decomposers and other soil decomposer organisms (Moorhead and Callaghan 1994)(Figure 2). The outcome of these processes is likely to be ecosystem and even species specific, although similar trends may occur within functional plant groups.
 

Fig. 5.3 Impacts of UV-B radiation on photodegradation and microbial decomposition of dune grassland litter. "amb" = ambient, "enh" = enhanced. The row labelled "UV-B exposure during plant growth" describes the conditions under which the grass was grown, i.e. under ambient or enhanced UV-B. The row "before/after decomposition" denotes whether the litter had not ("before") or had ("after") been subjected to decomposition in litter bags on a dune grassland under ambient or enhanced UV-B. The row "UV-B radiation during decomposition" describes the conditions under which the litter was decomposed, i.e. under ambient or enhanced UV-B. For example, the last bar indicates the lignin content of the litter from grass grown under enhanced UV-B and then subjected to enhanced UV-B during the period of decomposition. The primary effect observed was an increase in lignin content and a reduction in decomposition rates for the leaf litter of plants grown under enhanced UV-B. Exposure of the decomposing leaf litter to enhanced UV-B decreased the increase in lignin content, indicating that lignin photodegradation was stimulated. From Rozema et al. 1997a.

Indirect Effects of UV-B on Decomposition. Litter chemistry. Chemical changes in leaves (e.g. increased lignin) of the dune grassland species Calamagrostis epigeios, a grass, were induced by elevated UV-B and resulted in reduced decomposition (Figure 5.3) (Rozema et al. 1997b). (Here and throughout section 5.2 the term ‘elevated UV-B’ corresponds to a 15% reduction in the total ozone column at the location of the experiment or source material). In contrast, UV-B supplementation over three growing seasons had no significant effects on plant chemistry (nitrogen concentration (g N per g dry weight), C:N ratio and in some cases water soluble phenolic compounds) in Rubus chamaemorus, a perennial herb, or Calluna vulgaris, heather, (Moody et al., submitted). Nevertheless, chemical analysis at the end of the experiment revealed an indirect UV-B effect on nitrogen concentration, where nitrogen loss was significantly greater from litter of plants grown at enhanced UV-B (Moody et al., submitted), even though there were no significant differences in mass loss.

    Such differences in type and magnitude of litter chemistry response among plant species or functional types may be a function of the protective barrier in leaves (cuticle, epidermis), leaf angles or relative amount of self-shading. All these factors can modify the exposure of internal leaf tissues to UV-B.

    It is commonly accepted that exposure to enhanced UV-B leads to accumulation of flavonoids in leaf tissue (e.g. Gwynn-Jones and Johanson 1996). Such accumulation results from up-regulation of phenylpropanoid metabolism, which may also lead to an increase in a wide range of phenolic compounds. We have limited knowledge about the role of these compounds in decomposition although some are known to inhibit this process.

    Rate of Decomposition and Mass Loss. Elevated UV-B reduced the rate of decomposition of Vaccinium uliginosum leaf litter by changing the chemical composition of the litter (Gehrke et al. 1995). Similarly a 3% lower mass loss of oak litter at enhanced UV-B was attributable to altered leaf chemistry as carbon content was 5% less compared to ambient arrays (Newsham et al. 1997b) and the mass loss of High-Arctic Salix polaris leaf litter was retarded by field exposure to enhanced UV-B (Johanson et al. pers. comm.).

    More recently, no differences have been shown in the decomposition rates of Rubus chamaemorus leaf material field- grown for 12 months and decomposed at enhanced UV-B for over 24 months (Moody et al. submitted). However, when litter was produced under enhanced UV-B for two seasons and then decomposed in the field under these same conditions, there was a small (5.5%) but significant increase in mass loss compared with controls. Although, when present, the impacts of increased UV-B on decomposition and mass loss are small, long periods of decomposition and responses of geographically widespread ecosystems will amplify the importance of the impacts.

Direct Effects of UV-B on Decomposition

Microbial Diversity. Several researchers have shown changes in microbial communities associated with decomposing litter following exposure to enhanced UV-B (Gehrke et al. 1995, Newsham et al. 1997a, Newsham et al. 1997b, Moody et al. submitted). It could be hypothesized that observed changes in microbial respiration measured during the controlled decomposition of litter in microcosms are a result of UV-B induced changes in the microbial decomposer community. Evidence has been provided in support of this hypothesis (Gehrke et al. 1995; Moody et al. submitted). In the latter study the fungal decomposer community isolated from litter decomposing under ambient UV-B was significantly different from that of litter from elevated UV-B. Of the two most frequently isolated species, Aspergillus fumigatus was found to dominate at ambient UV-B while at elevated UV-B Penicillium purpurogenum was dominant.

    Photodegradation. Research has shown evidence of increased litter photodegradation at enhanced UV-B (Gehrke et al. 1995; Rozema et al. 1997b; Fig. 5.3). Nevertheless the impacts of such effects may sometimes be off-set by UV-B induced chemical changes in leaves (e.g. increased lignin), and the balance between the opposing processes might be difficult to relate to the causes.

    However, no significant direct effects of UV-B (i.e. effects of UV-B treatments imposed during decomposition) were found on decomposition measured as mass loss (Moody et al. submitted) of Rubus litter. This lack of direct effects was consistent in Rubus litter taken from plants grown under ambient and elevated UV-B for two growing seasons and subsequently decomposed in the field for 12 months, and in Rubus litter collected at Moor House (the site of origin of the plant material) and decomposed under ambient and elevated UV-B for 24 months.

    In practice, canopy structure and phenology (seasonal development of leaf expansion, etc.) determine whether the UV-B influences decomposition by changes in leaf chemistry or by direct photodegradation. Evergreen and wintergreen canopies in which senescent leaves and litter are shaded by living leaves would be expected to show indirect UV-B impacts as the living leaves attenuate UV-B. In contrast, simple seasonal canopies in which litter is exposed during Spring e.g. bracken fern, would be expected to show photodegradation.

Mineral Nutrient Cycling

In many natural ecosystems, mineral nutrient cycling is critical in supporting plant production. In agricultural and forest ecosystems, any constraint of increased UV-B radiation on mineral nutrient cycling can conceptually be modified by increasing fertilizer application and the outcome will rely on economics. Any change in mineral nutrient cycling could affect biodiversity of organisms which is often relatively high when mineral nutrient supply is limited.

    Mineral Nutrient Capture. In some natural ecosystems, nitrogen input via nitrogen fixation in free-living and symbiotic cyanobacteria is an important process. It has been shown that UV-B irradiation of soil in field experiments stimulated nitrogen fixation by 17 and 19% at irradiances of 0.34 and 0.49 W m-2 while irradiances of 0.77 W m-2 decreased nitrogen fixation by 13% when compared with untreated controls (Uralets 1991). In the High Arctic, recent preliminary results suggest that field exposure to enhanced UV-B significantly reduces cyanobacterial nitrogen fixation (B.Solheim pers. comm.).

    Studies on the nitrogen fixing cyanobacterium Nostoc showed that survival and growth were virtually arrested after 120 min of UV irradiation due to impairment of energy transfer from phycobiliproteins to the photosynthetic reaction centers (Sinha et al. 1995). Such results suggest that increased UV-B could reduce nitrogen availability to plants in natural ecosystems (e.g. polar and alpine and those of moist soils during early successional stages), and some agro-ecosystems (e.g. rice paddy fields). See also Chapter 4.

    There is evidence that UV-B radiation can also affect mineral nutrient capture by plants (Weih et al. in press). It is believed that this is mediated through the modification of the soil organism community. UV-B can affect rhizosphere organisms that are fed primarily by root-derived substances. In experiments on Acer saccharum mycorrhizae associated with the tree seedlings were inhibited by UV-B irradiation of the plants and there was a stimulation of the saprobe/pathogen soil organisms (Kilronomos and Allen 1996). Although implications for mineral nutrient capture by the plants was not apparently investigated, it can be assumed that reduced mycorrhizal activity would reduce mineral nutrient uptake by the plant while increased saprobe activity could result in mineral nutrient immobilization and competition between microbes and plant roots.

    Mineral Nutrient Allocation within Plants. There is much, and increasing, evidence that the exposure of green leaves to elevated UV-B radiation changes the allocation of plant mineral nutrients between various organic molecules (Rozema et al 1997a). Most commonly, UV-B induced pigment production and changes in phenols and tannins diverts nitrogen and carbon to refractory compounds which can affect herbivory (digestibility) and microbial decomposition of plant tissues. In recent decomposition experiments on dwarf shrub litters from a sub arctic heathland, elevated UV-B decreased tannin content, but did not significantly change the contents of nitrogen, phosphorus and the C:N ratio in contrast to CO2 at 660 ppm which changed all of these chemical components of the litters (Gwynn-Jones et al, unpublished). However, exposure of other plants to elevated UV-B increased tannin and also the refractory material lignin (Gehrke et al. 1995; Rozema et al. 1997a).

Interactions With Other Co-Occurring Environmental Change Variables

Enhanced UV-B is only one of many aspects of global environmental change. Other factors that are currently changing are well documented, including changes in atmospheric CO2 concentrations, surface temperatures (both cooling and warming trends have been recorded), and nitrogen deposition. Both high temperatures and elevated CO2 (660 ppm) concentrations can affect plant tissue chemical composition ("quality") (see Gwynn-Jones et al. 1996) while increased temperatures can increase decomposition rates thereby enhancing mineral nutrient turnover but reducing carbon storage. If the main effect of increased temperature is to reduce the moisture content of soil and plant litter, then the opposite effects occur. Possible synergism between increased temperatures, elevated CO2 and increased UV-B radiation could have significant consequences for biogeochemical cycling. Recent unpublished work. has shown that there is no combined effect of elevated CO2 (660 ppm) and elevated UV-B radiation on the CO2 respired from litters of the sub arctic dwarf shrubs Vaccinium myrtillus and V. uliginosum (Gwynn Jones et al., personal communication). However, both treatments applied singly affected decomposition in counter intuitive ways: both increased CO2 (660 ppm) and elevated UV-B increased decomposition. There are precedents for elevated CO2 increasing decomposition (e.g. Robinson et al. 1997) but the result for the UV-B treatment is opposite to that obtained for just one of the species, V. uliginosum. (Gehrke et al. 1995)

    Interactions between drought and elevated UV-B treatments in a limestone grassland community apparently increase competitive vigor under field conditions in those species which adopt a more compact, xerophytic growth form (C.Thorpe and J.P.Grime pers. comm.). Indeed, increased UV-B may pre-dispose certain plants to become drought tolerant and therefore more competitive during drought periods.

UV-B and Trace Gas Exchange

In addition to uptake of CO2 by photosynthesis and release of CO2 by respiration, living plants and litter release the chemically important gases CO and volatile organic compounds (VOC), such as isoprene, to the atmosphere (IPCC, 1992). VOC and CO react in the troposphere to influence ozone, other oxidants and aerosols. Isoprene, in particular, plays a key role in such reactions. In addition, CO and VOC emissions affect the oxidizing capacity of the troposphere (Chapter 6) thus possibly affecting the concentration of the greenhouse gases, methane and CFCs.

    Changes in UV-B have been shown to have no detectable effects on the release of isoprene from three temperate plant species (Harley et al. 1996). However, although direct effects of UV-B radiation have little effect, UV-B induced changes in the competitive balance of plants in terrestrial ecosystems could result in changes in species composition that could affect net fluxes of these chemically-important gases.

    Additional results have demonstrated that plant leaves produce CO on exposure to solar radiation and that senescent and dead leaves from temperate deciduous plants and tropical grasses produce CO more rapidly than living plant leaves (Tarr et al.,1995; Zepp et al., 1996; Schade et al., 1997). Estimates of the CO source from photodegradation of non-living plant matter indicate that 50 to 200 Tg of CO may be produced annually on a global basis. This source is sufficiently large that it should not be neglected when considering the global CO budget. Wavelength studies (>300 nm) indicate that the action spectra are species dependent and that UV-B radiation produces CO from the leaves with the highest efficiency, although UV-A radiation also induces CO formation. The floor of boreal forests that have experienced fire also have been shown to be net sources of CO to the atmosphere and the production of CO is in part attributable to UV-induced photodegradation of the charred organic matter (Zepp et al., 1997)

    Although terrestrial plants are believed to be an important sink of carbonyl sulfide, (OCS), the most concentrated sulfur gas in the lower atmosphere, no data are available on the effects of enhanced UV-B on plant uptake of OCS. Likewise, no results have become available on the UV-B mediated uptake of nitrogen oxides by vegetation during the past four years.


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