Aboveground tree responses

The principal observations that become evident from Table 11.3 are that:

(i) biomass increases under elevated [CO2] in all studies and in all tree species;

(ii) photosynthesis per unit leaf area is significantly stimulated, with some exceptions; (iii) there is considerable variability among different tree genera and species in the response reactions; and (iv) there are still very few long-term studies on adult or nearly adult trees. Results of studies of less than one growing season may not represent an acclimated or long-term response, and may exclude the influence of seasonality and ontology. Size constraints are often countered by the use of juvenile trees, yet large morphological and physiological differences often exist between juvenile and mature trees (Kozlowski and Pallardy, 1996). However, a number of CO2 enrichment studies have used adult trees equipped with branch bags (also included in Table 11.3), which allow a compromise between large tree size and tree maturity (Dufrene et al., 1993). In the following discussion, some response processes of trees to CO2 enrichment are briefly reviewed.

Photosynthetic responses and acclimation

The response of photosynthesis in trees to CO2 enrichment has been the subject of a number of recent reviews (Ceulemans and Mousseau, 1994; Gunderson and Wullschleger, 1994). In the majority of short-term CO2 enrichment studies on trees, photosynthesis is enhanced. The magnitude of the response varies widely from 0 to +216% (Table 11.3). This variation could be due to inter- and intraspecific differences, to variations in experimental growth conditions (such as nutrient and water treatments), or to the period of exposure to CO2 enrichment. Also, the variations in photosynthesis between juvenile plants and mature plants are particularly important in trees and may contribute to the large variation in the response of photosynthesis to elevated [CO2].

Acclimation is defined by a changed photosynthetic CO2 uptake when CO2-enriched plants are transferred back to the ambient [CO2]. A considerable number of papers have described photosynthetic acclimation in potted-tree experiments (Ceulemans and Mousseau, 1994), but free-rooted trees in open-top chambers might also show this phenomenon. Nevertheless, the studies summarized in Table 11.3 show that the overall effect of elevated [CO2] on photosynthesis in soil-grown trees is strongly positive, although this stimulation decreases after several years of treatment (Ceulemans et al., 1997).

Table 11.4. Effects of elevated CO2 on tree-mycorrhiza associations. Treatment conditions and duration, response of the percentage of mycorrhizal infection (% Inf), response of total amount of mycorrhizae (Total), fine-root responses and remarkable mycorrhizal features are reported.

Species

Plant growth conditions

Total

Other effects on mycorrhizae

Fine-root response

Reference

Betula alleghaniensis

Betula papyrifera

Betula papyrifera

Betula pendula

Liriodendron tulipifera Liriodendron tulipifera Pinus caribaea Pinus echinata

Pinus echinata

Pinus palustris

7 months, mesocosms (700 ml l-1) in competition with Betula papyrifera 7 months, mesocosms (700 ml l-1) in competition with Betula alleghaniensis 7-9 months, pots in GC (700 ml l-1)

4 years, OTC (700 ml l-1), no fertilizer 6 months, pots in GC

(+150 and +300 ml l-1) 1 year, pots in GC (660 ml l-1) 10 months, pots in GC (double CO2), no fertilizer

6 months, pots in GC (double CO2)

Altered morphotype assemblages, extraradical hyphal length î Altered species composition

Signif. T in % Inf. at 34 weeks, not at final harvest Signif. T in % Inf. at 6 weeks, not at final harvest No changes in morphotype assemblages, effect larger at low N and adequate water

Berntson and Bazzaz, 1998

Berntson and Bazzaz, 1998

Godbold and Berntson, 1997

Rey etal., 1997

O'Neill and Norby, 1991

O'Neill, 1994

Conroy etal., 1990 Norby etal., 1987

O'Neill etal., 1987

Runion etal., 1997

Pinus ponderosa Pinus ponderosa Pinus radiata Pinus strobus

2.5 years, OTC

(+175 and +350 ml l-1) 4 months, pots in GC (700 ml l-1)

1 year, pots in GC (660 ml l-1) 7-9 weeks, pots in GC (700 ml l-1)

EM ? Extraradical fungal hyphae Î, Area density ? Tingey et al., 1996;

Pinus sylvestris 3 months, Petri dishes in GC

Pinus sylvestris 7-9 months, pots in GC

(double CO2) Pinus sylvestris 3 months, pots in GC (700 ml l-1)

Pinus taeda 4 months, pots in GC

(double CO2) Populus 14 months, open-bottom pots in tremuloides OTC (700 ml l-1)

Populus hybrids 2 years, OTC (+350 ml l 1) Quercus alba 6 months, pots in GC (double CO2

Tsuga canadensis 7-9 weeks, pots in GC (700 ml l-1)

mycorrhizal turnover T

Altered morphotype assemblages EM T Extraradical mycelium T

No effect on total fungal mass

Elevated CO2 did not alleviate negative effects of NH3 and O3

Density ?

Extraradical mycorrhizal hyphal length ? under N-poor conditions, I under N-rich conditions

Increased mycorrhizal infec- Mass ? tion before increase in root mass, alterations in species abundance

Rygiewicz etal., 199Z DeLucia etal., 199Z

Conroy etal., 1990 Godbold etal., 199Z

Ineichen et al., 1995

Markkola etal., 199G

Lewis etal., 1994

Klironomos etal., 199Z; Pregitzer etal., 1995

Ceulemans and Godbold, unpublished

O'Neill etal., 198Z

O'Neill, 1994; Norby, 1994 Godbold etal., 199Z

OTC, open top chambers; GC, growth chambers; EM, enhanced (P <0.05).

ectomycorrhizae; AM, arbuscular mycorrhizae; =, no significant changes (P < 0.05); ?, significantly

Table 11.5. Effects of elevated CO2 on tree litter decomposition. Tree species and tissue, fumigation conditions and duration, decomposition conditions and duration, and changes in litter chemistry and decomposition rates are reported for every study.

Tree species and tissue

Decomposition Response of

Plant growth conditions conditions litter quality

Response of decomposition Reference

Acer pseudoplatanus, senesced leaves Betula pubescens, senesced leaves Betula pubescens, live roots < 2 mm

Castanea sativa, senesced leaves

Fraximus excelsior, senesced leaves Liriodendron tulipifera, senesced leaves

Liriodendron tulipifera, senesced leaves Picea sitchensis, senesced needles Picea sitchensis, live roots < 2 mm

1 season, 600 ml l-1, pots in solar domes

1 season, 600 ml l-1, pots in solar domes

1 season, 600 ml l-1, pots in solar domes

Fertilized Non-fertilized

1 season, 600 ml l-1, pots in solar domes

1 season, + 300 ml l-1, pots in GC, exposed to ozone

1 season, 600 ml l-1, pots in solar domes

1 season, 600 ml l-1, pots in solar domes Fertilized Non-fertilized

8 months, chambers

5 months, chambers

3 months, chambers with soil

6 months, chambers

Incomplete decomposer community

Complex decomposer community

6 months, chambers

1 year, litterbags in forest floor

2 years, litterbags in forest floor

5 months, chambers

3 months, chamber with soils

C/N T, lignin/N = C/N T, lignin/N = C/N T, lignin/N T N 4-, lignin T

Cotrufo et al., 1994 Cotrufo et al., 1994

Cotrufo and Ineson, 1995

Cotrufo et al., 1994

Boerner and Rebbeck, 1995

O'Neill and Norby,

1995b Cotrufo et al., 1994

Cotrufo and Ineson, 1995

OTC, open-top chambers decreased (P< 0.05).

GC, growth chambers; =, no significant changes (P< 0.05); T, significantly enhanced (P< 0.05); 4, significantly

Altered source/sink relationships and canopy processes

As a result of the increased CO2 assimilation rates, growth rates are enhanced and patterns of biomass allocation are altered (Eamus and Jarvis, 1989; Conroy et al., 1990). Hence, changes in the functional relationship between plant parts are frequently observed. The balance between sources and sinks changes during the growing season. It is also affected by other environmental variables, e.g. local climate, soil and plant nutritional levels, competition and solar radiation (Bazzaz and Miao, 1993). Unlike the pronounced seasonal allocation patterns of temperate tree species, many tropical tree species accumulate dry matter throughout the year if sufficient water is available (Goodfellow et al., 1997).

Although the major role of a leaf is to be a source, the production of new leaves may be considered as a sink. Very often an increase in total leaf area is found under elevated [CO2] (Table 11.3). This can be caused by enhanced new leaf production (Guehl et al., 1994; Ceulemans et al., 1995), increased individual and total tree leaf area (Guehl et al., 1994) or increased flush length and number of fascicles (Kellomäki and Wang, 1997b). This increased total leaf area may be displayed in various ways, which affect tree structure and architecture (Tissue etal., 1996; Ceulemans etal., 1995). Greater C assimilation in response to elevated [CO2] also affects canopy architecture through increased branch production, especially secondary branching (Idso et al., 1991; Ceulemans et al., 1995), increased shoot length (Teskey, 1995), or increased number of growth flushes produced during the growing season (El Kohen et al., 1993; Guehl et al., 1994). It has also been demonstrated that phenological processes can be affected by changes in atmospheric [CO2] and/or air temperature (Murray and Ceulemans, 1998). Remotely sensed observations indicate a lengthening of effective leaf area duration, likely leading to an accelerated growth of forest trees (Myneni et al., 1997).

Larger leaf area and/or altered crown architecture will result in earlier canopy closure, and thus enhanced competition (Kellomäki and Wang, 1997b). After 2 years of elevated [CO2] treatment, Scots pine showed a reduction of the initial growth stimulation. This was most likely due to earlier canopy closure (Jach and Ceulemans, 1999). Some studies suggest that under elevated [CO2] the altered vertical leaf display and crown structure might alter the red/far-red ratio of understorey tree seedlings, thereby affecting their growth pattern (Arnone and Körner, 1993). Increased understanding of branch morphological and crown characteristics should equate results of physiological studies to the tree or stand level, since whole-canopy function is an integration of both physiological processes and morphological characteristics at smaller scales.

Water relations and water-use efficiency

A reduction of stomatal conductance under elevated [CO2] might have a significant effect on water transport in trees, since the latter is roughly proportional to stomatal conductance. Hydraulic conductivity was reported to decrease with elevated [CO2] (Tognetti et al., 1996) but this effect is very species-specific (Picon et al., 1996; Heath et al., 1997). In the review by Scarascia-Mugnozza and de Angelis (1998), the reduction of stomatal conductance ranged from 20 to 90%. A decrease in stomatal conductance in response to CO2 enrichment is commonly observed in both short-term studies and experiments using tree seedlings grown in pots (Mousseau and Saugier, 1992; Berryman et al., 1994). Rooting restriction as a result of growing trees in pots has been suggested to result in reduced stomatal conductance either directly through drought stress, or indirectly through a feedback limitation of net photosynthesis that results from reduced root sink strength and increased internal [CO2] (Sage, 1994). However, stomatal conductance has also been found to decrease with CO2 enrichment when mature tree species are grown directly in the ground for more than one growing season (Kellomaki and Wang, 1997a).

The distinct structure and architecture of trees has led to the development of specific methods for measuring water transport in whole trees or branches (Granier et al., 1996). These methods are based on the relationship between water flux rates through the xylem and the dissipation of heat applied to the stem. Direct measurements of water-use efficiency (WUE) - defined as the ratio of assimilated C to the amount of water transpired - are now possible through the study of the stable C isotope composition of tree rings. This is thanks to the work of Farquhar et al. (1982), who have shown that plant discrimination of 13C (D13C) decreases as WUE increases, since both WUE and D13C are related to the difference in internal to external CO2 partial pressure ratio. Under elevated [CO2], the increase in WUE is usually greater than the reduction of stomatal conductance, especially under drought conditions (Scarascia-Mugnozza and de Angelis, 1998). Because instantaneous WUE is invariably enhanced with elevated [CO2], it is often thought that elevated [CO2] will increase drought tolerance (Tyree and Alexander, 1993). Increased [CO2] may thus alleviate moderate drought stress and might allow some extension of forests into drier areas. However, the hypothesized increase in drought tolerance may not always be the case in practice (Beerling et al., 1996): resistance to drought may also depend on a number of factors affecting the evaporative demand or the ability of stem and root systems to transport water. In trees, as well as in some agricultural crops, drought may induce cavitation, and the risk of cavitation is greater in tree species with large-diameter xylem vessels (Cochard et al., 1996).

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