216 R.]. Luxmoore et al.

Waring, R. H., and Schlesinger, W. H. (1985). The carbon balance of trees. In "Forest Ecosystems Conce~ts and Management" (R. H. Waring and w. H. Schlesinger, eds.) pp. 7-37. AcademiC Press, Orlando, Florida. '

Warri~gton, I. J., Roo~, D. A., Morgan, D. C., and Turnbull, H. L. (1988). The influence of Simulated shadehght and daylight on growth, development and photosynthesis of Pi­nus radzata, Agathzs australzs and Dacrydium cupressium. Plant, Cell Environ. 12:343-356

Wadt~on: M .. A., .and Casper, B. B. ( 1984). Morphogenetic constraints on patterns of carbo~ Istnbuuon m plants. Annu. Rev. Ecol. Syst. 15:233-258.

Webb, ~· L. ( 1977). Rate .of. current photosynthate accumulation in roots of Douglas-fi seedh~gs: Seasonal vanauon. In "The Belowground Ecosystem: A Synthesis of PI tr Associated Processes" (J. K. Marshall, ed.), Range Sci. Dept. Sci. Ser. No. 26., pp. ,:~~ 152. Colorado State Umv., Fort Collins Colorado. w .k . ,

ei e~t, R. M., Welder, M., Lippert, M., Schramel, P., and Lange, 0. L. (1989). Photos n-t7etiC performance, chloroplast .pigments, and mineral content of various needle Ige c asses of spruce _(Pzcea abies) with and without the new flush: An experimental a _ proach for analyzmg forest decline phenomena. Trees 3: 161-172. p

Went, F. W. (1974). Reflections and speculations. Annu. Rev. Plant Physiol. 25: 1-26 Whittaker, R. H., and Woodwell, G. M. (1968). Dimension and production relations of w·~~ees and shrubs m t~e Brookhaven forest, New York.]. Ecol. 56:1-25.

I , G. M., and Hodgkiss, P. D. (1977). Influence of nitrogen and phosphorus stresses on the. growth and form of radiata pine. N. z. J For. Sci. 7: 307-320.

W~lsvi.nkel, P. (1985). ~hl~em unl~a?in.g and turgor sensitive transport: Factors involved m smk control of assimilate partitionmg. Physiol. Plant. 65: 331-339.

Wood,. G. B. (1968). Photosy~thesis and growth in Pinus radiata D. Don as affected b envi~onmental factors and mherent qualities. Ph. D. Thesis, Australian National u ·~ versity, Canberra. m

Wo~dr~w, I. E., and Berry, J. A. (1988). Enzymatic regulation of photosynthetic C02

fixa­z tlon m C, plants. Ann~. Rev. Plant Physiol. Plant Mol. Bioi. 39:533-594.

ahner, R. (1~62). Ter~mal growth and wood formation by juvenile loblolly pine under two sod moisture regimes. For. Sci. 8: 345-352.

Zahner, R. ( 1968) .. Water deficits and growth of trees. In "Water Deficits and Plant Growth" (T. T. Kozlowski, ed.), pp. 99-110. Academic Press, New York.

7 _____ _

Carbon Allocation and Accumulation in Conifers

Stith T. Gower, J. G. lsebrands, and David W. Sheriff

I. Introduction

Forests cover approximately 33% of the land surface of the earth, yet they are responsible for 65% of the annual carbon (C) accumulated by all terrestrial biomes (Schlesinger, 1991). In general, total C content and net primary production rates are greater for forests than for other biomes, but C budgets differ greatly among forests. Despite several decades of research on forest C budgets, there is still an incomplete understanding of the factors controlling C allocation. Yet, if we are to understand how changing global events such as land use, climate change, atmospheric N deposition, ozone, and elevated atmospheric C02 affect the global C budget, a mechanistic understanding of C as­similation, partitioning, and allocation is necessary. Numerous abiotic and biotic factors influence C allocation patterns, which in turn affect the capacity of plants to obtain resources from the atmosphere and soil.

Although reviews on various components of conifer forest C budgets, such as photosynthesis (see Chapter 4, this volume) and detritus produc­tion (Vogt et al., 1986), are available, a synthesis of the influence of abi­otic and biotic factors on leaf, canopy, and stand-level C budgets of co­nifer forests is lacking. The objective of this chapter is to review the major factors that influence C allocation and accumulation in conifer trees and forests. In keeping with the theme of this book, we will focus primarily on evergreen conifers. However, even among evergreen co-

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218 Stith T. Gower et al.

nifers, leaf, canopy, and stand-level C and nutrient allocation patterns differ, often as a function of leaf development and longevity (Gower and Richards, 1990; Gower et al., 1993a; Reich et al., 1994).

The terminology related to C allocation literature is often inconsistent, confusing and inadequate for understanding and integrating past and current research. For example, terms often used synonymously to de­scribe C flow or movement include translocation, transport, distribu­tion, allocation, partitioning, apportionment, and biomass allocation. A common terminology is needed because different terms have different meanings to readers. In this paper we use C allocation, partitioning, and accumulation according to the definitions of Dickson and Isebrands (1993). Partitioning is the process of C flow into and among different chemical, storage, and transport pools. Allocation is the distribution of C to different plant parts within the plant (i.e., source to sink). Accu­mulation is the end product of the process of C allocation.

II. Distribution of Conifer Forests

Evergreen conifers occur from tropical to boreal forests and from temperate rainforests to dry woodlands (Walter, 1979; Kikuzawa, 1991). Conifers commonly dominate in temperate regions only after distur­bance or where soil infertility is low. The dominance of evergreen coni­fers in the Pacific Northwest of the United States can be explained partly by the unique environmental conditions of dry, warm days and cool nights in the summer and mild, moist conditions in the winter (Waring and Franklin, 1979). Although conifers occur on many of the major soil orders, conifers, especially pines, are more abundant on nutrient-poor soils (Miller et al., 1979). Nitrogen availability commonly limits net pri­mary production (NPP) in temperate and boreal conifer forests whereas phosphorus availability limits NPP in subtropical and tropical environ­ments (Ballard, 1984). In summary, temperature, moisture, and nutrient conditions differ greatly among evergreen conifer forests.

To help understand assimilation and allocation patterns of conifer forests, we briefly contrast the major differences in C and nutrient cycles of evergreen conifer and deciduous forests. Perhaps the most striking difference between evergreen conifers and broadleaf deciduous species is leaf longevity, which is correlated to a number of leaf, canopy, and ecosystem structural and functional characteristics (Gower et al., 1993a; Reich et al., 1994). Even among conifer species leaf longevity ranges from less than a year for Larix spp. and Taxodium spp. to greater than 40 years for Pinus longaeva. Leaf litterfall nitrogen content is often less for evergreen conifers than for broadleaf deciduous forests (Vogt et al.,

7. Carbon Allocation and Accumulation 219

1986). The litter of evergreen conifers. decomp?ses. slow~r than. th~t from deciduous trees due to its lower htte~ qu~hty (I.e., higher hgmn and lower nitrogen concentrations), resultmg m great~r forest floor mass beneath conifer forests compared to broadleaf deodu~us forests. Soil nitrogen availability is commonly lower in evergreen comfer. forests than in deciduous forests due to the positive feedback of leaf htt~~fall quality on soil nitrogen dynamics (Gower an~ Son, 1992). In addltlon, evergreen conifer forests have lower annual mtrogen uptake rates com­pared to deciduous forests (Gosz, 1981; Nadelhoffer et al., 1984; Son and Gower, 1991).

111. Controls on Carbon Assimilation

Net primary production of a seedling, tree,. or forest is t~e ~alance between total canopy photosynthesis (gross pnma_ry pro.du~twn, GPP) and the amount of Clost via respiration (autotrophiC r.es~:nr~twn). There are numerous direct and indirect feedbacks on C assimilatiOn a~d. allo-

. · c t (FI·g 1) For example net canopy photosynthesis IS the catwn In 10res s . . , . . product of net photosynthetic rate and p~otosyn~hettc surface ar~a m­tegrated over selected daily and seasonal tlme penods, and ne~ pnmary production is the difference between net canopy phot~synthesis and ~u­totrophic respiration of woody tissues. Clearly, net I:'nmary p~oductwn cannot be estimated without a complete understandmg of the mftuen~e of biophysical controls on physiological processes such as photosynthesis and respiration (Chapter 4) and ?n the allocation of C to components such as leaf area and live woody bwmass . .

Under optimal environmental ~onditio.ns; NPP IS lmearly related to intercepted photosynthetically active radiatwn (IP~R) for cro~ plant~ and tree seedlings (Monteith, 1977) and for.ests (~I~der, 1985, Lands berg, 1986). Factors that influence the relati?nship mclude ( 1) canopy reflectance, (2) canopy architecture and persistance of the canopy dur­ing the year, and (3) light use efficiency (Cannell, 1989_)- The first factor that affects the relationship between NPP and IPAR IS canopy albed~, which averages 15-20% for deciduous forests versus .1~-15% for com­fer forests (Rosenberg et al., 1983). Canopy charactenst~cs related to ar­chitecture include rate of leaf area development, maxim~m leaf a:ea, and leaf area duration. The rate of leaf area development m t?e sprmg, although important for deciduous species, is gradually less Important for evergreen conifers that retain their nee~les for. a greater numbe~ of ears because the new foliage comprises an mcreasmgly sm.aller fr~~twn

~f the total leaf area (Gower et al., 1993a). Leaf area duratwn positively influences annual IPAR. For example, Cannell et al. (1987) reported a