Sec1Fig1Sec2Sec3Tab1Sec4Sec5Sec6Sec7Fig2Fig3Fig4Sec8Sec9Fig4Fig4Sec10Tab2Tab3Sec11Sec12Tab4AckBibCR1CR2CR3CR4CR5CR6CR7CR8CR9CR10CR11CR12CR13CR14CR15CR16CR17CR18CR19CR20CR21CR60CR22CR23CR24CR25CR26CR27CR28CR29CR30CR31CR61CR32CR33CR34CR35CR36CR37CR38CR39CR40CR41CR42CR43CR44CR45CR46CR47ECOPHYSIOLOGYMirjam K. R. Wu¨rth Æ Susanna Pela´ez-RiedlS. Joseph. Wright Æ Christian Ko¨rnerNon-structural carbohydrate pools in a tropical forestReceived: 31 March 2004 / Accepted: 2 November 2004 / Published online: 1 December 2004 Springer-Verlag 2004Abstract The pool size of mobile, i.e. non-structuralcarbohydrates (NSC) in trees reflects the balance be-tween net photosynthetic carbon uptake (source) andirreversible investments in structures or loss of carbon(sink). The seasonal variation of NSC concentrationshould reflect the sink/source relationship, provided alltissues from root to crown tops are considered. Usingthe Smithsonian canopy crane in Panama we studiedNSC concentrations in a semi-deciduous tropical forestover 22 months. In the 9 most intensively studied species(out of the 17 investigated), we found higher NSC con-centrations (starch, glucose, fructose, sucrose) across allspecies and organs in the dry season than in the wetseason (NSC 7.2% vs 5.8% of dry matter in leaves, 8.8/6.0 in branches, 9.7/8.5 in stems, 8.3/6.4 in coarse and3.9/2.2 in fine roots). Since this increase was due tostarch only, we attribute this to drought-constrainedgrowth (photosynthesis less affected by drought thansink activity). Species-specific phenological rhythms(leafing or fruiting) did not overturn these seasonaltrends. Most of the stem volume (diameter at breastheight around 40 cm) stores NSC. We present the firstwhole forest estimate of NSC pool size, assuming a200 t ha1forest biomass: 8% of this i.e. ca. 16 t ha1isNSC, with ca. 13 t ha1in stems and branches, ca. 0.5and 2.8 t ha1in leaves and roots. Starch alone (ca.10.5 t ha1) accounts for far more C than would beneeded to replace the total leaf canopy without addi-tional photosynthesis. NSC never passed through aperiod of significant depletion. Leaf flushing did notdraw heavily upon NSC pools. Overall, the data imply ahigh carbon supply statu s of this forest and that growthduring the dry season is not carbon limited. Rather,water shortage seems to limit carbon investment (newtissue formation) directly, leaving little leeway for adirect CO2fertilization effects.Keywords Biodiversity Æ Carbon balance Æ Globalchange Æ Seasonality Æ Wood reservesIntroductionPlants produce, store, invest and lose carbon com-pounds. The size of the mobile fraction of these com-pounds at a given time may (1) reflect passiveaccumulation for no other reason than a periodic dis-parity between net-uptake and need; (2) it may representa required, in part transitory pool of solutes (transport,metabolic and osmotic requirements); (3) it may be tiedto defense compounds; or (4) represent ‘‘intentionally’’stored reserves (Chapin et al. 1990). Except perhaps fordefense compounds and osmotics, the size of the mobileC-pool is always likely to mirror a plant’s overall carbonsupply status, with the greatest fraction of this poolcommonly present as non-structural carbohydrates(NSC, largely starch and sugars). It is well established(review by Chapin and Wardlaw 1988) that this poolbecomes larger when active sinks are removed, forinstance when trees are debudded or girdled, or whensources become stronger, for instance through photo-synthetic stimulation by atmospheric CO2-enrichment orhigh compared to low light (Wong 1990;Ko¨rner andArnone 1992; Graham et al. 2003). Sink limitationcauses source activity to decline (‘end product inhibi-tion’), whereas active sinks stimulate source activity(e.g., Neals and Incoll 1968; Wardlaw 1990; Stitt andKnapp 1999; Fig. 1).On a whole tree basis, NSC concentrations indicate atree’s actual C-supply status and reflect its capital forM. K. R. Wu¨rth Æ S. Pela´ez-Riedl Æ C. Ko¨rner (&)Institute of Botany, University of Basel,Scho¨nbeinstrasse 6, 4056 Basel, SwitzerlandE-mail: [email protected]. J. WrightSmithsonian Tropical Research Institute,Apartado, 2072 Balboa, PanamaOecologia (2005) 143: 11–24DOI 10.1007/s00442-004-1773-2flushing and reproduction and its buffering capacity withrespect to replacement of lost tissue (e.g., after massiveherbivory or wind damage). Given that almost half ofthe world’s forests are in the tropics (Brown and Lugo1982) and that these forests have been supposed torepresent a net C-sink in response to atmospheric CO2-enrichment (e.g., Taylor 1993; Malhi and Grace 2000;Canadell and Pataki 2002), knowing their current C-supply status is of particular interest. Here we apply thisapproach to a broad samp le of tree species in a talltropical forest in central Panama and make use of theclimate-induced seasonal variation of source and sinkactivity.Tree carbon reserves are known to exhibit seasonaltrends (Kramer and Kozlowski 1979) , although theamplitude of such variations may have diminished inrecent decades as a consequence of higher atmosphericCO2concentrations (Hoch et al. 2003). Seasonal NSCvariations can be induced by seasonal temperature orwater regimes or by phenological patterns these regimesinduce. In the case presented here, water is the over-arching driver. It is often difficult to separate effectsof climate and phenology, because they typically arecorrelated and relationships differ widely across species.For example, in branch-wood of the drough t-decidu-ous, Mediterranean Aesculus californica, NSC concen-tration dropped during fruit production in fall andre-growth in early spring, whereas evergreen Quercusagrifolia showed little change throughout the year(Mooney and Hays 1973). For a subtropical seasonalclimate, Bullock (1992) reports an increase in stemNSC at the end of the wet season for Jacaratia mexi-cana, in contrast to Spondias purpurea which showedhardly any change across seasons. In a tropical sea-sonal climate vines showed increasing NSC in stems asseasonal drought developed (Mooney et al. 1992).Similarly, Tissue and Wright (1995) report for ever-green Psychotria species maximum NSC early in thedry season. Young trees of five agro-forestry tree spe-cies in Nigeria showed a dry season maximum and awet season minimum of NSC in stems (Latt et al.2001). In the forest where the present survey wasconducted, two fast growing, distinctly