Chapter 2 ABOVEGROUND BIOMASS INVESTMENTS AND LIGHT INTERCEPTION OF TROPICAL FOREST TREES AND LIANAS EARLY IN SUCCESSION

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1 Chpter 2 ABOVEGROUND BIOMASS INVESTMENTS AND LIGHT INTERCEPTION OF TROPICAL FOREST TREES AND LIANAS EARLY IN SUCCESSION with Niels Anten, Roelof Oomen, Mrc Mtthies nd Mrinus Werger Abstrct Bckground nd ims We relted crown structure nd boveground biomss investment to light interception of trees nd lins growing in six month old regenerting forest. Methods The verticl distribution of totl boveground biomss, height, dimeter, stem density, lef ngles nd crown depth were mesured for individul plnts of three short-lived pioneers (SLPs), four long-lived pioneers (LLPs) nd three lins. Dily light interception per individul (Φ d ) ws clculted with cnopy model. The model ws then used to estimte light interception per unit of lef mss (Φ lef mss ), totl boveground mss (Φ mss ) nd crown structure efficiency (E, the rtio of bsorbed vs. vilble light). Key results The SLPs Trem nd Ochrom intercepted higher mounts of light per unit lef mss (Φ lef mss ) becuse they hd shllower crowns, resulting in higher crown use efficiency (E ) thn the other species. These SLPs (but not Cecropi) were lso tller nd intercepted more light per unit lef re (Φ re ). LLPs nd lins hd considerbly higher mounts of lef mss nd re per unit boveground mss (LMR nd LAR, respectively) nd thus ttined Φ mss vlues similr to the SLPs (Φ mss = Φ re * LAR). Lins, which were mostly self-supporting, hd light interception efficiencies similr to those of the trees. Conclusions These results show how, due to the trde-off between crown structure nd biomss lloction, SLPs, nd LLPs nd lins intercept similr mount of light per unit mss which my contribute to the bility of the ltter two groups to persist. 17

2 Key words: Bolivi, cnopy model, crown structure, lef mss rtio, lins, light interception, pioneers, specific lef re, tropicl forest 18

3 INTRODUCTION Abndoned griculturl fields in tropicl forest res re typiclly colonized by pioneer trees nd lins. Along the spectrum of life histories nd ecologicl requirements pioneer trees re brodly clssified into short nd long-lived species (Whitmore, 1989; Finegn, 1996). In fllowed fields short-lived pioneer species rpidly develop closed cnopy nd dominte for bout 10 to 30 yers, with their pek in bundnce during the first 2 to 4 yers of succession (Peñ-Clros, 2003). Long-lived pioneers re lredy present in the first yer of succession but initilly occupy lower lyers of the cnopy nd become dominnt lter on (Finegn, 1996; Peñ-Clros, 2003). While lins lso occur in mture forest, their pek in bundnce is lso erly in succession (De Wlt et l., 2000; Schnitzer & Bongers, 2002; Gerwing, 2004). Interestingly, most lins go through self-supporting seedling phse (Putz, 1984; Cblle, 1998), lthough the height ttined by selfsupporting individuls vries mong species (Putz, 1984). But, how does the seedling growth hbit of lins enble them to compete with trees? Moreover, wht chrcteristics enble short-lived pioneer trees to chieve erly dominnce nd how re long-lived pioneers ble to persist below the short-lived ones erly in succession? During the erly stges of secondry forest succession, stnding biomss rpidly increses. Given this condition, it follows tht strong verticl light grdient is creted, long which some trees grow in the shde of others. Competing for light probbly plys n importnt role in determining the course of succession (Werger et l., 2002). Mny studies hve compred light requirement for growth nd survivl between pioneers nd lte-successionl shde-tolernt species (Pomp & Bongers, 1988; Venekls & Poorter, 1998; Chzdon, 1992; King, 1994; Kitjim, 1994; Vlldres et l., 2000), but reltively few hve compred this between different pioneer species (but see Peñ-Clros, 2001; Dlling, et l. 2004; Poorter et l., 2006). Most studies tht ttempted to relte individul trits of different tropicl forest tree species to their bility to cpture light, focused on interspecific differences in crown chrcteristics nd lef morphologies (King, 1994; Kitjim, 1994; Vlldres, 2002; Flster et l., 2003; Poorter, 1999; Poorter et l., 2006). Crown structure is obviously n importnt determinnt of light cpture (Horn, 1971) nd tropicl species show lrge vrition in lef morphology nd crown structure (Bongers & Popm, 1990; Vlldres, 2002; Poorter et l., 2006). Bsed on the concept of optiml crown structure for mximum light cpture severl reserchers predicted tht shde-tolernt trees should hve brod crowns with little lef overlp to minimize self-shding, wheres light-demnding trees should hve nrrow crowns with numerous lef lyers (Horn, 1971; Kohym, 1987; King, 1990). These predictions hve not been confirmed in the field; generlly crowns of shde-tolernt trees were deeper with more lef lyers thn those of the pioneer trees (e.g. Poorter, 1998; Sterck et l., 2001; Kitjim et l., 2005). The production of brod crown with miniml self-shding while mintining n efficient ngulr lef disply with respect to the previling light direction requires substntil investment in support 19

4 (dditionl brnches), which might be prohibitively expensive for trees growing under shded conditions (Vlldres et l., 2002). For light-demnding species, investing biomss for height growth improves ccess to light, but comes t the expense of folige nd brnch growth nd my require continuous remobiliztion of resources from older to younger leves (Smith, 1982; Gilbert et l., 2001; King, 1994). For these resons, such species would be expected to hve shllower crowns. Models tht include only crown geometry or lef morphology re pprently not ble to predict species competitive interctions in tropicl forests nd thus there is need for more integrted pproch tht considers biomss expenditure. Hirose & Werger (1995) developed such n pproch tht reltes biomss lloction ptterns nd crown structure with light interception nd then pplied this method to dense temperte grsslnd vegettion (Hirose & Werger, 1995). They clculted tht tll dominnt species bsorbed more light per unit of lef re (Φ re, the light cptured per unit of lef re) thn subordinte ones. Surprisingly, the mount of light cptured per unit of boveground mss (Φ mss ) by subordinte species ws similr or higher thn tht of the tll dominnt species. With this pproch they demonstrted tht during succession in grsslnds, erly dominnce is closely ssocited with high rtes of stem elongtion nd internode length, while persistence t low irrdince ws ssocited with high SLA nd comprtively high Φ mss (Werger et l., 2002; Hirose, 2005; Anten, 2005). As noted, these studies were conducted in grsslnds where boveground mss ccumulted for only one seson. In contrst, in woody vegettion, boveground mss ccumultes continuously for mny yers. Trde-offs relted to crown structure nd biomss investments for light interception tht enbles some species to ttin dominnce nd to others to coexist lower in the cnopy my therefore be different in herbceous thn in woody vegettion. We predict tht short-lived pioneer trees possess biomss lloction trits tht fcilitte high dily light interception per unit mss (Φ mss ) reltive to long-lived pioneers. The lins, while self-supporting very erly in succession, will eventully strt climbing relying on other plnts for support. Consequently, they my not hve to invest s much in durble support structure s trees. We predict tht they will llocte more mss to leves thus chieving higher Φ mss vlues. To test these hypotheses we modified grsslnd cnopy model (Hirose & Werger, 1995; Anten & Hirose, 1999), to incorporte specific fetures of forest trees. In combintion with field mesurements this model enbles us to relte interspecific differences in biomss lloction nd crown structure to light interception. We focus on very young stnd (6 months since cesstion of griculturl ctivity) becuse t this stge individuls of the three different groups re still not very different in size, nd smll differences in light cpture nd height growth gretly influence species composition nd size hierrchies in the subsequent 5 to 10 yers. 20

5 MATERIALS AND METHODS Study site nd plnt mteril We studied secondry forest stnd growing ner Riberlt, in the Bolivin Amzon (11 S 66.1 W). The re ws slshed nd burned, cropped with rice, mize nd cssv in sequence of three yers nd then bndoned. We conducted the study six months fter lnd bndonment. A plot of pproximtely 2/3 of h ws selected for the study. The plot ws locted t distnce of t lest 20 m. from the edge of the stnd to void the influence of the surrounding vegettion. The re ws surrounded by old growth forest. In the regrowing vegettion, which formed very homogenous cnopy, ten of the most common species were selected bsed on previous study on species diversity nd bundnce long chronosequence (Peñ-Clros 2003; Sely unpublished dt) Trem micrnth, Ochrom pyrmidle nd Cecropi ficifoli re present from the time of lnd bndonment to 4-25 yers lter nd denoted s short-lived pioneers (SLPs). Courtri guinensis, Rinoreocrpus uleii, Pseudolmedi levis nd Brosimum lctescens re found from lnd bndonment to old growth forest, with pek in bundnce between 30 to 100 yers, nd thus we refer to those s long-lived pioneers (LLPs). The lins Uncri guinensis, Hippocrtecee sp. nd Bignonicee sp. re present from erly stges of succession nd persist till the old growth forest nd in this study they re treted s seprte group due to the climbing growth hbit they develop lter in life. In our field however lmost ll lin individuls were still self-supporting. Herefter species re nmed by generic (or fmily) nmes. We selected individuls of different heights per species such tht they covered the height rnge with which ech species occurred in the 6 month old stnd. All individuls hd grown from seed. We crefully voided resprouts s these my hve different crbon blnce thn seedlings. Cnopy structure nd light (PPFD) distribution Cnopy structure nd light distribution were determined in October, t the beginning of the riny seson. The min plot ws subdivided in 63 subplots of 9 m 2 ech. Individuls were selected inside the subplots. Photosynthetic Photon Flux Density (PPFD, nm) ws mesured t verticl increments of 25 cm in ech subplot tht contined t lest one of the selected individuls. An SF 80 Line Sensor (Decgon devices Ltd. UK) ws used to mesure the PPFD in the cnopy nd simultneous mesurements of PPFD bove the cnopy were tken with point Li- 190 SA Quntum sensor (LiCor NE, U.S.A), connected to dt logger LI1000 (LiCor NE, USA). Averge lef re index (LAI, m 2 m -2 ) nd verge lef ngle distribution in ech subplot were estimted with LAI-2000 Plnt Cnopy Anlyzer (LiCor NE, USA). An bove-cnopy mesurement followed by four below-cnopy mesurement, viewing from ech subplot corner to the centre ws tken. A view cp of 45º ws used to restrict the lens field of view. PPFD nd LAI mesurements were tken under n overcst sky or t sunset. The verticl distribution of leves in the cnopy ws mesured using the point method, lifting scled pole from the bottom to the top of the cnopy nd recording the height t which the tip of the pole touched 21

6 lef. The procedure ws repeted t the centre of every squre meter of the 9 m 2 subplot, for nine replictes per subplot. Stem llometry, crown structure nd boveground biomss lloction Totl height, height to the first lef or brnch with leves, stem dimeter t 30 nd 130 cm height were mesured. When individuls were <30 cm tll, stem dimeter ws mesured t 10 cm. Individuls were strtified into horizontl lyers of 25 cm nd the inclintion ngles of five rndomly selected leves were mesured in ech lyer using hnd-held protrctor. The distribution of the boveground biomss ws determined by destructive hrvesting. Individuls were hrvested nd clipped into 25 cm height segments. Stems, brnches, petioles, nd leves were put seprtely in plstic bgs. Digitl photogrphs of representtive smple of leves were tken to obtin lef re of the individul. Lef re ws clculted using the Sigm Scn Pro 5 (SPSS Inc). Fresh mteril ws oven dried t 70ºC for bout 5 dys nd weighed to obtin dry mss. For ech individul the stem mss rtio (SMR, stem mss per boveground mss, g g -1 ) nd stem density (dry mss per volume, in g cm -3 ) were clculted. Stem density ws estimted for segments tht rnged between 30 to100 cm in length. The volume ws clculted s 0.25D^²πL, where L is the segment length nd D the dimeter mesured t the middle of the segment. Crown depth (frction of totl length with leves, in %) ws clculted. Lef mss rtio (LMR, lef mss per boveground mss, in g g 1 ), specific lef re (SLA, lef re per lef mss, in cm 2 g -1 ) nd lef re rtio (LAR, lef re per boveground mss, in cm 2 g -1 ) were lso clculted. Model The model works with 9 m 2 subplots to ccount for the vegettion heterogeneity nd divides the subplots into 25 cm horizontl lyers i. The subplots contin individul plnts of the selected species. Two illumintion clsses re distinguished: shded nd sunlit lef re (Depury & Frquhr, 1997). The PPFD (mol m -2 s -1 ) intercepted by the shded lef re of individul plnts in lyer i (I shp, i ) is given s: I shp, i Idifp, i + Isctp, i = (1) where, I difp,i nd I sctp,i re the diffuse-sky irrdince nd the scttered-bem irrdince (light scttered by leves in the cnopy), respectively. I difp,i nd I sctp,i cn be clculted using the pproximte exponentil expressions: I I difp, i sctp, i kdifveg, i αf cum ( γ) αk e kblveg, i αfcum kblveg, ifcum αkblp, i [( 1 γ) e αe ] = I (2) dif 1 difp, i = I (3) dir where, I dif is the diffuse PPFD bove the cnopy, k difp nd k difveg re the extinction coefficient of diffuse light of the trget plnt nd tht of the vegettion, respectively, 22

7 nd F cum the cumultive LAI bove the point in lyer i. Cnopy reflection γ is ssumed to be 5% nd the men lef bsorbnce α is ssumed to be 0.8 (Goudrin, 1977). In eqution (3) I dir is the bem PPFD bove the cnopy; k blp,i nd k blveg,i re the extinction coefficients for direct light of the plnt nd the vegettion, respectively. The model distinguishes between the extinction coefficient of the vegettion nd tht of individul plnts within the vegettion becuse they my differ in lef ngle distributions. Sunlit leves receive both direct-bem nd diffusesky irrdince. The light intercepted by single sunlit lef of n individul in cnopy lyer i (I slp,i ) is clculted s: I slp, i = Idirp, i + Ishp, i (4) I dirp, i = I dirkblp, iα (5) where, I dirp,i, is the direct bem, nd I shp,i is the diffuse light. I dir is the bem irrdince bove the cnopy, k blp,i is the coefficient of extinction of the plnt, nd α is the lef bsorbnce. The extinction coefficients of vegettion nd individuls were clculted following Goudrin (1988) nd using equtions 3-5 from Anten (1997). The light intercepted per unit of lef re in lyer i, (I p,i, mol s -1 ) is the sum of light intercepted by sunlit nd shded lef re integrted over the cumultive LAI from the top to the bottom of the lyer: I p, i Fcum, i Lp, i = + ( L i F cum, i 1 f slp, i * I slp, i + (1 f slp, i ) I shp,i ) df cum (6) f slp = e k blveg F cum (7) where, L p,i is the lef re of n individul plnt in lyer i nd L i is the totl lef re in tht lyer both in m 2, f slp,i is the frction of sunlit leves t certin depth in the cnopy nd I shp,i nd I slp,i re the light intensity on shded nd sunlit leves res respectively. The proportionl LAI (L p,i ) per 25 cm vegettion lyer ws clculted by multiplying the proportion of touched points in tht lyer (determined with the point method) with the totl LAI. To obtin the dily light cpture per lyer I dp,i (mol plnt -1 d -1 ), the vlue of I p,i is integrted over the dy from sunrise to sunset: 12 + dyl = p, 12 1 i dyl I I dt (8) dp, i where, dyl is the dy length on 15 th October, the medin dy of our hrvesting period nd ltitude of 11 S 66.1 W. The dyl nd the solr inclintion ngle re 23

8 clculted ccording to Gtes (1980). The totl dily light cpture per plnt Φ d is clculted s: Φ d Idp, i = (9) Light interception efficiencies We clculted the dily mount of light intercepted per unit of lef re (Φ re, ), lef mss (Φ lef mss ), nd totl boveground mss (Φ mss ) ll in mol g -1 dy -1 s: Φ d Φ re = (10) A Φ d Φ lef mss = (11) LM Φ Φ d mss = (12) M with A, LM, nd M the lef re, lef mss, nd totl boveground mss of plnt, respectively. Φ lef mss nd Φ mss cn be further expressed s: Φ = Φ *SLA, Φ = * LAR lef mss re mss Φ re (13) with SLA nd LAR specific lef re nd lef re rtio respectively. The reltionships between plnt height, crown structure nd biomss lloction on the one hnd nd light cpture per unit mss on the other cn be defined s: E = Φ I re H(height) (14) with E the crown efficiency of light bsorption the rtio of bsorbed vs. vilble light which is function of lef ngles nd lef re distributions (see Vlldres et l., 2002), I H(height) the totl dily irrdince on horizontl plne right bove the plnt. Note tht the list of the most importnt symbols is in Tble 1. 24

9 Tble 1. List of the most importnt symbols Vribles Definition Units LAI Lef re index m 2 m -2 SLA Specific lef re cm 2 g -1 LMR Lef mss rtio g g -1 LAR Lef re rtio cm 2 g -1 SMR Stem mss rtio g g -1 PPFD Photosyntheticlly photon flux density mol m -2 s -1 Ф d Light ttined by n individul plnt per dy mol plnt -1 dy -1 Ф re Light per lef re per dy mol cm -2 dy -1 Ф lef mss Light per lef mss per dy mol g -1 dy -1 Ф mss Light per totl boveground mss per dy mol g -1 dy -1 E Crown structure efficiency index mol mol -1 t the horizontl plne Sttistics The effect of species on height, biomss, dimeter, stem density, crown depth, Φ d,, Φ re, Φ lef mss nd Φ mss were tested with ANOVA. Vribles were log trnsformed, except LMR, to meet the ssumption of homogeneity of vrinces of the Levene s test (p<0.05).when the results of Levene s test were significnt fter dt trnsformtion, the ANOVA Welch test ws used. Pir-wise post hoc Sidk tests were used to test differences mong species nd Gmes-Howell tests for vribles with significnt differences in vrince. A second-order polynomil regression ws done to test the linerity of the reltionships between height, biomss, nd dimeter. ANCOVA ws pplied with height s the dependent vrible nd dimeter nd boveground biomss s covrites, nd n ANCOVA with LMR, LAR, Φ re or SLA s the dependent vrible nd either height or Φ re s the covrite with species s discrete fctor. The sme procedure ws pplied for Φ d,, Φ re, Φ lef mss or Φ mss s the dependent vrible nd either height or mss s covrites with species s discrete fctor. In the regression nlysis the choice of independent vs. dependent vrible ws bsed on cusl reltionships ssumed in the model. Height nd light interception were nlyzed s dependent vribles ginst mss or dimeter (in the cse of height) s dependent, to indicte how efficient given mount of mss is converted into height or used for light interception. LMR nd LAR were nlyzed ginst height, becuse we ssumed tht due to biomechnicl constrints, s plnt grow tller they hve to invest disproportionlly in support t the expenses of leves. 25

10 Finlly, SLA ws nlyzed reltive to Φ re becuse SLA it is usully negtively correlted with light intensity. RESULTS Cnopy structure nd light verticl distribution The vegettion ws on verge 1.8 m tll (mximum 2.25 m). The men lef re index (LAI) ws 1.63±0.05 nd tended to be concentrted between 0 nd 75 cm bove the ground (Fig. 1). Light (PPFD) vilbility t the forest floor ws bout 34 % nd incresed to 75 % t 75 cm bove ground level. Height (cm) Fig. 1. Men reltive LAI nd PPFD distribution s function of height (cm) of 63 subplots in 6 month old regenerting tropicl secondry forest stnd. Brs denote Stndrd Errors LAI PPFD PPFD reltive 26

11 Tble 2. Men plnt height, boveground mss, stem dimeter, stem density, crown depth, SLA, LMR, LAR, Φ d nd Φ re of short-lived (SLPs), longlived (LLPs) pioneer trees nd lins (L) in 6 month old secondry forest stnd. Significnce levels (p vlues) of the overll species effect re shown. Different letters indicte significnt differences (p < 0.05) between species pirs (test type indicted). 27 Species Group Height p <0.001 Plnt mss p <0.001Dimeter p <0.001 Wood density p <0.001 Crown depth p <0.001 n (cm) S.E. Sidk (g) S.E. Sidk (cm) S.E Sidk (g cm -3 ) S.E. Sidk (cm cm -1 ) S.E. G-H Trem SLPs bc bc bc b Ochrom SLPs c c Cecropi SLPs bcd bc b e c Courtri LLPs bc b cd bc Rinereocrpus LLPs bcd bc cde bc b Brosimum LLPs cd c e bc Pseudolmedi LLPs e d f b b Bignonicee sp L bc bc cde Hippocrtecee sp L bc bc cde Uncri L d bc de bc b Species Group SLA p <0.001 LMR p <0.001 LAR p <0.001 Ф d p <0.001 Ф re p <0.001 n (cm 2 g -1 ) S.E. Sidk (g g -1 ) S.E. Sidk (cm 2 g -1 ) S.E. G-H (mol d -1) S.E. G-H (mol cm -2 d -1) S.E. Sidk Trem SLPs b d d b Ochrom SLPs c d d Cecropi SLPs c d d bc b Courtri LLPs d b cd bc bc Rinereocrpus LLPs c b bc cd Brosimum LLPs c b bc c de Pseudolmedi LLPs b d f Bignonicee sp L d b d bc bc Hippocrtecee sp L d b d bc b Uncri L b b bc ef Chpter 2

12 The species differed in height, totl boveground biomss, stem dimeters, stem density nd crown depth (Tble 2). Two short-lived pioneers (SLPs) Trem nd Ochrom were on verge tller thn the other species, while Ochrom plnts hd the highest men boveground mss of ll species. Cecropi plnts were similr in height nd boveground mss to the long-lived pioneers (LLP) nd lins. Species height nd boveground mss, nd height nd stem dimeter were positively correlted (Fig. 2A nd 2B). Trem ttined the gretest height t given mss nd stem dimeter. The nlysis of covrince (ANCOVA) showed heterogeneity in the slopes of the reltionship between height nd mss wheres homogeneity of slopes ws found for the reltionship between height nd dimeter (Tble 3). All regressions of height nd mss by species were significnt except for Bignonicee sp (Tble 4). The qudrtic terms in the second order polynomil regression of height vs. dimeter nd height vs. mss ws not significnt (p vlues rnging from 0.07 to 1) indicting tht these reltionships were liner. The SLPs hd lower stem densities nd shllower crowns thn the other trees (Tble 2). LMR nd LAR vlues decresed with plnt height within species (Fig. 3A nd 3B). Species did not differ in the slopes of the reltionships between LMR nd height nd LAR nd height but they differed in intercepts. LMR vlues for given height were lower for SLPs thn the other species. LAR vlues for given height were lso lower for SLPs but the differences with the other species were not significnt (Tble 3) (Sidk t p<0.05). 250 A B Height (cm) Tr Oc Ce Co Ri Br Ps Bi Hi Un 0, Plnt mss (g) Dimeter (cm) x Tr Oc Ce Co Ri Br Ps Bi Hi Un Fig. 2. (A) Reltionships between height nd boveground mss nd (B) height nd dimeter. Lines denote species with significnt regression (p<0.05). Symbols denote species with no significnt correltion. Species re indicted s Tr (Trem), Oc (Ochrom), Ce (Cecropi) ll 3 short-lived pioneers, Co (Courtri), Ri (Rinereocrpus), Br (Brosimum), Ps (Pseudolmedi) ll 4 long-lived pioneers, Bi (Bignonicee sp.), Hi (Hippocrtecee sp.) nd Un (Uncri) ll 3 lins. Dt were log trnsformed. 28

13 Regressions per species of LMR nd LAR with plnt height were not significnt for 2 nd 3 out of the ten species, respectively (Tble 4). All three SLPs hd pproximtely two-fold lower LMR vlues thn the LLPs nd lins when compred t the sme height. LMR vlues were lso two-fold different between the LLPs. The men specific lef re of plnts (SLA) exhibited negtive correltion with the men light intensity per unit of lef re (Ф re ). However, t the species level SLA nd Ф re were only significntly correlted in Uncri (Tble 4). Most of the species held most of their leves horizontlly oriented (0 to 30 degrees) (dt not shown) (ANOVA F (9,138)= 2.68 p=0.007). Only Ochrom displyed hlf of its leves t ngles between the 30 nd 60 degrees. 1.0 A B LMR (g g -1 ) Tr Oc Ce Co Ri Br Height (cm) x Ps Bi Hi Un LAR (cm 2 g -1 ) Tr Oc Ce Co Ri x Br Ps Bi Hi Un Height (cm) Fig. 3. Regression lines per species of (A) plnt height with LMR (lef mss rtio), (B) plnt height nd LAR (lef re rtio). Lines denote significnt regression t p<0.05. Symbols denote species with no significnt correltion (see Fig. 2 for codes). Light (PPFD) interception The species differed in the bsolute dily mount of light cptured per individul plnt (Ф d ) nd per unit lef re (Ф re, Tble 2) with these vlues being higher for the two tllest species (Trem nd Ochrom) thn for the others. Ф d nd boveground mss were highly correlted (Tble 4). Species did not differ in the slopes of the reltionships between Ф d nd plnt mss, but they differed in the intercept with SLP (Trem) nd LLP (Rinereocrpus) intercepting more light for given mss thn the other species (Tble 4). The Ф re incresed with height (Fig. 4A). Species differed in the slopes of the reltionship between Ф re nd height with shorter individuls of SLPs hving higher Ф re thn the shorter individuls of the LLPs nd lins with the exception of Hippocrtece sp. nd Bignonicee sp. Species differed in the slopes of the reltionship Ф re nd in E nd height (Tble 3). 29

14 E decresed with height (Fig. 4B), but much less so in the SLPs thn in the other species. Consequently mong plnts > 0.5 m tll the SLPs hd the highest E vlues. Tble 3. Results of nlysis of covrince (ANCOVA) of plnt height, LMR (lef mss rtio), LAR (lef re rtio), Фd (bsolute light per dy), Фre (light per lef re per dy) nd E (crown efficiency index), SLA (specific lef re) s dependent vribles with plnt boveground mss, height, dimeter, nd Фre s covrites nd species s discrete fctor. n denotes not pplicble. Slope effect Intercept effec Dependent Covrite F vlues p p Height Mss F (9,140) = n Height Dimeter F (9,123) =1.45 ns <0.001 LMR Height F (9,140) =0.76 ns <0.001 LAR Height F (9,140 )=1.30 ns <0.001 Ф d Mss F (9,147) =1.70 ns <0.001 Ф re Height F (9,147 )= n E Height F (9,143 )=5.79 <0.001 n SLA Ф re F (9,147 )= n Φre (mol cm -2 d -1 ) A Tr Oc Ce Co Ri Br Ps Bi Hi Un Height (cm) Height (cm) Fig. 4. Regression lines per species of (A) plnt height nd Фre (light per lef re per dy) nd (B) E (Crown efficiency index) nd height. Lines denote significnt regression t p<0.05. Symbols denote species with no significnt correltion (see Fig. 2 for codes). E E (Crown efficiency index index mol.mol mol -1-1 ) ) B Tr Oc Ce Co Ri Br Ps Bi Hi Un The species differed with respect to light per lef mss Ф lef mss nd Ф mss. The SLPs Trem nd Ochrom ttined the highest Ф lef mss vlues of ll species 30

15 (ANOVA p<0.005, Tble 2). Pir-wise comprisons showed no differences in Ф mss between the SLPs nd LLPs nd lin species (Fig. 5). 0.6 Фlef mss Ф lef mss nd Ф mss (mol g -1 d -1 ) b c b c d e b c b c d e b c e b c e c Фmss c d b c d e b c 0 Tr Oc Ce Co Ri Br Ps Bi Hi Un SLPs LLPs L Fig. 5. Men instntneous Фlef mss (light per lef mss per dy) solid brs nd Фmss (light per boveground mss per dy) open brs of species (see Fig. 1 for codes). Brs denote Stndrd Errors. Similr letters denote no significnt differences between species t p<0.05. SLPs, LLPs nd L denote short-lived, long-lived pioneers nd lins respectively. DISCUSSION The prediction tht short-lived pioneers (SLPs) possess morphologicl trits tht fcilitte high light interception per unit mss ws not clerly supported by our dt. Per dy, the SLPs Trem nd Ochrom intercepted more light per unit lef mss (Φ lef mss ), but not in terms of (Φ mss ) thn the other species. The LLPs nd lins intercepted similr mount of light per unit of totl boveground mss (Φ mss ) thn SLPs. 31

16 Tble 4. Coefficients of the llometric reltionships between plnt height nd boveground mss, height nd dimeter, LMR nd height, LAR nd height, nd Фd nd height nd SLA nd Фre. p vlues of the regression nlyses re shown. r 2 denotes correltion coefficient. SLPs, LLPs nd L denote short-lived, long-lived pioneers nd lins respectively. 32 Species Group Height vs mss Height vs dimeter LMR vs Height LAR vs Height Constnt () Slope (b) p vlue r 2 Constnt () Slope (b) p vlue r 2 Constnt () Slope (b) p vlue r 2 Constnt () Slope (b) p vlue r 2 Trem SLPs < < Ochrom SLPs < < Cecropi SLPs < Courtri LLPs < < < Rinereocrpus LLPs < Brosimum LLPs < < < Pseudolmedi LLPs < Bignonicee sp. L Hippocrtecee sp. L Uncri L < < < Species Group Ф d vs mss Ф re vs height SLA vs Ф re Constnt () Slope (b) p vlue r 2 Constnt () Slope (b) p vlue r 2 Constnt () Slope (b) p vlue r 2 Trem SLPs < Ochrom SLPs < < Cecropi SLPs < Courtri LLPs < Rinereocrpus LLPs < Brosimum LLPs < Pseudolmedi LLPs < Bignonicee sp. L < Hippocrtecee sp. L < Uncri L < Chpter 2

17 High Φ lef mss nd Φ mss cn result from being tll or by hving n efficient crown structure (E). In this wy plnts cn position their leves fvourbly reltive to the light climte in the cnopy relizing high light interception per unit lef re (Φ re ) (Vlldres et l., 2002). In our study differences in Φ re between the tllest SLPs (Trem nd Ochrom), nd the other species lower in the cnopy were significnt but smll. This result contrsts with those reported for herbceous stnds (2-5 fold) (Hirose & Werger, 1995; Anten & Hirose, 1999; Werger et l., 2002). This is probbly becuse in the very young stnd we studied here, height differences were still smll nd the LAI ws low (1.6) so tht reltively lrge mount of light penetrted deep into the cnopy (Fig. 1). Tll stture cn be chieved through n efficient conversion of biomss to height growth by producing thin stems mde of low density wood. With the exception of Trem, the SLPs however, did not chieve greter height for given mss thn trees from the other groups. Trem plnts lso hd the most slender stems for their height of ll species in the study. Slender stems fcilitte rpid height growth but lso imply reduced mechnicl stbility (Putz et l., 1983 Nikls, 1992; King, 1994; Montgomery & Chzdon, 2001). It is possible tht the ltter issue is not mjor problem for Trem, becuse in dense vegettion, trees of this species hve short life spns, nd therefore do not need to invest s much in mechnicl stbility s the longer-lived Ochrom nd Cecropi. Observtions in our study re showed tht Trem ws not common in plots > 4 yers old wheres Ochrom nd Cecropi were found even fter 25 to 30 yers of succession. The SLPs hd higher crown efficiencies for light cpture (E ) thn the other species, t lest when compring mong individuls > 0.5 m tll. As lef inclintion ngles did not differ significntly between the groups, this greter E ws minly due to the fct the SLPs hd shllower crowns with leves being concentrted towrds the top of the plnt where they re fvourbly positioned reltive to the light climte. These differences in E between SLPs nd the other species probbly reflect their different light requirements (Vlldres et l., 2002). The SLPs, being shde intolernt, need to continuously produce new leves t the top of the cnopy to prevent being shded by neighbours. Older leves re dropped probbly to provide resources for new lef production nd becuse once they re shded their net photosynthetic contribution to the plnt is smll. By contrst, this continuous redeployment of resources is cost tht shde-tolernt plnts cnnot fford (King, 1994; Poorter & Werger, 1999; Vlldres et l., 2002). Keeping this rgument, we observed tht SLPs hd considerbly shorter lef longevities thn other species. Aprt from mintining high Φ re, efficient cquisition of light cn lso be chieved through high lloction of mss to leves nd the formtion of thin leves with high SLA, leding to lrge lef re per unit of plnt mss (LAR; Hirose & Werger, 1995). Previous studies report tht pioneers hve higher lef mss rtio (LMR) thn shde tolernt species especilly under shded conditions (see Poorter, 2005). In contrst to these studies, we found short-lived pioneers to hve much lower men LMR vlues thn the other tree groups lso when LMR ws compred between plnts of the sme height (see lso Sterck et l., 2001). This discrepncy in 33

18 the LMR dt is probbly relted to the fct tht previous studies (Poorter, 1999; 2005) were conducted with plnts grown in grden experiment t reltively low density while our study ws conducted on plnts growing in dense nturl vegettion. Plnts typiclly respond to the close proximity of neighbours by incresing mss investment to height growth (Smith, 1982; Anten & Hirose, 1998; Poorter, 1998), but the mgnitude of this response cn differ considerbly between species. Among both temperte nd tropicl forest trees erly successionl species hve been observed to exhibit greter responses to crowding thn lte successionl ones (Kitjim, 1994; Gilbert et l., 2001). Tller stture is usully ssocited with lower lef lloction becuse in order to mintin mechnicl stbility, tller plnts hve to invest disproportionte mounts of mss in stems for support (Nikls, 1992; Anten & Hirose 1998). However, low LMR cn lso be the result of low lef longevity (King, 1994; Clveu et l., 2005). As noted bove the progressive production of leves t the top of the cnopy by SLPs nd ssocited redeployment of resources cn result in reduced lef longevity. If the SLPs did not hve higher efficiency of biomss use for light cpture, re there other chrcteristics tht my contribute to their erly competitive dvntge? Grden experiments hve shown tht under reltively high light conditions, the differences in growth rtes of pioneers reltive to other successionl ctegories is more closely ssocited with growth per unit of lef re (NAR) thn with LAR (Poorter, 1999). The former, in turn, is strongly determined by physiologicl trits such s lef photosynthetic cpcity (P mx ). It hs indeed been shown tht the SLPs hve higher P mx vlues thn lter successionl species (Ellsworth & Reich, 1996). A study on grsslnds (Anten & Hirose, 2003) reveled tht the competitive dvntge of certin species in dense vegettion re ssocited not only with morphologicl trits tht fcilitte high Φ mss, like those studied here, but lso with physiologicl ones tht enble plnts to efficiently use bsorbed light for growth. It should be noted tht our nlysis is bsed on boveground mss nd tht interspecific differences in root mss frctions were not ccounted for. This difference my not be severe given tht studies of erly pioneers nd intermedite species t medium nd high irrdince levels showed tht the two groups do not differ in root mss frction (Venekls & Poorter, 1998). The hypothesis tht lins, owing to the fct they (will eventully) climb, need to llocte less biomss for support nd cn therefore use more resources to produce leves or for dditionl height growth tht results in higher Φ mss ws not supported by our dt. Φ mss nd LMR vlues of lins were not different from those of the tree species. We lso found tht self-supporting, young lins hd stem densities tht were similr to similrly sized tree seedlings. It ws further reported tht the trnsition from the self-supporting to climbing growth is ssocited with substntil chnges in wood ntomy (Gllenmüller et l., 2004) nd other wood properties (J. Putz, pers. comm.). Hence, it is possible tht becuse lins go through self-supporting seedling phse, very erly in succession they re not very different from trees in terms of mss lloction nd efficiency of light cpture, but my become more efficient once they hve strted climbing. 34

19 In conclusion, during the first yer of succession crown structure nd morphologicl chrcteristics of the SLPs did not result in greter efficiency of biomss use for light cpture (Φ mss ) compred to the LLPs nor were the lins more efficient thn the trees. While the SLPs were tller nd hd shllower crowns with less self-shding resulting in higher crown disply efficiencies (E ), the LLPs nd lins exhibited lrger lef mss nd re rtios. Thus, due to the trde-off between crown structure nd biomss lloction SLPs, LLPs nd lins intercept similr mount of light per unit mss which my contribute the bility of the ltter two groups to persist. ACKNOWLEDGMENTS We thnk Rene Boot nd Jck Putz for their vluble suggestions on the mnuscript. We re lso indebted to Ofeli L Fuente nd Kti Cuellr (Universidd Técnic del Beni) nd to Adhemr Sucedo, Luis Apz, Nzreno Mrtinez, Rene Armyo, Nico Divico, Miguel Cudiy, Victor Agud, Gbriel Ricldi nd Jorge Okit (PROMAB) for their ssistnce. Edurdo Chono kindly llowed us to use his plot. 35

20 36 Chpter 2