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A new look at computation of the complexity index in mangroves: do disturbed forests have clues to analyze canopy height patchiness?

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... 1997). Dagua faces strong pressure by felling of trees (Blanco and Cantera ... completely removed. Maximum and modal complexity indices were similar only in Aguacate 1, San Antonio 5 and Punta Soldado 25. The first ...
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  Wetlands Ecology and Management   9:  91–101, 2001.© 2001  Kluwer Academic Publishers. Printed in the Netherlands.  91 A new look at computation of the complexity index in mangroves: dodisturbed forests have clues to analyze canopy height patchiness? ∗ Juan F. Blanco † , Adriana C. Bejarano, Jairo Lasso & Jaime R. Cantera Centro de Investigaciones Marinas y Estuarinas (CIME), Departamento de Biolog´ıa, Facultad de Ciencias, Uni-versidad del Valle, A. A. 25360, Cali, Colombia; E-mail: jfblanco@mafalda.univalle.edu.co † Current Address: Graduate Program of Biology, University of Puerto Rico, PO Box 23360, SJ, PR 00931-3360 Received November 1998; accepted in revised form December 1999 Key words:  Forest structure, stressors, tropical brackish swamps Abstract This paper proposes some guidelines to compute complexity index in those mangroves where either seasonal orstrong disturbanceshave occurred.We surveyed31 mangrovelocalities in BuenaventuraBay, Central Pacific Coastof Colombia, where structural parameters were measured within a 0.1 ha plot. Also, most likely disturbances werenoted for each plot. Complexity index was calculated in its classical form using the arithmetic mean of the threetallest trees (maximal mean) and alternatively using: a) the total mean: the height average of all trees recorded ineach plot; and b) the mode: the most frequent tree height class (10 cm intervals) within each plot. Afterwards,we compared the three computations and discussed the reliability of each one according to the current state byplot. In addition, all structural parameters were sorted in two diameter at breast height (dbh) cohorts (2.5–10 cmand  ≥ 10 cm) to figure out which contributes more to the forest structure. We conclude the following: (a) Themean of the three tallest trees is not a good estimator of forest development when seasonal or strong disturbancesoccur since complexity index based in it always overestimates forest structure. (b) Seasonal disturbances andrecruitment produce mosaic forests. The best estimator of this condition is the mean height which encompassesboth central tendency and variability. (c) The modal height is also helpful to establish the dominant cohort whenforests show two or more storied-canopies, or intermediate cohorts are missing. It also applies when a new stock of recruits is entering in a mature forest (the modal or maximal heights can be used interchangeably in this case).(d) Maximal height is the best estimator for uniformly developed forests with closed canopies and/or a singledominant tall-cohort. (e) If one is not confident about which height type to include to compute the complexityindex, we recommend to sort out structure data by dbh-cohorts and calculate indices for both of them. This willshow which cohort is contributing most to forest complexity.Finally, we suggest to exclude non-mangrovespeciesfrom the complexity index computation since they mostly do not contribute significantly to forest basal area. Introduction It has beena frequentconcernforbotaniststoestablisha reliable algorithm representing spatial structure anddevelopment of forests. Holdridge (1967) proposedwhat he called a complexity index (CI) that integratedfloristic and structural features of terrestrial forests. Itis computed as follows: CI   = (s)(d)(b)(h) 10 − 3 (1) ∗ CIME contribution No. 66 where (s) is number of species, (d) stand density,(b) basal area, and (h) height. Afterwards, it wasimplemented by Lugo and Cintrón (1975) to meas-ure mangrove forest structure and development. Treessmaller than 2.5 cm diameter at breast height are notincluded. Utilization of this index became popular infurther studies of mangrove vegetation (Pool et al.,1977).Nevertheless, mangrove forests are usually mosa-ics of different tree cohorts as product of differentsuccessional stages within the stand. This situation  92brought Lugo (1980) to discuss whether mangrovesare successional or steady-state. He proposed that thedifferent mangrove settings result from different pro-cesses of succession due to the geomorphology andhydrology of the areas where they develop. Cintrónand Schaeffer-Novelli (1983) noticed this and triedto incorporate such phenomenon into the complex-ity index. They suggested that forest height must bethe arithmetic mean of the forest’s three tallest treeswhereas forests have non-uniformcanopies.Afterwards, Jiménez and Soto (1985) introducedtwo calculations of forest height, mean and maximal,realizingcanopyheightpatchinessinmangrovesunderdry environments. They actually found that maximalheightis greaterthanmeanheightbutthis differenceisslight for forest under humid climates. There is no fur-ther reference in the literature regarding measurementof canopy height patchiness of mangrove forest.Mangroves also are permanently stressed systemsdue to natural and human-induced sources (Lugo,1980; Lugo et al., 1980). Disturbances acting atvarious levels of energy pathways of this ecosys-tem drive a continuous process of tree mortality andseedling/sapling colonization not necessarily neitherbalanced nor continuous throughout the time (Lugoet al., 1980; Jiménez, 1990). Each topographic orphysiographic type of mangrove is sensitive to differ-ent stressors due to its givenspecies composition,geo-morphologyand hydrodynamics(Lugo,1980; Lugoetal., 1980). For example, mangrove types like fringe,bar-built (behind beach ridges) and overwashed islandare more disturbed than riverine or basin types, be-ing thus more successional (Lugo, 1980; Woodroffe,1983; Hatton and Couton, 1992). Moreover, climateis another factor determining mangrove seasonality. Itclearly controls water, nutrient and sediment balancewithin the forest (Cintrón et al., 1978). Therefore, cli-mate influences mangrove succession since it limitsenergy and matter inputs to the system conditioningreproductionto those seasons when nutrient and waterare more available (Jiménez, 1990). Forests under aridconditions(Poolet al., 1977; Cintrónet al., 1978;Sotoand Jiménez, 1982; Jiménez and Soto, 1985; Jiménez,1990) show marked phenologic cycles since nutrientsare restricted during most of the year. Then, repro-duction can occur only during rainy season or floods.Mortality is also seasonal. In contrast, forests underhumid climate with permanent freshwater and nutri-ent availability exhibit flowering throughout the year.However,mangroveforestsunderthisclimateoverridenewly settled seedling to remain latent in the under-story until they get the middle canopy or a light gap isopened. Consequently, all this variability in mangrovedevelopmentand succesion accordinglywith both nat-uralandman-induceddisturbancescanbeexpressedinterms of canopy height.This work is aimed at discussing how canopyheight patchiness in disturbed mangrove forests canbe represented within the algorithm of the complex-ity index and to propose some directions for easyestimations of stand structure and development. Methods The study was conducted at Buenaventura Bay, Cent-ral Pacific coast of Colombia (Figure 1), from 1990to 1994. The Buenaventura Bay is a drowned val-ley partially located on sediments from the Tertiary(sedimentary rocky cliffs and platforms) and the Qua-ternary (depositional platforms). Its central and in-terior parts show extensive mud flats, accumulat-ing organic matter, where riverine and fringe man-groves have developed. Most common species are  Rhizophora mangle  (Linnee),  R. racemosa  (Benthan,Hooker, Hutchinson),  Avicennia germinans  (Jacques),  Laguncularia racemosa  (Linnee) Gaertn and  Pellici-era rhizophorae  (Triana and Planchón). Tides aresemidiurnal and tidal amplitude is very wide, rangingaround 3 m vertically (Cantera, 1991). Air temper-ature is nearly steady throughout the year (25.6  ◦ C).Climate is very humid and rainfall is over 6000 mmyr − 1 though values of 8000 mm yr − 1 have been re-corded. Rainy season comprises August to November(835 mm) while lesser rainfall is from January toMarch (320 mm). Freshwater discharge accounts fora mean of 427 m 3 s − 1 contributedby three main rivers(Cantera, 1991). Seawater movingback and forth withtides is about 1254 m 3 s − 1 .We studied 31 localities of mangrove forest withinthe bay (Figure 1). Each one consisted of a 0.1 ha(100 × 10 m) plot that was subdivided into 10 subpar-cels whereall individualsgreaterthan 2.5cm diameterat breast height (dbh) were measured and recorded.The measured parameters were a) diameter at breastheight (dbh), b) tree height (h), c) specific density (d)and, d) basal area (b) (Cintrón and Schaeffer-Novelli,1983).Afterwards,Holdridge’scomplexityindexwas cal-culated in its classical form: using the arithmetic meanof the three tallest trees (hereinafterwe call it maximalmean) in each plot (Cintrón and Schaeffer-Novelli,  93 Figure 1.  Location of sampling plots (diamonds and numbers) for forest structure at Buenaventura Bay, Pacific Coast of Colombia. Mangrovesare uniformly distributed along the southeastern coastline. Dotted areas are sand banks exposed during low tide. 1983; Pool et al., 1977). Alternatively, complexityindex was computed based on two additional heighttypes: a) total mean: the average of all trees recor-ded in each plot; and b) mode: the most frequenttree height class within each plot. We sorted treeheights into 10 cm classes since it have been determ-ined that sapling-shoot and propagule annual growthranges from 10 cm under canopy shade to 15–20 cmundersun-exposure(Jiménez, 1990;DukeandPinzón,1992). The physiographic type, and the most likelynatural and anthropic disturbances were also recordedfor each mangrove forest. Why mean and modal heights? The idea of a complexity index is to enclose currentforest state, mean condition or steady-state in a singlevalue. Nevertheless, complexity index based on thethree tallest trees is just representing the maximumforest development, commonly, in absence of any dis-turbance. However, as mangrove forests are actuallyvery dynamic and successional (Lugo, 1980), one canfind a wide range of tree heights in a forest. A givenparameter must represent both height central tendencyand variance. The mean of the three tallest trees isclearly unreal because they occur in the upper side of the height distribution, assuming normality. The total  94 Figure 2.  Sketch of idealized mangrove height profiles and how current condition (both central trend and variability) can be expressed by astatistic. A. Maximal height, B. Modal height, and C. Mean height. Arrows show the approximate position of the statistic. mean could be a good parameter for forests in inter-mediate successional stages where height distributionis nearly normal (Figure 2c), but it must be borne inmind that it is verysensitive to extremevalues. Forestsstrongly disturbed by clear-cutting, where most of thetallest trees have been removed, and the standing-cropis being recolonized by saplings, can be representativeof this situation. The mean cannot be applied if anintermediate cohort is absent (Figure 2b).Unlike mean, modal height is not biased by ex-treme values and it could be useful for disturbedforestwhere a dominant cohort has been removed and a newone entered (Figure 2b). However, if felling of treesis strong or tree height is very heterogeneous, modalintervals or classes must be constructed as explainedin the methods section. If height-class frequencies arelow we recommend to discard this parameter.  95One would expect that complexity index computedby using maximal mean, modal and total mean heightscoincides as forests approach to climax or steady-state. The stronger the disturbances, the bigger thedifferences between maximal mean and the other twoestimators.We have identified some additional weaknesses incomputing complexity index. Computation of com-plexity index by sorting data in two cohorts ( ≥ 2.5 cmand  ≥ 10 cm) is widespread in literature (see reviewby Cintrón and Schaeffer-Novelli, 1983). Neverthe-less, tree height has been commonly considered asa single value for the forest and used indistinctly tocompute both  ≥ 2.5 cm and  ≥ 10 cm complexity in-dices. It clearlymisestimates bothcohorts’complexityindex since mean tree height is not being calculatedfor each one. All parameters must be calculated on thebasis of the same cohort. To achieve a more realisticestimation complexity index can be computed in twoways: a) to sort out data by tree cohort or, b) to poolall cohort data. We consider both are accurate, each,with specific biological significance. Pooled complex-ity indexis helpfulforenclosingwholeforeststructureand development information in a single value, whilesorted complexity index is more informational aboutcontribution of each cohort to forest. Furthermore, thecomputation method must always be stated in order tofacilitate data comparisons among researchers and toprevent misinterpretations. Results and discussion General remarks Structure data and complexity indices for the studiedlocalities are summarized in Table 1. When compar-ing classical calculation of complexity index amongour study localities we found some contrasting res-ults. Unlike those previously reported in literature(Lugo and Cintrón 1975, Pool et al., 1977; Cintrónand Schaeffer-Novelli,1983; Jiménez and Soto, 1985)riverine mangroves are not the more complex. Unex-pectedly, mangroves behind beach ridges (bar-built)(mean ±  s.d., 55.7 ±  66.3) were more complex thanriverine (19.8 ± 20.7). It can be explained by the factthat riverine mangroves are the most exploited by tim-ber extraction for fire wood and poles (Cantera, 1991;Prahl et al., 1990; Alvarez-León, 1993; Blanco andCantera, 1995; Cantera and Arnaud, 1997). Daguafaces strong pressure by felling of trees (Blanco andCantera, 1995) while Veneno and Anchicaya remainslightly disturbed (Cantera, 1991; Lasso and Cantera,1995). Target size is usually dbh ≥ 10 cm, thus forestsabruptly become smaller and younger.In our study, the tallest trees in bar-built man-groves are not commonly disturbed either by naturalor anthropogenic factors. Thus, the oldest cohort stillcontributes to forest structure and complexity. It iscommon to find the tallest trees in the inward forest,protected from waves and storms. One of the mostnoticeable features of these forests is that sedimenttrapping promoted steady seaward mangroveprograd-ation and shoaling of the canal between mangroveandridge.Woodroffe(1982,1983)foundthe samein man-groves of Cayman Islands. The other mangrove typecomplexityindicesareconsistentwithdataofpreviousworks (see review by Cintrón and Schaeffer-Novelli,1983).  Modal and mean versus maximal height  Classical computation of complexity index (i.e. basedin maximal height) was always greater than calcula-tions based on mode and total mean (Table 1). Thisreflects that maximal height is considerably higherthan mode and total mean in most of the cases.Only four of a total of 31 localities (namely Dagua12; Anchicaya 27, 28; Pajaro 31) showed similarcomplexity index as any height type was used forcalculations. Although, they belong to different topo-graphic types, the three computations of complexityindex were similar and low (excepting Anchicaya, alittle higher). This may be due to the low developmentof some forests (i.e. they are in early successionalstages) because either a natural colonization or a re-covery after a strong stressor had taken place (Dagua12; Pajaro 31). It seems that a past clear-cut took off a great number of tall trees (as witnessed by apatchy distribution of the invasive fern  Acrostichumaureum ) followed by a sudden or mass recolonizationas suggested by the similar values of modal and meanheights. Forests seem to be resized (Figure 2b).A second group corresponds to those localities ex-hibiting similar modal and mean complexity indices(as a direct consequence of likely modal and meanheights) (Aguacate 2; San Antonio 3, 4; Dagua 6,8, 9, 10, 11; Veneno 15; Punta Soldado 18, 19, 20,21, 22, 23; Anchicaya 26, 29, 30). Modal and meancomplexity indices were markedly lower (approxim-ately one half) than the classical form of computing.In these localities, tree height is normally distributed
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