Flooding and profuse flowering result in high litterfall in novel Spathodea campanulata forests in northern Puerto Rico
2011; Wiley; Volume: 2; Issue: 9 Linguagem: Inglês
10.1890/es11-00165.1
ISSN2150-8925
Autores Tópico(s)Forest ecology and management
ResumoEcosphereVolume 2, Issue 9 art105 p. 1-25 ArticleOpen Access Flooding and profuse flowering result in high litterfall in novel Spathodea campanulata forests in northern Puerto Rico Oscar J. Abelleira Martínez, Corresponding Author Oscar J. Abelleira Martínez International Institute of Tropical Forestry, United States Department of Agriculture, Forest Service, 1201 Calle Ceiba, Jardín Botánico Sur, Río Piedras, Puerto Rico 00926 and Departamento de Biología, Universidad de Puerto Rico, Río Piedras, Puerto Rico 00931 Present address: College of Natural Resources, University of Idaho, Moscow, Idaho 83843 USA.E-mail:ojabelleira@gmail.comSearch for more papers by this author Oscar J. Abelleira Martínez, Corresponding Author Oscar J. Abelleira Martínez International Institute of Tropical Forestry, United States Department of Agriculture, Forest Service, 1201 Calle Ceiba, Jardín Botánico Sur, Río Piedras, Puerto Rico 00926 and Departamento de Biología, Universidad de Puerto Rico, Río Piedras, Puerto Rico 00931 Present address: College of Natural Resources, University of Idaho, Moscow, Idaho 83843 USA.E-mail:ojabelleira@gmail.comSearch for more papers by this author First published: 29 September 2011 https://doi.org/10.1890/ES11-00165.1Citations: 3 Corresponding Editor: C. Kwit. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract The African tulip tree, Spathodea campanulata, dominates many post-agricultural secondary forests in the moist tropics. Some consider these novel forests have no ecological value, yet they appear to restore ecosystem processes on degraded sites. This study describes the litterfall mass and seasonality, canopy phenology, and microclimate of S. campanulata forests on alluvial and karst substrates in northern Puerto Rico. These substrates have different water drainage properties and I hypothesized that (1) annual leaf fall mass and seasonality would differ between substrate types; because (2) leaf fall would be related to water availability and seasonality. I used analysis of variance to compare annual and biweekly litterfall mass across three sites on each substrate type, and multiple linear regression analysis to relate biweekly litterfall to environmental variables. Litterfall mass was high (13.8 Mg·ha−1·yr−1, n = 6, SE = 0.60) yet its components did not differ by substrate type except for reproductive part mass which was higher on karst due to more S. campanulata flowers. Leaf fall had a bimodal seasonality and was negatively related to the number of dry days indicating it occurs when water is readily available or in excess as during floods. Observations show systematic leaf senescence in this deciduous species can be caused by water and nutrient demand from flowering. Litterfall mass and seasonality of novel S. campanulata forests is similar to that of native forests in Puerto Rico, yet flower fall appears to be higher than that of tropical forests worldwide. The environmental variables that affect litterfall seasonality and canopy phenology are similar to those in tropical forests in Puerto Rico and elsewhere. Litterfall seasonality and canopy phenology regulate understory microclimate, and influence the establishment and growth of juvenile trees and other organisms within S. campanulata forests. This study illustrates how forest ecosystem processes and properties restored by novel S. campanulata forests facilitate tree species establishment, growth, and turnover in deforested, abandoned, and degraded agricultural lands in Puerto Rico. Introduction Most ecosystems on Earth have been altered by humans and terms such as "homogeocene" have been used to describe the times we live in (Vitousek et al. 1997, Lugo 2009). In the Caribbean island of Puerto Rico, the development of a fossil fuel-based economy in the mid-20th century caused the abandonment of agricultural lands and this was followed by the spontaneous growth of secondary forests (Rudel et al. 2000). Many of these secondary forests are dominated by introduced species and are considered to be novel ecosystems that have originated from deforestation, land use, and abandonment (Hobbs et al. 2006). The processes taking place in novel tropical forests have not been adequately studied and there is a need to understand these ecosystems, which are becoming more common as a natural response to anthropogenic change. The African tulip tree, Spathodea campanulata Beauv. (Bignoniaceae), has been widely introduced outside its natural range and invades disturbed to deforested moist tropical lands worldwide, especially in islands (Francis 2000, Haysom and Murphy 2003). In Puerto Rico, it is the most abundant tree species and dominates secondary forests in the moist regions (Lugo and Helmer 2004, Brandeis et al. 2007). Novel S. campanulata forests have been studied in Hawaii (Mascaro et al. 2008) and Papua, New Guinea (Novotny et al. 2004). Although they accumulate biomass, novel S. campanulata forests in the Hawaiian lowlands lack regeneration of native trees due to the extirpation of remnant native forest and native avian seed dispersers (Foster and Robinson 2007, Mascaro et al. 2008). In Papua, various species of Lepidoptera larvae profit as herbivores on S. campanulata and other introduced tree species in novel forests, where these trophic webs have been described in detail (Novotny et al. 2004). Other examples of novel forests come from the invasion of Ligustrum spp. on abandoned farmlands in Argentina (Lichstein et al. 2004) and on human-disturbed native forests in La Réunion island (Lavergne et al. 1999). In Argentina, native birds disperse introduced Ligustrum lucidum to abandoned farmlands where its dominance limits recruitment of other trees, although it allows the establishment of native trees in degraded lands (Lichstein et al. 2004, Ferreras et al. 2008). In La Réunion, introduced Ligustrum robustum invades heavily human-disturbed native forest where it dominates the canopy and acts as source of invasion to native undisturbed forest remnants through introduced avian seed dispersers (Lavergne et al. 1999, Mandon-Dalger et al. 2004). Perhaps the most striking example of a novel forest ecosystem is that described by Wilkinson (2004) on Ascensión island. This island was barren of trees in the early 19th century, yet management practices introduced a myriad of tree species that resulted in lush cloud forests in current times. Most plant species endemic to Ascensión are missing from these novel forests yet observations suggest they provide ecosystem services by increasing soil depth, and capturing water from clouds and atmospheric carbon (Wilkinson 2004). The common denominator of these studies is the focus on the description of species assemblages and plant-animal interactions in novel forests, with no detailed focus on the ecosystem processes that inhibit or facilitate the establishment or interactions of species within novel forests. In Puerto Rico, many native tree species are becoming established in novel S. campanulata forests and some suggest that this tree's deciduousness results in sunlight and litterfall pulses that enhance the photosynthetic capacity and nutrient status of juvenile trees in the understory (Aide et al. 2000, Lugo 2004, Abelleira Martínez 2010, Abelleira Martínez et al. 2010). Besides pulses of increased sunlight, the understory microclimate of these forests could facilitate the establishment and growth of native trees by regulating temperature and relative humidity extremes, and increasing soil nutrient, organic matter, and water content. On the other hand, compositional changes due to introduced invasive tree species can alter ecosystem processes and this can affect native biota (Vitousek 1990, Crowl et al. 2008). The dominance and deciduousness of novel S. campanulata forests could result in litterfall mass and seasonality different from native evergreen and species-rich old-growth forests of the moist regions of Puerto Rico. In consequence, ecosystem processes that depend on litterfall fluxes, such as development of forest floor litter, soil organic matter, and nutrient cycling from the canopy to the forest floor, could differ from native forests (Prescott 2002, Lugo 2004). This can have consequences for biota that depend on litterfall such as invertebrates and soil microorganisms. Lowland tropical forest phenology and litterfall is closely related to water seasonality, particularly due to leaf senescence and fall pulses that coincide with droughts or floods (Reich and Borchert 1982, Medina 1983, Kinnaird 1992, Kozlowski 1997, Haase 1999, Parolin et al. 2004, Haugaasen and Peres 2005, Kozlowsky 1997). In moist northern Puerto Rico, S. campanulata forests located on deep alluvial soil in flat riparian zones have a constant water table supply from river inputs and suffer periodic floods due to high and long rainfall events in the watershed (Abelleira and Lugo 2008). In contrast, S. campanulata forests on karst depressions have shallower alluvial soils underlain by porous and well-drained calcareous bedrock on sloping to nearly flat topography, similar to a soup bowl in shape, and receive most water inputs during local rain events (Monroe 1976). Thus, better drainage and inconsistent water inputs in S. campanulata forests on karst depressions can result in litterfall mass and seasonality different from those on alluvial valleys. In particular, I hypothesized that (1) the leaf fall of S. campanulata forests would differ between alluvial and karst substrates; because (2) leaf fall would be linked to water availability and seasonality. Methods Study sites I sampled six lowland S. campanulata forests (Fig. 1; Table 1) located on abandoned agriculture and grazing lands on alluvial and karst geological substrates in the subtropical moist life zone of northern Puerto Rico (18° N, 65° W; Holdridge 1967, Ewell and Whitmore 1973, Bawiec et al. 2001). Between 1971–2000, mean annual temperature and precipitation were 25°C and 1443 mm, respectively, and precipitation in 2006 was 1612 mm at Manatí, which is within 20 km from all sites (NOAA 2002, USGS 2007). In the Cibuco river, which is adjacent to alluvial sites, high rainfall events result in floods (USGS 2007). Climate diagrams for Aguadilla and San Juan, which lie 75 km west and 50 km east of the sites, respectively, show climate in the northern coast is moist through the year with a drier period from February to March (Rivas Martínez 2009; Lugo et al. in press). Figure 1Open in figure viewerPowerPoint Map of Puerto Rico and location of Aguadilla (Ag), San Juan (SJ), and Manatí (Mn) climate stations with respect to the enlarged section of the island, which shows the location of the study sites and the Cibuco river (CR) monitoring station. Squares denote alluvial sites, circles denote karst sites, and triangles denote stations. Site codes are as follows; CI: Cibuco I, CII: Cibuco II, JN: Juan Nieves, OC: Ollas y Calderas, PI: Paso del Indio, and Pu: Pugnado. Table 1. Study site characteristics by geological substrate type. The valleys where alluvial sites are located were used for sugar cane plantations from the 17th to mid-20th century (Table 1; Picó 1937, Wadsworth 1950, Picó 1988). A crop annual cycle consisted of cane growth, harvesting, burning, and re-growth, sometimes interspersed with fallow years. Cattle plow lines are still evident on the forest floor of these sites. The depressions where karst sites are located might have been used for sugar cane as well, but if so, it was abandoned much earlier than on the alluvial sites (Picó 1988). Crops planted before abandonment at karst sites were small scale commercial to subsistence agriculture consisting of citrus, avocado, plantains, cassava, and yams. Grazing occurred in fallow years or after abandonment at most sites (Table 1). The soil atop karst bedrock is a mixture of bedrock material with sedimentary alluvial deposits that originated in higher elevations, which are akin to soils at alluvial sites (Bawiec et al. 2001). Two sites on each substrate type were located on mollisols (calcic arguidolls and fluvaquentic hapludolls) and the others were on entisols (typic ustorthents) and inceptisols (vertic endoaquepts; Table 1). Mollisols are well-developed and amongst the most fertile soils in the tropics compared to inceptisols and entisols, which are relatively younger and less fertile (Beinroth et al. 2003). However, I assumed no differences in soil fertility between sites because of similar sedimentary origin and disturbance by previous land use. In addition, soil bulk density at 0–15 and 15–30 cm depth is very similar across sites on both substrate types (Table 1). Tree structure is similar across sites, but alluvial sites have lower species richness and higher dominance by S. campanulata compared to karst sites (Table 1). Data collection Six 0.25 m2 baskets were placed randomly along line transects set haphazardly on each site for a total of 36 baskets. The baskets were lined with 1 mm screen mesh, placed 1 to 2 m off the ground, and at least 15 m from the forest edge. I collected biweekly litterfall from each basket starting in 10 January 2006, and ending in 7 May 2007, for a total of 30 sampling dates. Litterfall samples from each basket were oven dried to constant weight at 65°C, separated into S. campanulata senesced and green leaves, other leaves, wood, reproductive parts, and miscellaneous components, and each component was weighed separately. The compound leaf rachis was included with wood. Thus, leaves actually represent leaflets, but are referred to as leaves throughout this paper. The number of S. campanulata open flower calyxes per basket was recorded as a proxy of the number of flowers found in reproductive parts because the rest of the corolla broke apart easily during sampling and processing. I summed the litterfall mass and number of flowers per basket for the 26 sampling dates starting in 10 January 2006, and ending in 11 January 2007, to estimate annual rates per component (Mg·ha−1·yr−1 and flowers·ha−1·yr−1). Litterfall mass and number of flowers per basket were divided by the number of days that integrated each sampling date to report biweekly rates (g·m−2·d−1 and flowers·m−2·d−1). Canopy phenology was determined from observations on seven upper canopy trees selected randomly at each site for each sampling date starting in 26 July 2006. The percentage of leaf canopy closure and the presence of flowers for each tree were determined visually from the ground. Canopy closure was estimated on a scale of 0 to 100% that corresponded to leafless to fully flushed trees, respectively. The percentage of canopy closure and of trees in flower reported is the mean value of observations on the seven trees per site for each sampling date. Weather stations (OCC 2010) were placed at four sites in 17 May 2006 to monitor in-situ microclimate. The stations were placed on the two sites of each substrate type that were on mollisols (Table 1). Each station had sensors for temperature, relative humidity (RH), soil water content (SWC), and photosynthetic active radiation (PAR). The sensors were installed on or near large diameter S. campanulata trees and therefore all microclimatic data is spatially biased to the environment near trees. Temperature and RH sensors were placed at ∼3 cm from tree trunks and ∼2 m from the ground. The SWC sensor reached into 20 cm of soil depth, the PAR sensor was placed at 1 m above ground, and both were located between 1 to 2 m from the tree. The reason for this setup was to provide discretion because all sites had public access, suffered occasional vandalism, and preliminary conspicuous sensors were stolen or displaced. Thus, care should be taken when comparing this to other datasets. Reported values for in-situ temperature, RH, SWC, and PAR are the means of readings that integrated each sampling date. Data analysis I used ANOVA to compare annual and biweekly litterfall and flower fall using the mean of six baskets per site (n = 3 sites per substrate type), and biweekly in-situ microclimatic means per station per site (n = 2 sites per substrate type) using substrate type as single factor. ANOVA was also used to compare annual and biweekly litterfall within substrate type (n = 6 baskets per site) using site as single factor. I used the Shapiro-Wilks test to assess data normality, the F-max test to assess variance homogeneity, and the Kruskal-Wallis test to compare non-normal data. I used stepwise multiple linear regression analysis (SMLRA) to determine which variables related to biweekly litterfall rates at the substrate type (n = 3 sites per substrate type) and site (n = 6 baskets per site) levels. I used P < 0.05 as minimum for the inclusion of each variable in the SMLRA model. I tested the number of dry days (DD), daytime temperature (DT), daytime relative humidity (DRH), wind speed (WS), and day length (DL) as independent variables. Data for DD, DT, DRH, and WS were daily averages for 1961–1990 at San Juan (Fig. 1; USEPA 2002). The DD were the sum of days where pan evaporation exceeded rainfall in 1961–1990 daily averages for days that integrated each sampling date. Biweekly means for DT, DRH, and WS were the mean of daily values that integrated each sampling date. Data for DL were from San Juan for 2006–2007 (USNO 2008). Mean biweekly maximum and minimum values for DT, DRH, and WS (USEPA 2002) were each interchanged with corresponding variable means, and daily rainfall at Manatí (mm) and Cibuco river discharge (m3/s) for 2006–2007 (Fig. 1; USGS 2007) were each interchanged with DD, on separate analyses and reported if the significance of the model improved. Modes of litterfall components with bimodal seasonality were analyzed separately. I used Infostat for all analyses (Di Rienzo et al. 2003). Results Litterfall The mean total annual litterfall of S. campanulata forests was 13.8 Mg·ha−1·yr−1 (n = 6, SE = 0.60) and did not differ between substrates type in 2006 (Table 2). About half the total annual litterfall mass was from S. campanulata leaves which constituted 85 and 75% of all leaf mass on alluvial and karst sites, respectively. Senesced leaves constituted most S. campanulata leaf mass and green leaves amounted to ∼10%. Annual litterfall of karst sites had higher reproductive part mass than alluvial sites (F [1, 4] = 18.64, P = 0.01) but there were no differences in other components between substrate types. The difference in annual reproductive part mass was due to higher S. campanulata flower production at karst sites (2.17 × 106 [SE = 0.29] vs. 2.91 × 106 [SE = 0.05] flowers·ha−1·yr−1 at alluvial and karst sites, respectively; F [1, 4] = 6.28, P = 0.07). Annual litterfall mass did not differ between sites on the same substrate type except for S. campanulata senesced leaf mass which was lower on the alluvial site Cibuco II (CII; F [2, 15] = 8.34, P = 0.004) and the karst site Ollas y Calderas (OC; F [2, 15] = 8.60, P = 0.003) compared to the other sites (Table 3). Table 2. Annual litterfall mass of Spathodea campanulata forests on alluvial and karst geological substrates in 2006. Table 3. Spathodea campanulata senesced leaf fall mass by site in 2006. Biweekly total litterfall ranged from 1 to 6 g·m−2·d−1 except for dates were wood increased this value up to 12 g·m−2·d−1 (Fig. 2; Appendix A). Senesced S. campanulata leaves and reproductive parts had different biweekly values between substrate types for the most sampling dates, yet these only amounted to 10 to 17% of all dates. Within substrate type, differences in biweekly litterfall were found in ≤30% of sampling dates except for senesced leaf fall, which was lower in 50% of all dates on OC compared to the other karst sites. Figure 2Open in figure viewerPowerPoint Mean and standard error of biweekly (month and day) Spathodea campanulata litterfall for (A) all components, (B) S. campanulata senesced leaves, and (C) reproductive parts spanning from 24 January 2006 through 7 March 2007. Each value is the mean of six baskets per site for each sampling date. Empty symbols denote alluvial sites and dark symbols denote karst sites. Site codes follow Fig. 1. Asterisks denote significant differences between substrate type (ANOVA; df = 1, 4; P < 0.05; F [all components] = 8 March 2006: 11.86, 21 March 2006: 28.23, 27 June 2006: 11.02, and 6 February 2007: 27.90; F [senesced leaves] = 8 March 2006: 29.82, 21 March 2006: 36.87, 19 April 2006: 18.31, and 27 June 2006: 24.99; F [reproductive parts] = 24 January 2006: 9.56, 5 April 2006: 11.96, 13 December 2006: 33.26, 28 December 2006: 38.13, and 10 January 2007: 8.78). At most sites, S. campanulata senesced leaf fall had a bimodal seasonality with peaks within March to August and September to March, and a dip around March and August common to all sites (Fig. 2B). The March to August mode reached higher rates and peaked earlier at alluvial sites. The September to March mode also reached higher rates at alluvial sites, but was more irregular within sites of the same substrate type and was nonexistent at the karst site OC. The equations derived by SMLRA of S. campanulata senesced leaf fall to environmental variables explained between 31 to 72% of the seasonality on sites combined by substrate type (Table 4). Senesced leaf fall was negatively related to DD in equations for all sampling dates and for the March to August mode. Senesced leaf fall was also negatively related to DRH and positively to DL, but these relationships were inconsistent. No significant equations were found for the September to March senesced leaf fall mode for sites combined by substrate type. Table 4. Statistically significant regression equations found for biweekly litterfall mass (g·m−2·d−1) and flower fall (flowers·m−2·d−1) rates in Spathodea campanulata forests on alluvial and karst substrates derived from stepwise multiple linear regression analysis to environmental variables. Most S. campanulata green leaf fall occurred between January and July, when it reached up to half the rate of senesced leaf fall, and almost no green leaf fall occurred between September and December (Fig. 2B; Appendix A). On alluvial sites, S. campanulata green leaf fall was negatively related to maximum DRH but no relationship was found on karst sites (Table 4). The seasonality of other leaves, wood, and miscellaneous components resembled that of S. campanulata senesced leaves, but were more variable and no SMLRA equations were found for other leaves and wood. Reproductive part fall had a bimodal seasonality distinct from the other litterfall components with one mode in November to May of greater magnitude than a shorter mode in June to November (Fig. 2). The November to May mode coincided with S. campanulata flower fall (Fig. 3) and the smaller June to November mode corresponded to seedpod fall. The November to May flowering mode occurred slightly earlier and reached higher rates on karst sites (Figs. 2C and 3). The SMLRA equations explained between 51 to 68% of the variation in reproductive parts and S. campanulata flowers on sites combined by substrate type (Table 4). Reproductive part and S. campanulata flower fall were negatively related to DT in most equations, negatively related to DL and positively to DD in some equations, and related to other variables inconsistently. No significant equations were found for the June to November mode of reproductive part fall. Figure 3Open in figure viewerPowerPoint Mean and standard error of biweekly (month and day) Spathodea campanulata flower fall spanning from 24 January 2006 through 7 March 2007. Each value is the mean of six baskets per site for each sampling date. Empty symbols denote alluvial sites and dark symbols denote karst sites. Site codes follow Fig. 1. Asterisks denote significant differences between substrate type (ANOVA; df = 1, 4; P <0.05; F [flowers] = 24 January 2006: 14.62, 19 April 2006: 12.96, 13 December 2006: 33.99, and 28 December 2006: 10.40). Canopy phenology The canopies of S. campanulata forests lost their leaves by July 2006, leading to full canopy opening (Fig. 4A). Full canopy closure occurred from October to January at all sites yet lagged in time on OC. The onset of flowering coincided with full canopy closure in November on karst sites and slightly later on alluvial sites (Fig. 4B). The onset of canopy opening occurred from January to February and preceded the peak of flowering by one to two sampling dates. Figure 4Open in figure viewerPowerPoint Mean and standard error of (A) percentage of canopy closure and (B) percentage of trees in flower for seven Spathodea campanulata trees on each site spanning from 26 July 2006 through 7 May 2007. Empty symbols denote alluvial sites and dark symbols denote karst sites. Site codes follow Fig. 1. Asterisks denote significant differences between substrate type (ANOVA; df = 1, 4; P <0.05; F [canopy closure] = 13 December 2006: 27.15, 24 January 2007: 12.03; F [trees in flower] = 29 November 2006: 16.02). Microclimate In-situ temperature biweekly means ranged from 21 to 26°C and were significantly higher at alluvial sites for 38% of all monitored sampling dates (Fig. 5A). In-situ temperature was highest in May to September and lowest in January to February, which matched the minimum and maximum percentage of canopy closure, respectively (Fig. 4B). In-situ RH means ranged from 85 to 105% and were significantly higher at karst sites in 19% of all monitored sampling dates (Fig. 5B). After the minimum values that occurred in May, RH had a steady increase up to September when relatively constant values ranging from 93 to 95% at alluvial sites and from 97 to 100% at karst sites were maintained until January. Subsequently, there was another RH raise in February to March that coincided with peak S. campanulata flowering and flower fall (Figs. 2C, 3, and 4B). In-situ SWC had the greatest variability within sites of the same substrate type, and more so at karst sites (Fig. 5C). In spite of this, SWC appeared to be consistently higher at alluvial sites where means ranged from 0.21 to 0.38 m3/m3 compared to karst sites, which ranged from 0.15 to 0.31 m3/m3. There were two drops in SWC that occurred in October and February, which coincided with high values of canopy closure and flowering, respectively (Fig. 4). In-situ PAR means ranged from 2 to 58 μmol·m−2·s−1, did not differ between substrate types, and was highest in May to June and lowest in November to January, which coincides with lowest and highest percentage of canopy closure, respectively (Figs. 4A and 5D). Figure 5Open in figure viewerPowerPoint Mean and standard error of in-situ (A) temperature, (B) relative humidity (RH), (C) soil water content (SWC), and (D) photosynthetic active radiation (PAR) measured inside two alluvial (Cibuco II and Paso del Indio) and two karst (Ollas y Calderas [OC] and Pugnado) Spathodea campanulata forest sites spanning from 17 May 2006 through 7 March 2007. For each sampling date, n = 2 on each substrate type except for all variables in 19 October 2006, temperature during the dates after 24 January 2007, and RH during the dates after 13 December 2006 on karst for which n = 1 (OC). Asterisks denote significant differences between substrate type (ANOVA; df = 1, 2; P < 0.05; F [temperature] = 17 May 2006: 166.22, 31 May 2006: 21.53, 14 June 2006: 32.99, 27 June 2006: 19.19, 9 August 2006: 31.12, 23 August 2006: 24.31, 20 September 2006: 20.65, 13 December 2006: 28.08; F [RH] = 17 May 2006: 214.06, 14 June 2006: 831.51, 6 September 2006: 28.22, 20 September 2006: 19.90; F [SWC] = 26 July 2006: 27.77; F [PAR] = 14 June 2006: 219.57). Figure 6Open in figure viewerPowerPoint Continued. Discussion The hypothesis that leaf fall mass and seasonality would be different in S. campanulata forests on alluvial and karst substrates is rejected. However, S. campanulata senesced leaf fall was negatively related to the number of dry days, supporting the hypothesis that leaf fall relates to water seasonality (Table 4). The significant SMLRA equations found indicate leaf fall occurs when water is readily available or in excess, as during floods. Two mechanisms associated to water deficit or excess can cause leaf senescence and fall: (1) excessive water loss from high transpiration rates that cannot be compensated by xylem conductance due to low soil water potential during droughts (Medina 1983); and (2) water saturated soils that result in anoxic conditions in the roots impeding water uptake and xylem flow to leaves during floods (Kozlowski 1997). The absence of physiological drought (rainfall − evaporation < 0, Lüttge 1997) and occurrence of leaf fall during high water availability suggests the second mechanism. Floods were observed during February, April to May, and July to December at all sites except for OC, which does not flood because surface water runoff from higher ground is blocked by a karst wall (Monroe 1976). Based on observations, floods at alluvial sites covered more area, lasted longer for up to two weeks, and ranged in depth from 0.1 to 1.5 m, which can explain earlier and higher senesced leaf fall rates at alluvial sites in both modes. From February to June of 2010, floods were monitored with sensors on CII, Paso del Indio (PI), and Pugn
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