INTRODUCTION
Natural lignocellulosic fibres obtained from plants are important examples of renewable and biodegradable materials whose use may mitigate the generalized pollution due to discard of non-degradable synthetic materials. In addition, lignocellulosic fibres are less expensive than their synthetic counterparts and provide social benefits as a major source of income to communities in developing countries where they are exploited (Monteiro et al., Reference Monteiro, Lopes, Ferreira and Nascimento2009).
Palms are great producers of fibres, food, waxes and oils and represent common components of tropical regions around the world (Lorenzi, Reference Lorenzi2004). Piassava palm (Attalea funifera Mart.) is endemic of the coastal zone (up to 60 km inland) of the State of Bahia, Brazil, where it has been mostly exploited in its wild state as part of a secondary forest cut and burn cycle, without technological inputs (Vinha and Silva, Reference Vinha and Silva1998). This species produces a natural, long (up to 4 m) fibre with unique characteristics, such as elasticity, flexibility, impermeability, which is used for fabrication of many products such as industrial and domestic brooms, thermal isolants and brushes. Due to these characteristics, the interest in piassava fibre has increased in regard to construction of natural fibre-reinforced composites for domestic and industrial uses (Monteiro, Reference Monteiro2009). A sub-product of fibre extraction, locally known as ‘bôrra’, is used usually as thatch for cabanas and bars along the beach (Vinha and Silva, Reference Vinha and Silva1998).
Piassava palm is frequently associated with shade environments of secondary forest formation. The palms can reach full sunlight in the forest canopy when it is growing in spodosols, but remain in understorey when it is growing on latasolic soils (Vinha and Silva, Reference Vinha and Silva1998). Moreover, piassava usually grows in areas that are open for pasture development. When the exploited palms are conducted with canopy cleaning and weed control, it is known as ‘cultivated’ piassava. Otherwise, they are exploited as wild piassava (Melo et al., Reference Melo, Souza, Nakagawa, Silva and Mori2000).
According to local collectors, piassava fibres must be harvested once a year, so that the plant can produce long fibres of a high commercial value. Shorter harvest periods can lead to low quality of fibres as well as to a reduced longevity of the palm. Piassava fibre is part of a fibrous lignocellulosic twisted tissue, locally known as strip, that helps in fixing leaves and inflorescences as well as in protection of the stem. This tissue is present along the edges of leaf petiole and sheath. The standardization of the fibre for industry requires the preparation of a product with higher quality, according to market requirements. Increasing domestic production and the participation of piassava fibre in the composition of various products can be obtained with the implementation of rational cultivation systems, such as in agroforestry systems or in restocking or enrichment of forest or open areas (Melo et al., Reference Melo, Souza, Nakagawa, Silva and Mori2000). There are recent initiatives for rational cultivation of piassava palm under shade environments (forest understorey) or under full sun (forest clearings, pastures). In all these situations, the main product has been the fibre. However, the positive and negative aspects of growing A. funifera under different environments have not been studied.
Information about the efficiency of harvesting methods is practically inexistent, with only some non-experimental observations available. In the most common harvesting method, a set number of leaves are cut, the strips are separated from the petioles and the fibres are separated from the strips (d'Almeida et al., Reference d'Almeida, Aquino and Monteiro2005). Although this procedure may cause injuries to the stem, which in turn may attract insect pests, harvesting with defoliation has been quoted by the local collectors as more efficient and less laborious than harvesting without defoliation. Nevertheless, fibre collection without defoliation has been practiced by some collectors in order to preserve the palm leaf area.
The responses of trees to natural or artificial defoliation have been extensively studied and reviewed (Barry et al., Reference Barry, Quentin, Eyles and Pinkard2012; Eyles et al., Reference Eyles, Smith, Pinkard, Smith, Corkrey, Elms, Beadle, Mohammed and Whitehead2011; Iqbal et al., Reference Iqbal, Masood and Khan2012). Leaf removal usually results in the increased photosynthesis rate of remaining leaves due to greater irradiance availability following the removal of young (upper) leaves, to biochemical adjustments of the photosynthetic apparatus or both (Anten and Ackerly, Reference Anten and Ackerly2001). This compensatory photosynthesis can result from defoliation-induced changes in the source–sink ratio, although the reduced competition between remaining leaves for water, nutrients or hormones supplied by the roots, as well as the occurrence of severe climatic events, cannot be ruled out (Iqbal et al., Reference Iqbal, Masood and Khan2012; Martinez-Ramos et al., Reference Martínez-Ramos, Anten and Ackerly2009).
The results of leaf removal on palm species health are quite diverse, from beneficial to negative effects to vegetative and/or reproductive traits (Rosenfeld, Reference Rosenfeld2009). Defoliation has been related to increased leaf production in some palm species from tropical forests (Oyama and Mendoza, Reference Oyama and Mendoza1990), as well as to lower inflorescence production in others (Cunningham, Reference Cunningham1997). Defoliation generally does not increase mortality, unless in completely defoliated juveniles (Mendoza et al., Reference Mendoza, Piñero and Sarukhán1987). Post-defoliation leaf production has been demonstrated to be equal in some species (Chazdon, Reference Chazdon1991) and was more than doubled in others (Oyama and Mendoza Reference Oyama and Mendoza1990); the results depend on the defoliation intensity and frequency. Similarly, reproductive effort was found to be severely reduced in some species-treatment combinations (complete defoliation in Mendoza et al., Reference Mendoza, Piñero and Sarukhán1987) and increased in others (partial defoliation, Oyama and Mendoza Reference Oyama and Mendoza1990). Moreover, palm leaf harvesting can lead to overexploitation, which can result in negative impacts on growth, reproduction and survival of individuals, and, finally, on population's structure and dynamics (Flores and Ashton, Reference Flores and Ashton2000; Endress et al., Reference Endress, Gorchov, Peterson and Serrano2004). The importance of such changes should receive more attention, especially in regard to the conservation of A. funifera and to its rational exploitation under changing environmental conditions.
The objective of the present work was to investigate defoliation-related changes in leaf-level photosynthetic characteristics and their implication for the fibre production of A. funifera, cultivated in full sun or in an understorey. We tested the hypotheses that (1) old leaf removal during fibre harvesting leads to an increase in net photosynthesis of remaining leaves and (2) the increase in net photosynthesis and the associated changes in fibre production are dependent on the irradiance availability.
MATERIAL AND METHODS
Experimental conditions
The research was conducted in a private farm at the municipality of Itacaré, southeast region of the Bahia state, Brazil (14°16′39″ S, 38°59′48″ W and 100 m above sea level). Two contrasting light environments were chosen, full sun and forest understorey (shaded site of a disturbed area of the Atlantic Coastal Rainforest). The climate of the region is classified as tropical humid of the type Af of Köppen, characterized by high temperatures (24–27 °C) and rainfall (above 2000 mm per year). Photosynthetically active radiation (PAR) was monitored in the two environments during three non-consecutive days in April (14 and 28) and May (8) of 2009 using an SLIA-M003 quantum sensor coupled to a HOBO weather station (Onset Computer, Bourne, Massachusetts, USA). Air temperature (Tair) and relative humidity (RH) were also monitored, using Hobo H8 ProSeries sensors (Onset Computer, Bourne, Massachusetts, USA). The weather station was programmed to collect data every 1 minute, always between 8:00 and 17:00 h. Average values of the descriptive characteristics of the conditions of irradiance, such as mean PAR (μmol photons m−2 s−1) and total daily PAR (mol photons m−2 day−1), were calculated. Vapour pressure deficit (VPD) was calculated as the difference between saturation (es) and actual (ea) vapour pressure of the air, using measured values of air temperature and relative humidity (Buck, Reference Buck1981).
Photosynthetic characteristics
The photosynthetic characteristics were evaluated by measurements of the leaf gas exchange, using a Portable Photosynthesis System LI-6400 (LI-COR, Lincoln, Nebraska, USA) equipped with an artificial light source of 6400-02 B RedBlue. Measurements were performed between 8:00 and 12:00 h in two leaflets of the middle of one healthy and fully expanded leaf per plant (leaf number three counting from the top). Net photosynthetic rate (A), stomatal conductance to water vapour (gs), transpiration rate (E), the ratio between internal and atmospheric concentrations of CO2 (Ci/Ca), the intrinsic water use efficiency (A/gs) and the apparent carboxilation efficiency (A/Ci) were measured or calculated eight days after the second harvesting procedure in five individuals of defoliated and non-defoliated palms in both environments. Irradiance response curves of net photosynthesis were carried out with twelve levels of PAR (0, 5, 10, 25, 50, 100, 200, 400, 600, 800, 1000 and 1200 μmol photon m−2 s−1) in decreasing order, with 1–2 minute intervals between each reading. The maximum coefficient of variation to save each reading was set at 0.2%. The CO2 flux was adjusted to maintain a concentration of 400 μmol mol−1 inside the chamber. Chamber temperature was fixed at 27 ± 1 °C using the coolers of the equipment. For each individual replicate, non-linear regression for exponential equation was used to estimate the photosynthetic parameters. The following equation was used (Iqbal et al., Reference Iqbal, Rao, Rasul, Wahid and Pessarakli1997): A = Amax(1 − exp(−α (PAR/Amax)) − Rd, where Amax is the light-saturated photosynthetic rate, α is the apparent quantum yield of CO2 assimilation and Rd is the dark respiration. The exponential model was demonstrated to be suitable for this purpose in coconut palm (Gomes et al., Reference Gomes, Oliva, Mielke, Almeida and Leite2006). For each replicate, saturation (IS) and compensation (IC) irradiances were calculated from the respective exponential function, considering A = 0.9Anmax and A = 0, respectively. Anmax is the light-saturated net photosynthetic rate, where Anmax = Amax–Rd.
Fibre harvesting methods and leaf production rate
Sixty healthy and uniform palms of approximately 13 years were selected, 30 in the understorey of a disturbed area of the Atlantic Coastal Rainforest and 30 growing in an open sky area near to (~ 150 m) this forest formation (full sun). The palms were all planted from seeds in each environment, following a population enrichment project conducted by the farmer. In each environment, 15 palms were submitted to fibre harvesting with leaf removal, the others 15 were harvested without leaf removal. In defoliated palms, three older leaves were removed leaving one expanded leaf per plant. Strips were removed from the margins of petiole of each leaf, cleaned to separate the fibre from the ‘bôrra’ and all the parts were dried in an oven at 70 °C until constant weight. The procedure was performed at the beginning of the experiment (first harvest) and after 12 months (second harvest). At the moment of the first harvesting, there were four leaves at the crown of the palms, three of which were removed during harvesting to characterize the defoliated treatment. The total number of leaves per plant was counted quarterly during one year, aiming to evaluate the leaf production.
Statistical analysis
A completely randomized experimental design in a factorial scheme was adopted, with four treatments (two harvesting methods (with and without defoliation) × two environments) with 15 replicates in each treatment. The measured or estimated data of leaf production, fibre production and leaf gas exchange were submitted to a two-way ANOVA, using environments and defoliation as main factors.
RESULTS
PAR in the understorey was 49 μmol m−2 s−1 on average, with peaks along the day (maximum of 600 μmol m−2 s−1). At full sun, the mean values were 675 μmol m−2 s−1, with a maximum of 2300 μmol m−2 s−1 around 12:00 h (Figure 1a). Air temperature (T) and VPD in the understorey varied from 21 to 28 °C and from 0.1 to 1.1 kPa, respectively. Full sun values varied from 24 to 29 °C and from 0.2 to 1.2 kPa, respectively (Figures 1b and c). It must be noted that T and VPD were higher in full sun than in understorey before noon but the values of the two environments matched after noon. Total energy input (from 8:00 to 17:00 h) in each environment (daily PAR) was, on average, 1.5 and 23 mol m−2 day−1 in the understorey and full sun, respectively, showing a strong contrast between the environments.
Figure 1. Diurnal courses of (a) PAR, (b) air temperature (T) and (c) VPD measured in full sun (grey) and forest understorey (black) at the experimental site. Values are mean of three non-consecutive days in the autumn of 2009.
In both environments, defoliation led to significant (p < 0.05) increase of measured net photosynthesis (A), stomatal conductance to water vapour (gs), transpiration (E), internal to atmospheric CO2 concentration ratio (Ci/Ca) and to a decrease of intrinsic water use efficiency (A/gs) in remaining leaves after one year from the first defoliation event (Table 1). The apparent carboxylation efficiency (A/Ci) increased significantly only in understorey palms. Moreover, values of leaf gas exchange variables in palms of the two treatments were significantly higher (or smaller for A/gs) in full sun than in understorey (Table 1). The values of A in the understorey were 72 and 64% of that in full sun defoliated and non-defoliated palms, respectively.
Table 1. Maximum (PAR > 800 μmol m−2 s−1) measured values of leaf gas exchange variables in A. funifera Mart. cultivated in full sun or forest understorey and submitted or not to leaf removal during the harvest of fibre. Values are mean (s.e.) of 5 replicates.
A: net photosynthesis (μmol m−2 s−1); gs: stomatal conductance to water vapour (mol m−2 s−1); E: transpiration rate (mmol m−2 s−1); Ci/Ca: intercellular to atmospheric CO2 concentration ratio; A/Ci: apparent carboxylation efficiency (mol m−2 s−1); A/gs: intrinsic water use efficiency (μmol CO2 mol−1 water vapour). For each variable, means followed by different capital letters (comparing environments within each treatment) or lower case (comparing treatments within each environment) are different by the F test (p < 0.05).
Field data of light response curves of photosynthesis showed good fit to an exponential function in palms of all four treatments (Figure 2). There were no significant effects of defoliation in light response parameters of full sun palms (Table 2). In understorey palms, a significant (p < 0.05) increase in Anmax and α was observed in defoliated as compared with non-defoliated individuals. When the environments were compared, full sun palms of the two harvesting methods showed higher values of all parameters, although significant (p < 0.05) differences were detected only for Anmax and IS (defoliated palms only) (Table 2).
Table 2. Light response curve parameters derived from the exponential adjustment to field data of A × PAR, measured in leaflets of A. funifera Mart. cultivated in full sun or forest understorey and submitted or not to leaf removal during the harvest of fibre. Values are mean (s.e.) of five replicates.
Anmax: light-saturated net photosynthetic rate (μmol m−2 s−1); α: apparent quantum yield of CO2 assimilation (μmol CO2 μmol−1 photons); Rd: dark respiration rate (μmol m−2 s−1); IC: compensation irradiance (μmol m−2 s−1); IS: saturation irradiance (μmol m−2 s−1). For each parameter, means followed by different capital letters (comparing environments within each treatment) or lower case (comparing treatments within each environment) are different by the F test (p < 0.05).
Figure 2. Response of net photosynthesis (A) to PAR in leaflets of A. funifera cultivated in (a) full sun or (b) forest understorey and submitted or not to leaf removal during the harvest of fibre. Points are measured values of 5 replicates and lines are the exponential function (see text for details).
Leaf production was significantly (p < 0.01) reduced in defoliated palms of the two environments (Table 3). Leaf production in defoliated palms was also significantly (p < 0.01) reduced in the understorey (3.1 leaves per year) as compared with full sun (3.5 leaves per year). Although non-significant, the contrary was observed in non-defoliated palms, leaf production being lower (4.5) in full sun than in the understorey (4.9) (Table 3).
Table 3. Leaf emission rate (per year) in adult individuals of A. funifera Mart. cultivated in full sun or forest understorey and submitted or not to leaf removal during the harvest of fibre. Values are means (s.e.) of 15 replicates.
**p < 0.01; ns: non-significant by the F test.
The production of fibre and ‘borra’ in the first harvesting was significantly higher in defoliated than in non-defoliated understorey palms (76 and 118%, respectively) (Table 4). The harvesting method did not differ significantly in full sun palms. When their environments were compared, a significantly higher production of ‘borra’ and fibre was obtained from understorey defoliated palms (Table 4). In non-defoliated palms, high production of fibre and ‘borra’ was obtained from full sun palms, although significance was detected only for ‘borra’.
Table 4. Dry weight (g plant−1) of dreg and fibre and percentage of fibre in the harvested material (dreg + fibre) in A. funifera Mart. cultivated in full sun or understorey and submitted or not to leaf removal during the harvest. In each year, values are mean (s.e.) of 15 replicates.
*p < 0.05; **p < 0.01; ns: non-significant by the F test.
In the second harvesting (12 months later), there were no significant effects of the harvesting method or environment. The exception was the significant (p < 0.05) increment of fibre production in non-defoliated palms of the understorey (180 g) as compared with full sun (105 g) (Table 4). The proportion of fibre in the total weight of the strip (fibre + ‘borra’) was significantly higher in the understorey (77–80%) as compared with full sun (69–74%) (Table 4). The harvesting method did not affect this trait.
DISCUSSION
The removal of leaves, partial or complete, has been defined as defoliation, an old practice in many parts of the world. Palms are probably the most widely used plant family for non-timber forest products collection, with a large number of useful species that yield many different types of products from all plant parts, such as leaves for roofing and basketry, trunks for construction and furniture, apical meristem for palm heart, fruits for juices, and roots for medicine. Considering that leaves of some understorey palm species are intensively used for thatching, basketry and ornamental purposes, among others, the sustainability of palm leaf extraction over longer periods depends on the ability of individuals to survive defoliation and recuperate their leaf area after leaves have been removed.
There have been some controversial results on the effects of leaf removal in vegetative and reproductive growth of palms (Rosenfeld, Reference Rosenfeld2009; Zuidema and Weger, Reference Zuidema, Werger and Zuidema2000). Studies have shown that leaf harvesting can have a minor effect on leaf production and reproduction (Endress et al., Reference Endress, Gorchov, Peterson and Serrano2004; Navarro et al., Reference Navarro, Galeano and Bernal2011) and that mild defoliation can even stimulate leaf production (Endress et al., Reference Endress, Gorchov, Peterson and Serrano2004). However, Flores and Ashton (Reference Flores and Ashton2000) showed that leaf harvesting reduced the production of new leaves and inflorescences. Moreover, a variation in light availability and the occurrence of severe droughts can strongly aggravate the negative effects of leaf harvesting in Chamaedorea elegans, an understorey palm species, and should be considered when assessing the ecological effects and sustainability of leaf harvesting (Lopez-Toledo et al., Reference Lopez-Toledo, Anten, Endress, Ackerly and Martínez-Ramos2012; Martinez-Ramos et al., Reference Martínez-Ramos, Anten and Ackerly2009).
Responses of Geonoma deversa to defoliation varied widely among vital rates: no increase in mortality was observed after defoliation; leaf production of defoliated ramets decreased by 16 and 9% (first and second years, respectively); stem growth by 43 and 29%; probability of reproduction by 40 and 60% and rate of vegetative production of new ramets by an estimated 70% for both years. Undisturbed small ramets in defoliated clones were not affected by the treatment, as they showed no changes in growth rate and reproductive effort (Zuidema and Weger, Reference Zuidema, Werger and Zuidema2000). Single-occasion pruning of oil palm (Elaeis guineensis), to retain 17 youngest leaves, significantly reduced the length of new leaves produced in the ensuing months (Calvez, Reference Calvez1976).
In the present study with A. funifera, it was demonstrated that leaf removal resulted in lower leaf production in the following 12 months. If that reduction remains in the following years, fibre harvesting with leaf removal should result in lower production of fibre, since the harvested product (fibre + ‘borra’) is part of the leaves. In this context, defoliation-induced increment in the area of individual remaining leaves, as observed in other monocotyledons (Nomura et al., Reference Nomura, Lima, Fuzitani, Modenese-Gorla da Silva, Garcia and Tombolato2011), although not measured in the present experiment, would be of low practical importance, unless the area is related to leaf (petiole) length as demonstrated for Euterpe edulis (Carvalho et al., Reference Carvalho, Martins and Santos1999). Moreover, in Chamaedorea elegans, an understorey palm species of tropical rainforest, leaf area and production were reduced due to chronic defoliation (Lopez-Toledo et al., Reference Lopez-Toledo, Anten, Endress, Ackerly and Martínez-Ramos2012).
Height growth of the stem is slow in palms and occurs by continuous leaf production from a single terminal meristem, which is protected by the imbricated leaf sheaths (Tomlinson, Reference Tomlinson1961). As the palm is unbranched, the growth of the petiole is the only way for the photosynthesizing surface to rise above the forest floor, reaching greater quantities, and more suitable qualities, of light (Carvalho et al., Reference Carvalho, Martins and Santos1999). A. funifera can reach full sunlight in the forest canopy when it is growing in spodosols, but remains in an understorey when it is growing on latasolic soils (Vinha and Silva, Reference Vinha and Silva1998). Longer leaves produced in understorey individuals of A. funifera, as observed in the present (data not shown) and previous (Oliveira et al., Reference Oliveira, Oliveira and Gomes2008) works, seem to partially explain the higher fibre production in understorey palms in the second harvesting, despite the low leaf production in this environment.
Short-term studies do not reveal the real effect of leaf harvest, as palms have reserves that allow them to tolerate a reduction in leaf area (Chazdon, Reference Chazdon1991). Six years of leaf harvest in Chamaedorea radicalis led to an increase in mortality and a decrease in reproductive activity, growth, and population growth rate (Endress et al., Reference Endress, Gorchov and Berry2006). In a study with other palm species of the same genus, Lopez-Toledo et al. (Reference Lopez-Toledo, Anten, Endress, Ackerly and Martínez-Ramos2012) showed that cumulative effects of chronic defoliation concomitantly reduced leaf traits (leaf persistence, leaf production rate, leaf size and leaf area), survival, growth and reproduction, and this effect was stronger in female than in male palms, independent of plant size. Although the initial observation presented here for A. funifera may lead to some insights for the management of palm population in the field, they are derived from a short-term study, the morphological and physiological changes after 3–4 years remaining unknown.
Like A. funifera, the Mexican palm species Sabal yapa Wright ex Becc., whose leaves are harvested for roof thatching, the traditional Maya house has been managed within four different regimes: under mature and secondary forest formation, spared in maize fields, spared in pasturelands and maintained or promoted in complex agroforestry systems that combine a great variety of trees and shrubs, both wild and cultivated (Martínez-Ballesté et al., Reference Martínez-Ballesté, Martorell, Martínez-Ramos and Caballero2005). According to the authors, Maya achieves sustainable use of this species under diverse scenarios by managing the great plasticity of the species. It is noteworthy that, as a tradition, piassava fibre harvesting has been performed with old leaf removal for a long time, at 12–18-month intervals (Vinha and Silva, Reference Vinha and Silva1998).
The first harvesting with defoliation led to significantly higher fibre and ‘borra’ production in understorey palms, apparently due to great length of leaves in the understorey, when compared with that of full sun palms. After one year, increased leaf-level carbon assimilation rates, as a result of defoliation in the two environments, were not related to increments in fibre production. However, high gs in defoliated palms led to increased loss of water by transpiration, which results in lower A/gs in this treatment. Even considering that the whole plant reduction of leaf area (defoliation) can compensate the enhanced water loss by leaf area unit, leaf removal during harvesting was not advantageous for fibre production, at least after one year of evaluation. In areas where drought is a concern, the removal of some of the older leaves may help make limited water resources available to younger leaves (Heichel and Turner Reference Heichel and Turner1983; Magat et al., Reference Magat, Canja and Margate1994). Magat et al. (Reference Magat, Canja and Margate1994) theorized that the loss of water through leaf transpiration in coconut could be reduced by 25–50% if some older green leaves were removed, although recommended that at least 18 opened and functional leaves be retained in the crown to maintain productivity. Nevertheless, the impact of severe drought events should be taken into account when quantifying the effects of defoliation (Martínez-Ramos et al., Reference Martínez-Ramos, Anten and Ackerly2009)
Changing the discussion from water to carbon budget, it must be noted that photosynthesis and transitory carbon reserve pools located in vegetative organs constitute the two carbon sources that feed organ growth in higher plants. Transitory storage plays a major physiological function in plant metabolism (Chapin et al., Reference Chapin, Schulze and Mooney1990). Arboreal palms usually show high longevity, mainly due to longevity of tissues (Tomlinson, Reference Tomlinson2006). In the present experiment, removed leaves would last one or two more years, sustaining an active source of carbohydrate for the palm, a carbon reserve pool for metabolic activity, as well as an important carbon stock in the ecosystem as a whole.
There are many reports showing that leaf removal results in an increased photosynthesis rate of remaining leaves due to greater irradiance availability for remaining leaves following the removal of young (upper) leaves, to biochemical changes in the photosynthetic apparatus or to both (Anten and Ackerly, Reference Anten and Ackerly2001; Iqbal et al., Reference Iqbal, Masood and Khan2012). In the present experiment, both diffusive, as indicated by increased gs in plants of both environments, and biochemical factors, as indicated by increased A/Ci in understorey plants may explain the observed increments in A. It is worth remembering that old lower leaves were removed, so that the light availability for the remaining upper leaves was not changed. Nevertheless, changes in the availability of indirect (reflected) irradiance due to removal of old leaves should not be discarded.
The leaf-level light-saturated net photosynthetic rate (Anmax) estimated here for full sun palms (around 10 μmol m−2 s−1) was similar to those reported for other arboreal palms such as oil palm (8–14 μmol m−2 s−1; Legros et al., Reference Legros, Mialet-Serra, Clement-Vidal, Caliman, Siregar, Fabre and Dingkuhn2009) and coconut (10–20 μmol m−2 s−1; Gomes et al., Reference Gomes, Oliva, Mielke, Almeida, de, Leite and Aquino2008). Mialet-Serra et al. (Reference Mialet-Serra, Clement-Vidal, Roupsard, Jourdan and Dingkuhn2008) observed that severe sink and source limitation in coconut, induced experimentally by fruit and leaf pruning, strongly affected radiation use efficiency but only to a small extent non-structural carbohydrate (NSC) reserve dynamics. They concluded that CO2 assimilation rates of coconut are probably in part driven by physiological demand. Legros et al. (Reference Legros, Mialet-Serra, Clement-Vidal, Caliman, Siregar, Fabre and Dingkuhn2009), on the contrary, reject this hypothesis for oil palm because leaf Anmax was not negatively affected by fruit pruning and the resulting oversupply of assimilates, as indicated by the accumulation of NSC in the stem caused by the treatment. In fact, leaflet NSC concentration did not vary between treatments, indicating that the sink limitation caused by fruit pruning did not restrict leaf assimilate export. So, this study established the absence of sink feedbacks on leaf photosynthetic rates as a major potential adjustment mechanism to source–sink imbalances in oil palm, contrary to the initial working hypothesis derived from observations on a closely related species, coconut (Mialet-Serra et al., Reference Mialet-Serra, Clement-Vidal, Roupsard, Jourdan and Dingkuhn2008).
CONCLUSIONS
In this experiment, total attention was given to the individual-level impact of two leaf removal events in A. funifera, so that population-level consequences of leaf cutting practices remain unclear for this species. The results indicated that A. funifera shows some degree of plasticity, with photosynthetic adjustment under high light conditions. This is important if this palm tree species can be considered as a component of agroforestry systems, or in enrichment of open areas (pasturelands). The results presented for A. funifera support the hypothesis that old leaf removal during fibre harvesting leads to increase in net photosynthesis of remaining leaves due to diffusive as well as biochemical changes. The second hypothesis, i.e. the increase in net photosynthesis and the associated changes in fibre production depend on the irradiance availability, was not proved. It was demonstrated, for the first time in A. funifera, that fibre harvesting with leaf removal was not advantageous in terms of either fibre production or water and carbon budgets.
Acknowledgements
The authors are grateful to Mr. Carlos Alex Guimarães for allowing the use of his property in Itacaré, Bahia. Financial support for the investigation was provided by the Universidade Estadual de Santa Cruz (UESC) and the Fundação de Amparo à Pesquisa do Estado da Bahia (FAPESB). Marcelo S. Mielke and Fábio P. Gomes gratefully acknowledge the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil, for the concession of a fellowship of scientific productivity.
