Sustainable harvesting of non-timber forest products based on ecological and economic criteria

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This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/1365-2664.12384

This article is protected by copyright. All rights reserved.

Received Date : 01-May-2014 Revised Date : 25-Nov-2014 Accepted Date : 27-Nov-2014 Article type : Standard Paper Editor : Yann Clough Sustainable harvesting of non-timber forest

products based on ecological and economic

criteria

Juan C. Hernandez-Barrios1*, Niels P.R. Anten2, and Miguel Martınez-Ramos1 1Centro de Investigaciones en Ecosistemas, Universidad Nacional Autonoma de Mexico, Campus Morelia, Antigua Carretera a Patzcuaro 8701, 58190, Morelia, Michoacan, Mexico; 2Centre for Crop System Analysis, Wageningen University, PO Box 430, 6700 AK, Wageningen, The Netherlands. *Correspondence author. Email: [email protected] Short title: Sustainable harvest of non-timber forest products

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Summary

1. Harvesting of highly valuable non-timber forest products (NTFPs) has been considered a win-win strategy where local people profit while conserving forest biodiversity and ecosystem services. Nevertheless the sustainability of NTFP harvesting has been debated as the nature of NTFPs, harvesting regimes, and scale of commercialization are highly heterogeneous and few studies have evaluated the cumulative ecological and economic effects of such regimes. Here, we assessed the medium-term (10 years) sustainability of NTFP harvesting, using Chameadorea palm leaves, a major NTFP from Mesoamerica that is highly valued in the international floral industry, as a case study. 2. We used an experimental ecological study and an economic assessment to analyse the sustainability of leaf harvesting in C. ernesti-augustii. A four-year leaf removal experiment was conducted to assess effects of increasing levels of defoliation (0%, 25%, 50%, 75%, 100% leaf removal, biannually) on palm survivorship, leaf production and leaf quality. Results of this experiment were combined with estimations of harvest economic value to make projections of the availability of leaves and profit per unit area. Finally, we determined harvesting regimes that maximize profit while maintaining medium-term viability of exploited populations. 3. Palms tolerated up to 50% chronic defoliation, but higher defoliation levels reduced survivorship, leaf production, and leaf quality. In the long-term, this 50% defoliation level maximized harvest volume and profit without significantly affecting palm survival and leaf quality. Our results show that harvesters face the dilemma of either maximizing short-term income leading to rapid exhaustion of stocks, or maintaining exploited populations but maximizing income in the long-term. 4. Synthesis and applications. Our study shows that intermediate harvesting levels (≤ 50% leaf removal) are needed to achieve long-term sustainability of Chameadorea palm leaves. Results of this study have an immediate application for the amendment of the official

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Mexican law, which enables higher harvesting intensities of Chamaedorea leaves, and for the design of sustainable management strategies. Applications of such strategies should consider community-based management, fair markets, regulating norms, as well as a thorough communication among stakeholders. Keywords: plant demography, Chamaedorea ernesti-augustii, tropical rainforest, defoliation,

socio-ecological sustainability, Mexico

Introduction

The harvesting of valuable non-timber forest products (NTFPs) has been considered an ecologically sound alternative for the non-degrading commercial exploitation of natural forests, because it would generate profits to local people while conserving biodiversity and ecosystem functioning (Peters et al. 1989; Ruiz-Pérez & Arnold 1996). Indeed, a recent review about the sustainability of NTFPs harvesting states that a majority of practices are considered ecologically and economically sustainable (Stanley, Voeks & Short 2012). However, several studies have questioned these win-win strategies since intense harvesting can have detrimental effects at all biological levels, (Dove 1993; Salafasky, Dugleby & Terborgh 1993; Ticktin 2004; Schmidt et al. 2011). This controversy may arise because of the wide variation in the nature of NTFPs (e.g. species life form, life-history, harvested parts) and the way they are harvested and sold. For example, sustainability is more difficult to achieve when harvested plant parts (e.g. roots, stems, or whole individuals) are critical for the long-term survival than when less vital parts, (e.g. flowers, seeds or fruits of long-lived plants) are harvested. Similarly the difficulty of achieving sustainability will increase with the intensity and frequency at which products are harvested and with the extent to which they are sold internationally rather than locally (Belcher & Schreckenberg 2007). Futhermore, effects of NTFP harvesting are usually assessed over short

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periods (Ticktin 2004) and a harvesting regime that may appear sustainable in the short-term may not be sustainable in the long-term. Thus, to assess sustainability it is neccessary to analyse the cumulative ecological effects and the economic consequences of harvesting over sufficiently long time-spans, which rarely is done.

The commercial harvesting of NTFPs can be detrimental due to several biological causes, including: low natural abundances of exploited species (e.g. van Valkenburg 1997; van Dijk 1999); low demographic resilience to harvesting (i.e. negative fitness effects due to loss of extracted parts or individuals; e.g. Schumann et al. 2010); specifics of the reproductive system (e.g. obligatory outcrossing, lack of clonal reproduction, biotic pollination; e.g. Barford, Hagen & Borchsenius 2011); degradation and loss of habitat (Shahabuddin & Prasad 2004); and the lack of adequate ecological baselines for sustainable harvesting (Boot & Gullison 1995). A number of socio-economic factors can also contribute to overharvesting, such as: increasing demand (Belcher & Schreckenberg 2007); unequal commercialization chains (Current 2006; Wilsey & Radachowsky 2007); economic incentives to extract all available resource without controlling for quality (Crook & Clapp 1998); uncertainty in land ownership (Ostrom 2000; Kusters et al. 2006); lack of communal agreements and governance for resource use (Ostrom 2000); and inadequate regulatory norms and policies for NTFP harvesting (Morsello 2006; Guariguata et al. 2010). Thus, the assesment of the sustainable use of commercial NTFPs must include the analysis of harvesting effects on both the biological attributes of the species, and the key socioeconomic components of its exploitation and commercialization. The current study thus focuses on these two main aspects of NTFP exploitation: i) the cumulative demographic effects of harvesting at different intensities and ii) the potential economic profit obtained. We explored this issue for the case of Chamaedorea palms whose leaves are extracted in high quantities from

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the tropical forests of Mesoamerica (Hodel 1992). The leaves of several Chamaedorea species are harvested mostly from natural populations and exported to the floral industry in the United States and Europe, representing annual revenues of several million dollars (CEC 2003). Thus, several studies have analysed Chamaedorea species regarding their genetic diversity (Luna,Epperson & Oyama 2007; Cibrian-Jaramillo et al. 2009), functional performance (Anten & Ackerly 2001; Cepeda-Cornejo & Dirzo 2010), demography (Endress, Gorchov & Berry 2006; Valverde, Hernández-Apolinar & Mendoza-Amaro 2006; Jansen et al. 2012), response to defoliation (Hernández-Barrios et al. 2012; López-Toledo et al. 2012), as well as the effects of harvesting on the local economy of rural people (López-Feldman, Mora & Taylor 2007, López-Feldman & Wilen 2008). Understorey palms, including Chamaedorea species, have the capacity to tolerate single severe defoliation events without significant reductions in survivorship, growth or reproduction (Chazdon 1991; Oyama & Mendoza 1990; Anten, Martínez-Ramos & Ackerly 2003). Nevertheless, intense and chronic defoliation reduces these demographic rates (Endress, Gorchov & Berry 2006; Hernández-Barrios et al. 2012), negatively impacting the viability of populations (Martínez-Ramos, Anten & Ackerly 2009; López-Toledo et al. 2012). Since leaf harvesting is usually conducted at high intensities over several years (CEC 2003), its demographic impact should be assessed under scenarios of chronical defoliation (Endress, Gorchov & Berry 2006). Furthermore, the economic analysis of NTFPs extraction seldom considers the dynamics of resource availability (López-Feldman & Wilse 2008; Nelson et al. 2011). Here, we define sustainability of leaf harvesting as the defoliation intensity at which economic profit is maximized under the constraint that survival and growth of individuals are not significantly reduced (when compared to non-defoliated plants). We used a field defoliation

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experiment with different harvesting intensities applied twice a year over three years, to obtain medium-term projections (10 years) of the stock growth function and potential profit per unit area. The main questions that we addressed were: i) how do different levels of defoliation affect demographic rates of Chamaedorea ernesti-augustii ii) What is the relationship between defoliation level, stock volume and profit dynamics? And, iii) is it possible to define sustainable harvesting thresholds? We constructed two indices that quantify leaf stocks and their associated profits per unit area over time. Our analyses provide stakeholders with a tool to assess the ecological and economic consequences of alternative harvesting regimes, a fundamental topic in forest use research given the importance of environmental incomes for rural livelihoods (Vedeld et al. 2007). Materials and Methods

Species description. Chamaedorea ernesti-augustii H. Wendl. is a dioecious species occurring in the understorey of tropical forests, mostly on karstic soils, in southeastern Mexico, Belize, Honduras and El Salvador (Hodel 1992; Bridgewater et al. 2006). Chamaedorea ernesti-augustii has a monopodial growth form reaching heights of 2.5 to 3 m. Leaves are simple and bifid with a lamina length of 15–60 cm. Leaves with a 15-cm rachis are marketable (J.C. Hernández-Barrios personal observation). A detailed description of the species can be found elsewhere (Hodel 1992). Site description. A field experiment was conducted in one locality (16º37’15” N, 90º58’ 22” W) in the Lacantún Biosphere Reserve, southeastern Mexico, covered with evergreen tropical forest. Mean annual temperature is 25 ºC and mean annual precipitation is ca. 3000 mm with a short dry season (monthly precipitation <100 mm) from March to May; the soil type is karstic (limestone).

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Local people harvest C. ernesti-augustii leaves from natural populations (CEC 2003). This activity represents an important cash income especially for the poor (López-Feldman, Mora & Taylor 2007). The prevailing land property regime is characterized as common property with exclusive exploitation rights of NTFPs (including Chamaedorea leaves) for the Lacandon community, a native ethnic group from this region (Sánchez-Carrillo & Valtierra-Pacheco 2003; Romaní-Cortés 2006). Also, the Lacandones grant access to external harvesters, though illegal harvesting is frequent due to the difficulty to oversee an area of nearly 3000 km2. The Mexican Environmental Federal Agency (SEMARNAT) defines a maximum leaf-harvesting intensity of 75% per mature palm per harvesting event, leaving 20% of the palms in mature stage unharvested as a seed source for regeneration (SEMARNAT 2003). The harvesting of leaves is labour intensive and has minimal capital requirements besides opportunity costs (López-Feldman & Wilen 2008).

Defoliation experiment. Two plots (50 × 50 m) were established in an undisturbed population of C. ernesti-augustii in January 2006. We selected and tagged 270 reproductive palms (both female and male). Stem length, leaf number, and rachis length of the newest mature leaf (as indicated by leaf color and position) were measured for each palm. The newest leaf was tagged at this and all subsequent censuses to record leaf production. Each palm was then assigned to one of five defoliation treatments (0% control, 25%, 50%, 75% or 100% leaves removed) applied approximately every six months, from January 2006 to December 2008 (six defoliation events in total). To defoliate we selected only fully mature leaves with a leaf rachis ≥ 15 cm in length. Harvesters can return to the same plant once or twice a year and occasionally even more (Sol-Sánchez 2007), removing 30–100% of all mature leaves present in each plant (Reining & Heinzman 1992; J.C. Hernández-Barrios personal observation). Thus, our defoliation protocol brackets the whole range of the defoliation levels just described.

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At each census we registered survivorship, leaves produced per palm, and rachis length of the newest mature leaf. In December 2009 an additional census was done to register survivorship. Full details of our experimental protocol are given in Hernández-Barrios et al. (2012), which reported the results of the two first years from our 4-year experiment. Population density of C. ernesti-augustii was estimated from eight 30×30 m plots added in 2007 to study population dynamics of this species. Definition of the stock growth function and potential economic profit per unit area. Following Nelson et al. (2011), we defined two indices to analyse the stock growth function (i.e. leaf yield) and the potential economic profit per unit area over time, at each defoliation treatment. These indices estimate harvest volumes and economic value when harvest rates do not necessarily equal stock growth and forest stocks can change over time. The index “Harvestable Leaves per Unit Area” (HLUAi) estimates the average number of marketable leaves per harvesting event (i) and unit area. It incorporates the initial average palm density (D, in our case 998 adult palms ha-1), survivorship probability (PS), average leaf production palm-1 6-months-1 (LP), proportion of leaves harvested per palm per defoliation event (p), and the fraction of marketable leaves (f). As impacts of defoliation on leaf production differ between sexes in C. ernesti-augustii (Hernández-Barrios et al. 2012), we estimated sex-specific HLUAi indices. We defined a normal (Gaussian) probability density function for the leaf rachis length, at each defoliation event and defoliation treatment, to estimate the percentage of marketable leaves available. Finally, all the parameters were multiplied to obtain the average number of leaves per unit area per harvesting event as follows: HLUAi = D*PS*LP*p*f (1)

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We aimed to simulate the cumulative volume of harvested leaves over time per defoliation level, therefore reflects the cumulated amount of leaf harvested after 0, 1, 2, …n, harvesting events; in our case n corresponded to our five harvesting events.

The index for estimating the “potential economic profit per unit area” (EPUAi) incorporates the estimation of HLUAi, the total number of leaves per commercial unit (LCUi) or “gruesa” (equivalent to a bundle of 110 leaves), and the price of the gruesa paid by the local intermediary (CUP):

EPUAi = (HLUAi/LCUi) * CUP (2) The price of a gruesa was based on testimonies of local harvesters which were taken throughout the study period (January 2006 to October 2013). The local intermediary defines the price at each harvesting season (Sol-Sánchez 2007; Pineda-Morales 2010). As leaf size is also an important criterion for pricing (Sol-Sánchez 2007), we defined specific prices for each leaf rachis length categories (x, cm): i) 15 ≤ x < 20 cm at $1.9 USD, ii) 20 ≤ x ≤ 25 at $2.2 USD, and iii) x > 25 at $2.6 USD (J.C. Hernández-Barrios personal observation; exchange rate was 13.4 Mexican pesos per US dollar on April 2014). Then we calculated a specific EPUAi,x for each category. The proportion of leaves at each category was obtained from the general probability density function estimated for the HLUAi index at each defoliation level. Finally, all partial EPUAi,x’s were added to get a compound EPUA index for each defoliation treatment.

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Data analysis Effects of defoliation on demographic rates. We used binomial GLM analyses to test isolated and interactive effects of defoliation treatment, sex, and time on survivorship probability following the statistical protocol described in Hernández-Barrios et al. (2012). The effects of defoliation treaments on leaf production rate and leaf rachis length were analysed with repeated-measures ANOVA. In this analysis the repeated subjects over time were the experimental palms surviving until the last census date. Leaf production data was standardized to exact 6-month periods (due to differences in the span of some censuses) from January 2006 to December 2008. It was not possible to estimate leaf production for males in the 100% defoliation treatment at the last census because of the small sample size (n < 4). Leaf production rates were log (x+1)-transformed to meet homoscedasticity and parametric criteria (Sokal & Rohlf 1998). We assessed differences in leaf production rate and leaf rachis length among defoliation treatments (D), sex (S), and census (C), and inspecting the D × C and S × C interaction terms.

Stock volume and potential profit dynamics. We used the results from the analyses described above to project the behaviour of HLUAi and EPUAi over a 10-year period. This period was established considering that most palms from the 100% treatment would have died after 10 years (see Results section). The projection of survivorship for the HLUAi index was calculated as PST = ST; here, PST is the survivorship probability at time T (months) since the first harvesting event, and S corresponds to the mean monthly survivorship rate. We assumed that survivorship rate was constant although further studies are needed to test this because cumulative effects of recurrent harvesting events may increase mortality over time, especially at high harvesting levels. Because palms began to die several months after the initial defoliation (cf. Fig 1), we assumed S = 1 during the period without mortality, which varied with defoliation treatments. For example, for females in the 0% defoliation treatment mortality started after 36 months while for those in the 100% defoliation it started after 6 months. After this mortality-free

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period, we estimated S = (Nt/No)1/t, where No is the number of original palms, Nt is the number of surviving palms after four years and t is the number of months elapsed since mortality started until the last defoliation event.

Leaf production rate (LPR) and leaf rachis length (LRL) projections for the HLUAi index, for a given defoliation level and sex, were based on the repeated-measures ANOVA results described above. When the time × defoliation treatment effect was significant, we adjusted a regression model to the data of each defolation treatment. In the case of LPR we used a semi-log function of the form LPR = a + b*ln(T) and for the case of LRL a linear function of the form LRL = a + b*T was used. These functions assume that the cumulative effects of defoliation are stronger on leaf size than on leaf production, i.e. palms respond to defoliation stress by producing smaller rather than fewer leaves (Anten, Ackerly & Martínez-Ramos 2003). In cases where LPR or LRL were independent of time, we used the mean value of the variable. To obtain the probability density function for leaf rachis length, at a given time and defoliation level, we estimated its mean and standard deviation from the corresponding adjusted regression model. This gave us the fraction of leaves at each size-category previously listed. Once we obtained HLUAi values we estimated the corresponding EPUAi according to equation (2). LCU value remained constant. The CPU price varies according to demand and availability (Sol-Sánchez 2007). In our study area the average CUP price increased 266% in eight years (from $2.30 to $6.15 USD; J.C. Hernández-Barrios personal observation) or 32.5% annually, including an annual inflation rate of 3.5% (Mexico’s inflation rate for this period). We believe that this increment was derived from a reduced leaf supply, caused by declining palm populations and more strict harvesting regulations (SEMARNAT 2003, 2010). Based on this figure, we projected the EPUAi over ten years considering four experimental defoliation treatments (light: 25% leaf removal every six months; medium: 50%; high: 75%; and severe:

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100%) and two economic scenarios. The first scenario assumes that CPU price increases annually only with inflation. The second assumes that price is negatively related to leaf supply. At 100% defoliation the leaf stock was reduced 69% over the experimental study period. We assumed this as the maximum decrease in leaf supply and, correspondingly, we assigned a 32.5% annual increment to CPU price at this defoliation level. By scaling the percentual reduction in leaf stocks produced by a given defoliation level relative to that of the 100% defoliation, and multiplying the product by 32.5%, we assigned an 8.8% price increase for the ligth, 5% for the intermediate, and 7.2% for the high defoliation level. As suggested by Markandaya & Pearce (1991), the analysis of the sustainability of natural resource exploitation demands the application of low discount rates to future profits because of the uncertainty associated to the risks of environmental degradation. Accordingly, we used 0%, 3.5% and 5% discount rate (r) values in our calculations of net present value (NPV) for each economic scenario described above. We used NPV = EPUAtt=10

t=1(1+ r)t

, where t is the number of years since first defoliation event and EPUAt is the profit obtained each year.

Finally, to explore the harvesting regime that minimizes demographic effects and maximizes potential economic profit, we related the observed survirvorship probability in four years to cumulative EPUAi over the course of three years.

Results

Effects of defoliation on survivorship and growth Survivorship was greatly reduced only at the most intense defoliation treatments with females suffering higher mortality than males over time (significant defoliation × time × sex interaction

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term, Table 1). The 100% defoliation treatment reduced survivorship more than 50% for both sexes at the end of the fourth year (Fig. 1). Leaf production rate was higher in males than in females, and declined significantly with defoliation level and time independently of sex (Table 2, Fig. 1). Also, leaf rachis length was reduced over time affected depending on defoliation treatment and sex (Table 2). Leaf rachis was reduced in females in the 50–100% defoliation treatments while in males only at the 100% defoliation (Fig. 2a,b). On average, leaves produced by plants in the 100% defoliation treatment were 24% smaller than those in the 0% defoliation treatment. Overall, the fraction of non-marketable leaves (<15 cm of rachis length) increased over time in the 75% and 100% treatments, being about 25% and 50%, respectively, by the end of the experiment (Fig. 2c). Stock volumes and potential profit dynamics

Figure 3a,b shows the number of harvestable leaves per unit area (HLUAi) for female and male palms, at each harvesting event per defoliation treatment. For female palms the 50% defoliation treatment produced the highest HLUAi at the end of the study period, while for males this was the case for the 75% defoliation treatment. Under high defoliation levels HLUAi decreased notably over time. For instance, between the second and the last harvesting event the stock reduction was 17% at the intermediate defoliation level (both sexes combined) while a 69% reduction was observed at the100% treatment. Figure 3c,d shows the cumulative HLUAi and total potential profit (EPUAi) per defoliation level at the end of the experiment, for each sex. Males produced an EPUAi value twice that of the females. The 100% defoliation provided the highest number of leaves (5,363 ha-1,

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combining sexes) with a potential economic profit of $145.6 USD ha-1 after three years, while the 75% defoliation treatment provided 95% of that yield and 88% of the profit. At the 50% defoliation treatment the amounts were 71% and 64%, respectively. Table 3 shows the regression models adjusted to survivorship, leaf production rate and leaf size used for the 10-year projections of HLUAi and EPUAi. The 50% regime resulted in the highest yield per harvesting event regardless of sex (Fig. 4a,b). The maximum cumulative yield (both sexes combined) after ten years would be obtained with the 75% regime, with the highest yield for male palms (Fig. 4c,d). Regarding the cumulative EPUA after 10 years, profits were similarly high for the 50–100% defoliation levels (Fig. 4c,d). Projected EPUA values showed that the 100% defoliation level produced the highest profit in the short-term, while the 50% defoliation mantained a more stable annual income over time, and after four years it produced the highest yearly profit of any economic scenario (Fig. 5). The EPUA value for the 75% defoliation treatment, summed for both sexes, amounted to US$193.8 ha-1 in ten years, while EPUA values for the 50% and 100% defoliation levels reached 96% and 88% of that profit, respectively. Males provided higher cumulative leaf yields and profits than females at all defoliation levels. Net present value (NPV) derived from our 10-year projections with discount rates (r) of 0% to 5%, for both sexes and economic scenarios, are shown in Figure 5. Ecological sustainability and potential economic return There was a trade-off between EPUA and survivorship of defoliated palms. As harvesting volume increases profit increases but survivorship declines (Fig. 6). Economic profit obtained

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from males was twice that of females due to their higher survivorship, leaf production rate, and leaf size. Discussion

Following our definition of sustainability, we found the intermediate defoliation level (i.e. removal of 50% of newly produced leaves biannually) to be sustainable because economic profit was maximized without affecting palm fitness relative to non-defoliated palms. This defoliation threshold is lower than that established by the official Mexican norm (75%); therefore, an amendment to this norm is required considering that harvesting frequency is at least twice a year in areas where Chamaedorea palms are harvested (CEC 2003; Bridgewater et

al. 2006). Although the 75% defoliation level would provide the largest cumulated profits after 10 years, harvested populations would ultimately collapse under such harvesting intensity. The EPUA index makes our sustainability definition operational because it integrates the basic ecological and economic components associated with the harvesting of NTFPs (Hall & Bawa 1993). Defoliation effects on survivorship and growth

The fact that our C. ernesti augustii plants tolerated up to 50% defoliation twice a year, suggests that such understorey species are highly tolerant to chronic damage produced by falling canopy debris and biotic agents (Anten, Martínez-Ramos & Ackerly 2003). The progressive reduction in fitness caused by defoliation rates beyond this level can be explained by the loss of compensatory mechanisms and the depletion of stored resources (Anten & Ackerly 2001). Tolerance to defoliation was higher in males than in females as observed in other Chamaedorea palms (e.g. Cepeda-Cornejo & Dirzo 2010) and other dioecious species (Obeso 2002). Such sex difference could be utilized in designing harvesting regimes, e.g. by harvesting more from males

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and less from females thus maintaining the reproductive function of the latter (Hernández-Barrios et al. 2012). Such harvesting regimes could further also consider the recently documented persistent differences in performance between individual males and females (Jansen et al. 2012).

The increase of mortality at high chronic defoliation levels compromises the long-term persistence of C. ernesti-agustii populations. For example, mortality of palms under a sustained 75% defoliation was 0.69 after four years, and at this mortality rate an initial population of 1000 palms ha-1 would disappear in 30 years. Besides reduction in survival, reproduction (flower and seed production) of C. ernesti-augustii palms strongly decreases (almost to zero) after two years of chronic and intense defoliation (Hernández-Barrios et al. 2012). A diminished fecundity due to intense harvesting impacts seedling recruitment in our study palm (van Lent et

al. 2014) compromizing the regeneration of populations, as shown in Bertholletia excelsa, an important tropical NTFP in Brazil (Peres et al. 2003). Stock volume dynamics and ecological sustainability

The HLUA and EPUA indices allowed us to explore the dynamic effects of different defoliation treatments on stock volumes and its potential economic returns over time. Since conventional estimates are mainly based on static measurements of yields and profits (Boot & Gullison 1995; Nelson et al. 2011), our dynamic approach constitutes a novel study of the ecological and economic sustainability of harvesting practices. A comprehensive economic assessment would involve additional parameters such as: profitability, market behavior, analysis of the commercialization chain, opportunity and labour costs, economic relevance for livelihoods, adaptive harvest strategies or land ownership (López-Feldman, Mora & Taylor 2007; López-

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Feldman & Wilen 2008; Nelson et al. 2011). Nonetheless, the EPUA index represents an initial approach for exploring the immediate economic consequences of harvesting regimes. Defoliation effects on mortality were the main determinant of leaf stock dynamics and profits. Thus, the most intensive defoliation level (100%) produced a larger leaf volume and higher profit in the short term, but severely compromised the persistence of the harvested population by causing the highest mortality. In contrast, the intermediate defoliation level (50%) provided steady leaf volumes over time without affecting survival. Although the 50% and the 75% treatments projected profits that equal or surpass, respectively (Fig. 4c,d), those of the 100% treatment, such benefit only emerges after 6-10 years.

The HLUA index components can be refined to encompass different dimensions of variability, such as spatial and temporal variability (Newton, Watkinson & Peres 2012; Schmidt et al. 2011), different climatic conditions (Bridgewater et al. 2006; Gaoue & Ticktin 2008), harvesting frequency and intensity (see Endress, Gorchov & Berry 2006 for Chamaedorea

radicalis), or stock condition (harvested vs. non-perturbed). By adjusting the index specifications, its elementary design would be suitable for different species with different habits or life histories. Furthermore, while we focused on the impact of defoliation intensity, future studies should include the effects of changing defoliation frequency to explore optimal harvesting regimes. Endress, Gorchov & Berry (2006) explored the effects of different defoliation frequencies on demographic rates and leaf size, and found rather small effects. Future studies should examine potential interactive effects of defoliation intensity and frequency.

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Few studies assessing harvesting regimes of commercial NTFPs have considered spatial contexts, i.e. metapopulation approaches. Berry et al. (2008) and Van Lent et al. (2014) have shown for Chamaedorea species the importance of seed dispersal from undefoliated populations for the mantainence of defoliated ones. These studies indicate that an ecologically sustainable harvesting strategy should consider the protection of a subset of the population to function as source of seeds. By modifying the HLUA index it could be possible to assess spatially structured harvesting strategies. For example, different subpopulations i (or patches) can be subjected to contrasting harvesting levels leaving a fraction of the metapopulation as a source of seeds. In this case, the goal would be to maximize the total leaf volume and profit obtained per unit area (HLUAT) through the optimal combination of patch-specific HLUAi values, while ensuring the persistence of the metapopulation. Finally, leaf size, as a proxy for leaf quality, constitutes a critical trait that influences profits. Our results show that leaf size declines strongly at the most intensive defoliation levels, reducing the availability of marketable leaves. This result emphasizes the need to incorporate not only stock size but also quality in sustainability assesments. Also, markets could premium leaf quality over leaf volume, fostering better harvesting practices and the preservation of natural populations.

Economic sustainability and management trade-off

Nelson et al. (2011) underline three parameters that are important for analysing the economics of NTFP harvesting: i) stock level per unit area; ii) the harvest production function (the cost of harvesting a stock in terms of capital invested and opportunity costs); and iii) harvesting effort. Estimates of stock level and effort were both incorportated in the HLUA index, the former by estimating palm population density and the number of harvestable leaves per harvesting event,

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and the latter as the total harvesting intensity rather than the number of hours man-1 as in López-Feldman (2011). Since the harvesting of leaves only implies the use of simple knifes and rustic bags, the capital investment in this activity is negligible. Opportunity costs are heterogeneous (López-Feldman & Wilen 2008), therefore we decided not to include a harvest production function in our analysis (see Nelson et al. 2011). It would be important to estimate labour costs of harvesters on a spatial basis so it could be substracted from our EPUA index, which is spatially explicit.

All potential profit estimations, regardless of discount rate, were positive since the harvesting of leaves does not require significant monetary investments to initiate. However, it would be necessary to estimate labour and opportunity costs to obtain a more precise estimation of the potential economic return per unit area. Also, to make a proper opportunity costs analysis, other productive activities should be considered (Salafasky, Dugleby & Terborgh 1993). However, since in our study region the harvesting of palms is carried out mainly within natural protected areas, the alternative productive activities would be restricted to the explotation of other NTFPs. Our ecological and economic analysis enables us to highlight one of the central dilemmas that underlie the extraction of NTFPs, especially when dealing with common property and open access sytems (Ostrom 2000). That is, leaf harversters face the choice of either extracting all the available leaves from a certain area having a high profit in the short time but depleting the populations; or establishing a management strategy based on intermediate harvesting intensities with lower profits but preserving resource stocks over time (Figs 5 & 6). There are few incentives for harvesters to conserve in the long-term when operating in open access systems (Berkes et al. 1989).

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To achieve a sustainable harvesting, pertinent scientific information and governmental regulation are necessary. However, these conditions are clearly insufficient; community-based management strategies are fundamental, those imply: secure long-term ownership rights, effective communication between owners and/or users, efficent sanctioning systems, conflict resolution agreements, and self-determined regulations (Ostrom 2000). The current exploitation of Chamaedorea palms entails several difficulties for achieving such conditions. Harvesting areas are extensive making it very difficult to monitor extractive activities. Such situation turns to a large extent a “common resource use system” into a de facto “open resource use system”. The chain of commercialization is very large and is dominated by few trade companies. The harvesting activity constitutes a supplementary income for most harvesters, so they tend to focus more on their main economic activities (López-Feldman & Wiley 2008). Finally, there exist several land tenure conflicts. Nonetheless ecological and basic economic information exist for some Chamaedorea species, and it is fundamental to address the existing socio-economic dimension for designing viable sustainable commercial regimes for this important NTFP. CONCLUSIONS Intermediate levels of leaf harvesting (≤ 50% defoliation, applied biannually) in C. ernesti-

augustii are sustainable as such levels enable the persistence of harvested populations, the maintainance of leaf quality, and medium-term economic profitability. Our results provide stakeholders with tools for making informed decisions about the rational use of important NTFPs. An immediate application could be the amendment of the official Mexican norm NOM-006-SEMARNAT-1997 to reduce the maximum level of leaf harvesting. While our study incorporates two major components of sustainability, a successful implementation of our analysis should consider the social dimension of management, promoting the autonomy of resource owners and/or users regarding management decisions (Ostrom 2000) and

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incorporating all stakeholders interests (including the academic and governmental institutions for research and regulation). It is fundamental a comprehensive perspective for designing sustainable NTFPs harvesting regimes. Acknowledgments

We thank José and Billy Chankayub for field assistance. This work was supported by grants PAPIIT-DGAPA (IN-2297507) and CONACYT-SEMARNAT (2002-C01-0544). JCHB acknowledges to CONACYT for a PhD Scholarship grant, and to Utrecht University for a short-term fellowship. Data accessibility

Data available from the Dryad Digital Repository: doi:10.5061/dryad.6j19m

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region, Cameroon: a socio-economic and ecological assessment. Tropenbos-Cameroon Series No. 1 (Netherlands) pp. 209 pp. van Lent, J., Hernández-Barrios, J.C., Anten, N. & Martínez-Ramos, M. (2014) Defoliation effects on seed dispersal and seedling recruitment in a tropical rain forest understorey palm. Journal of

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sustainable forest use. Tropenbos Series 16. The Tropenbos Foundation, Wageningen, the Netherlands. Vedeld, P., Angelsen, A., Bojö, J., Sjaastad, E. & Kobugabe, B. (2007) Forest environmental incomes and the rural poor. Forest Policy and Economics, 9, 869-879. Wilsey, D.S. & Radachowsky, J. (2007) Keeping NTFPs in the Forest: Can certification provide an alternative to intensive cultivation? Ethnobotany Research & Applications, 5, 045-058. Table 1. Results of the GLM assessing the individual and interactive effects of defoliation, sex, and time on survivorship of Chamaedorea ernesti-augustii palms. Only the significative terms are shown. The term Time2 assesses changes in mortality rate over time

Trait/ factors df SS MS F P Time 1 32.69 32.68 8.28 0.0041Time2 1 53.16 53.16 13.47 0.0003Time*Time2 1 31.66 31.66 8.02 0.0047Defoliation*Time2 4 131.30 32.82 8.31 <0.0001Defoliation*Time*Time2 4 46.32 11.58 2.93 0.0198Sex*Time 1 47.36 47.36 11.99 0.0005

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Table 2. Results of the repeated-measures ANOVA for testing the effects of defoliation treatment, sex, and census on leaf production rate (logx+1-transformed) and leaf rachis length of the understorey palm Chamaedorea ernesti-augustii

Defoliation*Sex*Time 4 138.37 34.59 8.76 <0.0001Defoliation*Sex*Time*Time2 4 97.06 24.26 6.14 <0.0001Error 1596 6300.46 3.95 Total 1616 8237.12

Trait/ Source df SS MS F P Leaf production rate Defoliation 3 0.348 0.116

3.773 0.0125 Sex 1 0.174 0.174 5.680 0.0187 Subject 120 3.686 0.031 1.769 <0.0001 Census 5 5.826 1.166 67.105 <0.0001 Defoliation*Census 15 0.367 0.024 1.406 0.1386 Sex*Census 5 0.022 0.004 0.258 0.9360 Error 600 10.417 0.017

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Table 3. Average values and regression parameters for projected HLUA index values for female and male palms of Chamaedorea ernesti-augustii. Only significant regression functions are provided, otherwise average values are shown Trait Defoliation treatment Average rate/ Regression function R2 (adj.) % F P

Survivorship

Total 749 20.814 Leaf rachis length Defoliation 4 1349.85 337.46 6.173 0.0001 Sex 1 1288.04 1288.04 23.560 <0.0001

Subject 142 7763.27 54.67 5.098 <0.0001 Census 6 1711.82 285.30 26.606 <0.0001

Defoliation*Census 24 707.86 29.49 2.750 <0.0001 Sex*Census 6 222.55 37.09 3.459 0.0022

Error 852 9136.32 10.72 Total 1035 21883.1

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Females 25% 0.991 50% 0.994 75% 0.987 100% 0.972 Males 25% 0.997 50% 0.995 75% 0.993 100% 0.976 Leaf production rate (leaves ind-1 census-1)

25% LP = 1.408 – 0.333 ln(t) 1.4 4.47 0.036 50% LP = 1.428 – 0.410 ln(t) 2.8 8.21 0.004 75% LP = 1.546 – 0.562 ln(t) 5.2 14.2 0.0002 100% LP = 1.391 – 0.569 ln(t) 4.6 5.89 0.017 Leaf rachis length (cm) Females 25% 21.4 50% RL = 21.169 – 0.628 (t) 2.4 4.3 0.040 75% RL = 20.790 – 0.096 (t) 4.8 6.6 0.012 100% RL = 19.751 – 0.155 (t) 15.9 8.59 0.006 Males

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25% 23.2 50% 22.8 75% 20.8 100% RL = 22.877 – 0.199 (t) 14.5 8.3 0.006

Figure 1. Effects of defoliation and sex on survivorship probability and leaf production rates of Chamaedorea ernesti-augustii palms. Panels (a) and (c) for female palms, (b) and (d) for males. Fitted lines are best-adjusted GLM models or linear regressions for each defoliation level (see Table 3). Dots represent observed survivorship values and vertical bars correspond to 1 standard error. Figure 2. Effects of defoliation and sex on leaf rachis length [(a) females, (b) males], and proportion of non-marketable leaves (rachis length < 15 cm) availability per defoliation treatment (c). Fitted lines are best-adjusted GLM models or linear regressions for each defoliation level (see Table 3). Vertical bars correspond to 1 standard error. Figure 3. The change of leaf stocks (HLUAi) and profit (EPUA) with defoliation levels and time in Chamaedorea ernesti-augustii palms. Panels (a) and (b) show changes in HLUAi over time at different defoliation levels in females and males, respectively. Panels (c) and (d) show cumulative values of HLUA and EPUA after 5 defoliation events (over three years) at different defoliation levels in females and males, respectively. The inserted boxes in (a) and (b) indicate HLUAi values at the first defoliation event.

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Figure 4. The change of projected values of leaf stocks (HLUAi) and profit (EPUAi) with defoliation levels over 10 years in Chamaedorea ernesti-augustii palms. Panels (a) and (b) show changes in HLUAi over time at different defoliation levels in females and males, respectively. Panels (c) and (d) show cumulative values of HLUA and EPUA at different defoliation levels in females and males, respectively. The inserted boxes in (a) and (b) indicate HLUAi values at the first defoliation event.

Figure 5 Projected values of the EPUA index for ten consecutive years under two economic scenarios. Panels (a, females), (b, males) and (c, both sexes) correspond to annual EPUA values per defoliation level considering a constant annual inflation rate of 3.5%. Panels (d, females), (e, males) and (f, both sexes) correspond to annual EPUA values that consider a proportional increment of the commercial unit price (CUP, see equation 2) inversely related to stock size (see Methods section). Inside panels (c) and (f), net present values (NPV) for each defoliation level combining both sexes and at three discount rates (r) are shown. Figure 6. Trade-off between palm survivorship and total profit (EPUA, including an average annual inflation rate of 3.5%) across different leaf defoliation regimes; (a) female and male palms, and (b) averaged values for both sexes. Trend lines were fitted by hand.

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Sustainable harvesting of non-timber forest products based on ecological and economic criteria