Regenerating mahogany (Swietenia macrophylla King) on clearings in Mexico’s Maya forest: the effects of clearing method and cleaning on seedling survival and growth

Forest Ecology and Management 189 (2004) 143–160

Regenerating mahogany (Swietenia macrophylla King) on clearings in Mexico’s Maya forest: the effects of clearing method and cleaning on seedling survival and growth$ Laura K. Snooka,*, Patricia Negreros-Castillob a

Center for International Forestry Research (CIFOR), P.O. Box 6596, JKPWB, Jakarta 10065, Indonesia Natural Resource, Ecology and Management Department, Iowa State University, Ames, IA 50011, USA

b

Received 6 February 2003; received in revised form 13 April 2003; accepted 24 July 2003

Abstract To mimic catastrophic disturbances which have favored the establishment of natural stands rich in mahogany (Swietenia macrophylla), two 5000 m2 clearings were established in each of four locations using each of three treatments: complete felling; slashing, felling and burning; and machine-clearing, which uprooted all prior vegetation. One to three months later, and again after an additional 12 months, twenty 4-month-old mahogany seedlings were planted in the center of each clearing, and, simultaneously, nearby, under the forest canopy. One year after the first planting and again 7 months later, vines and competing vegetation were cleaned from around the seedlings on one of each type of clearing in each location. Fifty-eight months later, only 5% of mahogany seedlings survived under the canopy, as compared to 32% on felled clearings and 50% on burned or machine-made clearings. Average annual growth of seedlings planted the year clearings were opened was approximately double that of seedlings planted a year later, after natural regeneration of other species had become established. At 58 months uncleaned trees averaged 352 cm in height (and the tallest 600 cm) on burned clearings, 324 cm on machine-made clearings, and 195 cm on felled clearings. Surviving seedlings planted under the forest canopy had grown less than 30 cm during the same period. On burned and machine-made clearings the effect of cleaning on growth was not statistically significant, but on felled clearings cleaning increased growth by 120%, to rates similar to those on burned clearings. Attack by the Hypsipyla grandella shootborer was significantly affected by cleaning. After 58 months, only 12% of seedlings on uncleaned plots had been attacked, compared to 44% of seedlings on cleaned plots. Cleaning also significantly increased vines, particularly on seedlings planted the year after clearings were created: 36% of all seedlings on cleaned plots had vines, as compared to 19% of uncleaned seedlings. In summary, planting mahogany seedlings under the forest canopy cannot be expected to regenerate mahogany trees. However, mahogany seedlings survive and grow well on clearings, with no subsequent interventions, if planted shortly after these are opened by machine or burning. This approach to regeneration could be expected to yield densities of 100 commercialsized mahogany trees/ha among a matrix of 400 naturally regenerated trees/ha of other species. At this rate, regenerating mahogany on clearings equivalent to 3% of the annual cutting area intervened at each harvest, could provide for replacement of mahogany trees harvested from the permanent forest reserves in the region. # 2003 Elsevier B.V. All rights reserved. Keywords: Hypsipyla; Cleaning; Release cutting; Slash and burn; Fire; Sustainability; Shootborer; Silviculture; Vines; Clearcut; Patchcut; Yucatan $

The views expressed in this publication are those of the author, and not necessarily those of CIFOR. Corresponding author. Tel.: þ62-251-622-622; fax: þ62-251-622-100. E-mail address: [email protected] (L.K. Snook). *

0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2003.07.038

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1. Introduction Bigleaf mahogany (Swietenia macrophylla King) is the most important timber species of the Neotropics, and has been harvested from the forests of the Yucatan peninsula for centuries (Chaloner and Fleming, 1850; Mell, 1917; Record, 1924; Lamb, 1966; Snook, 1998). To this day, mahogany represents a major source of timber-derived revenue among 46 forest communities and a number of small property owners in Quintana Roo who control approximately 500,000 ha of commercial forest (Richards, 1991; Snook, 1991, 1998; Argu¨ elles, 1993; Flachsenberg, 1993a; Bray et al., 1993; Kiernan and Freese, 1997). In the past, mahogany harvests in this region have been sustained through progressive expansion into the forest as skidding technologies changed, and progressive dropping of diameter limits as markets changed (Snook, 1998, 2003). Today, for the forest-owning ejidos of Quintana Roo, there is no longer an untapped forest frontier from which new sources of commercial size mahogany trees can be obtained. The only way to sustain mahogany harvests into the future is to ensure that harvested trees are replaced by the growth of existing pre-commercial trees, and the establishment of regeneration in each annual cutting area. Mahogany regeneration represents a challenge due to the interplay between its ecology and selective harvesting practices (Snook, 1996, 2000, 2003). Mahogany has been found to regenerate naturally after catastrophic disturbances which destroy most associated species and produce large open areas (Lamb, 1966; Wolffsohn, 1967; Snook, 1993, 1996, 2000, 2003; Gullison et al., 1996). In the Yucatan peninsula, the typical disturbance regime favoring mahogany regeneration seems to have been hurricanes followed by wildfire, resulting either from lightning strikes (Wolffsohn, 1967) or spreading from burns used to clear agricultural fields. As a result of this strategy, mahogany trees tend to occur in essentially even-aged stands, although these may include some older individuals that survived the stand-initiating disturbance. These post-disturbance stands may include as many as 50 mahogany trees/ha (>30 cm) among 450 trees/ha of other species (Snook, 1993, 2000, 2003). These conditions may also be produced by shifting agriculture, which has been

practiced in the Maya forest for millennia (Hammond, 1982; Edwards, 1986). Previous authors have noted that mahogany trees commonly regenerate naturally on abandoned agricultural fields (Stevenson, 1927; Wolffsohn, 1961). Since no other species have equivalent timber value or demand, mahogany is ordinarily the only species harvested, though in a few areas, in recent years, individuals of several other species have been extracted as well (Flachsenberg, 1993a,b; Dickinson et al., 2000; Sills and Romero, 2002). All mahogany trees above the current commercial diameter limit (55 cm) are harvested, while most trees of other species are left standing. Because all mahogany trees in an aggregation tend to reach commercial size at about the same time, mahogany harvests may remove 95% of all mahogany trees in the focal stand (Gullison et al., 1996). Since these are mixed-species stands, this selective harvesting impedes mahogany regeneration in two ways: by depleting mahogany seed sources (Gullison et al., 1996), and by maintaining shady conditions inimical to the survival of mahogany seedlings (Snook, 1996; Whitman et al., 1997; Dickinson and Whigham, 1999). Mahogany seeds do not retain their viability longer than a few months (Rodrı´guez y Pacheco and BarrioChavira, 1979; Parraguirre, 1994; Morris et al., 2000), so there is no mahogany seed bank in the soil. However, the lack of seed sources can be overcome by sowing mahogany seed or planting mahogany seedlings. Enrichment planting has been carried out in the forests of Quintana Roo and elsewhere in Latin America, but its success has rarely been systematically evaluated. One study in Quintana Roo found that an average of only 22% of planted mahogany seedlings survived 1–3 years after planting (Negreros-Castillo and Mize, 2003). Since mahogany regenerates in response to disturbance, various experiments have evaluated ways of favoring mahogany regeneration by increasing canopy opening through gap creation (Stevenson, 1927; Wolffsohn, 1961; Negreros-Castillo and Mize, 1993; Gerhardt, 1994; Negreros-Castillo and Hall, 1996; Weaver and Sabido, 1997; Dickinson and Whigham, 1999). Those studies focused on degree of canopy opening and the results of direct seeding, planting, and natural regeneration under different levels of light and root competition. The current study evaluates the survival of mahogany seedlings on clear-

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ings created to mimic the conditions which have favored the natural regeneration of mahogany in the past, in particular hurricanes followed by wildfire.

2. Study area This study was carried out in the state of Quintana Roo, on Mexico’s Yucatan peninsula (Fig. 1). Quintana Roo is 49% forested, and produces 32% of Mexico’s precious tropical timbers (mostly mahogany as well as some cedar, Cedrela odorata) (INEGI, 1990:94). Forty-three percent of Quintana Roo is controlled through communal land ownerships called ‘ejidos’ (INEGI, 1990:95). Each of the 46 ejidos involved in commercial forestry in the state controls from 3000 to over 55,000 ha of land. These include 1000–30,000 ha of permanent forest reserve managed for the production of timber and other forest products (Flachsenberg, 1993a; Snook, 1991, 1998). The experiments described here were established in the permanent forest reserves of three ejidos and one

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private property in the Maya Zone of central Quintana Roo, between 888040 and 888320 W longitude and 198060 and 198430 N latitude. These seasonal tropical forests are part of the Maya Forest, which extends into Belize and Guatemala and is the most extensive contiguous tropical forest area north of the Amazon. They are composed of over 100 canopy tree species (Medina et al., 1968), of which the currently most abundant are Manilkara zapota, which occurs at densities of 15–60 individuals per ha, and Brosimum alicastrum (Miranda, 1958; Argu¨ elles, 1991). Canopy height averages 20–25 m (Snook, 1993). Mahogany is more common in this forest type than any other in Mexico (Pennington and Sarukha´ n, 1968). While mahogany trees are typically found in clumps, average densities of about one commercialsized tree/ha have been estimated from forest inventories (Argu¨ elles, 1991; Flachsenberg, 1993b). Soils are calcareous, derived from uplifted marine sediments, and topography is relatively flat (Wilson, 1980). Average annual rainfall of 1100–1500 mm/year falls mostly between May and October (SARH, 2001)

Fig. 1. Area within which the experimental areas were located, in the ejidos of Naranjal Poniente, Xpichil, Limones/Cafetal, and on the private property known as ‘‘Rancho Grande’’.

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and varies considerably from year to year depending on storms (Wilson, 1980). During the dry season, which becomes most extreme between February and April, many species drop their leaves for a short time (Pennington and Sarukha´ n, 1968). Hurricanes hit Quintana Roo almost every year, periodically knocking down or defoliating thousands of hectares of forest (de Landa, 1566; Jauregui et al., 1980; Wilson, 1980; Escobar, 1981). Forest fires, often spreading from slash and burn agricultural fields, commonly occur in the years following hurricanes, when fuel is abundant, and may burn hundreds of thousands of hectares of forest (Lundell, 1938; Lamb, 1966; Lo´ pez-Portillo et al., 1990; Whigham et al., 1991; Snook, 1993, 1998, 1999). As a result of the frequency and scale of past disturbances, the forest consist of a mosaic of different-aged aggregations of trees (Snook, 1993, 2000, 2003), so an annual cutting area or a permanent forest reserve may include younger aggregations of trees, including pre-commercial mahoganies, revealed in forest inventories (Argu¨ elles, 1991; Flachsenberg, 1993b).

3. Methods These experiments compared the effects on the survival and growth of planted mahogany seedlings of two factors, clearing treatment and cleaning, applied according to a complete randomized block design (Fig. 2). Planting seedlings on experimental plots in two successive years added a third factor, time lag between treatment and planting. In each of four locations, eight 50 m  100 m experimental areas were laid out on an east–west axis within the forest. Two 5000 m2 areas per location were allocated at random to each of three clearing treatments, yielding eight experimental clearings per treatment. Subsequently, seedlings were cleaned on one of the two clearings per location produced using each of the three methods, yielding four replicates of each combination of factors. In one clearing treatment, referred to hereafter as felling, the understory was slashed and all trees were felled. Some were removed by hand, but most were left on site. In another, referred to as machine-clearing, rubber-tired Tree Farmer skidders or tracked bulldozers were used to knock over and push vegetation to

Fig. 2. Schematic diagram of blocked experimental design with two factors (clearing treatment and cleaning). Small squares represent 10 m  10 m control plots located under the forest canopy. Rectangles represent 50 m  100 m clearings (see Fig. 3). Within each location, plots were not physically located with this regularity, and allocation of treatments among plots was randomized.

the edges of the experimental area. On a few clearings, some trees could not be uprooted and were either felled using chainsaws or left standing. Where clearing operations were observed, it seemed that little soil was removed by the process of uprooting and pushing trees to the side: blades of tree farmers and bulldozers were kept above the soil surface. In the third treatment, which will be referred to as burning, the understory was slashed with machetes and trees were cut with machetes, axes, or occasionally, chain saws. The resulting debris was allowed to dry, then it was burned, using the practices applied by local farmers to clear their agricultural fields. Some burns were more complete than others, as evidenced by the amount of

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unburned debris remaining on site afterwards. Treatments were carried out between April and June 1996, with the exception of the central portion of one machine-clearing, which was delayed due to machinery breakdown. In July of that year, 20 mahogany seedlings were planted on 100 m2 plots in the center of 23 of the 24 experimental clearings and on nearby control plots under the forest canopy. These seedlings are referred to as ‘‘cohort 0’’. Because of the delay in completing one of the machine-clearings, no seedlings were planted there in the first year. Cohort 0 seedlings for three locations were obtained from the nursery run by SEDENA (Secretarı´a de Defensa Nacional), near Chetumal, while seedlings for one location were obtained from the nursery run by SEMARNAP (Secretarı´a de Medio Ambiente, Recursos Naturales y Pesca) in Felipe Carrillo Puerto. Cohort 0 seedlings for each location were selected separately from the nursery just prior to planting. Farmers who had carried out the burns were permitted to plant agricultural crops on the experimental clearings. This took place only in two locations, on four of the burned clearings and one of the clearings opened by machine. Since the spacing between seedlings was wide, and annual crops (corn, beans and squash) were produced only the first year and perennial crops (chili peppers) only the second, we assumed that interplanted crops would not affect seedling growth. In light of mortality rates observed 6 months after planting, it was decided to increase the number of seedlings on each plot, and in July of the following year, an additional cohort of 20 seedlings was planted, using the same spacing, in a 4 m wide extension around each central plot on all 24 clearings, and on nearby control plots under the forest canopy. These seedlings are referred to as ‘‘cohort 1’’ (Fig. 3). These seedlings had been produced for this purpose, from seed, in each location. Seedlings of both cohorts were 4 months old at the time of planting, and averaged 20 cm in height. At each location, seedlings on one of each pair of experimental clearing treatments were cleaned 1 year after planting cohort 0, at the time of planting cohort 1; and again 7 months later. Cleaning consisted of cutting vines and all stems on the central plots, except the mahogany seedlings, near ground level with a machete. Seedlings planted on control plots under the forest canopy were not cleaned.

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Fig. 3. Layout of experimental plots. Clearing treatments were applied over the whole 5000 m2 area; cleaning treatments were applied only around the seedlings planted in the center.

At intervals (initially 6 months, then 1 and finally 2 years), heights of all mahogany seedlings were measured. The number of vines and attacks by Hypsipyla grandella shootboring caterpillars on each tree were counted and recorded. Analyses of rates of survival, growth and degree of infestation by damaging agents were carried out using Systat 7.01 for Windows (SPSS, 1997), which automatically compensates for missing data (i.e. one machine-cleared plot of cohort 0 seedlings). General linear models procedures for randomized complete block designs, considering location as block, were used to analyze differences among plot averages by clearing treatment, cleaning and cohort, complemented by Tukey pairwise comparisons to determine which treatments were significantly different from others. When controls and clearing treatments were compared, only uncleaned plots were included. When cleaning effects were evaluated, only plots on experimental clearings were included in the analysis. Proportional values (survival, mortality, percent of seedlings attacked by the shootborer, and percent seedlings infested by vines) were arcsine-transformed before GLM analysis, using Anscombe’s (1948) formula, as recommended by Zar (1984:240). Growth was the difference between the height of seedlings at planting and their heights at the last measurement date. Height growth over this period was linear (see Fig. 5), and both data and residuals from GLM analyses were normally distributed, so data was not transformed (Grafen and Hails, 2002). Height increment over the full measurement period was divided by the number of months and multiplied by 12 to give an average annual growth rate, used to combine and compare the two cohorts.

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4. Results 4.1. Survival General linear models comparing seedlings on controls and uncleaned seedlings on the three different clearing treatments revealed that the percent of seedlings surviving on each plot at the time of the last measurement (58 months for cohort 0 seedlings, 46 months for cohort 1 seedlings) was significantly affected by location ðP ¼ 0:004Þ and treatment ðP < 0:001Þ, but differences between cohorts were not significant (P ¼ 0:092, R2 ¼ 0:772) (Table 1; Fig. 4). Survival on controls (5%) was significantly lower than survival on clearings ðP < 0:003Þ, and survival of uncleaned seedlings on felled clearings (24%) was significantly lower than on machine-made clearings (53%) ðP ¼ 0:001Þ. On burned clearings, 40% of uncleaned seedlings survived at the last measurement. When the effects of clearing treatments and cleaning were analyzed together for both cohorts combined, survival was found to be significantly affected by clearing treatment ðP ¼ 0:024Þ, but not by cleaning ðP ¼ 0:080Þ, cohort ðP ¼ 0:757Þ or location (P ¼ 0:105, R2 ¼ 0:367). The interaction between treatment and cleaning was not significant ðP ¼ 0:241Þ. Tukey tests confirmed the significant difference between survival on burned and machine-made clearings and survival on felled clearings (P ¼ 0:044 and 0.045, respectively). When cohorts were analyzed separately, cleaning was found to have significantly affected survival for cohort 1 ðP ¼ 0:031Þ, but not for cohort 0 ðP ¼ 0:986Þ seedlings.

The patterns of mortality over the period of measurement varied by cohort and by interval (Fig. 3). Mortality rates over the first 6 months after planting were significantly lower ðP < 0:001Þ for cohort 1 seedlings (18%) than for cohort 0 seedlings (42%), but did not vary significantly among treatments (P ¼ 0:152, R2 ¼ 0:519). Mortality rates over the first year were significantly affected by location, cohort, and treatment (P ¼ 0:002, 0.025 and 0.038, respectively, R2 ¼ 0:459) with higher rates among cohort 0 seedlings, on felled clearings, and on cleaned plots. Comparisons between the two cohorts during their first year after planting incorporate differences between years. Mortality rates over the last 2 years of measurement, which were concurrent for both cohorts, were significantly affected by location, cohort, and cleaning, but not by treatment (P ¼ 0:002, 0.003, 0.024 and 0.998, respectively), and the patterns of the first year were reversed. During this interval, mortality rates were lowest in the location where they had been highest during the previous period, cohort 0 mortality rates were lower than those of cohort 1, and mortality was lower on cleaned plots than on uncleaned plots. Over the last 2 years of the study, annual mortality rates among cohort 0 seedlings were 2.5% on cleaned plots and 5.2% on uncleaned plots. Annual mortality rates for cohort 1 seedlings were 5.9% on cleaned plots and 14% on uncleaned plots. 4.2. Growth When both cohorts of uncleaned seedlings on controls and clearings were analyzed, general linear models revealed that cohort and treatment significantly

Table 1 Average percent survival by cohort, treatment and cleaning, plus or minus standard errorsa

Survival (%), cohort 0 (58 months) Survival (%), cohort 1 (46 months) a

Control (A) Uncleaned

Felled (B)

Machined (C)

Burned (C)

Cleaned

Uncleaned

Cleaned

Uncleaned

Cleaned

Uncleaned

52 43

35  4 45  17

28  13 21  9

43  17 51  9

54  8 51  15

48  8 71  14

50  7 30  9

Different letters next to values indicate statistically significant differences at a ¼ 0:05 based on GLM and Tukey tests on plot averages. Control plots were not cleaned and were analyzed only in relation to other uncleaned plots. To evaluate differences in survival among clearing treatments, data from the two cohorts was combined and analyzed together (indicated by A, B, C). Although GLM revealed that cleaning had a significant effect on survival of cohort 1 seedlings, Tukey tests on the effects of clearing treatment and cleaning by cohort did not reveal significant differences.

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Fig. 4. Percent of mahogany seedlings surviving over time by cohort, clearing treatment, and cleaning over the period of the study. Vertical lines indicate cleanings.

affected annual growth (for both, P < 0:001, R2 ¼ 0:832) (Table 2; Fig. 5). Growth on controls was significantly lower than growth on clearings ðP < 0:011Þ, and growth of seedlings on machinemade clearings tended to be higher than the growth of seedlings on felled clearings ðP ¼ 0:053Þ. When analyses were carried out on both cohorts of cleaned and uncleaned seedlings on clearings, GLM revealed that cohort, treatment, cleaning, and the treatment  cleaning interaction all had significant effects on annual growth (P < 0:001, 0.015, <0.001, 0.005, respectively, R2 ¼ 0:679). Seedlings grew an average of 91–209% faster on uncleaned plots and 32–75% faster on cleaned plots, when planted shortly after

clearings were created, as compared to a year later. Growth on burned clearings was significantly higher than growth on felled clearings ðP ¼ 0:015Þ. Over 58 months, uncleaned cohort 0 seedlings on burned clearings grew an average of 328 cm, achieving average heights of 352 cm and maximum heights of 600 cm, as compared to growth of 302 cm and average heights of 324 cm on machine-clearings, where the tallest trees reached 518 cm. Over the same period, trees on felled clearings grew 168 cm, to achieve average heights of 195 cm and maximum heights of 353 cm on uncleaned plots. For cohort 0 seedlings, differences in annual growth due to cleaning were significant ðP ¼ 0:039Þ, as was

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Table 2 Average annual height growth (derived from the difference between seedling height at planting and height at last measurement) by cohort, treatment and cleaning, plus or minus standard errorsa

Annual growth (cm), cohort 0 Annual growth (cm), cohort 1

Control (A) Uncleaned

Felled (B) Cleaned

31a 63a

77  6 d 44  9 f, g

Machined (B, C)

Burned (C)

Uncleaned

Cleaned

Cleaned

Uncleaned

35  11 e 16  3 f, g

50  17 d, e 63  7 d, e 36  8 f, g 33  8 f, g

83  7 d, e 63  11 f

68  11 d, e 22  3 g

Uncleaned

a Different letters next to values indicate statistically significant differences at a ¼ 0:05 based on GLM and Tukey tests on plot averages. Control plots were not cleaned, and were analyzed only in relation to other uncleaned plots. To evaluate differences in growth rates among clearing treatments, data from the two cohorts was combined and analyzed together (indicated by A, B, C). Results of separate analyses of the effects of clearing treatment and cleaning on each cohort are indicated by the d, e, f and g.

the interaction between clearing treatment and cleaning ðP ¼ 0:028Þ. Tukey tests revealed that growth differences due to cleaning were significant for cohort 0 seedlings only on felled clearings ðP ¼ 0:034Þ,

but not on burned ðP ¼ 0:826Þ or machine-made clearings ðP ¼ 0:967Þ. On machine-clearings, growth on uncleaned plots was higher than on cleaned plots (though this difference was not statistically significant)

Fig. 5. Growth of planted mahogany seedlings over time, by cohort, clearing treatment and cleaning over the period of the study. Vertical lines indicate cleanings.

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(Table 2). For cohort 1 seedlings, although cleaning had a significant effect on growth ðP ¼ 0:003Þ, treatment did not ðP ¼ 0:291Þ, and the treatment  cleaning interaction was not statistically significant ðP ¼ 0:085Þ. However, Tukey tests revealed that growth differences between cleaned and uncleaned seedlings were statistically significant only on burned clearings ðP ¼ 0:025Þ, but not on felled ðP ¼ 0:245Þ or machine-made clearings ðP ¼ 1:000Þ. On burned clearings, the increase in annual growth due to cleaning was 22% for cohort 0 and 186% for cohort 1; on felled clearings the growth increase was 120% for cohort 0 and 175% for cohort 1 (Table 2). 4.3. Attack by H. grandella The shootboring insect H. grandella is a moth that lays its eggs on the terminal shoots of mahogany trees. Its larval caterpillars eat into the growing stem, causing adventitious branching, reduced growth, or mortality. When general linear models were used to test the effect of cohort, treatment and cleaning on the percent of surviving seedlings attacked by the shootborer, only cleaning ðP ¼ 0:013Þ was found to have had a significant effect. The treatment  cleaning interaction was not significant (P ¼ 0:972, R2 ¼ 0:331). For all treatments combined, the proportion of seedlings attacked by the shootborer on cleaned plots (44% for cohort 0; 18% for cohort 1), was three times or more the rate on uncleaned plots (12% for cohort 0; 6% for cohort 1) (Fig. 6). Although treatment did not emerge as a significant factor when cohorts were combined and plot averages were compared, the proportion of uncleaned cohort 0 seedlings attacked was more than triple on burned clearings (23%) the rate of attack of uncleaned seedlings on felled or machine-clearings (6%). Among cohort 1 seedlings, rates of attack of uncleaned seedlings on burned and machine-made clearings was 8% as compared to 2% on felled clearings. On machine-clearings, the proportion of seedlings attacked was 9–10 times greater (for cohorts 0 and 1, respectively) on cleaned plots (54 and 21% respectively) than on uncleaned plots (6 and 2%, respectively). The cumulative number of attacks per surviving seedling confirmed and emphasized these patterns. When general linear models procedures were used to analyze plot averages, both cohort and cleaning were found to have significant effects (P ¼ 0:045 and

Fig. 6. Percent of mahogany seedlings attacked at least once by the H. grandella shootborer over the period of the study, by cohort and cleaning (yes or no), plot averages with standard errors.

0.040, respectively, R2 ¼ 0:286), but treatment did not ðP ¼ 0:438Þ. The average level of attack on cohort 1 seedlings was only 18% of the cumulative number of attacks on cohort 0 seedlings, while, on average, seedlings on cleaned plots experienced 4–6 times as many attacks as uncleaned seedlings (for cohorts 1 and 0, respectively). Although differences among treatments were not statistically significant, the cumulative number of attacks per seedling on burned clearings was triple the rate of attack on felled clearings (Fig. 7). Cleaned seedlings on burned clearings experienced four times as many attacks as uncleaned seedlings. On machine-cleared and felled clearings, the cumulative number of attacks on cleaned seedlings was 10 times higher than the number of attacks on uncleaned seedlings. 4.4. Vine load General linear models revealed that the proportion of seedlings with at least one vine at the last measurement differed significantly among locations ðP ¼ 0:018Þ, between cohorts ðP ¼ 0:017Þ and as a result of cleaning ðP ¼ 0:001Þ, but not by treatment (P ¼ 0:114, R2 ¼ 0:480). The treatment  cleaning interaction was not significant ðP ¼ 0:699Þ. An average of 37% of cohort 0 seedlings had at least one vine, as compared to 18% of cohort 1 seedlings. For cohort 0 seedlings, infestation on cleaned plots was 40% as compared to 34% on uncleaned plots (Fig. 8).

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Fig. 7. Average cumulative number of attacks per cohort 0 mahogany seedling by clearing treatment and cleaning, plot averages with standard errors.

The proportion of cohort 1 seedlings affected by vines was 10 times higher on cleaned plots (32%) than the proportion affected on uncleaned plots (3%). The average number of vines per seedling at the last measurement followed a similar pattern: cohort and cleaning were found to have significant effects (P ¼ 0:006 and 0.005, respectively), as did location ðP ¼ 0:020Þ; but treatment did not, nor was the treatment  cleaning interaction significant (P ¼ 0:591 and 0.305, respectively, R2 ¼ 0:465). However, the impact of cleaning on vine load appeared to be more pronounced on felled clearings, particularly for cohort 0 seedlings (Fig. 9).

5. Discussion

Fig. 8. Percent of mahogany seedlings with at least one vine at the time of the last measurement, by cohort and cleaning (yes or no), plot average with standard errors.

Fig. 9. Average number of vines per mahogany seedling, at the time of the last measurement, by clearing treatment and cleaning (both cohorts combined), plot averages with standard errors.

This experiment established clearings that increased light levels and reduced initial competition for mahogany seedlings. The techniques used to create the clearings differentially affected the reestablishment and growth of natural regeneration of dozens of associated forest species, from seed and sprouting, and thus the conditions influencing the survival and growth of mahogany seedlings. Cleaning surrounding vegetation from around the mahogany seedlings additionally modified these conditions.

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5.1. Survival The 95% mortality after 4 or 5 years of mahogany seedlings planted under the forest canopy was not surprising (see Lamb, 1966:116; Negreros-Castillo and Mize, 2003). Over the same 46–58 months, the mortality of an average of 68% of mahogany seedlings on felled clearings as compared to 50% on those cleared by felling and burning or machine-clearing reflects a higher degree of competition on felled clearings, due to abundant sprouting from stumps and roots of preexisting vegetation. Many species in this forest sprout after felling (Rewald, 1989; Snook, 1993; Dickinson et al., 2000; Negreros-Castillo and Hall, 2000), but sprouting was inhibited on machinemade clearings, where trees were uprooted, and by fire on burned clearings. This explains, too, the significant effect of cleaning, which diminished the competition from preexisting vegetation. The significant effect of location on survival of cohort 0 seedlings may reflect variation in the height and quality of the seedlings, which were selected separately for each location and came from two different nurseries. The lower rate of mortality among cohort 1 seedlings as compared to cohort 0 seedlings during their respective first years of growth may reflect differences in precipitation between the 12-month periods, July 1996–June 1997 and July 1997–June 1998. Data from the nearby city of Felipe Carrillo Puerto reveal that rainfall was <1200 mm during the first 12 months after the planting of cohort 0 seedlings, and >1550 mm during the comparable period for cohort 1 seedlings (SARH, 2001). Also, light shade resulting from natural regeneration on the clearings probably reduced initial stress on cohort 1 seedlings. The reversal, during the final 2 years of measurement, of patterns of mortality observed during the first year, may indicate that mortality and survival equilibrate at different rates in response to different conditions. 5.2. Growth Growth varied more in response to light and competition than did survival. Seedlings on clearings grew up to 23 times faster than seedlings under the forest canopy (Table 2; Fig. 5). Higher growth on burned and machine-made clearings as compared to felled

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clearings probably reflected the reduction in competition from sprouts. Burning probably additionally favored the growth of mahogany the same way it favors the growth of crops on agricultural fields created using similar techniques: by releasing nutrients in the form of ash. Whereas cohort 0 seedlings were planted on bare ground, those planted the year after clearings had been created were introduced into an environment where competing vegetation had already become established, and exceeded the 20 cm height of the new mahogany seedlings. It was not surprising that the annual height growth of these seedlings was only 32–52%, on uncleaned plots, and 57–76%, on cleaned plots, of the rate of seedlings planted the year before, depending on the clearing treatment (Table 2). It is clearly important to plant mahogany shortly after clearings are opened to give the seedlings a head start in the competition with natural regeneration of other species. However, by releasing them from competition, cleaning greatly improved growth rates of seedlings planted the year after clearings were opened: on burned clearings growth approached (92%) and on felled clearings it exceeded (120%) that of uncleaned seedlings planted the previous year on the same clearings. However, growth still fell short by a minimum of over 30% of that of cleaned seedlings planted immediately after clearings were opened. The relative impact of cleaning on clearings produced in different ways (the significant clearing treatment  cleaning interaction for cohort 0 seedlings) reflected the different levels of competition resulting from different rates of regrowth or regeneration of competing vegetation after the different clearing treatments. Where previous vegetation had been felled without burning, resprouting from stumps was rapid and abundant, and the growth of mahogany seedlings planted that same year was more than doubled when competing vegetation was cut back. Cleaning did not significantly increase the growth of cohort 0 seedlings on burned or machine-made clearings. Both burning and machine-clearing apparently inhibited regrowth sufficiently that mahogany seedlings planted the same year were able to hold their own within the mixture of species that became established naturally on the clearings, as has been observed previously for naturally regenerated mahogany (Snook, 1993, 2000, 2003).

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It is noteworthy that cleaning, which doubled the rates of growth of cohort 1 seedlings on burned clearings, did not significantly increase their growth on felled or machine-made clearings. The lack of statistical significance despite the nearly 3-fold difference in growth between cleaned and uncleaned cohort 1 seedlings on felled clearings may reflect the high mortality on those clearings, which had reduced the number of uncleaned plots with live seedlings from 4 to 3, thus increasing the error. The lack of significant differences in growth between cleaned and uncleaned cohort 1 seedlings on machine-made clearings suggests that machine-clearing, which uprooted preexisting vegetation, may have more effectively reduced the rate of recolonization by other species than did burning, so mahogany seedlings planted a year later could still compete effectively. An alternate explanation for the similarity in growth rates between cleaned and uncleaned seedlings of both cohorts on clearings opened by machine, may be the higher rate of shootborer attack on cleaned seedlings, as described below. Growth rates of cohort 0 seedlings on uncleaned machine-made and burned clearings compare favorably to those reported for mahogany seedlings planted in canopy openings in natural forest settings elsewhere in its native range. Ramos and del Amo (1992) reported mean height growth of 60 cm per year among mahogany seedlings planted in 1000 m2 patch clearcuts created in 10-year-old forest fallow, which had been weeded initially every 3 months, and subsequently twice a year. Assuming 20 cm initial seedling heights, these seedlings would have reached heights of 310 cm in 58 months. D’Oliveira (2000) reported average heights of 331 cm for 5-year-old mahogany seedlings planted in cleaned felling gaps 100–800 m2 in size. The higher growth of seedlings planted on our burned clearings is particularly noteworthy in light of the fact that these other sites, one in Brazil and the other elsewhere in Mexico, received 25–240% more rainfall, respectively, than the experimental sites in our study. While mahogany growth rates have been shown to increase in response to increased precipitation (Whigham et al., 1998), these comparisons indicate that maximizing light availability and reducing initial competition and the potential for resprouting, as we did by producing 5000 m2 clearings using fire or uprooting, provides an effective stimulus to mahogany growth.

5.3. Attack by H. grandella Attack by the shootborer has been widely perceived as a major impediment to the successful regeneration of mahogany by planting (Newton et al., 1993a,b; Mayhew and Newton, 1998). However, over the 58 months of this study, we found that only 12% of the cohort 0 mahogany seedlings planted on clearings where the vegetation was allowed to regrow, were attacked at least once; only 6% of cohort 1 seedlings on uncleaned plots were attacked over 46 months of observation. The lower rate of attack on cohort 1 seedlings may reflect one less year of sampling and/ or the lower growth rates of seedlings in this cohort (Table 2), which would have reduced the abundance of long, fresh shoots favorable to shootborer larvae (see, for example, Dickinson and Whigham, 1999). The tripling of the proportion of seedlings attacked and the 4–6-fold increase in the number of attacks per seedling, in response to the cleaning of surrounding vegetation from around the mahogany seedlings, might reflect increased seedling growth due to cleaning. However the 10-fold increase in attack rates due to cleaning on machine-made clearings was not reflected in a corresponding increase in growth (perhaps because new growth was attacked and fell off). These results parallel observations described in Mayhew and Newton (1998:134–135), that ‘unweeded’ plantations, those where mahogany stems are surrounded by vegetation, were less likely to be attacked by the shootborer. This pattern suggests that individuals in the surrounding vegetation matrix serve as a physical or chemical ‘‘shield’’ against the Hypsipyla moth. The substantial increase in the rate of attack by the shootborer as a result of cleaning may indicate that some shield species were diminished by cutting, or did not resprout and were lost from cleaned plots. The differences in levels of attack among treatments, although not statistically significant, follow the same kind of pattern. The fact that the proportion of cohort 0 seedlings attacked was three times higher and the frequency of attacks was eight times higher on burned clearings than on felled clearings may reflect either the faster growth of seedlings on burned clearings, or the differential survival and regeneration, in response to the clearing treatments, of ‘‘shield’’ species which somehow protect mahogany seedlings from the shootborer. It may be that burning impeded the

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regeneration of species that serve as particularly effective shields. Such species might have been represented among the sprouts of previous vegetation, which were abundant on felled clearings. These possibilities are supported by studies of the species composition of natural regeneration in one location a year after the clearings were created. Felled clearings were found to have had the highest species diversity, including 21 species not found on other kinds of clearings, while burned clearings had the fewest exclusive species (8) (Valdez, 1999). The fact that both proportions and rates of attack of uncleaned cohort 1 seedlings did not differ among the three clearing treatments may simply reflect the fact that the proportion of seedlings attacked (2–8%) was too low to reveal such patterns. The extraordinary increase in rates of attack on machine-clearings as a result of cleaning (from 6 to 54% of cohort 0 seedlings and from 2 to 21% of cohort 1 seedlings attacked, with an 8-fold and 13-fold increase, respectively, in average frequency of attacks per seedling) may reveal that some species that regenerated particularly successfully on machine-made clearings and created an exceptionally effective chemical or physical ‘‘shield’’ against the Hypsipyla moth, did not resprout when cut back. One year after clearing, machine-made clearings had 23 species not found on other kinds of clearings (Valdez, 1999). 5.4. Vine load Over 30% of uncleaned cohort 0 seedlings had vines as compared to 3% of uncleaned cohort 1 seedlings (Fig. 8). This probably reflects the fact that most vines, which regenerated within months of the opening of clearings, had already attached themselves to hosts by the time the cohort 1 seedlings were planted. The 10fold increase in the percent of cohort 1 seedlings with vines as a result of cleaning (as compared to a 6% increase in the proportion of cohort 0 seedlings with vines) suggests that vines that had originally colonized cohort 0 seedlings, and were cut during cleaning, were just as likely to recolonize a cohort 1 seedling afterwards. The significant increase in vines after cleaning has been observed before (see Dawkins, 1961 in Lamb 66:139), and probably reflects the high resprouting capacity of vines after cutting and the increase in available sunlight after cleaning. The greater increase in vines after cleaning on felled clearings probably

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reveals that many of the vines on these clearings were originally of sprout origin, and resprouted after cleaning more vigorously than those that had recolonized burned and machine-made clearings, which were more likely to have regenerated from seed.

6. Conclusions and implications for sustaining mahogany in natural forests These experiments confirmed that enrichment planting with mahogany seedlings under the forest canopy is not worthwhile. Seedlings survived and grew well on experimental clearings 5000 m2 in size, but different ways of producing those clearings and different intervals between clearing and planting yielded different results. Complete felling alone favored abundant resprouting from stumps of preexisting vegetation, creating a competitive environment where mahogany seedlings suffered about 70% mortality over 46–58 months, and low growth. Both machine-clearing and slashing and felling followed by burning yielded similarly good rates of survival (approximately 50% after 5 years), and similar growth rates, though those on burned clearings were somewhat higher. This experiment was designed around the expectation (Snook, 1993, 1998, 2003) that given the size of mahogany crowns at the commercial diameter of 55 cm (100 m2; Snook, 1993), only one of the fastest growing seedlings would eventually grow to commercial size and occupy each 100 m2, a final survival rate of 5%. If the 5% per year mortality rate determined over the last 22 months of the measurement period for uncleaned cohort 0 seedlings were projected into the future, all of the surviving planted seedlings might be expected to die within 20 years. This projection may be inappropriate, and can be evaluated through continuing data collection. However, the density planted (20 seedlings/100 m2) was determined for reasons of experimental design, not production. It might be necessary to slightly increase that density in operational regeneration treatments to ensure 1 crop tree/ 100 m2. Regenerating mahogany trees on clearings in this way, to yield 100 mahogany trees/ha among about 400 naturally regenerated canopy trees/ha of other species at the time the mahoganies reach commercial diameters (55 cm dbh), would essentially recreate the

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structure of natural high-density mahogany stands in this forest, which have regenerated in the past after fires (Snook, 1993, 2000, 2003). Cleaning was found not to significantly increase the growth of seedlings planted on burned or machinemade clearings a few months after they were opened, and had significant negative effects, multiplying several-fold the rate of attack by the H. grandella shootborer and increasing the vine load, particularly on seedlings planted the year after clearings were established. Both of these unintended consequences of cleaning are likely to negatively affect the future growth of cleaned seedlings, as compared to uncleaned seedlings. These outcomes reveal that when appropriate methods are used to create clearings favorable for regeneration, and seedlings are planted shortly after clearings are established, it is neither necessary nor desirable to make subsequent investments in cleaning. At 58 months, 10 seedlings, of which the tallest averaged over 5 m in height, survived on each uncleaned 100 m2 plot on machine-made and burned clearings. Of these, only 6 and 23%, respectively, had been attacked by shootborers, leaving at least seven unaffected. Although future attack rates and effects on form are unknown, this would seem to indicate that shootborer attack does not represent a constraint to successfully regenerating mahogany on clearings. Based on growth data obtained from natural stands, it seems likely that about 1/3 of the population of mahogany trees on a stand are likely to reach the current commercial diameter of 55 cm in 80 years (Snook, 2000, 2003). If one were to apply this proportion to stands resulting from plantings on clearings, and were to consider only those trees that would reach harvestable size in 80 years, it would seem reasonable to assume that 1 ha of clearing with 100 surviving mahogany trees could provide, in 80 years, for the replacement of about 33 commercial-sized mahogany trees, a density equivalent to that found, on average, over 33 ha of forest in this region. This implies, at its simplest, that populations of harvestable mahogany trees (if not current harvest volumes), could be sustained over the long term if regeneration treatments of this sort were applied to 1 ha per 33 ha, or 3% of each cutting area, at the time of each harvest. In ejidos which currently harvest timber, clearings are opened by machine for use as log yards.

These have been calculated to cover between <1% (Francisco Javier May, pers. commun. 2003; Snook, 1993) and 5% (Argu¨ elles, 1991) of each annual cutting area. In some ejidos, these clearings have been deliberately located near mahogany seed trees, or mahogany seedlings have been planted on the log yards after harvesting operations are concluded. These are worthwhile efforts, particularly if planting is carried out the same year the clearings are abandoned. Growth on log yards, which have been compacted as a result of use, might be expected to be lower than rates observed in this study. However, growth rates calculated by subtracting an assumed 20 cm seedling height from reported heights at 5–6 years of planted mahoganies on 10 log yards 300–3600 m2 in two neighboring ejidos, were found to average 58 cm per year, a rate less than 10% lower than what we measured on machine-made clearings over a similar period (Synnott et al., 1995; Table 2). Clearings could be extended beyond the areas required for yarding, or additional clearings could be made and planted elsewhere in the cutting area, to ensure mahogany regeneration on 3% of each annual cutting area. Permanent forest reserves managed by local communities for timber production range from 1000 to 30,000 ha per ejido (Flachsenberg, 1993a), divided into annual cutting areas (based on the current 25-year cutting cycle) ranging from 40 to 1200 ha. Three percent of the annual cutting area would represent from just over 1 to 360 ha, depending on the ejido. Given canopy heights and mahogany’s responsiveness to available light, clearings should probably not be smaller than 5000 m2 (Snook, unpublished data), but they could probably extend to several hectares without impeding natural regeneration processes or floral–faunal relationships. In these forests, felling gaps tend to measure <400 m2, less than a tenth of the size of our experimental clearings (Dickinson et al., 2000). The poor survival and growth of mahogany seedlings on clearings produced by felling suggests that expanding gaps by felling additional trees around the edges would be unlikely to provide conditions favorable to mahogany survival and growth unless some complementary treatments, such as oiling stumps, uprooting trees by machine, or burning, were carried out to limit sprouting and reduce competition from advance regeneration.

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Clearing 3% of each annual cutting area would mean that a total of 6–9% of the forest would be affected by regeneration clearings over the course of the 2 or 3 cutting cycles prior to harvesting the regenerated trees. Under the current 25-year cutting cycle followed in Quintana Roo, this harvest would take place 75 years after the first, after three cutting cycles; if the cutting cycle were longer (for example, 40 years, as in neighboring Belize), this harvest would occur 80 years after the first, after two cutting cycles. Clearing 6–9% of the forest to provide for mahogany regeneration would be more likely to contribute to sustaining forest biodiversity, which includes many shade-intolerant species that require canopy openings or clearings to regenerate (Snook, 1993; Dickinson et al., 2000), than to threaten it, as has been suggested by Rice et al. (1997). Current mahogany harvests in Quintana Roo still include trees considerably larger than 55 cm dbh, that became established centuries ago (Snook, 1993, 1998, 2003). Ensuring the replacement of these by an equal number of mahogany trees 55 cm in diameter will not guarantee that future harvests yield volumes equivalent to current harvests. If managers sought to compensate for the smaller size of younger trees which will provide the bulk of harvests after the imminent conclusion of the first cutting cycle, during which all ancient trees are being removed (Snook, 1998), they could increase the number of future harvest trees by clearing and regenerating mahogany on larger areas. Doubling the 6–9% area proposed for regeneration would still leave at least 80% of the forest disturbed only by relatively small felling gaps that favor the regeneration of shade-tolerant species like M. zapota and B. alicastrum, which currently dominate the forest (Argu¨ elles, 1991; Snook, 1993; Dickinson et al., 2000). Techniques used locally to create agricultural fields called ‘milpas’, where corn and other crops are grown for a year or two before abandonment to naturally regenerated forest fallow, could be expected to provide even more favorable conditions for mahogany regeneration. A number of forest ejidos in the region control between 140 and 280 ha of agricultural and forest land per family (Flachsenberg, 1993a). Given the average size of a ‘milpa’ (3 ha; Murphy, 1990, 1994), it would be feasible to plant mahogany seedlings along with agricultural crops, and then to let the seedlings grow

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for decades before harvesting the mahogany trees and clearing the land again (if these economies still depended on subsistence agriculture). The fact that thousands of ejido farmers are producing agricultural clearings each year represents an opportunity to ensure mahogany regeneration by planting seedlings, along with crops, the year clearings are burned. However, planting mahogany in ‘milpas’ in Mexico’s Maya forest will not ensure the replacement of harvested mahogany in the permanent forest reserves managed for timber production, since agricultural activities are zoned outside these areas. To ensure that the 500,000 ha of community forest reserves in the state of Quintana Roo continue to represent an economically viable land use, it is important to sustain their value by providing for the regeneration of mahogany, by far the most valuable timber species, within these permanent forest reserves.

Acknowledgements Thanks to the interest of Guillermo Castilleja, this work and its publication were made possible through financial support provided by the US Forest Service, US Department of Agriculture, under the terms of agreement No. USDA-95-CA-118 with the World Wildlife Fund, US. The opinions expressed herein are those of the authors and do not necessarily reflect the views of the US Department of Agriculture. Financial support was also provided by the Biodiversity Support Program, a consortium of World Wildlife Fund, The Nature Conservancy and the World Resources Institute, with funding by the US Agency for International Development. The opinions expressed herein are those of the authors and do not necessarily reflect the views of BSP or the US Agency for International Development. Additional financial support was provided by the US Department of Education and Duke University through a Title VI Faculty Travel Grant, as well as through Duke University’s Vice Provost for Academic and International Affairs/International Affairs Committee, the Rockefeller Foundation Mexico through their Natural Resources Management Program, Iowa State University through its Research Extension office; and by the Center for International Forestry Research (CIFOR). Thanks to the interest of Robert C.

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Petterson, Caterpillar Inc. contributed the time of bulldozers to create the machine-clearings. These studies could not have been carried out without the collaboration of the Organizacio´ n de Ejidos Productores Forestales de la Zona Maya (OEPFZM), particularly their technical advisors, Victoria Santos Jimenez, Marcelo Carreo´ n Mundo and Rosa Ledezma Santos. Experiments could not have been established without the willing participation of the ejidos of Cafetal/Limones, Naranjal Poniente and Xpichil, as well as the owners of ‘‘Rancho Grande’’, the late Don Antonio Uh and his family. Experiments were established with the help of the students Mario E. Sua´ rez Mota, Francisco de Jesu´ s Martinez Va´ zquez, and Erika Melina Lira Charco; and with the invaluable help of numerous community members, of whom we only have space to mention those who contributed the most: in Naranjal, Manuel Ake´ Pat, Ebelio Che Ciau, Augusto Tuyu´ , Hermelindo Che, Felipe Tzuc Yam, and Salvador Utzil (machineclearing); in X-Pichil, Andres Kanul, Marcelino Chuc Pech, Juan Can Ko, Santiago Motul, Gregorio Miss, Simon Miss, Maximo Can, Mario Balam, and special thanks to Sr. Jose´ Garcı´a (machine-clearing); in Limones, Gilberto Paredes, Porfirio Gongo´ ra Ku´ and Martin Cervantes; and in Rancho Grande, Antonio Uh, Jose Uh, Nacho Uh, Porfirio Ramirez, and Martin Cervantes. Thanks, too, to the women who provided hospitality in the towns where we worked, notably Lorenza Yam and Elvira Che in Naranjal; Constancia Lauriano Chuck, Matilde Chuc, special thanks to Edilberta Miss in Xpichil; Rosa Maria Paredes in Limones; and Don˜ a Enedina Uh in Rancho Grande. Over the course of 5 years, many people helped with periodic measurements. We thank Andres Kanul, Ebelio Che, Gilberto Paredes, Miguel Torres, Guadalupe Ramirez, Antonio Uh, Jorge (cunado Uh), Janet Uh, Jose Uh, Nacho Uh, Martin Uh, Alicia Uh, Dina Uh, Melissa Morris, Juan Damas, Miguel Ake S., Dawn Robinson, Pablo Sarmientos, Enrique Vazquez, Luisa Camara, Miche`le van Kooijk, Mirna Valdez, Christopher Smid, Victor Ku, David Schoch, Josh Sneiderman, Jennifer O’Connor, Douglas Spinacka, Gregory Buppert, Chantalle Clark, Matthew Mize and Ann Snook. Thanks, too, to Nadene Sorenson, for her generous support whenever requested, and to Hans van Kooijk, for obtaining the climatic data. Thanks to Douglas Sheil, Timothy O’Brian, Bruce Campbell and

Ramadhani Achdiawan, for their advice on statistical analysis; to Robert Nasi and Cesar Sabogal for their comments on an early draft; and to two anonymous reviewers for their useful suggestions.

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