a pilot study from the Highlands of Chiapas, Mexico

68 Embracing epiphytes in sustainable forest management: a pilot study from the Highlands of Chiapas, Mexico J. H. D. Wolf University of Amsterdam, Amsterdam, the Netherlands

ABSTRACT

et al., this volume). Information on epiphytes in montane cloud forests in succession is still scanty (Barthlott et al., 2001; Ko¨hler et al., this volume) and there is an equally urgent need to obtain more insight into the response of epiphytes to anthropogenic disturbance (Hietz-Seifert et al., 1996; Werner et al., 2005; No¨ske et al., 2008) so that this important ecosystem component may be included in forest management plans aiming for ecological sustainability (cf. Hietz, this volume; Nadkarni, this volume). The felling of trees inevitably reduces epiphytes on a groundsurface area basis. Of particular interest is the question of how epiphytes on remnant or regenerating trees respond to the disturbance (cf. Werner et al., 2005). This chapter reports the results from epiphyte inventories made on a comparable number of trees in forests that vary in the manner and intensity of anthropogenic disturbance in the Highlands of Chiapas, southern Mexico. Attention is also paid to the geographic position of the respective sites within the landscape. The results are used to develop recommendations for improved forest management to help preserve epiphytes. Offering an alternative to wood extraction, the results are also employed to help define criteria for the ecologically sustainable harvesting of ornamental epiphytes (bromeliads). The chapter integrates the results from two previously published papers in which more details can be found (Wolf and Konings, 2001; Wolf, 2005).

Vascular epiphyte biomass and species richness were investigated in 16 anthropogenically disturbed pine– oak forests within an area of ~400 km2 in the Highlands of Chiapas, southern Mexico. Epiphyte biomass on 35 host oak trees in six diameter classes varied from 0.8 to 243 kg dry-weight and comprised 13–34 species. The observed variation in epiphytes could be attributed to type and intensity of past forest disturbance as it affects present-day stand structure, as well as to site position within the landscape. To help preserve the diverse regional epiphyte vegetation it is recommended to abstain from cyclic clear-cutting, to spare a sufficient number of large “rescue” trees, and to consider epiphyte conservation at a large spatial scale. As an alternative to logging, various prerequisites are proposed for the sustainable harvesting of bromeliads from natural populations.

INTRODUCTION In the footsteps of early explorers, successive epiphyte researchers have concentrated primarily on old-growth forests. From these studies it is evident that in pristine wet mountain forests epiphytes may contribute up to more than half of the vascular plant diversity and green biomass in the forest (Hofstede et al., 1993; Wolf and Flamenco-Sandoval, 2003; cf. Ko¨hler et al., 2007; Gradstein 2008). Thus, epiphytes are possibly key to high-mountain forest ecosystem functioning. At the same time, in many populated mountain areas altered forests prevail over old-growth forests which are increasingly trimmed down to isolated fragments (Sarmiento, 1997; Cavelier et al., 1998; Williams-Linera et al., 2002; Cayuela et al., 2006; Munõz-Villers and Lo´pez-Blanco, 2008; cf. Barradas et al., this volume; Bae´z

MATERIALS AND METHODS Study area The Highlands of Chiapas form a well-delimited physiographic area in the south-west of Mexico (Breedlove, 1978). The southwestern part of the Highlands around San Cristo´bal de Las Casas is known as the Central Plateau (c. 16 42’ N, 92 37’ W). This undulating plain has an elevation between 2100 and 2500 m.a.s.l. with a few peaks up to 2900 m.a.s.l. It is composed of marine limestones with extrusions of volcanic rock on the higher peaks.

Tropical Montane Cloud Forests: Science for Conservation and Management, eds. L.A. Bruijnzeel, F.N. Scatena, and L.S. Hamilton. Published by Cambridge University Press. # Cambridge University Press 2010.

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The vegetation on the peaks has been classified as cloud forest (Breedlove, 1978). On the plain, however, clouds are less frequent. Climatic records (1978–1995) from San Cristo´bal de Las Casas at an elevation of 2276 m.a.s.l. show an annual average temperature of 14.8 C and an average annual precipitation of 1042 mm with less than 30 mm month1 between December and April (Comisioń Nacional del Agua, unpublished data).

Pine–oak forests and anthropogenic disturbance On the Central Plateau, pine–oak forest prevails. In the Highlands, this formation occurs at altitudes between 1300 and 2500 m.a.s.l. wherever the dry season exceeds 3 months (Breedlove, 1978). Pines and oaks dominate the woody vegetation, but other temperate genera such as Alnus, Carpinus, Crataegus, Ostrya, Prunus, Sambucus, and Viburnum are also frequently present. Vascular epiphytes, particularly bromeliads, are most conspicuous because of their abundance and variety, especially during the dry season when the oaks shed their leaves. In total, 608 species of vascular epiphytes are known from the pine–oak forest (Wolf and Flamenco-Sandoval, 2003). Lichens and mosses are more abundant than liverworts which are mostly confined to the understory. The pine–oak forests on the Central Plateau have been logged to create arable lands and pastures at an estimated annual rate of >2% (1984–1990; Ochoa-Gaona and Gonza´lez-Espinosa, 2000). The remaining stands fulfil the demand for wood and humans have altered all residual forests on the Central Plateau to some degree. Pines are the most important source of timber, and trees that exceed commercial sizes are rare nowadays. Oaks, the preferred host tree for epiphytes, are exclusively used for fuelwood. Two main types of harvesting are practiced in the area: (i) periodic (20–30 years) clear-cutting, mostly for the production of charcoal, and (ii) the continuously and apparently haphazard extraction of trees for subsistence purposes. Both types of exploitation take place at varying intensities and typically in small areas of less than a hectare. This has resulted in a fine mosaic of forest stands of variable structure. Forest regeneration is left to natural processes. Oak tree stumps may produce several stems. In contrast, pines rely on seed dispersal and an open forest canopy favors seedling establishment. Grazing by sheep and surface fires in the understory also affect forest regeneration (cf. Asbjornsen and GarnicaSańchez, this volume). The net result of frequent anthropogenic disturbance is mostly that the dominance of pines over oaks is increased (Ramı´rez-Marcial et al., 2001). However, pure (though highly disturbed) oak stands are also present, possibly due to pine seed limitation or selective cutting. In this study, the proportion of sprouted oaks is used to approximate the degree of disturbance, although it is recognized that to obtain a good understanding of the management history of a site, additional structural parameters would also have to be taken into account.

Sampling Between 1995 and 1999, 16 sites were selected along an extended anthropogenic disturbance gradient, ranging from oldgrowth forest to heavily disturbed forest. Site selection was based on forest structural parameters and field observations with respect to the occurrence of recent logging, fires, and grazing of the understory. Variation in altitude between sites was kept at a minimum (2160–2490 m.a.s.l.) whereas geographic distance between any two sites varied from about 1 to 25 km. At each site, the structure of the forest was determined in a flat area of 30 30 m. All trees with a diameter at breast height (DBH) > 5 cm were identified and measured. Because it was not possible to confidently distinguish Quercus crispipilis Trel. from Q. segoviensis Liebm., the two species were lumped. Similarly, Q. crassifolia H. & B. was grouped together with Q. rugosa Nee. Tree heights, tree densities, basal areas, diameter frequency distributions, and the frequency of sprouted oaks were used to parameterize site disturbance history. Observations on the occurrence of logging, grazing, and forest fires were also recorded. For epiphytes, the tree was used as the sampling unit, because at this scale it is easier to attain a sample of similar size between structurally divergent forest types (cf. Gradstein et al., 2003; Wolf et al., 2009). Only oak trees were considered since epiphytes on pines are rare, possibly due to the lower water-holding capacity of the bark (Castro-Hernańdez et al., 1999). Thirty-five oak trees per stand were sampled for epiphytes, 560 in total. To obtain a good distribution of tree sizes, five host trees were chosen in each of five classes of increasing DBH, at 5-cm intervals ranging from 5 to 30 cm. In addition, ten host trees with DBH > 30 cm were sampled. After sampling an initial randomly chosen first tree, the nearest neighbor trees were selected until a particular size class was filled. For each tree, three size parameters were obtained: height, number of bifurcation points with a branch diameter >5 cm, and DBH. Field trials on climbed trees showed that these variables could be estimated with little error. Epiphyte abundance was expressed as dry-weight. The clonal growth of many species made it impossible to discern individuals. An essentially non-destructive sampling method was applied in which dry-weight was estimated from counts of leaves, fronds, or rosettes, depending on the growth form of the species. Taller trees were climbed using rope-climbing techniques (Mitchell et al., 2002). Vouchers were deposited in the herbarium of El Colegio de La Frontera Sur at San Cristo´bal de Las Casas.

Analysis To compare epiphyte dry-weight and the number of species between sites, an ANCOVA was carried out, controlling for differences in tree size. Richness (alpha diversity) and biomass values were subjected to square-root transformation to homogenize variances. A biomass model (r2 ¼ 0.75, p < 0.001) included

654 DBH and the number of bifurcation points as covariates, after elimination of tree size parameters that did not contribute significantly (p < 0.05), whereas a richness model (r2 ¼ 0.69, p < 0.001) only included DBH (see Wolf, 2005 for details). The spatial distribution of epiphytes is influenced by spatially structured environmental variables, historic biogeography, and biological processes that are spatially heterogeneous, such as dispersal (e.g. Legendre et al., 2005). Multivariate data analysis was used to discriminate between the contribution of environmental and spatial variables, following Borcard et al. (1992). Environmental and spatial variables are partially correlated and pure space is defined as that part of the spatial variation that is not correlated with the environmental variables used in the analysis. The following quantitative environmental variables were used: altitude, forest height, the proportion of sprouted oaks, oak host tree species frequency, and the basal areas and densities of pines, oaks, and other broad-leaf trees. Forest height was taken as the average of the five tallest oak trees in the inventory. Large oaks with trunk DBH > 45 cm were separated from smaller oaks. In addition, some nominal values were considered: grazing, fire, and the presence of tree stumps. For spatial descriptors of the sites, the terms of a cubic trend-surface equation were used that was derived from the geographic coordinates of the sites (Borcard et al., 1992; Legendre and Legendre, 1998). Species biomass values were log-transformed. Spatial heterogeneity was analyzed further with Mantel’s test. Mantel’s regression coefficients were determined from a Euclidean distance matrix, computed from the geographic coordinates of the sites, and a species distance (dissimilarity) matrix. For the latter, the probabilistic coefficient of Raup and Crick was used, as suggested by Legendre and Legendre (1998). The computations were performed using Systat (version 5.03), the R Package for Multivariate and Spatial Analysis (4.0), and CANOCO (4.02) (Ter Braak, 1988; Casgrain and Legendre, 2001).

RESULTS AND DISCUSSION Forest disturbance, epiphyte richness and epiphyte biomass Epiphyte biomass on the 35 oak trees varied between 0.8 and 243 kg, and richness between 13 and 34 species (Table 68.1). Forest disturbance, measured as the proportion of sprouted oaks, not only had a negative effect on epiphyte biomass and alpha diversity per ground surface area (Wolf, 2005) but also on biomass (Pearson’s r ¼ 0.55, p ¼ 0.02) and alpha diversity (Pearson’s r ¼ 0.75, p ¼ 0.001) of the epiphyte vegetation on the remaining and regrowing trees (Table 68.1). This pattern was sustained after correcting for differences in tree size, as shown by the post-ANCOVA Tukey classification of the sites in cohorts of

J. H. D. WOL F

similar biomass and species alpha diversity; the less disturbed sites near the top of Table 68.1 belong to the epiphyte-high biomass and species-rich cohorts (As). Conversely, the coppiced stands listed at the bottom of the table are poor in epiphytes, particularly sites that have experienced nearly complete clearcuts on a cyclic basis and comprise more than 80% of sprouted trunks. This form of management is similar to that of tree plantations, which are also poor in epiphytes (Merwin et al., 2003). Colonization from outside the coppice is apparently a slow process, which is in accordance with the rapid decay in seed rain with distance from the source (Garcıá-Franco and Gray, 1988). Dispersal limitation may also explain why the epiphytic vegetation at site Chilil-2 was more affected as compared with the stands at Costik and Mitzitoń. The forests of these three sites had a similar proportion of sprouted oaks and comparable oak and pine tree basal areas, suggesting a similar management history. Nevertheless, at Chilil-2 epiphyte alpha diversity and biomass were significantly lower than at the two other sites where several large trees were apparently spared (Figure 68.1). Such heavily loaded “rescue” trees possibly serve as a nearby seed source, thereby facilitating the recovery of the epiphyte vegetation.

Spatial heterogeneity of epiphytes Twenty-five percent of the species variation between sites was left unexplained by the multivariate analysis (Figure 68.2). This amount is similar to values reported for other ecosystems and may be attributed to unmeasured environmental variables and random fluctuations (Borcard et al., 1992). Interestingly, spatial variables explained most of the distribution of epiphytes in the landscape. Pure space, i.e. that part of the spatial variation not correlated with environmental variables, even explained 32.3%. Space is usually not considered in epiphyte landscape ecology. A Mantel correlogram (Mantel’s r ¼ 0.44, p ¼ 0.002) visualizes the spatial dependence of epiphytes in the landscape (Figure 68.3). Thus, sites on the Central Plateau at distances less than 10 km proved to be similar in terms of their epiphytes. This finding is of considerable importance for the scale at which epiphyte conservation efforts should be contemplated.

Epiphytes and sustainable harvesting: theory After the felling of oaks to be used as fuelwood, the attached epiphytes are simply discarded, despite a worldwide growing market for ornamental plants such as bromeliads (Dimmitt, 2000). Could the harvesting of bromeliads from the pine–oak and oak forests of Chiapas offer an alternative source of income to wood extraction and thus be employed as a tool for forest conservation? An answer requires studies of the commercial, social, and

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Table 68.1 Site forest structure and epiphyte biomass and richness (alpha diversity) on 35 oak trees in six trunk diameter classes; the sites are arranged by increasing proportion of sprouted oaks, which approximates a gradient of increased anthropogenic disturbance Locality

La Florecilla-1 Chilil-1 El Chivero Basom-1 Costik Chilil-2 San Antonio Basom-2 Mitzitoń Carrizal San Jose´ Rancho Nuevo Las Flores La Florecilla-2 La Florecilla-3 Milpoleta

Forest structure

Epiphyte community

Altitude (m.a.s.l.) Sprouted oaks (%)

Height (m)

Oaks

Pines

Others

Total

Big oaks

2350 2300 2360 2490 2346 2290 2370 2450 2420 2160 2350 2400 2375 2350 2350 2425

8.6 11.4 11.4 14.3 14.3 20.0 31.4 31.4 34.3 40.0 51.4 51.4 60.0 74.3 82.9 94.3

22.6 23.2 22.2 29.2 21.8 19.4 21.8 28.6 17.2 19.4 15.9 23.0 17.4 17.6 20.4 28.8

31.6 20.3 17.7 45.3 8.7 8.0 25.9 14.6 7.2 27.4 21.4 10.4 37.9 21.9 22.6 41.6

6.5 7.0 15.2 4.3 26.9 21.2 8.2 56.8 24.6 0.0 0.2 31.5 10.7 13.5 20.8 0.0

0.2 2.6 3.3 10.2 4.2 0.1 12.5 1.1 1.8 0.5 0.7 0.1 4.5 7.5 0.0 9.5

38.3 29.9 36.2 59.8 39.9 29.3 46.6 72.4 33.7 27.9 22.3 42.0 53.0 42.9 43.4 51.1

Basal area (m2 ha1)

S

Biomass (kg)

17.5 3.2 9.3 32.6 1.3 0.6 9.9 4.9 2.4 9.5 0.0 10.4 7.8 8.0 0.0 10.9

27 35 34 23 24 16 29 24 24 16 18 23 18 15 14 13

Richness cohort

Biomass cohort

243.9 74.7 97.6 103.3 72.5 0.8 98.9 113.7 108.1 69.7 83.8 9.7 41.3 65.5 16.7 16.0

B A A A B C A B B C B C B C C B

A1 A2 A2 A2 B C2 A2 A2 A2 A2 A2 C2 B B C1 C1

Notes: Forest height is the average of the five tallest oak trees in the inventory. Big oaks are oaks with a trunk DBH > 45 cm. The division of sites in terms of epiphyte richness and biomass cohorts is based on the probabilities of a Tukey pairwise comparison of the means, following an ANCOVA that factored out the differences in tree size between the sites (r2 ¼0.69, p < 0.001). As to species richness (S), cohort A comprises species-rich sites and cohort C species-poor sites. Within each of the two cohorts, site species richness is not significantly different, in contrast to a comparison between cohorts A and C (p < 0.05). Cohort B assumes an intermediate position. The biomass at La Florecilla-1 (cohort A1) is significantly higher than at any other site. The sites in cohort A2 showed a significantly higher biomass than the low biomass sites of cohorts C1 and C2. Again, cohort B is intermediate

Basal area (m2 ha–1)

3.5

Chilil-2

Costik

Environment 21%

Mitzitón

3.0 2.5 2.0

Unexplained 25%

1.5 1.0 0.5 0.0 5–15

15–25

25–35 35–45 DBH class (cm)

> 45

Figure 68.1. Basal area (m2 ha1) of oak trees in various trunk diameter classes (DBH) at three disturbed sites, based on 900-m2 inventories on the Central Plateau, Chiapas Highlands, southern Mexico.

ecological sustainability of such activities. The remainder of this chapter addresses the ecological aspects of epiphyte harvesting. When defining criteria for sustainable harvesting, two approaches may be followed. By means of demographic and/or genetic studies and modeling one may attempt to establish the minimum viable population size (MVP) and propose

Space 32%

Environment + Space 22%

Figure 68.2. Partitioning of the variation in epiphyte communities between sites that may be explained by environmental and spatial variables, following Borcard et al. (1992).

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J. H. D. WOL F

Mantel’s r 0.4 0.3 0.2 0.1 0

0

1

2

3

4

5

Lag

–0.1 –0.2 –0.3

Figure 68.3. Mantel correlogram showing epiphyte spatial dependence between sites. The correlations (i.e. Mantel’s r) are calculated from a species similarity matrix, using the Raup and Crick probabilistic coefficient as a measure of species similarity, and a Euclidian distance matrix computed from the geographic coordinates of the 16 sites. Species biomass values were log-transformed. The lags are equidistant at 5 km. Black symbols indicate a significant value of Mantel’s r ( p < 0.05).

management interventions based on population viability analysis (PVA) (Menges, 2000). However, the MVP has no universal value (Soule´, 1987) whereas a reliable PVA also requires observations to be made over a long period of time. For the immediately threatened bromeliads, Wolf and Konings (2001) therefore advocated a more empirical approach to guarantee sustainability of yield. On the basis of generally accepted ecological principles, they proposed that harvesting may only be considered for populations that: (i) are close to carrying capacity (see explanation below), (ii) have high densities (see also below), and (iii) grow both in the lower stratum of the forest and in the high canopy. As to the latter guideline, it is advisable to harvest only plants in the understory so as to limit the effect on the reproductive capacity of the population. Similar to the above-mentioned “rescue” effect of large residual trees, the intact part of the epiphyte population within the canopy serves as a nearby source of seeds that should facilitate recolonization of the lower strata. For the first two prerequisites thresholds were set that, whilst being stringent, are also arbitrary. Therefore, frequent monitoring should be an essential component of any harvesting program. THE CARRYING CAPACITY THRESHOLD

As noted above, for oak-based epiphytes in the Chiapas Highlands, the poor proliferation of epiphytes in oak coppices, the “rescue” effect of large remnant trees on overall epiphyte community structure, and the similarity in epiphytes at nearby sites within the landscape could well be explained from a dispersal assembly perspective. In this view, the absence of epiphytes on oaks results primarily from a lack of arrivals rather than unfavorable habitat properties (Hubbell, 2001). Accordingly, when a

bromeliad species is less abundant in certain parts of the forest, i.e. not distributed homogeneously in space, the population is considered not to be at carrying capacity (yet). For an ecosystem with its epiphytes at carrying capacity this would mean that in any part of a (relatively environmentally homogeneous) forested area the same correlation between tree size and its epiphyte abundance should exist. In other words, the better the size of the tree explains the amount of epiphytic biomass that it supports, the closer is the epiphyte vegetation to its carrying capacity. The corresponding squared correlation coefficient may therefore be used as an index of spatial heterogeneity (ISH) to assess the closeness to carrying capacity of epiphyte populations (Wolf and Konings, 2001). It should be noted that a perfect correlation between tree size and epiphyte abundance is not to be expected, amongst others because the suitable bark area on a tree will not depend linearly on the size of the tree. The level of the ISH threshold is set by its value in an old-growth stand and will depend on species as well as type of habitat. The anthropogenic disturbance study by Wolf (2005) supported the assertion that the tree-size–abundance correlation may be employed to assess the closeness to carrying capacity of epiphyte populations in the study area. Epiphyte biomass was positively correlated (p < 0.001) with the size of the host tree at all sites, except at the heavily disturbed Chilil-2 plot (r2 ¼ 0.16, p ¼ 0.02). More significantly, at the nine sites with high epiphytic biomass (i.e. cohorts A1 and A2), the correlation was substantially higher (r2 ¼ 0.85, on average) than at the more disturbed sites which had fewer epiphytes (r2 ¼ 0.59, on average). THE POPULATION DENSITY THRESHOLD

Harvesting from a small population might negatively affect the local survival of a species, and should be avoided (e.g. Shaffer, 1981). Unfortunately, little information is available about the minimal size of viable populations of bromeliads. During three consecutive years, Benzing (1978) followed an apparently stable Tillandsia circinnata population in Florida. Based on the results of that study it is prudently proposed that harvesting may only be contemplated for populations that are at least ten times larger than that in Florida, i.e. comprising over 9000, rosettes ha1.

Epiphytes and sustainable harvesting in practice: a bromeliad pilot study In a pilot study at La Florecilla, the above-mentioned guidelines to attain a sustainable yield of epiphytes have been put into practice (Wolf and Konings, 2001). La Florecilla comprises 160 ha of continuous pine–oak forest at various degrees of disturbance by humans. Tillandsia vicentina Standley is the dominant epiphyte and the subject of this study. Only species of oak having similar bark properties were considered as host trees. First, the correlation between tree size and square-root

657

Square root number of rosettes


35 30 25 20 15 10 5 0 –2

0

2

4

6 Tree size

8

10

12

14

Figure 68.4. The square root of the number of Tillandsia vicentina rosettes >20 cm plotted against Tree Size. Tree Size is the sum of diameter at breast height and the number of bifurcation points, after standardization to zero mean and unit variance. Only sample points above the fitted line (solid circles) were used in the calculation of the correlation index (r2 ¼ 0.92).

transformed epiphyte biomass was determined at La Florecilla-1, an old-growth patch of forest in the area: r2 ¼ 0.91. This is the carrying capacity prerequisite threshold value (ISH). Next, the species density and the correlation index for the whole area were estimated from a transect inventory. A random sample of 35 points was selected along parallel transects that were evenly distributed over the forest. At each point, four oak trees were selected applying the plotless point-centered quarter method (Cottam and Curtis, 1956). The DBH of each of the trees was measured and the number of bifurcation points with a branch diameter >5 cm counted. In addition, the number of T. vicentina rosettes >20 cm tall were counted per tree, using binoculars. To simplify the analysis, the variables DBH and the number of bifurcation points were added only after standardization to zero mean and unit variance, to obtain a single variable to represent tree size, called Tree Size (Figure 68.4). At La Florecilla, small patches of forest of a few hectares each – which varied greatly in structure due to differences in degree and type of previous anthropogenic disturbance – formed a small-scale heterogeneous mosaic. The suggestion to restrict harvesting to old-growth forests was rejected because it was problematic for the local population to fairly rank and map stands. Instead, epiphytes in the entire forest were considered for harvesting. Non-stratified sampling of randomly selected transect points is more objective, but because heavily disturbed stands of forest such as that of La Florecilla-3 (Table 68.1) were also sampled, the ISH threshold determined for old-growth stands would certainly not be reached. On the other hand, it would be unrealistic to require that the carrying capacity prerequisite is met everywhere in such a large area as La Florecilla (160 ha). Therefore, it was, again arbitrarily, decided that harvesting might be contemplated if the prerequisite would be met for only approximately half the trees in the forested area. To

select the trees to be used for the calculation of the Tree Size– epiphyte abundance correlation, a partitioned regression analysis was performed. First, a line was fit through the entire cloud of data points representing all trees in the transect inventory. Second, the data sub-set above the line was selected (i.e. the positive residuals). Next, the correlation between Tree Size and the number of rosettes was analyzed to yield ISH. The transect analysis showed that T. vicentina met all prerequisites, having both a satisfactory population density of c. 24 000 large (>20 cm) rosettes ha1 on oaks, and an ISH of 0.92. The population density value corresponds to the lower limit of the 95% confidence interval of the mean bromeliad density (39 718 rosettes ha1), as calculated from the average occupation and density per hectare of oak trees. A separate inventory of T. vicentina on all species of host tree in the forest and including terrestrial rooted plants showed that 27% of all rosettes occurred in the understorey up to a height of 6 m. A little less than half of the epiphytic T. vicentina plants in the understory (2680, rosettes ha1) occurred there as solitary rosettes (genets). These are especially attractive because of their symmetric growth. Field observations suggested that T. vicentina plants need about 5 years to develop from seed to the commercial size of 20 cm. Because many of the smaller juveniles are already present at the time of harvesting, it is assumed that a period of 4 years will allow such rosettes to attain at least 20 cm. Adhering to a 4-year harvesting rotation scheme, it would be possible in La Florecilla to sustain an annual harvest from the lower stratum of c. 700 epiphytic single rosettes of T. vicentina ha1. Thus, from the 160-ha forest at La Florecilla an annual yield of 112 000 particularly attractive rosettes may be contemplated, equivalent to ~3% of the number of large-sized rosettes in the forest. The large number of annually harvestable plants suggests that such an activity may be commercially viable and that T. vicentina may be used as a tool to help conserve the forest. It should be remembered, however, that the thresholds of the prerequisites were, whilst stringent, arbitrarily set. The same is true for the omission of half of the trees in the carrying capacity assessment. Moreover, no data are available on the restoration capacity of the bromeliad population, nor on the effect of harvesting T. vicentina on other epiphyte species, amongst others. Therefore, yearly monitoring should be an essential component in any management plan for sustainable harvesting.

IMPLICATIONS FOR FOREST MANAGEMENT On the Central Plateau of Chiapas, pine–oak forests are a source of timber and fuel for subsistence purposes. This vital function is not likely to change in the near future. This study has shown, however, that tree harvesting practices of some indigenous communities

658 (whereby several large trees are left standing) have preserved the rich epiphyte vegetation better than the nearly complete clearcutting practiced by other communities. Accordingly, it is recommended to abstain from the cyclic clear-cutting of the oak forest and, in particular, to spare large “rescue” trees during logging that may serve as a source of epiphytes during the recolonization process. Furthermore, based on the observed spatial heterogeneity it is wise to address epiphyte conservation issues at larger spatial scales, e.g. per physiographic unit. Finally, it is proposed to further explore whether and to what extent ornamental epiphytes may be employed as a tool for forest conservation.

ACKNOWLEDGEMENTS The assistance rendered in the field by Teresa Santiago-Vera, Mariana T. Toledo-Aceves, and Henry E. Castan˜eda-Ocanã is gratefully acknowledged. Funds and other support were provided by Stichting Het Kronendak, El Colegio de la Frontera Sur (ECOSUR), and the Comisioń Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO), grants B060 and L050.

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