The role of shaded cocoa plantations in the maintenance of epiphytic orchids and their interactions with phorophytes

生境丧失和破碎化是热带森林生物多样性的主要威胁。遮荫的可可种植园(SCP)等农业生态系统为热带森林生物群提供了庇护。然而,在这些转化后的生境中是否还维持种间生态的相互作用,目前尚鲜为人知。我们评定附生兰花群落的多样性、繁殖状态和光合代谢(CAM或C₃),以及与热带雨林(TRF)相比,它们与SCP中寄主树种(附生植物)之间的相互作用。在墨西哥东南部,对TRF和SCP中各三个采样地点进行研究。每个采样地点建立了4个400平方米的样地,调查记录所有兰花及其附生植物。我们依据花/果实(或残体)是否存在来确定每个兰花个体的繁殖(成体)或非繁殖(幼体)状态,并根据文献确定每种兰花的光合作用途径。我们采用真正的分集和生态网络的方法分别分析兰花的多样性以及兰科与附生植物间的相互作用。我们一共记录了47个兰花种的607个个体。在TRF (19个有效物种)中的兰花多样性高于SCP (11个有效物种),两个生境之间仅共享7个物种。SCP (53%)中的CAM兰花物种比TRF (14%)更常见。在群落水平上,SCP维持了非生殖兰花和生殖兰花的比例以及TRF兰科附生植物网络的嵌套结构和特异化水平。然而,SCP中仅保留一部分的TRF附生兰花,突显出保护TRF的重要性。尽管存在这种差异,诸如SCP类型的遮荫农业生态系统仍然可以维持天然林的一些多样性和功能,因为SCP附生兰花群落主要由CAM物种组成,其附生植物构成了一个嵌套的相互作用网络,对干扰形成了更强的抗性。     

Endogamy costs and reproductive biology of <Emphasis Type="Italic">Laelia autumnalis</Emphasis>, an endemic orchid of Mexico

It has been hypothesized that the fitness of selfed progeny must be lowered by inbreeding depression. Most research into the breeding systems of orchids shows a similar fruit set from outcrossing and self-pollination, but few studies have measured seed production and viability. In five populations of Laelia autumnalis, in central México, we studied the species reproductive system and the early costs of endogamy. We performed spontaneous self-pollination, apomixis, assisted self-pollination, cross-pollination, and exogamous pollination (using pollen from a different population) treatments of bagged flowers and measured fruit set, seed production, and germination. No fruits were produced in the apomixis and spontaneous self-pollination treatments. Reproductive success from self-pollination was lower than that from cross- and exogamous pollination and no difference was found between the latter two treatments. The ratio between self-pollination and cross-pollination success in different traits ranged from 0.80?±?0.18 (fruit set) to 0.22?±?0.13 (seeds with embryo). The latter value suggests a high endogamy costs in the production of viable seeds. The concatenated success of the different traits studied showed that the relative fitness of self-pollination was 63% lower than with cross-pollination. Laelia autumnalis is a self-compatible non-autogamous species in which the cost of endogamy occurs at seed production and cannot be compensated for at other early stages. This also occurs in other orchid species and is likely to be a general pattern.     

The effect of substrate abundance in the vertical stratification of bromeliad epiphytes in a tropical dry forest (Mexico)

Diversity of epiphytes is associated with niche partitioning, through vertical strata and host preferences. However, abundance of substrate offered by hosts differs between vertical strata, misleading if epiphytes prefer a stratum or are randomly distributed. In a tropical dry forest of San Andres de la Cal Morelos, central Mexico, we tested the null hypothesis, that epiphytes follow the abundance of the substrate, rather than showing preference for a particular vertical stratum, and tested whether microclimatic variables, seed germination and seedling survival match with observed epiphyte distribution. Our data show that epiphytes could be randomly distributed inside some host; but in some host species, certain structures presented either a deficit or an excess of all, atmospheric, or tank epiphytes. In the hosts Bursera copallifera and Bursera glabrifolia, distribution of epiphytes was biased towards the upper strata, with a deficit of epiphytes in the lower strata. In Conzattia multiflora, Sapium macrocarpum and Ipomoea pauciflora, epiphyte distribution was biased towards the lower strata. Vertical gradation of light, seed germination and seedling survival did not generally match with epiphyte distribution and did not support the notion that the microclimatic gradient governs the vertical distribution of epiphytes. Our data indicate that vertical distribution of epiphytes in such tropical dry forests is mainly driven by the distribution of the structures, which apparently influence dispersion of the seeds and by the lifespan of branches, which allow the concentration of epiphytes in the stratum that optimizes seed capture and the clonal growth of epiphytes.     

Is Allelopathic Activity of Ipomoea murucoides Induced by Xylophage Damage?

Herbivory activates the synthesis of allelochemicals that can mediate plant-plant interactions. There is an inverse relationship between the activity of xylophages and the abundance of epiphytes on Ipomoea murucoides. Xylophagy may modify the branch chemical constitution, which also affects the liberation of allelochemicals with defense and allelopathic properties. We evaluated the bark chemical content and the effect of extracts from branches subjected to treatments of exclusion, mechanical damage and the presence/absence of epiphytes, on the seed germination of the epiphyte Tillandsia recurvata. Principal component analysis showed that branches without any treatment separate from branches subjected to treatments; damaged and excluded branches had similar chemical content but we found no evidence to relate intentional damage with allelopathy; however 1-hexadecanol, a defense volatile compound correlated positively with principal component (PC) 1. The chemical constitution of branches subject to exclusion plus damage or plus epiphytes was similar among them. PC2 indicated that palmitic acid (allelopathic compound) and squalene, a triterpene that attracts herbivore enemies, correlated positively with the inhibition of seed germination of T. recurvata. Inhibition of seed germination of T. recurvata was mainly correlated with the increment of palmitic acid and this compound reached higher concentrations in excluded branches treatments. Then, it is likely that the allelopathic response of I. murucoides would increase to the damage (shade, load) that may be caused by a high load of epiphytes than to damage caused by the xylophages.     

Is branch damage by xylophages related to the presence of epiphytes?

Host identity influences the guilds (epiphytes and xylophages) that interact within canopies. Host species can be categorized as either limiting or preferred hosts based on epiphyte load. It is possible that, depending on the host category (limiting or preferred), galleries bored by xylophages would affect the quality and availability of space for epiphytes. The objective of this study was to determine, among and within limiting and preferred hosts, the relationship between the damage inflicted by insects to branches and epiphytic bromeliads. We collected two branches each (with and without epiphytes, respectively) from limiting hosts (Bursera fagaroides, Ipomoea murucoides and I. pauciflora) and preferred hosts (Bursera copallifera and B. glabrifolia). The variables measured were: number and species of epiphytes, number of holes, number and taxonomical group of insects, percentage of epiphyte cover and percentage of area damaged by insects. These variables were compared among and within hosts and the significant correlations, where present, determined. We identified five bromeliad species and six taxonomical groups of insects. I. murucoides showed a higher proportion of damage and a larger number of insects. For the three limiting hosts, there was a negative relationship between (1) epiphyte cover and damaged area, (2) number of epiphyte individuals and number of xylophages and (3) number of epiphyte individuals and damaged area. Within species, B. copallifera, B. glabrifolia and I. pauciflora had more holes in branches that supported epiphytes than in branches without. We hypothesized that, inter-specifically, xylophages would interfere with the establishment of epiphytes by facilitating the release of allelopathics, but this possibility needs to be examined in more detail. Intra-specifically, it is possible that holes made by xylophages do not cause sufficient damage in hosts and, consequently, a possible repercussion on epiphytes is not reflected.     

Do cloud forest tree species differ in their suitability as a substrate for epiphytic bromeliads?

There is evidence for the existence of varying degrees of host preference in vascular epiphytes; certain tree species can be positively, neutrally, or negatively associated with epiphytes. The objective of this study was to evaluate whether tree species of the cloud forest differ in their suitability as a substrate for epiphytic bromeliads. To evaluate the association between epiphytic bromeliad cover and host tree species, we sampled 62 plots (each of 200 m²) in four cloud forest fragments in Veracruz, Mexico. For all trees ≥10 cm in diameter at breast height (DBH), we recorded species name, DBH, and percentage cover of bromeliads in categories of tree coverage. In total, 587 trees belonging to 52 species were recorded. All of the 10 tree species used to assess differences in epiphyte cover (each with a minimum of nine individuals) supported bromeliads, but mean bromeliad cover differed significantly among the tree species. The tree species that concentrated the highest bromeliad cover were Quercus sartorii (29.86%) and Liquidambar styraciflua (21.72%). Our results indicate that, while none of the tree species analyzed was a limiting host for epiphytic bromeliads in general, varying levels of bromeliad cover occur depending on the host species in tropical montane cloud forest fragments suggesting that certain tree species are better hosts than others. The implications for conservation efforts of differential tree species suitability as epiphyte hosts are discussed.     

Diversity and genetic structure of the Mexican endemic epiphyte Tillandsia achyrostachys E. Morr. ex Baker var. achyrostachys (Bromeliaceae)

BACKGROUND AND AIMS: The monoecious, bird-pollinated epiphytic Tillandsia achyrostachys E. Morr. ex Baker var. achyrostachys is an endemic bromeliad of the tropical dry forests of Mexico with clonal growth. In the Sierra de Huautla Natural Reserve this species shows a host preference for Bursera copallifera (Sessé & Moc ex. DC) Bullock. As a result of deforestation in the study area, B. copallifera has become a rare tree species in the remaining forest patches. This human-induced disturbance has directly affected the population densities of T. achyrostachys. In this study the genetic consequences of habitat fragmentation were assessed by comparing the genetic diversity, gene flow and genetic differentiation in six populations of T. achyrostachys in the Sierra de Huautla Natural Reserve, Mexico. METHODS: Allozyme electrophoresis of sixteen loci (eleven polymorphic and five monomorphic) were used. The data were analysed with standard statistical approximations for obtaining diversity, genetic structure and gene flow. KEY RESULTS: Genetic diversity and allelic richness were: HE = 0.21 +/- 0.02, A = 1.86 +/- 0.08, respectively. F-statistics revealed a deficiency of heterozygous plants in all populations (Fit = 0.65 +/- 0.02 and Fis = 0.43 +/- 0.06). Significant genetic differentiation between populations was detected (Fst = 0.39 +/- 0.07). Average gene flow between pairs of populations was relatively low and had high variation (Nm = 0.46 +/- 0.21), which denotes a pattern of isolation by distance. The genetic structure of populations of T. achyrostachys suggests that habitat fragmentation has reduced allelic richness and genetic diversity, and increased significant genetic differentiation (by approx. 40 %) between populations. CONCLUSIONS: The F-statistic values (>0) and the level of gene flow found suggest that habitat fragmentation has broken up the former population structure. In this context, it is proposed that the host trees of T. achyrostachys should be considered as a conservation priority, since they represent the limiting factor to bromeliad population growth and connectivity.     

Head shape variation in cerambycid saproxylic beetles as a function of host plant selection

Saproxylic insects depend on deadwood for larval development, and a certain degree of specialization may be involved in their choice of host plants and/or wood in a particular stage of degradation. The plant species chosen for oviposition in turn act as an environmental pressure on the head morphology of larvae and it is expected that head shape plasticity varies directly with the number of woody plant species used for larval development in each insect species. We analyzed head shape variation in saproxylic beetles with respect to host plant species, maximum time of larval emergence and season of the year when insects colonized branches. Generalist species in the use of host plants showed significant variation in head shape and size. Time of emergence and season did not appear to affect head shape, although season was a determinant factor of abundance and possibly head size variation.     

The relationship between bark peeling rate and the distribution and mortality of two epiphyte species

Tree bark characteristics influence epiphyte establishment and survival and consequently the way in which epiphytes are distributed on trees. Tree species with peeling bark have been reported as poor epiphyte hosts. We analyzed the distribution and seedling mortality of two Tillandsia species (Bromeliaceae) in relation to rate of bark peeling of Bursera fagaroides (Burseraceae). The highest peeling rate (0.12% per day) took place on the trunk and the lowest rate on twigs (0.04% per day; branches ≤2 cm in diameter). The highest proportion of Tillandsia plants appeared on twigs. The distributions of juvenile and adult plants on twigs were higher than those expected based on the distribution of first-year seedlings, suggesting that on twigs, survival could be greater than on trunks and branches, canopy areas where peeling is faster. On the trunk and branches, in contrast, the proportion of juveniles and adults were similar to or less than that expected for first-year seedlings. The main cause of mortality was peeling and the area of minor overall mortality was the trunk, suggesting that this area should be favored as the main distribution area for the Tillandsia species but is not. Our results show that the peeling rate of B. fagaroides depends on branch size and suggest that the Tillandsia distribution depends not only on peeling rate but also on seed dispersion. We suggest that to colonize B. fagaroides epiphytes would either have adaptations to counteract the peeling rate or should occur in the areas of lowest peeling rate located in the exterior crown of trees.     

Effects of floral display and plant abundance on fruit production of Ryncholaelia glauca (Orchidaceae)

Flowering plant density can increase number of visits and fruit set in multi-flowering plants, however this aspect has not been studied on few flower species. We studied the effects of individual floral display and plant density on the fruit production of the epiphytic, moth-pollinated orchid, Ryncholaelia glauca, in an oak forest of Chavarrillo, Veracruz, Mexico. Species is non-autogamous, and produced one flower per flowering shoot each flowering season. We hypothesized that orchids with more flowering shoots and those on trees with clumps of conspecific should develop more fruits than isolated ones. R. glauca population flowers synchronouly, and individual flowers last up to 18 days, with flowers closing rapidly after pollination. Individuals produced few flowers per year, although some plants developed flowers in both seasons and fewer of them developed fruits both years. There was no relationship between flower number per orchid, or per host tree, with the number of fruits developed per plant. Host trees with flowering and fruiting orchids were randomly dispersed and the pattern of distribution of flowering and fruiting plants was not related. Apparently, pollinators visit the flowers randomly, with no evidence of density dependence. The fruit set of R. glauca was as low as fruit set of multi-flowered orchids moth pollinated, suggesting that fruit set on moth pollinated orchids could be independent of the number of flowers displayed.