Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

Tropical dry forests (TDFs) are often degraded and fragmented through human impact, which is also the case in Southern Ecuador, where land-use pressure is high. In this context we studied tree species composition and forest structure in a protected and adjacent disturbed TDF at altitudes between 560-1080 m asl. Fabaceae and Malvaceae were identified as the most important tree families in both forest types. The disturbed forest displayed lower tree species richness than the protected forest, and the gap in species richness between the two forest types increased with increasing altitude. Ten species of the protected forest were not recorded in the disturbed forest, two of them endemic. The disturbed site was further characterized by a lower number of stems but with larger diameters, in comparison with the protected forest. The majority of the most abundant tree species in the disturbed forest had rather low wood densities, but also the combination of high wood density with browsing tolerance and high resprouting capacity was encountered, and seems to be advantageous for getting established in such sites. Although certain tree species were well represented in the disturbed forest, some endemic species with relatively low abundances (e.g. Simira ecuadorensis, Prockia crucis) should receive more conservation attention.

Abstract:

Forest recovery on disturbed areas is of special significance in the Ecuadorian Andes, where deforestation is a serious problem. Natural diachronic succession was evaluated on three large plots or sites, differing in their land use and vegetation composition, one is dominated by grass species on an abandoned pasture (Pasture site), the other two are post-fire vegetation dominated by bracken (Bracken site) and various shrubs (Shrub site). Additionally, we assessed the effectiveness of manual removal of competitive herbaceous species to accelerate forest recovery. Monitoring was done in 2003, 2005 and 2007 on 48 subplots of 116 m2 each recording species richness and woody-species density. Results showed that the Pasture site demonstrated a competitive effect of exotic grasses on woody species recruitment with much lower species recruitment and density, suggesting serious inhibition of natural forest regeneration and an unclear successional trajectory. The Bracken and Shrub sites became significantly similar floristically and there is evidence for a marked facilitation of woody plant recruitment correlated with light availability on the ground. Both sites showed characteristics of classic succession, with Shrub showing a higher species richness and density while late-successional species are poorly represented on the Bracken site. However, NMDS ordination using species density showed that the two trajectories may not be converging towards a common ‘‘final state’’. Manual weeding was ineffective for accelerating forest recovery. These results suggest that the main limiting factor for the recruitment of woody species on the Pasture site is strong grass competition and must be addressed before seed availability, while seed availability seems to be theconstraining factor for Bracken and Shrub site development, thus direct seeding or planting may be effective in accelerating forest recovery.

Abstract:

Field studies have shown a wide array of responses of vascular epiphyte diversity to human disturbance-assem- blages of disturbed habitats range from largely unchanged to severely impoverished when compared with intact forest. This variability is not well understood. We explored the hypothesis that the relative impoverishment of disturbed-habitat epiphyte assemblages is a function of local climate, by analyzing the available literature on epiphyte diversity on isolated trees as a model system. We found that assemblages of moist and moderately seasonal areas experience considerably stronger impov- erishment than those of aseasonally wet or distinctly dry areas. We argue that the integrity of the vertical microclimatic gradient is more crucial for the maintenance of epiphyte diversity in moderately seasonal forests than in distinctly dry or aseasonally wet forests.

Abstract:

Tank bromeliads are common epiphytic plants throughout neotropical forests that store signi?cant amounts of water in phytotelmata (tanks) formed by highly modi?ed leafs. Methanogenic archaea in these tanks have recently been identi?ed as a signi?cant source of atmospheric methane. We address the effects of environmental drivers (temperature, tank water content, sodium phosphate [P], and urea [N] addition) on methane production in anaerobically incubated bromeliad slurry and emissions from intact bromeliad tanks in montane Ecuador. N addition ? 1 mg g
1 had a signi?cantly positive effect on headspace methane concentrations in incubation jars while P addition did not affect methane production at any dosage (? 1 mg g
1 ). Tank bromeliads (Tillandsia complanata) cultivated in situ showed signi?cantly increased ef?uxes of methane in response to the addition of 26 mg N addition per tank but not to lower dosage of N or any dosage of P (? 5.2 mg plant
1 ). There was no signi?cant interaction between N and P addition. The brevity of the stimulatory effect of N addition on plant methane ef?uxes (1–2 days) points at N competition by other microorganisms or bromeliads. Methane ef?ux from plants closely followed within-day temperature ?uctuations over 24 h cycles, yet the dependency of temperature was not exponential as typical for terrestrial wetlands but instead linear. In simulated drought, methane emission from bromeliad tanks was maintained with minimum amounts of water and regained after a short lag phase of approximately 24 h. Our results suggest that methanogens in bromeliads are primarily limited by N and that direct effects of global change (increasing temperature and seasonality, remote fertilization) on bromeliad methane emissions are of moderate scale.

Abstract:

1. Ridges of tropical mountains often differ strikingly from neighbouring ravines in terms of forest structure, productivity and species composition. This heterogeneity is poorly understood despite its critical role in biodiversity maintenance, carbon and nutrient budgets. 2. We examined measures of tree biomass and productivity, foliage and litter quality (nutrient concentrations, specific leaf mass, phenolics), herbivory and leaf litter decomposition in each six plots laid out in upper and lower slope position in a tropical montane moist forest in southeastern Ecuador. 3. Productivity, quality of foliage and litter as well as herbivory were significantly lower in upper slope position, and closely correlated with soil nutrient concentrations and accumulated humus. The decomposition of upper slope leaf litter (decomposition rate k) was substantially lower than in litter from lower slope forest, whereas the site of decomposition (slope position) only had a marginal effect on the decomposition rate. 4. Our results suggest that the differences in stand structure, productivity, foliar quality, herbivory and decomposition between slope positions are ultimately due to stronger nutrient limitations in upper slope forest. We propose a general conceptual model that explains origin and maintenance of contrasting forest types along topographical gradients through down-slope fluxes of nutrients and water, and a nutrient-driven positive feedback cycle.

Abstract:

The spatial heterogeneity of tropical forest epiphytes has rarely been quantified in terms of biomass. In particular, the effect of topographic variation on epiphyte biomass is poorly known, although forests on ridges and ravines can differ drastically in stature and exposure. In an Ecuadorian lower montane forest we quantified epiphytic biomass along two gradients: (1) the twig-branch-trunk trajectory, and (2) the ridge-ravine gradient. Twenty-one trees were sampled in each of three forest types (ridge, slope, ravine positions). Their epiphytic biomass was extrapolated to stand level based on basal area?epiphyte load relationships, with tree basal areas taken from six plots of 400 m 2 each per forest type. Our results document the successional addition and partial replacement of lichens by bryophytes, angiosperms and finally dead organic matter along the twig-branch-trunk trajectory. Despite having the highest tree basal area, total epiphytic biomass (mean ± SD) of ravine forest was significantly lower (2.6 ± 0.7 Mg half 1) than in mid-slope forest (6.3 ± 1.1 Mg half 1) and ridge forest (4.4 ± 1.6 Mg half 1), whereas maximum bryophyte water storage capacity was significantly higher. We attribute this pattern to differences in forest dynamics, stand structure and microclimate. Although our study could not differentiate between direct effects of slope position (nutrient availability, mesoclimate) and indirect effects (stand structure and dynamics), it provides evidence that fine-scale topography needs to be taken into account when extrapolating epiphytic biomass and related matter fluxes from stand-level data to the regional scale.

Abstract:

Forest fragmentation can negatively affect tropical epiphyte diversity, but the processes leading to such impoverishment are insufficiently understood. Due to a lack of experimental studies, the relative influence of dispersal constraints vs. growth conditions remains particularly controversial. This paper addresses the fate of late juvenile and adult vascular epiphytes in response to severe forest disturbance in montane southern Ecuador. Plant growth and survival on trunks and lower branches of isolated remnant trees was studied for the first three years following clear-cutting. Overall epiphyte mortality was substantially increased on remnant trees (72% over 3 years) relative to undisturbed forest (11%). Mortality on remnant trees was higher during the first year (52%) than during the second (20%) and third year (26%). Pteridophytes and dicots suffered higher losses than monocots. Plants surviving on remnant trees generally showed a marked negative growth regarding maximum leaf length, whereas the annual increment in leaf number varied more strongly among taxa (families). The present study provides the first field-experimental evidence for the adverse effects of forest disturbance on the performance of later, well-established life stages of vascular epiphytes. The results suggest that growth conditions may often be a more important predictor of epiphyte diversity in disturbed habitats than dispersal constraints. Similar plant responses can be expected to occur along forest edges. Therefore, the retention of scattered green trees, narrow strips or small fragments of forest are unlikely to be sufficient management tools for the conservation of epiphyte diversity in tropical landscapes.

Abstract:

Although vascular epiphytes contribute substantially to the biodiversity of tropical montane forests, it is unclear how their diversity and community composition is affected by forest alteration. We studied the response of vascular epiphyte assemblages to different intensities of land-use in a montane wet forest of northeastern Ecuador: (1) unmanaged mature forest; (2) mature forest with mid- and understorey opened for cattle grazing; and (3) isolated remnant trees in cattle pastures. The numbers of individuals and species of epiphytes per host tree did not differ significantly between land-use types, neither did total epiphyte species richness (n = 30 trees). However, total species richness of pteridophytes was significantly lower on isolated remnant trees compared to unmanaged forest, whereas several taxa rich in xerotolerant species (Bromeliaceae, Orchidaceae, Piperaceae) exhibited the opposite trend. An analysis of floristic composition using ordination (NMS) and randomisation techniques (MRPP) showed that epiphyte assemblages on isolated remnant trees were significantly distinct from unmanaged forest while managed forest was intermediate between those two vegetation types. Ordination analysis further indicated reduced floristic heterogeneity in disturbed habitats. These results suggest considerable, rapid species turnover since land-use change 6 years prior to study, with pteridophytes being replaced by more xerotolerant taxa. We attribute this floristic turnover primarily to changes in microclimate towards higher levels of light and desiccation stress associated with forest disturbance. Our results support the notion that community composition offers a more sensitive indicator of human disturbance than species richness.

Abstract:

A new species of mountain viscacha, Lagidium ahuacaense, is described based on a specimen and a mitochondrial cytochrome b (cyt b) sequence obtained from a second individual from Cerro El Ahuaca, Loja Province, Ecuador. In several external and craniodental measurements, the new species differed significantly from the three congeneric species (greatest length of skull, basilar, nasal length, palatilar length, length of diastema, least interorbital breadth, breadth of rostrum and skull height). The cyt b sequence of the Ecuadorean viscacha differed by 14 exclusive nucleotide substitutions from all other sequences of Lagidium examined. Kimura 2-parameter (K2P) genetic distances of the Ecuadorean sequence were 8.1?11.0% to L. peruanum, 7.9?9.9% to L. viscacia and 9.7% to L. wolffsohni. The single known population of the newly described species may not comprise more than a few dozen individuals and warrants urgent conservation actions.

Abstract:

While tropical deforestation proceeds at high pace, conservation biologists remain divided over the extent to which anthropogenic habitats will be able to offset the loss of biodiversity from primary forests. Case studies addressing disturbance effects on epiphyte assemblages have yielded contrasting patterns, and underlying mechanisms remain unclear. At present, one of the questions most widely debated is the importance of growth conditions vs. dispersal limitations for the persistence of epiphyte populations following disturbance. The present dissertation aims at contributing to our understanding of the processes that determine epiphyte diversity in anthropogenic landscapes, focussing primarily on assemblages of vascular epiphytes on isolated trees. Isolated trees offer an excellent model system since they constitute the smallest possible forest fragment, exposed to multiple edge effects and dispersal limitations. Field work was done from 2003?2006 at two lower montane sites in the Ecuadorean Andes: (1) moist forest (2200 mm rain/yr; 1800?2200 m elevation) and adjacent matrix habitat around Estación Científica San Francisco (ECSF) on the eastern Andean slope (Zamora-Chinchipe Province), and (2) perarid Interandean forest (530 mm rain/yr; 2300m elevation) and adjacent pastures at Bosque Protector Jerusalén (Pichincha Province). At the moist forest site (ECSF), I studied the diversity of vascular epiphytes on remnant trees ca. 10?30 years after their isolation in pastures (chapter 2). The objective of this study was to document the importance of remnant trees for the survival of vascular epiphyte communities following forest clearance. Trees were divided into five zones following Johansson (1974) and climbed with the single rope technique. Abundance and diversity of epiphytes were significantly lower on remnant trees than on forest trees. Several families rich in xerophilous taxa (Bromeliaceae, Orchidaceae, Piperaceae, Polypodiaceae) were relatively well represented on remnant trees in terms of species richness and abundance, whereas essentially hygrophilous families such as Dryopteridaceae, Ericaceae, Grammitidaceae and Hymenophyllaceae were poorly represented or absent. Impoverishment was greatest on the stem base and in the outer crown, and least in the inner crown of the host trees. These results suggest that the observed impoverishment on isolated trees is closely related to microclimatic changes. Experimental work done at the same site gave some insight into the mechanisms behind the pronounced impoverishment of epiphyte assemblages. In one field trial (chapter 6) I studied abundance, diversity and floristic composition of epiphyte seedling establishment on isolated and adjacent forest trees. All vascular epiphytes were removed from plots on the trunk bases of a common tree species (Piptocoma discolor). Newly-established epiphyte seedlings were recorded after two years, and their survival after another year. Seedling density and the number of taxa (families and genera, respectively) per plot were significantly reduced on isolated trees relative to forest trees, as was the rarefied total number of epiphyte taxa. Seedling assemblages on trunks of forest trees were dominated by hygrophilous understorey ferns whereas assemblages on isolated trees were dominated by xerophilous canopy taxa. Colonisation probability was significantly higher for plots closer to forest but not for plots with greater canopy or bryophyte cover. Seedling mortality on isolated trees was significantly higher for hygrophilous than for xerophilous taxa. These results show that altered recruitment can explain the long-term impoverishment of post-juvenile epiphyte assemblages on isolated remnant trees. Altered establishment is attributed to a combination of dispersal constraints and the changed microclimate that was documented by measurements of temperature and humidity. Although isolated trees in anthropogenic landscapes are key structures for the maintenance of forest biodiversity in many aspects, my results show that their value for the conservation of epiphyte diversity can be limited. Abiotic seedling requirements may increasingly constitute a bottleneck for the persistence of vascular epiphytes in the face of ongoing habitat alteration and atmospheric warming. In a second field trial at the same site (chapter 7) I studied the response of assemblages of well-established vascular epiphytes to severe forest disturbance. Individual plants were marked on isolated remnant trees (up to 5 m above ground) in a fresh clearing and in undisturbed forest (controls), and their growth and survival was followed during three consecutive years. Wind-throw and branch breakage caused the loss of 24% of plants from isolated trees but only of < 1% in the forest. Epiphyte mortality on the remaining, intact phorophytes was substantially higher on remnant trees than on forest trees, averaging 72% vs. 11% after 3 years. Mortality on isolated trees was greatest after the first year (52%) and among ferns and dicots, and lowest after the second year (20%) and among monocots (aroids, bromeliads, orchids). Mortality differed significantly between taxa but was elevated significantly for most taxa after the first year relative to forest levels. Plants that survived on isolated trees commonly showed a marked decrease in maximum leaf length. The annual increment in leaf number varied more widely, both between and within epiphyte families. This study provides first experimental, field-based evidence that increased physical exposure affects the performance of well-established vascular epiphytes. The results suggest that growth conditions may often be a more influential driver of vascular epiphyte diversity in disturbed habitats than dispersal. It is argued that elevated desiccation stress is only one of several mechanisms by which increased physical exposure may limit epiphyte performance in disturbed habitats. The two trials showed that the marked impoverishment of vascular epiphyte assemblages at the study site results from both lowered recruitment and survival of established plants, and that mid-term effects of changes in growth conditions can be drastic. At the dry forest site (Bosque Protector Jerusalén) I studied the development of epiphyte diversity and abundance across a disturbance gradient (chapters 3 and 4). Epiphytic bryophytes and vascular plants were sampled on 100 trees of the dominant tree species, Acacia macracantha, in five habitats: closed-canopy mixed and pure acacia forest (old secondary), forest edge, young semi-closed secondary woodland, and isolated trees in grasslands. Host trees were divided into four zones (cf. Johansson 1974). Species density and surface cover of vascular epiphytes were determined for each zone, bryophytes were only sampled in the inner crown. Total species richness in forest edge and on isolated trees was significantly lower than in closed forest types. Species density of vascular epiphytes did not differ significantly between habitat types. Species density of bryophytes, in contrast, was significantly lower in forest edge and on isolated trees than in closed forest. Surprisingly, edge habitat showed greater impoverishment than semi-closed woodland and similar floristic affinity to isolated trees and to closed forest types. Assemblages were significantly nested; habitat types with major disturbance held only subsets of the closed forest assemblages, indicating a gradual reduction in niche availability. This conclusion is lent further support by patterns of epiphyte abundance. Bromeliads were xerophilous and early-successional, being most abundant in the middle crown, whereas ferns were skiophilous and late-successional, being essentially restricted to the inner crown. While the former were significantly more abundant on isolated trees than in closed forest, the latter were restricted to closed forest and semi-closed woodland. Bryophyte cover was significantly lower on isolated trees and in forest edge than in closed forest. Total species density and covers of epiphytes on isolated trees were not related to the distance to forest, but total species density and bryophyte cover were closely correlated with crown closure as a measure of canopy integrity. These results suggest that microclimatic changes are key determinants of the observed impoverishment of epiphyte assemblages following disturbance, and that epiphytic cryptogams are sensitive indicators of microclimate and human disturbance in montane dry forests. Patterns of abundance and diversity further suggest that the ?similar gradient hypothesis? (McCune 1993) applies to tropical dry forest assemblages of epiphytes. The substantial impoverishment of edge habitat underlines the need for fragmentation studies on epiphytes elsewhere in the Tropics. Case studies on vascular epiphyte assemblages have failed to find a common, general response to human disturbance. In chapter 5, a hypothesis is proposed and explored that aims to explain and unite these conflicting patterns in the light of a non-linear effect of local climate: while the diverse assemblages of moist, slightly seasonal forests rely on the integrity of forest canopies during droughts, this dependency may be less pronounced in aseasonally wet or distinctly dry forests. This hypothesis is supported by the results of a literature survey.

Abstract:

Ziel dieser Arbeit war es, die Epiphytenvegetation bezüglich ihrer Abundanz, Diversität und floristischen Zusammensetzung in einem Bergregenwald in Südecuador zu untersuchen. Dafür wurde zum einen ein junger Sekundärwald mit Primärwald und zum anderen Unterwuchs und Kronendach eines gestörten bzw. älteren sekundären Waldes miteinander verglichen. Es wurden verschiedene Umweltparameter aufgenommen, um einen Einfluss dieser auf die Epiphytenvegetation zu untersuchen. Lediglich bei den Umweltparametern Hangneigung, Kronendachöffnung und Moosdeckung im jungen Sekundärwald konnte ein möglicher Einfluss auf Abundanz und Artenzahl der Epiphyten festgestellt werden. Nach Einteilung der Untersuchungsgebiete in vertikale Zonen konnte insbesondere im Unterwuchs des gestörten bzw. älteren sekundären Waldes eine vertikale Stratifizierung festgestellt werden. Die Abundanz und Artenzahl war in der Zone 1 (>0,25m - 1,25m) signifikant höher als in der Zone 2 (>1,25m - 2,25m). Dies wurde hauptsächlich auf mikroklimatische Unterschiede zurückgeführt. Auch hinsichtlich der floristischen Zusammensetzung gab es Unterschiede zwischen den beiden Zonen. Die Abundanz der Farne nahm mit zunehmender Höhe besonders stark ab. Lichtliebende und trockentolerante Gattungen der Familien Orchidaceae und Bromeliaceae hatten hingegen in der Zone 2 eine höhere Abundanz. Im jungen Sekundärwald stellte sich die durchschnittlich 4,5m hohe Zone 3 (>2,25m - Kronenhöhe) verglichen mit den beiden Zonen 1 und 2 als arten- und individuenärmer heraus. Dies wurde insbesondere auf das geringe Alter und das Fehlen von geeigneten Etablierungsmöglichkeiten für vaskuläre Epiphyten sowie auf deren oft langsames Wachstum zurückgeführt. Beim Vergleich der Epiphytenvegetation des Unterwuchses und des Kronendaches stellte sich heraus, dass trotz einer etwa doppelt so hohen Abundanz im Kronendach, die Anzahl der Arten im Unterwuchs und im Kronendach fast gleich hoch war. Hinsichtlich der Artenzusammensetzung unterschieden sich Unterwuchs und Kronendach stark voneinander. Von den insgesamt 229 aufgenommenen Arten kamen etwa 40% ausschließlich im Unterwuchs vor und nur 20% waren sowohl im Unterwuchs als auch im Kronendach vertreten. Im Vergleich des jungen Sekundärwaldes mit Primärwald zeigten sich ebenfalls starke Unterschiede. Im jungen Sekundärwald waren Abundanz und Artenzahl sowie der Anteil zoochor ausgebreiteter Arten um ein Vielfaches niedriger als im Primärwald. Die monokotylen Epiphyten, darunter insbesondere Orchidaceen, hatten im Primärwald die höchste Abundanz und Artenzahl. Im jungen Sekundärwald waren die monokotylen Epiphyten zwar auch die artenreichste Klasse, ihre relative Abundanz war jedoch wesentlich geringer. Trotz der hohen relativen Abundanz der Farne im jungen Sekundärwald hatten besonders feuchtigkeitsliebende Farnfamilien eine sehr geringe Abundanz. Dies wurde auf das weniger konstant-feuchte Mikroklima des jungen Sekundärwaldes mit seinem noch nicht so stark geschlossenen Kronendach (hohe Kronendachöffnung) zurückgeführt. So war beispielsweise die feuchigkeitsliebende Farnfamilie Hymenophyllaceae, die im Primärwald nach den Orchidaceen die zweithöchste Abundanz hatte, im jungen Sekundärwald mit einer nur sehr geringen Abundanz vertreten. Diese Ergebnisse verdeutlichten zum einen die Bedeutung der Unterwuchsepiphyten für die Diversität der gesamten Epiphytenvegetation tropischer Bergregenwälder. Zum anderen zeigte sich die besondere Sensibilität dieser Unterwuchsepiphyten gegenüber den mikroklimatischen Bedingungen ihrer Standorte. Dies zeigte die Abnahme ihrer Abundanz und Artenzahl mit Abnahme der konstanten Luftfeuchtigkeit in höheren Stammbereichen. Charakteristische Unterwuchsepiphyten sind wahrscheinlich deshalb im jungen Sekundärwald noch vergleichsweise schwächer vertreten als charakteristische Kronendachepiphyten, weil sich die mikroklimatischen Bedingungen, wie sie in älteren Wäldern und besonders in Primärwäldern im Unterwuchs vorherrschen noch nicht wieder eingestellt haben. Selbst der untersuchte etwa 30 bis 40 Jahre alte, gestörte bzw. sekundäre Wald (Gebiet ECSF) hatte im Vergleich zum Primärwald eine immer noch deutlich geringere Abundanz und Artenzahl. Dies deutet darauf hin, dass die Sukzession der Epiphytenvegetation ein sehr langsamer Prozess ist. Es stellt sich die Frage, ob es möglich ist und wie lange es in diesem Fall dauert, bis sich wieder eine primärwaldähnliche floristische Zusammensetzung und eine hohe epiphytische Diversität entwickelt.

Abstract:

We sampled the diversity of epiphytes (lichens, bryophytes, vascular plants) and moths (Geometridae, Arctiidae) in mature and recovering forest and in open vegetation in the montane belt in Ecuador. No uniform pattern of change in species richness was detected among the different taxonomic groups with increasing disturbance. Species richness of epiphytic bryophytes and vascular plants declined significantly from mature forest towards open vegetation. In contrast, species richness of epiphytic lichens did not change with increasing forest alteration, while that of geometrid moths was significantly higher in recovering forest compared with mature forest and open habitats. Arctiidae were significantly more species-rich in recovering forest and open vegetation than mature forest. Hence, for some organisms, modified habitats may play an important role for biodiversity conservation in the Andes, whereas others suffer from habitat disturbance. However, trends of changes in species composition following deforestation were surprisingly concordant across most studied epiphyte and moth taxa.

Abstract:

The impact of human disturbance on colonisation dynamics of vascular epiphytes is poorly known. We studied abundance, diversity and floristic composition of epiphyte seedling establishing on isolated and adjacent forest trees in a tropical montane landscape. All vascular epiphytes were removed from plots on the trunk bases of Piptocoma discolor. Newly established epiphyte seedlingswere recorded after 2 years, and their survival after another year. Seedling density, total richness at family and genus level, and the number of families and genera per plot were significantly reduced on isolated trees relative to forest trees. Seedling assemblages on trunks of forest trees were dominated by hygrophytic understorey ferns, those on isolated trees by xerotolerant canopy taxa. Colonisation probability on isolated trees was significantly higher for plots closer to forest but not for plots with greater canopy or bryophyte cover. Seedling mortality on isolated trees was significantly higher for mesophytic than for xerotolerant taxa. Our results show that altered recruitment can explain the long-term impoverishment of post-juvenile epiphyte assemblages on isolated remnant trees.We attribute these changes to a combination of dispersal constraints and the harshermicroclimate documented by measurements of temperature and humidity. Although isolated trees in anthropogenic landscapes are considered key structures for themaintenance of forest biodiversity inmany aspects, our results show that their value for the conservation of epiphytes can be limited.We suggest that abiotic seedling requirementswill increasingly constitute a bottleneck for the persistence of vascular epiphytes in the face of ongoing habitat alteration and atmospheric warming.

Abstract:

Question: Disturbance effects on dry forest epiphytes are poorly known. How are epiphytic assemblages affected by different degrees of human disturbance, and what are the driving forces? Location: An inter-Andean dry forest landscape at 2300 m elevation in northern Ecuador. Methods: We sampled epiphytic bryophytes and vascular plants on 100 trees of Acacia macracantha in five habitats: closed-canopy mixed and pure acacia forest (old secondary), forest edge, young semi-closed secondary woodland, and isolated trees in grassland. Results: Total species richness in forest edge habitats and on isolated trees was significantly lower than in closed forest types. Species density of vascular epiphytes (species per tree) did not differ significantly between habitat types. Species density of bryophytes, in contrast, was significantly lower in edge habitat and on isolated trees than in closed forest. Forest edge showed greater impoverishment than semi-closed woodland and similar floristic affinity to isolated trees and to closed forest types. Assemblages were significantly nested; habitat types with major disturbance held only subsets of the closed forest assemblages, indicating a gradual reduction in niche availability. Distance to forest had no effect on species density of epiphytes on isolated trees, but species density was closely correlated with crown closure, a measure of canopy integrity. Main conclusions: Microclimatic changes but not dispersal constraints were key determinants of epiphyte assemblages following disturbance. Epiphytic cryptogams are sensitive indicators of microclimate and human disturbance in montane dry forests. The substantial impoverishment of edge habitat underlines the need for fragmentation studies on epiphytes elsewhere in the Tropics.