UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)
UvA-DARE (Digital Academic Repository)
Flora, vegetation and ecology in the Venezuelan Andes: a case study of Ramal
de Guaramacal
Cuello Alvarado, N.L.
Publication date
2010
Link to publication
Citation for published version (APA):
Cuello Alvarado, N. L. (2010). Flora, vegetation and ecology in the Venezuelan Andes: a case
study of Ramal de Guaramacal. Universiteit van Amsterdam, Institute for Biodiversity and
Ecosystem Dynamics (IBED).
General rights
It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons).
Disclaimer/Complaints regulations
If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible.
Chapter 7
Synthesis
7.1 AIMS
The study of flora and vegetation of Ramal de Guaramacal, located in the Andes of Venezuela, was conducted with the general aim to study montane rain forest and páramo by exploring their structure, botanical composition and diversity and relating these vegetation types to main environmental factors changing along gradients. A specific goal was to examine the patterns of forest diversity along altitudinal gradients with regard to plant functional traits. To elucidate the phytogeographical patterns of the wet páramo flora, these vegetation types were compared to other páramo areas in Ecuador, Colombia and the Talamancas of Central America. This study contains basic knowledge for the conservation and biodiversity management in the region.
This thesis provides information about forest and páramo vegetation along altitudinal gradients using a floristic and phytosociological approach (chapters 2, 3 and 4); information about the analysis of the phytogeography of the Guaramacal páramo flora, and relationships to floras of other páramos (chapter 5), and finally information on montane forest diversity along an altitudinal gradient by means of a plant functional approach (chapter 6). The results of the different chapters are synthesized below.
7.2 ALTITUDINAL ZONATION
The TWINSPAN analysis of forest vegetation along an elevational gradient in Ramal de Guaramacal showed an altitudinal zonation of forest types. Forest types are grouped into discrete zones corresponding to the lower montane rain forest (LMRF), upper montane rain forest (UMRF), and subalpine rain forests (SARF) classes of Grubb (1977). Alternatively there is a correspondence to the subandean forest, Andean forest, and high Andean forest, respectively, following Cuatrecasas (1934, 1958). However, forest zonation was found variable between the northern and southern slopes of Guaramacal.
LMRF of Ramal de Guaramacal can be found from 1350 m on the southern slope and from 1650 m on the northern slope, to about 2300 m. However, in downslope direction LMRF is limited to 1800 m, determined by the Park boundaries, in most areas on the northern slope. Below 1800 m, disturbed areas occupy the potential LMRF zone. UMRF is present from 2300 to ~2800 m on the northern slope of Guaramacal. However, on southern or northwestern slopes near the tops of small ranges, UMRF occurs as low as 2100 m. In Ramal de Guaramacal SARF is present at the same altitudes as páramo vegetation, viz. from 2800 to 3050 m. In this altitudinal range a zone with subpáramo vegetation, according to Cuatrecasas (1934, 1958), is recognized. Subpáramo vegetation is classified into a lower subpáramo or shrub páramo, and an upper subpáramo or dwarfshrub bunchgrass páramo, following Cleef (1980, 1981). On the windward southern slopes, forest zones of UMRF tend to reach lower elevations than on the opposite and drier northern slopes and there the sequence of forest zones is shortest in distance. Higher temperatures, almost permanent humidity, and frequent landslides on the steeper and wetter southern slopes at mid-elevation may play a role.
_______________________________________________________
The surveys along the forest-páramo border in Ramal de Guaramacal detected an upper forest line (UFL) position around 2950 m. This is remarkably low in comparison with UFL positions elsewhere in the equatorial Andes and the Costa Rican Talamancas. The low UFL is apparently caused by the “top effect” (Grubb 1971) with UMRF (including SARF) found at lower altitudes (Grubb 1977). A top effect may also cause a compression of the montane forest zones. The history of the low UFL in Guaramacal range may be explained by paleoecological studies of lake sediments, e.g. the promising peat land at c. 2000 m near the Park Rangers house of the Guaramacal National Park. Also an analysis of climatic data from the Davis weather station near Antenas (data available from December 2006 onwards), and analysis of soil surface temperatures on the top of Páramo de Guaramacal may be helpful to explain the UFL position. Additional comparative analysis of the vegetation ecology at the UFL would also provide more clues for understanding the low altitude of the UFL on Ramal de Guaramacal.
7.3 FLORISTIC COMPOSITION AND DIVERSITY
Montane forests of Ramal de Guaramacal show different patterns of species diversity, family composition and vegetation structure along altitudinal gradients on the drier as well as on the wetter slopes. In LMRF the Rubiaceae, Lauraceae and Melastomataceae are the most species rich families of woody plants, which is the same trend as observed in other Andean forests (Gentry 1992, 1995, Rangel-Ch. 1991). In UMRF the Lauraceae family is the most diverse in species, which is exceptional, followed by the Melastomataceae and Myrtaceae. In SARF Asteraceae and Ericaceae are the most species rich families.
In páramo vegetation Asteraceae and Poaceae rank as most species rich. This is the case in almost all páramo vegetation and in the flora‟s of other alpine areas (Rangel-Ch. 2000c; Vargas & Sánchez 2005; Rivera-Diaz 2007; Rangel-Ch. et al. 2008; Briceño & Morillo 2002, 2006; Lozano et al. 2009), followed by Ericaceae and Orchidaceae. Pteridophytes are also species diverse, with Grammitidaceae, Lycopodiaceae, and Dryopte-ridaceae as the most species rich families.
Species diversity and composition of montane forests of Ramal de Guaramacal varies along the altitudinal gradient with some bias depending slope exposure. Species richness generally decreases with elevation. However, an increase in species richness based on 0.1 ha plots was locally observed between 2300 and 2400 m on the northern slope of Guaramacal. At this elevation is the transitional zone from LMRF to UMRF, and here species richness may be related to the increasing humidity from the dry interandean Boconó Valley to the top of the mountain. This diversity pattern supports a proposed “third pattern” of altitudinal species richness claiming highest biodiversity in the middle of an altitudinal zone (Lomolino 2001). Wolf (2003) already pointed at high richness of epiphytic bryophytes and lichens in the mid-altitudinal range of a zone.
With a limited altitudinal span (2820-3130 m) and a small surface area of ca. 10 km2, the Páramo de Guaramacal counts some 200 vascular páramo species (alpha
diversity). Compared to the number of 1544 vascular species reported for all Venezuelan páramos [1437 angiosperms species reported by Briceño & Morillo Flora, vegetation and ecology in the Venezuelan Andes
(2002, 2006) plus 107 fern species reported by Luteyn (1999)] the low species number of Páramo de Guaramacal seems proportional. At an elevation of about 2200 m there is a small direct connection to the Mérida Andes.
Up to date, about 50 endemic vascular species are known from Ramal de Guaramacal which represent ca. 4% from a total of about 1400 vascular species. Repeated isolation in the past probably triggered the development of endemic species. The highest species diversity of Ruilopezia rosettes reported for the Venezuelan Andes to date is in our study area and it may be speculated that during the Pleistocene repeated isolation and merging of populations gave rise to endemics.
7.4 VEGETATION PHYSIOGNOMY
The structure of the montane forests of Ramal de Guaramacal is more compressed towards higher elevations, with an increase in stem density and a decrease in stem diameter and canopy height. LMRF is dense and of medium height, with canopies up to 25 m tall, while UMRF canopies reach up to 18 m, and those of SARF to 6-8 (10) m only.
Diversity and density of growth forms varies with elevation. More diversity and density of palms, lianas and climbers is clearly observed in LMRF. Although diversity and density of lianas decrease with altitude, still a substantial percentage of the total species richness of SARF is represented by liana species. Tree ferns show highest density in the LMRF, but highest species diversity is observed in UMRF.
Zonal páramo vegetation is represented by shrub páramo, bunchgrass páramo and, in our study area most commonly by bamboo páramo. Bamboo páramos are mainly dominated by woody growth forms, particularly upright shrubs with bamboo groves and clumps, which give an overall appearance of a mostly shrub páramo vegetation. Low bunchgrass páramo vegetation, devoid of shrubs and with a high density of small ground rosettes, cushion grasses and few bamboos, is found in limited areas above 2900 m. In the study area the most representative life form, in terms of both number of species and cover, are the phanerophytes, especially those belonging to the microphanerophytic type, followed by hemicryptophytes with a caespitose life form.
The dominance of shrubby growth forms in páramo of Ramal de Guaramacal may be partly explained by the high relative humidity, the low altitudinal range, the close proximity of the dwarf forests near the upper forest line, past disturbance events and fire dynamics.
Azonal páramo vegetation is represented by patches of azonal bunchgrass,
Sphagnum bogs, aquatic communities, and boggy bamboo páramo. 7.5 PHYTOSOCIOLOGICAL CLASSIFICATION
The phytosociological classification of the vegetation of Ramal de Guaramacal was based on floristic composition and species abundance or cover. Results
_______________________________________________________
revealed a total of eight new alliances and eighteen vegetation communities at association level, which are distinguished and described according to the Zürich-Montpellier method (Braun-Blanquet 1979; Westhoff & van der Maarel 1973). We recognized and documented three new subandean forest (LMRF) communities, and four new Andean forest (UMRF) and high Andean forest (SARF); three new associations of lower subpáramo or shrubby páramo, and two new associations of upper subpáramo or bunchgrass páramo dominated by rosettes and tussock plants; one new azonal bunchgrass páramo association, two new Sphagnum bog associations, one new bamboo páramo („chuscal‟) association, and two submerged aquatic communities. A synoptic syntaxonomical scheme for classification of the vegetation of Ramal de Guaramacal runs as follows:
I. Montane forest group of Meliosma tachirensis – Alchornea grandiflora
A. The alliance Geonomo undatae – Posoquerion coriaceae Cuello & Cleef 2009, which contains the following subandean forests associations:
1. Simiro erythroxylonis – Quararibeetum magnificae Cuello & Cleef 2009 2. Conchocarpo larensis – Coussareetum moritzianae Cuello & Cleef 2009 B. The alliance Farameo killipii – Prunion moritzianae Cuello & Cleef 2009. This alliance contains one subandean forest community and one Andean forest community:
3. Croizatio brevipetiolatae – Wettinietum praemorsae Cuello & Cleef 2009 4. Schefflero ferrugineae – Cybianthetum laurifolii Cuello & Cleef 2009 C. The alliance Ruilopezio paltonioides – Cybianthion marginatii Cuello & Cleef 2009. This includes one Andean and two high Andean forest communities:
5. Geissantho andini – Miconietum jahnii Cuello & Cleef 2009
6. Gaultherio anastomosantis – Hesperomeletum obtusifoliae Cuello & Cleef 2009
7. Libanothamnetum griffinii Cuello & Cleef 2009
II. Zonal humid lower páramo of Ruilopezio lopez-palacii – Chusqueetalia angustifoliae Cuello & Cleef 2009 (prov. Ord.)
D. The alliance Hyperico paramitanum –Hesperomeletion obtusifoliae Cuello & Cleef 2009, groups the shrubby páramo associations:
8. Ruilopezio paltonioides –Neurolepidetum glomeratae Cuello & Cleef 2009 9. Disterigmo acuminatum –Arcytophylletum nitidum Cuello & Cleef 2009 E. The alliance Hyperico cardonae – Xyridion acutifoliaeCuello & Cleef 2009, groups one shrubby páramo and two open grass páramo associations:
10. Cortaderio hapalotrichae –Hypericetum juniperinum Cuello & Cleef 2009 11. Puyo aristeguietae –Ruilopezietum lopez-palacii Cuello & Cleef 2009 Flora, vegetation and ecology in the Venezuelan Andes
12. Rhynchosporo gollmeri –Ruilopezietum jabonensis Cuello & Cleef 2009 III. The order of azonal páramo peat bog vegetation of Geranio stoloniferum – Caricetalia bonplandii Cuello & Cleef 2009
F. The alliance Sphagno recurvi – Paepalanthion pilosi Cuello & Cleef 2009, groups a bunchgrass páramo association and the both new Sphagnum bog associations:
13. Paepalantho pilosi – Agrostietum basalis Cuello & Cleef 2009 14. Sphagno recurvi – Caricetum bonplandii Cuello & Cleef 2009 15. Sphagno sparsi – Caricetum bonplandii. Cuello & Cleef 2009
G. The alliance Carici bonplandii – Chusqueion angustifolia Cuello & Cleef 2009, contains a bamboo páramo („chuscal‟) association:
16. Carici bonplandii – Chusqueetum angustifoliae Cuello & Cleef 2009 H. The alliance Districho submersi – IsoëtionCleef 1981 is represented by:
17. The submerged aquatic community of Sphagnum cuspidatum 18. Isoëtetum karstenii Cleef 1981
For both montane forest and páramo vegetation classes could not yet be defined on the basis of the present number of relevés and other available information, neither due to the lack of similar data to characterise the region and from montane forests and Chusquea angustifolia bamboo páramos elsewhere in Venezuela and adjacent Colombia. Comparison of zonal páramo communities of Chusquea angustifolia is at present impossible.
More relevés, in particular in the zones of LMRF and UMRF may provide helpful information to better classify the alliance Farameo killipii - Prunion moritzianae, which includes forest associations of both LMRF and UMRF. More studies with comparable aims in the surrounding mountains enable a forest classification at order and class level.
This study represents the first attempt to classify the vegetation phytosociolo-gically based on a quantitative data set from an entire mountain range in the Vene-zuelan Andes. Despite of the relatively low number of relevés and methodological constraints, we arrived at a clear forest classification for the montane forests of Ramal de Guaramacal.
Zonal páramo vegetation of the Guaramacal range was described on the basis of a relatively low number of relevés from the most accessible páramo areas of Ramal de Guaramacal (sector of Las Antenas of Páramo de Guaramacal). The most
different physiognomic formations are found in sector Las Antenas in relatively close proximity. This sector shows a larger altitudinal range (2820~3130 m). However it has a past history of disturbances and fires, which may have affected the spatial distribution of vegetation communities and consequently the current situation of the upper limit of the forest. The resulting zonal páramo classification
_______________________________________________________
at association level may be representative of most páramo areas of Ramal de Guaramacal.
The azonal páramo vegetation has been described on the basis of a limited number of relevés and from two peat bogs and a small pond only located in two páramo areas of Ramal de Guaramacal. The latter includes the Páramo de Guaramacal and the Páramo El Pumar. The limited accessibility of the study area throughout most of the year, together with high precipitation levels and the frequency of mist, made the exploration of peat bog areas of Guaramacal difficult. As other unexplored peat bogs are known to exist in the area other azonal vegetation communities may be present in the páramos of Ramal de Guaramacal.
An integrated study of regional importance of the Sphagnum bogs of the northern Andes is still lacking as most studies report on local peat bog types only. Despite their presence in the páramos of the Sierra Nevada de Mérida Sphagnum bog communities have not yet been formally reported. Sphagnum bogs in the páramos of Ramal de Guaramacal have shown two new associations belonging to the new alliance Sphagno recurvi - Paepalanthion pilosi. Peatbog communities in Colombia share Carex bonplandii, Sphagnum magellanicum and S. sancto-josephense with the Sphagnum bog communities of Guaramacal. We have not found other shared species as a basis to establish relationships. Isolation, low altitude, and an inadequate number of phytosociological studies account for the observed assemblage of species in Ramal de Guaramacal.
The phytosociological classification of montane forest and páramo vegetation in the Venezuelan Andes has just started with the present study. For a proper management and conservation this mission needs to be continued in order to develop a strong tool for vegetation mapping.
7.5 PHYTOGEOGRAPHY
The phytogeographical composition at genus level of páramo flora of Ramal de Guaramacal shows a relatively high proportion of neotropical-montane elements (ca. 40%) compared to other páramo floras. This characteristic is considered a consequence of the humid climate and the low altitude of the Guaramacal range. The latter could also be the cause of the low proportion of endemic páramo taxa. It is plausible that the wet climatic conditions on the low range of Guaramacal since Holocene time (Van der Hammen 1974; Salgado-Laboriau 1979, 1980) has served as a filter preventing the arrival and survival of dry páramo species originating from the Mérida páramos. The low representation of Andean-alpine (0.9%) and Holarctic (3.7%) genera is in support of this suggestion.
The presence of Isoëtes karstenii in a small glacial lake, a submerged species known from grass páramo up to the highest lakes in the superpáramo in Colombia (Cleef 1981; Salamanca et al. 2003) and Venezuela (Fuchs-Eckert 1982; Small & Hickey 2001) suggests that páramo vegetation with some form of superpáramo, nowadays completely absent, could have occurred during glacial times at lower altitudes along the steep slopes of the uppermost parts of Guaramacal range. Roches moutonnées are commonly present along the ridges and are a testimony the past glaciations on the top of Guaramacal ridge.
Flora, vegetation and ecology in the Venezuelan Andes
The vascular flora of the Páramo de Guaramacal is largely composed of (1) a group of neotropical widespread species (31%), (2) a group of Andean distributed species (49%), a part of this group is confined to the northern Andes and another part is widespread in the Andes from Colombia to Bolivia, and (3) a group of Venezuelan endemics (20%).
The vascular páramo flora of Ramal de Guaramacal shows neither close relationship to the flora of the dry páramos of the extended and high elevated central part of the Sierra Nevada de Mérida nor to some other nearby páramos of Colombian Cordillera Oriental. At the genus level some similarities may be found between the páramo flora of the Podocarpus National Park in southern Ecuador and the flora of the páramo/SARF mosaic of Guaramacal. Both locations do have some genera in common and show a high proportion of neotropical-montane elements. Both páramo areas have a permanent high humidity level, are relatively low in altitude, and have a smooth topography. In southern Ecuador also a gradual transition from SARF into shrub páramo has been observed.
7.6 FOREST FUNCTIONAL DIVERSITY AND ALTITUDE
Increasing deforestation and global warming are potential threats for Andean forests. Losses in forest cover and biotic attrition might be exacerbated by degradation in functional diversity, i.e. the variety of life-history traits presented by an assemblage of organisms (Mayfield et al. 2005; Girao et al. 2007). Considering the role of temperature changes in ecosystem functioning along mountain slopes (Chapin & Körner 1995; Colwell et al 2008; Svenning & Condit 2008) and the importance of analysing changes of functional traits along altitudinal gradient as of potential value for predicting the effects of environmental changes on ecosystem functioning (Díaz & Cabido 1997; Díaz et al. 1999; Duckworth et al. 2000; Lavorel & Garnier 2002; McGill et al. 2006), functional diversity was studied in relation to altitude in undisturbed Andean forests of Ramal de Guaramacal (Guaramacal National Park). The aim was to contribute with reference information for studies of degraded Andean systems.
Information of the vascular plant species composition of forest relevés sampled along the altitudinal gradient (Chapter 2) was linked to different species functional traits related to the energy balance (growth form, leaf shape and leaf size) or fragmentation (dispersal and pollination modes, fruit type, and flower and fruit size). This information was obtained from the literature and herbarium studies. Information of species traits and altitude from plots surveys were summarized by means of ordination analysis to detect the principal variation. Randomizations of the species assemblages in relevés (Legendre et al. 1997; Dray and Legendre 2008) were used to test if the composition and diversity of energy-related traits and fragmentation-related traits changed with elevation.
Results show that functional diversity of fragmentation-related traits decrease with elevation (more underdispersion at higher elevations), and the energy-related traits increase (more overdispersion at higher elevations). Overdispersion occurs when better adapted species outcompete functionally to related species from the local community (Weiher & Keddy 1995; Mayfield et al. 2005). Leaf size contributed
_______________________________________________________
substantially to the altitudinal variation in energy balance related traits. The smaller leaf size at higher altitudes in wet tropical forests has been repeatedly reported (Grubb et al. 1963; Vareschi 1966; Sugden 1985). In the absence of pronounced dryness, as is the case in the situation along the slopes of the Ramal de Guaramacal, smaller leave size can be explained by a lower temperatures upslope, less hydraulic conductance of stems and associated lower mineral supplies, lower nutrient availability, and an increased frost frequency (Cavelier 1996). Overall, nitrogen becomes more limiting at higher altitudes in montane wet forests (Grubb 1977; Cavelier 1996). Therefore, our results suggest that competition for resources, which is mostly related to the capture of radiation (heat) and the uptake of minerals and nutrients, is an important driver of species composition.
The association between traits showing a functional diversity of energy balance and traits related to elevation further suggests that a temperature increase due to Global Change will affect forest functionality of Andean forests. Higher temperatures in the coming decades might reduce functional diversity of energy balance along the slopes of the Andes. This projection should be treated with caution, however, because effects of changing temperatures on species migration and the way ecological filtering has impact on species interactions, is unknown. Results also suggests that even in undisturbed Andean forest functional diversity varies significantly, which implies that in the nearby future a lower degree of functional diversity at a certain elevation does not necessarily endangers ecosystem well-being.
The negative altitudinal association of the main variation in fragmentation trait state composition is considered mostly due to the presence of wind-dispersed, wind pollinated, and fern species from SARF (e.g. species from Alsophila,
Cyathea, Dicksonia, Diplazium, Baccharis, Diplostephium, Pentacalia, Mikania).
Since SARF and páramo vegetation occur in the study area in a spatially well mixed mosaic (Chapters 2 and 3), the predominance of the trait states in the samples at highest elevation related to wind dispersal can be explained by the functional role of wind as an edge effect. There is a tendency that species with larger fruits and flowers occurr at relatively low elevations (e.g. Symbolanthus
vasculosus, Zygia bisingula, Drymonia crassa, Tabebuia guayacan, Macrocarpaea bracteata, Inga edulis), which may be related to a more
pronounced role of mammals (including large bats) at lower elevations versus a more dominant role of birds in pollination and seed dispersal at elevations above 2100 m.
Several SARF plots showed an outlying functional diversity compared to the trends in the forest relevés at lower elevations. This suggests that the plants in the SARF relevés contained markedly more trait states relative to the number of species, and/or more plant individuals relative to the variety in trait states as is the case at lower elevations. Both phenomena may be caused by the increased wind vigor in SARF enhancing the number of plants with traits related to pollination and dispersal by wind.
The diversity of fragmentation related traits tend towards a negative relationship with elevation, visible in both species-based and individual-based indices of the montane forests. Hence, our results indicate that along an undisturbed altitudinal gradient in Andean rain forest ecological differentiation driven by disturbance is Flora, vegetation and ecology in the Venezuelan Andes
decreasing in upslope direction. Possibly, the increased level of underdispersion in upslope direction is due to a higher fragmentation of the forest matrix related to more frequent terrain dissections. More information on forest dynamics and forest disturbance in relation to mass movements and slope instability in the Guaramacal area is needed to better understand mechanisms that explain the upward decrease of functional diversity of disturbance related traits.
According to our expectations we concluded that in the Guaramacal area functional diversity of undisturbed Andean forest is changing with altitude. This implies that in the nearby future a temperature rise might affect the functionality of Andean forest, but not necessarily in a harmful way. Functional diversity related to energy balance traits increases in upslope direction, pointing to increased levels of ecological differentiation. We explained this by assuming more upslope competition in the Andean forests regarding capture of radiation and the uptake of minerals and nutrients. Diversity in fragmentation related traits showed an opposite pattern (more underdispersion upslope), which might relate to discontinuities in the forest matrix due to the geomorphology in the study area. SARF diverged from the observed altitudinal trends in fragmentation related traits, probably as a consequence of edge effects created by wind in the SARF-páramo mosaic.
7.7 CONSERVATION IMPLICATIONS
Tropical montane forests and páramos are fragile ecosystems and hold a high and exceptional biodiversity. In the Venezuelan Andes there is a net of national parks and reserves that have kept UMRF and páramo ecosystems relatively well protected. However, LMRF is most affected by human intervention and has been largely converted into areas with an agricultural land use (Ataroff 2000). Outside protected areas montane ecosystems have been severely affected and fragmented, leaving natural montane forest as remnants only.
Fortunately, Ramal de Guaramacal and its montane ecosystems is among the best conserved national parks in Venezuela. Results of this thesis are important contributions to the knowledge of floristic diversity and botanical composition of our forests and páramos. The information from this mountain range is basic for conservation planning and restoration of these ecosystems. However, more studies on ecosystem dynamics from other mountain systems are needed to develop a proper management planning and conservation on a wider regional scale.
