Environmental gradients, mountain forest types and characteristic species

Elevation together with annual mean temperature, annual mean precipitation and soil pH were determined as the principal environmental factors explaining the occurrence of the different mountain forest types in Armando Bermúdez National Park. All factors correlated significant-ly with the scores of the first axis of ordination: Altitude and annual mean precipitation showed high positive correlations and annual mean temperature high negative ones. The first axis was called the altitudinal gradient. Considering the high correlations between altitude and annual mean temperature and altitude and annual mean precipitation, the floristic changes along the altitudinal gradient can be explained by a varying temperature and precipitation re-gime. Altitude increased by a factor of 4.45 over the sampled plots, annual precipitation by a factor of 1.7 and annual mean temperature decreased by a factor of 3.2. The identification of the altitudinal gradient together with temperature and precipitation as proxies to explain dif-ferences in woody species composition in tropical mountain forests was consistent with the results of previous studies realized in tropical mountain forests (GENTRY 1988; HAGER &Z A-NONI 1993; LIEBERMAN et al. 1996; SCHRUMPF et al. 2001; SHERMAN et al. 2005; HOMEIER

2008; HÄGER 2010). Altitudinal variation of tree species composition and vegetation structure is generally explained by nutrient limitations (TANNER et al. 1998; HOMEIER 2008; WILCKE et al. 2008), heterogeneous topographical effects (HOMEIER 2008), decreasing temperatures, reduced incoming radiation due to elevated cloud and fog cover (GRUBB 1977; HAGER &Z A-NONI 1993; GEROLD 2008) and an increase of humidity (BRUIJNZEEL 2001). Here neither the topographical factors slope and aspect, nor incoming solar radiation or soil moisture had ex-planatory potential for the determination of the floristic compositions of the sampled plots in the ordination diagram. It is assumed that these data were not differentiated enough to deliver explanatory results. Additional data on air humidity and soil nutrients and higher resolution data on air temperature, precipitation and soil humidity could contribute to improve the ex-planation of the vegetation patterns along the altitudinal gradient and the spatial predic-tions (KAPPAS 1999; WANGDA &OHSAWA 2010). Collection or generation of these data at the (high) spatial resolution needed is not trivial.

The vertical precipitation profile in the Cordillera Central is influenced by the north eastern trade winds and the trade wind inversion (TWI). Precipitation maxima occur below the base

of the TWI which is considered to be at approximately 2,150 m a.s.l. (map for the Caribbean in SCHUBERT et al. 1995 according to tabular data of GUTNICK 1958). Above the TWI rainfall and humidity decrease (SCHUBERT et al. 1995). However, the Worldclim annual precipitation data model steadily increased with altitude (data not shown). Thus, the base of the TWI was not adequately represented in this model. Although there are no climate stations inside of Ar-mando Bermúdez National Park, MAY (1997a) assumed that the annual mean precipitation must be between 1,500-1,800 mm around 3,087 m a.s.l. and between 2,100-2,400 m a.s.l at the base of the TWI. The Worldclim data model contained precipitation values of 2,150 mm at 3,000 m a.s.l. and 1,700 mm at the base of the TWI.

Soil pH-values correlated negatively with the first axis of ordination and with elevation (Pear-son, 0.460, p<0.001). Up to 2,000 m a.s.l. pH-values decreased significantly (Pear(Pear-son, r=-0.533, p<0.001). Our findings were consistent with the results of MAY (2007) in Armando Bermúdez National Park. In upper montane forests in the tropics soils are typically wet and close to saturation so that decomposition of organic matter is impaired and acidity increases (BRUIJNZEEL & PROCTOR 1995). WILCKE et al. (2008) found correlations between altitude, duration of water logging and acidity in montane forests in southern Ecuador.

Above 2,000 m a.s.l. the pH-values measured on the plots did not fallow a clear spatial pattern (Pearson, r=0.376, p<0.001) and ranged between 4.50 and 6.10. The changing precipitation pattern above the base of the TWI with drier conditions would suggest less acid soils and in-creasing pH-values. MAY hypothesized that fire could also be a reason for increased soil pH-values of the upper soil horizons at high elevations (pers. communication MAY 2011). The causes why partially also low pH-values were measured on the plots above the base of the TWI could not be assessed.

High correlations with the first ordination axis and with altitude were calculated for the NDVI. The index reflected the variation of photosynthetic activity and biomass production in the different forest types. The highest vegetation belt in the Cordillera Central is composed of subalpine pine forests. Here the lowest NDVI values were obtained. NDVI decreased with altitude by a factor of 3.28.

Mountain forest types and characteristic species

The four mountain forest types discriminated with the help of the ordination diagram are hu-mid broadleaf/gallery forest, cloud forest, mixed forest and pine forest. SHERMAN et al. (2005) identified in the eastern part of the Cordillera Central secondary riparian, broadleaf, mixed, cloud and four different subtypes of pine forests using ordination techniques. Their main

ex-planatory gradients (elevation, steepness, terrain shape index and pH-mineral) resulted from plot based observations and measurements.

Here gallery forests could not be distinguished floristically from the humid broadleaf forests.

Consequently they were treated as one forest type. PEGUERO et al. (2007) stated that the floris-tic composition of gallery forests either resembles the florisfloris-tic composition of humid broad-leaf or cloud forests, depending on the altitudinal position in the water basin.

Two principal transitions in floristic composition were observed in this study along the altitu-dinal gradient: a.) the transitions between humid broadleaf/gallery forests and cloud forests, b.) the transition between cloud forests and pine forests.

The transition between humid broadleaf/gallery forest and cloud forests occurred between 1,150-1,500 m a.s.l. or 16-19˚C annual mean temperature (Table IV.1). Patches of cloud fo-rests that occurred under suitable microclimatic conditions (e.g. in creeks and on wind ex-posed slopes) were observed within areas of general humid broadleaf cover. GRUBB &W HIT-MORE (1966) stated that the transition between lower to upper montane forests in Ecuador took place where cloud condensation became more persistent. We could not confirm this with hard data as there was no information on cloud formation frequency or on heights of cloud bases.

Species-rich families observed in the humid broadleaf/gallery forests were Myrtaceae and Lauraceae. Lauraceae was also listed by RICHTER (2008) as an important family in lower montane forests. Ocotea, Miconia and Ilex were the most species-rich genera in the here stu-died plot data of the mountain forests and also in the mountain forests of Costa Rica, Nicara-gua and Mexico (GENTRY 1995).

Characteristic humid broadleaf/gallery forest species such as Alchornea latifolia have hygro-morphic leaves appropriate to adapt to humid and shady environments with a high supply of water. Hygromorphic leaves are soft, have thin cuticles and epidermis (ELLENBERG &

LEUSCHNER 2010). Stomata are inverted out of the epidermis to facilitate the transpiration and absorption of new minerals (ELLENBERG & LEUSCHNER 2010).

The second transition zone was the one between cloud and pine forests, located around 1,800-1,900 m a.s.l. (Table IV.1). This zone is not a gradual one like the first one described above, it is abrupt. Low annual mean temperature is one explanatory factor for the change of vegetation types at these altitudes (Figure IV.4) (BRUIJNZEEL 2001; JARVIS & MULLIGAN 2010). The annual mean temperature isotherm was 14.2˚C at 1,900 m a.s.l. in our data (Table IV.1).

HAGER &ZANONI (1993) estimated that the annual mean temperature above 2,000 m a.s.l had to be lower than 12˚C. According to their opinion precipitation and humidity did not have a

large effect on the occurrence of Pinus occidentalis (HAGER & ZANONI 1993). Pinus occiden-talis occurred in the study area over a wide precipitation range on sites that received between 1,230-2,160 mm of annual mean precipitation.

However, for the occurrence of the cloud forests, annual mean precipitation, soil and air hu-midity and air temperature are crucial (STADTMÜLLER 1987; HAGER &ZANONI 1993; S HER-MAN et al. 2005; SCATENA et al. 2010). Decreasing precipitation above the TWI and a higher variability in humidity (SHERMAN et al. 2005; PEGUERO et al. 2007; HAGER &ZANONI 1993) are supposed to affect the distribution of the cloud forests besides the decreasing annual mean temperatures. Due to the low resolution Worldclim precipitation data the complex relationship between cloud forest distribution and change of precipitation patterns above the TWI could not be assessed. Therefore a high resolution precipitation climatic model based on local sta-tion data would have been needed. Another important factor for determining the distribusta-tion of cloud forests is the frequency of cloud and fog formation and the height of the cloud base (BRUIJNZEEL &PROCTOR 1995; LAWTON et al. 2010). None of these data were available and it would have been very difficult to gather these data by field work or from remote sensing data.

Cloud forests were richer in ferns and epiphytes of the families Orchidaceae and Bromelia-ceae than the humid broadleaf/gallery forests. Together with trees, epiphytes are the most fre-quent life form in tropical montane forests (RICHTER 2008). In the cloud forests of Armando Bermúdez the principal plant families recorded were Cyrillaceae, Clusiaceae, Melastomata-ceae, AraliaMelastomata-ceae, AquifoliaMelastomata-ceae, Euphorbiaceae and Myrsinaceae. Tree species with small leaves in the cloud forest were Cyrilla racemiflora, Weinmannia pinnata, Ilex microwrigh-tioides, Podocarpus aristulatus and Ditta maestrensis. Leaf area and leaf sizes decrease with increasing altitude to minimize extreme transpiration rates and radiation maxima (R ICH-TER 2008).

Pinus occidentalis was the only tree species recorded above 2,300 m a.s.l. during field work.

No other woody species follows Pinus occidentalis in the natural succession above 2,200-2,300 m a.s.l. (ZANONI 1993). Saplings of Pinus occidentalis were commonly observed at these elevations during field work. Their heights depended on the years that had passed since the last fire event (pers. communication RADHAMES 2011).

Mature pine trees are protected from fire by the hard and resistant bark (DARROW &Z ANO-NI 1993). Around 2,200 m a.s.l. Brunellia comocladifolia was the first tree species observed during field work associated with Pinus occidentalis on sites with a long fire absence (pers.

communication RADHAMES 2011). Fires are caused naturally by lightning or have their origin in uncontrollable man-made fires in the valleys (HORN et al. 2001). Brunellia comocladifolia

is a short lived post-fire successor in cloud and pine forests that declines after 30-40 years (MAY 1997b).

Above 2,500 m a.s.l just small growing (1-2 m) woody species were observed and Ericaceae and Asteraceae gained importance. These families were also described by RICHTER (2008) for high elevation tropical mountain forests. The main woody species observed besides Pinus occidentalis above 2,300 m a.s.l. were: Myrsine coriacea, Baccharis myrsinites, Eupatorium illitum, Lyonia truncata var. montecristina, Lyionia stahlii var. costata, Gaultheria domingen-sis, Rondeletia ochracea, Ilex microwrightioides, Ilex tuerckheimii, Garrya fadyenii, Wein-mannia pinnata, Myrica picardae, the bunch grass Danthonia domingensis and the fern Glei-chenia bifida. Danthonia domingensis and Gleichenia bifida are fire resistant by sprouting ability (pers. communication MAY 2011). Myrsine coriacea, Myrica picardae, Garrya fadye-nii and Baccharis myrsinites are considered to be common postfire colonists in these altitudes (MAY 1997b;MAY 2000a; HORN et al. 2001). According to HORN et al. (2001) the pine and shrub dominated vegetation of the higher elevations of the Cordillera Central are well adapted to fire and the limit between the cloud forests and the monotypic pine forest is a result of fire pattern (MARTIN et al. 2007).