DESCRIPCIÓN GENERAL DE LA PROPUESTA DIDÁCTICA

The microclimatic fluctuations recorded on the Weinmania sp. generally agree with findings of other studies conducted predominantly in lowland forests (Freiberg, 1997; Szarzynski and Anhuf, 2001). One contrasting report is that of Catchpole (2004) conducted at a nearby site on the Yanachaga-Chemillén Range, which highlights the heterogeneity of the montane forest canopy and the strong influence crown architecture and permeability can have on the microclimate and the epiphyte population it supports.

Opposing gradient of temperature, humidity and radiation intensity existed between the lower trunk and the outer-crown periphery of the Weinmannia host. Mean lowest temperature and highest humidity occurred near the ground. The lowest proportion of visible sky, highest leaf area index and lowest photosynthetically active radiation intensity was also recorded on the lower trunk. The microclimate in the outer crown was most

Chapter 6. Conclusion

varied, while the diurnal range of temperature and relative humidity near the ground was smaller than in any other microhabitat on the Weinmannia.

The diurnal fluctuations of temperature and relative humidity throughout the Weinmannia

were affected by the prevailing macroclimatic conditions. Wet and cloudy weather compressed the otherwise clear gradients between the lower trunk and the crown periphery. Mean hourly values revealed a complex interaction of temperature, relative humidity and radiation, influenced by the structure of the host (Fig. 16). Highest day temperatures were recorded between 1 and 3 pm in the highly irradiated mid-outer crown, just 0.03˚C warmer than the crown periphery. The slightly higher temperature in zone 4 than in zone 5 may have been the result of lower wind speeds and greater surface area and mass of the branches lower in the canopy. The air above the crown at the warmest time of the day was 0.48˚C cooler. Lowest daytime temperature was recorded along the upper trunk. Decoupling of the crown from the atmosphere closer to the ground induced by higher radiation interception and warming of the crown did not appear as pronounced as observed in lowland forest, possibly due to greater mixing of the below canopy atmosphere facilitated by the more open canopy characteristic of montane forests on steep slopes. However, the gradient of relative humidity remained strong and an average of 14% difference separated the lower trunk and the upper crown by the middle of the day. The average temperature difference between the same sections of the host was 1.1˚C at 3 pm. The profile begun to cool after 4 pm. Radiative cooling in the outer to mid crown forced a reversal of the temperature gradient and the coolest night temperature was recorded in the mid-crown at 6 am. The air above the crown had a higher night temperature than any habitat on the Weinmannia. As the sun warmed the crown in the early morning, the temperature and relative humidity gradients re-established the daytime pattern.

A different pattern of diurnal fluctuation was found on a Ficus sp. by Cathchpole (2004). Lowest temperature occurred near the ground and above the crown. During the day, the mid-crown was cooler than the above crown temperature. The contrasting tree

Chapter 6. Conclusion

different crown microclimate to that found in the present study. Difference in canopy structure was suggested as a possible cause of variation in epiphytic bryophyte composition between two types of temperate montane rainforest in Tasmania (Dalton, 1998). The heterogeneous canopy of montane forests appears to provide a rich array of microclimates, reflected by the exceptional diversity of both vascular and non-vascular flora in the tropical Andes.

6.3.2 Environmental parameters

Height above the ground and diameter had the strongest correlations with the first two ordination axes of species and environmental variable data recorded on the Weinmannia

host. Both of these variables are inherent components of all environmental gradients measured. Height above the ground is an essential reference when describing temperature, relative humidity, light interception and radiation intensity throughout a tree host. Diameter was strongly correlated with height above the ground and itself summates a host of gradients like bark age, exposure to colonisation opportunities (Van Leerdam et al., 1990) and perhaps most importantly, the horizontal gradient of temperature and humidity extending from the crown centre to the periphery (Freiberg, 1996), unfortunately not measures in the present study. The strong correlation of height above the ground or the related host Johansson (1974) zone with species data has also been reported by other studies of epiphytic bryophytes (Wolf, 1994; Catchpole, 2004; Kelly et al., 2004). However, like diameter, it does not offer any new information beyond confirming already accepted variation of epiphytic community composition with distance from the ground and canopy centre. It is proposed here that height above the ground and diameter are ‘nuisance’ variables and their continued interpretation as important for the distribution epiphytic flora detracts from more meaningful parameters.

Below crown diffuse radiation and relative humidity were the second most strongly correlated variables to the ordination axes. Light is a basic requirement for autotrophic organisms and its quantity and quality has been identified in ecophysiological studies to influence growth and preferred location of epiphytic bryophytes along the declining gradient from the outer crown to the lower trunk (Hosokawa et al., 1964; Tobiessen et al.,

Chapter 6. Conclusion

1977; Proctor, 1981). Relative humidity may have limited direct impact on the physiology of epiphytic bryophytes (León-Vargas et al., 2006), but it does influence the duration of leaf wetness and consequently, the duration of active photosynthesis by bryophytes between periods of desiccation. Distribution of bryophyte species along moisture gradients has been suggested by other research (Barkman, 1958, Hosokawa et al., 1964; Tobiessen et al., 1977; Proctor, 1981; Sillett et al., 1995; Jarman and Kantvilas, 1995).