There was significantly more epiphytic biomass suspended on trees in LCF than in LRF. We found less epiphytic biomass in the investigated forests, however, than is reported from other tropical lowland forests (Hietz-Seifert et al. 1996, Freiberg & Freiberg 2000). The discrepancy may be explained by the different methodologies used in our study and those of other authors. While our study only focused on holoepiphytes (Schimper 1888), Hietz-Seifert et al. (1996) also included the biomass of hemiepiphytes and climbers, and this probably holds true for the study
of Freiberg and Freiberg (2000) as well. Although the amount of vascular epiphyte biomass analyzed in the present study was limited, we found that abundance of vascular epiphytes in LCF greatly exceeded that in LRF, both in terms of biomass and cover. In the latter habitat, epiphytic angiosperms and pteridophytes were very scarce, while occurring with high frequency in LCF.
As to biomass of epiphytic lichens and bryophytes, the amounts measured in LRF crowns are similar to those reported for lowland Ecua-dor (Freiberg & Freiberg 2000). Trees in LCF, however, hold over 30 per-cent more biomass than those in LRF. The amount of bryophyte biomass on LCF trunks is similar to that found in montane forest above 1000 m in the Andes of NE Peru (Frahm & Gradstein 1991). Comparison of the epiphytic bryophyte cover in LCF with that measured along altitudinal transects in Colombia (van Reenen & Gradstein 1983) and Borneo (Frahm 1990) yields even more striking results and shows a similarity of LCF with moist montane forest at 2000 m. The latter data coincide with those for species richness of liverworts, which in LCF are as high as in Colombian forests at 2000 m (Gradstein 2006, Gradstein et al. 2010).
We propose that the similarities between LCF and moist tropical mountain forests reflect the relatively high air humidity and occurrence of fog in the two forest types, in spite of the obvious differences in air temperature and radiation intensity. The general increase of bryophyte biomass with elevation has been explained by various climatic factors including precipitation, air humidity, frequency of fog, temperature, light intensity, and combinations of these (e.g., Seifriz 1924, Grubb &
Whitmore 1966, Bayton 1969, Richards 1984). Apart from the obvious importance of moisture availability to bryophyte growth (Hosokawa et al.
1964), bryophytes reach their highest rates of net assimilation at temper-atures below 25°C and light intensities between 500 and 900 lx. There-fore, production of biomass is considered to be restrained in lowland for-ests with temperatures above 26°C and light intensities below 500 lx (Frahm 1990). High (day and night) temperatures cause high rates of
Chapter 2 Epiphyte Biomass and Canopy Microclimate
31 dark respiration (Lambers et al. 1998), causing bryophytes in hot condi-tions to lose greater parts of their assimilated carbon. With increasing elevation, bryophyte growth is considered to be favored by lower tempera-tures coupled with higher light intensities and longer periods of high humidity, as seen in tropical montane forests (Richards 1984, Zotz et al.
2003).
Nonvascular epiphytes are known to successfully colonize all height zones of trees in the humid tropics, but in terms of microclimate many bryophytes prefer the more shaded, humid habitats, where VPD is low, while lichens generally thrive on exposed bark, their majority being less tolerant against water over-saturation (Proctor 2000, Sillett & Antoine 2004, Green et al. 2008). The microclimate data gathered in this study demonstrate that RH is higher in LCF than in LRF, particularly at night and early mornings. We attribute the higher humidity in LCF to the prev-alence of radiation fog in this forest type. During fog events, the moist environment should facilitate bryophyte growth in LCF but causes exces-sive water saturation in lichens, inhibiting photosynthesis and thus bio-mass gain (Lange et al. 1993, 2000; Zotz et al. 1998). As the day pro-gresses, RH decreases, VPD increases, and lichens may again take up CO2 and become photosynthetically active. For the majority of bryo-phytes, on the other hand, the ability to engage in photosynthesis is in-hibited during periods of decreased air humidity (Proctor 2000). The oc-currence of fog events in LCF, however, reduces the daily decrease of air humidity and the increase of VPD and, thus, would shorten the period of photosynthetic inactivity of the bryophytes. This, in turn, may explain why biomass of bryophytes in LCF is higher than in LRF. We suggest that the prolonged availability of high air humidity in LCF and the additional input of liquid water through fog, enhance epiphyte growth by shortening the desiccation period and lengthening the period of photosynthetic activ-ity of the plants. The greater amount of nonvascular biomass in LCF, re-sembling that found in montane forests, may be explained by enhanced growth of bryophytes in response to additional water input by fog. The
fog events may result in prolonged periods of photosynthetic activity in these organisms and thus improve conditions for bryophyte growth.
Since lichens are water over-saturated during early morning in both for-est types, the increased humidity observed in LCF would not affect these organisms.
The data on bryophyte and lichen abundance in the two forest types are paralleled by species richness, which is more strongly increased in bryophytes of LCF than in lichens, with exception of cyanolichens (Normann et al. 2010). Future studies may focus on the processes deter-mining the high diversity and biomass of epiphytes that characterizes the tropical LCF.