The largest portion of tropical forest biomass is contained within the woody plants. However, as shown in the previous section on epiphyte diversity, epiphytic plants (including bryophytes) make up a large portion of rain forest canopies. In one Costa Rican rain forest, epiphyte biomass was equivalent to approximately 30 percent of the total canopy foliar biomass (Nadkarni 1984). The mineral capital contained within this biomass is roughly equivalent to 45 percent of that contained within the foliar biomass (Nadkarni 1984). The mobility of the nutrients contained within the epiphytic plants may be important in tropical forests, where much of the nutrient capital is locked within the woody biomass of trees.
Bryophytes have been shown to release minerals and
nutrients upon re-wetting after periods of desiccation. This is an
effect of increased solute accumulation in intracellular pools (Gupta 1977).
Because bryophytes lack roots and possess no anatomical features to regulate water loss (such as cuticle, stomata), these plants are entirely dependent upon direct precipitation for maintenance of water balance. According to Coxson et al., the accumulation of solutes within the cells is thought to be important for maintaining metabolism and in the stabilization of membrane function during periods of desiccation. Many bryophytes commit a large proportion of recent photosynthate to storage rather than immediate growth and reproduction; these stored reserves are then accessible during periods of low water availability. Upon a precipitation event, these sugars and minerals are susceptible to leaching as the bryophyte body is rehydrated (1992).
It has been shown that bryophytes can reabsorb these
lost solutes given a long enough period of hydration (Gupta 1977), however,
for canopy epiphytes rainfall is likely to wash the released solutes to
other parts of the canopy (Coxson 1990). This table (below) adapted
from Coxson (1990) illustrates the amount of standing nutrient capital
in bryophyte mats and the amount of nutrients (Ca, P, Na, K, Mg, N) released
to throughflow in a tropical montane forest on Guadeloupe. Field
trial measurements were done primarily in the upper canopy with natural
|Ca||P||Na||K||Mg||N||Foliage biomass (kg/ha)|
|Standing nutrient capital|
|Lower canopy bryophytes||46.7||1.7||1.7||40.9||22.5||---||10126|
|Upper canopy bryophytes||13||0.5||0.4||11.3||6.3||---||2210|
|Net annual pulse release on rehydration of upper canopy bryophytes|
|Intact mats, no prerinse||1.7||1.4||32||80.1||1.7||11.8||na|
|Intact mat, in-situ canopy||6.7||0.2||16.5||28.7||2.1||---||na|
|Other tropical rain forests|
|Net annual throughflow|
It seems clear that epiphytic bryophytes play an
important role in nutrient cycling in tropical forests. How might
the phenomenon of pulse release of nutrients and sugars from epiphytic
bryophytes be important to the addition of new nutrients to tropical ecosystems?