The role of bryophytes in mineral and sugar cycling in the phyllosphere.

    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).


 
 
This figure (below) from Gupta (1977) illustrates solute loss from four bryophyte species after exposure to various moisture regimes and rewetting.

 


 

    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 rainfall events.
 
 
Nutrient retention and solute release (rehydration response only) for canopy epiphytes (mosses and hepatics) from tropical montane rain forest on La Suofriere, Guadeloupe and net throughflow estimates from tropical montane rain forest in Puerto Rico, New Guinea and Venezuela
Element (kg/ha)
  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
Experimental trials              
Intact mats, no prerinse 1.7 1.4 32 80.1 1.7 11.8 na
               
Field trials              
Intact mat, in-situ canopy 6.7 0.2 16.5 28.7 2.1 --- na
Standard error 1.9 0.05 7.7 8.8 0.74    
               
  Other tropical rain forests
Net annual throughflow              
Puerto Rico 25.8 --- 249 79 47.5 --- na
New Guinea 17.2 2.8 --- 81.4 10.6 30 na
Venezuela 7 1.4 --- 70 3 8 na

 
 

This figure  (Coxson 1990) illustrates the same phenomenon after artificial rewetting.

 

 
 


 
The following figure from Coxson et al. (1992) illustrates the amount of standing sugar and polyol capital in bryophyte mats and the amount released to throughflow in the same forest in Guadeloupe.

 
 

 
 
 
 
This figure (below) (Coxson et al. 1992) shows a differential release rate for upper canopy (primarily liverwort dominated) and lower canopy (primarily moss dominated) regimes.

 
 

    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?
 
 

Specifically, how might it affect nitrogen fixation rates of cyanobacteria?

Data on nutrient load in throughfall.