Habitat suitability for terrestrial orchids in a tropical forest: best sites for survival differ from those for reproduction

Background/Question/Methods

Understory herbaceous plants can be an important component of forest diversity yet their patterns of spatial distribution and the factors that influence them are largely unknown, especially in the tropics.  For this study we asked: 1) Can specific trees or shrubs indicate suitable habitat for understory species? 2) Do abiotic factors have a greater influence on species abundance and plant size than historic land use disturbance?  3) Is leaf size a predictor of reproductive effort? We used Prescottia stachyodes, a common terrestrial orchid, to address these questions.  Within the 16 ha Luquillo Forest Dynamics Plot in Puerto Rico we surveyed six 10 m x 500 m transects (1200 subplots) for orchid occurrence, abundance, and measured the largest leaf per individual as an indicator of plant size.  First we performed an Indicator Species Analysis for orchid occurrence, and then examined the relationship between habitat characteristics, orchid abundance and plant size.  Lastly, we assessed the correlation between plant size and reproductive effort. We expected shade-intolerant shrubs (possible indicators of canopy gaps) to be associated with orchid occurrence, and that land use history would be more important than abiotic factors for explaining orchid density and plant size (predicted to be associated with reproduction).

Results/Conclusions

Orchids were found in 90 out of 1200 subplots, with a clumped distribution pattern, suggesting either dispersal limitation or habitat specificity. There were fewer indicator species for the photosynthetic orchid P. stachyodes than an achlorophyllous mycoheterotrophic orchid within the same study area. Woody species with positive associations for both orchids might be associated with mycorrihizal fungi needed for nutrient uptake or germination.  Slope and topographic position were significantly associated with abundance of P. stachyodes, with the highest orchid densities in areas with flat terrain.  Land use history was not associated with orchid abundance, but was associated with plant size.  These results differ from dominant factors shaping the distribution of many tree and shrub species within the same wet subtropical forest type.  More disturbed sites, soil with poor drainage, had orchids with larger leaves.  The variation in orchid size across sites may have been due to differences in understory light availability, or higher moisture within reach of the orchid's shallow roots.  We also found a strong positive correlation between orchid leaf size and reproductive effort.  Sites that are the source of seeds may not be optimal for establishment.  Thus, factors influencing species abundance can differ from conditions that influence plant size.