Ecophysiology of Cecropia schreberiana saplings in two wind regimes in an elfin cloud forest: growth, gas exchange, architecture and stem biomechanics.

The hypothesis that the short, stunted development of elfin cloud forest trees is influenced by strong winds was tested by studying growth, gas exchange and biomechanics of potted saplings of Cecropia schreberiana Miq. ex. C. peltata L. (Cecropiaceae) exposed to two natural wind regimes (exposed and protected) at high elevation sites in the Luquillo Experimental Forest, Puerto Rico. The wind-exposure treatment produced several thigmomorphogenetic responses, including reductions in plant stature and crown area, changes in allocation patterns, and increased root to shoot ratio, leaf abrasion and leaf epinasty. Wind-exposure decreased maximum photosynthetic rate and respiration on an area basis, but not on a leaf-mass basis. Wind-exposed plants had lower apparent quantum yields, and higher light compensation points than wind-protected plants. Photosynthetic nitrogen-use efficiency was lower in wind-exposed plants, but such plants had higher leaf nitrogen concentration than wind-protected plants. There were no effects of treatments on stomatal conductance, transpiration rate and water-use efficiency. Stems of wind-exposed plants had lower second moment of area, apparent modulus of elasticity, flexural stiffness and stem density, but higher water content than wind-protected plants. Tissue-density-specific stiffness and the calculated critical height were not affected by the treatments. Wind-exposed plants were biomechanically less predisposed to bending and failing under their own weight than wind-protected plants because their safety factors were smaller, indicating that maintenance of an ontogenetically less developed structure enables plants to cope with wind loading. Windward trees showed a lower scaling component of the allometric relationship between diameter and height than leeward trees.