Tolerance in Western hemlock

Needle anatomy, water response and gas exchange characteristics of juvenile ecophenes or habitat forms of Western hemlock (Tsuga hetero-phylla) from sites of varying degrees of exposure were compared and contrasted. This was done to gain information about the seemingly broad adaptative spectra displayed by ecophenes of this species within its natural habitat. Some differences were found in needle anatomy and chlorophyll content; the ratio of needle area to needle dry weight increased with the amount of shelter and chlorophyll content was less in the exposed ecophene. A scanning electron microscope was used to observe needle surfaces and cross sections were examined through a light microscope. The exposed ecophene had a thicker cuticle, thicker cell walls, more surface wax and a more circular cross section than sheltered ecophene. A series of experiments, both in the field and in the laboratory was run to test the postulate that response to changes in environmental water demand was different between the ecophenes. Results supported the postulate and suggested that the sheltered ecophene was unable to control transpirational loss under high evaporative demand. A relationship was established between carbon uptake and relative turgidity. Laboratory experiments were run with juvenile whole trees to test the postulates that diffusion resistances, carbon uptake, light compensation, and carbon uptake under different degrees of evaporative demand were different for the two ecophenes. Carbon uptake by the sheltered ecophene did not reach light saturation as rapidly as uptake by the exposed ecophene. Increase of carbon uptake by the sheltered ecophene vas probably limited by diffusional processes. Increase in uptake by the exposed ecophene at high light was limited by non-diffusional processes. Although the shade ecophene displayed a higher carbon uptake and light saturation intensity than the sun ecophene on a unit area basis, the two ecophenes were statistically inseparable when uptake was expressed as a percentage of maximum uptake per tree. Lowered relative humidity lowered carbon uptake and raised transpiration rates of both sun and shade ecophenes.