Comparison of growth temperature responses of Pinus ponderosa from northern California and eastern Oregon seed sources

Background/Question/Methods

Plant shoot elongation occurs progressively later in spring and summer as growth site elevation increases. Controlled environment studies on Pinus ponderosa (PP) were conducted to test the hypothesis that temperature response of plant growth rates would reflect growth season climate of source locations. Four source locations were tested. Two were from the John Day region of Malheur National Forest Oregon. One source was from a higher elevation (OHE), 1680 m to 1980 m. The second had a lower elevation (OLE) 1370m to 1680m.  Two sources were from California seed zone 524/5, higher elevation (CHE) 1525m and lower elevation (CLE) 915m.  Five seedlings from each source were tested. Trees were sown in the spring and moved into controlled environment chambers in September. Mean initial source heights (mm) were CLE 257.0, OHE 236.4, CHE 194.4 and OLE 163.2.  The trees were subjected to 20 months of temperature manipulation.

Results/Conclusions

Final mean source heights (mm) were CLE 530.4, OHE 407.0, CHE 483.0 and OLE 391.4.  CLE started and remained the tallest.  Initially, OHE were second tallest but during the study CHE grew faster.  OLE started and remained shortest.  Fastest height growth rates occurred at different temperatures for each source.  OLE had maximum growth rates at lowest temperatures, 20 to 21^oC.  CHE had maximum growth rates at intermediate temperatures, 21-23^oC.  OHE had maximum growth rate at warmest temperatures, 22 to 25^oC. CLE had maximum growth rates over the widest temperature range, 18 to 27^oC. In general low elevation sources grew faster at lower temperatures than corresponding higher elevation sources.  This may be forced by loss of soil moisture earlier in spring at lower elevations causing elongation to occur earlier when ambient temperatures are lower. Higher elevation sources then elongate later at higher temperatures due to later retention of soil moisture permitting faster growth and greater protection from frost damage. CHE growth temperatures indicate a more moderate temperature range when moisture is available. Wider range of maximum height growth rate temperatures in CLE trees is consistent with broad distribution of PP at that source and elevation.