Variation in temperature responses among populations of Betula papyrifera
Silva Fennica 30(23): 145158
How will global warming affect southern populations of boreal trees? In paper birch, Betula papyrifera (Betulaceae), alpine trees with an evolutionary history of relatively cool summers may be more sensitive to climate warming than valley populations. We evaluated this scenario by growing seedlings from different populations in four temperature treatments (mountain field site, valley field site, and two greenhouse rooms). Populations from low elevations germinated earlier and had higher germination success than populations from high elevations (16.8 vs. 22.0 d; 72% vs. 11%). At the valley site, seedlings from native populations grew faster than seedlings from higher elevations (mean ▒ SE = 0.25 ▒ 0.02 vs. 0.09 ▒ 0.04 mm · cm–1 · d–1) while at the mountain site, all seedlings grew at similar rates. Seedlings grown in cooler environments had higher root : shoot ratios, perhaps to compensate for temperature limitations in nutrient uptake by roots. Leaf area varied among populations but was not affected by environmental differences across the field sites. Net photosynthetic rates at valley temperatures were higher for seedlings grown in the valley than for seedlings grown in the mountains or the warm greenhouse (12.0 vs. 10.3 and 5.8 Ámoles · m–2 · s–1), perhaps due to adaptive phenotypic adjustments. Climatic warming could rapidly produce important phenotypic changes in birch trees (e.g., decreased root : shoot ratio, reduced growth in alpine populations). On a longer time scale, warming could also result in genetic changes as natural selection favors valley genotypes in alpine sites where they are presently rare.
Received 15 September 1995 Accepted 12 June 1996