Environmental determinants of subalpine treelines in the Valles Caldera, New Mexico

Subalpine grasslands are prominent but poorly understood features of the southern Rocky Mountains. Grasslands frequently occur in broad valleys, forming abrupt subalpine treelines where they abut forested mountain slopes. Nearly 10,000 ha of such grasslands occur in the Valles Caldera National Preserve (VCNP) in the Jemez Mountains of northern New Mexico. To assess the causes of this pattern I conducted gradient analyses, observations on adult tree growth and moisture relations, experimental tests of tree seedling establishment, and a reconstruction of historic landscape dynamics using aerial photographs and tree establishment dates from increment cores.; Gradient analysis revealed that the position of subalpine treelines in the VCNP was most strongly associated with spatially coupled shifts in the thermal regime due to cold air drainage and pooling in the valleys (partial Mantel r = 0.76, P < 0.001) and edaphic factors, particularly increased % sand on the forested slopes and increased clay in the valleys (partial Mantel r = 0.12, P < 0.05). Adult ponderosa pine (Pinus ponderosa var. scopulorum) trees growing at the inverted valley bottom treelines did not show decreased growth relative to trees on forested slopes above, as would be expected if these treelines were analogous to alpine treelines. In fact, adult trees exhibited greater rates of radial growth and lower moisture stress at treeline.; Ponderosa seedlings transplanted into 32 sites in valley bottoms and mountain slopes showed very strong decreases in growth at sites experiencing lower minimum temperatures and greater incidence of growing-season frosts. Measurements of chlorophyll fluorescence suggested that cold-induced photoinhibition in tree seedlings was frequent in grassland sites. Transplanted seedling survival was greatest on sandy soils and at warmer sites, though this pattern was substantially weakened by high mortality from golden-mantled ground squirrels ( Spermophilus lateralis) on warmer sites in forest clearings.; Between 1935 and 1996, the area of grasslands in the Valles Caldera declined by 18%. Tree invasion was most pronounced, and more continuous, on high, steep, south-facing slopes where grassland maintenance was most dependent on frequent ground fires. Tree invasion in valley bottoms was relatively less severe, and increased during periods of higher temperatures.