Climate Linked Range Shift in a California Endemic Conifer
Authors:Daniel Mendes, Peter Pellitier
Mentor:Frances Hanzawa, Professor, Pomona College
In response to continuing climatic warming, plant species worldwide have shifted in geographic distribution to be in suitable growing conditions. In tree populations, lower soil moisture that results from warmer, drier conditions can affect recruitment and stand structure in two ways: by reducing the survival of seedlings and saplings and by increasing fire frequency. We hypothesized that climate warming and fires are affecting the current distribution and recruitment of the montane tree Pseudotsuga macrocarpa. We predicted that populations at the lower elevations would show reduced recruitment relative to populations at higher elevations where temperatures are cooler. P. macrocarpa is a large, long-lived conifer endemic to the Southern California mountains. P. macrocarpa is found between 550m and 2500m, and is the dominant conifer species through much of this elevation range. We examined 11 stands from 595-2000m on Mt. Baldy. In each stand we recorded the number of seedlings, saplings, and adults. For adult trees the diameter at breast height (DBH) and incidence of fire marks were recorded. Soil moisture, soil temperature, and ambient temperature were measured at each stand. Hottest and driest soils were found at lowest elevations 595m-965m. These stands had the lowest proportion of juveniles (seedlings and saplings), 13.6%. Low elevation stands had significantly more fire marked trees, 67%. At mid elevations (1190m-1760m), soil and air temperatures were cooler. Stands at mid elevations had a significantly lower proportion of fire-marked trees than the low elevation stands; recruitment was highest, 28% juvenile. At the highest elevations >1950m, soil and air temperature were the coldest observed, yet we had data from only one stand and could not draw conclusions about rate of recruitment. These findings suggest that in the future P. macrocarpa may undergo a range contraction on Mt. Baldy with low elevation populations going locally extinct.