Southern Appalachian forest growth responds most strongly to precipitation variability

Background/Question/Methods

Climate change poses challenges for forest management and conservation to determine how to maintain forests under novel climates. Temperature increases and precipitation changes, in particular, are likely to become a major factor limiting tree species growth and distribution. Trees growing under the same climatic conditions may differ according to local site-related variations in soil nutrients and moisture. We evaluated the climate driven patterns of growth for dominant hardwood species in the southern Appalachians. We hypothesized that there would be species-specific differences in growth responses to climate variability, and that these responses would depend on whether the species were growing in mesic or dry topographic positions within the same climatic regime. We extracted increment growth cores from Acer, Betula, Liriodendron, and Quercus trees across the elevation gradient in the Coweeta Basin, in western North Carolina. We used standard dendrochronological methods to cross-date and measure tree-ring widths, and to calculate annual basal area increment (BAI). We examined the relationships between on-site, long-term climatic data and standardized tree-ring chronologies from 1935 to 2003. Climate variables included precipitation, minimum and maximum daily air temperature, solar radiation, Palmer Drought Severity Index (PDSI), streamflow, dry spell length, and number of storms. 

Results/Conclusions

Species differed in BAI growth over time depending on their physiology and how they responded to topographic moisture conditions.  Quercus species growing on dry, upslope sites had higher BAI than corresponding species growing on mesic sites; whereas, Acer, Liriodendron and Betula had lower BAI on dry sites than mesic sites. All species, except Betula, were positively related (p<0.05) to winter minimum temperature. Cool, wet springs were negatively correlated (p<0.05) with Quercus growth, due in part to the decreased available energy during cool, wet springs. The single best predictor of growth across all species was the number of storms in the growing season (p<0.05), not PDSI or annual precipitation. In our long-term record, the number of growing season storms has been significantly decreasing over time. Precipitation distribution, rather than the total amount, will likely regulate future southern Appalachian forest productivity.