Influence of climate on annual changes in Douglas-fir stem taper

Planting sites that are cooler, with more precipitation, early springs, high TD and hot dry summers reduce bole taper in young Douglas-fir trees. Seed-source climates that are drier, with high TD and low snowfall produce seed-sources with lower taper. Analysis of a 10 year reciprocal transplant study was performed to determine the influence of seed-source and local planting site climates on the bole taper of 8995 Douglas-fir (Pseudotsuga menziesii var. menziesii) trees. Trees were planted at 9 sites across Oregon and Washington, with 120 known families taken from 12 seed-source regions across California, Oregon, and Washington. Diameter at breast height (DBH) ranged from 0.1 to 22.9 cm, with tree ages ranging from 2 year-old seedlings to 12 year-old trees. Changes in Gini coefficients (∆G) of diameters along tree boles, as a surrogate for changes in taper, were modeled as a function of age, site climate, and seed-source climate (Wang et al., in PLoS One 11, 2016) using universal response functions (URF) (Wang et al., in Ecol Appl 20:153–163, 2010). Lower Gini coefficients come from less tapered boles, i.e., more cylindrically shaped boles. There was significant influence on taper from five site climate variables: mean annual temperature (MAT), mean annual precipitation (MAP), beginning of frost-free period (bFFP), difference between minimum and maximum monthly temperatures (TD), and summer heat-moisture index (SHM). These results suggest that cooler years with more precipitation, early springs, large TD and hot dry summers reduce tree taper. There was significant influence on taper from three seed-source climate variables: percent precipitation as snow (PAS), Hargrave’s climate moisture deficit (CMD), and TD. These results suggest dry regions with large ranges in monthly temperatures and low snowfall will produce seed-sources with lower Gini coefficients. Projections under future climates using an ensemble model show areas where G estimates are currently highest showed increases in G, while areas with the lowest G estimates decreased. Most of the Douglas-fir region shows declines in Gini coefficients under high emissions scenarios. These predicted changes in taper have direct implications for wood volume and carbon mass estimates of trees under future climates.