Changes in the Growth Response of Larch to Climate Variability Along a Latitudinal Gradient in Central Siberia

The boreal forest is expected to be highly sensitive to future climate warming. Patterns of response to past warming however, exhibit a degree of spatial and temporal complexity with responses to temperature varying over time among species and among sites around the boreal forest. Dendrochronological studies have indicated that negative growth responses to warming in which tree growth has slowed as it has warmed over the last several decades have become more prevalent. The causes of negative growth responses however remain uncertain: drought stress, heat stress, global dimming and pollution have all been suggested as causes. In this study, we examined spatial patterns of tree response to warming along a latitudinal gradient in central Siberia. We sampled trees of three species at six sites along the Lena River from Yakutsk to Zhigansk, a span of approximately 5.5° of latitude. Tree cores were measured and crossdated ring-widths from each tree were compared to CRU climate data for each site. We used bootstrapped correlation coefficients to quantify climate response of each tree.

The prevalence of negative responses to temperature declined from south to north. At the southernmost sites near Yakutsk, fewer than 15% of trees responded positively to warming. At the three northernmost sites near Zhigansk in contrast, more than 75% of the trees responded positively to warming. The proportion of positively responding trees within a population was significantly and positively correlated with latitude for larch (rs=0.9, P<0.05, n=5 populations) and when all species were combined (rs=0.898, n=9 populations). Latitude is in turn, significantly correlated with a number of climatic parameters. Summer precipitation is significantly and negatively correlated with latitude (r=-0.926, P<0.05), indicating that positive responses to temperature are more prevalent in areas with drier summers. This argues against drought stress as an explanation for temperature-related growth declines. Spring (r=-0.995, P<0.05), summer (r=-0.994, P<0.05) and fall (r=-0.98, P<0.05) temperatures all decline along the latitudinal gradient; positive responses to temperature thus become increasingly prevalent as warm-season temperatures decline. This observation is consistent with the heat stress hypothesis and suggests that even as far north as Yakutsk temperatures may have exceeded the climatic optima of Siberian tree species in recent years.