University of Wyoming Laramie, Wyoming, United States
Abstract: Spatial synchrony, or positive covariation in geographically distinct populations fluctuating through time, is a ubiquitous phenomenon. Strong spatial synchrony is expected to increase species’ extinction risk, as populations are more at risk of simultaneous decline. Spatial synchrony is often driven by spatially correlated environmental patterns (i.e., the Moran effect); therefore changes in climate will likely have cascading effects on species’ synchrony. Theoretical tests show a link between changes in climate regimes and increased synchrony—yet this link has little empirical validation. Further, determining the drivers of synchrony can be challenging due to multiple interacting drivers operating on different timescales. Here we seek to understand; A) how changes in climate empirically alter spatial synchrony and B) if drivers of spatial synchrony vary in their strength through time and across timescales. Using a 119-year time series of whitebark pine (Pinus albicaulis) growth, we applied wavelet phasor mean fields to characterize the timescale dependence of synchrony within 20 plots across the Sierra Nevadas. We then investigated spatial correlations in drivers of synchrony using wavelet coherence techniques. Here we show evidence of increased spatial synchrony in whitebark pine populations over time and that the degree of synchrony varies across timescales. Specifically, synchrony in growth doubled (40% - 80% of possible events were synchronous) on timescales of 5 - 10 years and almost tripled (25% - 85% synchronous events) on timescales of 20 - 30 years after 1950. Our results suggest that precipitation and temperature are both drivers of synchrony and can operate simultaneously at certain timescales. However, these two drivers shift the timescales at which they are most important through time. Before 1940, precipitation was a significant driver at shorter timescales (2-5 years), while temperature was not significantly coherent at any timescale. After 1940, precipitation became the main driver of synchrony for longer timescales of 5-10 years and temperature became a driver at timescales of 10-20 years. In conclusion, our results show evidence of increased synchrony across whitebark pine populations through time and that the role of temperature and precipitation as drivers of synchrony has shifted through time. These patterns indicate that climate change may be altering the relationships between climate and spatial synchrony and suggest that whitebark pine populations may be more vulnerable to climate-related disturbances under future warming.