Long-term Dynamics of Forest and Aquatic Net Primary Productivity Inferred from Tree-Ring and Limnological Records in a Sub-Alpine Lake

Terrestrial and aquatic ecosystems are connected through the exchange of nutrients, energy, and organisms. Investigating the spatio-temporal synchronicity (i.e., coupling and decoupling) of Net Primary Productivity (NPP) across these ecosystems is essential for understanding their responses to current and future environmental changes. While tree rings provide a robust proxy for reconstructing terrestrial NPP (TNPP) and its historical fluctuations under varying climatic and environmental conditions, a comparable approach for freshwater ecosystems is hindered by the lack of long-term records of aquatic NPP (ANPP). In this study, we compared annually resolved time series of TNPP, derived from ring-width chronologies of white fir (Abies concolor) and lodgepole pine (Pinus contorta) in the Castle Lake basin (USA), with ANPP records from 1961 to 2020 collected by the long-term ecological research program at the lake. Our analysis focused on identifying patterns of synchronicity between TNPP and ANPP and their climatic drivers across high- and low-frequency domains. Our results revealed a one-year lagged negative effect of TNPP on ANPP, potentially linked to nutrient uptake by vegetation, and a delayed influence of ANPP on TNPP, with a lag of 5–10 years. In the low-frequency domain, we identified a pronounced episode of decoupling (1961–1988), followed by a phase of coupling (1989–2012). These dynamics appear to be driven by contrasting climatic sensitivities: TNPP was negatively influenced by June–July temperatures and drought stress throughout the growing season, whereas ANPP was positively associated with April temperatures and constrained by winter precipitation. This study highlights the value of long-term monitoring in disentangling the complex interactions between terrestrial and aquatic ecosystems. Our research suggests that the response of aquatic and terrestrial ecosystems to climate change might be characterized by complex patterns of synchronicity, highlighting the importance of cross-disciplinary research. Measurements that connect fundamental processes across the terrestrial to aquatic ecosystems are needed to understand the connections between lake, watershed, and climate, particularly given the certain future of warming in the region.