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The pace and trajectory of ecosystem development are governed by the availability and cycling of limiting nutrients, and anthropogenic disturbances such as acid rain and deforestation alter these trajectories by removing substantial quantities of nutrients via titration or harvest. Here, we use six decades of continuous chemical and hydrologic data from three adjacent headwater catchments in the Hubbard Brook Experimental Forest, New Hampshire—one deforested (W5), one CaSiO3-enriched (W1), and one reference (W6)—to quantify long-term nutrient and mineral fluxes. Acid deposition since 1900 drove pronounced depletion and export of base cations, particularly calcium, across all watersheds. Experimental deforestation of W5 intensified loss of biomass and nutrient cations and triggered sustained increases in streamwater pH, Ca2+, and SiO2 exports over nearly four decades, greatly exceeding the effects of direct CaSiO3 enrichment in both duration and magnitude. We detect no long-term changes in water yield or water flow paths in the experimental watersheds, and we attribute this multidecadal increase in weathering rates following deforestation to biological responses to severe nutrient limitation. Our evidence suggests that in the regrowing forest, plants are investing photosynthate into belowground processes that amplify mineral weathering to access phosphorus and micronutrients, consequently elevating the export of less limiting elements present in silicate parent material. Throughout decades of forest regrowth, enhanced biotic weathering has continued to deplete the acid buffering capacity of the terrestrial ecosystem while the export of weathering products has elevated the pH of the receiving stream. 

Increasing salt production and use is shifting the natural balances of salt ions across Earth systems, causing interrelated effects across biophysical systems collectively known as freshwater salinization syndrome. In this Review, we conceptualize the natural salt cycle and synthesize increasing global trends of salt production and riverine salt concentrations and fluxes. The natural salt cycle is primarily driven by relatively slow geologic and hydrologic processes that bring different salts to the surface of the Earth. Anthropogenic activities have accelerated the processes, timescales and magnitudes of salt fluxes and altered their directionality, creating an anthropogenic salt cycle. Global salt production has increased rapidly over the past century for different salts, with approximately 300 Mt of NaCl produced per year. A salt budget for the USA suggests that salt fluxes in rivers can be within similar orders of magnitude as anthropogenic salt fluxes, and there can be substantial accumulation of salt in watersheds. Excess salt propagates along the anthropogenic salt cycle, causing freshwater salinization syndrome to extend beyond freshwater supplies and affect food and energy production, air quality, human health and infrastructure. There is a need to identify environmental limits and thresholds for salt ions and reduce salinization before planetary boundaries are exceeded, causing serious or irreversible damage across Earth systems. 

Abstract Resilience is the ability of ecosystems to maintain function while experiencing perturbation. Globally, forests are experiencing disturbances of unprecedented quantity, type, and magnitude that may diminish resilience. Early warning signals are statistical properties of data whose increase over time may provide insights into decreasing resilience, but there have been few applications to forests. We quantified four early warning signals (standard deviation, lag-1 autocorrelation, skewness, and kurtosis) across detrended time series of multiple ecosystem state variables at the Hubbard Brook Experimental Forest, New Hampshire, USA and analyzed how these signals have changed over time. Variables were collected over periods from 25 to 55 years from both experimentally manipulated and reference areas and were aggregated to annual timesteps for analysis. Long-term (>50 year) increases in early warning signals of stream calcium, a key biogeochemical variable at the site, illustrated declining resilience after decades of acid deposition, but only in watersheds that had previously been harvested. Trends in early warning signals of stream nitrate, a critical nutrient and water pollutant, likewise exhibited symptoms of declining resilience but in all watersheds. Temporal trends in early warning signals of some of groups of trees, insects, and birds also indicated changing resilience, but this pattern differed among, and even within, groups. Overall, ∼60% of early warning signals analyzed indicated decreasing resilience. Most of these signals occurred in skewness and kurtosis, suggesting ‘flickering’ behavior that aligns with emerging evidence of the forest transitioning into an oligotrophic condition. The other ∼40% of early warning signals indicated increasing or unchanging resilience. Interpretation of early warning signals in the context of system specific knowledge is therefore essential. They can be useful indicators for some key ecosystem variables; however, uncertainties in other variables highlight the need for further development of these tools in well-studied, long-term research sites. 

This dataset contains confirmed observations of fish species at the Hubbard Brook Experimental Forest and adjacent Mirror Lake. The original list was published in Holmes, R. T. and G. E. Likens. 1999. Organisms of the Hubbard Brook Valley, New Hampshire. USDA Forest Service, Northeastern Research Station, General Tech. Report NE-257. 32 pp. The list is updated here (January 2021) to include additional species observed since the original publication, update taxonomic classifications, and provide annotations on distribution. 

This dataset contains confirmed observations of reptile species at the Hubbard Brook Experimental Forest and adjacent Mirror Lake. The original list was published in Holmes, R. T. and G. E. Likens. 1999. Organisms of the Hubbard Brook Valley, New Hampshire. USDA Forest Service, Northeastern Research Station, General Tech. Report NE-257. 32 pp. The list is updated here (January 2021) to include additional species observed since the original publication, update taxonomic classifications, and provide annotations on distribution. 

This dataset contains confirmed observations of amphibian species at the Hubbard Brook Experimental Forest and adjacent Mirror Lake. The original list was published in Holmes, R. T. and G. E. Likens. 1999. Organisms of the Hubbard Brook Valley, New Hampshire. USDA Forest Service, Northeastern Research Station, General Tech. Report NE-257. 32 pp. The list is updated here (January 2021) to include additional species observed since the original publication, update taxonomic classifications, and provide annotations on distribution. 

This dataset contains confirmed observations of lepidoptera species at the Hubbard Brook Experimental Forest. The original list was published in Holmes, R. T. and G. E. Likens. 1999. Organisms of the Hubbard Brook Valley, New Hampshire. USDA Forest Service, Northeastern Research Station, General Tech. Report NE-257. 32 pp. The list is updated here to include additional species observed since the original publication, updated taxonomy, and observation notes. 

This dataset contains confirmed observations of mammal species at the Hubbard Brook Experimental Forest and adjacent Mirror Lake. The original list was published in Holmes, R. T. and G. E. Likens. 1999. Organisms of the Hubbard Brook Valley, New Hampshire. USDA Forest Service, Northeastern Research Station, General Tech. Report NE-257. 32 pp. The list is updated here (January 2021) to include additional species observed since the original publication, with annotated comments by R.T. Holmes, H. ter Hofstede (bats) and L. Christenson (from motion-detecting cameras, 2014-2019).