Date of Award

August 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Charley T. Driscoll

Keywords

biogeochemistry, climate change, harvesting, Hubbard Brook Experimental Forest, managed forests, PnET-BGC

Subject Categories

Engineering

Abstract

Global and regional environmental disturbances, including harvesting and climate change, can lead to integrated and interactive effects on forest ecosystems, altering their structure and function, and therefore long-term sustainability. Understanding both short- and long-term impacts of harvesting practices (e.g., cutting rotation length, intensity) on forest dynamics is a key factor in developing criteria and guidelines for sustainable forest management practices. Process ecosystem models are useful tools to improve predictive understanding of complex, interacting ecological process and their response to disturbance. Few studies have rigorously tested model simulations against field measurements which would provide more confidence in efforts to quantify logging impacts over the long-term. The biogeochemical model, PnET-BGC has been used to simulate forest biomass, and soil and stream chemistry at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. Previous versions of PnET-BGC could accurately simulate the longer-term biogeochemical response to harvesting, but were unable to reproduce the marked changes in stream NO3- immediately after clear-cutting which is an important impact of this disturbance regime. Moreover, the dynamics of nutrients to and from major pools, including mineralization and plant uptake, were poorly predicted.

The overall goal of this dissertation was to develop a simulation tool to evaluate short and long-term effects of harvesting on the hydrology and biogeochemistry of the northern forest. In the first phase of dissertation, PnET-BGC was modified and tested using field observations from an experimentally whole-tree harvested northern hardwood watershed (W5) at HBEF. In the second phase of dissertation, the parametrized/modified model was applied to other experimentally cut watersheds at the HBEF; including a devegetation experiment (W2; devegetation and herbicide treatment) and a commercial strip-cut (W4) to confirm the ability of the model to depict ecosystem response to a range of harvesting regimes. In the third phase of dissertation, the confirmed model was used as a heuristic tool to investigate long-term changes in aboveground biomass accumulation and nutrient dynamics under three different harvesting intensities (40%, 60%, 80% watershed cutting) for three rotation lengths (30, 60, 90 years) under both constant (current climate) and changing (MIROC5-RCP4.5) future climate through the year 2200.

In this dissertation, the model was modified and parametrized allowing for a lower decomposition rate during the earlier years after the clear-cut and increased NH4+ plant uptake with the regrowth of new vegetation to adequately reproduce hydrology, aboveground forest biomass, and soil solution and stream water chemistry in response to a whole-tree harvest of a northern hardwood forest watershed (W5) at the HBEF. Revisions of algorithms of PnET-BGC significantly improved model performance in predicting short- and long-term dynamics of major elements for evaluating effects of various forest cutting strategies at the HBEF. The comparison among cut watersheds showed that around 15 years after the cuts, W5 biomass accumulated at a faster rate than W4 and W2. Despite some initial differences in species composition and biomass accumulation rates among the cut watersheds, simulations of total biomass for all three treated watersheds (W2, W4 and W5) are consistent with the expected growth trajectory of a second- growth watershed (W6) at the HBEF. These results suggest that though the different harvesting practices influence initial forest composition and growth, the overall impact on total aboveground biomass is minimal over the long-term at the HBEF.

The modified two-soil-layer PnET-BGC was capable of capturing the immediate increase in stream concentrations of NO3-, Ca2+, Mg2+ and Na+ as well as enhanced adsorption of SO42- following the treatments and indicated a greater response for the devegetated W2 and the whole-tree harvested W5 than the strip-cut W4. Modeled soil solution Bs horizon and stream water chemistry successfully captured the rapid recovery of leaching nutrients to pre-cut levels after the treatments. Accurate simulation of vegetation regrowth allowed for improved prediction of the chemical response of soil and streamwater to cutting disturbance, indicating the important role of plant uptake in regulating the recovery of the forest ecosystem. Simulations for W2 showed more intense NO3- leaching associated with the herbicide treatment resulting in an accelerated decline in soil base saturation, to values lower than those anticipated from the effects of acid atmospheric deposition alone, and a slower recovery pattern during forest regrowth by the end of the simulation period (2100). A first-order sensitivity analyis showed that simulations by the model to a given level of perturbation of input parameters are more sensitive under mature forest (pre-cut) conditions than for an aggrading forest (post-cut conditions).

Simulations of the interactions between forest harvest practices and future climate change for W5 demonstrated the greater sensitivity of forest ecosystem nutrient pools to logging strategies under climate change which included fertilization effects of atmospheric carbon dioxide, relative to constant climate conditions. These effects are accentuated with a shortening of the length of cutting interval and increasing forest harvesting intensity. Simulations of both constant and varying climate conditions considered showed greater sensitivity to varying the length of cutting period than altering cutting intensities. My simulations suggest that tree harvesting under constant current climate should affect living tree biomass and woody debris more than soil carbon, while under climate change, loss of soil organic matter pools may adversely affect site fertility. Depletion of soil base cations is accelerated under climate change due to increases in soil mineralization, coupled with increased plant uptake and enhanced biomass accumulation. Nitrogen is predicted to be the element which experiences the greatest relative loss over both short- and long- periods under different harvesting strategies, particularly with changing climate. Simulations show that all management options under climate change enhance both timber production and overall carbon storage in comparison to stationary climate, but with greater potential for a reduction in long-term soil fertility.

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