Measures of sustainability in plantation forest ecosystems
Keywords: Earth System Science.

Globally, planted forests account for around 7% of forested land, a proportion that is increasing as people seek to maintain the benefits of forests while expanding other land uses such as food crops. Forest planting includes "afforestation" of previously non-forested ecosystems, as well as industrial plantations—intensive tree farms that provide over 30% of the world's non-fuel wood on less than 2% of its forested area. In sustainable planted forests, tree growth must be maintained over time, and over repeated harvests in the case of production forestry. However, trees take up large quantities of nutrients and other resources from the soil, and depletion of these resources could limit the growth of future forests. My dissertation assesses long-term trends in productivity and soil resources in the most extreme cases of forest management, combining a global analysis of afforestation with regional and plot-scale studies of the world's most productive plantation forest system, eucalyptus in southeastern Brazil. To quantify effects of afforestation on soil resources, I conducted a global meta-analysis of stocks and availability of a crucial plant nutrient, phosphorus, in afforested soils. Across 220 sites from 108 studies, planted forests decreased total phosphorus stocks in the top 20 cm of soil relative to the previous vegetation type, but could increase phosphorus availability on previously degraded land such as abandoned pastures. To assess whether productivity is sustained over repeated harvests in industrial plantations, I analyzed 33 years of remote sensing data across the Brazilian state of Minas Gerais. Using machine-learning methods, I classified thousands of plantation stands—forest management units—on the landscape. For each stand, I extracted a time-series of satellite observations of the vegetation productivity index NIRv. I then developed an algorithm to divide each time series into harvest cycles, or rotations, based on patterns in the NIRv. I used data from plantation companies to statistically relate harvestable wood volume to the length and pre-canopy closure NIRv of each rotation, and analyzed trends in the modeled wood volume. Changing management practices appeared to maintain wood production over three rotations, as increasing annual productivity offset decreasing rotation lengths. However, NIRv also increased in native vegetation, suggesting that both management intensification and environmental factors such as increasing atmospheric CO2 may be contributing to sustained production. Finally, I conducted field sampling and laboratory analysis to address the impacts of established plantations on soil nutrients. I collaborated with researchers at the Federal University of Viçosa, Brazil, who compared soil carbon stocks in eucalyptus plantations and native vegetation in 2004. In 2016, 1-2 rotations later, we resampled the same stands to evaluate changes in soil carbon and nutrient stocks. Although nitrogen removals in harvested wood frequently exceed fertilizer applications, we observed either increases or no significant change in soil stocks of carbon, nitrogen, phosphorus, potassium, and calcium to depths of 20 or 100 cm. Fertilizer additions and recycling of bark and leaf litter appeared to sustain soil nutrient stocks over the span of two harvest cycles. Nutrient stocks also varied substantially in space and time under native vegetation, demonstrating the uncertainty inherent in using unmanaged vegetation as a static indicator of pre-plantation conditions. Overall, my work suggests that nutrient depletion may not be the primary sustainability concern in the extreme planted forests I studied, and that intensively managed plantations can maintain high rates of wood production over successive harvests. My research also demonstrates the importance of comparing alternative land uses over time to assess whether vegetation trends stem from management or environmental changes. As planted forests expand, my work can help to quantify their long-term effects on sustained productivity of soils and forests.