Factors Influencing Responses of Loblolly Pine Stands to Fertilization

Abstract

Fertilization of pine plantations has increased dramatically in the last decade. Over 600,000 hectares are being fertilized annually to overcome chronic widespread nitrogen (N) and phosphorus (P) limitations. However, responses to fertilization vary widely since specific responses after fertilization for any particular stand will be the result of complex interactions of nutrients and rates applied, stand and site conditions at time of application, years since application, and climatic conditions after application.

Stand, soil and forest floor (FF) responses to fertilizations were assessed at three different sites located in the 'flatwoods' area of southeast Georgia and northeast Florida after five years of repeated fertilizer additions. Significant leaf area index (LAI), stemwood growth and FF responses were found at all three sites. Leaf area index was double for some treated plots as compared to control plots (from 1.4 to 3.0), five year cumulative growth on treated plots almost tripled that of the control plots (32 to 89 cubic meter per hectare). Soil N availability increased dramatically soon after fertilization however, it decreased few months after application. Several nutrients affected growth at these three sites, N and P at all three sites and potassium (K) and manganese (Mn) at the Georgia study sites.

Factors affecting growth efficiency (GE) of loblolly pine plantations across the southeast were examined using 86 studies sites with different climatic, edaphic and stand conditions. Two modeling approaches were developed, one where GE would change with levels of LAI (non-linear using Gompertz model) and a second where GE was independent of the level of LAI (linear model), in both cases significant reduction in RMSE (>200%) was achieved when parameters in the models were allowed to be functions of edaphic, climatic and stand characteristics. In conclusion, GE is a dynamic rather than static stand parameter; it changes with stand age, drainage, soil texture and climate. Current year climatic variables were better predictors of GE than long term climatic averages, indicating that the inter-annual variation on temperature and rainfall exerts great influence on GE.

Further modeling efforts were undertaken to determine factors affecting the variation in growth responses to N+P fertilization in loblolly pine stands. By utilizing a standardization procedure on the original data, 66% of the variation in the standard response was explained by LAI, Foliar N, and growth efficiency. These variables are ecophysiological variables proposed as drivers of stand response to fertilizer application(s). Two other variables, quadratic mean diameter (Dq), and stand age were also significant predictors in the model.