Optimal nitrogen application rates for three intensively-managed hardwood tree species in the southeastern USA

Abstract

a b s t r a c tForest production can be limited by nutrient and water availability, and tree species are expected to respond differently to fertilization and irrigation. Despite these common expectations, multi-species comparisons are rare, especially ones implementing a range of fertilization rates crossed with irrigation. This study compares the response of three forest hardwood species to numerous nitrogen (N) fertilization levels and water availability using a novel non-replicated technique. A range of N levels was included to determine how N affected the growth response curve, and statistical procedures for comparing these non-linear response functions are presented. We used growth and yield data to calculate the Land Expectation Value (LEV) for these intensive management treatments, and to determine the optimal growing conditions (accounting for tree productivity and grower expenses). To accomplish these objectives, we used a series of cottonwood, sycamore, and sweetgum plots that received a range of N fertilization with or without irrigation. Regression is an economical approach to define treatment responses in large-scale experiments, and we recommend >3 treatment levels so the response of any single plot does not disproportionally influence the line. The non-replicated plots showed a strong positive N response below 150 kg N ha À1 yr À1, beyond which little response was observed. However, different amounts of fertilization were required for the greatest biomass accumulation rate in each tree species. Cottonwood and sycamore growth was optimized with less than 150 kg N ha À1 yr À1 while sweetgum growth was optimized with less than 100 kg N ha À1 yr À1. Results from this experiment should be representative of many of the nutrient-poor soils in the Coastal Plain in the southeastern USA. The LEVs were not positive for any treatment  genotype combination tested when using irrigation or liquid fertilizer, but our analysis showed that several non-irrigated treatments in sycamore and sweetgum did result in positive LEVs when fertilized with granular urea.

Growth responses of narrow or broad site adapted tree species to a range of resource availability treatments after a full harvest rotation

“…We hypothesized that increased resource availability would positively influence above and belowground growth, productivity, and biomass accumulation of each species tested (e.g. Cobb et al, 2008;Brinks et al, 2011;Coyle et al, 2013). We also hypothesized that previously reported differences in growth, productivity, and biomass accumulation among species (Coyle and Coleman, 2005;Coyle et al, 2008) would be maintained throughout the rotation.…”

Ecosystem Services of Woody Crop Production Systems

“…Two pure P. deltoides clones ('ST66' of Mississippi origin and 'S7C15' of East Texas origin) were included in this study that measured growth and productivity responses to varying levels of irrigation and fertilization. In a non-replicated experiment designed to find optimal nitrogen fertilization rates [28], the optimal nitrogen fertilization rates were 131 and 71 kg N ha −1 year −1 for irrigated and non-irrigated 'ST66', respectively. Maximum aboveground biomass differed little, however, between irrigated and non-irrigated trees (i.e., range from 3.6 to 4.3 dry Mg ha −1 year −1 ).…”

“…In the southeastern USA, the growth and productivity of sycamore and sweetgum grown with a range of water and nutrient availability were evaluated in two separate experiments on the Upper Coastal Plain of South Carolina. After 8 years, Coyle et al [28] found that the optimal fertilization rates were 147 and 141 kg N ha −1 year −1 for irrigated and nonirrigated sycamore, respectively. The productivity of sycamore receiving irrigation was 6.5 dry Mg aboveground woody biomass per hectare per year, a 20 % increase over the biomass production rate without irrigation.…”

Dryland management regimes alter forest habitats and understory arthropod communities

“…Research using a similar experimental design in temperate forests of North America tends to find that fertilisation has stronger effects on tree growth, whereas the effects of irrigation are weaker and often only manifest themselves when fertilisation is applied (Coyle & Coleman, ; Coyle et al, ). This highlights how dryland forests are likely to respond differently to management regimes than those of temperate regions, which are more likely to be nutrient‐limited than water‐limited (Coyle et al, ). Previously, Eucalyptus species grown in drylands showed positive responses to increased water availability (Tomé et al, ; Reed & Tomé, ), whereas those grown in humid conditions frequently do not (Misra et al, ; Adams et al, ).…”