The Garden of Eden Experiment
Ponderosa pine is the most widely planted forest tree throughout California and southwest Oregon and is the usual reforestation species of choice following wildfire. The geographic abundance of ponderosa pine and and its capacity to respond quickly to treatment underscores an immense potential for meeting societal needs for fiber. This potential could reduce harvest pressures on natural forests. But managing plantations for sustainably higher yields requires a basic understanding of how it responds to release from competition, improved fertility, and freedom from insect pests. The Garden of Eden experiment was established in 1986 to help us understand the response potential of planted ponderosa pine when competition, fertility, and insect pests are controlled from establishment through crown closure.
California forest industry provided sites for the experiment. Locations represent a gradient in site quality from the poorest to the best within the natural range of this forest type in the Coast Ranges, Klamath Mountains, southern Cascades, and Sierra Nevada. Industry cooperators cleared and prepared each site for planting with genetic families of pine known to perform exceptionally well at each specific seed zone and elevation. Trees were planted at a 2.4-m spacing to 0.04 ha plots. Plots consited of 8 factorial combinations of extreme vegetation control (complete or none), fertilization (N, P, K, Ca, Mg, S, Zn, Cu, B applied at an exponential rate or not applied), and insect control (acephate or dimethoate applied to trees each spring vs no application) were applied both at planting, and through the first 6 years of stand development. Each treatment was replicated 3 times for a total of 24 treatment plots completely randomized per location. Each treatment plot has been maintained and measured regularly at 2-year intervals. Measurements include overstory and understory growth and chemical composition of foliage. Other more fundamental measurements also have been made.
Results show that the potential of ponderosa pine to respond to combination treatments is far greater than previously imagined. On the poorest site qualities, vegetation control spells the difference between successful plantation establishment and failure. On moderate sites and better, volume growth is readily doubled through the first 10 years. This tells us that soil moisture availability dominates all processes on the poorest sites, but works in concert with nutrient availability on better sites. On the best sites, fertilization stimulates plantation as much or greater than vegetation control, alone.
The secret to sustained growth response is the ability to increase leaf mass (or volume or area). In young plantations, this is achieved by improving soil moisture and/or nutrient availability, depending on which is the dominant limiting factor. Increasing crown mass, volume, or area leads to predictable increases in tree growth. The phenomenon is independent of site quality. Findings have been reported in several publications, most notably Powers and Ferrell (1996), Powers and Reynolds (1999, 2000), and Busse et al. (2001).
The installations continue to have high demonstration value to students, scientists, and practitioners throughout the United States and have drawn visits from scientists of Australia, Canada, Germany, and New Zealand. But the Garden of Eden experiment has a finite lifespan because the trees soon will be too large for the plots. But together with other field experiments established by this Unit, they can be the basis for addressing other, more fundamental questions. For example, what is the potential of intensive plantation management for sequestering atmospheric carbon above and below ground? Do treatments alter the proportion of fixed carbon that goes to roots? Does this phenomenon vary by site quality? If silvicultural treatment has fundamentally altered soil fertility, is there a carryover effect into the second rotation? These questions are being addressed under current research.