TREES IN THE INTENSIVE CARE WARD: UPDATE ON GLOBAL CHANGE RESEARCH
We visited and reported in detail on the Global Change Research Project
in Scotland County, North Carolina, in our April 1993 issue, before irrigation
began, and since the trees being studied were obviously unable to capture
the site's precipitation before it trickled down into the deep sand, we
couldn't wait to return with Dr. Phillip M. Dougherty and see how well they
did on irrigated plots. Changes in appearance were remarkable as you shall
see below. But equally remarkable was the increase in the number of measuring
devices; they are now even measuring how the earth and the trees breathe.
The white instruments behind Phil in the picture are sealed to the tree
and to the earth to capture CO2 being released in respiration. Here's what
we learned in this visit:
| JMV: | Please go over again when you started these treatments. |
| PMD: | We selected the site at the end of 1991. We laid out the treatments and started fertilization in early 1992. In 1993 we began branch CO2 exposure treatments in March, irrigation treatments and measures of root production and physiological responses to the CO2 treatments in April, and measures of woody tissue and "soil" respiration in May. |
| JMV: | Has there been any response to fertilization alone? |
| PMD: | Yes. In fact, adding fertilizer has had the biggest impact of all the treatments. We can definitely say that, on this site, nutrients are more limiting than anything else. The main missing nutrient is nitrogen. We've seen little growth response from irrigation alone. It's only when we take care of the major limiting factor, nitrogen, that we get a big response from irrigation. |
| JMV: | Did that surprise you? |
| PMD: | A little bit, but not so much on this site because it was so nutrient poor. Our pre-treatment nutrient analysis showed that the level ofnitrogen in the foliage was 0.9% on an oven-dry weight basis. That's close to the low end of what you would ever expect to find in loblolly pine. A few other nutrients were also close to their established critical levels. Nutrients were so low that there was no hope of increasing productivity by other means until you took care of the nutrient deficiency. |
| JMV: | So a landowner with some of these sandhills shouldn't try any treatment until he runs a nutrient analysis of the foliage? |
| PMD: | Right. In fact, I would do that, not just on the sandhills where you know that the soil has low organic matter and low nutrients. It is essential, on most upland sites, to make a foliar analysis during the dormant season. In any soil series that you might deal with, even one that the Soil Survey says has a high percentage of organic matter and you expect productivity to be high, nutrition may be below its optimum. Past land use has a tremendous impact on the nutrient status of the soil and the organic-matter reserve available to be turned over to supply nitrogen and other needed nutrients. Within any soil series you can have a wide range of nutrient availability for trees. |
| JMV: | Now that you are adding water and nutrients, what has been the response to elevated CO2? |
| PMD: | What we're doing is exposing branches and branch foliage to elevated CO2; then we assess the impact upon physiological processes. Elevated Co2 dramatically increases the photosynthate production. So we conclude that, under the range of resource conditions (water, nutrients) that we have established at this site, loblolly pines will increase their production of carbohydrates, i.e., the food supply that they will have available to grow, to protect themselves, and to use for other purposes. Increased atomsperic Co2 has the potential to increase productivity of loblolly pine stands in the southern U.S. The extent this will occur will depend on how much temperature, rainfall, and nutrient availability regimes change in the future. |
| JMV: | This site originally had an exhibited site index (age 25) of 45, but you and Lee predicted that you could raise it to 90 with fertilizer and irrigation. Do you want to change your prediction? |
| PMD: | No. I believe we are on course. I haven't gone back and looked at the second-year data on height and tried to relate that to site index. But just knowing that we have better than doubled the leaf-area index, which is often very closely related to productivity potential, I think that we are pretty close to doubling productivity on the site right now. In fact I still regard the trees as adjusting to the treatments that we've applied. It has taken more than two years for them to begin growing at the rate that I think they will grow at if we continue the treatments. The following chart from our recent report shows comparative changes in annual volume increment due to the main effect treatments: |
| JMV: | Look, Here's a loblolly pine with four needles in a fascicle. Isn't this an unusual way to increase leaf-area index? |
| PMD: | Yes. There does appear to be a higher frequency of fascicles with four needles in the fertilized treatments. |
| JMV: | Has there been any increase in understory vegetation? |
| PMD: | No, but the reason is that we cut down or kill all such vegetation. Allowing it to develop would introduce another unknown into our work. |
| JMV: | You are now adding to the organic matter and nutrients of this site. Won't this increase its productivity? |
| PMD: | We will surely increase material on the forest floor. I believe that we will do likewise in the mineral part of the soil due to production and turnover of more roots, but there's not enough information to know yet. This soil is very sandy, and there's always plenty of oxygen between the grains of sand to convert the organic matter in the soil to CO2, which will then escape as a gas. On a site that is less well aerated, I'm sure that the increased forest production due to our treatments would, over time, increase soil organic matter. On this site, we won't know until we get more data, but we would expect increases in soil organic matter to be a slow process.
However, Jim, your question is directed at sustainability of productivity, and trying to answer it is one of the reasons for the intensity of our research. On this site, and throughout the south, I do think that there is some upgrading of sites due to converting of a site from agriculture to forest. All over the world and certainly over much of the Piedmont, we use forests to upgrade land that has been depleted. We are hoping that the forest will build up the forest floor and the production of fine roots will build up the soil organic matter and improve the structure of the soil. I think it will. On the other hand, frequent harvesting of the crop may reduce some critical element and thus limit forest productivity in the future. We may have to add some nutrient to maintain productivity, especially if we remove a large volume of biomass on a frequent basis. It's the same in agriculture; when you farm intensively year after year, you have to add something back to maintain productivity. |
| JMV: | So the intensive forestry of JMV&CO plantations could cause reductions in site index? |
| PMD: | Yes. If there's an existing forest on the site before you establish one of your plantations, gain as much information from it as you can. The site index and nutrient status shown by it can be very important for determining what to expect from the next crop. These existing trees are the best integrators of all the factors that control growth. When the trees are gone, you've lost all the information they contained.
I would assess the nutrient status of the stand in the dormant season before it is clearcut to make way for the next crop. If it is OK, i.e., shows no lack of nutrients, I would take another look at it after the trees begin serious competition with each other, say at 12 to 15 years. If a deficiency does exist, you can then take some corrective action. On poorly-drained sites, where phosphorus is often limited, it is important to take care of this problem at the time of planting. |
| JMV: | Does winter-time yellowing of the needles indicate nutrient deficiency? |
| PMD: | It can, but trees normally experience some loss of chlorophyll in the winter. Chlorophyll is both produced and lost daily, and in some winters more is lost than produced, causing the foliage to be yellow. If this condition is due to weather, it does not indicate a nutrient deficiency, requires no immediate action, and will be corrected in the spring. I would not depend on foliage color as an indicator of nutrient status. |
| JMV: | Dr. David O'Malley of the Forest Biotechnology Group at North Carolina State University has just installed some research plots down here. What is he seeking? |
| PMD: | Dave O'Malley and Steve McKeand are studying how genetics, environment, and tree physiology interact to control productivity of Loblolly pines. In the stand in which Lee Allen and I have established our irrigation and fertilization treatments, we know that we have ten half-sib families, but we don't know what family is represented by each tree. The genetic pool on our site is very narrow: ten North Carolina Piedmont improved half-sib families. On this adjacent site David and Steve have established plots of trees with a much broader genetic base, but each individual's genetic makeup is known in detail. They plan to irrigate and fertilize one-half of these trees and supply no water or nutrients to the other half. They will then determine which individuals of each family perform well under low resource conditions and high resource conditions. Once they answer this question, they will identify the traits that caused its good or poor productivity performance. David and Steve are trying to find the specific gene combinations that code for traits that lead to high productivity under high or low resource growing conditions. |
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