VARDAMAN VIRTUAL FORESTRY COMPANY

The Most Direct Link to Knowledge Workers in the Southeast Forest Economy

THE MOST UNUSUAL AND EXCITING FOREST IN THE SOUTH

The most unusual forest we have ever seen is the Southeast Tree Research and Education Site (SETRES) in Scotland County, North Carolina. It was originally laid out in 1992 in an eight-year-old loblolly pine plantation on a very infertile, excessively-drained soil with per-acre survival of 510 trees 11.1 feet tall and 1.8 inches in DBH. The exhibited SI-25 was in the upper 40s. It looked awful.

It was divided into four blocks of four plots each. Four were not treated and served as controls; four were irrigated only; four were fertilized only; four were both irrigated and fertilized. Certain trees were exposed to elevated CO2. They are now finishing their 12th growing season. When the research began, the scientists measured almost everything that can be measured; there are so many measuring devices underground, on the ground, and in the trees that you must always be careful where you step. The distinguished scientists in charge of this project are Phillip M. Dougherty and Lance Kress of U. S. Forest Service and H. Lee Allen of North Carolina State University. We recently went to the site and interviewed them for the third year in a row, and here's what we learned.

JMV: Before we get started on your findings, give us some background on the project.

PMD: We are part of an international research network that has as its major objective to understand what will happen to managed forests around the world, those forests that society lives off of, under future climate and pollution conditions. In addition to meeting this objective, we will contribute new knowledge about how to manage the forests under current conditions so productivity is enhanced in an ecologically sustainable manner.

At SETRES Lee and I were charged specifically with the mission of finding out how loblolly pine forests will function under future conditions. We knew that, if nutrition and water availability are altered due to climate change, productivity could change tremendously because across the southern U.S. these are the two factors that are limiting productivity. We also know that atmospheric CO2 is increasing, and in the agriculture sector, CO2 has been shown to have a tremendous influence on plant carbon-gain potential. Our challenge was to set up an integrated study that would permit us to evaluate the combined effects of nutrition, water, and carbon dioxide on loblolly pine forests.

HLA: There are or have been studies with similar objectives and study designs using Scots Pine in Sweden, Radiata Pine in Australia, Norway Spruce in Sweden, eucalyptus in Portugal, and others are working on related aspects in Scotland and New Zealand. We work closely with more than 20 counterparts in these countries. Some of the species are much less responsive to environmental stress than loblolly pine. We speak of loblolly pine as being a real weed, i.e., being extremely elastic in its response to changes in its environment.

PMD: Loblolly pine is very elastic in terms of adaptability. It has a very wide geographical distribution and does well across a wide range of soils and conditions. It does well on poor soils, and on very good soils, not many species can outgrow it. It is probably the best-adapted pioneer species in the south.

JMV: This plot certainly shows that it can make it under poor conditions.

HLA: The whole purpose of our project is to find out what are the limiting factors that slow down the fixation of carbon. Our original hypothesis was that on this site the most limiting factor was water. What we have found is that at certain times of the year it is nutrients. We didn't realize that nutrients were more important than water on this type of site.

You will notice that, although there are many trees per acre, well above 350, they don't have many needles, there is plenty of light all around; light is not limiting growth. Nevertheless, the lower limbs are dying. Why? There are not enough nutrients to grow any more needles that they presently have. These needles contain about l% nitrogen. When the tree runs out of nitrogen, it won't produce any more needles.

PMD: One way I like to think about this is that the tree must optimize its carbon-gain potential with the limited nutrients that it has. There is light reaching the lower part of the crown here, but better light in the upper part of the crown, so the tree will re-mobilize and move some of its nitrogen up there. When this happens, lower branches lose their ability to maintain themselves. The tree is much better off under nitrogen-limiting conditions to have its limited foliage in the crown position where light interception is optimized. This is a good example of loblolly's elasticity in coping with its environment.

LK: If we could stop the growth of the top, either cut it off or install a barrier up there, the nitrogen would stay down here, and these branches would continue to grow. One thing that was surprising to me is that with fertilization we don't get greater longevity of the needles; we are just getting a whole lot more needles being produced.

JMV: How do you decide what to add in the way of nutrients?

HLA: From our earlier work with loblolly, we have a very good idea of what is the appropriate nutrient balance. Each month we sample the foliage, determine the absolute concentration and the relative balance of nutrients. About once a year we fertilize the trees to try to maintain the foliar nitrogen levels at 1.4% and the others in proper proportion. We use the tree as our guide rather than the soil.

JMV: Does that mean that private landowners shouldn't use soil analyses to determine the need for fertilizers?

HLA: The techniques that we have traditionally used for soil analysis are almost worthless for that purpose. Foliage analysis isn't perfect, but it's the best we have. Here's why it's not perfect. Let's take that poor little tree that we talked about earlier. If the site contained about ten pounds of nitrogen per acre, the concentration in the needles would be about 1% If we added ten more pounds, the tree would probably put on more needles, but the concentration would remain the same. If we added 50 pounds, then the number of needles as well as the concentration would rise.

We know that there is a pattern of growth. The trees grow above ground at certain times of the year and below ground at other times. On a bi-weekly basis we measure the nutrient demand of these trees and where it's coming from. Is it being met from the soil? About 70% of the nitrogen in a needle is withdrawn back into the tree before the needle dies and is used for other purposes.

PMD: What it comes down to is we want to be able to make an assessment of the status of nutrient-supplying material in the soil and on the forest floor relative to the demand for nutrients by the forest stand and to be able to predict what will happen to the nutrient-supplying capacity and stand demand as the stand develops.

If you look at our graphs, you would have to question whether it's better to make a foliar analysis or measure leaf area for a given basal area. If I had to choose between the two methods, I'd measure leaf area. It's better to measure both of them, because it's the product of the two that determines the nutrient capital of the site.

JMV: That tree has some holes like a big yellow-bellied sapsucker would make. What are they for?

HLA: We are very much interested in carbon fixation from the standpoint of growth, but we also want to learn how the tree uses carbon to produce secondary defense compounds. Jeff Warren, a masters student, samples the phloem and the resin periodically. That's what the holes are for. For example, much work has been done on assessing which phenolics are not palatable to insects and the role of oleoresin exudate in making the tree much more resistant to SPB attack.

PMD: In the first year we found that nutrition didn't change the phenolic compounds of foliage that much, but that elevating the CO2 concentration caused high carbohydrate production, lower nitrogen concentration, and higher production of phenolics. Fitz Booker with North Carolina State University did the research on determining foliage phenolics.

JMV: What's the purpose of these things that look like rain gauges?

PMD: We want to know when irrigation is needed on certain plots. 80% to 85% of precipitation falls through the foliage, 5% runs down the trunk of the tree, and the rest is intercepted and evaporates before it reaches the ground. The intercepted portion offsets the loss of some water from inside the tree, but it is not effective from the standpoint of contributing to soil biological processes or mass movement of nutrients into the tree. The water that comes down the trunk is very important. You get a large volume placed in a small area, so it tends to be driven to a greater depth and is less likely to be lost by direct evaporation from the forest floor.

HLA: The water that flows down the stem also contains a higher nutrient content. An important amount of nutrients comes from dry deposition. Ash from a fire in the vicinity or even dust from a nearby farm or road may blow nutrient-containing particles onto the tree. Nitrogen is a component of air pollution, and the tree's branches and needles are filters that catch some of it. When it rains, some of the intercepted dry particulate matter is dissolved and runs down the trunk. On the way to the ground, the rain may also leach some potassium from green needles.

LK: You also have the bark that is flaking off all the time, and that contains nutrients that are leached. Scientists at the Tennessee Valley Authority who are working on air-pollution effect of coal-fired power plants have estimated that the atmospheric deposition of sulphur and nitrogen may be more important than anything we do in fertilization in terms of putting sulphur and nitrogen back in our southern soils. Dry deposition occurs much more often than wet deposition, so it's quite important.

JMV: Many people would look upon dry deposition as bad because it comes from pollution, but it actually might be good. Right?

PMD: This is a real important issue so far as what's going to happen to these forests. People often say, "If climate changes." But it's not if; climate is already changing. As population goes up, nitrogen inputs in the system go up, CO2 goes up. In parts of Europe today nitrogen inputs may amount to 40 to 50 pounds per acre per year, whereas annual nitrogen inputs in the southern U.S. may currently be only eight to ten pounds per acre, but they are certain to increase with increasing population. You will see today the powerful effects of these two resources (nitrogen and carbon dixoide) on our forests.

JMV: So nutrition is important. What about water?

HLA: Nutrition may be more important to the productivity of loblolly pine forests; water may be more important in survival of the trees. A tree will die if it doesn't get enough water, but it can get all the water in the world, and it won't grow if the nutritients are not there. We have been surprised in that nutrition has been more important in areas where we first thought that water was more important.

PMD: This study has already shown that trees are different from humans and trees are different from crops. Trees handle nutrition differently than seedlings and totally differently than crops. Trees are very different from seedlings in terms of their nutrient and carbon-storage capacity. Seedlings don't have much nutrient-or carbon-storage capacity; they depend solely on whatever they are able to produce that day. That's not at all true of trees; storage is very important for them to get through hard times. We believe that the important role of storage compounds in the growth and maintenance of loblolly forests has been underestimated.

HLA: Yes. Arthur Sampson, a scientist on our team, has worked with a model constructed by a cooperator in Australia to grow trees one day at a time. Using this model, we hypothesize that trees fix more carbon than they need up until about June 1 and then, for a period of about 75 days of fast growth and high temperatures, fix much less. We think that a tree can store carbon early in the year and then draw down this stored carbon to maintain growth during the hot summer, and our measurements are testing this hypothesis now. So far we seem to be correct. Apparently fertilization reduces the size of the carbon deficit.

PMD: This has important implications for our work on climate change and for stand management. If this is really true and this carbohydrate recharge period is important, how climate changes seasonally is tremendously important. If we have high CO2 and more moderate winter conditions, stored carbon will be high going into the growing season and productivity will be increased. If on the other hand, spring becomes much drier, the draw-down of the carbon reserve will be much more serious. Therefore to estimate the effect of climate change, we must have data on seasonal changes.

From a management standpoint, we need to do what is needed to keep this stored carbon high, so that the tree goes into each growing season well-equipped to make strong growth. We need to be especially alert for damage from insects or fires that might occur during the draw-down period, for such damage might seriously limit the tree's capacity to recover.

JMV: How much has fertilization increased leaf area?

HLA: Peak leaf-area occurs about Sept. 1. One plot has achieved a leaf area of more than 3.0, i.e., the area of leaves is 3.0 times the area of land under the tree, and we expect to increase leaf area to about 3.5 with fertilization alone and to reach 4.0 to 4.5 with addition of irrigation. This is just a hypothesis that we are now testing. If we can't get above 3.5 without water, that's the upper limit of what we can achieve by nutritional management. Greater foliage production increased peak leaf area index (LAI) on fertilized stands when compared to nonfertilized stands by 67% in 1992, 58% in 1993, and 88% in 1994.

The next question is, "Does growth increase with leaf area?" And it does. Stem volume production increased dramatically with optimum nutrition, showing increases over control of 60% in 1992, 85% in 1993, and 120% in 1994.

We've increased the growth, but we've also increased the amount of wood produced per unit of leaf area. What do we need to do in management to get high leaf area as fast as possible? We know we can get large amounts of leaf area quickly with site-prep and competition control, but I would argue that we need fertilizer to maintain high leaf area as stands approach full stocking.

JMV: That's fascinating, but we didn't discuss fertilizer when we developed the Vardaman 1993 Hypothesis, and I'm curious about how it fits in. We add a boost for genetic improvement, one for competition control, one for morphologica1ly-improved seedlings, etc. Can we also add something for fertilizer? If we can, the future of growing loblolly pines is absolutely glorious.

HLA: There's no easy answer to your question. Sometimes the benefits are additive, and sometimes they aren't. I'll give you an example. The extra growth that you get from genetically-improved seedlings may be due to the improved ability to acquire nutrients or to utilize nutrients. If there are no nutrients on the site, any ability to acquire them is useless. There are so many factors involved here, some of which are unknown, that I can't give you the answer. Many of our findings will help.

JMV: Now tell us about the effect of elevated CO2 on the trees.

PMD: We hypothesized up front that the increased photosynthate production with increased CO2 would depend on the level of site resources. After exposing these branches to elevated CO2 for about 18 months, we got about the same percent increase in photosynthate production whether it was in a control, a fertilized, an irrigated, or a fertilizer-irrigated plot. This surprised us. With exposure to 1.5 times the current level of CO2, photosynthate production went up about 45%. When we doubled CO2, which is what is supposed to happen about two-thirds of the way into the next century, we increased photosynthate production about 90%. In comparison we found that the most effective cultural treatments that we can employ (fertilization and irrigation) would increase photosynthate production between 20% and 30%. Therefore, increases in CO2, increase production three or four times as much as we can get with fertilizer and water.

But you must ask, "What's the other effect of nitrogen?" As we mentioned earlier, it greatly increases the leaf area. CO2, on the other hand, doesn't increase the leaf area; it increases the production per unit of leaf area. I believe that these results point to a close coupling between the nitrogen inputs into this system and the CO2 inputs.

We also thought that, if CO2 went up, the stomata would close to conserve some of the tree's moisture and shut down photosynthesis. That's what has been reported in agriculture. We didn't see that at all. There was no stomatal closure with increased CO2.

The other thing that we found was that, on a per-unit basis, the increase in photosynthesis due to CO2 was independent of changes in rates of photosynthesis due to water or nutrition. They really were additive on a per-unit leaf area basis.

HLA: You must remember that foliage and roots are what really cost in terms of nutrients. If foliage produces more carbon, then theoretically this will go into stemwood, which has low nutrient costs. We therefore might see dramatic increases in stemwood and in the production of secondary compounds.

PMD: I have no doubt that carbon-fixation capacity of these trees will go up as CO2 goes up. One would have to say that, across the south, CO2 is probably the most limiting factor. Lots of people don't want to accept this.

PMD and HLA: From a biological standpoint, we'd get the biggest bang, not from water or fertilizer, but by raising CO2.