Projects
My current and long-range research interests focus on examining in situ root system function, in particular how:
- Root systems of long-lived perennials respond to natural soil stresses and resource heterogeneity, and anthropogenic stresses;
- Root system responses may account for the wide variation in above-ground growth that is generally observed in tree populations;
- Rhizophere biota alter root system and belowground ecosystem function.
Carbon strategies of fast- and slow-growing trees
Loblolly pine (Pinus taeda L.) is the most widely planted tree species in the Atlantic Coastal Plain, with more than 12.3 million hectares in both natural and planted stands. Because of its wide geographic distribution, it is not uncommon to find significant genotypic variation in growth of various populations. Populations of loblolly pine from the western extent of its physiographic range, i.e. Texas, are often described as slow-growing, but known for their drought tolerance and resistance to many natural pests/pathogens. Eastern populations from North and South Carolina are generally faster-growing, but less resistant to drought and natural pathogens such as fusiform rust and tipmoth.
We have been examining whether differences in aboveground growth between the various populations are a function of differences in whole-tree carbon source/sink relationships, particularly root system carbon demands. We have found that slow-and fast-growing populations of loblolly have similar photosynthetic rates, whole-tree biomass allocation and similar whole-tree partitioning towards carbon storage. Of the traits measured, only fine root system production and lifespan differed between the fast- and slow-growing populations. Because fine root system production and maintenance costs in conifers may be as much as 75 percent of total net primary productivity, it is not unreasonable to postulate that variation in aboveground yield may be associated with genetic differences in belowground carbon demands.
The majority of fine roots in trees are in the upper 10 centimeters of soil. Soil resources such as nutrients are generally highest in this part of the soil horizon. In this picture, soil in the upper horizon has been blasted away to reveal the reticulate nature of fine root growth in a sandy soil.
Impact of smog on tree health
Atmospheric pollutants have been implicated as a causal factor in the decline of forest health and productivity occurring in many regions of North America and Europe. Tropospheric ozone (ground-level smog) is the most widespread air pollutant in the United States and concentrations are increasing at 0.5-1 percent per year. Ozone probably has the most negative impact of all air pollutants on tree vigor and growth because once in the leaf, ozone reacts with water to form free radicals, H2O2, and hydroxyl ions. These highly reactive compounds oxidize proteins and membrane phospholipids, increase membrane permeability and can directly inhibit photosynthesis. Whole-plant carbon balance can be altered because of reductions in photosynthesis and leaf growth, and increased carbohydrate demands in the shoot resulting from accelerated leaf senescence and turnover, membrane repair processes and synthesis of antioxidants, with allocation of carbohydrates to roots perhaps most affected.
Sugar maple (Acer saccharum Marsh.) is an important forest species that is widely distributed in eastern North America where elevated levels of ozone occur, and some forests are undergoing decline. Sugar maple has been described as a moderately ozone-tolerant species, but research we have conducted suggests that this may only be true under high irradiances. Leaves in the mid and lower canopies are growing in a light environment that may be less than 10 percent of full sunlight. We have found that these shaded leaves are more susceptible to ozone injury because of their leaf anatomy and low carbon gain. Ozone accelerates senescence-like injury of sugar maple leaves and reduces their photosynthetic life span by 25 to 30 percent. Although ozone has no effect on net photosynthesis until the end of the growing season, our study indicates that it affects leaf metabolism immediately, with recently-fixed carbon being diverted to injury repair processes (and not towards growth). Interestingly, we did not find that ozone impaired transport out of leaves as other studies have postulated. If transport to roots is affected, it is more a function of a reduced supply available for transport and not the process per se.
Ground-level smog accelerates senescence in leaves of many tree species, including sugar maple. This shortens the growing season for trees, and may be a contributing factor in sugar maple decline in the northeast.
