Philip M. Wargo

Tree Health and Physiology in a Changing Environment

A tree is a large, long-lived, perennial, compartmented, woody, shedding, walling plant. This definition is based on new tree biology concepts (Shigo, 1986a,b, 1991) and explains much about how mature trees function through their unique structure. When the tree begins its life, it is mostly leaf in mass (Fig. 7.1a). As a tree grows in stature, it becomes mostly stem in mass and the foliage represents only a few percent of the total mass. Roots remain relatively constant at about one-fifth the total mass as a tree grows from a small sapling to a mature standard in the forest canopy. Branches represent only a small fraction of total mass, which decreases over time, as older branches are shed. Also shed are leaves, roots, and outer bark. However, aging wood cannot be shed, but dies internally as sapwood is transformed into a core of protection wood, often called “heartwood” (Fig. 7.1b,c; Table 7.1).

Effects of climate change on forest insect and disease outbreaks

General circulation models (GCMs) predict dramatic future changes in climate for the northeastern and north central United States under doubled carbon dioxide (CO2) levels (Hansen et al., 1984; Manabe and Wetherald, 1987; Wilson and Mitchell, 1987; Cubasch and Cess, 1990; Mitchell et al., 1990). January temperatures are projected to rise as much as 12°C and July temperatures as much as 9°C over temperatures simulated at ambient C02 (Kittel et al., 1997). Projections of precipitation are quite variable over the region, ranging from 71 to 177% of ambient levels in January and 29 to 153% of ambient in July among several GCMs (Kittel et al., 1997). Such climate changes clearly may affect the growth and species composition of our northern forests directly in ways discussed in previous chapters. In contrast with the discussions in previous chapters, this chapter steps up one trophic level to consider the effects of climate change on the populations of microorganisms, fungi, and insects that feed in and on forest trees.

Effect of chronic nitrogen additions on soil nitrogen fractions in red spruce stands

The responses of temperate and boreal forest ecosystems to increased nitrogen (N) inputs have been varied, and the responses of soil N pools have been difficult to measure. In this study, fractions and pool sizes of N were determined in the forest floor of red spruce stands at four sites in the northeastern U.S. to evaluate the effect of increased N inputs on forest floor N. Two of the stands received 100 kg N ha-1 yr-1 for three years, one stand received 34 kg N ha-1 yr-1 for six years, and the remaining stand received only ambient N inputs. No differences in total N content or N fractions were measured in samples of the Oie and Oa horizons between treated and control plots in the three sites that received N amendments. The predominant N fraction in these samples was amino acid N (31-45% of total N), followed by hydrolyzable unidentified N (16-31% of total N), acid-soluble N (18-22% of total N), and NH4+ (9-13% of total N). Rates of atmospheric deposition varied greatly among the four stands. Ammonium N and amino acid N concentrations in the Oie horizon were positively related to wet N deposition, with respective r2 values of 0.92 and 0.94 (n = 4, p < 0.05). These relationships were somewhat stronger than that observed between atmospheric wet N deposition and total N content of the forest floor, suggesting that these pools retain atmospherically deposited N. The NH4+ pool may represent atmospherically deposited N that is incorporated into organic matter, whereas the amino acid N pool could result from microbial immobilization of atmospheric N inputs. The response of forest floor N pools to applications of N may be masked, possibly by the large soil N pool, which has been increased by the long-term input of N from atmospheric deposition, thereby overwhelming the short-term treatments.

Atmospheric Deposition Effects on Surface Waters, Soils, and Forest Productivity

When acid rain was discovered to be a regional problem in North America in the 1970s, initial concerns focused on surface-water acidification. Some of the earliest acidic deposition research found that fish populations in some lakes and streams in the Adirondack Mountains of NY had been eliminated by acidification (Schofield, 1976). In the 1980s, research in the U.S. expanded greatly through the National Acid Precipitation Assessment Program (NAPAP) to include soils and forests, as well as aquatic ecosystems. Because little environmental monitoring had been done before the start of NAPAP, however, information on changes that led to the conditions observed in the 1970s and 1980s was limited. As a result, NAPAP research focused on assessments of current conditions, short-term experimental manipulations, reconstructions from paleolimnological evidence, and mathematical modeling, to investigate past and possible future changes caused by acidic deposition. This program yielded conclusive evidence that acidic deposition had acidified poorly buffered surface waters, resulting in the loss of fish populations and other aquatic organisms, although uncertainties remained about the extent of these effects (NAPAP, 1991). The NAPAP Integrated Assessment Report (NAPAP, 1991) also concluded that acidic deposition may have affected soil chemistry but effects on forest health were not apparent, except for high-elevation spruce-fir forests where stand dieback was attributed to acidic deposition.