Donald I. Dickmann

Repeated insect defoliation effects on growth, nitrogen acquisition, carbohydrates, and root demography of poplars

Large-scale outbreaks of defoliating insects are common in temperate forests. The effects of defoliation on tree physiology are expected to cascade through the entire forest ecosystem, altering carbon, nitrogen, and water fluxes, and subsequently affecting nitrogen cycling and plant-herbivore interactions. If these post-defoliation changes are largely driven by N deficiency, tree root system responses to defoliation should be central to regulating the long-term effects of defoliation; N fertilization should reverse the effects. We examined these phenomena in a 3-year large-scale replicated manipulative field experiment in a hybrid poplar plantation, where we regulated defoliation by gypsy moths as well as nitrogen availability. To our knowledge, this is the first manipulative field experiment at this scale to examine the effects of severe insect defoliation on whole-tree physiology. Defoliation decreased tree growth and increased the rate of top dieback in the stand. Defoliation led to transient declines in carbon allocation to starch in fine roots, trunk, and twigs in the year of heaviest defoliation. Root production and root mortality were unaffected by the heaviest defoliation, but nitrate and ammonium uptake were strongly depressed. N fertilization increased tree growth, but did not alter defoliation effects on starch accumulation or top dieback. Defoliation and fertilization treatments did not interact. In this system, defoliation effects on tree recovery of leaf nitrogen lost to herbivory were primarily driven by effects on nitrogen uptake, rather than effects on root production or mortality.

Characterizing historical and modern fire regimes in Michigan (USA): A landscape ecosystem approach

We studied the relationships of landscape ecosystems to historical and contemporary fire regimes across 4.3 million hectares in northern lower Michigan (USA). Changes in fire regimes were documented by comparing historical fire rotations in different landscape ecosystems to those occurring between 1985 and 2000. Previously published data and a synthesis of the literature were used to identify six forest-replacement fire regime categories with fire rotations ranging from very short (<100 years) to very long (>1,000 years). We derived spatially-explicit estimates of the susceptibility of landscape ecosystems to fire disturbance using Landtype Association maps as initial units of investigation. Each Landtype Association polygon was assigned to a fire regime category based on associations of ecological factors known to influence fire regimes. Spatial statistics were used to interpolate fire points recorded by the General Land Office. Historical fire rotations were determined by calculating the area burned for each category of fire regime and dividing this area by fifteen (years) to estimate area burned per annum. Modern fire rotations were estimated using data on fire location and size obtained from federal and state agencies. Landtype Associations networked into fire regime categories exhibited differences in both historical and modern fire rotations. Historical rotations varied by 23-fold across all fire rotation categories, and modern forest fire rotations by 13-fold. Modern fire rotations were an order of magnitude longer than historical rotations. The magnitude of these changes has important implications for forest health and understanding of ecological processes in most of the fire rotation categories that we identified.

Multiple successional pathways on recently disturbed jack pine sites in Michigan

Abstract Jack pine communities in northern lower Michigan recently disturbed by clearcutting, deliberate burning, or wildfire were studied over three growing seasons, and were compared to undisturbed jack pine stands. Newly disturbed sites generally had more vascular plant species than mature forests. Many of these species did not persist, especially on burned sites, and species richness declined sharply the second year after fire. In several cases annual and biennial species dominated first-year burns but were unimportant thereafter. Several pathways of early successional development were evident on the disturbed sites, which was facilitated by jack pine regeneration failure on all but one of the disturbed sites. Unburned clearcuts rapidly developed into Carex meadows, with few other species of any importance. This pathway was also followed on some deliberately-burned and wildfire sites. Other burned areas developed a stratified canopy of shrubs and early successional hardwoods, and were rich in species and high in cover. Stand replacement by jack pine occurred on one mature jack pine site burned by wildfire. Jack pine regeneration failure has been attributed to numerous factors involving: seed loss due to fire and predation, and low germination and seedling survival due to inadequate seedbed, drought, competition, pathogens, grazing and high surface temperatures. Index of similarity comparisons showed that different sites of the same post-disturbance age were significantly (P

Response of Understory Vegetation to Fertilization on Mature and Clear-cut Jack Pine Sites in Northern Lower Michigan

A study was established on jack pine sites in northern lower Michigan to investigate the effects of fertilization on the relative growth of understory vegetation on clear-cut and undisturbed sites. The primary objective was to discern if the dramatic increases in Carex pensylvanica following disturbance (e.g., fire and clear-cutting) were in part due to its ability to exploit newly available nutrient resources. Total live shoot biomass as well as live shoot biomass of Carex and blueberry were significantly increased on fertilized blocks in mature jack pine and on clear-cut sites. Fertilized to control biomass ratios showed that Carex consistently had an above-average response to fertilization in both mature jack pine and clear-cut sites compared to the community average. Biomass of Carex species from a recent prescribed burn site greatly exceeded that of any other species. During this experiment, neither Carex pensylvanica nor any other species appeared to greatly suppress or exclude neighboring species. Although C. pensylvanica showed a preferential uptake of nutrient fertilizer, it is apparent that its expansion following clear-cutting and burning is in response to other factors such as increased space, light, soil temperatures and available moisture, as well as increased nutrient availability.

Surface burning in a mature stand of Pinus resinosa and Pinus strobus in Michigan: effects on understory vegetation

Beginning in 1991, periodic surface fires (frontal fire intensities 1 m, ≤ 1.9 cm dbh) and 3944 saplings (2.0–5.9 cm dbh) per ha, consisting of 23 woody angiosperm taxa. Plots burned once contained 60% of the large seedlings, 7% of the saplings, and 6 fewer taxa than unburned plots. No large seedlings and few saplings were found in plots burned biennially. Cover of low (<1 m) woody and herbaceous vegetation in plots burned once or three times was twice that of unburned plots, even in the growing season immediately following the May 1995 re-burn. Recovery of low vegetative cover in the re-burned plots was rapid, exceeding that in once-burned or unburned plots by late summer following the burn. Species richness of low vegetation was 20–25% higher in burned than unburned plots, except in the year immediately following reburning. Taxa dominating this site following burning were Sassafras albidum (Nutt.) Nees, Rubus spp., Phytolacca americana L., and Dryopteris spinulosa (O.F. MUll.) Watt. Restoration of low-intensity surface fires to ecosystems dominated by mature red pine or white pine is feasible, but major changes in understory structure and composition will occur.

Management of exotic timber trees in temperate regions of eastern North America: an assessment.

Abstract Based on data in the forestry literature and experiences at Kellogg Forest in southwest lower Michigan and Jericho Research Forest in northwest Vermont, the potential of various exotic conifers and hardwoods as timber trees in temperate eastern North America is assessed. Site adaptability, preferred seed sources and pest problems of several promising exotics are discussed and growth comparisons between exotic and native timber trees are made. It is suggested that wood production on many sites could be improved by planting exotics such as Norway spruce, the larches, Scotch pine, and English oak. However, further comparison trials with natives and exotics are needed before wide-scale planting of exotics can be recommended.

Modeling Adventitious Root System Development in Trees: Clonal Poplars

Mechanistic growth modelers have recently become interested in exploring the mysteries of tree root systems. At the outset of these modeling efforts, some understanding of the morphology, ecology and physiology of roots, and how they correlate with the aerial parts of the tree is essential. The growing tree is an integrated system, with water, minerals, nitrogenous compounds, carbohydrates, growth regulators and other organic substances moving freely, though often phasically, between the roots and the shoots. A perturbation or stress in one part of the tree is sensed and reacted to in all others. In addition, roots grow in a complex, heterogeneous soil environment. If we are to improve upon existing mechanistic and predictive models of tree growth, or to build more responsive new models, the physiology and ecology of this integrated shoot-root-soil system must be better understood.

Net assimilation and photosynthate allocation of Populus clones grown under short-rotation intensive culture: Physiological and genetic responses regulating yield

The overall objective of this project was to determine the differential responses of poplar clones from sections Tacamahaca and Aigeiros of the genus Populus to varying levels of applied water and nitrogen. Above- and below-ground phenology and morphology, photosynthate allocation, and physiological processes were examined. By manipulating the availability of soil resources, we have been able to separate inherent clonal differences from plastic responses, and to determine genotype-environment interactions. We also have been able to make some contrasts between trees grown from hardwood cuttings and coppice sprouts. Our overall hypothesis was that carbon allocation during growth is greatly influenced by interactions among moisture, nitrogen, and genotype, and that these interactions greatly influence yield in short-rotation plantations. As is true of any project, some of our original expectations were not realized, whereas other initially unforeseen results were obtained. The reduced funding from the Biofuels Feedstock Development Program (BFDP) during the last few years of the project slowed us down to some extent, so progress was not been as rapid as we might have hoped. The major problem associated with this funding shortfall was the inability to employ skilled and unskilled student labor. Nonetheless, we were able to accomplish most of our original goals. All of the principal investigators on this project feel that we have made progress in advancing the scientific underpinning of short-rotation woody biomass production.