Marc D. Abrams

FIRE AND THE DEVELOPMENT OF OAK FORESTS

ak (Quercus) represents one of the most dominant species groups in the eastern deciduous forest of North America (Table 1). In certain eastern regions, oak dominance reflects the importance of this genus in presettlement forests (Abrams and Downs 1990, Spurr 1951, Whitney and Davis 1986). Yet, in other regions, the current distribution of oak greatly exceeds that of the original vegetation (Abrams 1986, Howell and Kucera 1956, Nowacki et al. 1990). Thus, the development of oak species has occurred through a variety of ecological pathways and disturbance conditions. The dominance of oak in presettlement forests is particularly interesting because most oaks are considered early to midsuccessional species. Indeed, most recent studies indicate the potential for widespread oak replacement by more shade-tolerant species in mature forests (Christensen 1977, Lorimer 1984, Nowacki et al. 1990). However, this phenomenon may vary with regional and edaphic factors and be more pronounced on mesic rather than on xeric sites (Abrams 1986, Host et al. 1987, McCune and Cottam 1985, Nowacki and Abrams 1991). These observations have led researchers to ask the following questions about oak ecology in eastern

The Demise of Fire and “Mesophication” of Forests in the Eastern United States

ABSTRACT A diverse array of fire-adapted plant communities once covered the eastern United States. European settlement greatly altered fire regimes, often increasing fire occurrence (e.g., in northern hardwoods) or substantially decreasing it (e.g., in tallgrass prairies). Notwithstanding these changes, fire suppression policies, beginning around the 1920s, greatly reduced fire throughout the East, with profound ecological consequences. Fire-maintained open lands converted to closed-canopy forests. As a result of shading, shade-tolerant, fire-sensitive plants began to replace heliophytic (sun-loving), fire-tolerant plants. A positive feedback cycle—which we term “mesophication”—ensued, whereby microenvironmental conditions (cool, damp, and shaded conditions; less flammable fuel beds) continually improve for shade-tolerant mesophytic species and deteriorate for shade-intolerant, fire-adapted species. Plant communities are undergoing rapid compositional and structural changes, some with no ecological antecedent. Stand-level species richness is declining, and will decline further, as numerous fire-adapted plants are replaced by a limited set of shade-tolerant, fire-sensitive species. As this process continues, the effort and cost required to restore fire-adapted ecosystems escalate rapidly.

UIML: an appliance-independent XML user interface language

Abstract Todays Internet appliances feature user interface technologies almost unknown a few years ago: touch screens, styli, handwriting and voice recognition, speech synthesis, tiny screens, and more. This richness creates problems. First, different appliances use different languages: WML for cell phones; SpeechML, JSML, and VoxML for voice enabled devices such as phones; HTML and XUL for desktop computers, and so on. Thus, developers must maintain multiple source code families to deploy interfaces to one information system on multiple appliances. Second, user interfaces differ dramatically in complexity (e.g, PC versus cell phone interfaces). Thus, developers must also manage interface content. Third, developers risk writing appliance-specific interfaces for an appliance that might not be on the market tomorrow. A solution is to build interfaces with a single, universal language free of assumptions about appliances and interface technology. This paper introduces such a language, the User Interface Markup Language (UIML), an XML-compliant language. UIML insulates the interface designer from the peculiarities of different appliances through style sheets. A measure of the power of UIML is that it can replace hand-coding of Java AWT or Swing user interfaces.

The Red Maple Paradox What explains the widespread expansion of red maple in eastern forests

O ne of the most dramatic changes that has occurred in forests of eastern North America during the twentieth century is the increase in the dominance of red maple (Acer rubrum L.; Lorimer 1984, Abrams 1992). Red maple has become nearly ubiquitous across sites of widely varying light, moisture, and nutrient availability. This distribution is in stark contrast to the limited distribution of red maple reported in pre-European settlement forests, where it occurred mainly in poorly drained areas. Red maple has increased after a wide range of disturbances and as a late successional species in many forest types. It now dominates the understory and mid-canopy of many oak (Quercus), pine (Pinus), and northern hardwood forests; moreover, it will probably continue to increase in dominance in the overstory during the next century, causing widespread replacement of the historically dominant trees of the forests of the eastern United States. Surprisingly, red maple exhibits rather modest levels of leaf physiological responses (e.g., gas exchange and osmotic adjustment) to various environmental conditions and has leaf structural characteristics and nitrogen levels that are not particularly conducive to a robust leaf physi-

Removal policies in network caches for World-Wide Web documents

World-Wide Web proxy servers that cache documents can potentially reduce three quantities: the number of requests that reach popular servers, the volume of network traffic resulting from document requests, and the latency that an end-user experiences in retrieving a document. This paper examines the first two using the measures of cache hit rate and weighted hit rate (or fraction of client-requested bytes returned by the proxy). A client request for an uncached document may cause the removal of one or more cached documents. Variable document sizes and types allow a rich variety of policies to select a document for removal, in contrast to policies for CPU caches or demand paging, that manage homogeneous objects. We present a taxonomy of removal policies. Through trace-driven simulation, we determine the maximum possible hit rate and weighted hit rate that a cache could ever achieve, and the removal policy that maximizes hit rate and weighted hit rate. The experiments use five traces of 37 to 185 days of client URL requests. Surprisingly, the criteria used by several proxy-server removal policies (LRU, Hyper-G, and a proposal by Pitkow and Recker) are among the worst performing criteria in our simulation; instead, replacing documents based on size maximizes hit rate in each of the studied workloads.

RADIAL-GROWTH AVERAGING CRITERIA FOR RECONSTRUCTING DISTURBANCE HISTORIES FROM PRESETTLEMENT-ORIGIN OAKS

A novel dendroecological procedure was developed to elucidate canopy disturbances spanning a >300-yr period for oak (Quercus) forests of central Pennsylvania. Running comparisons of sequential 10-yr ring-width averages may effectively neutralize both short-term (i.e., drought) and long-term growth trends associated with climate while enhancing detection of abrupt and sustained radial-growth increases characteristic of canopy disturbance. Thinning-response studies revealed the conservative tendencies of overstory oak, with substantial basal area reductions (>1/3) required to attain moderate and consistently detectable growth increases. Based on empirical evidence, a minimum growth-response threshold of 25% was established to depict canopy disturbances. This is in contrast to the 50–100% sustained radial-growth release often used to detect disturbance using understory trees in closed forests. Our default threshold was adjusted higher as necessary for those trees highly correlated to climatic trends (as repr...

Where Has All the White Oak Gone

Abstract Before European settlement, vast areas of deciduous forest in what is now the eastern United States were dominated by oak species. Among these species, white oak (Quercus alba) reigned supreme. White oak tended to grow at lower elevations but was distributed across a broad range of sites, from wet mesic to subxeric, and grew on all but the wettest and most xeric, rocky, or nutrient-poor soils. A comparison of witness tree data from early land surveys and data on modern-day forest composition revealed a drastic decline in white oak throughout the eastern forest. By contrast, other dominant oaks, such as red oak (Quercus rubra) and chestnut oak (Quercus prinus), often exhibited higher frequency in recent studies than in surveys of the original forest. The frequency of red oak witness trees before European settlement was quite low, generally under 5% in most forests. Red oaks distribution was apparently limited by a lower tolerance to fire and drought and a greater dependence on catastrophic disturbances than that of white oak. During the late 19th and early 20th centuries, much of the eastern forest was decimated by land clearing, extensive clear-cutting, catastrophic fires, chestnut blight, and then fire suppression and intensive deer browsing. These activities had the greatest negative impact on the highly valued white oak, while promoting the expansion of red oak and chestnut oak. More recently, however, recruitment of all the dominant upland oaks has been limited on all but the most xeric sites. Thus, the dynamic equilibrium in the ecology of upland oaks that existed for thousands of years has been destroyed in the few centuries following European settlement.

Variation in radial growth responses to drought among species, site, and canopy strata

Abstract Radial growth responses to drought were examined in the tree-ring records of six species growing within two locations of differing land-use history and soil moisture characteristics, and in overstory and understory canopy positions in northern Virginia. Tree species experienced differential ring-width reductions during or immediately following four severe drought periods occurring from 1930 to 1965 and were influenced by climatic variables including annual and summer temperatures, annual precipitation, and annual Palmer Drought Severity Index. Relative growth comparisons averaged across species before and after drought years indicated that understory trees on dry-mesic sites grew 11% faster after drought compared to pre-drought rates while mesic site trees in both canopy positions grew approximately 4% slower. Superposed epoch analysis indicated that Liriodendron tulipifera growing on mesic sites experienced greater ring-width reductions associated with drought than co-occurring, more drought-tolerant Quercus alba and Q. velutina. On dry-mesic sites, L. tulipifera also experienced greatly reduced growth as a result of drought but exhibited significant growth increases following individual drought events. Quercus alba was the only species that exhibited a consistent, significant ring-width decrease associated with all droughts on dry-mesic sites. In contrast, Pinus virginiana was least impacted by drought on dry-mesic sites but was much more impacted by drought on mesic sites, indicating a drought×site interaction for this species. Overstory Carya glabra and Q. alba experienced larger growth decreases during drought on dry-mesic versus mesic sites. Understory tree growth reductions did not differ between site types but were often significantly larger than overstory responses of the same species on mesic sites. Following drought, most trees exhibited growth reductions lasting 2–3 years, although several species experienced reductions lasting up to 6 years. The results of this study suggest that tree rings represent an important long-term proxy for leaf-level ecophysiological measurements of growth responses to drought periods.

Proxy caching that estimates page load delays

Abstract Do users wait less if proxy caches incorporate estimates of the current network conditions into document replacement algorithms? To answer this, we explore two new caching algorithms: (1) keep in the cache documents that take the longest to retrieve; and (2) use a hybrid of several factors, trying to keep in the cache documents from servers that take a long time to connect to, that must be loaded over the slowest Internet links, that have been referenced the most frequently, and that are small. The algorithms work by estimating the Web page download delays or proxy-to-Web server bandwidth using recent page fetches. The new algorithms are compared to the best three existing policies—LRU, LFU, and SIZE—using three measures-user response time and ability to minimize Web server loads and network bandwidth consumed—on workloads from Virginia Tech and Boston University.

Historical variation in fire, oak recruitment, and post-logging accelerated succession in central Pennsylvania

ABRAMS, M. D. AND G. J. NOWACKI (School of Forest Resources, Ferguson Building, Pennsylvania State University, University Park, PA 16802). Historical variation in fire, oak recruitment, and post-logging accelerated succession in central Pennsylvania. Bull. Torrey Bot. Club 119: 19-28. 1992.-Composition, structure and radial growth patterns were studied in relatively undisturbed, mature mixed-oak (Quercus), valley floor forests and in similar forests extensively logged between 1936-1946 in central Pennsylvania. These data were analyzed in relation to presettlement forest composition and historical fire records to investigate temporal variation in Quercus recruitment versus accelerated succession of more shade tolerant species following logging. Presettlement valley floor forests in the study area were dominated by Quercus alba and Pinus strobus. Recurring logging and fire between 1780-1900 associated with charcoal iron furnace activity increased Quercus and decreased Pinus dominance in second-growth forests established during that period. Between 1908-1989 the total area burned by wildfire throughout Pennsylvania decreased by >99% (from >400,000 ha to <3500 ha per year). The decreased influence of logging and fire this century facilitated recruitment of later successional Acer and Prunus species in Quercus forest understories. Logging of forests in this condition rapidly accelerated the rate of obtaining canopy dominance for A. rubrum, A. saccharum and P. serotina in area forests. This form of disturbance-mediated accelerated succession should be anticipated in a wide variety of forest types with an overstory dominated by early successional species and an understory comprised mainly of later successional species.