M. H. Unsworth

Resource Capture by Crops.

Interplant competition for capture of the essential resources for plant growth i.e. light, water and nutrients, strongly affects the performance of natural, semi-natural and agricultural ecosystems. Ecologists have studied competition to understand the diversity and stability of plant communities, succession patterns of vegetation, and to help define management strategies for semi-natural ecosystems. Agroecologists have studied competitive phenomena to optimize plant densities of crops, to optimize intercropping systems and to quantify crop-weed interactions to improve weed management systems with minimum herbicide inputs. Similar approaches have been used to study interplant competition in natural and disturbed systems. However, because of the complex nature of interplant competition, it has taken a long time to develop generalized concepts and theories. The first systematic approaches for studying competitive phenomena were developed in the 1960s. For monocultures, much of our current understanding is based on the work of a Japanese group (e.g. Shinozaki and Kira, 1956) whereas de Wit (1960) developed the first systematic approach to study competition in mixtures. He introduced an experimental design (the replacement series in which the mixing ratio varies, but total density remains constant) with a model to analyse the results. These approaches were based on a hyperbolic relationship between plant density and yield, and have been used extensively in agricultural and ecological sciences to study competition between plants, plant population dynamics, and component contributions of intercropping systems (see reviews by Trenbath, 1976; Harper, 1977; Radosevich and Holt, 1984; Grace and Tilman, 1990). Recently, several papers discussed the disadvantages and pitfalls of the replacement series approach, such as its total density dependence ( cf. Connolly, 1986; Taylor and Aarssen, 1989). Only in the early 1980s, approaches were developed to describe competition over a range of population densities with varying mixing ratios and at a range of total densities, generally also based on the hyperbolic yield-density relationship (Suehiro and Ogawa, 1980; Wright, 1981; Spitters, 1983a, b; Cousens, 1985; Spitters, Kropff and de Groot, 1989). Similar approaches have been developed using the neighbourhood approach, in which the number of neighbours of an individual plant in a predefined area is related to the weight of the central plant (Silander and Pacala, 1985; Firbank and Watkinson,

Review and synthesis

Studies of trace gases in the atmosphere, their sources, sinks, and mechanisms of transport, have developed rapidly in the last few decades. This has been driven partly by increasing recognition that particular gases are associated with problems such as acidification, eutrophication, and global warming, but also by the developing enthusiasm for multidisciplinary research in which scientists from many disciplines collaborate to explore biological, geochemical, and atmospheric cycles and to understand how human action disturbs such cycles. Major international programs such as the International Biological Program (IBP) and the International Geosphere-Biosphere Program (IGBP) have been very influential in generating and encouraging this new way of working. To answer these complex environmental research questions, scientists have needed to develop new field instrumentation, or at least to modify instruments normally used in the laboratory. Equally, mathematical simulation models are increasingly being used at scales ranging from cellular to global to allow complex computations that would have been unthinkable even a decade ago. The papers in this proceedings illustrate some of the exciting developments taking place in the study of the exchange of trace gases between the atmosphere and the land. They describe new understanding of the processes in soils, plants and the atmosphere that control gases important in the carbon and nitrogen cycles, they summarize new techniques and instrumentation that allow field studies at scales ranging from soil grains to landscapes, and they present results of mathematical models that allow us to explore consequences of global changes that may yet come.

Acid deposition at high elevation sites

There is no shortage of general books on the subject of acid rain, or of symposium proceedings reviewing work ranging from atmospheric chemistry and deposition processes to freshwater acidification and effects on vegetation. In contrast, the collection of papers from this Workshop is focussed on a much smaller subject, the processes of acid deposition at high altitude sites. Interest in deposition at high elevation sites comes largely from observed vertical gradients in the degree of forest damage at sites in the Federal Republic of Germany and the eastern United States. These gradients show that damage to Norway spruce and fir increases with altitude at sites in Bavaria and the Black Forest, and that Red spruce are declining at high elevation sites in the Appalachian Mountains. With the large scale of scientific interest in forest decline, cany research groups, during the last five years, have been examining atmospheric chemistry, deposition processes, and effects on vegetation and soils at upland sites. In particular there have been many recent studies of cloud and precipitation chemistry, which show much larger concentrations of all ions in cloud water than in rain or snow. These studies have also shown that processes of wet and dry deposition and also the chemistry of the air at hill tops are modified strongly by orographic effects.

Perspectives on aerosol deposition to natural surfaces: interactions between aerosol residence times, removal processes, the biosphere and global environmental change

Abstract This paper summarizes the state-of-the-art and the research needs in the areas of aerosol residence-time assessments, deposition modelling, and understanding of aerosols in biogeochemistry. Research needs are emphasized from a systems perspective of global environmental change. Although fundamental quantitative knowledge is lacking, some qualitative linkages between source strengths, residence times, removal processes and the biosphere can be identified. It turns out that not only are the respective areas as such challenged by new problems, superimposed on the fairly well established conventional ones, but these areas also face mutually operating sets of feedbacks between residence times and sink/source characteristics of ecological systems subject to stress. To evaluate the sensitivity of these feedback loops, and to assess whether they are negative (stabilizing interactions) or positive (destabilizing) are important and potentially urgent tasks. Such studies should focus on a common goal, such as developing empirical and theoretical understanding of aerosol resuspension, transport and deposition for application in large-scale circulation models. A basic link in such advancements is that structural change of vegetated surfaces of the biosphere caused by deposition itself, and by changing land-use and climate, be understood and assessed.

USING NOCTURNAL COLD AIR DRAINAGE FLOW TO MONITOR ECOSYSTEM PROCESSES IN COMPLEX TERRAIN

This paper presents initial investigations of a new approach to monitor ecosystem processes in complex terrain on large scales. Metabolic processes in mountainous ecosystems are poorly represented in current ecosystem monitoring campaigns because the methods used for monitoring metabolism at the ecosystem scale (e.g., eddy covariance) require flat study sites. Our goal was to investigate the potential for using nocturnal down-valley winds (cold air drainage) for monitoring ecosystem processes in mountainous terrain from two perspectives: measurements of the isotopic composition of ecosystem-respired CO2 (delta13C(ER)) and estimates of fluxes of CO2 transported in the drainage flow. To test if this approach is plausible, we monitored the wind patterns, CO2 concentrations, and the carbon isotopic composition of the air as it exited the base of a young (approximately 40 yr-old) and an old (>450 yr-old) steeply sided Douglas-fir watershed. Nocturnal cold air drainage within these watersheds was strong, deep, and occurred on more than 80% of summer nights. The depth of cold air drainage rapidly increased to tower height or greater when the net radiation at the top of the tower approached zero. The carbon isotope composition of CO2 in the drainage system holds promise as an indicator of variation in basin-scale physiological processes. Although there was little vertical variation in CO2 concentration at any point in time, we found that the range of CO2 concentration over a single evening was sufficient to estimate delta 13C(ER) from Keeling plot analyses. The seasonal variation in delta 13C(ER) followed expected trends: during the summer dry season delta 13C(ER) became less negative (more enriched in 13C), but once rain returned in the fall, delta 13C(ER) decreased. However, we found no correlation between recent weather (e.g., vapor pressure deficit) and delta 13C(ER) either concurrently or with up to a one-week lag. Preliminary estimates suggest that the nocturnal CO2 flux advecting past the 28-m tower is a rather small fraction (<20%) of the watershed-scale respiration. This study demonstrates that monitoring the isotopic composition and CO2 concentration of cold air drainage at the base of a watershed provides a new tool for quantifying ecosystem metabolism in mountainous ecosystems on the basin scale.

Wet, occult and dry deposition of pollutants on forests

Abstract Mechanisms by which air pollutants are transferred to forests by wet and dry deposition and in wind-driven cloud water (occult deposition) are reviewed. New methods of making deposition estimates are described. In upland forests, orographic cloud can enhance the amount and ionic concentration of precipitation so that wet deposition may be 4–5 times larger than in nearby valleys. Recent work supports the use of a simple aerodynamic relation to estimate rates of occult deposition. Occult deposition rates to forests in identical weather conditions may vary by an order of magnitude depending on forest structure. Evaporation of deposited water may cause the solute concentration of water on foliage to be substantially larger than in cloud. Dry deposition of HNO 3 and HCl is controlled by aerodynamic properties of the canopy. Several kg N ha −1 year −1 are deposited to forests by this path. Transfer of SO 2 , H 2 O 2 and O 3 is restricted by surface and internal resistances of foliage. Controls of dry deposition of ammonia and nitrogen oxides and of particles are not well known. A canopy transfer model “Maestro” was modified to include gaseous pollutants, and agrees well with measured SO 2 fluxes to a forest. Estimates of wet, occult and dry deposition on Keilder Forest, England, and Whitetop Mountain, U.S.A., show substantial differences in occult deposition which are only partly explained by differences in cloud frequency. Regional cloud chemistry probably differs between the sites, and different models of occult deposition also disagree.

Simulation of Water and Energy Fluxes in an Old-Growth Seasonal Temperate Rain Forest Using the Simultaneous Heat and Water (SHAW) Model

In the Pacific Northwest (PNW), concern about the impacts of climate and land cover change on water resources and flood-generating processes emphasizes the need for a mechanistic understanding of the interactions between forest canopies and hydrologic processes. Detailed measurements during the 1999 and 2000 hydrologic years were used to modify the Simultaneous Heat and Water (SHAW) model for application in forested systems. Major changes to the model include improved representation of rainfall interception and stomatal conductance dynamics. The model was developed for the 1999 hydrologic year and tested for the 2000 hydrologic year without modification of the site parameters. The model effectively simulated throughfall, soil water content profiles, and shallow soil temperatures for both years. The largest discrepancies between soil moisture and temperature were observed during periods of discontinuous snow cover due to spatial variability that was not explicitly simulated by the model. Soil warming at bare locations was delayed until most of the snow cover ablated because of the large heat sink associated with the residual snow patches. During the summer, simulated transpiration decreased from a maximum monthly mean of 2.2 mm day21 in July to 1.3 mm day21 in September as a result of decreasing soil moisture and declining net radiation. The results indicate that a relatively simple representation of the vegetation canopy can accurately simulate seasonal hydrologic fluxes in this environment, except during periods of discontinuous snow cover.

Effects of heat balance stem-flow gauges and associated silicone compound on ash trees

Abstract Stem-flow gauges were used to estimate water use by young ash trees ( Fraxinus excelsior L.) growing in the field at the University of Nottingham, Sutton Bonington Campus, Loughborough. In 1992 and 1993 the trees were exposed to episodes of elevated ozone concentration or to clean air in open-top chambers. Trunk constriction was observed on trees fitted with gauges in 1992, resulting in impaired flushing and growth in the spring of 1993. Despite precautions, constriction was again observed in 1993. Also in 1993 an experiment was conducted to assess the effect on ash trees of the silicone compound used with gauges to enhance performance and aid fitting to the trunks. Trunk constriction was observed when the compound was applied to the trunks without the use of gauges or any other physical restriction to radial growth. Data which demonstrate the degree of constriction are presented.

The absorption and evaporation of water vapor by epiphytes in an old‐growth Douglas‐fir forest during the seasonal summer dry season: Implications for the canopy energy budget

Our goal was to determine how epiphytic lichens and bryophytes affect canopy latent heat fluxes in an old-growth Douglas-fir forest when the canopy was dry. The epiphyte water content (WCe expressed as a percent of dry weight) of representative epiphytic foliose lichens, fruticose lichens and bryophytes was measured in the laboratory after 1 to12 h of exposure at five different values of vapor pressure deficit (VPD). After 12 h of exposure, WCe increased 5-6 fold as VPD decreased from 1849 to 132 Pa. In addition, we measured WCe in the field using strain gauges. These field measurements were used to calibrate the models described below. Two models were created to estimate the potential latent heat flux from epiphytes at the canopy scale (LEe). The first model combined measured total biomass of epiphytes with a model that estimated the laboratory determined VPD-dependent changes in WCe of the lichens/bryophytes (VPD method). The second model estimated LEe by scaling the change in WCe of epiphyte-laden branches that were continuously monitored in situ in the canopy by a strain gauge (SG method). Both methods showed a strong diurnal trend in LEe when VPD was less than 645 Pa. Prior to sunrise the epiphytes absorbed water, corresponding to a latent heat flux of 5 to 15 W m-2 per unit ground area, whereas after sunrise, the epiphytes lost water at a rate of -10 to -20 W m-2. For short periods, epiphytes may contribute a significant portion of the latent heat flux from Douglas-fir forests. This article is protected by copyright. All rights reserved.

TLM; a technique with application in the numerical solution of diffusion problems.

Abstract Transmission Line Modelling (TLM) is a technique which allows the numerical solution, for voltage or current, of networks of transmission lines. The basic equation describing the behaviour of such networks in space and time is an attenuating wave equation. Under appropriate conditions this equation simplifies to the standard diffusion equation. Therefore TLM can be applied to any system where diffusion is to be simulated. The technique has advantages over the finite-element type of approach as it is simple to apply and robust in operation. In this paper the basic principles of the technique are outlined, and some examples of TLM presented.