Showing papers in "Advances in Ecological Research in 1997"

Age-Related Decline in Forest Productivity: Pattern and Process

Abstract: Publisher Summary This chapter reviews the evidence for the pattern of growth decline with age and discusses the evidence for the mechanisms that may be responsible. It begins with an overview of the proposed mechanisms. The chapter also presents a framework for understanding the changes in stand productivity with age, because many of the proposed mechanisms are linked and affect carbon allocation. The available information on the importance of various mechanisms behind growth decline, in the context of the stand carbon cycle is presented. The common patterns of a decline in stand leaf area and leaf photosynthetic capacity suggest a new model of carbon balance with stand development. In this model, photosynthesis and above-ground dry-matter production increase with canopy development. After the forest reaches a maximum leaf area, photosynthesis and dry-matter production decline as leaf area, photosynthetic capacity, and photosynthesis also decline. The model assumes that allocation to respiration and below ground to roots and symbionts is a constant fraction of assimilation over the life of a forest stand.

The Ecology of Root Lifespan

Abstract: Publisher Summary This chapter discusses the competing theories of root lifespan and reviews the evidence available to support them. New methods of root observation and analysis produce data appropriate to testing these theories, but the results till date are few and often conflicting. Tentative generalizations include a suggestion that small diameter roots with low tissue density tend to have short lifespan. Root lifespan appears to be longest in cold environments, but data are lacking for tropical species. There is a strong seasonal variation in lifespan, with roots produced in the fall surviving longest, at least in temperate climates. Species differences are difficult to quantify because of seasonal and interannual variation, but root lifespan of deciduous fruit crops seems to be shorter than those of temperate deciduous forest trees or citrus, a broadleaf evergreen. The current model of root efficiency omits some important factors that may exert control over root lifespan. Fine roots have other functions in addition to absorption, including transport of water and nutrients. Furthermore, seasonality of climate and the need for carbon and nutrient storage could constrain the root lifespan that optimizes plant fitness to differ from that which maximizes root efficiency.

Faunal Activities and Soil Processes: Adaptive Strategies That Determine Ecosystem Function

Abstract: Publisher Summary This chapter helps in understanding the role of soil fauna in soil function. Soils host an extremely diverse community of invertebrates that differ in their adaptive strategies and hence in the functions they fulfill in soils. The chapter illustrates the functional classification of soil organisms based on their adaptive strategies. Three major groups of invertebrates may be defined based on the nature of the relationship that they develop with soil microflora. The microfauna comprise invertebrates of less than 0.2 mm on an average, makes use of micro-organisms mainly through predation in micro-foodweb systems. Mesofauna and large arthropods comprise the group of litter transformers. Earthworms, termites and, to a lesser extent, ants, are “ecosystem engineers” that create diverse organo-mineral structures. It is suggested that the three systems defined, operate at nested scales of time and space and have decreasing overall effects on the determination of soil function in the order micro-foodwebs

Terrestrial plant ecology and 15N natural abundance : The present limits to interpretation for uncultivated systems with original data from a Scottish old field

Abstract: Publisher Summary This chapter discusses the present limits to interpretation for uncultivated systems with original data from Scottish old fields in reference to terrestrial plant ecology and I5N natural abundance. The use of I5N natural abundance (δ15N) is a powerful tool for obtaining insights through δI5N pattern analysis, and for deriving new questions to be tested, but it is not a reliable tracer of nitrogen fluxes in soils or in plants growing in soils. To interpret δ15N in terms of sources and sinks, one must know the source δ15N values, be able to quantify the fractionations occurring between source and sink, and frequently be able to estimate nitrogen fluxes. Given the widespread absence of such knowledge, question arises whether much of the published literature on δ15N for terrestrial systems can have been interpreted correctly. Moreover, the techniques of δ15N analyses are ideally suited to research in remote locations but not the interpretations. Nitrogen isotope interpretations are complicated and depend on a thorough knowledge of the site under study and on controlled experimentation. The isotopic values do not, themselves, supply the answers. The answers are supplied through examining patterns of isotopic signatures in the context of what is already known about the system's ecology and through testing hypotheses which arise from analysis of the δ15N patterns.

Nocturnal Insect Migration: Effects of Local Winds

Abstract: Publisher Summary This chapter discusses the effects of local wind systems on insect migration at night. Many insect species undertake migratory flights at night. Such flights are usually dominated by the wind because the insects climb to heights where the wind speed exceeds their own flying speed through the air. Consequently, the distance moved is essentially the product of wind speed and duration of flight. The results throw light on the occurrence of clouds of flying adults and on the clumped distribution of populations of migrants on the ground. Much of the evidence comes from field studies of a few pest species, but it is likely that many other species are similarly affected. Several types of wind system are considered: low-level jet streams, coastal winds, rainstorm outflows, down-slope and down-valley winds, and a variety of local winds caused by barrier effects of mountains. Furthermore, field studies need to put emphasis on atmospheric behavior through supplementary observations of winds and temperatures in the lowest kilometer of the atmosphere. This chapter discusses coastal winds, bores and solitary waves, rainstorm outflows, and mountain winds: drainage winds and barrier winds in reference to the nocturnal insects.