Elgene O. Box

Accuracy of the AVHRR vegetation index as a predictor of biomass, primary productivity and net CO2 flux

The Normalized Difference Vegetation Index (NDVI) or ‘greenness index’, based on the Advanced Very High Resolution Radiometer (AVHRR) aboard the NOAA-7 satellite, has been widely interpreted as a measure of regional to global vegetation patterns. This study provides the first rigorous, quantitative evaluation of global relationships between the NDVI and geographically representative vegetation data-bases, including field metabolic measurements and carbon-balance results from global simulation models. Geographic reliability of the NDVI is judged by comparing NDVI values for different surface types with a general global trend and by statistical analysis of relationships to biomass amounts, net and gross primary productivity, and actual evapotranspiration. NDVI data appear to be relatively reliable predictors of primary productivity except in areas of complex terrain, for seasonal values at high latitudes, and in extreme deserts. The strength of the NDVI-productivity relationship seems comparable to that of earlier climate-based productivity models. Little consistent relationship was found, across different vegetation types, between NDVI and biomass amounts or net biospheric CO2 flux.

Scale Effects in Landscape Studies

Landscape ecology cannot escape dealing with spatial analysis, spatial scale and scale-change effects. A landscape may appear to be heterogeneous at one scale but quite homogeneous at another scale, making spatial scale inherent in definitions of landscape heterogeneity and diversity. In analyzing disturbances and other aspects of landscape change, temporal scale (or temporal resolution of events) may also become an important factor, for similar reasons. The importance of integrating the disparate spatial and temporal scales in landscapes was emphasized by Risser in the preceding chapter.

Plant functional types and climate at the global scale

Abstract. Globally applicable sets of terrestrial plant functional types (PFTs) have been identified as a major need in the development of dynamic global vegetation models for use with global atmospheric models. Global sets of PFTs should represent the worlds most important plant types; characterize them through their functional behavior; and provide complete, geographically representative coverage of the worlds land areas. Three main schools of thought on PFTs have emerged: (1) a physiological focus on internal function, especially at the level of basic metabolism; (2) an ecological focus on function in relation to plant form and environmental conditions; and (3) a geophysical focus on how plant functions affect the adjacent atmosphere. A structural approach based on pheno-physiognomy permits ready identification of relatively familiar, recognizable plant types. Many of the criteria cited by other approaches also are intimately related to structure and its seasonal changes. An earlier global system of structural-functional PFTs and their climatic relations has been improved, including addition of less well-known plant types, and is briefly described. A more strictly ‘functional’ approach is proposed, in which major aspects of plant function, initially metabolism and water balance, are used to classify functional types and suggest how these are constrained by climate. Such functional considerations, however, are closely linked to structural manifestations - but also require other functional criteria for more completely functional classifications. A recent global model of potential natural vegetation types suggested ca. 15 major plant types as necessary to cover the worlds main terrestrial vegetation patterns. These essential types correspond well with a first-cut set of structural types implied by metabolic considerations.

World Patterns and Amounts of Terrestrial Plant Litter Production

Leaf and total plant litter amounts worldwide are estimated by climatic curve-fits and mapped by computer using a world climatic data base. Computer planimetry of the maps produced estimates of yearly terrestrial leaf litter (35.1 x 10 to the power of 9t) and total litter production (54.8 x 10 to the power of 9t). This is about 1.3% and 2.0% respectively, of the estimated world totals of detrital soil organic matter. (Refs. 20).

Predicting physiognomic vegetation types with climate variables

A quantitative terrestrial vegetation model was produced which consists of: (1) A world classification of important terrestrial plant growth forms (life forms); (2) A set of predictive variables representing the main climatic correlates of these forms; and (3) Empirically obtained hypothetical limiting values defining an ecoclimatic envelope for each plant form (relative to the climatic variables). A world classification of important terrestrial plant growth forms (life forms); A set of predictive variables representing the main climatic correlates of these forms; and Empirically obtained hypothetical limiting values defining an ecoclimatic envelope for each plant form (relative to the climatic variables). The model was applied to a world climatic data-base (1 225 sites) in order to substantiate the hypothesized life-form status of the plant types by accurately predicting their actual world distributions. Particular combinations of forms are interpreted as vegetation formation types by reference to growth-form dominance considerations. Model validation was attempted by comparing predicted and actually occurring vegetation at independent sites on all continents. Prediction accuracy of 85% for individual plant types and 50% for vegetation structure (exact combination of actually occurring dominant forms) suggests that general macroclimatic conditions are much more important than any other factors (such as complex specific interactions) in determining general ecological structure on most sites.

Plant functional types in Earth system models: past experiences and future directions for application of dynamic vegetation models in high-latitude ecosystems

BACKGROUND Earth system models describe the physical, chemical and biological processes that govern our global climate. While it is difficult to single out one component as being more important than another in these sophisticated models, terrestrial vegetation is a critical player in the biogeochemical and biophysical dynamics of the Earth system. There is much debate, however, as to how plant diversity and function should be represented in these models. SCOPE Plant functional types (PFTs) have been adopted by modellers to represent broad groupings of plant species that share similar characteristics (e.g. growth form) and roles (e.g. photosynthetic pathway) in ecosystem function. In this review, the PFT concept is traced from its origin in the early 1800s to its current use in regional and global dynamic vegetation models (DVMs). Special attention is given to the representation and parameterization of PFTs and to validation and benchmarking of predicted patterns of vegetation distribution in high-latitude ecosystems. These ecosystems are sensitive to changing climate and thus provide a useful test case for model-based simulations of past, current and future distribution of vegetation. CONCLUSIONS Models that incorporate the PFT concept predict many of the emerging patterns of vegetation change in tundra and boreal forests, given known processes of tree mortality, treeline migration and shrub expansion. However, representation of above- and especially below-ground traits for specific PFTs continues to be problematic. Potential solutions include developing trait databases and replacing fixed parameters for PFTs with formulations based on trait co-variance and empirical trait-environment relationships. Surprisingly, despite being important to land-atmosphere interactions of carbon, water and energy, PFTs such as moss and lichen are largely absent from DVMs. Close collaboration among those involved in modelling with the disciplines of taxonomy, biogeography, ecology and remote sensing will be required if we are to overcome these and other shortcomings.

Factors determining distributions of tree species and plant functional types

Plant functional types have been identified by the International Geosphere Biosphere Program as functionally similar basic plant types, especially trees, as needed for global ecological modeling. Based to some extent on an earlier set of pheno-physiognomically defined plant types, a Global Biome Model was produced but has not satisfied all the desired functional criteria posed by IGBP physiologists and modelers. This paper asks two questions: what are the main environmental factors which limit terrestrial plant types, especially tree types; and how many types of potential vegetation are needed to cover the worlds terrestrial vegetation patterns? Based on the main environmental factors recognized, a model of world potential dominant vegetation types was produced and used to estimate the minimal number of vegetation types needed. The resulting set of 40 potential dominant vegetation types may serve as an initial basis for a structural-functionally based set of world plant functional types.

Predicted Effects of Climatic Change on Distribution of Ecologically Important Native Tree and Shrub Species in Florida

A previously developed plant species-climatic envelope model was evaluated further and used to predict effects of hypothesized climatic change on the potential distribution of 124 native woody plant species in Florida, U.S.A. Twelve scenarios were investigated. These included mean annual temperature increases of 1 °C or 2 °C, achieved either by equal 1 °C or 2 °C increases on a monthly basis throughout the year, or by disproportionately larger seasonal increases in winter and smaller ones in summer. The various temperature increases were then combined with each of several precipitation changes, ranging from +10% to –20%, to produce the final set of scenarios. More detailed analysis involving six of the scenarios and a subset of 28 representative, ecologically important species suggested that (1) large decreases in the Florida range of many temperate species would result if 1 °C warming occurs predominantly in winter or with a 20% decrease in annual precipitation, or (2) if 2 °C warming occurs, with or without decrease in annual precipitation, and regardless of whether there is a uniform monthly warming pattern or one that is higher in winter than in summer. Available information concerning other factors that might also affect climatic-change responses suggests that these large predicted impacts on temperate Florida species may be underestimates. Subtropical Florida species will tend to move north and inland with warming but extensive human assistance may be needed, if they are to realize their newly expanded, potential natural ranges.

Plant types and seasonality of wild-plant foods, Tanzania to Southwestern Africa: Resources for models of the natural environment*

Pursuant to understanding the ecological adaptations of the African hominids, we have analysed some of the characteristics of 419 wild plant species exploited for food by humans in a broad portion of the African summer-rain climatic region running from Tanzania to southwestern Africa. With regard to general growth form: trees, arborescents (other woody forms that can mature as trees), and forbs provide the majority of edible species. Although the grasses are diverse and abundant, most appear not to provide food items for humans. Woody rosette forms (e.g. palms and cycads) and truc shrubs (as opposed to arborescents) are both species-poor. The woody vines and lianas appear to provide even fewer edible species. Stem succulents, parasites, and ferns provide almost no edible species. When scasonality and growth form are both taken into consideration, deciduous trees, perennial forbs, and mushrooms are the most species rich “edible” plant types. Deciduous trees primarily provide edible fruit/seed/pods, particularly during the rainy season, leaves in the rainy season and some underground parts on a more or less year-round basis. Perennial forbs provide flowers, fruit/seed/pods, or leaves in the rainy season, but most (78%) of the edible species also provide underground parts on a more or less year-round basis. The general pattern of species providing edible above-ground parts in the rainy season is reinforced by the mushrooms, evergreen/semi-evergreen trees, and arborescents, as well as the speciespoor plant types. The overall potential dictary pattern is that of a pronounced seasonal change in the quality of plant foods, from a variety of types of food items provided by several different types of plants during the rainy season to mainly fruit/seed/pods and/or underground parts in the dry season. This implies a predictable qualitative as well as quantitative scasonal shift in the wild-plant-food diet of prehistoric humans wherever the tropical summerrain climate imposed itself or remained in effect.

Vegetation analogs and differences in the Northern and Southern Hemispheres: A global comparison

Biogeographic comparisons help to identify similarities, differences, larger contexts, and useful hypotheses. Between the (extra-tropical) Northern and Southern Hemispheres taxonomy differs almost completely, but other useful bases for vegetation comparison include phenophysiognomy, form composition and vegetation architecture, subphysiognomic morphology, environmental limits of plant types, taxonomic richness, and some aspects of function. All major biome types occur in both hemispheres except the boreal forests and analogous montane coniferous forests of the Northern Hemisphere. Some vegetation and plant types appear clearly unique, including Eucalyptus, campo cerrado, and flat-cushions such as Azorella, all of these in the Southern Hemisphere. Different but parallel adaptations to different but analogous conditions include the tall submediterranean forests, temperate rainforests, and continental versus maritime temperate deserts. Climatic limits appear similar in the two hemispheres, but mechanisms for some differences remain unresolved. Per unit land area, the Southern Hemisphere has higher annual net primary productivity than the Northern. Dissimilar analogs continue to suggest hypotheses and research questions.