Pugnaire F.I. Valladares F. Functional Plant Ecology

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but Evans and Ashmore (1992) showed that filtration affected the forb component of a

seminatural grassland in a year when O

concentrations were relatively high (17% of days

with maximum hourly average >60 ppb). In contrast, there is much evidence that ambient

levels of SO

are high enough to change community structure in some regions, and several

excellent reviews are available on the subject (Kozlowski 1985, Bell et al. 1991, Armentano

and Bennett 1992, Taylor and Pitelka 1992).

CONCLUSIONS

In this chapter, we have attempted to evaluate some of the features that underlie the observed

variation in the resistance of plants to the most common gaseous air pollutants. However,

many key questions remain to be answered. Comparative analysis at the molecular level is

beginning to reveal evidence of similar defense-related responses to a range of stresses, and

the use of mutants and genetic transformation techniques, already of importance in our

improved understanding of specific genetic sequences, should eventually allow the dissection

of the molecular mechanisms underlying resistance, as well as yielding approaches that may

be used to manipulate the resistance of crop plants.

We have chosen to focus on O

since this is one of the most potent gases to which plants

are regularly exposed in the filed, and it is likely to constitute a continuing threat to vegetation

for the foreseeable future. Although there is growing literature documenting the effects of this

and other gaseous pollutants on wild species, most studies have measured a small number of

response variables, commonly under controlled or semicontrolled conditions, with little

regard to their ecological significance. This work demonstrates which aspects of growth and

reproduction are sensitive to pollutants and the potential range of responses that exist both

within and between species, but it is of limited value in assessing how pollutants affect the

fitness of plants in communities in the field where a range of additional factors must be taken

into consideration. The few studies that have been performed on simple plant communities in

the filed indicate that there may be winners and losers at the individual, population, and

community levels. These differential responses may result in the most fit genotypes predom-

inating in future generations in polluted regions, and there can be changes in the genetic

structure of populations and dominance relationships within individual communities. In

many cases, however, it is difficult to ascribe directional selection to individual pollutants,

as other factors of the physical and biological environments interact in a variety of ways that

may collectively influence the direction and extent of selection. Even in circumstances in

which the pollutant may not be the principal factor underlying evolutionary changes in

population structure, the existence of resistant populations in polluted regions is of consid-

erable ecological significance. A major challenge is to devise experiments in the future in

which the impacts of gaseous pollutants on natural ecosystems can be assessed; to use the

words of Smith (1990): ‘‘this will require the marriage—or at least the co-habitation—of

specialists in different disciplines (e.g. molecular biologists, biochemists, ecologists & physio-

logists), at present not the most congenial of bed-fellows.’’

ACKNOWLEDGMENTS

The authors acknowledge support from the Royal Society, the Spanish Ministry of Science

and Culture, and Spanish Interministerial Commission for Science and Technology, the

Natural Environment Research Council, the European Union, the Swales Foundation (admi-

nistered by Newcastle University), and the UK Overseas Development Agency. The chapter

was written during J.B.’s tenure as a Royal Society Research Fellow.

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626 Functional Plant Ecology

Canopy Photosynthesis

Modeling

Wolfram Beyschlag and Ronald J. Ryel

CONTENTS

Introduction ....................................................................................................................... 628

Model Overview and Perspectives ...................................................................................... 628

Single-Leaf Photosynthesis and Conductance Models ...................................................628

Whole-Plant=Canopy Models......................................................................................... 629

Uniform Monotypic Plant Stands............................................................................... 630

Uniform Multispecies Plant Canopies ........................................................................ 631

Inhomogeneous Canopies ........................................................................................... 631

Big-Leaf Models.......................................................................................................... 632

Examples......................................................................................................................... 633

Roadside Grasses ........................................................................................................633

Effects of Needle Loss on Spruce Photosynthesis....................................................... 634

Future Directions ........................................................................................................... 634

Model Development ........................................................................................................... 636

Single-Leaf Photosynthesis ........................................................................................ 636

Mechanistic ................................................................................................................. 636

Empirical..................................................................................................................... 638

Single-Leaf Photosynthesis ........................................................................................ 638

Mechanistic ................................................................................................................. 638

Empirical..................................................................................................................... 639

Stomatal Conductance.................................................................................................... 639

Coupled Models.......................................................................................................... 639

Uncoupled Models...................................................................................................... 640

Whole-Plant and Canopy Models .................................................................................. 640

Uniform Monotypic Plant Canopies...........................................................................640

Uniform Multispecies Plant Canopies ........................................................................ 642

Inhomogeneous Canopies ........................................................................................... 643

Parameterization............................................................................................................. 643

Model Parameters for Single-Leaf Photosynthesis ..................................................... 644

Canopy Structural Parameters.................................................................................... 644

Model Validation............................................................................................................644

Acknowledgments .............................................................................................................. 647

References .......................................................................................................................... 647

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627

INTRODUCTION

Photosynthesis models are an important development for estimating gas-flux rates of plants.

These models have been used to estimate fluxes from the level of the single leaf to community

carbon fluxes across the globe. Questions addressed with photosynthesis models involve

differences in carbon exchange among plant communities, plant community responses to

climate change, and basic ecological concepts concerning resource acquisition, competition

for light, and effects of stress.

Primary productivity of a whole plant or canopy is the cumulative carbon gain from all

photosynthetically active organs. Because of differences in age, physiology, and exposure to

microclimatic conditions, organs are not equally productive, and measurements of individual

elements generally do not represent the behavior of the whole plant or canopy. However, by

accounting for structural and microclimatic differences between foliage elements, photosyn-

thesis models can provide integrated estimates of photosynthesis and water vapor exchange

for the whole canopy.

In this chapter, a class of photosynthesis models is presented that scale up from single-leaf

estimates to the whole plant and entire canopies. A general description, perspectives on use,

and recent developments are presented as well as relevant details for model development.

Model formulations described here range from simple for homogeneous single-species can-

opies, to complex for diverse multispecies canopies, and are suitable for addressing a range of

ecological questions. This presentation is broken into two parts; the Section ‘‘Model Overview

and Perspectives’’ is designed to introduce the reader to the general approach used in con-

structing canopy photosynthesis models, whereas the Section ‘‘Model Development’’ contains

sufficient mathematical detail to aid in model use and development.

MODEL OVERVIEW AND PERSPECTIVES

Whole-plant and canopy photosynthesis models contain integrated submodels for (1) single-

leaf photosynthesis and (2) the linkage between foliage elements and the physical environment

(Figure 21.1). Whole-plant or canopy photosynthesis is modeled by dividing the canopy

structure into subregions of similar foliage characteristics (density, orientation, and physio-

logical properties). Microclimatic conditions affecting foliage elements within these subunits

are determined at regular intervals, and photosynthesis rates are then calculated for defined

foliage classes within each subunit. Whole-plant or canopy rates are calculated as sums of

rates for canopy subunits weighted by foliage density. Canopy photosynthesis models often

include three general assumptions: (1) the photosynthetic activity of a plant organ relates to

its maximum photosynthetic capacity, and reflects age, phenology, acclimation, and physio-

logical condition of the plant; (2) the photosynthetic rate of individual foliage elements

depends on interactions with microclimatic conditions (e.g., intercepted radiation, leaf temp-

erature, CO

partial pressure within the leaf ); and (3) microclimatic conditions within plant

canopies result from interactions of macroclimate above the canopy, structural and physical

properties of canopy elements, and relative position of foliage elements. An additional

assumption is often included: (4) the photosynthesis rate of each foliage element is independent

of the rates of the other elements.

SINGLE-LEAF PHOTOSYNTHESIS AND CONDUCTANCE MODELS

Single-leaf photosynthesis is the basic unit of canopy photosynthesis models, and carbon

assimilation for this basic unit depends on photosynthetic characteristics and surrounding

microclimatic conditions. These models are of two basic types: (1) empirical models,

where mathematical relationships are formulated between measured variables and (2) mech-

anistic models, where mathematical formulations are more closely linked to the physiology of

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628 Functional Plant Ecology

photosynthesis. Since mechanistic models are more closely linked to physiological processes,

they are more suitable for addressing how changes in environmental conditions affect photosyn-

thesis (e.g., Harley et al. 1992, Tenhunen et al. 1994). Because parameterization is often simpler,

however, empirical models are often used when questions concern canopy structure or light

competition (e.g., Ryel et al. 1990, 1994). Models exist for both C

and C

metabolic pathways

of single-leaf photosynthesis. Development details for mechanistic and empirical models for

single-leaf C

and C

photosynthesis are contained in the Section ‘‘Model Development’’.

Important companions to models for single-leaf photosynthesis are models for stomatal

conductance. Since CO

is supplied to the mesophyll through the stomata, and water vapor

exits the leaf through these same pores, stomatal diffusive conductance is an essential

component in modeling single-leaf photosynthesis. Although many factors affect stomatal

conductance (including irradiance, temperature, air humidity, CO

partial pressure, plant

water status, and endogenic rhythms), controlling mechanisms of stomatal regulation are not

fully understood. As a result, truly mechanistic models do not exist for stomatal conductance.

The existing empirical models can be divided into uncoupled models where stomatal

conductance is calculated as a function of external environmental conditions, and coupled

models where the rate of leaf photosynthesis is linked to conductance. Models for stomatal

conductance are usually linked with formulations that account for foliage boundary-layer

diffusional effects (see Gates 1980, Nobel 1983, Schuepp 1993).

WHOLE-PLANT=CANOPY MODELS

Whole-plant=canopy photosynthesis is accomplished by simultaneously calculating single-

leaf fluxes at multiple locations within the plant canopy. Since microclimatic differences in

Mesoclimatic conditions

above the canopy

(radiation fluxes, Temperature, air

humidity etc.)

Structural characteristics

of the canopy

(LAI, SAI, leaf and stem angles, spatial

distribution of canopy elements)

Physical laws

Spherical geometry

Microclimatic conditions

within canopy subunits

Species-specific

physiological

characteristics

Physical characteristics

of the canopy elements

(photosynthetic capacity etc.)

(absorbance reflectance, translucence,

heat capacity etc.)

Summing up over

all canopy subunits

Canopy photosynthesis

Single leaf photosynthesis

model

FIGURE 21.1 Flow diagram of a typical canopy photosynthesis model. Input parameters are in oval

and hexagonal boxes, the latter representing parameters provided for each canopy subsection. Variables

calculated by the model are shown within rectangular boxes.

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Canopy Photosynthesis Modeling 629