Landscape Evolution: Landforms, Ecosystems, and Soils

By Jonathan D. Phillips

Landscape Evolution: Landforms, Ecosystems and Soils asks us to think holistically, to look for the interactions between the Earth’s component surface systems, to consider how universal laws and historical and geographical contingency work together, and to ponder the implications of nonlinear dynamics in landscapes, ecosystems, and soils. Development, evolution, landforms, topography, soils, ecosystems, and hydrological systems are inextricably intertwined. While empirical studies increasingly incorporate these interactions, theories and conceptual frameworks addressing landforms, soils, and ecosystems are pursued largely independently. This is partly due to different academic disciplines, traditions, and lexicons involved, and partly due to the disparate time scales sometimes encountered. Landscape Evolution explicitly synthesizes and integrates these theories and threads of inquiry, arguing that all are guided by a general principle of efficiency selection. A key theme is that evolutionary trends are probabilistic, emergent outcomes of efficiency selection rather than purported goal functions. This interdisciplinary reference will be useful for academic and research scientists across the Earth sciences.

• Serves as a primary theoretical resource on landscape evolution, Earth surface system development, and environmental responses to climate and land use change
• Incorporates key ideas on geomorphic, soil, hydrologic, and ecosystem evolution and responses in a single book
• Includes case studies to provide real-world examples of evolving landscapes

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 30, 2021
• ISBN: 9780128232491

Jonathan D. Phillips

Jonathan Phillips is Professor of Earth Surface Systems and University Research Professor in the Department of Geography, University of Kentucky and an affiliate of the "Blue Cats" research team in the Forest Ecology unit of the Sylva Tarouc Institute, Brno, Czech Republic. He previously held faculty positions at East Carolina, Texas A&M, and Arizona State Universities. Phillips has been recognized with distinguished career awards from both the British Society for Geomorphology (Linton Medal) and the Geomorphology Specialty Group of the American Association of Geographers (Marcus Award), as well as several other research awards. He is author of more than 200 refereed research publications across the fields of geomorphology, pedology, hydrology, ecology, environmental science, and quantitative geography.

Book preview

Landscape Evolution

Landforms, Ecosystems, and Soils

Jonathan D. Phillips

Table of Contents

Cover image

Title page

Copyright

Preface

1. An integrated approach to landscape evolution

Introduction

Key concepts

Background: evolutionary pathways in landscapes

Comparison and contrast: integrated approach to landscape evolution versus traditional approaches

2. Earth surface systems as supraorganisms

Introduction: supraorganisms

State factor model

Ecosystem evolution

The upshot

3. Observing landscape evolution

Introduction

Methods and approaches

Indicators

4. It depends on the scale: scale contingency in landscape evolution

Overview of scale issues

Scale (in)dependence

Hierarchies and the vanishing point

Time—real and realized

Scale contingency

5. Historical contingency in landscape evolution

Memory, inheritance, and legacies

Succession and state transitions

Canalization

Extinction and reinforcement of evolutionary pathways

Maturation

Divergence and convergence

Evolutionary pathways and historical trajectories

Summary

6. Attractors and goal functions in landscape evolution

Introduction

Deterministic, single-outcome systems

Multiple path, multiple outcome concepts

Plasticity, degrees of freedom, and constraints

Goal functions and emergence

Multiple causality

Circular reasoning

Consilience?

7. Thresholds, tipping points, and instability

Introduction

Thresholds in the landscape sciences

Lessons from the past

Mode switches and meta-thresholds

Example: a hierarchy of thresholds

Conclusions

8. Selection and landscape evolution

Introduction

Ecosystem selection

Abiotic selection

Preferential flow

Efficiency selection

Selection is local

Why aren't landscapes always becoming more efficient?

Occam's selection

Example: Inner Bluegrass, Kentucky

9. The perfect landscape

The perfect storm

The perfect landscape

Triangles, badasses, and axioms

Evolutionary creativity

Evolution of landscape diversity

Conclusions

10. Landscape evolution and environmental change

Landscape evolution lessons

Transformational, reciprocal, emergent evolution: TREE

A churning urn of burning funk

Landscape evolution stories

Lower Sabine River

Trees and surface drainage in the Šumava Mountains

The last word

Index

Copyright

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Preface

Sometimes it is a useful thought experiment, or a convenient model assumption, to imagine a landscape as an isotropic plane, with some process acting unequally upon it. The featureless landscape then develops features as the inequalities manifest themselves. Literal or figurative peaks and pits, ridges and valleys, and so on emerge.

Now imagine the landscape with multiple processes acting unequally upon it, each creating its own (literal or figurative) topography. Sometimes these processes may be more or less independent; in other cases, they may reinforce or offset each other, and mutually influence each other in various other ways. Then, imagine that each of these processes also influences other aspects of the surface, such as, say, color, texture, and opacity. In each of these respects, the phenomena acting on our thought-experimental surface also may be independent, offsetting, or mutually reinforcing. What a complex, difficult to interpret or predict surface emerges!

But we are not done with our experiment. Figure that the initial surface was not isotropic—like unicorns and mermaids, isotropic surfaces in nature are imaginary! And figure that the hypothetical surface is just one layer in a volume, influenced by the layers above and below, which are comparably complicated.

The abstraction in the first paragraph, or something close to it, is the way we have traditionally looked at landscape evolution, at least partly due to the necessity of isolating and simplifying so as to try and make sense of a very complex phenomenon. Each additional now imagine or figure that brings us closer to the way actual landscapes evolve.

This is the view of landscape evolution that I am trying to present here, in all its glorious complexity—though I naturally fell short of even identifying all the glorious complexity, much less explaining it. But even so, I know what story I want to try to tell. But how to tell it?

Henry David Thoreau wrote: Every man will be a poet if he can; otherwise a philosopher or man of science. This proves the superiority of the poet. Every poet has trembled on the verge of science.

This is a science book. I wish that I could make it more of a book that is also art, literature, or poetry. Those are ways of experiencing and understanding Earth systems and landscapes and their changes that are complementary to science, and would do a better job of conveying why I—and many of my fellow scientists—devote ourselves to the study of Earth systems. I would like to think that I am a scientist with the soul of a poet. Alas, I am only a scientist who wishes he had the soul of a poet. I have tried to season the book with a bit of imagination, humor, and introspection, but my working vocabulary and communication skills are those of the scientist, not the novelist or the poet.

All of this is not so much to tamp down expectations of entertainment, or to prepare the reader for the cold, hard facts of science, the resolute acknowledgment of uncertainty, and the convoluted chains of explanation, interpretation, and speculation that follow. Rather, it is because I wish I could translate, or at least embed, the cold, the hard, the resolute, and the convoluted in some appreciation of the wonder and joy that accompanies them. Many people feel that science, representing nature via mathematics, statistics, data, diagrams, etc., dulls its appreciation. And sadly, I guess it does for many. But for me—and again, for many if not most of my professional peers—it has the opposite effect. For us, science causes wonder and joy to blossom into awe, astonishment, and delight—notwithstanding the pain and despair that sometimes comes with knowing what we humans are doing to our planet.

Arthur C. Clarke wrote that any sufficiently advanced technology is indistinguishable from magic. Riffing on that theme, I once gave a talk proclaiming any sufficiently improbable event is indistinguishable from the miraculous. Some definitions of miracle invoke the divine or supernatural, but I have in mind the definition as: an extremely outstanding or unusual event, thing, or accomplishment. The point is that due to the inescapable, irreducible role of geographical and historical contingency in Earth surface systems, all such systems (landscapes, ecosystems, soils, etc.) are unique in some respects (formal arguments along these lines are summarized in Chapter 9). Thus, the probability of existence of any given state of any given system at a given point in time is infinitesimally low. This exceedingly low probability makes nearly any landscape in some senses extremely outstanding and unusual, and thus a miracle.

Another take on miracles is from Alan Moore's Watchmen: In each human coupling, a thousand million sperm vie for a single egg. Multiply those odds by countless generations, against the odds of your ancestors being alive, meeting, siring this precise son; that exact daughter...until your mother loves a man ...and of that union, of the thousand million children competing for fertilization, it was you, only you...(it's) like turning air to gold... a thermodynamic miracle.

Earth surface systems (mountains, watersheds, forests, sinkholes, or whatever) have certain commonalities and are in part governed by principles that apply everywhere and always. But each also embodies the particular combination of environmental factors of a given location and a unique sequence of events. Like the contingencies affecting whether two people meet and reproduce, going back generation after generation, landscapes are affected by uncountable contingencies—events that did or did not happen; the occurrence and timing of meteor impacts, fires, floods, storms, earthquakes, bison herds, insect swarms, lightning strikes, droughts, landslides, gully erosion, dust deposition, human impacts, etc., etc., etc., over thousands to billions of years.

Knowing people is not just about knowing human biology, physiology, medicine, anthropology, psychology, sociology, and so on. It is about knowing individuals. The same goes for landscapes. Place matters, and history matters, and truly understanding one of them, like understanding a person, requires dealing with them one-on-one. That's the kind of approach to geosciences—integrating laws, place, and history—reflected in this book.

From James Still's novel, River of Earth: These hills are jist dirt waves, washing through eternity. My brethren, they hain't a valley so low but what it will rise agin. They hain't a hill standing so proud but hit'll sink to the low ground o' sorrow. Oh, my children, where air we going on this mighty river of earth, a-borning, begetting, and a-dying --- the living and the dead riding the waters? Where air it sweeping us? ...

When we think about the science of landscape evolution, we often, and for obvious and good reasons, think in terms of how did it get to be this way? or how will it change in the future? These are perfectly natural things to be curious about, and perfectly good starting points for thinking about landscape evolution. Those questions, logical as they may be, unconsciously (at least) privilege the present, as either the endpoint of history up to now or the starting point for things to come. To understand landscape evolution, we need to move past this to recognize that the state of a landscape now, however broadly now is defined, is just a momentary snapshot of constant change—analogous to a still frame from an endless (from the human perspective) film. Or—to a River of Earth.

We know this. But we forget. Because now is what we can directly observe, and the recent past is what we can directly recall. We unavoidably emphasize it. Because we sometimes want to change things from the way they are now or correct historical mistakes, we often valorize some before condition, real or imagined, as the way things ought to be. And that before condition—preindustrial, presprawl, precolonial, etc.—may indeed better by some criteria. But when we seek to restore it, we need to recognize that the way it was (or seems to have been) is not necessarily the way it would be now absent industry, sprawl, human population expansion, or whatever. There exists no single particular right or optimal way for landscape to be; that before state cannot be viewed as what was intended to be there for all time.

The constant, pervasive change represented by the flow of the River of Earth not only has no particular predestination but also no particular pathway. And what happens within an Earth surface system today effects its path tomorrow. The past and present intertwine and melt into the future. Earth and its systems are always becoming. Evolution is contingent, path dependent, and ongoing.

There is no destination; only a journey with many possibilities.

Thoreau again: The most distinct and beautiful statement of any truth (in science) must take at last the mathematical form.

An accomplished cook or a great musician might well thrive on improvisation; making on-the-fly adjustments and decisions. However, if they want to communicate their creative process to someone who was not in the room while they did it, they must also produce some sort of durable record—a recipe, a recording, and musical charts. That is why, though I recognize and even promote the virtues of nonstandardized approaches, I am dealing with formalisms.

Dictionary definitions of formalism are generally something along the lines of rigorous or excessive adherence to prescribed forms. By its own definition, of course, excessive is not a good thing. But rigorous, to a scientist, is. Prescribed forms, in this context, refer to mathematical, logical, and graphical representations. Note that I do not advocate communication only via formalisms, but that formalisms should accompany presentations in other forms (most commonly prose or narrative in my case).

Why are formalisms important? First, they provide an additional, crucial level of testability and falsifiability. Can the facts, evidence, data, and the arguments and interpretations based on them be presented in some rigorous, prescribed form? Second, they are divorced (at least as much as anything human can be) from the cultural and social baggage that often inhibits cross-cultural communication. Mathematical notations, for instance, have universally agreed-upon meanings independent of politics, culture, and time (or at least less dependent). Third, formalisms at least mitigate language barriers. As I write, English has become the lingua franca of science, giving an unearned advantage to those of us for whom it is a first language. For others, some of my clever turns of phrase, cultural references, stylistic nuances, and metaphors may be less effective, or even useless. However, if I accompany those words with a good enough formal statement of my ideas, those ideas will still be accessible. Likewise, many people may be quite adroit, ingenious, and subtle in communicating in their native language, but much more limited in subsequent languages. Formalisms allow them to convey their ideas to monolingual louts such as myself.

So, this book contains some mathematics, some logical arguments, and some axiomatic deductions. Sorry. But not really.

So many people deserve thanks and acknowledgments for this book and the several decades of work leading up to it. So many … and I hope they know it and feel it. But I will not try to list them, as the list would be so long, and my highly and increasingly fallible memory would surely omit some.

Instead I will acknowledge places—landscapes where I have had my conversations with Earth and been honored and delighted to help solve many mysteries, fail to solve many more, and encounter so even more yet to be solved.

I will start with the environments where I did much of the fieldwork in my career. These include the Inner Bluegrass karst region of central Kentucky and the Cumberland Plateau of eastern Kentucky, rivers of the Texas coastal plain (Sabine, Neches, Trinity, Brazos, Guadalupe, and San Antonio), and the Ouachita Mountains in southwestern Arkansas.

Then there are the places where I spent less time, but got out of my humid subtropical frame of reference and experienced much different landscapes, such as the old-growth forests of the Czech Republic, especially in the Sumava Mountains and the outer western Carpathians; the Sonoran Desert of Arizona; the White Desert of the Egyptian Sahara; the tropics of northeastern Queensland, Australia; the North Island, New Zealand.

I should also acknowledge the woods and fields of various places in central and eastern North Carolina, where I spend a good bit of my formative years happily roaming and accidentally learning.

Two places deserve special recognition as my spiritual homes, so to speak. Big Walker Mountain and Hungry Mother State Park in southwestern Virginia is a place where I have always had family connections and have returned repeatedly over my entire life. There are parts of it where I could, and did, and still could, navigate the forest trails in complete darkness, for no other reason than that I could, and it was another way to experience that miraculous (because they all are!) landscape.

The other is the area in and around Croatan National Forest in eastern North Carolina. There I have lived on several different occasions and am returning to as I complete this book. There I have done professional fieldwork for many years. There my children were born, and my grandchildren play. There many hours have been spent traversing the terrain by kayak, canoe, rowboat, foot, and bicycle. There many hours have been spent sitting or lying idly (at least physically) on the forest floor, the riverbank, and the beach. So, thanks to the sand, the mud, and peat. Thanks to the cypress, the tupelo, the beech, the oak, the longleaf, and even, begrudgingly, the loblolly pine. Thanks to the alligator, the great blue heron, the osprey, the bald eagle, and the canine that leaves tracks on the river edge almost every night and might be a red wolf but is probably just a big coyote. Thanks to the ravine swamps, the drowned river valleys, the sand ridges, the marshes, the barrier islands (modern and paleo-), the dunes, and the pocosins.

Finally, thanks to you for at least making a stab at reading this book.

It's been fun.

1: An integrated approach to landscape evolution

Abstract

An approach to landscape and Earth surface system evolution is outlined based on the inseparability of landform, soil, and ecosystem development, versus the traditional semi-independent treatment of geomorphic, ecological, pedological, and hydrological phenomena. Key themes are the coevolution of biotic and abiotic components of the environment; selection whereby more efficient and/or durable structures, forms, and patterns are preferentially formed and preserved; and the interconnected role of laws, place factors, and history. Existing conceptual frameworks for evolution of geomorphic, soil, ecological, and hydrological systems are reviewed and contrasted with the integrated approach.

Keywords

Coevolution; Earth surface systems; Ecology; Geomorphology; Hydrology; Pedology; Selection

Introduction

Landforms, topography, soils, and ecosystems affect, and are affected by, each other. We have long been interested in how Earth's environments change over time, and have long known that these environmental components influence each other. Yet, perhaps because it is intimidating to consider nature in all its complexity, and because of different interests, traditions, and training of scientists, we have generally studied their development over time separately.

Not in total isolation, mind you. Studies of soil formation and evolution, for example, have considered the effects of biota, geology, topography, and climate on soils since the birth of pedology. The ecosystem concept is based on the combination of organisms and their abiotic environment. And changes in, e.g., vegetation cover have figured prominently in studies of geomorphic landscape development. But traditionally, we have paid little attention to reciprocal interactions. That is, while many studied effects of soils on vegetation or of vegetation on soils, for instance, few were concerned with both simultaneously.

As a result, concepts and theories of the evolution of geomorphic, ecological, and soil landscapes have developed along separate lines. While these are not generally incompatible or irreconcilable, they are largely independent while the phenomena they seek to explain are strongly interdependent.

The term evolution is used in several different ways in Earth and environmental sciences. In some cases, it simply refers to development or change over time, usually implying simply that change is systematic or nonrandom. Evolution is also sometimes contrasted with development or other terms (e.g., succession, progression) implying single-path, deterministic development toward a single end-point. In this use, evolution signifies the possibility of multiple pathways and a role for randomness, chance, and path-dependence. A stronger notion of evolution further incorporates the role of selection processes. I use evolution in the latter sense, with terms such as development or succession applying to broader notions of change over time.

In the late 19th and early 20th century, all kinds of intellectual endeavors were strongly influenced by Darwin's evolutionary ideas as laid out in The Origin of the Species. Scientists were either looking for, or interpreting what they had already found, in terms of systematic changes over long spans of time. Osterkamp and Hupp (1995), for example, describe how theories of ecological succession, soil formation and change, and topographic development arose contemporaneously in this intellectual milieu. The main connecting theme was systematic change over time—other key ideas arising from evolutionary theory, such as the roles of selection, chance, and historical contingency, were far less prominent. As we will see, these were worked in later or appeared in theories widely viewed as rivals to Cowles' and Clements' notions of succession, Dokuchaev and Marbut's ideas of progression toward mature climax soils, and Davis' cycle of erosion (more on these below).

No individual in the midst of any historical trend can put it in proper context or identify it as any sort of revolution or transformative event, but they can at least testify as to its effect on their own work. For me, at least, the late 20th and early 21st century have been characterized by widespread recognition of and emphasis on interconnectedness.

Earth and its environments are characterized by dense networks of interactions over space and through time. Thus any changes to any component of the environment inevitably influence others. A principle arose that has been referred to as the First Law of Geography, the First Law of Ecology, and the First Law of Environmental Science (Tobler, 1970; Commoner, 1971):

Everything is connected to everything else.

A corollary: you can't change part of a system.

Strictly speaking, of course, not all things are related in the sense of direct, traceable, causal links. But everything is related to everything else in the sense that Earth surface systems (ESS) are typically characterized by multiple, interrelated components and controls. Straightforward cause: effect relationships are rarely sufficient in Earth system science, even at the pedagogic level. Earth systems are conceptualized and analyzed in terms of maps, webs, matrices, flow charts, multiple equation systems, and other representations of multiple, interconnected, mutually adjusting objects or phenomena. Places and environments are manifestations of multiple, interrelated forcings and controls, and complex histories.

In addition, all things are connected in the sense that everything Earth system science deals with is ultimately wired into the same Earth system and, somewhere along the line, shares a common history. Organisms share common genetic ancestors, and all play a role in the carbon/oxygen cycle, alongside a number of abiotic processes. These interconnections and the coevolution of earth systems are described in unique and insightful ways by Denis Wood (2003). Wood's major message: global change is all part of one big, continuing story, going on for five billion years (and counting).

The interconnectedness of biota, geophysical and geochemical phenomena, landforms, and soils has long been recognized. Milestones include Dokuchaev's (1883/1967) pioneering ideas of soils as the product of the combined influences of geology, climate, topography, and organisms, Vernadsky's (1926/1998) exposition of the biosphere concept and foundational work in biogeochemistry, and the idea of ecosystems (Tansley, 1935). Dokuchaev did not explicitly deal with reciprocal interactions, but did recognize that the factors of soil formation influence, and are influenced by, the other factors as well as the soil itself. The ecosystem notion (and possibly the term) predate Tansley, but his 1935 article introduced the concept to a broad audience.

Until relatively recently, however, studies tended to be one-way in terms of causality—for example, the effects of climate on geomorphic processes or the influence of topography on soil or vegetation distributions—rather than reciprocal interactions. The increasing focus on reciprocity, along with the recognition that biota, soils, landforms, hydrology, and climate are best understood as responding and evolving together, led to the rise of several subdisciplines focusing on these reciprocal interactions. These include biogeomorphology (or ecogeomorophology), which is concerned with the influence of landforms on the distribution and development of plants, animals and microorganisms; and with the influence of plants, animals and microorganisms on earth surface processes and the development of landforms (Viles, 1988). Ecohydrology is at the interface of hydrology and ecology, and according to the mission statement for the journal of that name, emphasizes interactions and associated feedbacks in both space and time between ecological systems and the hydrological cycle. The journal Geobiology describes that subfield as exploring the relationship between life and the Earth's physical and chemical environment. Hydropedology is portrayed as bridging traditional pedology with soil physics and hydrology (Henry Lin, 2003).

Recent decades have also seen the rise of the terms Earth system, climate, ecosystem, and critical zone science. Earth system science explicitly recognizes the dense interconnections of the atmo-, hydro-, litho-, and biospheres, and is particularly concerned with addressing these at very broad—including global—scales and a range of temporal scales, from fluid dynamics to Earth evolution. The term climate science seems to have emerged as an attempt to encompass climatology, paleoclimatology, and atmospheric sciences, and also the study of climate impacts on, and feedbacks with, human and other biophysical systems. Ecosystem science emphasizes that the field transcends ecology. According the mission statement of the journal Ecosystems, the scope of ecosystem science extends from bounded systems such as watersheds to spatially complex landscapes, to the Earth itself, and crosses temporal scales from seconds to millennia. Ecosystem science has strong links to other disciplines, including landscape ecology, global ecology, biogeochemistry, aquatic ecology, soil science, hydrology, ecological economics and conservation biology. In the US National Science Foundation Critical Zone program, the critical zone is defined as Earth's permeable near-surface layer… from the tops of the trees to the bottom of the groundwater (http://criticalzone.org/national/research/the-critical-zone-1national/). The critical zone (the term is in wide use outside the USA, with similar definitions) is an integrated approach to the study of rock, regolith, soil, water, biota, and atmosphere interactions near Earth's surface.

The emergence of these subdisciplines, terms, and concepts (along with more traditional ones such as biogeochemistry and geoecology) is clear evidence of recognition that (1) no component of Earth's system(s) can be fully understood in isolation; (2) ESS are characterized by constant internal and external feedbacks and reciprocal interactions.

Though rarely explicitly expressed, Svensson (2018) argues that biological evolutionary theory has implicitly recognized reciprocal causation for some time. Reciprocal causation (i.e., bidirectional cause-effect relationships) has been included in mathematical models of coevolutionary arms races, dynamics of selection, and eco-evolutionary dynamics.

So here's how you get to the approach taken in this book: Start with a notion of systematic change over time in topography, landforms, soils, ecosystems, and hydrological systems. Then figure in notions of selection, chance, and contingency. Selection is nonrandom, as it preferentially preserves and propagates the fittest, the most efficient, and the most resistant. However, it is also nonrandom—that is, probabilistic—in that the optimal individual elements are not always, invariably, perfectly selected. Chance includes the role of luck, whereby fitter entities may be destroyed and less fit ones perpetuated now and then. It also includes the role of disturbances, mutations, and happenstance, as in a particular combination of factors intersecting at particular times and places. Contingency reflects the fact that present and future conditions depend in part on the past. To this, add in the interconnectedness of environmental systems, whereby organisms, climate, soils, landforms, hydrological and other biogeochemical fluxes and climate (to name a few) affect, and are affected by, each other. Finally, consider that this all occurs in a context of constantly changing boundary conditions, such as solar inputs, bolide impacts, plate tectonics, and human agency. The last may or may not be external to ESSs in every respect, but are certainly influenced by factors generally treated separately, such as economics, politics, and culture.

Put all this together, and you get an approach that seeks to get at the interrelated coevolution of ESS, as illustrated in Fig. 1.1.

Examples

The interactions in ESS occur at time scales from planetary evolution to the rates of operation of physical, chemical, and biological processes. They are most evident, however, in systems that exhibit system-level changes over relatively rapid periods of years to decades.

Figure 1.1 Landscape system.Integrated approach to landscape evolution.

One example is coastal wetlands (e.g., salt marshes) subject to effects of rising sea level. These wetlands are a source of vast economic and esthetic values to humans, and ecosystem services. Coastal wetlands also play important geomorphological roles in sediment storage and buffering inland areas from storm impacts. These low-elevation coastal landforms are profoundly affected by submergence because of relative sea-level rise. The latter term incorporates any land subsidence, compaction, or uplift in combination with eustatic sea-level rise. Sea-level rise below refers to relative rise.

The single most important factor in determining wetland response is the rate of accretionary upbuilding relative to that of sea-level rise. But numerous other hydrological, ecological, and geomorphic factors and interactions are also influential. Applicable to any coastal wetland are general interrelationships among sea-level, hydroperiod (frequency and duration of inundation), sedimentation, vegetation productivity, and wetland surface elevation. In addition, any given coastal landscape is influenced by factors that vary with climate, such as presence or absence of ice or of tropical features such as mangroves or corals, and storm climatology. Wetland response is also affected by the local topographic context (e.g., elevation and slope of adjacent uplands) and hydrographic context (e.g., estuarine or deltaic flow, circulation, and sedimentation patterns; tidal regime; salinity). Biogeographic factors, themselves influenced by characteristics such as hydroperiod, salinity, climate, and wave exposure, also play a role. Wetland plants vary not only in their habitat requirements and preferences, but also in their broader ecological impacts, biogeomorphic effects, and dispersal and regeneration dynamics.

Like other ESS, salt marshes and other coastal wetlands are historically contingent—their condition and state or stage of development is strongly affected by past events and histories of development and disturbance. This includes the rate and variability (and over longer time scales, direction) of past sea-level change, the time period over which the system has been evolving, and storm history (timing, magnitude, frequency, sequence of events).

Relationships between vegetation and sedimentation illustrate the influence of local and regional place factors on general laws. Mineral sediment input effects on plants are general, in that plants are typically unable to tolerate complete burial. Yet wetland plants may be positively influenced by more moderate deposition rates, e.g (Corenblit et al., 2015; Morris, 2006; Reed, 1990), and threshold sedimentation rates may exist separating beneficial versus deleterious effects (Walters and Kirwan, 2016). However, the details of these responses are quite variable among hydrophytes (Corenblit et al., 2015; Roman et al., 1984; Dexter, 1981; Phillips, 1987).

Studies of coastal wetland response to sea level indicate six key components applicable to all cases: sea-level, hydroperiod, sediment deposition, vegetation cover (including biomass production and organic matter deposition), net vertical accretion (upbuilding from deposition and organic matter accumulation minus autocompaction or local subsidence), and wetland surface elevation.

Fig. 1.2 shows interactions among these components. Positive links indicate that a change (increase or decrease) in the component at the beginning of an arrow results in a change in the same direction in the component at the other end, other things being equal (as other interactions are accounted for by other links). An increase or decrease in net vertical accretion, for instance, is linked to an increase or decrease in elevation. Negative connections signify that change in the source component leads to a change in the component at the end of the arrow in the opposite direction. A decrease (increase) in surface elevation, for example, leads to an increase (decrease) in hydroperiod (Fig. 1.2).

Some relationships in Fig. 1.2 may be either positive or negative. Depending on the species involved, stage of wetland development or deterioration, and the degree of change in inundation, longer hydroperiods may