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Temperate and Boreal Rainforests of the World: Ecology and Conservation

ISLAND PRESS

Just What Are Temperate and Boreal Rain forests?

Dominick A. DellaSala, Paul Alaback, Toby Spribille, Henrik von Wehrden, and Richard S. Nauman

When most people think of rain forests, they think of lush, tropical "jungles" teeming with poison arrow frogs (Dendrobates spp.), toucans (e.g., Ramphastos sulfuratus), mountain gorillas (Gorilla gorilla beringei), and jaguars (Panthera spp.). Tropical rain forests are indeed special places, as they account for over half the terrestrial species on Earth (Meyers et al. 2000) while representing just 12 percent of the world's forest cover (Ritter 2008). Their temperate and boreal counterparts are another story, though, one yet to receive the kind of global recognition rightfully merited by tropical rain forests. Their story is told here, beginning with historical and recent accounts to define and map the temperate and boreal rain forests of the world.

Any discussion of rain forests must begin with what we mean by this term and how we map rain forests. Definitions and mapping standards are the mortar with which scientists visually construct biome delineations such as temperate and boreal rain forests. Consequently, the modeling techniques used in this chapter frame the entire book, as each of the regional chapters is built from the approaches set herein. In cases where it is necessary to deviate from globally based models and maps, explanations are given by regional authors of the book. Nevertheless, we now build on earlier approaches and definitions of temperate and boreal rain forests by providing a standardized modeling approach and a consistent methodology for mapping these rain forests. While it was our original intent that readers of this book would use our approach as the up-to-date standard for defining and delineating temperate and boreal rain forests, we note that this is a work-in-progress requiring further refinement and real-world verification as new data sets become available. Similarly, in Chapter 10, we present standardized mapping techniques aimed at determining just how much of this rain forest biome is in strict protection, a necessary step for developing a unifying vision for rain forests globally and for calling on decision makers to protect these rain forests as we do in Chapter 11. Because the process used in this opening chapter is central to the entire book, we put more emphasis here compared to the regional chapters that follow.

SCIENTIFIC HISTORY OF TEMPERATE AND BOREAL RAIN FORESTS

Throughout this book we refer to either temperate or boreal rain forests that differ mainly with respect to latitude, climate, and plant associations. For descriptive purposes we separate these rain forest types in this chapter but refer to them jointly throughout much of the book.

Temperate Rain Forests

Temperate rain forests have been recognized in some fashion by ecologists for nearly a century (Köppen 1918; Holderidge et al. 1971; Whittaker 1975; Jarmon and Brown 1983;Veblen 1985; Read and Hill 1985; Omernick 1987; Moore 1990; Hickey 1990; Alaback 1991; Kirk and Franklin 1992; Kellogg 1992, 1995; Gallant 1996; Lawford et al. 1996; Schoonmaker et al. 1997; Moen 1999). Most researchers classify them as distinct biomes based on broad differences in dominant vegetation and/or climate, or as inclusions within larger ecoregions (large areas distinguished by their dominant vegetation, climate, and land form). Yet a simple internet search for "temperate rain forest" yields inconsistencies in mapping locations due to gross differences in definitions and mapping techniques.

An earlier term, "high-latitude rain forest," was proposed by researchers to describe the pan-American portion of the biome (Lawford et al. 1996), since this is the most simple and unambiguous way to define temperate as contrasted with tropical (low-latitude) rain forests, but "high-latitude rain forests" has increasingly been replaced by "temperate rain forests," which generally have milder climates than boreal rain forests, due primarily to comparatively low latitudes. A number of temperate rain forest subtypes are described later in this chapter in order to distinguish rain forests from one another, and this terminology is used throughout this book.

Boreal Rain Forests

The border between boreal and temperate has traditionally been defined as the zone where conifer forests give way to deciduous forests, or, in drier regions, grasslands, roughly equated by Köppen (1918) with the–3°C January isotherm in the south (Tuhkanen 1984). The delineation of boreal versus temperate is blurred in montane regions, where temperate coniferous forest transitions seamlessly to boreal conifer forest. The important thing to note here is that boreal is a latitudinal zone and should not be conflated with terms such as continental; biogeographers are unanimous in recognizing some high-precipitation oceanic regions as part of the boreal zone. Tuhkanen (1984) compared a wide variety of different approaches to delineating the northern and southern limits of the boreal zone, and in the integrated classification he proposed that several of the rain forest regions treated here as "temperate" would be considered part of the boreal zone. Nonetheless, throughout this book, we use the term boreal to describe the cold northern rain forests of what in other studies have been more generally termed subpolar. As we will see later, these include the Pacific Coast of North America north of ~55°N latitude (chapter 2), the northern half of the inland rain forest of Northwestern North America (chapter 3), much of the wet forests of Eastern Canada (chapter 4), portions of Norway (chapter 6), and Inland Southern Siberia (chapter 9). Because there is no boreal zone in the Southern Hemisphere, relatively colder areas in this hemisphere are considered subpolar.

In reality, many temperate rain forests straddle the abiotic (nonliving chemical and physical factors) boundaries between temperate and boreal, both latitudinally and altitudinally, and more so for oceanic boreal systems. Thus, these rain forests serve as a phytogeographical bridge, facilitating the exchange of mesic (moist) floral elements among neighboring systems and as corridors of latitude- and slope- related south-to-north, north-to-south and slope-up, slope-down migrations of wildlife during periods of climate change. How much of the forests included in this book is boreal versus temperate depends on which classification system chosen. The fact that highly similar forest-species assemblages can be found on both sides of artificially drawn lines is a topic best reconciled to biogeography debates.

RAIN FOREST DEFINITIONS

Where and how to draw the line between temperate and boreal rain forests has changed over time as more and better data have become available regarding these unique rain forests and the conditions that have created them. Severalgeographers who developed classifications for the world's climate included a category for temperate rain forest based, for instance, on some combination of cool temperatures and high rainfall, or cool temperatures and a small annual range of temperatures (see below). Whittaker (1975) in his classic ecology text Communities and Ecosystems also identified a temperate rain forest type. Most of these early efforts separated the Southern Hemisphere forests into a broadleaf evergreen forest type, further complicating a comprehensive global definition. These classifications vary widely in how they portray the distribution of temperate rain forests, and especially what types of temperate rain forests occur on Earth.

The prevailing definition of temperate rain forest began with work in the 1980s, when the environmental group Ecotrust and its collaborators proposed a more precise definition so that more accurate global maps and conservation strategies could be developed (Alaback 1991, 1996; Kellogg 1992, 1995). The first iteration of this work included a definition for these rain forests consisting of: (1) annual precipitation exceeding 1,200 millimeters with 10 percent or more occurring during summer months; (2) mean July temperature of 16°C or less; (3) cool dormant seasons; and (4) infrequent fire that is an unimportant evolutionary factor (Alaback 1991). Soon it became apparent that this definition was too restrictive, and more important, it did not accurately characterize availability of moisture, since there was no direct link between evaporation and the required minimum amount of rainfall. The most biophysically precise method of doing this would be to calculate potential evapotranspiration, which corrects for latitude—with increasing latitude, less precipitation is required to maintain the same humidity levels (Stephenson 1990). Potential evapotranspiration was also later shown to precisely predict the distribution of at least one common rain forest tree in northwestern North America, western hemlock (Tsuga heterophylla), even including its distribution in interior rain forests of northwestern North America (Gavin and Hu 2006). In the absence of detailed models and global spatial coverages, a more inclusive definition was proffered by Alaback (1996). In this case, temperate rain forests meeting the original criteria for annual rainfall were divided into four subtypes (or zones, including boreal), analogous to subtypes of tropical forests, based on seasonality of precipitation and annual temperatures:

• Subpolar—summer rainfall is above 20 percent of the annual total, summers are cool, and snow is persistent in winter, with mean annual temperature below 4°C.

• Perhumid—summer rainfall is above 10 percent of the annual total, summers are cool, and typically transient snow is present in winter, with mean annual temperature of 7°C."Cool-temperate" also has been used in this context.

• Seasonal—summer droughts and fires can periodically occur, summer rainfall is less than 10 percent of the annual total, with mean annual temperature of 10°C.

• Warm-temperate—summer precipitation is less than 5 percent of the annual total, winter snow is rare, drought can occur during any season, and mean annual temperature is 12°C or above (Alaback 1996;Veblen and Alaback 1996;Alaback and Pojar 1997).

The threshold values of temperature and precipitation for each of the forest subtypes was determined by examining climatic conditions in areas along the west coast of North and South America that possessed key ecological characteristics associated with rain forests. This has been the prevailing set of definitional parameters for describing rain forest regions used throughout the chapters of this book.

A NEW GLOBAL RAIN FOREST MODEL

Building on concepts from Alaback (1991), we developed a strongly organism/ecosystem–driven model for temperate and boreal rain forests that has identified a very small amount of land surface of the earth within the same biome and sharing climatic characteristics and associated ecological processes that rightfully and generally can be called temperate and boreal rain forest. The processes described herein build on earlier work of rain forest ecologists by providing a broad suite of climatic criteria and a standardized approach to mapping rain forests globally.

In this chapter, we use computer modeling to develop defensible criteria for identifying temperate and boreal rain forests and to locate forests not widely recognized as rain forest but meeting our criteria. Further, we create a computer model with high-resolution climate data and compare it to maps created by regional experts.

Rain Forest Distribution Model

This book's chapter authors, from a wide range of rain forest regions, provided locations of sites they considered typical of temperate or boreal rain forest in their area. Based on this input, we used climate data for 117 localities from six regions for the initial modeling step: the Pacific Coast of North America (n = 55, mostly coastal); Chile and Argentina (n = 9); New Zealand (n = 10); Tasmania (n = 6); Norway (n = 15); and Japan (n = 22). These regions were selected because we had localities from collaborators, and because there was little dispute that the locations represent rain forests (especially the Pacific Coast of North America, Chile, and New Zealand). Baseline predictors were extrapolated from a global climate data set (Hijmans et al. 2005); redundancy in the model variables was reduced based on a principal-components analysis of the complete data set. The final model was constructed using a MaxEnt modeling approach (Phillips et al. 2006), consisting only of predictors that improved the model. This yielded 11 discrete climate-related parameters. We used the Max-Ent model since it is known to be more conservative compared to other presence-only models, which tend to overestimate occurrence of a particular variable of interest (in this case, temperate and boreal rain forest).

The model was evaluated with a bootstrapping method (Burnham and Anderson 2002), resulting in strong support of the predictive ability of the model (AUC = 0.90; values less than 0.5 indicate no predictive capabilities; see Phillips et al. 2006). Based on 100 repeated runs, we quantified the heterogeneity of the ground-truth climate data set, thus ensuring a demarcation of core zones with a high probability of rain forest occurrence in comparison to areas with a lower probability (for mapping simplicity, only high-probability areas were depicted).

The rain forest distribution model generated four additional regions with climate suitable for temperate and boreal rain forests: the Inland Northwest of North America (figure 1-1, middle-right portion of panel a—inland British Columbia), Eastern Canada (figure 1-1, panel b), Great Britain and Ireland (figure 1-1, western corner of panel d), and portions of the Alps (figure 1-1, lower middle of panel d). Notably, two of these regions have not been widely recognized as rain forest by scientists, including the wettest parts of Eastern Canada, which appeared in some form in all map iterations, and some valleys of the eastern Alps, in particular the Salzburg Alps and mountain ranges of western Slovenia. Interestingly, these regions support rain forest lichen assemblages remarkably similar to those of the Pacific Northwest of North America or coastal Norway.

Two lower-latitude regions often considered rain forest by some (e.g., Kellogg 1992), such as the Colchic (Georgia) and Hyrcanic (Iran) forests of the Western Eurasian Caucasus, and the forests of the southern cape of South Africa, were shown to be in a class of their own compared to the more definitive rain forests of the Pacific Coast of North America and Valdivia. Including these warmer and drier outliers in the model calibration invariably resulted in overestimating the global extent of these rain forests by also including SouthAmerican páramo, high-elevation African equatorial fog forests, and nearly half of the Alps. Retention of the eastern Black Sea region (Colchic), in particular, resulted in model inclusion of large areas of eastern North America, parts of which are indeed climatically similar, but did not agree with our initial criteria on several counts. We settled on a conservative definition of temperate and boreal rain forest based generally on the climate data (see table 1-1; figures 1-2,1-3) presented for nine regions (some were combined from the set above) as follows:

• Annual (minimum, maximum) temperatures from ~4 to 12°C.

• Annual (minimum, maximum) precipitation from 846 to 5,600 millimeters.

• Snowy winters in high latitudes.

• Significant precipitation (that is, up to 25 percent of annual precipitation) during the driest quarter.

• Low annual temperature fluctuation (based on low annual temperature variability).

• Temperature of warmest quarter (summer) from 7 to 23°C.

This is the first time a spatially explicit global data set was made available for the world's temperate and boreal rain forests that was based on a suite of climate variables obtained from a global data set (available in raster—or grid—GIS format), improvements in computer processing capacity, and statistical models. The model therefore represents an initial cut at producing a global rain forest map, requiring further refinements through the use of regional climate data sets, regional rain forest classifications, and regional maps. Notably, while the minimum precipitation and maximum temperature values reported seem extreme in comparison to earlier rain forest definitions, rain forest communities persist in these regions due to compensatory factors as discussed below and in the regional chapters of this book. This is why regional ground-truth of the model and further study of rain forest classifications are essential.

CLIMATIC PATTERNS OF TEMPERATE AND BOREAL RAIN FORESTS

Based on the rain forest distribution model, rain forests were clustered along precipitation and temperature gradients that distinguished them from one another and other forest types.

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Excerpted from Temperate and Boreal Rainforests of the World: Ecology and Conservation by Dominick A. DellaSala. Copyright © 2011 Island Press. Excerpted by permission of ISLAND PRESS.
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