Chapter 6. Fire and Climate Histories in Sequoia-Mixed Conifer Forests of the Sierra Nevada

Transcription

1 Chapter 6 Fire and Climate Histories in Sequoia-Mixed Conifer Forests of the Sierra Nevada 6. 1 Introduction Thomas W. Swetnam, Christopher H. Baisan and Anthony C. Caprio The fire history of Sierra Nevada forests provides a window on past ecological responses to climatic changes. We have reconstructed detailed chronologies of fire events within pine and mixed conifer forests by sampling and analyzing hundreds of fire scarred trees. The frequent surface fires that swept through Sierran forests for millennia before the arrival of European settlers were generally of low intensity, and so most mature pine and fir trees survived dozens of these events throughout their lifetimes. Many giant sequoias are over 2,000 and years old and have survived hundreds of surface fires. Even though surface fires were apparently widespread and frequent, only a small proportion of conifer trees consistently incurred fire scars at their bases. The fire scar record is fragmentary and non-randomly distributed across the landscape, but by sampling many fire scarred trees widely distributed within stands it is possible to compile relatively complete chronologies of widespread fires that occurred across a broad range of spatial scales. Because fire ignition and spread is partly controlled by weather and climatic factors, assessment of long fire chronologies allows us to detect and evaluate patterns that are indicative of past climate-fire relations. There are two fire regimes parameters that are particularly useful for assessing past climatic responses of ecosystems. These two parameters are the main focus of this chapter, and they are -- fire frequency and fire synchrony. Fire frequency the number of fires per time period -- is a commonly assessed aspect of fire regimes. A simplistic assumption might be that increased fire frequency occurs during droughts. However, because fire occurrence is contingent on both fuel condition (e.g., moisture content) and fuel amount and distribution, frequent or long lasting droughts may eventually serve to inhibit fire occurrence through limitation of available fuel for fire ignitions and spread. This may be especially true in high frequency fire regimes, where surface fires regularly consume fuels on the forest floor (e.g., grasses and tree leaves), so that ignition and spread of subsequent fires depends upon adequate re-growth and accumulation of fuels between fire events. Moreover, different permutations of warmer/drier, cooler/wetter (or warmer/wetter and cooler/drier) conditions may act to alter fire frequency in different and complex ways. Finally, climate varies across a range of temporal and spatial scales e.g., from seasonal to millennial scales, and from forest stands to regional scales and the variations at each of these scales may result in different fire regime responses. Fire synchrony is the temporal pattern of correspondence of fire events among multiple points in space. This fire regime parameter (and its underlying concept) has not been as widely or explicitly recognized by fire ecologists or fire historians (but see Swetnam and Betancourt 1990, 1998, Swetnam 1993, Grino-Mayer and Swetnam 1999, Veblen et al. 1999, Kitberger et al. 2001). The analyses of fire synchrony has developed

2 naturally in fire history studies of surface fire regimes because the primary record the fire scarred tree is by nature a point record. Surface fires are irregularly and incompletely recorded by fire scarred trees, and therefore it is not possible to reconstruct the precise perimeter of past fires, or to develop time since fire maps that might be used to reconstruct fire rotation or fire cycle estimates (e.g., Johnson and Van Wagner 198x). Instead, by sampling widely within relatively homogeneous stands, and then by compiling composite chronologies of fire scarred trees, it is possible to infer patterns of relative fire extent by assessing the degree of synchrony and asynchrony of fire dates among sampled fire-scarred trees. Highly synchronous dates are probably indicative of fire years when a relatively large proportion of the sampled area burned, whereas fire dates recorded by a small number or proportion of trees probably represents relatively smaller areas burned during those years. There are a number of important assumptions and potential biases inherent in this approach to fire history reconstruction and interpretation which we will discuss later in the chapter. At this point, it is mainly important to note that patterns of fire synchrony can be assessed at different spatial scales and resolutions. We argue that patterns of synchrony assessed at the broadest scale (regional, continental, etc.) are most likely to reveal climatic associations with fire because (1) extreme climatic events that affect fire ignition and spread, such as drought, are known to occur at regional and broader scales, (2) no controls other than climatic events are known to operate at these broad scales that would result in annual synchrony of fire events. In this chapter we present examples from our past and ongoing studies of fire history and climate-fire associations in pine and mixed conifer forests on the western slope of the Sierra Nevada. We use these examples to demonstrate how fire frequency and fire synchrony patterns are useful in identifying responses of fire regimes to past climate variability at the scale of forest stands to the region. We are developing extensive networks of fire chronologies along elevational networks. The details of the elevation/forest type-related fire patterns are described in other works (Caprio and Swetnam 199x, Swetnam et al in preparation), but we use examples from these collections to illustrate fire frequency and synchrony patterns at the stand to watershed spatial scales. Last, we use examples from our giant sequoia fire history work to show how inter-annual to centennial-scale climatic changes have affected surface fire regimes in the Sierra Nevada. 6.2 Study Areas Our fire history collections were obtained along four elevational transects on the west slope of the Sierra Nevada (Fig. 1). Each of these transects includes 10 to 15 stands where we have collected partial cross sections from 10 to 50 or more fire scarred trees. Each of the transects is also anchored by a collection within a giant sequoia grove (usually within the upper one third of the elevational gradient) (Fig. 2). The northernmost transect is located in Yosemite National Park, and the southernmost is in Mountain Home State Forest. The two middle transects are located in Sequoia and Kings Canyon National Parks. The transects span a broad range of elevations, aspects, slopes,

3 and forest types. In general, the transects are located in the middle elevations of the west slope, extending from about 1090 to 2680 m. The transects range in length from 7 to 15 km. The topography along the length of the transects is relatively steep (up to 100% slopes), although flat areas are also present within and between many of the collection sites. The aspects of the sites are primarily towards the west and south, although a few sites have northern or eastern aspects. The transects are situated on continuous topographic features, such as along ridge lines (or close to a ridge line), or along the midportions of slopes. These topographic features extend along the elevational gradient without major fire barriers (e.g., continuous rock escarpments, cliffs, or large rivers) between the collection sites, except for the Giant Forest transect, which is bisected by a major river canyon (Marble Fork of the Kaweah River). Because the topography and fuels (at present) are more-or-less continuous between collection sites, fire spread between most sites along the transects is possible. Fire spread between the three southern transects is possible but unlikely because of distances (i.e., up to 30 km) and major barriers such as deep river canyons and large, continuous rock exposures. At the lowest end of the transects the dominant over story trees include ponderosa pine (Pinus ponderosa) and black oak (Quercus kelloggii). Some of these sites are located on the ecotone of pine/oak forest and foothills chaparral types (see Nates?? chapter for discussion of Sierra Nevada vegetation types). In the middle portions of the transects white fir (Abies concolor), ponderosa pine, jeffrey pine (Pinus jeffreyii), and giant sequoia (Sequoiadendron giganteum) are dominant. This type is commonly referred to as mixed conifer (Rundel et al. 1977). At the highest end of the transects white fir, red fir (Abies magnifica), and jeffrey pine are dominant, with western white pine (Pinus monticola) and lodgepole (Pinus contorta) as occasional dominant trees in some sites. The fire season extends from April to November in the southern and central Sierra, with a maximum area burned in June, and a maximum in numbers of lightning caused fires in July (ref to Jan s paper?). 6.3 Methods 6.31 Site and Sample Selection Our previous fire history research in giant sequoia groves (Swetnam 1993) provided a database of fire regime reconstructions for the sequoia-mixed conifer portion of the elevational transects. In our search for suitable collection sites we followed the major topographical features (ridges and slopes) up and down from a set of four groves: Mariposa, Big Stump, Giant Forest, and Mountain Home. The transects follow elevational gradients, with the sequoia groves in the middle or near the upper end of the transects. We searched for forest stands that were relatively free of major canopy disturbance, such as complete overstory removal from past logging or recent crown fires that would have consumed the fire-scar record preserved in living trees, stumps, logs, and

4 snags. We also searched for forest stands that appeared relatively homogenous (composition and density) and were not dissected by potential fire barriers (e.g., continuous rock outcrops, rivers, etc.). The selected stands ranged in size from approximately 20 to 50 ha. These areas were defined by topographic features such as ridge tops and drainage bottoms, or obvious changes in the overstory structure (e.g., a change from forest to open meadow). We searched systematically within the stands for fire-scarred trees. Our aim was to thoroughly search the entire area of the stand and to identify a set of broadly distributed specimens that contained a well preserved and long historical record of past surface fires. We examined all potential fire-scarred specimens that we could locate within the selected stands. This searching and examination involved careful inspection of the fire-scarred surfaces of all potential specimens, and identification of those with well preserved wood, datable tree-rings, and maximum numbers of visible scars. Most of the collected specimens were full cross sections taken with a chainsaw from stumps, logs, and snags. A smaller number of partial sections were taken from living trees (Arno and Sneck 1977) to extend the fire history record to the present. A goal of our sampling strategy was to maximize the temporal completeness and length of the fire event record for each stand. This was not a statistical sampling of the population of fire events (or fire intervals), rather, our goal was to obtain an inventory of fire events that occurred within the stands that was as complete and as long as possible, given the fragmentary nature of the record and practical limitations in recovering it Laboratory Sample Processing and Analyses Dry cross sections were re-sectioned with a band saw to prepare a smooth surface for sanding. A belt sander was used with progressively finer grits from 150 to 400. All tree-rings were crossdated using variations in ring-width, latewood widths, and false ring patterns to accurately assign calendar years to each growth ring (Stokes and Smiley 1968, Swetnam et al. 1985). Calendar years and seasonal timing were determined for each fire scar by observing under magnification (10X to 30X) the relative position of the scars within the dated annual rings (Dieterich and Swetnam 1984, Baisan and Swetnam 1990, Ortloff 1996). Scar position was divided into six categories, dormant (D), early-earlywood (E), middle-earlywood (M), late-earlywood (L), latewood (A), and undetermined (U). The early-earlywood, middle-earlywood, and late-earlywood correspond approximately to a division of the earlywood by thirds (Baisan and Swetnam 1990). Based on our knowledge of cambial phenology of conifers (Fritts 1976, Baisan and Swetnam unpublished data, Parsons unpublished data), we estimated the approximate calendrical dates for each of the scars (except U positions) to within 4 to 6 week periods. The undetermined (U) category were scars with too much decay, resin, or suppressed (slow) ring growth in the area of the scar for reliable determination of intra-ring position.

5 6.33 Compositing Fire Chronologies and Assessing Patterns of Synchrony 6.34 Superposed Epoch Analyses, and Other Climate-Fire Assessments 6.4 Fire Frequency and Synchrony Patterns at the Stand and Watershed Scales: The fire Spread process, and Land Use History Figs 2, 3, and Fire Frequency and Synchrony Patterns at the Regional Scale Figs 5, 6, 7, Summary and Conclusions

6 Figure 1. Map of the western slope of the Sierra Nevada and locations of fire history transects and giant sequoia groves where fire histories have been compiled.

7 Figure 2. Examples of a fire scarred giant sequoia in Mariposa Grove (upper left), sampling a fallen sequoia log in the Giant Forest with chain saw (lower right), fire scars within sequoia tree rings from the Giant Forest (lower left) and in a ponderosa pine cross section from Yosemite National park (lower right). [these could be in black & white]

8 Figure 3. Examples of stand-level fire chronologies from a site on the Giant Forest transect at xxxxm (upper chart), and on the Mountain Home transect at xxxx m (lower chart).

9 Figure 4. A composite fire chronology chart from the Giant Forest transect. The horizontal lines are the stand-level composites from sites along the transect. The upper 3 sites (LM, CM, and HM) are from groups of fire scarred sequoia trees within the Giant Forest.

10 Figure 5. Fire frequency and synchrony in five giant sequoia groves in the Sierra Nevada, California. Fire frequency is summarized as number of fire events in nonoverlapping century-length periods for each grove (A), and for all groves combined during moving 50 and 20 year periods, plotted on the central year (B). Changing patterns of synchrony are show by the events when fires occurred in 3, 4 and 5 of the groves during the same years (C).

11 Figure 6. Comparison of tree-ring reconstructed winter-spring precipitation for California division 5 precipitation (Graybill 2000) with the fire years recorded in four (filled circles) and five (filled inverted triangles) giant sequoia groves.

12 Figure 7. Superposed epoch analyses of fire events in five sequoia groves compared to reconstructed winter-spring precipitation (see figure 6) during the period AD 1050 to The sets of years with no fires in any grove (0), and synchronous fire events in multiple groves (1, 2, 3, 4 or 5) were tested and plotted separately.

13 Figure 8. Comparison of smoothed fire frequency and temperature estimates from AD 500 to the late 20 th century. The fire frequency was the summed number of fire events in 20-year non-overlapping periods in all five groves. The short dashed line is summer temperature (smoothed 20 year non-overlapped means) reconstructed from Sierra Nevada foxtail pine tree-ring widths (Graumlich 199x) and the long dashed line is ring widths (smoothed 20 year non-overlapped means) from upper forest border bristlecone pine from Campito Mountain, White Mountains, CA (LaMarche and Harlan 197x).