Topoedaphic and Morphological Complexity of Foliar Damage and Mortality Within Western Juniper (Juniperus Occidentalis Var. Occidentalis) Woodlands Following an Extreme Meteorological Event

Aim An extreme early season freeze event in October 2002 resulted in significant foliar damage and/or mortality within western juniper woodlands. We identify the geographical patterns of tree damage based on morphological, topographical and edaphic parameters, and discuss the ecological implications of this event. Methods: We sampled trees on four matched pairs of disturbed and minimally disturbed study sites within the core area of tree damage. We collected information about age, morphology (height, basal area, level of foliar damage), and topographic position (elevation, slope, aspect) for each tree sampled using dendroecological and GPS/GIS procedures. We collected plot-level data on tree density and degree of cover for shrubs and grasses. We used a series of Mann– Whitney, Kruskal–Wallis and Wilcoxon tests and graphical analyses to determine if significant differences exist in our collected variables both between matched pairs and within our disturbed sites. Results: Topographically, we found that aspect was a critical element regulating damage, with trees in colder microenvironments (north-westerly slopes) experiencing less damage because they were further along in the annual process of cold-hardening. The majority of damaged trees were found in areas of higher density and more even age structure that are typical of recent juniper afforestation. Morphologically, we show that trees with greater basal areas and/ or less stature were less likely to have foliar damage, consistent with winter cavitation. Main conclusions: Our results show that the spatial pattern of damage was closely related to topography and tree morphology, parameters that, in turn, correspond closely to differences between the historical locales of western juniper and juniper woodlands that have emerged in a 100+ year period of rapid afforestation. This freeze event fits the criteria for a large, infrequent disturbance in that it was spatially complex and may act as an endogenous factor that can contribute to stand maintenance Peter T. Soule & Paul A. Knapp (2007) "Topoedaphic and Morphological Complexity of Foliar Damage and Mortality Within Western Juniper (Juniperus Occidentalis Var. Occidentalis) Woodlands Following an Extreme Meteorological Event" Journal of Biogeography Volume 34: pp.1927-1937. Version of Record Available From (www.onlinelibrary.wiley.com) infrequent disturbances’ (Turner & Dale, 1998: 493). These events can be temporally exceptional, such as the Mount St Helens eruption, which affected many long-lived species such as old-growth conifers, and/or geographically extensive, such as the widespread fires that burned approximately 36% of Yellowstone National Park (Stone, 1998). Despite the destructiveness of these events, they ensure vegetation heterogeneity (Turner et al., 2003; Dale et al., 2005), and provide reasons for complex vegetation patterns across a landscape. Here we document evidence of a recent large, infrequent disturbance to western juniper woodlands in the interior Pacific Northwest, and discuss the spatial complexity of the event. In late October 2002, extremely low temperatures occurred throughout much of the core range of western juniper (Juniperus occidentalis Hook. var. occidentalis). Temperatures below )15 C were recorded at numerous climate stations east of the Cascade Mountains from 40 to 45 N, with some stations recording temperatures below )20 C (Knapp & Soulé, 2005). While these low temperatures are common within the range of western juniper during winter, they are extremely rare during autumn. By late spring 2003, range management personnel noticed a dramatic change in the appearance of western juniper woodlands in some areas (T. Deboodt, OSU Crook County Extension Agent, personal communication; R. Halvorson, District Botanist, Prineville, OR, Bureau of Land Management, personal communication), with over half of the trees in the most affected areas experiencing either partial dieback on the outer portion of branches, complete dieback of the top one-third to one-half of the tree with the lower canopy needles also partially impacted, or complete mortality (Fig. 1). Plant pathologists from Oregon State University analysed samples of dead foliage collected after the freeze event and concluded that there ‘was no evidence of any diseases or pests’ (Savonen, 2003: 1). With no other logical cause of such large-scale impacts to juniper woodlands, the conclusion was that the extreme freeze event of the prior autumn was responsible Figure 1 Examples of foliar damage to or mortality of western juniper trees at the Salt Creek Disturbed site, July 2004 (photograph by authors). Introduction The spatial heterogeneity of many landscapes can be attributed in part to rare, but severe, ecological disturbances that affect large areas. Two events in the latter half of the 20th century, the eruption of Mount St Helens in 1980 and the wildfires in Yellowstone National Park 1988, serve as classic examples of what have been identified as ‘large, (Savonen, 2003; R. Halvorson, personal communication). Specifically, the affected trees were probably damaged by cavitation (Sharrow, 2004), where freezing of the xylem cells forces air into the xylem water column (Davis et al., 1999; Pittermann & Sperry, 2006). The subsequent embolism ‘impedes water transport’, leading to foliar injury (Pittermann & Sperry, 2006: 374). While conifers are less susceptible to freeze-induced cavitation than other tree species because their tracheids have a small diameter, multiple species of juniper have been shown to experience freeze-induced cavitation from a single freeze event when that event is preceded by drought conditions (Willson & Jackson, 2006). The timing of the cold event in the interior Pacific Northwest occurred before many western juniper trees were sufficiently cold-hardened. Thus, in the absence of this protective physiological mechanism for extremely cold conditions, trees were damaged or killed by late spring 2003. The impacts were readily apparent in summer 2004, when we conducted our investigation. While there were no detailed surveys taken to assess the spatial extent of the foliar damage, Bureau of Land Management (BLM) personnel estimated that c. 3% (T. Deboodt, personal communication), or 110,000 ha, of the 3.6 million ha of western juniper woodlands (Miller et al., 2005) were affected, with complex spatial patterns of dieback/mortality present. Western juniper trees are common throughout the interior Pacific Northwest. While historically relegated to higher elevations, steeper slopes and rocky outcrops, where fire cannot easily spread (Miller & Rose, 1999), within the past 100 years there has been an exceptional expansion of the species (Miller & Rose, 1995, 1999; Gedney et al., 1999; Soulé et al., 2004) such that substantial populations now occur on virtually every type of terrain present throughout their range. Because of the topographical and ecological diversity of the current distribution of western juniper, we theorized that the spatial pattern of foliar damage and mortality would be closely linked to a suite of micro-environmental conditions related to slope, elevation, aspect or edaphic conditions. However, our discussions with BLM personnel, and our initial observations of the spatial pattern of foliar damage, suggested that the pattern was more complex and not so easily classified, as some level of foliar damage was evident on most aspects, at higher and lower elevations, and on flat as well as steeply sloping terrain. In a prior study, we documented the meteorological parameters of the extreme freeze, showing that it was an unusual event with a statistical recurrence interval, calculated for daily low temperature on 31 October 2002, often in excess of 500 years (Knapp & Soulé, 2005). Disturbances of this magnitude are notable from an ecological perspective because they may help explain both species range distributions and complex landscape patterns not easily attributed to smaller and more frequent disturbances. For example, the distribution of creosote bush (Larrea tridentata Cov.) in central New Mexico is believed to be controlled, in part, by extreme freeze events (Martinez-Vilalta & Pockman, 2002). As these events ‘are not well understood’ (Turner & Dale, 1998: 493), examining the ecological consequences of the October 2002 freeze provides insight on an event that will leave a multi-decade or longer imprint on the western juniper woodlands of the interior Pacific Northwest.