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Postfire responses by southern Arroyo willow forest and by southern-willow scrub dominated by Salix lasiolepis in San Diego County, California.

Southern riparian forest dominated by arroyo willows (Salix lasiolepis) and southern-willow scrub are two riparian ecosystems in southern California. Southern riparian forest dominated by arroyo willows in San Diego County has an understory of herbaceous species (Hickman, 1993). This habitat occurs commonly in alluvium in depositional channels of intermittent and perennial streams and along terraces adjacent to floodplains that are subject to periodic high-intensity flooding and sustained inundation (Sawyer and Keeler-Wolf, 1995). Southern-willow scrub in San Diego County is dominated by small tree and shrubby forms of S. lasiolepis in open-todense thickets that commonly have an understory of deciduous shrubs, drought-deciduous subshrubs, and herbs. Poison oak Toxicodendron diversilobum is another common understory component that grows as a shrub and woody vine that is supported by S. lasiolepis in forest or scrub settings (Gartner, 1991). Southern-willow scrub tends to occur in alluvial deposits of streams that experience intermittent flooding and inundation (Sawyer and Keeler-Wolf, 1995; Vaghti and Greco, 2007).

Fire is an important process in riparian ecosystems of the western United States (Dwire and Kauffman, 2003), including southern California (Rundel and Sturmer, 1998). Periodic wildfires are natural components of chaparral and coastal-sage-scrub shrublands that occur in areas of southern California with Mediterranean climate. Intervals of fires in these shrublands are about every 2-3 decades, but historically, the interval may have been much longer (Keeley, 2000). Both southern riparian forest dominated by arroyo willows and southern-willow scrub occur within a fire-prone matrix of chaparral and sage scrub (Keeley, 2006), and tolerance to periodic wildfires is critical to their long-term survival within these shrublands. Wildfires affect structure and regeneration of stands, wildlife habitat, nutrient cycling, and livestock grazing (Allen-Diaz et al., 2007).

Little is known about the influence of wildfires on structural characteristics and resilience of both southern riparian forest dominated by arroyo willows and southern-willow scrub situated in riparian settings in southern California (Faber et al., 1989). Vaghti and Greco (2007) noted the dearth of information on response of riparian species to fires in the Great Valley of California. There is no study that contains detailed preburn and postburn comparisons of structure and composition of willow-dominated riparian forest and scrub.

In our study, we contrast preburn and postburn structural and compositional characteristics of southern riparian forest dominated by arroyo willows and southern-willow scrub that occur in riparian settings in San Diego County, California. Specific questions addressed were: did wildfire result in a change in structural characteristics of S. lasiolepis (i.e., overall mortality, average ground cover, height, diameter at breast height, density of stems, and closure of crown), and did overall ground cover by various growth forms change following the wildfire?

The study area was in Marine Corps Air Station Miramar, a 9,562-ha military facility ca. 16 km N San Diego, California. The eastern one-half of Marine Corps Air Station Miramar is characterized by steep terrain with most canyons aligned in a north-to-south direction. This region has a Mediterranean climate with ca. 93% of average annual precipitation occurring October-April. The area often is blanketed by a cool, moist marine layer whose influence is most pronounced in spring and early summer. During autumn, the study area and all of southern California experiences periodic Santa Ana winds: a foehn-type wind characterized by strong offshore winds and extremely warm and dry conditions. The study area was covered with chaparral and coastal-sage scrub with native and nonnative grasslands admixed. Riparian corridors occurred in larger canyons.

Nearly all vegetation in Marine Corps Air Station Miramar burned during the Cedar Fire in late October 2003 during a powerful episode of Santa Ana winds. This was a massive, intensive wildfire that burned an area of 113,470 ha (Keeley et al., 2004) before reaching its western terminus on the westernmost portion of Marine Corps Air Station Miramar

All five sampling plots were in southwestern Marine Corps Air Station Miramar, and all but one burned in the Cedar Fire. Two plots within southern riparian forest dominated by arroyo willows and three plots within southern-willow scrub were sampled in spring 1999 and resampled in spring 2008. Prior to sampling in 1999, the two plots of southern riparian forest dominated by arroyo willows had not burned since at least 1910 (Marine Corps Air Station Miramar, 2007). These plots were in the only stand of southern riparian forest dominated by arroyo willows on Marine Corps Air Station Miramar. The plot with southern-willow scrub in Murphy Canyon last burned between 1935 and 1945, and the two plots of southern-willow scrub in San Clemente Canyon had not burned since at least 1910. One of the southern-willow-scrub plots in western San Clemente Canyon did not burn in the Cedar Fire.

Each sampling plot measured 25 by 25 m and was in the middle of a representative stand of southern riparian forest dominated by arroyo willows or scrub. We recorded locations of the four corners of each plot using a submeter accuracy GPS. Ground cover of all species of vascular plants was measured along four 25-m line transects. Transects were placed parallel to each other in a stratified-random manner from a baseline, which was along one side of each plot. On each plot, the same locations of transects as in 1999 were reestablished in 2008. Bonham and Reich (2009) demonstrated that this replication procedure significantly minimizes cover-estimate errors associated with relocation of transects in monitoring plots through time. We recorded data using a modified point-intercept method (Bonham, 1989). We sampled 25 points along each of the four line intercepts beginning at the 0.5-m point and continuing at 1.0-m intervals to the 24.5-m point of each transect. Percentage cover for each species represents the total number of points intercepted by each species out of a possible 100 points on each plot. Foliar overlap by two or more species at each point can result in a plot having >100% ground cover. Individual transects were extended for 1 m on a side to create a 1 by 25-m belt transect. Density and size of woody plants rooted within each of the four belt transects were recorded. We measured heights of mature resprouts and dead-mature woody plants to the nearest decimeter.

We measured each tree and sapling within a plot for diameter at breast height (dbh), height, and closure of canopy. If an individual tree had multiple stems, height of the tallest stem was recorded, although measurements of dbh were taken of all stems. Individual woody plants were recorded as trees (dbh >10 cm or multiple stems) or as saplings (height >20 cm and dbh <10 cm). Percentage closure of canopy for crowns of trees was estimated visually by two observers and averaged. We placed estimated averages into one category for closure of canopy: 0-20, 21-40, 41-60, 61-80, and 81-100%. We recorded all living trees and all standing dead trees.

Data were reduced, captured, and analyzed statistically using SPSS 15.0 for Windows (SPSS, Inc., Chicago, Illinois). We calculated repeated-measures statistics for mean-paired differences between 1999 and 2008 separately for each riparian ecosystem following methods of underwood (1997) and zar (1999).

In the five growing seasons since the Cedar Fire, ground cover of S. lasiolepis in plots within southern riparian forest dominated by arroyo willows increased from zero to ca. 92%, nearly identical to its preburn cover in 1999 (Fig. 1). Median closure of crown of S. lasiolepis had recovered from its preburn value of 40-60% (n = 48 individuals) in 1999 to 40-60% (n = 47 individuals) in 2008. Both woody shrubs and subligneous subshrubs were unimportant components of understory on plots within southern riparian forest dominated by arroyo willows both before and after the Cedar Fire (Fig. 1). Mean ground cover by vines was attributable almost entirely to T. diversilobum, which increased significantly after the fire (Fig. 1). Herbaceous cover did not differ significantly between 1999 and 2008 and was almost entirely comprised of nonnative species such as pampas grass Cortaderia selloana, Italian plumegrass thistle Carduus pycnocephalus, and umbrella grass Cyperus involucratus.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

individuals of S. lasiolepis in plots within southern riparian forest dominated by arroyo willows experienced no significant long-term decrease in height (Fig. 2a); average height in 2008 was 94% of its preburn average. Mean dbh of S. lasiolepis decreased, although not significantly (Fig. 2b), as a result of a nearly five-fold increase in number of new stems (ramets) that resulted from resprouting (Fig. 2d). only minimal mortality of S. lasiolepis was observed in plots in 2008 (Fig. 2c). While no seedling or sapling of S. lasiolepis was noted in any belt transect on any plot in 1999, four postburn saplings were observed in 2008.

Postburn ground cover of S. lasiolepis on plots with southern-willow scrub was 36% of the value in 1999, but the difference was not statistically significant, likely due to the small sample (Fig. 3a). Shrub cover decreased from 1999 to 2008 (Fig. 3a), and in both years, the dominant shrub was mule-fat Baccharis salicifolia. While cover by vines was insignificant in 1999 and 2008, herbaceous cover increased from 24 to 181% postfire. Of herbaceous cover in 2008, about two-thirds was comprised of nonnative herbs such as Italian ryegrass Lolium multiflorum and longbeak stork's bill Erodium botrys (Table 1). However, one plot of southern-willow scrub had an increase in cover of native Lemmon's canarygrass Phalaris lemmonii from 0% in 1999 to 47% in 2008. Ground cover of the native sedge Juncus dubius increased dramatically on the same plot but plummeted in cover on the other plot (Table 1). No other substantial decrease in cover of herbs was noted on either plot. The unburned willow scrub plot exhibited decreases in ground cover of S. lasiolepis and shrub cover similar to that of burned plots (Fig. 3b). However, cover by vines comprised entirely of T. diversilobum increased from 11 to 28% (Fig. 3b). Herbaceous cover on this plot increased substantially with virtually all of the increase due to nonnative species, most notably red brome grass Bromus madritensis subspecies rubens (Table 1).

[FIGURE 3 OMITTED]

Results of our study indicate that S. lasiolepis exhibited marked resilience to an intense wildfire in southern riparian forest dominated by arroyo willows but less so in southern-willow scrub. Individuals of S. lasiolepis displayed vigorous and abundant resprouting that allowed the two plots with southern riparian forest dominated by arroyo willows to return to prefire values for percentage of ground cover, height, and closure of canopy. Salix lasiolepis displayed both epicormic and basal resprouting as other studies of this species elsewhere in the western United States have revealed (Stein et al., 1992; Dwire and Kauffman, 2003). The rapid postburn regrowth of the overstory of S. lasiolepis might have limited growth of herbaceous species. As expected, mean dbh decreased after the fire, but this species is recovering. Presence of four saplings within transects on plots following the Cedar Fire demonstrates the ability ofS. lasiolepis to resprout and produce sapling-sized progeny within 5 postburn growing seasons.

While counts of small trees and shrubby individuals of S. lasiolepis were not tallied in plots of southern-willow scrub, some mortality likely occurred as a result of higher intensity of fire experienced on those plots due to substantial cover of shrubs before the burn. However, reduction in cover of S. lasiolepis might not have been due completely to the fire but also a consequence of below-average precipitation in all but one of the first 5 years of precipitation following the Cedar Fire (climatic data from National Oceanic and Atmospheric Administration). This is supported by the unburned plot of southern-willow scrub that had substantial decreases in ground cover of S. lasiolepis and shrubs. Further, the spectacular increase in herbaceous cover on burned plots might not be due entirely to an opening of the shrub canopy following the Cedar Fire. Herbaceous cover on the unburned plot of southern-willow scrub increased from <1% in 1999 to 60% in 2008, with nearly all cover attributable to a proliferation of nonnative herbs. Proliferation of nonnative herbs on all southern-willow-scrub plots likely was aided by timing of precipitation in 2007-2008. Precipitation mostly fell earlier than normal in 2007-2008. Nonnative herbs tend to germinate earlier in the season than native herbs and shrubs (Eliason and Allen, 1997), and proliferation of nonnative herbs might not be attributable entirely to fire and the consequently more-open overstory of southern-willow-scrub plots.

Following the Cedar Fire, the most striking change in composition of the understory of both burned and unburned southern-willow-scrub plots was the increase in herbaceous vegetation, particularly by nonnative grasses. Burned plots of coastal-sage scrub, riparian forests dominated by coast live oaks (Quercus agrifolia), and woodlands dominated by California sycamores (Platanus racemosa) also displayed enormous increases in cover by nonnative species of grasses, a phenomenon that is becoming common in postburn forests (Franklin, 2010) and shrublands (Zedler et al., 1983; O'Leary and Westman, 1988; Keeley, 2001; Fleming et al., 2009). Clearing of shrublands coupled with disturbance of soil by grazing livestock has resulted in a gradual proliferation of nonnative herbs in southern California (Rejmanek et al., 1991). Although grazing by cattle has not occurred in the study area for >80 years, effects of grazing may persist for decades. The proliferation of nonnative herbs that we observed might represent the interaction of fire with both a lingering effect of grazing and earlier-than-usual precipitation in 2007-2008. The stands of southern-willow scrub we studied occurred within a heterogeneous landscape of nonnative grasslands, and disturbed and undisturbed shrublands were adjacent to one another. Reestablishment by nonnative grasses in riparian areas after the burn likely was augmented by additional propagules of grass dispersed from adjacent sources of seeds.

Fire is an important process in ecosystems in areas of southern California with Mediterranean climate. However, fires combined with drought and unfavorable timing of precipitation negatively impacted southern-willow scrub by significantly reducing its overstory for a protracted period, allowing for proliferation of nonnative herbs. Continued dominance by nonnative herbs might have other long-term implications for the future biological integrity of southern-willow scrub. Interspecific competition for moisture between nonnative herbs and seedlings of S. lasiolepis might negatively impact dynamics of recruitment of this species. Further, continued presence of nonnative herbs might reduce quality of habitat for wildlife and impact biodiversity in this and other riparian ecosystems. Southern riparian forest dominated by arroyo willow and southern-willow scrub represent two riparian ecosystems in southern California that humans have destroyed or heavily modified (Faber et al., 1989). Future long-term monitoring will further elucidate the dynamics of recovery and the integrity of these two endangered riparian ecosystems.

We thank the United States Navy for funding. Special thanks are extended to J. Kassebaum, B. Bell, and G. Clune of the Environmental Management Department, Natural Resources Division, at Marine Corps Air Station Miramar. Thanks also are extended to E. Fallon, A. Fojtik, C. Martin, J. Miller, and S. Whitford for assistance with sampling; to M. Chiu, L. Coulter, H. Johnson, and D. McKinsey for technical assistance; and to D. Deutschman for statistical advice. We thank J. Marquez for helping translate the abstract into Spanish. We also thank P. Crawford and three anonymous reviewers whose suggestions greatly improved the manuscript.

Literature Cited

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Submitted 8 June 2010. Accepted 24 March 2012. Guest Associate Editor was Priscilla H. C. Crawford.

John F. O'Leary * and William R. Bredemeyer

Department of Geography, San Diego State University, San Diego, CA 92182-4493

* Correspondent: lujastro@yahoo.com
Table 1--Percentage of ground cover of the six common species of plants
in southern-willow-scrub plots at Marine Corps Air
Station Miramar, San Diego County, California.

Taxon                    Burned                        Unburned

                         San Clemente   Murphy Canyon  San Clemente
                         Canyon                        Canyon

                         1999   2008    1999   2008    1999    2008

Lolium multiflorum (a)    0       9      0      66       0      0
Phalaris lemmonii         0      47      0       1       0      0
Bromus madritensis        0       2      0      14     0 (b)    45
subspecies rubens (a)
Juncus dubius             8      39      16    0 (b)     0      0
Erodium botrys (a)        0     0 (b)    0      20       0      0
Bromus diandrus (a)       0     0 (b)    0      19     0 (b)    3

(a) Indicates nonnative  species.
(b) Endcounterd on plot, but not a point interciept.
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Title Annotation:Notes
Author:O'Leary, John F.; Bredemeyer, William R.
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1USA
Date:Sep 1, 2012
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