111 ISLAND SCRUB OAK (QUERCUS PACIFICA) POPULATION STRUCTURE AND DYNAMICS ON SANTA CATALINA ISLAND Roland C. de Gouvenain* and Ali M. Ansary Department of Biological Sciences, Chapman University, One University Drive Orange, CA 92866 Present address of R. de Gouvenain: Department of Biology, Rhode Island College, Providence, RI 02908 Present address of A. Ansary: University of Cambridge, Downing College, Regent Street Cambridge, UK, CB2 1DQ *Correspondent: [email protected] ABSTRACT: The purpose of our study was to describe the demographic structure and project the demographic trend of a sample of populations of the island scrub oak (Quercus pacifica) on Santa Catalina Island, and to determine if the low regeneration observed in some populations was associated with low tree fertility or low seedling recruitment and/or survival. Matrix projection modeling using tree ring data and two different fertility estimates produced a range of population intrinsic growth rates with no conclusive association with location on the island or several environmental variables. Demographic projections differed between fertility estimates calculated from acorn production or from seedling recruitment. The discrepancy between the two suggests that acorn germination and/or seedling survival and growth are not sufficient in some populations to ensure long term population maintenance. The low recruitment observed in several populations that also have low intrinsic rates of growth is cause for concern. Further studies could determine whether this low recruitment is due to low acorn germination or low seedling survival, or both. KEYWORDS: Canonical correlation analysis, fertility, fire, matrix model, regeneration, soil. INTRODUCTION Monitoring of the island scrub oak (Quercus pacifica Nixon & Muller) woodlands of Santa Catalina Island by the Catalina Conservancy suggests that several populations have experienced tree dieback and low regeneration over the last decades, for reasons yet unknown (D. Knapp pers. com.). Possible reasons for the dieback include senescence, stress due to pollution, fungal infection, and lower rain or fog precipitation (or a combination of the above). Low regeneration may be caused by impacts of introduced animals such as mule deer and American bison, competition from exotic grasses, fungal infection, decline in moisture input, and/or fire suppression. Several species of oak have been declining throughout North America and Europe in the last decades, and a variety of causal factors have been proposed depending on the region and the species. Human-caused changes in fire regimes (Abrams 2003; Jacqmain et al. 1999; Lorimer 1980), grazing practices (Bakker et al. 2004; Kelly 2002) and drought (Faberlangendoen and Tester 1993) were all found to be possible factors, among others. In California, Griffin (1971) found that acorn germination and seedling survival for several native oak species were influenced by slope aspect, ambient temperature, and competition from grasses, seed burial activity of birds and squirrels, and seedling predation by deer and pocket gophers. Also in California, Momen et al. (1994) found that grazing history and grass density (which can co-vary), influence the success of blue oak (Quercus douglasii) regeneration. We investigated whether the putative decline of the island scrub oak is occurring island-wide or whether it is restricted to some of the populations, and whether poor regeneration is associated with low acorn production (low fertility) and/or lack of seedling establishment and survival. We hypothesized the following: Oak ecosystem restoration on Santa Catalina Island, California: Proceedings of an on-island workshop, February 2-4, 2007. Edited by D.A. Knapp. 2010. Catalina Island Conservancy, Avalon, CA. Q. pacifica Population Structure and Dynamics 112 1. If a given population of scrub oak is declining, or has experienced poor sapling survival, we should observe a population structure skewed toward older (or larger) age (or size) classes. 2. The intrinsic rate of population increase (λ) calculated from matrix population models should be <1 for declining populations. 3. If population decline is mostly connected to tree senescence, we should observe overall low tree fertility typically associated with old trees. The purposes of our study were to: (a) describe the demographic structure of a sample of scrub oak populations from the island-wide scrub oak metapopulation, (b) project demographic trends at the population and metapopulation scale, and (c) determine whether low reproduction is due to overall tree senescence and low fertility, or low germination/seedling survival. We also examined populations for records of past fires in the tree-ring record and in the soil profile to determine if fire had been part of the disturbance regime in the past. METHODS Field sampling Using a map of island scrub oak-dominated areas produced by the Catalina Conservancy, we selected 10 study areas distributed throughout Catalina Island (Table 1; Figure 1). Within each study area, populations that were clearly identifiable within the landscape from a high vantage point were numbered in the field, and one population was randomly selected. Within the selected population, we numbered five clearly identifiable Quercus pacifica trees > 10-cm diameter (10 cm above ground, since Q. pacifica often branches just above the ground) and selected one randomly as our sample starting point. Table 1. Geographic coordinates of the 10 populations sampled in this study. Three populations have no coordinates due to technical problems with GPS during sampling (refer to Figure 1). Population No. Latitude / Longitude 1 33º 21’ 15" N / 118º 26’ 52" W 2 33º 19’ 34" N / 118º 28’ 16" W 3 No data (see map) 4 33º 23' 29.5" N / 118º 23' 49.6" W 5 33º 21' 50.2" N / 118º 27' 54.8" W 6 33º 27' 22.2" N / 118º 31' 36.5" W 7 No data (see map) 8 33º 23' 52.5" N / 118º 23' 53.1" W 9 No data (see map) 10 No data (see map) Oak ecosystem restoration on Santa Catalina Island, California: Proceedings of an on-island workshop, February 2-4, 2007. Edited by D.A. Knapp. 2010. Catalina Island Conservancy, Avalon, CA. Q. pacifica Population Structure and Dynamics 113 N 6 9 3 8 4 2 5 7 1 10 0 10 km Scale Figure 1. Locations of the 10 sampled Quercus pacifica populations We used the wandering-quarter method (Catana 1963) to collect data on 4 adult trees > 10-cm diameter and on the population. The method was modified by making the transect bearing from one tree to the next variable within a 90° window, and we selected the bearing randomly each time from a set of possible bearings with unique numbers. From each of the four trees, we collected two cores approximately 40-50 cm from the base of the tree at right angle from each other to the center of the trunk, and noted height of each core above ground. Within a 5-m radius around each tree, we counted the number of Quercus pacifica seedlings < 50-cm tall to estimate tree fertility. We also estimated the number of acorns borne by each tree by first counting all acorns borne on a typical branch and multiplying by the number of branches; acorns on smaller branches were counted separately and added to the total. From approximately the middle of the wandering-quarter transect, we selected one additional random bearing constrained to a 90° window facing perpendicular to the transect and conducted a census of all Quercus pacific individuals within a 10×30-m belt transect in three basal diameter classes: < 5 cm (seedlings + saplings), 5-10 cm (young trees) and > 10 cm (mature trees). We repeated this procedure on the other side of the wandering-quarter transect, for a total of 600 m2 of belt transect sampling. At each population, we dug a 50×50×50 cm soil pit and described the soil profile according to USDA (1993). We looked for evidence of charcoal in the A and B soil horizons. When dead trees were present within a population, we collected stem cross-sections for ring analysis and to look for fire scars. Oak ecosystem restoration on Santa Catalina Island, California: Proceedings of an on-island workshop, February 2-4, 2007. Edited by D.A. Knapp. 2010. Catalina Island Conservancy, Avalon, CA. Q. pacifica Population Structure and Dynamics 114 Data analyses The seedling count for population 1 was excluded from the analysis, due to an error in counting root sprouts as seedlings. All tree cores (4 trees × 2 cores × 10 populations = 80 cores total) were dried, mounted, sanded and the best core from each tree was analyzed with a Velmex® tree-ring measurement instrument under a stereo boom microscope. Tree ring increment data were measured and recorded into ring width time series (40 series total) and we charted them in Excel to cross-date (synchronize) the series among each other. When we found a fire scar, we recorded its estimated year of occurrence. We used previously existing Quercus pacifica acorn germination and seedling growth data (Stratton, unpublished data) to determine: (a) seed germination rate (b) seedling survival rate, and (c) seedling growth rates. We then used the seedling growth rate information plus the growth rate, determined from tree ring analysis, to define a life cycle graph for Quercus pacifica which summarizes the general life history of that tree species, from seedling to mature tree (Figure 2). Stage survival rate (si) for stage 1 was estimated from our analysis of Quercus pacifica seedling survival data (Stratton, pers. comm.), and set as a constant for all 10 populations. We estimated si for stage 2 and 3 individually for each population from our census of living trees in each of the size classes corresponding to stage 2 through 4 in each -7 population. We set si for the terminal stage 4 at 110 (≈0) for all populations. F4 F3 G G G 1 1 2 2 3 3 4 P1 P2 P3 P4 Seedlings Saplings Young trees Mature trees Age (years) 0-6 7-20 21-40 41-80 Diameter (cm) 0 - 1 1.1 - 5.0 5.1-10 10.1-25 Figure 2.