Growth and reproductive responses of true mountain
mahogany to browsing
موضوع مقاله : تحقیقات عکس العمل رشد و رشد زایشی درخت ماهون در ارتفاعات کوهستانی
نوع کیفی مقاله درجه 1 برگرفته از پورنال مرتعداری و مراتع range management journal
ارائه این مقاله در دروس : مرتعداری اکولوژی فیزیولوژی بیوتکنولوژی اصلاح نباتات و اصلاح خصوصی
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True mountain mahogany (Cercocarpus montanus Raf.) compensates for annual growth lost to browsing under conditions of high resource availability. To develop better guidelines for its management for big game winter forage, twig demography was studied under natural herbivory and resource availability inside and outside exclosures at 1 site in the Wasatch Mountains and on 4 sites on the North Slope of the Uinta Mountains, Utah. Annual and previous years' twig lengths, as well as location and numbers of flowers and seeds were diagrammed on branches of browsed and unbrowsed shrubs in the spring or summer and fall between 1996 and 1999. Annual twig growth and flower and seed numbers of both browsed and unbrowsed shrubs were greatest in 1997 or 1998 when precipitation was highest. Utilization of annual growth varied among sites within a year and among years within a site and ranged from < 21 % to > 300 % when previous years' growth was browsed. Despite differences in utilization, browsed twigs compensated similarly for length lost to herbivory, so that total twig lengths remained the same over the course of the study. Although twigs on unbrowsed shrubs had less annual growth per unit branch length than those on browsed shrubs, lack of length lost to herbivory resulted in an increase in total twig length over time. Years of high resource availability are important in allowing grazing tolerant shrubs such as true mountain mahogany to compensate for years of heavy utilization. Flower and seed numbers were much higher (P < 0.05) on unbrowsed than browsed shrubs. Compensatory growth was enough to maintain, but not increase total twig lengths after high utilization (> 100 %) even on years of high resource availability True mountain mahogany (Cercocarpus montanus Raf.) is one of the most important browse species in the mountain brush zone (Holechek et al. 2001), and is considered tolerant of browsing (Shepherd 1971, Davis 1990). Low growth form and basal sprouting allow big game easy accessibility to its forage (Davis 1990). On the North Slope of the Uinta Mountains, it has been heavily used in winter primarily by moose (Alces alces) and elk (Cervus elaphus), but also by deer (Odocoileus hemionus) or pronghorn antelope (Antilocapra americana) (Turley et al. 2003). In that study, heavy browsing reduced canopy cover and shrub size of true mountain mahogany, but shrubs on some sites with over 300 % utilization of annual growth still maintained their size. The grazing optimization hypothesis proposes that at a certain grazing intensity, production is optimized (Briske and Heitshmidt 1991). An important basis for this hypothesis is that plants can compensate for tissue lost to herbivory, and that growth may even be stimulated by tissue removal through factors such as decreased shading of photosynthetic tissue, and activation and increased flow of photosynthates, water, and nutrients to meristematic tissue (Noy-Meir 1993). The grazing optimization hypothesis has probably been over applied (Briske andHeithsmidt 1991), and certainly misinterpretedby those who declare it false usingthe damage of heavy grazing as evidence (Patten 1993). However, compensation for tissue loss has been well established (Briske 1993). Compensatory growth varies among species and even populations (Painter et al. 1989), and with resource availability for a given species (Williamson et al. 1989, Wandera et al. 1992, Briske 1993). True mountain mahogany tolerates simulated browsing when resources are highly available because of its ability to initiate long-shoot growth from available buds (Wandera et al. 1992). However, ability of true mountain mahogany and other shrubs to compensate for herbivory should be tested under field conditions of actual herbivory and resource availability to best recommend utilization and management guidelines (Wandera et al. 1992, Briske 1993, Noy-Meir 1993, Trlica and Rittenhouse 1993). The purpose of this study was to measure twig and reproductive responses of browsed and unbrowsed true mountain mahogany under field conditions to provide guidelines for its use andmanagement.
Methods روش ها
Twig demography was studied on unbrowsed shrubs inside exclosures and browsed shrubs outside at 1 site in the Wasatch Mountains (N 39° 45' 111 ° 48.8') and at 4 sites on the North Slope of the Uinta Mountains, Utah (N 40° 58' W 110° 4.5-14.5'). Vegetation, soils, and other characteristics of these sites have been described by Turley et al. (2003). Precipitation and temperature data for Gardner Canyon in the Wasatch Mountains were from the Nephi, Utah weather station at 1,520 m elevation located southwest (< 8 km) of the site (NOAA 1995-1999). Precipitation data for the North Slope of the Uinta Mountains were from the Lonetree weather station (NOAA 1995-1999). Lonetree is located about 16 km north of the 4 sites with an elevation of 2,280 m. At Gardner Canyon, 10 mature true mountain mahogany shrubs in the exclosure and 10 in the open area were randomly chosen for branch diagramming (Roundy and Ruyle 1989). Long shoot growth was measured on 3 branches and short shoot growth was measured on 1 branch on each shrub. Short shoots were distinguished from long shoots by their lack of internode elongation for previous years' wood. Ultimately, 30 long-shoot and 9 short-shoot branches were diagrammed in the exclosure, and 29 longshoot And 12 short-shoot branches were diagrammed in the open area. At the 4 North Slope sites, 3 branch diagrams of long shoots were drawn on each of 3 to 9 randomly chosen shrubs both inside and outside the exclosures. The number of shrubs per site included Gregory Basin inside- 9, outside- 3, Poison Mountain inside- 5, outside- 7, Telephone Hollow inside- 6, outside- 6, Widdop Mountain inside- 5, outside- 5. Branches were diagrammed to record location and length of annual growth, second- year wood, third-year wood, and location of buds, flowers, and seeds. Branches were drawn freehand to scale on 20 by 28 cm paper. All lengths were measured to the nearest millimeter. Each paper was photocopied, and branches were rediagrammed in the spring after winter browsing, and in the summer or fall after summer growth. Branches were diagrammed for twig growth from 1996 through 1999. At all sites, the number of seeds was recorded on diagrams in the springs of 1997, 1998, and 1999. The number of flowers per branch was recorded in 1997 through 1998 at the North Slope sites and in 1997 and 1998 at Gardner Canyon. Number of branches diagrammed each year that produced annual growth, flowers, seeds, and that were browsed were counted to determine branch activity. Annual growth browsed as a percentage of total annual growth was calculated to give percent utilization (USDA Forest Service 1979). This results in > 100 % utilization when previous years' growth is browsed. To determine branch size, total twig length of individual twigs was summed for each branch diagram. To determine increases in branch size over time, total twig length for a sample date was divided by total twig length from initial branch diagrams and measurements made in summer and fall 1996 Annual growth estimates included annual growth length per branch and annual growth length at the end of the growing season per total twig length at the beginning of the growing season. Annual twig growth per branch was the current annual growth produced by the individual branch divided by the number of current annual twigs produced by that branch. Flower and seed data were analyzed as numbers per branch, and numbers per total twig length. The number of seeds per flower and branch was also calculated. The ratio of twig length produced in a given year to total twig length was calculated for 1995 through 1998 to determine differences in branch demography between treatments and over time. For example, ratios of 1998, 1997, and 1996 wood over total twig length and 1995 and older wood over total twig length were calculated. Total precipitation and mean temperature at Gardner Creek were tested for correlation with mean annual twig growth per branch, mean annual twig growth of growing twigs per branch, mean annual growth length per branch, and mean seeds per branch. Annual growth was tested for correlation for the years 1996 through 1999. Seeds per branch were tested for correlation for years 1997 through 1999. Correlation analysis of North Slope sites used years 1996 to 1999, except for seeds per branch, which used years 1997 to 1999. An ANOVA mixed model approach using SAS (Littell et al. 1996) was used to statistically analyze data from twig diagrams. Fixed effects included browsing compared to no browsing and site. The random effects included shrubs nested within browsing treatment at the Gardner Creek site and shrubs nested within site and browsing treatment at the North Slope sites. Individual branches were nested within shrubs, browsing treatments, and sites. Shrubs were nested within browsing treatment because they were not blocked within treatments and were a random sample of those plants within that treatment. Sites were kept as a fixed variable, in which case inferences should only be made to the communities that were sampled. Three types of analysis were done. The first was a simple ANOVA, in which browsed and unbrowsed treatments were compared within a single year. All responses were analyzed with this simple ANOVA to compare treatments within years. Ratio of annual wood was only analyzed in this within-year ANOVA. The second analysis was a repeated measures ANOVA with year as a class variable in which each year is considered independent of all other years. This was used in the case of the responses of annual growth length per branch, annual growth length per total twig length, annual twig growth per branch, annual twig growth of growing twigs per branch, length browsed per branch, flowers per branch, flowers per total twig length, seeds per branch, and seeds per total twig length. Because these response variables may react differently under different climatic conditions from year to year, each year was seen as a class variable and not continuous. All browsing responses for Gardner Canyon were analyzed with year as the only variable. All browsing responses for the North Slope sites were analyzed for site, year, and their interaction, and when site was significant, only year. The third type of analysis was a repeated measures ANOVA with year as a continuousvariable in which each year wasconsidered dependent on the other years. This was used with the variables total twig length per branch, and total twig length a given year divided by total twig length in 1996. All sites at the North Slope were analyzed together unless site was significant, in which case an ANOVA was done for each site individually and between treatments. All 3 types of analysis used a backwards elimination in which nonsignificant effects were taken out of the model to achieve greater sensitivity of the tests by estimating the error terms more accurately. Because P-values < 0.25 may show trends, nonsignificant effects were taken out only when the P-value was > 0.25 to be conservative. Random effects were eliminated first and then fixed effects when necessary. A square root transformation wasperformed on all data prior to analysis.
Twig Growth Precipitation variables were significantly (P < 0.05) positively correlated with variougrowth variables for browsed shrubs. At Gardner Canyon and Telephone Hollow annual growth per total twig length was correlated with total precipitation (r2 = 0.99, 0.96). At Gardner Canyonannual growth per branch was significantly correlated with June precipitation (r2 = 0.99). At Poison Mountain and TelephoneHollow, annual growth per branch wascorrelated with total precipitation (r2 =0.99, 0.96). At Gregory Basin, annual growth per branch was correlated with August precipitation (r2 = 0.98), while at Widdop Mountain annual growth per branch was correlated with June through August precipitation (r2 = 0.95). Only at Widdop Mountain was there a significantpositive correlation between annual growth and precipitation for unbrowsed shrubs. At that site, annual growth per branch was significantly correlated with June through August precipitation (r2=0.96). Greater total precipitation and higher summer precipitation for the Nephi station in 1998 and for the Lonetree station in 1997 and 1998 (Figs. 1,2, and 3) were associated with greater annual growth forthese years.