Direct and Indirect Effects of Salt Spray and Fire on Coastal Heathland Plant Physiology and Community Composition

RHODORA, Vol. 108, No. 933, pp. 32–42, 2006 DIRECT AND INDIRECT EFFECTS OF SALT SPRAY AND FIRE ON COASTAL HEATHLAND PLANT PHYSIOLOGY AND COMMUNITY COMPOSITION 1 2 MEGAN E. GRIFFITHS, ROBIN P. KEITH, AND COLIN M. ORIANS Department of Biology, Tufts University, Medford, MA 02155 1Current Address: Forest Biodiversity Research Unit, School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa e-mail: griffi[email protected] 2Current Address: Department of Geography, University of Colorado, Boulder, CO 80309 ABSTRACT. Coastal sandplain heathlands are managed largely by prescribed burning. While it is known that salt spray is an important natural disturbance contributing to the maintenance of coastal heathlands, it is unclear whether fire in coastal areas intensifies the detrimental effects of salt spray on plants growing close to the ocean. We carried out a field experiment to test the interactive effects of fire and salt spray on Martha’s Vineyard, Massachusetts. Following a prescribed fire, one-meter-square plots were established in burned and unburned areas and randomly assigned a control or salt spray treatment. After ten weeks of spray treatments we found that burning stimulated new growth and salt spray consistently increased plant water stress, increased leaf necrosis, and inhibited shoot elongation. Burning did not increase the negative effects of salt spray in individual plants; salt spray and control treated plants growing in burned areas showed less water stress and leaf necrosis than those in unburned areas. This may be due to increased water availability in the burned area resulting from lower biomass and therefore lower competition for water and lower evapotranspiration rates. Burning reduced plant canopy height, decreased vascular plant species richness, and stimulated new growth in plants. Our results suggest that fire indirectly reduces the damaging effects of salt spray by increasing soil water availability and decreasing plant water stress. Key Words: disturbance regime, physiological responses, prescribed burning, vascular plant community structure, water balance A fundamental principle of modern ecology is that disturbance maintains the structural and species diversity of communities (Pickett and White 1986). Coastal sandplain heathlands in the northeastern United States are characterized by their dependence on salt spray, burning, grazing, mowing, and plowing to remain in an early suc- cessional state. Recent studies have demonstrated that heathland communities developed in response to intensive agricultural and forest clearance by European settlers (Foster et al. 2002; Foster and Motzkin 32 2006] Griffiths et al.—Salt Spray and Fire in Heathlands 33 2003; Motzkin et al. 2002). In the present day, however, these land use practices are infrequent and the lack of large-scale disturbance has al- lowed succession by native woody species such as Pinus rigida P. Mill. and Quercus ilicifolia Wangenh. to proceed in many formerly shrub- dominated heathlands (Dunwiddie 1989). Current management in coastal heathlands emphasizes the use of prescribed burning to maintain both the characteristic low stature and the composition of these vascular plant communities (Dunwiddie and Caljouw 1990). While fire has been the primary focus of restoration efforts in these heathlands, salt spray has also been shown to play a role in maintaining the low stature of plants and the distinctive plant community composition of heathlands that grow in close proximity to the ocean and receive naturally high levels of salt spray (Griffiths 2003). Research from heathlands in coastal regions of Tasmania suggests that fire might act synergistically with salt spray, resulting in high levels of damage to burned plant communities in areas with intense salt spray (Thomas and Kirkpatrick 1996). Synergistic fire and salt spray effects have also been conjectured for coastal sandplain heathlands in the United States (Dunwiddie et al. 1997). Such interactions could occur on a very short-term basis because heathland species show rapid resprouting in the growing season immediately following a burn. The new tissue is expected to be more susceptible to damage by salt spray because young leaves have less well-developed cuticles and are therefore more easily penetrated by salt spray (Franke 1967). Consequently, we hypothesized that recently burned plants would exhibit higher levels of necrotic damage and water stress in response to salt spray, as compared with plants that had not been burned. The combined effects of fire and salt spray are important to consider when creating fire management plans because salt spray damage to plants may be increased in recently burned sites. This damage could limit aboveground growth in heathlands and selectively inhibit salt-intolerant species, thereby changing the stature and composition in these communities. The purposes of this research were therefore to: (1) establish whether burning exacerbates the morphological and physiological effects of salt spray applied at natural levels, (2) determine the mechanism of the response of individual plants to burning and salt spray, and (3) ascertain if burning and salt spray effects are expressed in changes in heathland community structure and composition. MATERIALS AND METHODS Field measurements. The plant species composition of a stand before burning is a major determinant of the plant community response 34 Rhodora [Vol. 108 to fire (Hobbs and Gimingham 1987). For that reason we used a single site for experimental work to control for differences in pre-fire plant community composition. The combined effects of burning and salt spray on vascular heathland plants were investigated at King Point in Edgartown, Martha’s Vineyard, Massachusetts (418219200N, 708329300W), a conservation area managed by the Sheriff’s Meadow Foundation. The site is situated approximately 2 km from the ocean and there was no natural salt spray accumulation on plants in the study area during the experimental period (data not shown), allowing us to manipulate the amount of salt spray a plant received. King Point was used as sheep pasture in the late 19th century but both grazing and fire had not occurred in the area since 1950 (R. Johnson, Sheriff’s Meadow Foundation, pers. comm.). The Nature Conservancy’s Fire Management Program burned approximately 6 ha of heathland at King Point on 14 April, 2000. The fire was of moderate intensity and resulted in 75–95% top kill of shrubs (J. Carlson, The Nature Conservancy Fire Management Program, pers. comm.). We randomly placed 48 one-meter-square (1 m2) plots throughout the 6 ha burned area and an equivalently sized unburned area of King Point following the prescribed fire. The experiment was designed to have 12 plots each in four different treatments: unburned þ control spray (UC), unburned þ salt spray (US), burned þ control spray (BC), and burned þ salt spray (BS). Control plots were sprayed every second day with deionized water using a handheld plant mister to account for any physical effect the spraying had on plants. For the salt spray treatment, filtered seawater (31 ppt) was collected off the southern shore of Martha’s Vineyard, and applied every second day at a concentration of 8 mg NaCl/dm2, which is equivalent to the accumulation rate found on plants growing close to the ocean (Griffiths 2003). Spray treatments began 12 June, 2000 and lasted for 10 weeks. Within each plot, we tagged individuals of three perennial heathland plant species: Solidago rugosa P. Mill., Gaylussacia baccata (Wan- genh.) K. Koch, and Vaccinium angustifolium Aiton. Plants were standardized by height for each species. Prior to the treatment period, shoot height and number of leaves were measured for each focal plant used in the experiment. Shoot height and leaf number were sampled again following the spray treatment period so that shoot elongation and leaf production could be calculated. Additional morphological measurements included mean area of the first three fully expanded leaves and the total number of fruits produced by the entire plant for G. baccata and V. angustifolium. 2006] Griffiths et al.—Salt Spray and Fire in Heathlands 35 Two aspects of water status were tested at the end of the treatment period: predawn xylem pressure potential and leaf water content. Predawn xylem pressure potential of stems was measured in the field at 04:00 using a pressure chamber (PMS Instrument Company, Corvalis, OR). Leaf water content was measured for the first three fully expanded leaves at the top of each plant. Leaves were dried at approximately 608C for 48 h to determine dry mass. Leaf water content was calculated using the following equation: leaf wet mass 2 leaf dry mass water content ¼ leaf dry mass Leaf necrosis was assessed on the same leaves used for water content measurements. It was measured using a grid and expressed as a pro- portion of the total leaf area with necrotic damage. Leaf necrosis serves as an effective index for salt spray damage because it occurs directly as an accumulation of chloride ions to levels toxic for plants (Parsons and Gill 1968). The overall plant community stature for each 1 m2 plot was determined as a mean of 15 randomly placed canopy height measures that included any vascular plant species. Vascular plant community composition of each plot was also surveyed to determine plant species richness. These surveys included measurements for all species of grasses, forbs, climbers, shrubs, and trees. Statistical analysis. Statistical analyses were carried out using general linear model procedures in SPSS 11.5.0 (SPSS, Chicago, IL). Preliminary analyses showed that the three focal species responded sig- nificantly differently to burning and spray treatments (F14, 254 ¼ 24.35, p , 0.001) therefore each species was analyzed with a separate MANOVA to determine the effects of burning, salt spray, and the interaction between treatments. We interpreted overall effects based on Wilks’ k statistics but also considered the ANOVA results for each individual plant trait that was measured. Responses of plant community canopy height and vascular plant species richness to burning and spray treatments were investigated using two-way ANOVAs.

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