Effects of Spine Removal on Coryphantha vivipara in Central Kansas

Abstract

Spine removal in the cactus Coryphantha vivipara resulted in no significant differences in stem-air temperature differentials relative to controls with spines in field and laboratory experiments. In addition, no significant differences between heights and diameters of clipped and unclipped plants were found in situ throughout a summer. In spite of these findings, spine removal resulted in higher (p<0.10) acid accumulation over a range of irradiance levels indicating that clipping allowed more irradiance penetration to the cactus surface. The lack of stem heating after spine removal may be related to increased levels of convective cooling-a result of the unusual morphology of this species. INTRODUCTION The massive succulent stems of desert cacti effectively delay the onset of severe water stress during extended droughts (Szarek and Ting, 1975; Levitt, 1980). As a further adaptation minimizing water loss, cacti undergo Crassulacean acid metabolism (CAM), opening their stomata mostly at night and assimilating CO2 when evaporative demand is low. A consequence of the subsequent daytime stomatal closure is minimal cooling by evaporation (Larcher, 1980). Also, the larger size of the photosynthetic organs precludes substantial cooling by convection (Gates, 1980). These two factors together result in a large (greater than 10 C) daytime increase in surface temperatures above that of the air (Gates et al., 1968; Gibbs and Patten, 1970; Baskin and Baskin, 1973). Such high daytime temperatures are potentially detrimental (Huber, 1932). On the other hand, high temperatures coupled with the high heat capacity of cactus tissues may benefit the plants by slowing the rate of cooling throughout the night. Maintenance of nocturnal stem temperatures higher than ambient has been implicated as a causal factor in the distribution of several cactus species (Shreve, 1911; Nobel, 1980). It has long been assumed that cactus spines deter herbivory (Haberlandt, 1884; Sumner, 1925; Stelfox and Vriend, 1977). Although this is undoubtedly true, spines may also alter the microclimate of the cactus surface to a physiologically important extent. Recently, Nobel and co-workers (Lewis and Nobel, 1977; Nobel 1978, 1983) showed that cactus spines decrease the irradiance on the stem surface, resulting in lower daytime surface temperatures. Also, spines increase the boundary layer depth on the stems, slowing the rate of nighttime cooling. Decreased daytime interception of light has been shown to lower rates of nocturnal CO2 uptake and thus the extent of acidification (Lewis and Nobel, 1977; Nobel, 1978, 1983). The majority of this research has been conducted in the laboratory using potted plants or computer simulations. Lewis and Nobel (1977) briefly mention testing their findings using one cactus with and without spines in the field. The purpose of this study is to test these predictions of the effects of spine removal on in situ and potted individuals of Coryphantha vivipara, a widespread cactus with extensive spine cover. MATERIAL AND METHODS Field research. -Stem temperatures, heights and diameters were measured throughout a summer on individuals of Coryphantha vivipara growing in a rocky prairie and on adjacent rock outcrops at the southern tip of Lucas Park at Wilson Reservoir, Russell Co., approximately 22 km N of Wilson, Kansas. Cacti were scattered among individuals of species of Bouteloua, Sporobolus, Yucca and Opuntia.