Abstract
Cycad plants possess uncommon morphological, chemical, and ontogenetic characteristics and they may introduce localized changes in soil traits that increase habitat heterogeneity. We used mature Cycas micronesica K.D. Hill trees growing in a range of soil types in Guam, Rota, and Yap to quantify differences between the soils beneath target trees and paired non-target soils away from cycad trees. The chronic presence of a C. micronesica tree introduced numerous localized changes in soil traits, increasing the heterogeneity of elemental stoichiometry in the community. Nitrogen, carbon:phosphorus, and nitrogen:phosphorus were increased in target soils among every soil type. Carbon increased and phosphorus decreased in most target soils. The habitats revealing the greatest number of elements with differences between target and non-target soils were the habitats with acid soils. The greatest number of metals exhibiting differences between the target and non-target soils occurred in the impoverished sandy habitat. This is the first report that indicates a cycad tree increases community spatial heterogeneity by localized changes in soil chemistry. Contemporary declines in cycad populations due to anthropogenic threats inadvertently decrease this spatial heterogeneity and its influences on primary producers in the landscape then cascading effects on the food web.
Highlights
We are aware of only two reports that included dedicated soil sampling with descriptions of proximity to the cycad plants that were being studied for green leaf nutrient content [45,46]. Continued research within this agenda must include descriptions of influential soil chemistry, and our results indicate the sampling sites must be constrained within close proximity to the studied cycad plants to enable the legitimacy of the methods
The long-lived C. micronesica tree is able to modify the elemental profile of the soils within the drip line of its leaf canopy
The potential causal mechanisms that a C. micronesica tree employs to modify the soils in direct contact with it were not analyzed in this study
Summary
Cycas micronesica K.D. Hill is an arborescent cycad species (Figure 1a) with a native range that includes Palau, Yap, Guam, and Rota Islands [1]. Hill is an arborescent cycad species (Figure 1a) with a native range that includes Palau, Yap, Guam, and Rota Islands [1] These Micronesian islands are positioned in the western belt of the Pacific Ring of Fire from ca. C. micronesica tree mortality [2,3,4]. This tree was the most numerous tree species on Guam at the time of the invasion [5], indicating that it served as a foundation species due to its abundance (Figure 1b). The selective purging of this single species from in-situ and urban forests could lead to substantial ecological changes [4]
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