Cycas micronesica Trees Alter Local Soil Traits

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.

Two Cycad Species Affect the Carbon, Nitrogen, and Phosphorus Content of Soils

“…The fieldwork and analyses were patterned after our previous methods [3,8]. The soils associated with the C. micronesica plants were collected at half the length of the pinnately compound leaves, and four subsample cores at each cardinal direction were collected in the 0-10 cm horizon for each specimen.…”

“…The direct influence of the gymnosperm Cycas micronesica K.D. Hill (Cycadaceae, Cycadales) on soil chemistry was determined in a variety of locations and soils [3]. The results revealed an increase in nitrogen and carbon and a decrease in phosphorus in the soils beneath the in situ cycad trees.…”

“…Herein we focused on those modifications in nitrogen, carbon, and phosphorus and the carbon:nitrogen:phosphorus stoichiometric traits. Moreover, the long list of minerals and metals that were determined in the earlier C. micronesica report [3] did not include labile biogeochemical variables such as the components of mineralization. We used this managed Tinian C. micronesica germplasm to determine the ephemeral changes in ammonium and nitrate and how the introduced cycad trees influenced these mineralization processes.…”

“…We also included Z. integrifolia in this report to expand the list of studied cycad taxa to two species, and because it is commonly used in the nursery and landscape horticulture industry. Our approach was to repeat the methods that were previously used for C. micronesica [3]. Because cycad roots associate with nitrogen-fixing endosymbionts, we predicted the changes in soil nitrogen concentrations associated with the newly introduced cycad plants would begin earlier than the changes in soil carbon or phosphorus.…”

Tree conservation can be constrained by agents from conservation permitting and funding agencies

“…Indeed, ecosystem changes can be misunderstood in insular settings if consistent monitoring is not used [50]. This wise approach of a funding agency enabled the accumulation of the only existing long-term C. micronesica survival data set throughout three islands and has resulted in numerous peer-reviewed articles that are available to inform future C. micronesica conservation decisions [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68].…”