Distribution of Elements along the Rachis of Cycas micronesica Leaves: A Cautionary Note for Sampling Design

Abstract

Cycas micronesica K.D. Hill trees on the island of Yap were used to determine the influence of position along the leaf rachis on macro- and micro-nutrient concentrations and how leaf age affected the results. The outcomes revealed improvements to sampling protocols for future cycad leaf research. The concentration of every element except carbon and copper was influenced by leaflet position along the rachis. Most elements exhibited similar patterns for the oldest and youngest leaves on a tree, but the influence of position along the rachis for nitrogen, phosphorus, calcium, zinc, and boron was highly contrasting for old versus young leaves. The elements with the greatest variability along the rachis were potassium, phosphorus, manganese, and zinc, with the difference in basal and terminal leaflets as great as four-fold. Sampling leaflets at one position on a cycad leaf may generate inaccurate elemental concentration results for most essential nutrients other than carbon and copper. We have added position of sampled leaflets within leaves as a mandatory component of what is recorded and reported for future cycad leaf tissue analyses. Leaflets that span the full length of the rachis should be included in cycad leaf samples that are collected for tissue analysis.

Incident Light and Leaf Age Influence Leaflet Element Concentrations of Cycas micronesica Trees

“…Therefore, a direct measure of leaf age or the determination of which historical leaf flush a sampled leaf resides within is a mandatory component of the methods for future studies on cycad species. This can be accommodated by sampling every leaf of a plant [24] or by explicating which leaf flush was used for obtaining the samples [4,5]. We have also shown herein that the incident light at the leaf level influenced mass-based nutrient concentrations for many of the essential elements and influenced the area-based nutrient concentrations for all 11 of the elements that we measured.…”

“…Most in situ cycad habitats and most botanical gardens do not provide full sun conditions for the resident cycad plants. For plants that are not growing in full sun, measuring the amount of incident light [4,5] is a mandatory component of the methods for all future studies. In cultivated conditions where commercial shade fabric is employed, a description of percent shade would be sufficient.…”

“…We argue that there is a need to standardize sampling protocols and conduct more case studies to fully understand what controls leaf nutrient status of cycad species and to compare these traits with other gymnosperm and angiosperm taxa [3]. The sampling protocols have been diverse and the level of detail in sampling methods contrasts among the reports [4][5][6][7][8][9][10][11][12][13][14][15]. We have recently shown that plant size [4] and position of leaflet along the rachis [5] are two plant traits that should be recorded to ensure repeatable methods in cycad leaf nutrient studies.…”

“…The sampling protocols have been diverse and the level of detail in sampling methods contrasts among the reports [4][5][6][7][8][9][10][11][12][13][14][15]. We have recently shown that plant size [4] and position of leaflet along the rachis [5] are two plant traits that should be recorded to ensure repeatable methods in cycad leaf nutrient studies. Additionally, the nutrient status of the soils within the leaf umbrella of the sampled plants needs to be quantified and reported, as this soil differs in nutrient content from that of the bulk community soil [6].…”

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].…”