Leaf nutrients of two Cycas L. species contrast among in situ and ex situ locations

Abstract

An understanding of leaf nutrient relations is required for tree conservation and horticulture success. The study of cycad leaf nutrient dynamics has expanded in recent years, but direct comparisons among reports remains equivocal due to varying sampling protocols. We used Cycas micronesica K.D. Hill and Cycas nongnoochiae K.D. Hill trees to determine the influence on leaf nutrient concentrations of in situ versus ex situ locations and orientation of leaves within the tree canopy. Nitrogen, phosphorus, and potassium concentrations of leaves from ex situ plants exceeded those from in situ plants, and the differences were not explained by soil nutrient differences. Calcium concentrations of leaves varied among the site pairs, with differences primarily explained by soil calcium. Magnesium concentrations of leaves were not different among all location pairs even though soil magnesium concentrations varied among the sites more than any of the other elements. Differences in leaf macronutrient concentrations among four C. micronesica provenances were minimal when grown in a common garden. Lateral orientation of leaves did not influence any of the essential elements for either of the species. These findings indicate that the lateral orientation of cycad leaves does not influence leaf nutrient concentrations, leaf nutrient relations of cycad plants in managed ex situ settings do not align with leaf nutrient relations in habitat, and these differences are not explained by soil nutrition for most elements. We suggest that leaf nutrient concentrations should be determined in all niche habitats within the geographic range of a cycad species in order to fully understand the leaf physiology of each species.

Leaf Nutrient Relations of Cycads in a Common Garden

“…Moreover, the global data (Wright et al, 2004) were obtained from natural settings rather than botanic gardens to ensure the data have ecological relevance. Most of the published cycad leaf element data have been generated in botanic gardens (Deloso et al, 2020), where leaflet concentrations of nutrients are generally greater than from competitive forests (Marler & Lindström, 2020). The common garden approach is important to ensure all phylogenetic comparisons are based on data obtained from the same edaphic and climatic conditions, but the historical focus on cycad garden studies must be augmented with more data from natural settings.…”