Size is not everything for desiccation‐sensitive seeds

Abstract

Summary Almost 50% of all seed plants produce desiccation‐sensitive seeds. For these species, pre‐germination survival may be equally as important a basis of seed trait selection as seedling establishment. However, few studies have explored correlations among seed traits considered influential in the retention of viability prior to germination. We examine four physiological traits: critical water content (Wr50), the mean seed water content when a cohort of seeds reaches 50% mortality, desiccation rate (t0.368), the time taken for each seed to reach a relative water content of 36.8%, mean time to germination (TG), the time taken for 50% of seeds within a cohort to germinate; and specific relative water content (WM), the proportion of available water to dry seed mass; two morphological traits: seed size (MD), dry seed mass and seed coat ratio (SCR), the proportional mass between the seed coat and dry seed mass; and one phenological variable: mean monthly rainfall at mean time of seed dispersal (RM) in 16 plant species that produce desiccation‐sensitive seeds from a seasonal tropical forest in Cairns, Queensland. We first test for relationships among species, and then assess relationships between species in trait space via principal component analysis (PCA). Regression analysis revealed a significant negative relationship between RM and SCR, so that species that had dispersal periods at times of high moisture availability invested proportionally less into seed coats. No other pairwise trait combinations were significantly related. PCA revealed two axes of trait space. The first axis was associated with the traits SCR, TG, t0.368, RM and WM, and explained 44.1% of the variation between species. There were strong positive loadings for SCR, TG and t0.368, and strong negative loadings for RM and WM. Traits with strong loadings on the second axis (total variance 21%) were MD and Wr50 and WM. There were strong positive loadings for Wr50 and strong negative loadings for MD and WM. Synthesis. Our findings reveal an axis of trait variability among desiccation‐sensitive seeds that is orthogonal to size. These traits might be important in surviving pre‐germination environmental conditions, independent of the advantages and/or disadvantages that size has been shown to confer on seed survival and seedling establishment. The implication of this finding is that seed size alone may not account for pre‐germination viability in desiccation‐sensitive seeds and may be inadequate to predict long‐term persistence of these species if climate changes occur on the scale predicted.