Seed Weight in Relation to Environmental Conditions in California

Abstract

Analyses of the California flora involving nearly 2,500 taxa are presented which show that there are correlations between the weights of individual seeds and environmental conditions in which their producers normally grow. These differences in seed weight appear to be adaptive and result from compromises between increased nutrition of the seedling which would result from larger food reserves in heavier seeds and increased dispersibility and increased reproductive output which are provided when smaller seeds are produced in larger numbers. Literature and experiments show a general positive correlation between seed weights and rates of shoot and root growth, at least within species. Herbs (annual and perennial), shrubs and trees are necessarily treated separately in the calculations in this paper (for seed weights increase progressively in a series from annual herb to tree when to California flora or any particular community type within it are considered). Raw seed weights are reduced to a series of classes (on a logarithmic basis) before means for floras or community types are calculated. Generalizations arrived at from considerations of "native" species in "natural" plant communities are confirmed by their applicability to "introduced" species now forming various kinds of "weed" communities in California. Finally, species lists from actual stand analyses, including both "native" and "introduced" species, are utilized to provide the data for more precise analyses. For herbaceous plants, seed weights are higher, on the average, for taxa whose seedlings are exposed to the risk of drought soon after establishment (giving faster root—development). Such a relationship can be demonstrated for species of a single genus or, on a combination basis, for community types as a whole and can be put on a quantitative basis by subjectively ordinating community types (in relation to the likelihood of drought stress hitting the seedlings) and making rank correlations with mean seed weight for each community type. The relation holds even for desert communities (where large—seeded perennials produce large root—systems but small—seeded ephemerals complete at least their seedling development in temporarily mesic conditions). In coastal communities, the importance of wind—dispersal of the seeds of species whose seedlings become established in rock crevices outweighs any droughtiness of the habitat in favoring smaller seeds than typify the community types generally. Correlations of herb seed—weight with likelihood of seedlings becoming established in shade or in conditions of severe competition are less marked for California than Salisbury found them to be in England. For shrubs, shading and competitive stress appear to be more influential factors in promoting increased mean seed weight but for trees moisture availability again appears to be most important. Another kind of correlation is established; between mean seed weight and altitude at which the plants occur. With decreasing length of the growing season as Californian mountains are a...

Fire‐stimulated reproduction in the resprouting, non‐serotinous conifer Podocarpus drouynianus (Podocarpaceae): the impact of a changing fire regime

“…These local nuances in resource availability may impact seed size and potential germinability (Schutte et al 2008). As the size and weight of seeds increases, so does seedling resilience to shading and competitive stress (Baker 1972;Stanton 1984). Seedlings of P. drouynianus established from the smaller seeds that are dispersed following spring burns may be less successful in the post-burn environment, having to compete with many other resprouter and seeder species (Bell 2001).…”

Tree recruitment at theNothofagus pumilioalpine timberline in Tierra del Fuego, Chile

“…This restriction could be imposed by pollination (Allen & Platt 1990) or the resources allocated to seed production, or both. Seed dry mass decreased signi®cantly at Balseiro mountain from 9.7 0.4 mg at 450 m to 3.6 0.5 mg at 690 m (Cuevas 1999), suggesting that resource limitation as a consequence of the short growing season and low photosynthetic rates as compared to lower altitudes (Baker 1972;Benecke & Nordmeyer 1982;Barclay & Crawford 1984) is involved.…”

Seed weight increases with altitude in the Swiss Alps between related species but not among populations of individual species

“…In the most detailed across-species study of variation in seed weight with environmental conditions, seed weight of herbaceous species of the Californian flora decreased with increasing altitude as well as with decreasing soil moisture (Baker 1972). As differences in altitude and moisture availability are confounded in the Mediterranean climate of California, the conclusions were questioned (Ko¨rner 2003).…”

“…As differences in altitude and moisture availability are confounded in the Mediterranean climate of California, the conclusions were questioned (Ko¨rner 2003). Phylogenetic relationships among species are important but were not considered by Baker (1972), whereas Landolt (1967) compared seed weight in pairs of congeneric lowland and alpine species in Switzerland. Qualitatively measured seed size tended to increase rather than decrease with higher altitude (Landolt 1967).…”