Desert shrubs growing in a heterogeneous mosaic of high and low-density patches of plants may experience differences in resource availability through interactions with neighboring plants. Resource limitations during seed production could alter either the number or size of seeds produced by an individual. Variation in seed masses within and between plants may affect both the fitness of individuals and the growth rate of the population. To assess whether the distribution of seed masses is affected by proximity to neighboring plants, we sampled seeds from Ambrosia dumosa (Asteraceae) within a long-term ecological research site in Joshua Tree National Park. Prior research at this site has shown demographic differences between Ambrosia with and without close neighbors. Due to this known spatial heterogeneity, we hypothesize that 1) variation within the range of seeds produced by an individual Ambrosia is smaller than variation within the population, and 2) the proximity to neighbors modifies the sizes and number of seeds produced.
We collected seeds from 60 randomly-selected Ambrosia initially tagged in 1984 or 1989, for which the age, size, growth rate, and proximity to neighboring plants is known. Seed collections per parent ranged from 58 to 502 seeds. The progeny of six plants were germinated to test for viability. The set of 9,557 seeds from the remaining 54 parents was weighed to a precision of 0.0001 g. Parents were considered not neighbored if no woody perennials grew within 1 meter. To test whether neighbors affected the mass of Ambrosia seeds, a linear model was fitted in R(v3.2.3). We controlled for potential confounding effects of plant age, size, growth rate, number of seeds produced, and percentage of seeds retained on plant at time of collection.
Results/Conclusions
Parentage affected seed mass (p<0.001) and explained 42% of the variation in seed mass. The mean seed masses from plants without close neighbors were 8.6% heavier than those with neighbors (p<0.001). This result remained significant when controlling for confounding factors and spatial autocorrelation. Younger plants produced larger seeds (p<0.001), and larger plants produced larger seeds (p<0.001). However, seeds were smaller from plants with a faster growth rate between 1999 and 2015 (p<0.001). We conclude that the proximity of neighbors affects seed mass of Ambrosia. A possible explanation for this result is competition-mediated maternal effects on seed size in Ambrosia dumosa.