Allometric relationships between seed mass and seedling characteristics reveal trade-offs for neotropical gap-dependent species

Abstract

A seed size-seed number trade-off exists because smaller seeds are produced in greater number but have a lower probability of establishment. This reduced establishment success of smaller-seeded species may be determined by biophysical constraints imposed by scaling rules. Root and shoot diameter, root growth extension rate (RGER) and shoot length at death for dark-grown seedlings are predicted to scale with the cube root of seed embryo and endosperm mass (m). We confirmed this expectation for ten neotropical gap-dependent tree species with an embryo and endosperm dry mass >1 mg. However, for nine smaller seeded species (m < 1 mg) with photoblastic germination, root and shoot diameters were larger than expected, and consequently, RQER was slower than expected. The maximum shoot thrust of seedlings from seeds with masses >1 mg was comparable to the estimated force required to displace overlying litter, supporting the hypothesis that photoblastic behaviour only occurs in seeds with insufficient shoot thrust to displace overlying leaves. Using the model soil water, energy and transpiration to predict soil drying in small and large gaps, we showed that: (1) gaps that receive a significant amount of direct sunlight will dry more quickly than small gaps that do not, (2) compared to the wet-season, soil that is already dry at depth (i.e. the dry-season) will dry faster after rainfall (this drying would most likely kill seedlings from small seeds) and (3) even during the wet-season, dry periods of a few days in large gaps can kill shallow-rooted seedlings. We conclude that the smaller the seed, the more vulnerable its seedling would be to both covering by litter and soil drying because it can only emerge from shallow depths and has a slow RGER-Consequently, we suggest that these allometrically related factors contribute to the reduced establishment success of smaller-seeded species that underpins the seed size-seed number trade-off.

Plant and Animal Reproductive Strategies: Lessons from Offspring Size and Number Tradeoffs

“…Therefore, plants may produce small seeds with high calorie nutrition (lipid rich), or with a thicker seed coat, or with activate mechanisms of dormancy, and such seeds are likely as viable as large seeds under stressful conditions. However, there are disadvantages as well, since small seeds are generally more vulnerable to early mortality (Daws et al, 2007). If the stressful conditions persist for several generations, then offspring mortality can be very high (especially in the case of annuals), since all offspring originated from seeds smaller than that of their parents.…”

Soil drying in a tropical forest: Three distinct environments controlled by gap size

“…For seeds that are newly germinated, the challenge is greater than for seedlings because, to survive, the seedling root growth rate must outpace the rate of soil drying. The roots of most seedlings on BCI grow <50 mm in 5 days and those of most pioneer species <10 mm in 5 days (Dalling and Hubbell, 2002;Pearson et al, 2003a;Daws et al, 2007), so the dangers of desiccation especially to small (e.g. pioneer) germinating seeds are considerable even in the wet season.…”

Caracterización de semillas de un bosque siempreverde tropical del oeste de Cuba: Correlaciones ecológicas entre rasgos

“…La correlación casi perfecta que se obtuvo en el presente trabajo entre la masa total (fresca y seca) y la masa distribuida a la cubierta seminal fue encontrada también por Daws et al (2005Daws et al ( , 2007 en especies de bosques tropicales de Panamá. Estos autores señalan, que la masa seca es una buena medida de la cantidad de materia absoluta que se asignó a cada parte de la semilla, lo cual podría justificar los máximos valores promedio de masa seca de cubierta en plantas arbóreas y los mínimos en arbustos.…”

“…La falta de asociación entre el porcentaje de asignación a cubierta y las masas seminales indica, que la cantidad de masa que se "destina" a cada componente de la semilla deberá ser posiblemente un compromiso individual de cada especie con sus requerimientos para la dispersión, defensa, germinación y el establecimiento de las plántulas, como se ha propuesto para especies de ecosistemas tropicales (Daws et al 2007, Sánchez et al 2009a.…”

“…En general, solo se encontraron trabajos a nivel de comunidad, que aunque involucran un gran nú-mero de especies, fundamentalmente arbóreas, no tienen en cuenta la forma de crecimiento de la planta (ver e.g. Daws et al 2005Daws et al , 2007. Por consiguiente, serán necesarios más ensayos experimentales para dilucidar el significado ecológico de las variables seminales estudiadas en este grupo funcional.…”