The data support the hypothesis that canopy species have developed mechanisms to prevent germination in open sun conditions. The results and data from the literature show that inhibition of germination at temperature regimes characteristic of open sun conditions can be found in fleshy-fruited species of widely divergent taxonomic groups. It is predicted that germination mechanisms to detect canopy shade based on temperature cues are widespread in species depending on nurse plants, especially bird-dispersed species.
The longer soil has to stay moist to allow germination the more likely that seedlings experience favourable moisture conditions. Since theory predicts that fitness variance-reducing traits will be negatively correlated, we tested the hypothesis that time to germination is negatively correlated with the ability of radicle growth to keep up with the drying front. We measured time to germination and root elongation rate (RER) in 14 Kalahari annuals. We controlled for habitat (canopy association and sand content), germinability, median base water potential for germination (psi(50)), seed mass and seed shape as a persistence surrogate. For species and phylogenetically independent contrasts (PICs), we did not find a relationship between time to germination and RER. However, we found a negative relationship of time to germination with RER for PICs when controlling for sand content and psi(50). Seed shape increased with time to germination which can be explained by reduced opportunities for germination in slow-germinating species that select for persistence. We found a positive relationship between time to germination and psi(50), suggesting a continuum of risky to cautious germination. psi(50) was not correlated with RER suggesting that variation in psi(50) reflects different drought-adapted traits. Probably the relationship of time to germination with RER is not mediated by seed mass, which was not correlated with time to germination as found by others, though RER was positively correlated with seed mass. Instead of the seed size-seed number trade-off, a trade-off with resource capture may explain variation in RER: more root hairs or rootlets may increase resource capturing surface while reducing RER. For habitat, we found a (positive) relationship with time to germination only for canopy association. This may be explained by maximization of resource capture at the expense of RER being favoured by the higher nutrient and water availability under canopies. Future studies should clarify which trade-offs govern variation in time to germination, focussing on a possible resource capture-RER trade-off.
This paper aims to contribute to the taxonomy and biogeography of Southeast Asian cicadas. It also provides a basis for biodiversity studies in cicadas of Southeast Asia, like those executed in Malaysia by Zaidi and co-workers (Zaidi 1996, 1997, Zaidi & Hamid 1996, Zaidi & Ruslan 1995). Since almost all male cicadas sing, assessment of biodiversity would be greatly enhanced if identification by sound, like is normal for birds, would be possible also in tropical rainforests and similar habitats despite of high biodiversity. During daytime , observation of tropical rainforest cicadas in their natural habitat, the canopy for most species, is very difficult and catching them is hardly feasible. Also samples collected at light are rather small and thought to be unrepresentative for the whole cicada fauna at a certain locality. For a quick and easy assessment of cicada biodiversity sound would be most suitable but first we should know the song patterns of the different cicada species. This paper is the first of a series of planned to integrate data from our research groups in the Zoological Museum, Amsterdam and the Prirodoslovni muzej Slovenije, Ljubljana, concerning cicada taxonomy and bioacoustics.
Questions Is kind of seed dormancy important in environmental filtering at the local scale? Are there differences among kinds of seed dormancy in life‐history traits like seed mass and adult life span? Location Southern Kalahari (Botswana, Namibia and South Africa). Methods Using mainly literature data, we determined kind of dormancy for each species in the Southern Kalahari flora and combined this data set with data on seed mass, life form and soil texture. Soil sand content was used as an environmental gradient because the risks associated with seedling establishment increase with soil sand content and would favour seed dormancy as a bet‐hedging strategy. Also, the rapid descent of water in very sandy soils may exclude species with morphological (MD) or morphophysiological (MPD) dormancy that require relatively long moist periods for dormancy breaking. Multinomial logistic regression and phylogenetically independent contrasts were applied to determine relationships of kind of dormancy with life‐history variables and soil texture. Results The proportion of species with some kind of seed dormancy was not correlated with soil sand content. However, the proportion of species with MD or MPD decreased with increasing sand content, and proportion of species with some kind of dormancy decreased with increasing seed mass and life span. Life form and seed mass, but not soil sand content, had significant effects in a multinomial regression model predicting the presence of kind of dormancy. With increasing seed mass, the probability of a species having physical dormancy (PY) increased relative to the probability of having physiological dormancy (PD). With an increase in life span, the probability of a species lacking dormancy increased relative to the probability of having PD. Conclusions We found little evidence that kind of seed dormancy plays a direct role in environmental filtering. Our results suggest that traits related to dormancy do not evolve independently and are part of a syndrome of co‐adapted plant traits.
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