Is it necessary that rate of evolution of longer lived trees will be lesser than that of annuals ? I understand that new individuals will come up faster in annuals and it may adapt to varying conditions faster but does it necessarily imply that long lived trees will evolve slower ?
That is true to a certain extent in normal environments, where there is not an excess prevalence of mutagenic elements. The "Generation Time Hypothesis" explains why a smaller generation time and hence a higher reproductive rate ( as in annuals) corresponds to a higher rate of mutation (substitutions in DNA) and therefore have a faster evolutionary rate as compared to slow reproducing and longer generation time individuals(for example, long-lived trees).
Since "fitness"-increasing adaptations produced by somatic variations are very less, the formation of new offsprings giving rise to individuals carrying a DNA which has been subjected to processes (Repeated divisions, replications and translations, amphimixis, and different environment) making it more vulnerable to mutation, becomes mandatory, to propel evolution. This process is much faster in annuals than in long lived trees like Sequoia. Moreover Sexual selection (especially in animals) operates at a much slower rate in longer-generation time individuals. Next, smaller individuals, having higher metabolic rates usually have shorter generation time (more pronounced in case of animals). Higher metabolic activity is linked to greater mutation rate (Due to metabolic intermediates acting as mutagens) and hence faster evolutionary rate. Higher metabolic rate also is linked to higher temperature (faster mutation) and these individuals have a geographic distribution favouring the tropics.(Again increases evolutionary rate)
All these, imply faster evolution for short lived individuals(which usually have higher reproductive rate) as compared to long lived trees. But under "normal" environmental conditions (not drastic and fast changes), the evolutionary rate of large individuals is fast enough to preserve their existence (as the Sequoia prevalence indicates). However, in fast changing environment, larger and long-lived individuals are more likely to be wiped out (probably, this applied to Dinosaurs!).