Clinal adaptation and adaptive plasticity in Artemisia californica: implications for the response of a foundation species to predicted climate change

Local adaptation and plasticity pose significant obstacles to predicting plant responses to future climates. Although local adaptation and plasticity in plant functional traits have been documented for many species, less is known about population-level variation in plasticity and whether such variation is driven by adaptation to environmental variation. We examined clinal variation in traits and performance - and plastic responses to environmental change - for the shrub Artemisia californica along a 700 km gradient characterized (from south to north) by a fourfold increase in precipitation and a 61% decrease in interannual precipitation variation. Plants cloned from five populations along this gradient were grown for 3 years in treatments approximating the precipitation regimes of the north and south range margins. Most traits varying among populations did so clinally; northern populations (vs. southern) had higher water-use efficiencies and lower growth rates, C : N ratios and terpene concentrations. Notably, there was variation in plasticity for plant performance that was strongly correlated with source site interannual precipitation variability. The high-precipitation treatment (vs. low) increased growth and flower production more for plants from southern populations (181% and 279%, respectively) than northern populations (47% and 20%, respectively). Overall, precipitation variability at population source sites predicted 86% and 99% of variation in plasticity in growth and flowering, respectively. These striking, clinal patterns in plant traits and plasticity are indicative of adaptation to both the mean and variability of environmental conditions. Furthermore, our analysis of long-term coastal climate data in turn indicates an increase in interannual precipitation variation consistent with most global change models and, unexpectedly, this increased variation is especially pronounced at historically stable, northern sites. Our findings demonstrate the critical need to integrate fundamental evolutionary processes into global change models, as contemporary patterns of adaptation to environmental clines will mediate future plant responses to projected climate change.

The effect of steepness of temporal resource gradients on spatial root allocation

Plants are able to discriminately allocate greater biomass to organs that grow under higher resource levels. Recent evidence demonstrates that split-root plants also discriminately allocate more resources to roots that grow under dynamically improving nutrient levels, even when their other roots grow in richer patches. Here, we further tested whether, besides their responsiveness to the direction of resource gradients, plants are also sensitive to the steepness of environmental trajectories. Split-root Pisum sativum plants were grown so that one of their roots developed under constantly-high nutrient levels and the other root was subjected to dynamically improving nutrient levels of variable steepness. As expected, plants usually allocated a greater proportion of their biomass to roots that developed under constantly high resource availability; however, when given a choice, they allocated greater biomass to roots that initially experienced relatively low but steeply improving nutrient availabilities than to roots that developed under continuously-high nutrient availability. Such discrimination was not observed when the roots in the poor patch experienced only gentler improvements in nutrient availability. The results are compatible with the notion that responsiveness to the direction and steepness of environmental gradients could assist annual plants to increase their performance by anticipating resource availabilities foreseeable before the end of their growing season. The results exemplify the ability of plants to integrate and utilize environmental information and execute adaptive behaviours which, until recently, were attributed only to animals with central nervous systems.

Interference competition and niche theory

A linear model of interspecific competition with separate parameters for exploitation and interference is deduced. Interference is assumed to have a cost and an effect. The interfering species realizes a "profit" if some resources, which the species interfered against would have utilized, are made available as a result of the interference. Interference is favored when its cost is small, its effect is high, and the resource overlap with the species interfered against is high. Interference is likely to be an alternative strategy to high exploitation efficiency. The incorporation of interference into niche theory clarifies the competitive phenomenon of unstable equilibrium points, excess density compensation on islands, competitive avoidance by escape in time and space, the persistence of the "prudent predator," and the magnitude of the difference between the size of a species' fundamental niche and its realized niche.