Is age just a number if you’re a plant? How plant responses to the environment change with age

As I recently discussed on Botany One in the context of epigenetics, studies of ecosystems often account extensively for variation between species, but less often examine the effects of variation within species (intraspecific variation). When intraspecific variation is considered, variation that may occur due to age or different developmental stages is seldom accounted for. In their recent paper in Annals of Botany, Jennifer Funk and colleagues investigate how trait variation in the Californian sagebrush Artemisia californica correlates compared to plant age compared to an environmental factor (water availability). The results they produce boost our understanding of the seldom-studied question of how plant traits may vary with age, as well as highlighting some of the difficulties faced when trying to understand the complex interactions found in natural ecosystems.

Funk and colleagues measure leaf physiology and morphological traits of Artemisia californica, as well as measures of plant performance such asflowering stalk length as a proxy for flowering effort, and canopy dimensions. They relate these to plant age and water availability. The authors use this data to address three questions. The first of these questions is how do measured leaf traits change with age and water availability?

The results gathered by Funk and colleagues show that both reduced water availability and increased age were associated with traits indicating more conservative use of resources. These include high leaf mass area but low leaf nitrogen and photosynthetic function. The correlation was particularly strong between these conservative traits and increased age. The author find that more conservative use of resources particularly seems to kick in after the first year of growth, the point at which Artemisia californica is considered to be mature.

The second question the authors pose is whether change in water availability impacts traits and growth more in older plants? The results of the study indicate that water availability has a greater ability to produce trait variation in younger plants compared to older plants, supporting a scenario in which younger plants seeking to establish themselves have greater growth plasticity that can adapt to possibly variable conditions.

The final question Funk and colleagues ask, and possibly the most complicated, is how water availability and age impact relationships between the traits and performance measurements they made. The authors admit that they expected to see strong correlations between traits and performance measurements in some conditions. However, this was surprisingly not the case and Funk and colleagues found few correlations between trait features and the performance measurements across ages or watering availabilities.

Why is this the case? One possible reason, the authors speculate, is that combinations of traits may be more influential on performance than individual traits. Funk and colleagues also concede that they do not measure any root traits in their experiments, which may be particularly influential in low-water systems. So it seems that, sadly, age is not just a number if you’re a plant – it has consequences for plant physiology, development and their responses to the environment. As Funk and colleagues conclude: ‘Our study demonstrates that plant age and environmental gradients create a sizable amount of intraspecific trait variation, which complicates the use of species means to model ecological processes’.