To survive, a young seedling must simultaneously invest in roots to establish adequate access to water while also investing in leaves to develop photosynthetic capacity. Tradeoffs in resource allocation between root and shoot growth can have large consequences for seedling survival and growth rates. In an arid environment with unpredictable rainfall, the optimal root/shoot allocation ratio depends on both present and expected future water availability. We investigated whether early-life cues of potential future drought stress affect the root/shoot ratio of seedlings of Ambrosia dumosa, a shrub of the Mojave and Colorado Desert ecoregions. Data from a long-term field site in Joshua Tree NP suggests that unlike most shrubs, Ambrosia seedlings can establish in both dry open sites and less-harsh facilitated microenvironments. Based on this broad seedling establishment niche, we hypothesized that Ambrosia seedlings set their root/shoot ratio in response to initial conditions, and predicted that seedlings experiencing initially more xeric conditions would maintain a higher root/shoot ratio to guard against future drought stress. Alternatively, seedlings could acclimate by shifting their root/shoot growth allocation in response to each pulse of water. We performed a greenhouse experiment in which the timing of water provided during an 8-week period was varied while the total quantity was held constant. Utilizing a randomized complete block design with 20 blocks of 24 seedlings/block, Ambrosia seedlings were germinated under identical conditions, but watered at 2, 4, or 8-week intervals. We measured aboveground growth biweekly, and harvested individuals at 4 and 8 weeks to measure root/shoot ratio from dry biomass. A linear mixed-effects model was analyzed in Rstudio (package lme4), with fixed effects of the timing of watering, initial water availability, and initial seedling size, and a random effect of blocks.
Results/Conclusions
We found a significant interaction between the effects of timing of watering and initial water availability (p<0.01), the timing of watering (p<0.01), and the initial seedling size (p<0.05) on root/shoot ratio after 4 weeks of growth. However, none of these parameters were significant after 8 weeks of growth, once the total quantity of water delivered was the same for all seedlings. Taken together, these results suggest that although Ambrosia dumosa does respond to early-life conditions, it can also continue to alter its root/shoot ratio as it develops. The broad seedling establishment niche of Ambrosia in both open and facilitated microsites may be due to its’ ability to rapidly acclimate to changing water conditions.