Extreme events such as prolonged drought can alter the relative importance of competition and facilitation in plant populations. We examine drought effects on the demography of a population of Ambrosia dumosa (Asteraceae), a dominant shrub in the Colorado Desert that suffered 68% drought mortality between 1999-2004. Prior research showed that within this population, conspecific-neighbored plants were demographically distinct from those without conspecific neighbors during an interval of historically average climate, 1984-1994. Competitive and facilitative feedbacks may have shifted in response to extreme drought, motivating an analysis of drought impacts on Ambrosia demography. We hypothesize that drought intensified competition among neighbored plants, reducing their contribution to population growth.
We used a size-structured matrix population model to compare neighbored and isolated subsets of Ambrosia mapped in a permanent 1-ha site in Joshua Tree National Park, censused every five years since 1984. Analyses included: 1) Population growth rate, λ, of the total population structured by neighbor state, 2) elasticity analysis to observe climate effects on λ, and 3) a life table response experiment (LTRE) examining the effects of neighbors and climate on λ. We used randomizations of initial neighbor state to quantify the significance of neighbors on population responses to drought.
Although λ for the neighbored and isolated population subsets significantly differed prior to the severe drought (1984-89, p<0.001; 1989-94, p<0.0001; 1994-99, p<0.01), there was no significant difference between neighbored and isolated subsets for the severe drought interval of 1999-2004. Similarly, the LTRE showed an interaction between the effects of climate and neighbor state on vital rates. Elasticity analysis forecasted an overwhelming decrease in the contributions of neighbored plants to λ during the drought, as well as a marked increase in the elasticity of λ to both neighbored and isolated juvenile survival with the drought.
Our results support the hypothesis that drought stress changes the responses of individuals to neighboring plants, leading to shifts in the demography of neighbored and isolated population subsets. The effect of conspecific neighbors shifted from positive (1984-89) to negative (1989-99) to not significant (1999-2004) when compared to isolated plants. These results imply that while facilitation may buffer plant communities from mild stress, severe droughts may lead to widespread death regardless of facilitative buffers. With droughts in the Southwest anticipated to increase in frequency and duration with climate change, major disruptions to the growth and persistence of facilitation-dependent species may occur.