Root contraction does not increase long-term population growth in Pediocactus bradyi, an endangered cactus of...

Pediocactus bradyi, a semi-globose cactus endemic to northern Arizona, displays a root-contraction mechanism to survive extreme drought: its roots contract, pulling the stem below ground during dry periods, re-emerging once rains return. To quantify how root contraction shapes population dynamics, we developed an integral projection model based on 31 years of demographic data from a P. bradyi population on the Navajo Nation Off-Reservation Trust Land. We explored two scenarios: one including root contraction and one excluding it. We found that, being ∼10% of the individuals and mostly confined to smaller individuals, root contraction did not have an effect on the long-term population growth [λ = 1.041 (1.039, 1.303) with contraction vs. λ = 1.044 (1.035, 1.289) without]. Also, we show that larger individuals have higher survival and reproductive rates, while growth declines beyond 35 mm in diameter. An elasticity analysis confirmed that survival and growth are the main vital rates affecting population growth, followed by root elongation after contraction. Thus, while root contraction may improve individual survival, elongation is in fact more important at the population level. Therefore, as with most cacti species, conservation efforts should focus on ensuring the survival of large individuals irrespective of their root contraction status.