Sensitivity of the fire-dependent species Tecate cypress to fire frequency, fragmentation, and connectivity among patches

Background/Question/Methods   Fire-prone environments usually encompass areas with different burn probabilities, which results in species having subpopulations with asynchronous dynamics. This asynchrony arises when subpopulations differ in their fire probability distributions or when despite sharing the same underlying probability, they burn at different times. We hypothesize that 1) fire-dependent species are highly sensitive to asynchrony among subpopulations and 2) this effect is mediated by the level of connectivity among subpopulations. To test these hypotheses, we constructed a size-structure demographic model with density dependent mortality for Cupressus forbesii (Tecate Cypress). This species is one of the twelve endemic cypresses in southwestern North America, and its current distribution is restricted to five populations within southern California, and several small and scattered stands in Baja California, Mexico. Seeding and recruitment in Tecate cypress stands occur only after a fire, which often kills all individuals in the population. We estimated the long-term total population growth rate and probability of extinction for this population under different fire regimes and when population is represented as source-sink subpopulations with different connectivity, and independent fire dynamics. We ran each simulation, with 1000 iterations, for 500 years.

Results/Conclusions   In all our simulations, the population showed negative growth with a fire return interval (FRI) of less than twelve years. When the FRI was above this threshold, growth rates were higher but highly variable. Long-term population growth rates peaked at a FRI of around 16 years but decreased to null growth when FRI was above 50 years. Growth rates showed low sensitivity when the population was divided into subpopulations with independent fire dynamics given that although total population growth rate was lower and more variable in fragmented than in unfragmented populations, these differences were not significant. Population growth rate did not change with increased connectivity among source and sink subpopulations, although connectivity had an effect when at least one subpopulation experienced a low FRI (less than 16 years). Total population growth rate was significantly lower whenever source subpopulations had the lowest FRI. This effect decreased as years between fire events among subpopulations increased. We conclude that the lack of sensitivity of population growth at larger FRI and connectivity reflects the idiosyncrasy of Tecate's life history, of strong density-dependent mortality on seedlings after fire and of decreasing seed viability with time. Fire-dependent species with relaxed density dependence and constant recruitment rates might be more sensitive than Tecate cypress to FRI and connectivity.