Minimum recruitment frequency in plants with episodic recruitment

Abstract

There is concern about the lack of recruitment of Acacia trees in the Negev desert of Israel. We have developed three models to estimate the frequency of recruitment necessary for long-term population survival (i.e. positive average population growth for 1,000 years and < 10% probability of extinction). Two models assume purely episodic recruitment based on the general notion that recruitment in arid environments is highly episodic. They differ in that the deterministic model investigates average dynamics while the stochastic model does not. Studies indicating that recruitment episodes in arid environments have been overemphasized motivated the development of the third model. This semi-stochastic model simulates a mixture of continuous and episodic recruitment. Model analysis was done analytically for the deterministic model and via running model simulations for the stochastic and semi-stochastic models. The deterministic and stochastic models predict that, on average, 2.2 and 3.7 recruitment events per century, respectively, are necessary to sustain the population. According to the semi-stochastic model, 1.6 large recruitment events per century and an annual probability of 50% that a small recruitment event occurs are needed. A consequence of purely episodic recruitment is that all recruitment episodes produce extremely large numbers of recruits (i.e. at odds with field observations), an evaluation that holds even when considering that rare events must be large. Thus, the semi-stochastic model appears to be the most realistic model. Comparing the prediction of the semi-stochastic model to field observations in the Negev desert shows that the absence of observations of extremely large recruitment events is no reason for concern. However, the almost complete absence of small recruitment events is a serious reason for concern. The lack of recruitment may be due to decreased densities of large mammalian herbivores and might be further exacerbated by possible changes in climate, both in terms of average precipitation and the temporal distribution of rain.

Radiocarbon Determination of Past Growth Rates of Living Acacia tortilis Trees from Two Arid Sites in Eastern Sahara

“…Successful recruitment seems to require several consecutive rainfall events (unpublished ACACIA interview data; Wiegand et al 2004). The fact that 4 of 5 trees at Nuqrus were established in a limited period might be due to how the sampled trees were selected, but might just as well indicate that the Nuqrus catchment had received more frequent rains during the time of establishment than later.…”

History matters: tree establishment variability and species turnover in an African savanna

“…Variability in rates of tree establishment, whether due to fire (Higgins et al 2000), herbivory (Dublin et al 1990, Prins and van der Jeugd 1993, Baxter and Getz 2005 or climate variability (Watson et al 1997, Wiegand et al 1999, Wiegand et al 2004, may indeed play a major role in shaping the long-term, apparently stable, coexistence of trees and grasses. Spatial patchiness in establishment, at both the stand and hillside level, suggests that landscape processes such as fire and herbivory impact strongly on rates of tree establishment.…”

“…Theoretical work has argued that variability in establishment rates of adult savanna trees is fundamental to promoting the coexistence of grasses (Higgins et al 2000). Modeling work has also shown that variability in climate, herbivory and fire can produce variability in tree establishment that tracks broad-scale patterns in ecosystem change (Dublin et al 1990, Watson et al 1997, Woodward and Beerling 1997, Wiegand et al 1999, Higgins et al 2000, Wiegand et al 2004, Baxter and Getz 2005. However, empirical work evaluating whether variability in savanna tree establishment actually occurs and investigating the types of demographic variability that characterize savanna tree populations is rare.…”

Interaction between ungulates and bruchid beetles and its effect on Acacia trees: modeling the costs and benefits of seed dispersal to plant demography

“…In the case of the Negev desert, for example, suitable conditions for germination occur every 3-4 years (Ashkenazi 1995). According to a semi-stochastic model, approximately two mast events of highly successful recruitment per century combined with a low annual probability of recruitment events are necessary to sustain A. raddiana populations in the Negev desert (Wiegand et al 2004). Maximum adult survival is about 50 years .…”