Environmental factors can influence spatial aggregation and acorn production in the endemic and endangered oak Quercus brandegeei in Mexico

Abstract

Background and study species: The endangered oak Quercus brandegeei Goldman, endemic to Baja California Sur, Mexico, is threatened because of its relictual distribution restricted to ephemeral riverbeds, declining populations, and a lack of seedling regeneration. Although there is evidence of viable acorn production, no seedlings or saplings have been found across the range of occurrence for this species. 
 Hypothesis: We tested for spatial aggregation and factors determining acorn production, hypothesizing that Q. brandegeei trees would be less distant from one another in regions with higher precipitation and that tree attributes and climatic variables would co-determine acorn presence.
 Study site and dates: Baja California Sur, Mexico, 2016-2017
 Methods: We conducted a Ripley’s K analysis to determine each population’s spatial pattern (random, uniform, clumped) across 12 populations and a total of 194 individuals. To find the impact of environmental factors on acorn production we conducted a series of generalized linear models on eight climatic variables. 
 Results: Ripley’s K analysis revealed that Q. brandegeei is a highly clumped species, with the highest rate of aggregation occurring within 0-25 meters. Region but not climatic cues were found to be important determinants of presence of acorns. 
 Conclusions: High aggregation in clumps and low evidence of acorn production in some populations suggests that there is a niche attrition for this species, which is confined to very specific microsites along ephemeral riverbeds. Suitable recruitment areas could be reduced even further under the hotter and drier climate predicted for the region.