Thermophilization – changes in community composition towards greater relative abundances of species associated with warmer environments – has been described for plants and animals in many locations around the world. Disturbances of various kinds have increased rates of thermophilization in temperate sites, and this has been proposed, but not demonstrated, for some tropical environments. In this study, we tested whether disturbance by a Category four hurricane in 1988 (Hurricane Gilbert) increased thermophilization in a Jamaican montane forest by using pre- and post-hurricane data collected over four decades (1974–2014). We analysed tree species composition in permanent plots at c. 1580 m elevation in Jamaica’s Blue Mountains. There were 66 tree species with stem diameters ≥3 cm at breast height. We used published data on the altitudinal distribution of 62 species (94% of genetic individuals (genets)) to calculate the mean community altitude scores (MCAS) of the trees recorded in each census, as well as the MCAS of the survivors, recruits and dead trees after each decade. We found that thermophilization did occur (i.e., MCAS decreased significantly over time), and that this was due both to a decreasing MCAS of recruits through the four decades (significantly lower than expected in the last three decades) as well as a high MCAS of trees that died. Thermophilization was fastest in the post-hurricane decade, during which time there was marked and significant increase in the MCAS of dead trees; this change was above and beyond expectations of long-term successional dynamics. The rate of compositional change equates to an overall decrease in MCAS of 1.6 m yr-1 over the forty-year study period. We conclude that this Jamaican montane forest is undergoing thermophilization (likely due to rising temperature) and that the hurricane-caused disturbance accelerated thermophilization through differential mortality.

This study was conducted using data from four closely-located plots (called ‘sites’ in previous papers) spanning a narrow altitudinal range at c. 1580 m, but differing in topographic position, forest structure and species composition. The four plots were in the Blue and John Crow Mountains National Park, Jamaica (18° 5´N, 76° 39´W, 1543–1610 m elevation; maps in Tanner 1977, Chai et al. 2012); the Blue Mountains have forest cover from c. 500 m to 2,256 m. Each study plot was 0.06 to 0.1 ha in area: Col forest (Gap forest of Tanner 1977, 0.09 ha sampled); Wet Slope forest (0.1 ha); Mull Ridge forest (0.1 ha) and Mor Ridge forest (0.06 ha); hereafter abbreviated to Col, Slope, Mull and Mor. The Col, Mull and Mor plots are on the ridge top, at 1580 m, 1580 m, and 1620 m respectively, the Slope plot is on the north side of the ridge at c. 1560 m.

In each of the four plots, all stems ≥3 cm diameter at breast height (dbh) were measured, labelled, marked with a paint ring and identified to species in 1974, 1984, 1994, 2004, and 2014; dead stems were located and tallied. Taxonomic nomenclature follows Adams (1972) and Proctor (1985), except where other authorities are listed. Stems (ramets) were grouped into genetic individuals (genets) for calculations of recruitment, mortality and thermophilization rates.

Mean altitude values, for Jamaica, were calculated for 62 of the 66 tree species (94% of genets) as the midpoint of the species’ ranges as reported in Adams (1972), Proctor (1985), Adams (2014), and Mill (2015). The vast majority of the altitude data come from Adams (1972), who was careful to include only data based on herbarium specimens or trusted authorities (pers. comm. 1974). 

References:

Adams, C. D. 1972. Flowering plants of Jamaica. - University of the West Indies, Mona, Jamaica.

Adams, R. P. 2014. Junipers of the World: the genus Juniperus, 4th Edition. - Trafford Publishing.

Chai, S.-L. et al. 2012. Evaluation of Forest Recovery over Time and Space Using Permanent Plots Monitored over 30 Years in a Jamaican Montane Rain Forest. - PLoS One 7: e48859.

Mill, R. R. 2015. A monographic revision of the genus Podocarpus (Podocarpaceae): II. The species of the Caribbean Bioregion. – Edinb. J. Bot. 72:61–185.

Proctor G. R. 1985. Ferns of Jamaica. - British Museum (Natural History), London, UK.

Tanner, E. V. J. 1977. Four montane rain forests of Jamaica: a quantitative characterization of the floristics, the soils and the foliar mineral levels, and a discussion of the interrelations. -  J. Ecol.  65: 883–918.

For use with the R codes provided in the paper "Hurricane disturbance accelerated the thermophilization of a Jamaican montane forest", the "jadata" sheet should be first converted into a .csv file and loaded into R

NA = missing or unknown values