• Energy Research

ABSTRACTTree growth–survival relationships link two demographic processes that individually dictate the composition, structure and functioning of forest ecosystems. While these relationships vary intra‐specifically, it remains unclear how this reflects environmental variation and disturbance. We examined the influence of a 700‐m elevation gradient and an Mw = 6.7 earthquake on intra‐specific variability in growth–survival relationships. We expected that survival models that incorporated recent growth would be better supported than those only using other factors known to influence tree survival. We used a permanent plot network that representatively sampled a monodominant Nothofagus forest in New Zealand's Southern Alps in 1974 and that was remeasured seven times through to 2009. The relationships were assessed using pre‐earthquake growth and survival, pre‐earthquake growth and post‐earthquake survival (0–5 years post‐earthquake), and post‐earthquake growth and survival (5+ years post‐earthquake). Survival was related to growth of 4504 trees on 216 plots using Bayesian modelling. We hypothesised there would be a positive, logistic relationship between growth and survival. Pre‐earthquake, we found a positive, logarithmic growth–survival relationship at all elevations. At higher elevations, trees grew more slowly but had higher survival than trees at lower elevations, supporting our hypothesised demographic trade‐off with elevation. The earthquake altered growth–survival relationships from those found pre‐earthquake and 0–5 years post‐earthquake survival held little relationship with growth. A strong, logarithmic growth–survival relationship developed 5+ years post‐earthquake because of enhanced survival of fast‐growing trees yet low survival of slow‐growing trees. Synthesis. Our findings demonstrate a trend in growth–survival relationships along an elevation gradient. If we assume a gradual climate warming is the equivalent of a forest stand shifting to a lower elevation, then data from our pre‐earthquake period suggest that tree growth–survival relationships at any elevation could adjust to faster growth and lower survival. We also show how these novel growth–survival relationships could be altered by periodic disturbance.

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 4 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 ca 1580 m above sea level 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.