A Growth-Survival Trade-Off Along an Elevation Gradient Is Altered by Earthquake Disturbance in a Monodominant Southern Beech Forest.

Abstract

Tree 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.