Earthquake disturbance alters a growth--survival trade-off found along
an elevation gradient in a monodominant forest
Abstract
1. Tree growth–survival relationships link two demographic processes
that dictate the com-position, structure and functioning of forest
ecosystems. While these relationships have been shown to 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 in 1994 on intra-specific
variability in growth–survival relationships. We expected that survival
models that incorporated recent growth would be better sup-ported than
those using other factors known to influence tree survival. 2. 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).
Sur-vival was related to growth of 4504 trees on 216 plots using
Bayesian modelling. We hy-pothesised there would be a positive, logistic
relationship between growth and survival. 3. Pre-earthquake, we found a
positive, logarithmic growth–survival relationship at all eleva-tions.
At higher elevations, trees grew more slowly but had higher survival,
supporting our hypothesised demographic trade-off with elevation. As we
expected, the earthquake al-tered the pre-earthquake growth–survival
relationships and 0–5 years post-earthquake survival held little
relationship with growth. Less expected was a strong, logarithmic
growth–survival relationship that developed 5+ years post-earthquake
because of en-hanced survival of fast-growing trees yet low survival of
slow-growing trees. 4. Synthesis. Our findings demonstrate there can be
trends 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
sug-gests 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.