Abstract:
Abiotic conditions are known to affect plant fruit crop characteristics, including annual fruit
abundance and the morphological traits of seeds and pulp that comprise individual fruits. How
these characteristics vary along abiotic gradients has important consequences for plant
regeneration with flow on effects for animal consumers of fruits and seeds. In Tuawhenua
forests in the central North Island of New Zealand, Tuawhenua elders have noticed a decline
in the size and abundance of tawa fruit (Beilschmiedia tawa) within the past several decades.
I investigated whether changes in abiotic conditions explain these observations.
My first aim was to determine abiotic correlates of fruit traits (size and mass) by sampling eight
populations across tawa’s range. I found that pulp traits, but not seeds traits were related to
variation in climate variables. This is probably because pulp is mostly water, while seeds are
mostly lipids and proteins, thus have different metabolic costs of production.
Secondly, I evaluated how rainfall and temperature variables during key stages of tawa’s
reproductive cycle influenced interannual variability in fruiting, using long-term fruit abundance
datasets for five sites. Mature fruit abundance was positively influenced by rainfall variables,
likely through facilitation of plant resource uptake. Cooler winters during pollinated ovule
dormancy also led to higher mature fruit abundance, possibly indicating a chilling requirement
for dormancy release. Warmer conditions during flower initiation also had a small positive
influence on mature fruit abundance, possibly through increased photosynthesis.
Finally, I attempted to construct a dated chronology using tawa ring width series from trees in
Tuawhenua. The objective was to examine climate relationships and the effects of forest
disturbance using total ring widths, thereby providing evidence of changes in tree growth which
may have reflected declines in tawa fruit crop. I was unable to complete a dated chronology
but gained valuable insights for future dendrochronological work with tawa.
Within my first two aims, I used my results to model the effect of past climate in Tuawhenua
on tawa fruit crop. I predicted that changing climate in the past century contributed to the
observed decline in fruit abundance, but not size. My findings demonstrate that fruit crop
characteristics are sensitive to abiotic gradients across tawa’s range, but further work is
required to better understand the specific mechanisms which drive these trends, especially for
interannual variation in fruit production.