How do abiotic conditions influence fruit crop characteristics in Beilschmiedia tawa?

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.