22 July 2022
Tawa (Beilschmiedia tawa) can be found growing as a canopy or sub canopy tree from the top of the North Island down to Kaikoura. Tawa fruit is the fourth largest of New Zealand’s endemic flora and is primarily eaten by kererū. Prompted by observations of Tūhoe Tuawhenua elders that tawa fruit has become smaller and less abundant in recent decades, my masters research project aimed to determine how tawa fruiting characteristics are related to key climate and soil variables.
We measured tawa fruit and seed size at 8 sites across tawa’s distributional range, representing a wide variety of different climates and soil types. Overall, 34,263 fruits were counted across the sites. We then used climate data for each site to see how key variables like rainfall and temperature across the fruit growing season affected the size of the fruit. On average, tawa at Maungatautari had the second largest fruit of any site. Soils at Maungatautari had the highest levels of nitrogen and the growing season was the warmest.
We found that tawa fruit pulp was related to the amount of rainfall that falls when the fruit is developing. So lots of rainfall equals a plump, juicy tawa fruit, perfect for kererū kai! However, after drying the fruit to remove all water content, the weight of fruit pulp was unrelated to rainfall. This suggests that while higher rainfall causes pulp to swell with water, it does not necessarily cause pulp to accumulate more nutrients. Rainfall didn’t affect the size of seed – this was more related to nutrients in the soil, specifically nitrogen.
We also used the DOC seed rain dataset (which collects annual data on fruit abundance from several tree species using seed traps) to look at long term patterns in tawa fruit abundance across 5 sites. Tawa showed a general pattern of having a big year of fruiting, followed by a low year of fruiting. Again, higher levels of rainfall during key times of fruit development were related to larger crops in tawa. Cool winters before fruit set also were related to larger crops in tawa. This finding suggests that if climate change creates drier years with warmer winters, then tawa fruit crops may be reduced. There are potentially signs of this at some of the seed rain sites, where very few tawa have fruited over the last 5-10 years. However, more work is required to fully understand causes of the fruiting failure.
Oscar Yukich Clendon, Masters student, University of Auckland