Partitioning above and below ground interactions and their effects on juvenile Dacrycarpus dacrydioides (kahikatea) and Podocarpus totara (totara)

Competitive and facilitative interactions play an important role in determining plant community structure and development. Historically, competitive interactions have been considered to be more prevalent in nature. However, in the past few decades strong facilitative interactions have been identified as being more important than competition in certain environments. Recent evidence has also suggested that interactions occurring in the above and below ground environments may be unevenly contributing to the net interaction effects between a target plant and nurses species. This study partitions the above and below ground interactions and determines their strength and directions in order to help better understand their relative importance to plant community dynamics. In Chapter 2 I develop species specific allometric models which aim to accurately estimate the total above- and below- ground biomass of individual D. dacrydioides and P. totara juveniles using measurements which are easily and non-destructively obtained in the field. The best model for each species is then used to construct total above and below ground biomass estimates for use in Chapter 3. Eight models using stem height, diameter, and volume either alone or in combination are examined for their predictive power and tested for their goodness of fit. Models using diameter alone are found to be less powerful in predicting total tree biomass, while models containing height either alone or in combination with diameter are more powerful. The absolute best model for predicting D. dacrydioides total biomass was BTOTAL = 0.0099(Height²)⁰˙⁸⁷⁴⁹, whereas the absolute best model for P. totara was BTOTAL = 0.2635((Height*Diameter)²)⁰˙⁵⁶⁹⁵. In Chapter 3 I use the Relative Interaction Index (RII) to determine the strength and direction of the net interactions affecting D. dacrydioides and P. totara juveniles. To partition the above ground interactions, I examined the effects of a conspecific or interspecific neighbour. I found that my two study species D. dacrydioides and P. totara showed different responses to the treatments that they received. D. dacrydioides showed net facilitation and gained biomass when it had access to the mycorrhizal network and a neighbour. Whereas, P. totara showed net neutral interactions and did not gain biomass. P. totara also showed net competition when it did not have access to the mycorrhizal network and was grown next to neighbours. The role of above ground interactions was found to be less important than below ground interactions, overall. In general, these results mean that D. dacrydioides juveniles should be expected to have higher growth, reproductive, and survival rates when grown next to nurse species in comparison to P. totara. Chapter 4 details the significance of this study for the restoration of Wairio wetland, and wetlands in general. Given the result in chapter 3 and the current restoration method at Wairio wetland, this study suggests that it may be worth exploring the benefit of planting new P. totara juveniles farther away from older woody species in order to avoid root competition.