Research was undertaken to determine the nature of the coexistence between kahikatea and grey willow in the Waikato Ecological Region. Three main questions were investigated. Is grey willow inhibiting recruitment of kahikatea? If this is occurring, which mechanisms (of interference) are involved? And does anthropogenic disturbance influence this interaction?
The first and third were addressed by measuring kahikatea and grey willow populations for age, stem frequency and diameter at six sites, to assess how populations were structured, to reconstruct the population histories at each site and to determine whether kahikatea is successfully regenerating and what possible factors influence regeneration. Results indicated grey willow was establishing after kahikatea populations at all but the Totara Park site, and that once grey willow had reached the canopy, no further recruitment of kahikatea into the canopy occurred. No kahikatea saplings were present in grey willow densities above two per 10m2 and sapling presence did not always result in canopy emergence. Sites containing the highest sapling frequencies were those most recently exposed to moderate to large-scale disturbance, and those closest to a large seed source, rather than those with long established kahikatea populations. This suggests anthropogenic disturbance of certain scale and frequency can promote the regeneration of both species.
The second question was investigated using a range of methods. Dendrochronology was employed to compare diameter growth rates between the species at each site and found grey willow only performed marginally better than kahikatea and its competitive advantage in reaching the canopy faster is likely exerted via more rapid height growth rates. Experimental methods involved measuring annual height and diameter growth rates of forty introduced and twenty naturally established seedlings in various grey willow canopy treatments and water table heights in Totara Park, Hamilton city. Hemispherical photography and water table measurements were used to quantify seedling microhabitats. The greatest seedling diameter and height growth rates were recorded in the open canopy introduced treatment and the lowest in the closed canopy naturally established treatment irrespective of water table height. The reduction in summer light levels from a grey willow canopy regularly caused apical stem death or reduced growth rates of kahikatea seedlings.
Interference effects from allelopathy were investigated as previous research indicated kahikatea litter is toxic to its own seedlings. A trial was set up using potted grey willow cuttings and Sinapus alba seedlings and regularly sprayed with kahikatea litter extract. Results were inconclusive and the natural concentrations of kahikatea litter found at Totara Park did not appear to affect the growth of either species.
Overall this research suggests grey willow is inhibiting kahikatea regeneration via overtaking kahikatea growth to the canopy, shading out further recruitment, and maintaining dominance through efficient vegetative reproduction. Active management is required to ensure the return to dominance of kahikatea in Waikato swamp forests and highly disturbed sites, close to abundant seed source may provide novel opportunities for restoration
