Abstract
Transgressive dune systems are dynamic environments that support a range of landforms and communities. These include deflation complexes – comprising wetland vegetation and dunes - within migrating parabolic dunes. The development of deflation complexes is controlled and shaped by abiotic-vegetation interactions that display a high level of spatial and temporal dynamism. However, the widespread stabilisation of transgressive dune systems threatens the flora of deflation complexes. Stabilisation restricts the natural dynamism of parabolic dunes with concomitant loss of habitat for early successional communities.
Deflation complexes represent an important component of dune system biodiversity in Aotearoa New Zealand. Deflation complexes are species-rich in comparison to the wider dune system, containing a unique assemblage of wetland and dune species, including several threatened species. These environments are also unique due to their ecosystem properties; their dynamism is driven by unique interactions between dune dynamics, hydrology, and vegetation. Deflation complexes generally only occupy small areas of the transgressive dune system but contain a range of geomorphic and vegetation diversity. However, there is little understanding of how abiotic-vegetation interactions influence their development and evolution.
This study is the first to study the abiotic-vegetation interactions and temporal development of deflation complexes in southern New Zealand. The study site, Mason Bay, provides an exceptional opportunity to study deflation complex dynamics in one of the few remaining and largely unmodified transgressive dune systems in Aotearoa New Zealand. The dune restoration programme at Mason Bay, Rakiura National Park, has protected the natural dynamism in parts of the Northern and Central dune systems, subsequently protecting the natural character of deflation complexes. The spread of Ammophila arenaria (marram grass) and Lupinus arboreus (tree lupin) in the dune system outside the management zone also provided the opportunity to examine how these species influence deflation complex development and their associated native flora. The research aims to examine the natural character of deflation complexes at Mason Bay, including (i) the abiotic conditions and vegetation communities; (ii) historical migration rates and developmental patterns; and (iii) how interactions between abiotic conditions and vegetation in early stages of development influence the development of deflation complexes.
Twelve deflation complexes (DC1-DC12) were examined across the Northern and Central Dunes systems of Mason Bay. Measurements of wind, dune dynamics (sand erosion and deposition), rainfall, and groundwater were used to describe the abiotic conditions of deflation complexes. UAV photogrammetry was used to investigate changes in the morphology of the deflation complex during the study period, and orthomosaics were used to map vegetation. The composition of the vegetation communities was described using botanical surveys across the twelve deflation complexes. Quadrats areas in seepage environments in DC9 and DC10 were monitored over an 18-month period to examine vegetation change and surface erosion/accretion. Georectification of historical imagery allowed the mapping of landforms and vegetation communities over a period of forty years to identify medium-term migration rates and developmental patterns for DC6-DC10.
Deflation complexes at Mason Bay migrate west to east at an average rate of 5.4 m yr-1 and develop an increasingly diverse range of vegetation communities over time. The earliest stage is the formation of a seepage environment as surface erosion intersects the groundwater table, and this surface is colonised by the earliest wetland community Isolepis cernua var. cernua (slender clubrush) cushionfield. This community is eventually replaced by turf-rushland-sedgeland and dune communities. Further migration of the deflation complex creates a new seepage environment, and the developmental sequence starts again.
Dune dynamics and groundwater hydrology are the key abiotic processes that influence landform and vegetation community development in deflation complexes. The predominant onshore winds promote the migration of deflation complexes from west to east. Offshore winds transport sand into the seepage environment and facilitate the development of gegenwalle ridges. Variations in groundwater hydrology are important for determining the distribution of wetland communities and deflation complexes, where lower groundwater tables promote the development of alternative wetland communities.
The abiotic-vegetation interactions within the seepage environment determine vegetation community and landform patterns in the deflation complex. Abiotic processes control the establishment of vegetation in the seepage environment. During the study period, wind erosion and low soil moisture (drought) led to the loss of I. cernua seedlings. However, once I. cernua and the dune-builders Ficinia spiralis (pīngao/ pikao) and Poa billardierei (sand tussock) establish in the seepage environment they act as biogeomorphic agents. The structures of I. cernua help stabilise the surface, allowing wetland and dune-building species species to colonise the seepage environment. The colonisation of the cushionfield by dune-builders promotes increased sand trapping, which may result in the formation of gegenwalle ridges. The formation of gegenwalle ridges protects wetland communities from sand burial during periods of offshore winds, which allows the development of turf and rushland/sedgeland communities. The frequent repetition of these processes as the deflation complex migrates creates a sequence of wetland and dune communities in the deflation complex.
The natural development of deflation complexes relies on dune dynamism; however, A. arenaria and L. arboreus impact these processes by stabilising deflation complexes. The stabilisation of deflation complexes restricts parabolic dune migration and habitat creation for I. cernua cushionfield. Stabilised deflation complexes also facilitate the spread of L. arboreus, which replaces turf and dune communities and accelerates succession to form later successional mixed shrubland communities. Therefore, maintaining dynamism in deflation complexes is required to support the full range of wetland and dune communities. Active management and restoration by the Department of Conservation - Te Papa Atawhai and other agencies will be necessary to protect the biodiversity of deflation complexes elsewhere in Aotearoa New Zealand, because of their vulnerability to exotic species.