Effects of Landscape Heterogeneity and Clearfell Harvest Size on Beetle (Coleoptera) Biodiversity in Plantation Forests
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Effects of landscape heterogeneity and clearfell harvest size on beetle (Coleoptera) biodiversity in plantation forests A thesis submitted in partial fulfilment of the Requirements of the Degree of Doctor of Philosophy in the University of Canterbury by S.M. Pawson University of Canterbury 2006 ii Paper produced from 84% FSC certified forest resources iii Abstract Compared to natural forests, fast-growing plantations of exotic species such as Pinus radiata are often perceived as marginal habitat or unsuitable habitat for most native species. By studying Coleoptera (beetles) in a variety of landscape elements (pasture, native forest and different aged Pinus radiata stands) in a highly modified and fragmented landscape in New Zealand I aimed to determine the value of exotic plantation forests for native biodiversity, and how these species are affected by different sized clearfell harvest areas. Pitfall trap sampling of beetles showed that plantation forest stands can provide suitable complimentary habitat to native forest for many species. Rarefied species richness of Carabidae, Scarabaeidae and Scolytinae was not significantly different between habitats, however, habitat types differed significantly in their beetle community composition. Comparing different production habitats, Pinus radiata stands had a beetle community composition most similar to native forest. However, a small minority of species, e.g., Dichrochile maura, were restricted to native forest habitat highlighting the importance of retaining indigenous ecosystems within plantations. Unlike human modified habitats, native forests did not provide suitable habitat for exotic species. Clearfell harvesting is controversial and its impact on biodiversity is a key constraint for many forest certification programs, such as that administered by the Forest Stewardship Council (FSC). Despite this, no replicated manipulative experimental studies of the impact of different sized clearfell harvest areas on biodiversity have been undertaken at scales relevant to the New Zealand forest industry. One potential model of the impact of different clearfell harvest sizes is the concept of a threshold size. A threshold scenario may occur where clearfell harvest impacts increase at a rate disproportionate to the change in clearfell size over a small range of harvest areas, but impacts remain relatively unchanged either side of the threshold zone. I sampled Coleoptera in experimentally created 0.01, 0.05, 0.5, 5.0, 50 and 500 ha clearfells within Pinus radiata plantations in the central North Island of New Zealand. The wide range of clearfell harvest sizes, including some very small areas, such as 0.01 ha was instigated in an attempt to document potential clearfell harvest size thresholds. Rarefied native beetle species richness was higher in harvest areas compared to adjacent mature plantation stands. The beetle species richness in 5 ha and 500 ha harvest areas was significantly greater species than that in small 0.01 – 0.5 ha harvest areas. Although, the high Paper produced from 84% FSC certified forest resources iv beetle diversity recorded in 500 ha clearfells should be treated with caution due to confounding spatial autocorrelation. The degree of change in beetle community composition increased with increasing clearfell harvest area. Beetle assemblages in large harvest areas were less similar to their paired adjacent mature forest than smaller harvest areas. Although, constrained multivariate ordination techniques did show a short-term change in beetle species composition between recently clearfelled harvest areas of as little as 0.05 ha and adjacent mature P. radiata stands. The colonisation by open-habitat disturbance-adapted species was a key driver of this change, some species dispersed into clearfelled stands in significant densities within days post-harvest. Overall, there were no distinct short-term trends to the change in species richness as a function of increasing harvest area that would suggest an ecological impact threshold response. If short-term outcomes of clearfell harvesting are ameliorated by successful recolonisation, the long-term spatial arrangement of different aged stands becomes more important for the maintenance of biodiversity at the landscape level than short-term consequences of harvesting. By sampling selected beetle taxa in 1, 2, 4, 8, 16 and 26 year-old stands, I found that the abundance of seven out of eight of the species selected for analysis recovered to levels similar to those in adjacent mature forest within the timeframe of a single harvest rotation. Individual species utilised different aged stands, indicating different life-history strategies. For example, open-habitat, disturbance-adapted species such as Cicindela tuberculata and Sitona discoideus were prominent in young stands, and forest species such as Pycnomerus sophorae and Paracatops phyllobius were highly abundant in older stands. These alternative life-history strategies highlight the benefits of maintaining a mixture of different aged stands to increase biodiversity at the landscape level. This thesis fills an important gap in our knowledge of biodiversity in production landscapes. I show that plantation forests have value as complimentary habitat to native forest and they make an important contribution to the maintenance of biodiversity at the landscape level. Although clearfell harvesting is a severe disturbance to the forest ecosystem, the long-term recovery of beetle populations suggests that harvesting is not the key limiting factor to the enhancement of biodiversity in the plantation forests studied. This unusual situation is possibly the result of prior land-use history, as many plantations were established on degraded pastoral land, and harvest-sensitive species are unlikely to have survived this initial land-use Paper produced from 84% FSC certified forest resources v change. As such, the severity of the long-term impacts of clearfell harvesting on biodiversity are likely to be context specific and will vary accordingly. The importance of spatial heterogeneity of habitat elements, including different aged plantation stands and native forest remnants, needs to be investigated in more detail to determine what limits biodiversity in this plantation landscape. Key points to consider are the proximity to, and proportion of, native forest cover in the landscape and the degree of connectivity among native remnants. It is these landscape-level attributes that may determine biodiversity at a regional scale, and more emphasis should be placed on landscape scale factors and there interaction with stand specific forest management practices. For example, the spatial mosaic of harvesting areas may need to be of a finer-scale when there are fewer native remnants within the landscape. Paper produced from 84% FSC certified forest resources vi Paper produced from 84% FSC certified forest resources vii Acknowledgements Writing the Acknowledgements section of a thesis is a difficult process, especially when the thesis has consumed the best part of 3 ½ years of my life, and although many friends, colleagues and acquaintances have come and gone they have all contributed in some way to my time as a PhD student. In particular I would like to thank the following people, companies and organisations for their support over the years. My supervisory team Raphael Didham, Eckehard Brockerhoff and David Norton have been invaluable. Three supervisors was daunting, at times I was given different competing perspectives on issues, however each was the result of slightly different research background. The end result was that I gained a greater breadth of knowledge during my studies that could not have been achieved with a smaller group. For this and the many other opportunities that my supervisors have presented me over the last few years I am highly grateful. Dave Lowry of Fletcher Challenge Forests was instrumental in this project and secured industry funding. I will be forever grateful to you Dave, you continually went further than necessary to assist my work. I especially wish to thank you for ensuring the continued support of the project by Kaingaroa Timberlands during sale of Fletchers Forests at a time that must have been personally very difficult, thank you! Colin Maunder of Kaingaroa Timberlands, you were not there at the beginning of the study, but half way through you stepped into the breach and provided the necessary assistance to see the field work of this project survive, many thanks! Being co-funded by Forest Research (now Ensis) I was fortunate to be given an office within the organisation. Many people contributed to my time within forestresearch, however the most outstanding supporter would have to be Joy Wraight, who helped me with every little problem I had (and there were many). There were many people who helped me as field staff through my time, there contribution is acknowledged in my declaration. As well I would like to acknowledge all the boys and girls of the Buena Vista Ilam Social Club who helped me through the weeks, and end of weeks, with the odd beer or two. Paper produced from 84% FSC certified forest resources viii Lastly I wish to acknowledge my friends and family: My friends; well I must say my introduction to the Canterbury University Tramping Club and its extended international family of visitors has provided me with the largest most diverse collection of friends I have ever had in my life. The trips I have been on in the last three years have defined my holidays (both here and abroad), my weekends, my flatmates (cheers Ben and Emily and the collection of random foreign students), and essentially my life; turning me into who I am today. The lasting joke of it is, I hand in my thesis, finish being a student and then become club captain of the CUTC, which is supposed to be a student organisation, oh well once a student always a student. My family have encouraged me through these past few years with gentle prods and verbal reminders of ‘when are you finishing’ to keep me going. Though the distance of the Cooks Strait has meant family visits only occur a few times a year, your long distance love and support have kept the fires burning and the finish line in sight.