Abundance and decline of isolated trees in the agricultural landscapes of central New South Wales, Australia

Abstract

Prior to this study, isolated trees were largely isolated from research. This study has provided a methodology, rigorous assessment of isolated tree density and distribution, and identified the potential ecological, social and economic importance of isolated trees, in a relatively small, but probably typical area of the wheat-sheep zone of eastern Australia. The abundance and decline of isolated native trees was measured by line-intersect sampling in the highly fragmented agricultural landscapes of the mid-Lachlan Valley of central New South Wales, Australia. A total of 7 000 trees were sampled along 5 678 km of transect on 441 aerial photographs. An isolated tree was defined as having no neighbouring tree within 25 m. The density of trees outside of remnants patches is low (0.3/ha) and has generally declined by 20% since the 1960s. The 1990s density of isolated trees equates to 275 000 trees across 830 000 ha of agricultural land not occupied by remnant vegetation patches larger than about 10 ha. The density of isolated trees was found to vary with land use with consistently fewer trees in cultivated areas compared to areas with no traces of cultivation. The isolated trees that remain within the agricultural landscape are not uniformly scattered. They exist as widely spaced clusters of isolated trees with 50% of trees having a nearest neighbouring tree within 25-49 m and less than 10% of trees had a nearest neighbour within a distance class greater than 100 m. Compared to the 1960s, isolated trees are now more isolated - the nearest neighbour distance has increased. The mean diameter of isolated tree crowns has significantly increased from 15 m in the 1960s to 18 m in the 1990s. The total canopy cover of isolated trees from the 1990s samples was 0.8% of the total study area below 400 m asl. A reversal in isolated tree decline will only occur if trees are replanted, or existing trees fenced to promote regeneration. Otherwise, isolated trees are dieing relicts of 150 years of clearing and intensive agriculture. This is of concern considering that we do not fully understand their value. We speculate on some of the ecological, economic and social values of these trees.