Tree establishment on post-mining waste soils: species, density, and mixture effects

Tree establishment to restore degraded boreal post-mining lands is challenged by low soil productivity, a harsh microclimate, and potentially high contaminant levels. Use of mixed vegetation can facilitate microclimate but increase competition for soil resources. A statistical accounting of plant-plant interactions and adaptation to multi-species conditions is hard to achieve in field experiments; trials under controlled conditions can distinguish effects of planting density and species interactions in the early stages of plant establishment. A greenhouse trial was established in containers (‘mesocosms’) with waste rock or fine tailings from gold mines. Pre-germinated (one-week-old) seedlings (Alnus viridis subsp. crispa, Picea glauca, Populus tremuloides, Salix arbusculoides) were planted using a Nelder density gradient design, modified for species combinations. A Relative Competition Effect (RCE) was estimated as a competitiveness index for each species combination, calculated as ratio of α coefficients in the Holliday growth equation. The Specific Leaf Area (SLA) was measured to indicate plant water stress adaptation. All species grew better in monoculture on fine tailings, while only P. tremuloides grew better in all mixtures on waste rock. Although net positive effects of density on SLA increment during early growth suggested microclimate improvement on fine tailings, no mixture provided advantages for both species in paired combinations.

Tree establishment and growth drive treeline advance and change treeline form on Pikes Peak (Colorado) in response to recent anthropogenic warming.

Treeline advance is a well-established response of treeline ecotones to climate change. However, the degree to which tree growth and establishment drives treeline movement is widely debated. We used GIS analysis of aerial photographs and dendrochronological analysis of tree growth and age structure to examine treeline dynamics on Pikes Peak (CO). The rate of treeline advance at the site (1938-2017) was 0.235m/year, and it accelerated through time. Several sites have transformed from abrupt to diffuse topology. Regional temperatures significantly increased after the 1890s, particularly in the last half-century. Tree growth was inhibited by late spring snow in the 1935-1985 window and enhanced by growing season temperature in the 1965-2009 window. Tree establishment above treeline appears to have transformed treeline topology and set up the potential for further treeline advance. We conclude that if current climatic trends and system relationships continue, treeline should continue to advance because (1) there are large numbers of seedlings and saplings above the treeline due to continuous significant seedling recruitment and (2) growth of trees in the treeline ecotone is positively related to growing season temperatures, which are increasing. Other limits to the system, such as drought or topographic barriers, may arise in the future.

Modification of Fire Regimes Inferred from the Age Structure of Two Conifer Species in a Tropical Montane Forest, Mexico

Research Highlights: Age structure was used to infer fire regimes in the Monarch Butterfly Biosphere Reserve. Uneven-aged structures in stands dominated and co-dominated by pine and fir species, which are distributed according to an altitudinal gradient, indicated a regime of frequent, low-severity, and low-intensity fires. Background and Objectives: Age structure analyses have been used to infer natural and disrupted fire regimes when field-based descriptions of fires are scarce or unavailable. In montane conifer forests, fire regimes typically vary according to an altitudinal gradient, shaping contrasting tree establishment patterns. In the Monarch Butterfly Biosphere Reserve, Mexico, the altitudinal distribution and fire regimes of sacred fir forests (Abies religiosa), smooth-bark Mexican pine forests (Pinus pseudostrobus), and mixed-conifer forests are poorly documented. The objectives of this study were to determine the altitudinal ranges occupied by mono-dominant and co-dominant stands and to reconstruct tree establishment history to infer historical fire regimes. Materials and Methods: Six altitudinal transects were established along the reserve, each one at elevations from 2400 to 3300 m, with sampling sites at every 150 m of elevation. In each site, increment cores were collected from the base of 25 mature trees. A total of 800 increment cores were collected and cross-dated. Results: P. pseudostrobus is dominant in stands between 2400 and 2850 m, A. religiosa between 3150 and 3300 m, and both species co-dominate between 2850 and 3150 m. The establishment pattern for both species has been continuous, represented by uneven-aged structures, suggesting that tree establishment in smooth-bark Mexican pine forests, mixed-conifer forests, and sacred fir forests, is likely to be associated with frequent, low-severity, and low-intensity fires. Conclusions: These fire regimes suggest, by the one hand, the disruption of natural fire regimes by human activities, limiting the occurrence of high-severity fires; on the other hand, a distinctive feature of these tropical montane forests.