Abstract
Microbes such as fungi and bacteria play fundamental roles in litter-decay and nutrient-cycling; however, their communities may respond differently than plants to climate change. The structure (diversity, richness, and evenness) and composition of microbial communities in climate transects of mature Douglas-fir stands of coastal British Columbia rainshadow forests was analyzed, in order to assess in situ variability due to different temperature and moisture regimes. We compared denaturing gradient gel electrophoresis profiles of fungi (18S-FF390/FR1), nitrogen-fixing bacteria (NifH-universal) and ammonia-oxidizing bacteria (AmoA) polymerase chain reaction amplicons in forest floor and mineral soil samples from three transects located at different latitudes, each transect spanning the Coastal Western Hemlock and Douglas-fir biogeoclimatic zones. Composition of microbial communities in both soil layers was related to degree days above 0°C (2725–3489), while pH (3.8–5.5) best explained shifts in community structure. At this spatial scale, climatic conditions were likely to directly or indirectly select for different microbial species while local site heterogeneity influenced community structure. Significant changes in microbial community composition and structure were related to differences as small as 2.47% and 2.55°C in mean annual moisture and temperature variables, respectively. The climatic variables best describing microbial composition changed from one functional group to the next; in general they did not alter community structure. Spatial distance, especially associated with latitude, was also important in accounting for community variability (4–23%); but to a lesser extent than the combined influence of climate and soil characteristics (14–25%). Results suggest that in situ climate can independently account for some patterns of microbial biogeography in coastal Douglas-fir forests. The distribution of up to 43% of nutrient-cycling microorganisms detected in forest soils responded to smaller abiotic gradients than host trees.
Highlights
Ecological niches are multidimensional; trees have adapted to both above- and below-ground environmental factors, whether abiotic or biotic
This study focused on eastern Vancouver Island, BC, Canada located in the Coastal South Pacific Cordilleran ecoclimatic region (Ecoregions Working Group, 1989), dominated by podzolic soils and the cool temperate wet forests of the coastal Western Hemlock (WH) and DFBEC zones (Meidinger and Pojar, 1991)
To some extent (26.7% of total inertia; Permutation test for correspondence analysis (CCA) P = 0.003), more acidic mineral soils were associated with more vanilla leaf and moss, and less salal, Oregon grape (OG) and fern cover; greater aeration (Co/F ratio) was associated with OG cover, despite pH being negatively correlated with the coarse and fine fraction mass and ratio (Co/F) ratio (r = −0.49; P = 0.002)
Summary
Ecological niches are multidimensional; trees have adapted to both above- and below-ground environmental factors, whether abiotic or biotic. Microbial communities provide key ecological functions, are usually well-adapted to a tree species genotype (e.g., Finzi et al, 1998; Ste-Marie and Houle, 2006), but can respond differently than above-ground flora to abiotic stressors (e.g., Nantel and Neumann, 1992; Kranabetter et al, 2009) and potentially to climate change. Co-adapted soil communities are important because longterm forest growth and resilience depends on below-ground processes such as appropriate organic matter degradation and nutrient cycling (van der Heijden et al, 2008). In this study we used polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) as an inexpensive, replicable technique to efficiently screen the principal constituents of nutrient-cycling microbial communities (Nicolaisen and Ramsing, 2002; Vainio et al, 2005; Fierer and Jackson, 2006; Oros-Sichler et al, 2007; Hoshino and Morimoto, 2010) accross climatic gradients within the same forest ecosystem
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