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Theoretical and empirical studies have long suggested that stability and complexity are intimately related, but evidence from long-lived systems at large scales is lacking. Stability can either be driven by complex species interactions, or it can be driven by the presence/absence and abundance of a species best able to perform a specific ecosystem function. We use 64 years of stand productivity measures in forest systems composed of four dominant conifer tree species to contrast the effect of species richness and abundance on three stability measures. To perform this contrast, we measured the annual growth increments of > 900 trees in mixed and pure forest stands to test three hypotheses: increased species richness will (1) decrease stand variance, (2) increase stand resistance to drought events, and (3) increase stand resilience to drought events. In each case, the alternate hypothesis was that species richness had no effect, but that species composition and abundance within a stand drove variance, resistance, and resilience. In pure stands, the four species demonstrated significant differences in productivity, and in their resistance and resilience to drought events. The two pine species were the most drought resistant and resilient, whereas mountain hemlock was the least resistant and resilient, and red fir was intermediate. For community measures we found a moderately significant (P = 0.08) increase in the community coefficient of variation and a significant (P = 0.03) increase in resilience with increased species richness, but no significant relationship between species richness and community resistance, though the variance in community resistance to drought decreased with species richness. Community resistance to drought was significantly (P = 0.001) correlated to the relative abundance of lodgepole pine, the most resistant species. We propose that resistance is driven by competition for a single limiting resource, with negative diversity effects. In contrast resilience measures the capacity of communities to partition resources in the absence of a single limiting resource, demonstrating positive diversity effects.

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A line intersect technique was used to determine the volume and mass of dead, naturally fallen logs at least 25 cm in diameter and 4 m or more in length over an 80-ha mesic, upland old-growth redwood forest in northwestern California. Each log was identified to species and placed in one of three decay classes: sound logs, moderately decayed logs, and logs in an advanced state of decay. Only logs of redwood (Sequoiasempervirens (D. Don) Endl.), Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), and western hemlock (Tsugaheterophylla (Raf.) Sarg.) met the dimension requirements of sampling. Redwood logs in the middle decay class had the greatest volume (528 m3/ha) and mass (116 t/ha). The total volume and mass of all logs were 957 m3/ha and 200 t/ha, respectively. The total volume and mass of decaying logs in this redwood stand exceeded averages typical of other kinds of low-elevation coniferous forests in the Douglas-fir region.

Questions: 1. Does resource use efficiency increase with increased species richness in conifer forests? 2. Do patterns found in resource use support niche differentiation/complementarity between species, or is any increase indicative of a selection effect? Location: All data were collected from upper montane (2200–2600 m a.s.l) conifer forests of the Desolation Wilderness in the central California Sierra Nevada, USA. Methods: We established 281 plots of varying levels of conifer richness throughout the wilderness area. Within each plot we used hemispherical photos to measure canopy closure and LAI, total soil carbon and nitrogen from the A‐horizon, and stand basal area. We used linear regression and ANOVA to analyse the relationship between stand species richness and resource availability. Results: We found no correlation of either soil nitrogen or carbon with stand biomass. Nor did soil nitrogen and carbon levels change with species richness. Canopy closure increased with species richness but also varied significantly between pure stands of different species. Pure Pinus monticola stands had the lowest canopy closure, Tsuga mertensiana stands the highest. Composition explained more canopy cover variation than did species richness. We found evidence supporting both the sampling effect and niche differentiation models at different stages of stand development. Conclusions: During initial stages of stand development, the interaction between the shade‐intolerant Pinus species and shade‐tolerant Abies magnifica and T. mertensiana followed the niche differentiation model, but switched to the sampling effect model during the competitive‐exclusion stage. In contrast, interaction between A. magnifica and T. mertensiana followed the niche differentiation model.

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