Species richness and stand stability in conifer forests of the Sierra Nevada.

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.

The effect of competition and salinity on the growth of a salt marsh plant species.

Young rhizome sprouts of the herbaceous perennial Jaumea carnosa were propagated from material collected in a salt marsh along the central California coast. The sprouts were transplanted to flats of sand sown with different densities of seeds of a representative glycophyte, Lolium perenne L. "Derby, turf type." Controls flats contained only Jaumea or Lolium. Three series of replicated flats were watered from above with dilutions of seawater in 1/10 strength Hoagland solution, such that dissolved salts were 400, 4000 or 11,600 ppm. Two other series were continuously subirrigated with 400 or 11,600 ppm salt water. After 61 days of treatment in a greenhouse with a 30/11°C thermoperiod (mean daily max/min), all plants were harvested and weighed. In the monospecific control flats, the growth of both species declined with increasing salinity, but the relative decline of Lolium was three times that of Jaumea. Jaumea's root: shoot ratio was also less affected by salinity. Both species grew well when subirrigated by 400 ppm salt water, but grew poorly when subirrigated by 11,600 ppm salt water, indicating that aeration alone is not the most significant factor in the marsh. The effect of interspecific competition on Jaumea was marked at low salinity, depressing growth by 52% compared to controls, but at high salinity the competitive effect was insignificant, whether the plants were watered from above or subirrigated. This supports the hypothesis that intolerant halophytes such as Jaumea are restricted in nature to salt marshes because they are poor competitors with glycophytes on non-saline soils.

Salt spray as a microenvironmental factor in the distribution of beach plants at point reyes, California.

Measurements of salt spray were made weekly for 15 spring weeks with a grid of 50 traps in a 32 m longx24m wide plot of beach vegetation along the central California coast. The trap centers were 14 cm above the ground, the traps could swivel to continuously face the wind, and trap area (16 cm2) was small in order to minimally affect wind flow. Regressions or analyses of variance of wind speed and direction, rainfall, tide level, plot topography, and plant distribution were performed against salt spray. The period was lengthy enough to include a wide range of climatic conditions and the time of maximum plant growth and flowering. An ecologically and statistically significant salt spray gradient, which correlated with distance back from, and elevation above, mean tide line, was revealed. Overall, the front-most, lowest traps received 4-5 times the spray that the back-most, highest traps received. Weeks with moderate wind and spray (7-14 kph, in contrast to weeks with lower or higher wind speed) resulted in even steeper gradients, up to 20+-fold. The highest trap reading for any one week was 70 mg salt deposited dm-2 of trap surface day-1. Plant distribution correlated with salts spray and there is some evidence that it is a functional relationship. Attempts were made to extrapolate from the overall mean trap load of 13 mg dm-2 day-1 down to the leaf surface just below the traps, and to data from salt spray studies with larger traps on the Atlantic Coast. It appears that the salt spray load just above the beach plant canopy ranges from less than 1 to nearly 200 mg dm-2 vertical trap surface day-1, depending on trap height, distance back from tide line, and wind speed. The plant canopy and sand surface receive an order of magnitude less spray than that received by vertical traps.