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Assessing the ecological importance of clouds has substantial implications for our basic understanding of ecosystems and for predicting how they will respond to a changing climate. This study was conducted in a coastal Bishop pine forest ecosystem that experiences regular cycles of stratus cloud cover and inundation in summer. Our objective was to understand how these clouds impact ecosystem metabolism by contrasting two sites along a gradient of summer stratus cover. The site that was under cloud cover ~15% more of the summer daytime hours had lower air temperatures and evaporation rates, higher soil moisture content, and received more frequent fog drip inputs than the site with less cloud cover. These cloud-driven differences in environmental conditions translated into large differences in plant and microbial activity. Pine trees at the site with greater cloud cover exhibited less water stress in summer, larger basal area growth, and greater rates of sap velocity. The difference in basal area growth between the two sites was largely due to summer growth. Microbial metabolism was highly responsive to fog drip, illustrated by an observed ~3-fold increase in microbial biomass C with increasing summer fog drip. In addition, the site with more cloud cover had greater total soil respiration and a larger fractional contribution from heterotrophic sources. We conclude that clouds are important to the ecological functioning of these coastal forests, providing summer shading and cooling that relieve pine and microbial drought stress as well as regular moisture inputs that elevate plant and microbial metabolism. These findings are important for understanding how these and other seasonally dry coastal ecosystems will respond to predicted changes in stratus cover, rainfall, and temperature.

Moisture inputs drive soil respiration (SR) dynamics in semi-arid and arid ecosystems. However, determining the contributions of root and microbial respiration to SR, and their separate temporal responses to periodic drought and water pulses, remains poorly understood. This study was conducted in a pine forest ecosystem with a Mediterranean-type climate that receives seasonally varying precipitation inputs from both rainfall (in the winter) and fog-drip (primarily in the summer). We used automated SR measurements, radiocarbon SR source partitioning, and a water addition experiment to understand how SR, and its separate root and microbial sources, respond to seasonal and episodic changes in moisture. Seasonal changes in SR were driven by surface soil water content and large changes in root respiration contributions. Superimposed on these seasonal patterns were episodic pulses of precipitation that determined the short-term SR patterns. Warm season precipitation pulses derived from fog-drip, and rainfall following extended dry periods, stimulated the largest SR responses. Microbial respiration dominated these SR responses, increasing within hours, whereas root respiration responded more slowly over days. We conclude that root and microbial respiration sources respond differently in timing and magnitude to both seasonal and episodic moisture inputs. These findings have important implications for the mechanistic representation of SR in models and the response of dry ecosystems to changes in precipitation patterns.

It is becoming increasingly clear that plant roots can impact the decomposition of existing soil C in the rhizosphere. Studies under controlled conditions suggest this impact may be plant species dependent, but whether this is the case in natural conditions or what factors underlie this variation is mostly unknown., With a novel field-based isotopic approach combining 13C-enriched glucose and 5-bromo-2-deoxyuridine additions, we compared in situ C decomposition of added labile C and native soil C (priming) among eight semi-arid grassland species' rhizospheres to investigate the factors driving inter-species variation. We examined the influence of several rhizosphere factors related to soil chemistry, microbial activity, microbial community, microbial stoichiometry, plant chemistry and root morphology., Plant species generated distinct microbial and chemical rhizosphere environments, which translated into differences in the direction, magnitude and temporal dynamics of the soil C priming. Soil C decomposition was positively related to soil C/P and soil N/P (via its influence on the bacterial community), which in turn were positively related to plant N/P. Plant C/N was also a significant factor via its negative influence on soil N/P. In contrast, the main direct predictors of labile C decomposition were microbial biomass, microbial C/N and the C-degrading enzymes, which in turn were linked to root morphology and C chemistry., Synthesis. Within this community, plant species' rhizospheres can vary in their susceptibility to C loss in response to changes in C availability. Soil stoichiometry, driven by plant chemical traits, appeared to be the strongest driver of priming. Our study suggests that shifts in plant communities involving increases in N relative to P have the greatest potential to lead to C loss. We provide evidence of root morphology and C chemistry as drivers of labile C processing in soil, a novel empirical contribution to our understanding of the role of plant traits below-ground. The contrasting regulation of different pools of soil C suggests observations of the regulation of simple C compounds should not be extrapolated to the whole C pool. Our findings provide support for rhizosphere-driven mechanisms by which shifts in plant community composition could have implications on the ecosystem-level C balance. [ABSTRACT FROM AUTHOR]