Small differences in arrival time influence composition and productivity of plant communities.

'Who comes first' is decisive for plant community assembly and ecosystem properties. Early arrival or faster initial development of a species leads to space occupancy both above and below ground and contributes to species success. However, regular disturbance (e.g. biomass removal) might permit later-arriving or slower-developing species to catch up. Here, artificial communities of grassland species belonging to the plant functional types (PFTs) herb, grass and legume were used to test the effect of stepwise arrival (sowing) of PFTs. Dramatic effects were found as a result of a 3 wk arrival difference on composition and above-ground biomass that persisted over four harvests and two seasons. Priority effects, such as unequal germination time (arrival), and thus differences in community age structure, had lasting effects on PFT biomass contribution and associated ecosystem functioning. These effects were robust against above-ground disturbance. Benefits of earlier root formation outweighed above-ground species interaction. Earlier space occupancy and bigger reserve pools are the likely causes. Natural populations commonly exhibit age diversity and asynchrony of development among taxa. In experiments, artificial synchrony of arrival (sowing) may thus induce assembly routes favouring faster-establishing taxa, with consequences for ecosystem functioning (e.g. productivity). Founder effects, such as those observed here, could be even greater in communities of slow-growing species or forests, given their longer generation time and minor disturbance.

Carbon flux and growth in mature deciduous forest trees exposed to elevated CO2.

Whether rising atmospheric carbon dioxide (CO2) concentrations will cause forests to grow faster and store more carbon is an open question. Using free air CO2 release in combination with a canopy crane, we found an immediate and sustained enhancement of carbon flux through 35-meter-tall temperate forest trees when exposed to elevated CO2. However, there was no overall stimulation in stem growth and leaf litter production after 4 years. Photosynthetic capacity was not reduced, leaf chemistry changes were minor, and tree species differed in their responses. Although growing vigorously, these trees did not accrete more biomass carbon in stems in response to elevated CO2, thus challenging projections of growth responses derived from tests with smaller trees.

Non-structural carbohydrate pools in a tropical forest.

The pool size of mobile, i.e. non-structural carbohydrates (NSC) in trees reflects the balance between net photosynthetic carbon uptake (source) and irreversible investments in structures or loss of carbon (sink). The seasonal variation of NSC concentration should reflect the sink/source relationship, provided all tissues from root to crown tops are considered. Using the Smithsonian canopy crane in Panama we studied NSC concentrations in a semi-deciduous tropical forest over 22 months. In the 9 most intensively studied species (out of the 17 investigated), we found higher NSC concentrations (starch, glucose, fructose, sucrose) across all species and organs in the dry season than in the wet season (NSC 7.2% vs 5.8% of dry matter in leaves, 8.8/6.0 in branches, 9.7/8.5 in stems, 8.3/6.4 in coarse and 3.9/2.2 in fine roots). Since this increase was due to starch only, we attribute this to drought-constrained growth (photosynthesis less affected by drought than sink activity). Species-specific phenological rhythms (leafing or fruiting) did not overturn these seasonal trends. Most of the stem volume (diameter at breast height around 40 cm) stores NSC. We present the first whole forest estimate of NSC pool size, assuming a 200 t ha(-1) forest biomass: 8% of this i.e. ca. 16 t ha(-1) is NSC, with ca. 13 t ha(-1) in stems and branches, ca. 0.5 and 2.8 t ha(-1) in leaves and roots. Starch alone (ca. 10.5 t ha(-1)) accounts for far more C than would be needed to replace the total leaf canopy without additional photosynthesis. NSC never passed through a period of significant depletion. Leaf flushing did not draw heavily upon NSC pools. Overall, the data imply a high carbon supply status of this forest and that growth during the dry season is not carbon limited. Rather, water shortage seems to limit carbon investment (new tissue formation) directly, leaving little leeway for a direct CO2 fertilization effects.