Environmental Controls of LAI

As climate (especially the mean and variations in annual precipitation and temperature) is the primary force shaping the major biomes of the world, much of the observed pattern of LAI distribution is initially driven by similar climatic factors. Second, biome distribution is controlled by edaphic conditions (water supply and soil fertility), which also control LAI. According to the resources optimization theory, LAI may adjust to climate and site potential. Reviews of plant science literature have computed the response of LAI to variations in soil moisture, soil fertility, and atmospheric CO2. LAI is probably co-limited by a number of resources, including water, nitrogen, and light. A linear response of LAI to N was reported for some crops or coniferous species, but is not uniform for all plant species, soil nutrients, or fertilization rates. Fertilization (nitrogen, phosphorus, or potassium) strongly increases LAI but the response could be a short-term one, after which an acclimatization of the canopy occurs. A new steady state is adjusted in the following growing seasons, especially in terms of leaf area versus root ratio. LAI sensitivity decreases (i.e., LAI saturates) indicating that something other than soil fertility is a limiting factor for canopy development. In many cases, water supply acts as a strong limitation resulting from LAI increase and related water uptake needs. The response to increasing soil water content is close to that of soil fertility. Increasing soil water availability in soils suffering from severe drought causes a significant increase in LAI. Interestingly, the response to increases in atmospheric CO2 is nonlinear. LAI curves for crops and plant communities indicate a strong response of LAI to increases in atmospheric CO2 up to the current ambient content. Afterward, the impact is more limited. At some resource thresholds, the addition of fertilizers or water will have no further influence on LAI. The saturation of LAI is likely to be indicative of light limitation, due to self-shading of leaves and negative carbon balances in low canopy layers. A large proportion of natural ecosystems lies below a threshold of optimal resource availability. LAI saturation is a threshold beyond which any further increase in LAI is compensated for by a negative carbon balance in shaded lower canopy leaves.

dieback and eventually plant mortality in the following years. Whatever the kind of natural disturbance or extreme event, the time needed to recover pre-event LAI could be used as an index of ecosystem resilience.

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