Plant competition, temporal niches and implications for productivity and adaptability to climate change in water-limited environments

1. We review plant competition in water-limited environments with focus on temporal niche dynamics and examine implications for diversity-productivity relationships and the response of ecosystem productivity to changes in water availability. The main theses under examination are that (i) plant functional types (PFTs) have distinct resource pulse use and coexist through mechanism of temporal resource use complementarity; and (ii) species of same PFT (functionally redundant species) coexist through distinct recruitment niches. 2. In water-limited systems, opportunities for plant resource uptake and growth fluctuate through time, dependent on precipitation patterns. Species differ in the sensitivities of germination, seedling mortality and adult productivity to pulses of water availability, and this generates opportunity for temporal niche diversification. We illustrate this in two case studies. 3. Case study I. Savannas: This is an example of niche separation between two distinct plant functional types (PFTs), trees and grasses. Several models suggest that the two PF s have complementary resource pulse use, which regulates their abundances, but other models suggest that tree abundance is regulated by the narrow recruitment niche of trees. Overly restrictive recruitment niches can cause a mismatch between resource availability, PF composition and ecosystem productivity. 4. Case study II. The tropical dry forest: Here, we examine niche separation between closely related species of same PFT. These species commonly have distinct temporal recruitment niches based on differences in seed and seedling traits. A diversification of recruitment niches may be necessary for sympatric speciation and has the effect of broadening of the recruitment 'portfolio' of a phylogenetic lineage and PFT. 5. Synthesis: Functional diversity, characterized by differences in adult resource use, optimizes ecosystem function in a pulsed resource environment only if PF abundances are regulated by adult resource use. Regulation through recruitment niches tends to uncouple plant productivity from resource availability. However, we hypothesize that a diversification of recruitment niches within PF s may help alleviate recruitment limitations and help communities attain a PF composition that optimizes resource use and permits adaptation to climate change.