Morphological and chemical leaf traits of tropical montane tree species and their responses to warming

Abstract: Leaf morphological and chemical traits of tropical trees vary along climate gradients, but it is currently unclear how they will respond to a rapidly warming climate and how this will vary among species. Considering this, tropical trees native to East and Central Africa were investigated in three complementary studies in Rwanda. Differently aged trees of both early (ES) and late (LS) successional species from two types of upland forest, Lake Victoria transitional rainforest (LVTF;1600-2000 m a.s.l.) and Tropical montane rainforest (TMF; 2000–3000 m a.s.l.) were included. The three studies were: (i) an elevation gradient study on young trees of 18 species planted at three sites, from 2400 m to 1300 m a.s.l (15.2-20.6 ºC mean annual temperature, MAT); (ii) an elevation gradient study on mature trees of four species growing at five sites from 2700 to 1700 m a.s.l (13.3-19.5 ºC MAT); and (iii) a study of 20 mature tree species growing in 15 permanent monitoring plots in Nyungwe TMF located between 2500 to 1950 m a.s.l. (14.1-16.1 ºC MAT). The variation in leaf morphology (LMA: leaf mass per area, LA: leaf size or area, leaf W/L: leaf width to length ratio or leaf shape) and leaf macro- and micronutrients were investigated. LA and LMA decreased while leaf W/L increased with decreasing elevation in most species. The decline in LMA with declining elevation was mostly due to decreasing leaf density rather than leaf volume per unit area. The leaf morphological elevation responses of young and mature trees were essentially the same for the four species included in all three studies. The elevation gradient site effects on leaf morphology are likely related to both increasing temperature and vapour pressure deficit (VPD) from high to the low elevation sites. The most limiting nutrient was P independently of ontogeny, successional strategy, and elevation, although for some species and age classes also K, S, Zn, Cu could potentially be co-limiting. The ratios of most nutrient elements (except K, Ca, Mn) to N were higher in mature compared to young trees. Leaf nutrient resorption efficiency varied among species but not between successional groups. The juvenile tree leaf concentration of most nutrients across species and sites correlated significantly with its soil concentration (N, P, K, Ca, Mg, Fe, Zn, and Cu) and soil pH (K, Ca, Mg, Fe, Mn, Zn, B, Cu, and Mo). Down the elevation gradient, mass-based leaf contents of Ca, Mg, Fe, B and Mo increased while N, P, Zn, and Cu decreased with most species responding similarly., while the elevation effect of other elements were more variable among species. The nutrients declining down the elevation were all potentially limiting and warming will therefore likely enhance nutrient limitation and possibly lead to reduced productivity of these species. However, no such elevation trend was observed for mature trees. The elevation effect may therefore be transient with age. In general, and independently of tree age, the variation in both leaf morphology and leaf nutrient content and ratios was considerably larger between species than between sites, despite large site differences in both soil and climate conditions. Several morphological and nutrient variables differed significantly between successional groups, but the species identity explained more the main variation between species than the successional strategy. This study therefore clearly shows the risk of predicting forest responses to climate change using only few species, highlighting the need for more data from tropical trees.