Introduction

Tropical montane rainforests in the Andes are threatened by a vast number of human activities including climate change and increasing nutrient. Increased nutrient availability in inherently nitrogen (N) and phosphorus (P) limited montane forests can change functional community composition and ultimately ecosystem functioning.

Objectives

This study investigated how community functional composition and tree diameter growth in highly diverse tropical montane rainforests changed under altered nutrient availability over a period of 12 years. A particular focus was on the contribution of demographic components (growth, recruitment, mortality) to functional shifts and on evaluating the tree growth response using species functional traits. Methods This study took advantage of the NUMEX (Ecuadorian nutrient manipulation experiment) project conducted in South Ecuadorian old-growth montane rainforests, which aimed to investigate the effect of long-term nutrient addition in this highly diverse ecosystem. The experiment was conducted in forests along an altitudinal gradient with study sites at 1000, 2000 and 3000 m. Starting in 2008, moderate amounts of N (50 kg h-1 yr-1), P (10 kg ha-1 yr-1) and a combination of both were applied to treatment plots of 400 m2 of size. Ten leaf and stem functional traits were used to characterize the tree communities in 2008 and changes until 2020. Diameter growth of > 2000 individual trees was monitored over the study period and related to tree functional properties.

Results

The assessment of tree communities prior to the first fertilization revealed a pronounced gradient from functionally rich communities exhibiting more acquisitive resource use strategies (fast nutrient uptake and high growth rates) at low elevation towards rather resource conservative communities at high elevation. After 12 years of N and N+P fertilization, analysis of demographic components (recruitment, growth and mortality) contribution to changes in abundance-weighted mean trait values revealed a shift towards tree communities exhibiting more acquisitive resource use strategies. Tree growth differed across elevation sites, but overall N+P addition performed best when looking at trait level growth as well as ecosystem productivity inside plots, suggesting that co-limitation of N and P varies with elevation. Acquisitive species showed increased growth as a response to nutrient addition. Species-specific response was diverse but no negative responses to nutrient addition were observed. In general, individual species responded most positively to N+P addition.

Conclusions

Our results confirm the importance of nutrient availability in these highly diverse and still poorly understood tropical montane forests, highlighting that potential community shifts are likely to occur