Alpine grassland plants grow earlier and faster but biomass remains unchanged over 35 years of climate change

Abstract

Satellite data indicate significant advancement in alpine spring phenology over decades of climate warming, but corresponding field evidence is scarce. It is also unknown whether this advancement results from an earlier shift of phenological events, or enhancement of plant growth under unchanged phenological pattern. By analyzing a 35-year dataset of seasonal biomass dynamics of a Tibetan alpine grassland, we show that climate change promoted both earlier phenology and faster growth, without changing annual biomass production. Biomass production increased in spring due to a warming-induced earlier onset of plant growth, but decreased in autumn due mainly to increased water stress. Plants grew faster but the fast-growing period shortened during the mid-growing season. These findings provide the first in situ evidence of long-term changes in growth patterns in alpine grassland plant communities, and suggest that earlier phenology and faster growth will jointly contribute to plant growth in a warming climate.

Keywords: alpine grassland; biomass production; climate warming; ecosystem function; functional group composition; phenology; plant growth; the Tibetan Plateau.

Figure 1

Conceptual representation of the mechanisms of the advancement in spring phenology under climate warming. Four potential scenarios responsible for this advancement are presented, including earlier shift of phenological pattern (a), enhancement of growth under the same phenological pattern (b), both earlier shift of phenology and enhancement of growth (c), and earlier shift of phenology, enhancement of growth, and compression of growth period (d).

Figure 2

Long‐term (1980–2014) changes in annual and seasonal aboveground biomass production at Haibei research station. The temporal trends in annual (a), spring (b), summer (c) and autumn (d) biomass production. Solid and dashed regression lines indicate statistically significant and non‐significant trends at the 0.05 level, respectively.

Figure 3

Long‐term (1980–2014) changes in plant growth patterns. (a) Data are mean values in 1980s (n = 7 years), 2000s (n = 7 years) and 2010s (n = 4 years). (b) The temporal trends in the start, end and length of fast‐growing phase. (c) The temporal trends in rate and timing of maximum growth. Solid regression lines indicate statistically significant trends at the 0.05 level.

Figure 4

Controls of air temperature on the start of the fast‐growing phase and soil moisture on the end of the fast‐growing phase. Seasonal dynamics in monthly mean air temperature (a) and soil moisture at the 5 cm depth (b) and their changing trends. Lines with circles indicate monthly mean values; bars indicate their changing rates, as expressed by the slopes of linear regressions between years and monthly averages, and * indicates statistically significant at P < 0.05. The insets in (a) and (b) indicate the interannual trends in annual mean air temperature and soil moisture, respectively. Relationships between pre‐ and early‐season growing degree days (January–May) and the start of the fast‐growing phase (c) and between mid‐season soil moisture (June–August) and the end of the fast‐growing phase (d). Solid and dashed regression lines indicate statistically significant and non‐significant trends at the 0.05 level, respectively.

Figure 5

Comparisons of growth patterns of different plant functional groups. Data are mean values during 1980–1983 (a) and 2007–2010 (b). The insets in (a) and (b) indicate relative abundance of different functional groups.

Figure 6

Illustration of mechanisms of long‐term changes in plant growth patterns in a Tibetan alpine grassland. Climate warming shifted the start of fast‐growing phase earlier and enhanced rate of maximum growth while reduction in mid‐season soil moisture accelerated the end of fast‐growing phase. In contrast, a shift in functional group composition towards grasses induced by reduced soil moisture contributed less to community phenology and growth rate (black dotted arrows in Figure). The positive effect of enhanced maximum growth on biomass production of fast‐growing phase was cancelled out by the negative effect of shortened growth period. The earlier phenology led to an increase in spring biomass production and a decrease in autumn biomass production. The increased spring biomass production contributed to reduced mid‐season soil moisture due to more water demand of plant growth.