Forest biomass density across large climate gradients in northern South America is related to water availability but not with temperature

Understanding and predicting the likely response of ecosystems to climate change are crucial challenges for ecology and for conservation biology. Nowhere is this challenge greater than in the tropics as these forests store more than half the total atmospheric carbon stock in their biomass. Biomass is determined by the balance between biomass inputs (i.e., growth) and outputs (mortality). We can expect therefore that conditions that favor high growth rates, such as abundant water supply, warmth, and nutrient-rich soils will tend to correlate with high biomass stocks. Our main objective is to describe the patterns of above ground biomass (AGB) stocks across major tropical forests across climatic gradients in Northwestern South America. We gathered data from 200 plots across the region, at elevations ranging between 0 to 3400 m. We estimated AGB based on allometric equations and values for stem density, basal area, and wood density weighted by basal area at the plot-level. We used two groups of climatic variables, namely mean annual temperature and actual evapotranspiration as surrogates of environmental energy, and annual precipitation, precipitation seasonality, and water availability as surrogates of water availability. We found that AGB is more closely related to water availability variables than to energy variables. In northwest South America, water availability influences carbon stocks principally by determining stand structure, i.e. basal area. When water deficits increase in tropical forests we can expect negative impact on biomass and hence carbon storage.

理解并预测生态系统对气候变化的潜在响应，是生态学与保护生物学领域的核心挑战。这一挑战在热带地区尤为严峻——热带森林的生物量储存了全球大气总碳储量的一半以上。生物量由其输入（即林木生长）与输出（即林木死亡）之间的动态平衡所决定。因此我们可以推测，有利于高生长速率的环境条件，如充足的水分供应、适宜的温度以及养分丰富的土壤，往往与较高的生物量储量呈正相关。本研究的核心目标是描述南美洲西北部不同气候梯度下主要热带森林的地上生物量（Above Ground Biomass, AGB）储量分布格局。我们在该区域内共收集了200块样地的调查数据，样地海拔跨度为0至3400米。研究基于异速生长方程，以及样地水平上以胸高断面积加权得到的茎密度、胸高断面积与木材密度数值，估算得到各站点的AGB储量。我们选用了两组气候变量：年平均气温与实际蒸散量作为环境能量的替代指标，年降水量、降水季节变率与水分可利用性作为水分可利用性的替代指标。分析结果显示，AGB储量与水分可利用性变量的相关性显著高于其与环境能量变量的相关性。在南美洲西北部区域，水分可利用性主要通过调控林分结构（即胸高断面积）来影响森林碳储量。当热带森林的水分亏缺程度加剧时，其生物量乃至整体碳储存能力均会受到负面影响。