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The influence of soil properties and nutrients on conifer forest growth in Sweden, and the first steps in developing a nutrient availability metric

Abstract. The availability of nutrients is one of the factors that regulate
terrestrial carbon cycling and modify ecosystem responses to environmental
changes. Nonetheless, nutrient availability is often overlooked in
climate–carbon cycle studies because it depends on the interplay of various
soil factors that would ideally be comprised into metrics applicable at large
spatial scales. Such metrics do not currently exist. Here, we use a Swedish
forest inventory database that contains soil data and tree growth data for
> 2500 forests across Sweden to (i) test which combination of
soil factors best explains variation in tree growth, (ii) evaluate an
existing metric of constraints on nutrient availability, and (iii) adjust
this metric for boreal forest data. With (iii), we thus aimed to provide an
adjustable nutrient metric, applicable for Sweden and with potential for
elaboration to other regions. While taking into account confounding factors
such as climate, N deposition, and soil oxygen availability, our analyses
revealed that the soil organic carbon concentration (SOC) and the ratio of
soil carbon to nitrogen (C : N) were the most important factors explaining
variation in “normalized” (climate-independent) productivity (mean annual
volume increment – m3 ha−1 yr−1) across Sweden. Normalized
forest productivity was significantly negatively related to the soil C : N
ratio (R2 = 0.02–0.13), while SOC exhibited an empirical optimum
(R2 = 0.05–0.15). For the metric, we started from a (yet
unvalidated) metric for constraints on nutrient availability that was
previously developed by the International Institute for Applied Systems
Analysis (IIASA – Laxenburg, Austria) for evaluating potential productivity
of arable land. This IIASA metric requires information on soil properties
that are indicative of nutrient availability (SOC, soil texture, total
exchangeable bases – TEB, and pH) and is based on theoretical considerations
that are also generally valid for nonagricultural ecosystems. However, the
IIASA metric was unrelated to normalized forest productivity across Sweden
(R2 = 0.00–0.01) because the soil factors under consideration were
not optimally implemented according to the Swedish data, and because the soil
C : N ratio was not included. Using two methods (each one based on a
different way of normalizing productivity for climate), we adjusted this
metric by incorporating soil C : N and modifying the relationship between
SOC and nutrient availability in view of the observed relationships across
our database. In contrast to the IIASA metric, the adjusted metrics explained
some variation in normalized productivity in the database
(R2 = 0.03–0.21; depending on the applied method). A test for five
manually selected local fertility gradients in our database revealed a
significant and stronger relationship between the adjusted metrics and
productivity for each of the gradients (R2 = 0.09–0.38). This study
thus shows for the first time how nutrient availability metrics can be
evaluated and adjusted for a particular ecosystem type, using a large-scale
database.
